# Fileset

[2025A01543G_manuscript.pdf](https://mdr.nims.go.jp/filesets/a577c4e4-3e5c-4d48-b9ae-72b00ff8a48b/download)

## Creator

Marzia Cuccu, Tommaso Venanzi, Edith Wietek, Xiaoxiao Sun, Raul Perea-Causin, [Takashi Taniguchi](https://orcid.org/0000-0002-1467-3105), [Kenji Watanabe](https://orcid.org/0000-0003-3701-8119), Ermin Malic, Manfred Helm, Stephan Winnerl, Alexey Chernikov

## Rights

©2025 American Physical Society[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

## Other metadata

[Terahertz-induced population transfer between exciton complexes in monolayer                    <math>                      <msub>                        <mi>WSe</mi>                        <mn>2</mn>                      </msub>                    </math>](https://mdr.nims.go.jp/datasets/231ccbc6-ff86-489f-a45f-8317c9f06731)

## Fulltext

Terahertz-induced population transfer between exciton complexes in monolayer WSe2Marzia Cuccu*,1 Tommaso Venanzi*,2 Edith Wietek,1 Xiaoxiao Sun,3 Raul Perea-Causin,4 TakashiTaniguchi,5 Kenji Watanabe,6 Ermin Malic,7 Manfred Helm,3, 8 Stephan Winnerl,3 and Alexey Chernikov1, ∗1Institute of Applied Physics and Würzburg-Dresden Cluster of Excellence ct.qmat,TUD Dresden University of Technology, Dresden, Germany2Center for Life Nanoscience, Istituto Italiano di Tecnologia, Rome, Italy3Institute of Ion Beam Physics and Materials Research,Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany4Department of Physics, Stockholm University, Stockholm, Sweden5Research Center for Materials Nanoarchitectonics,National Institute for Materials Science, Tsukuba, Japan6Research Center for Electronic and Optical Materials,National Institute for Materials Science, Tsukuba, Japan7Department of Physics, Philipps-Universität Marburg, Marburg, Germany8Institute of Applied Physics, TUD Dresden University of Technology, Dresden, GermanyTwo-dimensional van der Waals semiconductors feature a variety of stable Coulomb-boundelectron-hole complexes, which determine the optical response of the materials and serve as primarycarriers of energy and spin-valley encoded information. Importantly, transitions between differentexcitonic states are found in the terahertz spectral range, motivating the use of strong THz radiationfor their manipulation on ultrafast timescales. In this work, we apply this technique to efficientlytransfer populations within the manifold of excitonic complexes in monolayer WSe2, combiningpulsed optical injection with a perturbation induced by a THz free-electron laser source. Monitor-ing time-resolved photoluminescence we show conversion between different Coulomb-bound speciesacross biexcitonic and excitonic regimes. Depending on the lattice temperature, these processesinvolve both short-lived bright and long-lived dark states. Combining experimental findings withtheory support, we outline possible dissociation and formation pathways of charged excitons andbiexcitons induced by the THz radiation. Finally, we demonstrate the access to formation dynamicsof charged biexcitons under controlled conditions of thermalized populations of their constituents,avoiding complications of excess energies that otherwise occur after non-resonant optical excitation.Keywords: excitons, trions, biexcitons, terahertz, two-dimensional materials, ultrafast microscopyI. INTRODUCTIONLow-dimensional semiconductors, such as monolayertransition-metal dichalcogenides (TMDCs), feature a va-riety of excitonic complexes with large binding energiesthat form due to strong Coulomb interactions [1, 2].These include neutral excitons, trions [3–5] and Fermipolarons [6–9], neutral and charged biexcitons [10–14] aswell as increasingly exotic states [15] up to the Mott tran-sition [16, 17]. Excitonic complexes dominate both theoptical properties of these materials and serve as pri-mary carriers for energy and spin-valley-encoded infor-mation. Consequently, it became important to find ef-fective means for their manipulation and tuning of theirproperties. A variety of successful approaches have beenemployed, which make use of external electric [18], mag-netic [19], optical [20], and strain [21, 22] fields, changesin the dielectric environment [23] and chemical function-alization [24].In this context, a particular challenge is to trigger con-version of one type of the excitonic species into another∗ alexey.chernikov@tu-dresden.deon ultrafast time-scales, especially from equilibrated pop-ulations of low-energy states. A useful approach, demon-strated for conceptually similar quantum well systems, isthe employment of low-frequency radiation following theinjection of excitons using optical pulses. Due to compar-atively low binding energies of the excitonic complexes ofa few meV in these systems, this was achieved using eitherstrong microwave radiation [25–27] or frequencies on theorder of a THz [28–30]. The underlying mechanisms in-volved impact ionization, multi-photon processes or cou-pling to the intra-excitonic transitions. For TMDCs, sim-ilar to bulk CuO2 [31], the relevant energy scales are onthe order of a few 10’s up to 100’s of meV instead. There-fore, many-THz photon energies provide access to theproperties of excitonic states and their manipulation inatomically-thin materials [32–36].Consequently, radiation in this range can be used toinduce transfer between different excitonic species. Asa proof of concept, we have demonstrated how a com-bination of optical injection with strong THz excita-tion by a free-electron laser can rapidly convert excitonsdressed by free electrons into neutral excitons in mono-layer MoSe2 [37]. In close analogy to the light-inducedionization of molecules and atoms, this transient processinvolves photodetachment of an electron from the com-mailto:alexey.chernikov@tu-dresden.de2posite trion state tunable by an externally set delay be-tween the optical and THz pulses. While highly promis-ing, the general applicability of this approach for othertypes of TMDCs with a more complex excitonic struc-ture involving long-lived dark states remains an openquestion. Similarly, it is unclear whether higher-particlestates such as biexcitons could be manipulated in thesame manner. The capacity to control biexcitons, in par-ticular, is especially interesting given their importancefor quantum information technologies and entanglement-based sources of photons [38, 39].Here we apply the optical-pump/THz-push approachto WSe2 monolayers, which host a rich excitonic struc-ture involving dark states and higher-order excitoniccomplexes. After the optical injection and subsequentequilibration, a THz pulse with a set delay triggers atransient transfer of exciton populations within picosec-onds. Monitoring time-resolved photoluminescence, weobserve almost complete quenching of the dominant emis-sion from charged biexcitons and n-type trions. Thisis accompanied by a transient brightening of neutralbiexcitons and bright exciton states followed by subse-quent reformation of charged biexcitons within a few upto tens of ps. Supported by theoretical considerationsextending the description of the exciton-trion scenarioto biexcitonic complexes, we discuss possible pathwaysand demonstrate the applicability of the approach overa broad temperature range up to 100 K. In this manner,we also gain access to the temperature dependent forma-tion time of the charged biexcitons, avoiding the compli-cations of substantial excess energies after more typicalnon-resonant optical injection conditions.II. EXPERIMENTAL DETAILSThe investigated sample was a mechanically exfoliatedWSe2 monolayer encapsulated in hBN to reduce disor-der [40, 41] and deposited on a diamond substrate usingvisco-elastic transfer [42]. The transparency of diamondto THz radiation prevents additional heating of the struc-ture from the residual THz absorption. The initial opti-cal characterization of the monolayer was performed withthe sample held in a microscopy-cryostat at the tempera-ture 5 K and using a continuous-wave laser with a photonenergy of 2.3 eV for excitation. The laser was focused to aspot of 1µm diameter by a 60x objective. For the hyper-spatial mapping the power density was set to 74 W/cm2,corresponding to an estimated steady-state electron-holepair density on the order of 1010 cm−2. The step sizewas 1µm in both x- and y-directions. The resulting pho-toluminescence (PL) signals were recorded with a CCDafter passing through an imaging spectrometer. A mi-crograph of the sample is shown in Figure 1 (a) togetherwith a map of the bright exciton peak energy extractedfrom the hyperspatial photoluminescence mapping. Theobserved energy shifts on the order of a few 10’s of meVare typical for this type of samples. They are attributedto the roughness of the diamond leading to mechanicalstrain that is inhomogeneously distributed in the mono-layer. For the subsequent measurements we identifiedsufficiently large homogeneous regions with reasonablynarrow linewidths of the exciton resonances.The experimental configuration for the optical-pump/THz-push experiments is schematically illustratedin Fig. 1 (d). The sample was mounted on a cold-fingermicroscopy cryostat and cooled down to temperatures be-tween 5 and 130 K. To create the initial population of theexcitonic states, we used an optical pulse with a photonenergy of 3.1 eV obtained from the second-harmonic of apicosecond Ti:sapphire laser. The laser was focused to a2µm spot on the sample using a 50x objective, resultingin typical fluence of several 10’s of µJ/cm2. Narrow-bandpicosecond THz pulses (see Appendix A) generated withan infrared free-electron laser (FEL) [43] were then usedas a perturbation to the system after an externally settime delay. PL emission from the sample was dispersedin an imaging spectrometer and detected in time by astreak camera. The THz pulse duration was approxi-mately 5 to 10 ps and the nominal time-resolution of thePL detection, extracted from the scattered light of theoptical pulse using the streak camera, was determinedas 5 ps. The experiments were performed as a functionof temperature, THz excitation power, and for severalselected THz photon energies.III. THZ-INDUCED TRANSITIONS BETWEENEXCITONIC STATESTypical low-temperature PL spectra of the investi-gated sample obtained under continuous-wave excitationare presented in Figure 1 (b). The spectrum recordedat a low power density shows the characteristic, spec-trally narrow features of an hBN-encapsulated mono-layer WSe2 with small residual n-type doping [44–46].The features include neutral excitons (X0) and biexcitons(XX), bright trions (X−), neutral (D0) and charged (D−)dark states and phonon sidebands (P) as well as chargedbiexcitons (XX−). Both neutral and charged biexcitonsbecome more pronounced with increasing power density[10, 11, 13], as evidenced in the spectrum recorded at ahigher excitation power presented in Figure 1 (b). Thisis reflected in the extracted ratios of neutral and chargedbiexcitons compared to the neutral exciton. At low powerdensity these are XX−/X0 = 0.04 and XX/X0 = 0.01,increasing substantially for the spectrum at high powerdensity, where XX−/X0 = 1.9 and XX/X0 = 0.4.Corresponding streak camera image of the time- andspectrally-resolved PL is presented in Figure 1 (c). Thephoton energies of the peaks are plotted on a scale rela-tive to the X0 resonance at 1.72 eV. The emission spec-trum matches the main resonances of the continuous-wave measurement aside from potential photodoping andadditional broadening. The latter stems from the largerexcitation area, smaller spectral resolution of the setup30200400PL (arbitrary units)X0X-D0XX-P, D-XX-XX X0X-Time (ps)(b)0-25-50Energy relative to X0 (meV)(d)Time (ps)-25 0-50(c)THz(e)PL10200300XX10 µmX0 (eV) (a)1.70 1.72 1.74hBNhBNDiamondDtXX- XX X0X-Energy relative to X0 (meV)w/o. THz with THz1L WSe22 kW/cm244 W/cm2T = 5 KFIG. 1. (a) Top panel: optical micrograph of the sample with the highlighted WSe2 monolayer. Bottom panel: spatial mapof the bright exciton energy peak extracted from the PL spectra at 5 K under continuous wave excitation, superimposed onthe optical image. The black circle marks the area where the optical-pump/THz-push measurements were performed. (b)A representative PL spectra of the WSe2 sample under continuous wave excitation at high (2 kW/cm2) and low (44 W/cm2)power densities. (c) Spectrally- and time-resolved streak camera image in absence of terahertz pulse and the correspondingtime-integrated PL spectrum obtained for pulsed excitation using the fluence of 28µJ/cm2 per pulse. The shaded white areaindicates the time range studied in the subsequent measurements involving THz excitation. (d) Schematic illustration of theexperimental setting for the optical-pump/THz-push experiments. ∆t indicates the time delay between the optical and theTHz pulses. (e) Streak camera image of the PL in the presence of a strong THz pulse, with a fluence of 30µJ/cm2, that arriveswith a time delay of 250 ps after the optical excitation.for time-resolved experiments and, most importantly,higher optical pump density. In particular, the fluenceof the pulsed laser of 30µJ/cm2 corresponds to the av-erage power density of 6 MW/cm2, which is three ordersof magnitude higher than that used for the high-powerPL spectrum shown in Figure 1 (b). This leads to a fur-ther enhancement of the PL intensity ratio of XX− andXX compared to X0). The spectrum in Fig.1 (b) (bot-tom), yields PL ratios of XX−/X0 = 3.2 and XX/X0= 0.8, both of which are significantly higher than thecorresponding values extracted from the spectra undercontinuous-wave excitation. The absence of the D0 fea-ture is attributed to both high fluence with the dominantXX- emission and the limited observation time windowin the spectrally- and time-resolved measurements.After the optical excitation at t = 0 ps, the brightstates X0 and X− (representing the trion doublet) de-cay within a few 10’s of picoseconds, while charged biex-citon, that is composed of a bright and a dark state,decays much slower over a few 100’s of picoseconds. Forthe experiments involving THz excitation, we thus focuson this later time range, as indicated by the shaded re-gion in the bottom panel of Fig. 1 (c). This enables us toinvestigate the case of thermalized excitonic populations[47, 48] when only the long lived XX− state and a resid-ual trace of bright trions are visible. We also note, that,while the image presented in Figure 1 (c) was recordedas a general overview, the overall acquisition time wasincreased substantially for all subsequent measurementsto ensure high signal-to-noise ratios.When the THz pulse arrives, at tTHz = 250 ps after theoptical excitation, the emission dynamics changes sub-stantially, as shown in Figure 1 (e). For this measure-ment, the photon energy and fluence of the THz beamwere set to 14 meV and 30 µJ/cm2, respectively. Themagnitude of the changes is overall similar to the pre-viously studied case of the MoSe2 monolayer [37], albeitboth the time delays are substantially longer and the ori-gin of the emission involves biexcitons in the present case.Here, the THz radiation causes a strong quenching of thecharged biexciton and residual trion populations and in-duces a brightening of both neutral exciton and biexcitonstates. The quenching occurs on the same timescale forcharged biexcitons and trions within the time-resolutionof the setup (see Appendix B). The impact of the THzpulse resembles an effective reset to the initial conditionsof the system at time t = 0, corresponding to the exci-tation by the optical pulse. This effect is evident in thebroadening of the trion emission, potentially related to ahigher temperature of the electronic distribution [36] and4Brightening B016 (b)THz fluence (µJ cm-2)0 10 20 30250 300Time (ps)XX-X0XXX-PL (arb. u., offset)(a)Quenching Q-1.0-0.50X0XXX-XX-0 50t – tTHz (ps)(c)(1)X-X0e-+Trion(2) (3)XX-XXe-+D0X-+Charged biexcitonTHzTHz8100502575125FIG. 2. (a) PL transients extracted at the energies of theexcitonic resonances in the time window of the THz pulsearrival presented together with the corresponding fit curves.The characteristic time constants for the THz pulse arrivalare of similar values for all fit curves within the temporal res-olution of 5 ps. Maximal duration of the THz pulse is addedfor comparison. The fluence of the THz pulse 30µJ/cm2 (b)Top panel: brightening factor B of the bright exciton X0 andneutral biexciton XX as a function of the THz fluence. Bot-tom panel: quenching factor Q of the negative trion X− andthe charged biexciton XX−; the gray dashed line separatesthe linear regime from the saturation regime. (c) Schematicillustration of the possible THz-induced dissociation pathwaysof the trions and charged biexciton. Gray shading indicatescontributions of the spin-dark states.its impact on the trion lineshape via recoil effect [49]. Inaddition, the increase of the intensity at the high energyside of the trion could similarly stem from the transientredistribution of the PL intensity within the spin-splittrion doublet [50] (see Appendix C). Here, the higher en-ergy trion initially exhibits higher PL intensity that shiftstowards the lower energy trion at later times [51]. Wethus speculate that the arrival of the THz pulse returnsthe distribution of the PL within the trion-doublet to theconditions resembling those immediately after the opticalexcitation.Most importantly, however, the simultaneous change indynamics demonstrates an effective population transferbetween charged and neutral excitonic complexes, illus-trated by the corresponding PL transients presented inFigure 2 (a). These are obtained by integrating the PLsignal over the spectral region of interest of each peak,after the background subtraction of the streak cameraimage. We also add the time scale t − tTHz, relative tothe arrival of the THz pulse, and use it for the rest ofthe presented data. Following the THz-induced redistri-bution of the PL intensity, the initial PL counts prior tothe THz pulse arrival recover within 10’s of ps. We alsonote that the initial changes of the dynamics occur onthe same time-scale during THz-pulse arrival. Slight dif-ferences in the positions of the minima and maxima areattributed to the interplay of depopulation and popula-tion dynamics, as discussed in more detail below, as wellas differences in the reformation times that are either fastor slow, depending on the given state.For the analysis of the THz-induced dynamics we note,that the full picture of interacting charged and neutralexciton and biexciton states is a very complex problemand is currently not accessible in a unified, microscopicframework. We thus use both qualitative comparisonwith the generic picture of a charged-to-neutral trans-fer scenario further below and phenomenological fittingof the experimental data. In particular, the transients ofthe excitonic peaks are fitted with a single exponentialfunction multiplied by a Heaviside function, convolutedwith a Gaussian to account for the limited time resolutionof the experiment given by the streak camera resolutionand the pulse lengths. The resulting fitting function isthe following:PL(t) =γNH(t− t0) e−(t−tTHz)/τ + a t+ b,where the parameter γ is the quenching (if negative) orbrightening (if positive) factor, N is a normalization fac-tor to the intensity prior to the THz pulse arrival attTHz, τ is the time characteristic constant of the recov-ery. H(t− t0) indicates the modified Heaviside functionerfc(− (t − t0)/√2σ + σ/√2τ), where σ is the stan-dard deviation of the Gaussian function. The linear term(a t+ b) approximates the PL emission dynamics in thisvery short time-window compared to the overall long de-cay time of the signal.Using this fit function we extract the brightening B ofthe X0 and XX states and the quenching Q of the X−and XX− states. The extracted quenching and bright-ening factors are subsequently normalized with respectto the PL intensity of the corresponding populations attime tTHz in the absence of THz radiation, enabling aquantitative analysis of the impact of the FEL on eachexcitonic population. The resulting values are shown inFigure 2 (b) as a function of THz fluence. The initial lin-ear dependence of both quenching and brightening fac-tors indicates a predominant one-photon origin of theunderlying processes. At higher fluences, it is likely thatthe finite doping density in the material contributes tothe saturation of the quenching. We note, however, thatdue to the potential competition of the THz-induced dis-sociation and filling dynamics of the trion state in partic-5ular, quantitative assessment of the saturation behavioris likely to be more complex. The brightening of the XXalso saturates, while the intensity of the X0 state contin-ues to increase at higher powers. While this differencestems from the data point for the highest THz fluence,one could in principle consider THz-induced dissociationof the bright trions to serve as an additional pathway tocreate neutral excitons.In general, there are several possible dissociation chan-nels for the initial populations of trion and charged biex-citon states at the arrival of the THz pulse, as schemati-cally illustrated in Fig. 2 (c). Upon THz photon absorp-tion, a trion separates into a bright exciton and a freeelectron (e−):X− → X0 + e− . (1)For the charged biexciton, being a five particle state, inanalogy to the negative ion of the hydrogen molecule H2−[52, 53], two pathways are in principle possible. It canseparate into a neutral biexciton and a free electronXX− → XX + e−, (2)or, alternatively, dissociate into a (dark/bright) excitonand a (bright/dark) trion, considering the typical com-position of the charged biexciton binding an electron, abright and a dark exciton states [54]:XX− → D0 +X − . (3)The process of trion photodetachment (1) accounts forthe quenching of the trion population and should alsoserve as the primary source for the observed brighten-ing of the neutral exciton. We note, however, that thisprocess may not be highly efficient in absolute terms asthe energy of THz pulse of 14 meV is below the trionbinding energies in the range of 30 meV [50]. The sec-ond process of charged biexciton dissociation, (2), thenacts as a main source for the THz-induced population ofneutral biexcitons. Similar to X0, the low intensity of thebrightening is attributed to the different THz photon andcharged biexciton binding energies. The latter is similarto the trion binding energy of about 30 meV as extractedfrom the energy separation between the XX− and theXX states. Finally, the dissociation process (3), with abinding energy in the range of 20 meV, contributes to thestrong decrease of the XX− states while simultaneouslyincreasing the trion population. This should account forthe bleaching of the trion being less pronounced com-pared to that of the charged biexciton. Moreover, chan-nel (3) may also produce bright excitons either directlyor from the dissociation of trions.In addition, higher-order processes, cannot be fully ex-cluded as a contribution to the brightening of the neu-tral exciton, especially at elevated THz fluence. We alsonote that similar observations are obtained for the lower(9 meV) and higher (35 meV) photon energies of the THzpulse (see Appendix D). This may further indicate theadditional role of higher-order processes, while preclud-ing reliable statements regarding the resonance behaviordue to the coarse energy intervals between these mea-surements.In principle, the two dissociation processes of thecharged biexciton, (2) and (3), can be conceptually con-sidered as the biexcitonic versions of the trion photode-tachment, (1). Thus, the model used to describe the lat-ter [37] can be adjusted by changing the effective massesof its constituents. In that case biexcitons and trionsare taking the role of excitons and electrons, respec-tively, in this simplified description (see Appendix E).Then, the matrix element for the THz-induced dissoci-ation depends only on the Bohr radius and the reducedmass of the composite exciton states involved. Com-paring the case of the trions and charged biexcitons,the reduced masses are µX− = mXme/(mX +me) andµXX− = 2mXme/(2mX + me), respectively. Assumingme = mh, that reasonably applies for WSe2, the result-ing differences in the reduced masses are rather smallwith µX− = 2/3me and µXX− = 4/5me. Thus, despitemuch heavier total mass, the reduced mass of the chargedbiexciton is only 20 % higher than that of the trion, ren-dering the processes (1) and (2) very similar. For thedissociation pathway (3) into an exciton and a trion thedifference is factor of two, rendering it even more efficientfor sufficiently high THz photon energies compared to (1)(see Appendix E).We note that we addressed the dissociation of chargedexcitonic particles in terms of photon energy rather thanTHz-field-driven ionization. This choice is qualitativelymotivated by estimating the THz electric field strengthrelative to the relevant binding energies. The maximumelectric field in air is about 60 kV/cm at 30 µJ/cm2 an 87µm, corresponding to a ponderomotive energy of aboutUp =e2E2THz4m∗ωTHz= 3meV. Considering the exciton bindingenergy of a about Ip = 300 meV, the Keldysh parameteris√Ip2Up∼ 10 [55]. This condition is far from the field-regime and supports the use of a photon-energy-baseddescription.IV. TEMPERATURE-DEPENDENT DYNAMICSThe observed interplay between different excitoniccomplexes in WSe2 triggered by THz radiation moti-vates temperature-dependent measurements to changethe population of the initial states. Streak camera im-ages at selected temperatures are presented in Figure 3(a) together with the extracted transients at individualresonances and corresponding fits shown in (b). Thecomplete data set is presented in Appendix F. The PLdecays of the four resonances are obtained by summingover the energy regions of Figure 3 (a), each centered atthe energy of the corresponding excitonic state. Overall,it shows that THz-induced population transfer in WSe2monolayers is a robust effect with respect to temperature,60-50-4040-404010 KXX- XX5070100(a)-4040-4040PL10(a)50 K70 K100 K0000PL (arb. u., offset)0 50100 K70 K50 K10 K(b)XX-XXX-X0XX-XXX-X0XX-XXX-X0XX-XXX-X0X0X-X0X-Time relative to the THz pulse, t –t THz(ps)t – tTHz (ps)Energy relative to X0 (meV)50100501005010050100FIG. 3. (a) Streak camera images of the spectrally- and time-dependent PL for different temperatures in the time rangeof the THz pulse arrival. The energy scale on the x-axis isrelative to the X0 resonance at 5 K. (b) Transients extractedat the energies of charged biexciton, negative trion, neutralbiexciton and bright exciton together with the fit curves. Thearrows indicate dominant THz-induced dissociation processesat 5 and 100 K. The THz pulse fluence used for the tempera-ture dependent measurements is 20µJ/cm2observed up to 130 K. Two distinct regimes are identified:at low temperatures the predominant process is the dis-sociation of the charged biexcitons, while, at higher tem-peratures only the interplay between bright trions andexcitons remains.Based on these observations, all three dissociationchannels outlined above are active up to 50 K. Around50 K, a transition regime begins where the charged biex-citons become increasingly less populated due to the el-evated temperature and their emission intensity gradu-ally decreases [10, 11]. This interplay is particularly ev-ident at 70 K. At this temperature, also the brighteningof the neutral biexcitons is completely suppressed, as fur-ther illustrated in the corresponding transient presentedin Fig. 3 (b). Simultaneously, the gradual depopulationof charged biexcitons leads to a decrease of dissociationchannel 3, resulting in reduced trion formation. This, inturn, induces a more pronounced quenching of the trionpopulation. For even higher temperatures of 100 K andabove, the only states that remain in the system are thebright excitons X0 and trions X−. The quenching dy-namics extracted at the spectral position of the XX−resonance can thus stem from the shoulder of the broad-ened X− peak in the 100 K data. Consequently, the pro-cess triggered by the arrival of the THz radiation is theone associated with (1). This result closely resembles thebehavior of MoSe2 monolayers at low temperature [56]further supporting the direct dissociation of trions intobright excitons and free electrons, as discussed in SectionIII.For the analysis, fits of the transients provide quench-ing and brightening factors of the four states for in-creasing temperature, as presented in Figs. 4 (a) and (b)for bright excitons and charged biexcitons, respectively.These allow for the quantitative assessment of the twotemperature-dependent regimes, as shown in Figure 4 (c).Notably, a crossover of the quenching factors of XX− andX− is observed around 70 K, originating in the thermaldissociation of the biexciton states, as discussed above.Specifically, the quenching factor of XX−, which domi-nates at low temperatures, strongly decreases due to thegradual vanishing of the dissociation channels 2 and 3caused by the thermal instability of the charged biexci-ton. In principle, one can also consider the heating ofthe excitonic complexes by the THz radiation, leading todecreased recombination rate and thus quenching. If thelattice temperature is already high, the additional heat-ing should be less pronounced. Moreover, increasinglyfaster reformation dynamics, as shown in panel (d), ap-proaching the time-scales of the THz-pulse length shouldrender the quenching less pronounced due to simultane-ous refilling. In contrast, the initially higher populationof the trions with increasing temperature attributed tothe thermal dissociation of biexcitons also leads to an in-crease in the quenching factor of X−. At the same time,the gradual suppression of the brightening factor of X0reflects the strong increase of the bright exciton popu-lation, which becomes dominant at higher temperatures.The brightening of the neutral biexciton, XX, slightly de-creases and vanishes completely at temperatures around50 K.Particularly interesting is the extracted time τform forthe re-formation of the charged biexciton as a functionof temperature, shown in Figure 4 (d). The values areextracted from the fits to the rising profile of the XX−after the arrival of the THz pulse and are substantiallylarger than the temporal resolution of the measurements.Importantly, here charged biexcitons are created with alow kinetic energy due to the quasi-resonant conditionwith the THz radiation. Therefore, we are able to ana-lyze the re-formation times in a pristine manner, avoid-ing any cooling and relaxation processes that otherwisewould occur after optical excitation. The overall behav-ior and the obtained time scales resemble the character-istics of trion and free carrier cooling times [49, 56] aswell as relaxation timescales typically found for excitoniccomplexes[57, 58]. These values are thus indicative ofcharged biexciton formation mediated by phonon scatter-ing. As the temperature increases, the phonon-assistedprocesses become more likely due to the occupation of7(b)Quenching Q-1.0-0.50Brightening B024Temperature (K)0 50 100X-XX-X0XX0 500 50PL (arb. u., offset)(c)XX- quenchingτ form(ps)01020(d)XX- formation80 K60 K5 K20 K40 K100 K(a)X0 brighteningt – tTHz (ps)t – tTHz (ps)80 K60 K5 K20 K40 K100 K120 K120 K50100150200FIG. 4. Transients of the bright exciton (a) and the charged biexciton (b) PL for increasing temperature with correspondingfit to the curves. (c) Top: brightening factor B of bright exciton (dark red) and neutral biexciton (red) as a function oftemperature at the time of arrival of the THz pulse. Bottom: quenching factor Q of negative trion (light blue) and chargedbiexciton (blue) as a function of temperature at the same time as for the upper panel. Brightening and quenching factors arenormalized with respect to the PL of the peak in the absence of the THz pulse. The gray dashed line indicates two regimes:the left one, in which the population is dominated by dark states, and the right one, in which only bright states are left in thesystem. (d) Re-formation time of the charged biexciton extracted from the fits in (b) as a function of temperature.both optical and acoustic zone-edge modes [59]. In ad-dition to phonons, however, the re-formation of chargedbiexcitons under the emission of photons could also be inprinciple possible. This would correspond to the reversepathways of (2) and (3) mechanisms, involving either acapture of an electron by a neutral biexciton or the bind-ing of an exciton to a trion, with the consequent emissionof a THz photon.V. CONCLUSIONWe have demonstrated transient population transferon picosecond time-scales between charged and neutralexcitonic complexes via THz absorption in WSe2 mono-layers. At highest THz fluences we report almost com-plete depletion of the charged biexciton and trion pop-ulations leading to the emergence of the biexciton andexciton luminescence. We discuss possible dissociationchannels and demonstrate the phenomenon for a broadrange of temperatures, from 4 up to 130 K. Two charac-teristic regimes are observed, dominated by the THz radi-ation coupling primarily to either charged biexcitons andtrions as a direct consequence of the changes in thermalpopulations. In this manner, we also gain access to thetemperature-dependent formation time of the chargedbiexciton during the relaxation of the excitonic system.Altogether, these findings show the general applicabilityof using THz radiation to switch between populations ofdifferent excitonic states, including exciton molecules inatomically-thin semiconductors. Both the manipulationof the exciton complexes on ultrafast time-scales as wellas the additional insights into the quasiparticle formationdynamics are expected be useful for a wide range of exci-tonic materials. One can also consider the application ofthe approach to excitons coupled to correlated electronicstates as well as consider its use for non-linear optics andphotonics merging optical and THz excitations in ultra-thin crystals.VI. ACKNOWLEDGMENTSWe thank M. M. Glazov and L. Baldassarre forhelpful discussions. Financial support by the DFGvia Emmy Noether Initiative (CH 1672/1, project ID287022282, A.C.), SFB 1277 (project B05, project ID:314695032, A.C.) and SFB1083 (project B09, projectID 223848855, E.M.), DFG project Propagation dy-namics of exciton electron complexes in atomically-thinsemiconductors (project ID 542873285, E.M., A.C.), theWürzburg-Dresden Cluster of Excellence on Complexityand Topology in Quantum Matter (ct.qmat) (EXC 2147,project ID 390858490, A.C.) is gratefully acknowledged.8-1000 1000Space (μm)Intensity (arb. u.)0FWHM 323 μm 00.51.0Intensity(a)(b)14.0Photon energy (meV)PL (arb. u.)14.5FWHM 0.22 meV(c)1 pixel = 80 μmFIG. 5. (a) Pyroelectric camera image of a representativeFEL beam with the photon energy of 14 meV (3.4 THz). Thesize of one pixel corresponds to 80 um. (b) Extracted profilefrom the image in (a) along with a Gaussian fit to the data.From the fit, a full-width-at-half-maximum of 323µm was ob-tained at the sample position for an FEL beam at energy of14 meV. (c) Normalized spectrum of the THz pulse at photonenergy of 14 meV. The extracted full-width-at-half-maximumof 0.22 meV was extracted from the Gaussian fit in red.Parts of this research were carried out at ELBE at theHelmholtz-Zentrum Dresden-Rossendorf e.V., a memberof the Helmholtz Association. We thank A. Wagner andthe ELBE team for support. A.C. acknowledges fund-ing from ERC through CoG CoulENGINE (GA num-ber 101001764). K.W. and T.T. acknowledge supportfrom the JSPS KAKENHI (grant numbers 20H00354,21H05233 and 23H02052) and World Premier Interna-tional Research Center Initiative (WPI), MEXT, Japan.M. C. and T. V. contributed equally to thismanuscript.Appendix A: THz spectrum and spot sizeTo capture the FEL beam size at the sample position,a pyroelectric infrared camera was used. The dimensionof the beam varies within the range of hundreds of µmdepending on the energy of the THz pulse. Figure 5 (a)250 300Time (ps)Normalized PLXX-X-0 50t – tTHz (ps)FIG. 6. Normalized (in intensity) transients of the chargedbiexciton (XX-) and trion (X-) emission corresponding to thedata from Figure 1 (e) of the main text. The data is showntogether with phenomenological fit curves modeling both riseand decay with exponentials.shows an exemplary image of the FEL spot size at an en-ergy of 14 meV, which corresponds to the energy used forthe results presented in the main manuscript. The cor-responding intensity profile, along with a Gaussian fit tothe data, is shown in Figure 5 (b), yielding a full-width-at-half-maximum of 323µm at the sample position. Forreference, a representative normalized spectrum of theTHz source centered at 14 meV is presented in Figure 5(c), along with the Gaussian fit and the extracted full-width-at-half-maximum of 0.22 meV, corresponding to apulse duration of approximately 5 ps.Appendix B: Trion and charged biexciton transientsWith the arrival of the THz radiation, both chargedbiexciton and trion populations undergo strong quench-ing on similar timescales, as shown in Figure 6. Here,the time zero, defined as the half-time of the initial THz-induced intensity decrease, is extracted from the fits tothe data using the function described in the main text.The extracted values are the same for both excitonic pop-ulations within the experimental uncertainty of severalpicoseconds. This indicates that the initial quenchingoccurs simultaneously, but due to the faster reformationdynamics of the trions, their minimum appears at earliertimes compared to that of the charged biexciton tran-sient.Appendix C: PL spectra before and after the THzpulseFrom the streak camera image in Figure 1 (e), the PLspectra are extracted at the times just before and afterthe arrival of the THz pulse, as shown in Figure 7. Thisdirect comparison reveals an additional broadening of the9before THzafter THzXX-X0XXX-PL (arb. units)1.64 1.68Energy (eV)1.72FIG. 7. PL spectra before (black) and after (red) the arrivalof the THz pulse, as extracted from the streak camera imageshown in Figure 1 (e) of the main text. The arrows indicatethe two spin-split trions.trion (X-) emission by approximately 10 meV in the spec-trum extracted after the arrival of the THz pulse, alongwith the appearance of a shoulder on the higher-energyside. The broadening could be associated with the in-crease of the trion PL flank related to the increase ofthe electronic temperature and recoil effect [36, 49]. Aprobable cause to the increased PL at the higher-energyflank is the consequence of the trion dissociation and thesubsequent reformation. Specifically, the trion in WSe2 issplit into a doublet peak due to exchange interaction [50].After optical excitation, the higher-energy trion usuallyexhibits stronger PL, followed by the subsequent redis-tribution into the lower-energy trion [51]. Here, the THzpulse acts as an effective reset of the initial conditions ofthe system at the time of the optical excitation, leadingto the reappearance of the higher-energy trion, observedas a shoulder in the red spectrum in Figure 7.Appendix D: Dissociation dynamics for differentTHz photon energiesThe presented spectrally- and time-resolved measure-ments were repeated at 5 K also for two additionalTHz photon energies, one below (9 meV) and one above(35 meV) the value presented in the main text. In bothcases, the results were similar to those obtained for the14 meV photon energy. Specifically, with the arrival ofthe THz pulse, we observe a transient quenching of thechanrged biexciton and negative trion populations, ac-companied by a brightening of the bright exciton andneutral biexciton populations. Brightening B of brightexciton and neutral biexciton as well as quenching Q ofnegative trion and charged biexciton are extracted fromthe fits to the transient profiles and normalized as de-scribed in the main text. The obtained values are shownper unit fluence in Figure 8 as a function of the THzphoton energy. The data does not show any strongBrightening B/ µJ cm-2(b)THz energy (meV)10 20 30Quenching Q/ µJ cm-2-0.10X0XXX-XX-(a)10FIG. 8. (a) Brightening factor B per unit fluence of brightexciton and neutral biexciton as a function of the THz photonenergy at the time of arrival of the THz pulse. (b) Quench-ing factor Q per unit fluence of negative trion and chargedbiexciton as a function of the THz photon energy at the sametime as for (a).dependence on the THz energy, indicating that the ef-ficiency of the transfer processes between neutral andcharged states remains constant in the studied range.This behavior could indicate that additional contribu-tion of higher-order processes may be involved in theTHz-induced transfer processes. Nonetheless, due to thecoarse energy scan between the measurements, the datadoes not allow to draw definitive conclusions regardingspecific resonance conditions.Appendix E: Adaptation of the trion dissociationmodel to charged biexcitonsIn Ref. [37] we modeled the conversion from chargedinto neutral excitons induced by a THz field with thematrix element dk = e0∫d2rψ∗(r) rϕk(r). The lat-ter describes the transition of an electron bound to theexciton into a free electron with relative momentum k.The bound and unbound states correspond to chargedand neutral excitons and are phenomenologically mod-eled by the wave functions ψ(r) =√2/πa2exp(−|r|/a)and ϕk(r) = exp(ik · r)/√A, respectively, with the Bohrradius a and the system area A. Here, we extend thismodel to account for the conversion from charged biexci-tons into neutral biexcitons. In the same spirit, an elec-tron bound to a biexciton can be described by the wavefunction ψ(r), and the same approach can be employed.10The conversion rate readsγi(ℏω) =2πℏ∑k|E · dk|2δ(ℏω −∆i −ℏ2k22µi), (E1)where i denotes either charged excitons (i = X−) orcharged biexcitons (i = XX−). Here, we have intro-duced the charged exciton and biexciton reduced massesµX− = mXme/(mX+me), µXX− = mXXme/(mXX+me),the binding energy ∆i, and the THz photon energy ℏω.Using the analytical expression for the matrix element,dk ∝ a2(ak)/[1 + (ak)2]5/2, the conversion rate readsγi(ε) =18πℏ|e0E|2a2i ξiξiε[1 + ξiε]5Θ(ε), (E2)with ε = ℏω − ∆i and ξi = 2µia2i /ℏ2. This ap-proach can also be used to describe the conversion froma charged biexciton into a neutral and a charged ex-citon by having the charged exciton take the role ofthe electron above, i.e. considering the reduced massµ′XX− = mXmX−/(mX +mX−).Appendix F: Temperature dependentmeasurements: complete data setHere, we present the complete data set of the temper-ature dependent measurements. Streak camera imagesof the spectrally- and time-resolved PL measured withtemperature between 130 and 5 K are shown in Figure9 from the top left (130 K) to the bottom right (5 K)panel. Note that, as mentioned in the main text, theenergy scale is relative to the bright exciton X0 reso-nance at 5 K and the time scale is relative to the timearrival of the THz pulse. The extracted transients forall temperatures, with corresponding fits, are illustratedin Figure 10 for neutral biexciton XX0 (a) and negativetrion X− (b), and in Figure 4 of the main text for brightexciton X0 (a) and charged biexciton XX− (b). Thetransition between biexcitonic and excitonic regimes areclearly identified. Specifically, at 5 K all four resonances(X0, XX, X−, XX−) are present, with the predominantTHz-induced process being the dissociation of chargedbiexcitons, following the pathways (2) and (3) describedin Section III of the main text. These two channels, to-gether with the one related to the trion (1), coexist upto around 60 K. Above this temperature, as illustratedin the transient profiles of Figure 10 (a), the brighten-ing of the charged biexciton is suppressed and with thisalso the dissociation channels (2) and (3). At the sametime, neutral exciton and negative trion populations gainin intensity, dominating the dynamics at higher temper-atures. Therefore, the only process activated by the THzradiation in this regime is the one related to the triondissociation (1). Overall, the THz-induced populationtransfer in monolayer WSe2 is shown to be robust withrespect to temperature, persisting up to 130 K.[1] G. Wang, A. Chernikov, M. M. Glazov, T. F. Heinz,X. Marie, T. Amand, and B. Urbaszek, Colloquium :Excitons in atomically thin transition metal dichalco-genides, Reviews of Modern Physics 90, 021001 (2018).[2] R. Perea-Causin, D. Erkensten, J. M. Fitzgerald, J. J. P.Thompson, R. Rosati, S. Brem, and E. Malic, Exciton op-tics, dynamics, and transport in atomically thin semicon-ductors, APL Materials 10, 10.1063/5.0107665 (2022).[3] K. F. Mak, K. He, C. Lee, G. H. Lee, J. Hone, T. F.Heinz, and J. Shan, Tightly bound trions in monolayerMoS2., Nat. Mater. 12, 207 (2013).[4] J. S. Ross, S. Wu, H. Yu, N. J. Ghimire, A. M. Jones,G. Aivazian, J. Yan, D. G. Mandrus, D. Xiao, W. Yao,and X. Xu, Electrical control of neutral and charged ex-citons in a monolayer semiconductor., Nat. Commun. 4,1474 (2013).[5] R. Perea-Causin, S. Brem, O. Schmidt, and E. Malic,Trion Photoluminescence and Trion Stability in Atomi-cally Thin Semiconductors, Physical Review Letters 132,036903 (2024).[6] M. Sidler, P. Back, O. Cotlet, A. Srivastava, T. Fink,M. Kroner, E. Demler, and A. Imamoglu, Fermi polaron-polaritons in charge-tunable atomically thin semiconduc-tors, Nature Physics 13, 255 (2017).[7] Y.-C. Chang, S.-Y. Shiau, and M. Combescot, Crossoverfrom trion-hole complex to exciton-polaron in n -dopedtwo-dimensional semiconductor quantum wells, PhysicalReview B 98, 235203 (2018).[8] D. K. Efimkin, E. K. Laird, J. Levinsen, M. M. Parish,and A. H. MacDonald, Electron-exciton interactions inthe exciton-polaron problem, Physical Review B 103,075417 (2021).[9] M. M. Glazov, Optical properties of charged excitons intwo-dimensional semiconductors, The Journal of Chemi-cal Physics 153, 034703 (2020), arXiv:2005.05829.[10] Y. You, X. Zhang, T. C. Berkelbach, M. S. Hybertsen,D. R. Reichman, and T. F. Heinz, Observation of biexci-tons in monolayer WSe2, Nat. Phys. 11, 477 (2015).[11] S.-Y. Chen, T. Goldstein, T. Taniguchi, K. Watanabe,and J. Yan, Coulomb-bound four- and five-particle inter-valley states in an atomically-thin semiconductor, NatureCommunications 9, 3717 (2018).[12] Z. Ye, L. Waldecker, E. Y. Ma, D. Rhodes, A. Antony,B. Kim, X.-X. Zhang, M. Deng, Y. Jiang, Z. Lu,D. Smirnov, K. Watanabe, T. Taniguchi, J. Hone, andT. F. Heinz, Efficient generation of neutral and chargedbiexcitons in encapsulated WSe2 monolayers, NatureCommunications 9, 3718 (2018).[13] Z. Li, T. Wang, Z. Lu, C. Jin, Y. Chen, Y. Meng, Z. Lian,T. Taniguchi, K. Watanabe, S. Zhang, D. Smirnov, andS.-F. Shi, Revealing the biexciton and trion-exciton com-plexes in BN encapsulated WSe2, Nature Communica-tions 9, 3719 (2018).[14] M. Barbone, A. R.-P. Montblanch, D. M. Kara,C. Palacios-Berraquero, A. R. Cadore, D. De Fazio,B. Pingault, E. Mostaani, H. Li, B. Chen, K. Watan-https://doi.org/10.1103/RevModPhys.90.021001https://doi.org/10.1063/5.0107665https://doi.org/10.1038/nmat3505https://doi.org/10.1038/ncomms2498https://doi.org/10.1038/ncomms2498https://doi.org/10.1103/PhysRevLett.132.036903https://doi.org/10.1103/PhysRevLett.132.036903https://doi.org/10.1038/nphys3949https://doi.org/10.1103/PhysRevB.98.235203https://doi.org/10.1103/PhysRevB.98.235203https://doi.org/10.1103/PhysRevB.103.075417https://doi.org/10.1103/PhysRevB.103.075417https://doi.org/10.1063/5.0012475https://doi.org/10.1063/5.0012475https://arxiv.org/abs/2005.05829https://doi.org/10.1038/nphys3324https://doi.org/10.1038/s41467-018-05558-xhttps://doi.org/10.1038/s41467-018-05558-xhttps://doi.org/10.1038/s41467-018-05917-8https://doi.org/10.1038/s41467-018-05917-8https://doi.org/10.1038/s41467-018-05863-5https://doi.org/10.1038/s41467-018-05863-5110-500-50Energy relative to X0 (meV)-4040-4040-4040-4040-4040-4040-4040PL10Time relative to the THz pulse, t –t THz(ps)5 K10 K20 K30 K40 K50 K60 K70 K80 K90 K100 K110 K120 K130 KXX- XX X0X-X0X-0000000FIG. 9. Streak camera images of the spectrally and time-dependent PL for temperature between 5 and 130 K in thetime range of the THz pulse arrival. The energy scale is rel-ative to the bright exciton resonance at 5 K.abe, T. Taniguchi, S. Tongay, G. Wang, A. C. Ferrari,and M. Atatüre, Charge-tuneable biexciton complexesin monolayer WSe2, Nature Communications 9, 3721(2018).[15] D. Van Tuan, S.-F. Shi, X. Xu, S. A. Crooker, andH. Dery, Hexcitons and oxcitons in monolayer WSe2,arXiv (2022), 2202.08375.[16] A. Chernikov, C. Ruppert, H. M. Hill, A. F. Rigosi, andT. F. Heinz, Population inversion and giant bandgaprenormalization in atomically thin WS2 layers, NaturePhotonics 9, 466 (2015).[17] A. Steinhoff, M. Florian, M. Rösner, G. Schönhoff, T. O.Wehling, and F. Jahnke, Exciton fission in monolayertransition metal dichalcogenide semiconductors, NatureCommunications 8, 1166 (2017).[18] T. Mueller and E. Malic, Exciton physics and device ap-plication of two-dimensional transition metal dichalco-genide semiconductors, npj 2D Materials and Applica-tions 2, 29 (2018).[19] A. Arora, Magneto-optics of layered two-dimensionalsemiconductors and heterostructures: Progress and80 K60 K5 K20 K40 K100 K120 KX- quenching0 50PL (arb. u., offset)t – tTHz (ps)(b)80 K60 K5 K20 K40 K100 K120 K(a)0 50t – tTHz (ps)XX brightening5010015050100150200PL (arb. u., offset)FIG. 10. Transients of the neutral biexciton (a) and the nega-tive trion (b) PL for increasing temperature with correspond-ing fit to the curves.prospects, Journal of Applied Physics 129, 120902(2021).[20] E. J. Sie, J. W. McIver, Y.-H. Lee, L. Fu, J. Kong, andN. Gedik, Valley-selective optical Stark effect in mono-layer WS2, Nature Materials 14, 290 (2015).[21] E. Blundo, E. Cappelluti, M. Felici, G. Pettinari, andA. Polimeni, Strain-tuning of the electronic, optical, andvibrational properties of two-dimensional crystals, Ap-plied Physics Reviews 8, 021318 (2021).[22] R. Rosati, K. Wagner, S. Brem, R. Perea-Causín,J. D. Ziegler, J. Zipfel, T. Taniguchi, K. Watanabe,A. Chernikov, and E. Malic, Non-equilibrium diffu-sion of dark excitons in atomically thin semiconductors,Nanoscale 13, 19966 (2021).[23] A. Raja, A. Chaves, J. Yu, G. Arefe, H. M. Hill, A. F.Rigosi, T. C. Berkelbach, P. Nagler, C. Schüller, T. Korn,C. Nuckolls, J. Hone, L. E. Brus, T. F. Heinz, D. R.Reichman, and A. Chernikov, Coulomb engineering ofthe bandgap and excitons in two-dimensional materials,Nature Communications 8, 15251 (2017).[24] S. Bertolazzi, M. Gobbi, Y. Zhao, C. Backes, andP. Samorì, Molecular chemistry approaches for tuning theproperties of two-dimensional transition metal dichalco-genides, Chemical Society Reviews 47, 6845 (2018).[25] E. Lifshitz and L. Bykov, Microwave modulated and ther-mal modulated photoluminescence studies of 2H-lead io-dide, The Journal of Physical Chemistry 97, 9288 (1993).[26] Z. Chen-Esterlit, E. Lifshitz, E. Cohen, and L. N. Pfeif-fer, Microwave modulation of circularly polarized exci-ton photonluminescence in GaAs/AlAs multiple quan-tum wells, Physical Review B 53, 10921 (1996).[27] R. Guliamov, E. Lifshitz, E. Cohen, A. Ron, andH. Shtrikman, A Study of Semiconductor QuantumStructures by Microwave Modulated Photolumenescence,MRS Proceedings 573, 119 (1999).[28] I. Y. S. Ganichev, A. Dmitriev, S. Emel’yanov, Y. Ter-ent’ev, I. Yaroshetskii, Impact ionization in a semicon-ductor in a light wave, JETP Lett 40, 948 (1984).https://doi.org/10.1038/s41467-018-05632-4https://doi.org/10.1038/s41467-018-05632-4http://arxiv.org/abs/2202.08375https://arxiv.org/abs/2202.08375https://doi.org/10.1038/nphoton.2015.104https://doi.org/10.1038/nphoton.2015.104https://doi.org/10.1038/s41467-017-01298-6https://doi.org/10.1038/s41467-017-01298-6https://doi.org/10.1038/s41699-018-0074-2https://doi.org/10.1038/s41699-018-0074-2https://doi.org/10.1063/5.0042683https://doi.org/10.1063/5.0042683https://doi.org/10.1038/nmat4156https://doi.org/10.1063/5.0037852https://doi.org/10.1063/5.0037852https://doi.org/10.1039/D1NR06230Ahttps://doi.org/10.1038/ncomms15251https://doi.org/10.1039/C8CS00169Chttps://doi.org/10.1021/j100139a006https://doi.org/10.1103/PhysRevB.53.10921https://doi.org/10.1557/PROC-573-11912[29] S. Zybell, H. Schneider, S. Winnerl, M. Wagner,K. Köhler, and M. Helm, Photoluminescence dynam-ics in GaAs/AlGaAs quantum wells under pulsed in-tersubband excitation, Applied Physics Letters 99,10.1063/1.3615298 (2011).[30] W. Rice, J. Kono, S. Zybell, S. Winnerl, J. Bhat-tacharyya, H. Schneider, M. Helm, B. Ewers,A. Chernikov, M. Koch, S. Chatterjee, G. Khitrova,H. Gibbs, L. Schneebeli, B. Breddermann, M. Kira, andS. Koch, Observation of Forbidden Exciton TransitionsMediated by Coulomb Interactions in PhotoexcitedSemiconductor Quantum Wells, Physical Review Letters110, 137404 (2013).[31] S. Leinß, T. Kampfrath, K. V.Volkmann, M. Wolf, J. T.Steiner, M. Kira, S. W. Koch, A. Leitenstorfer, andR. Huber, Terahertz Coherent Control of Optically DarkParaexcitons in CuO2, Physical Review Letters 101,246401 (2008).[32] C. Poellmann, P. Steinleitner, U. Leierseder, P. Nagler,G. Plechinger, M. Porer, R. Bratschitsch, C. Schüller,T. Korn, and R. Huber, Resonant internal quantum tran-sitions and femtosecond radiative decay of excitons inmonolayer WSe2, Nature Materials 14, 889 (2015).[33] E. J. Sie, C. H. Lui, Y.-H. Lee, L. Fu, J. Kong, andN. Gedik, Large, valley-exclusive Bloch-Siegert shift inmonolayer WS 2, Science 355, 1066 (2017).[34] F. Langer, C. P. Schmid, S. Schlauderer, M. Gmitra,J. Fabian, P. Nagler, C. Schüller, T. Korn, P. G. Hawkins,J. T. Steiner, U. Huttner, S. W. Koch, M. Kira, andR. Huber, Lightwave valleytronics in a monolayer of tung-sten diselenide, Nature 557, 76 (2018).[35] C.-K. Yong, M. I. B. Utama, C. S. Ong, T. Cao,E. C. Regan, J. Horng, Y. Shen, H. Cai, K. Watan-abe, T. Taniguchi, S. Tongay, H. Deng, A. Zettl, S. G.Louie, and F. Wang, Valley-dependent exciton fine struc-ture and AutlerTownes doublets from Berry phases inmonolayer MoSe2, Nature Materials 18, 1065 (2019).[36] T. Venanzi, M. Selig, S. Winnerl, A. Pashkin, A. Knorr,M. Helm, and H. Schneider, Terahertz-Induced EnergyTransfer from Hot Carriers to Trions in a MoSe 2 Mono-layer, ACS Photonics , acsphotonics.1c00394 (2021).[37] T. Venanzi, M. Cuccu, R. Perea-Causin, X. Sun, S. Brem,D. Erkensten, T. Taniguchi, K. Watanabe, E. Malic,M. Helm, S. Winnerl, and A. Chernikov, Ultrafast switch-ing of trions in 2D materials by terahertz photons, NaturePhotonics 18, 1344 (2024).[38] O. Benson, C. Santori, M. Pelton, and Y. Yamamoto,Regulated and Entangled Photons from a Single Quan-tum Dot, Physical Review Letters 84, 2513 (2000).[39] Y.-M. He, O. Iff, N. Lundt, V. Baumann, M. Davanco,K. Srinivasan, S. Höfling, and C. Schneider, Cascadedemission of single photons from the biexciton in mono-layered WSe2, Nature Communications 7, 13409 (2016).[40] A. Raja, L. Waldecker, J. Zipfel, Y. Cho, S. Brem, J. D.Ziegler, M. Kulig, T. Taniguchi, K. Watanabe, E. Malic,T. F. Heinz, T. C. Berkelbach, and A. Chernikov, Dielec-tric disorder in two-dimensional materials, Nature Nan-otechnology 14, 832 (2019).[41] D. Rhodes, S. H. Chae, R. Ribeiro-Palau, and J. Hone,Disorder in van der Waals heterostructures of 2D mate-rials, Nature Materials 18, 541 (2019).[42] A. Castellanos-Gomez, L. Vicarelli, E. Prada, J. O. Is-land, K. L. Narasimha-Acharya, S. I. Blanter, D. J. Groe-nendijk, M. Buscema, G. a. Steele, J. V. Alvarez, H. W.Zandbergen, J. J. Palacios, and H. S. J. van der Zant,Isolation and characterization of few-layer black phos-phorus, 2D Materials 1, 025001 (2014).[43] M. Helm, S. Winnerl, A. Pashkin, J. M. Klopf, J.-C.Deinert, S. Kovalev, P. Evtushenko, U. Lehnert, R. Xi-ang, A. Arnold, A. Wagner, S. M. Schmidt, U. Schramm,T. Cowan, and P. Michel, The ELBE infrared and THzfacility at Helmholtz-Zentrum Dresden-Rossendorf, TheEuropean Physical Journal Plus 138, 158 (2023).[44] Z. Li, T. Wang, C. Jin, Z. Lu, Z. Lian, Y. Meng, M. Blei,S. Gao, T. Taniguchi, K. Watanabe, T. Ren, S. Tongay,L. Yang, D. Smirnov, T. Cao, and S.-F. Shi, Emergingphotoluminescence from the dark-exciton phonon replicain monolayer WSe2, Nature Communications 10, 2469(2019).[45] M. He, P. Rivera, D. Van Tuan, N. P. Wilson, M. Yang,T. Taniguchi, K. Watanabe, J. Yan, D. G. Mandrus,H. Yu, H. Dery, W. Yao, and X. Xu, Valley phonons andexciton complexes in a monolayer semiconductor, NatureCommunications 11, 618 (2020).[46] M. Yang, L. Ren, C. Robert, D. Van Tuan, L. Lombez,B. Urbaszek, X. Marie, and H. Dery, Relaxation anddarkening of excitonic complexes in electrostaticallydoped monolayer WSe2: Roles of exciton-electron andtrion-electron interactions, Physical Review B 105,085302 (2022).[47] M. Selig, G. Berghäuser, M. Richter, R. Bratschitsch,A. Knorr, and E. Malic, Dark and bright exciton for-mation, thermalization, and photoluminescence in mono-layer transition metal dichalcogenides, 2D Materials 5,035017 (2018).[48] C. Robert, R. Picard, D. Lagarde, G. Wang, J. P.Echeverry, F. Cadiz, P. Renucci, A. Högele, T. Amand,X. Marie, I. C. Gerber, and B. Urbaszek, Excitonic prop-erties of semiconducting monolayer and bilayer MoTe2,Physical Review B 94, 155425 (2016).[49] J. Zipfel, K. Wagner, M. A. Semina, J. D. Ziegler,T. Taniguchi, K. Watanabe, M. M. Glazov, andA. Chernikov, Electron recoil effect in electrically tun-able MoSe2 monolayers, Physical Review B 105, 075311(2022).[50] E. Courtade, M. Semina, M. Manca, M. M. Glazov,C. Robert, F. Cadiz, G. Wang, T. Taniguchi, K. Watan-abe, M. Pierre, W. Escoffier, E. L. Ivchenko, P. Renucci,X. Marie, T. Amand, and B. Urbaszek, Charged excitonsin monolayer WSe2 : Experiment and theory, PhysicalReview B 96, 085302 (2017), arXiv:1705.02110.[51] J. Zipfel, K. Wagner, J. D. Ziegler, T. Taniguchi,K. Watanabe, M. A. Semina, and A. Chernikov,Lightmatter coupling and non-equilibrium dynamics ofexchange-split trions in monolayer WS2, The Journal ofChemical Physics 153, 10.1063/5.0012721 (2020).[52] N. A. Cordero, N. H. March, and J. A. Alonso, Ionizationpotentials of neutral atoms and positive ions in the limitof large atomic number, Physical Review A 75, 012505(2007).[53] B. Jordon-Thaden, H. Kreckel, R. Golser, D. Schwalm,M. H. Berg, H. Buhr, H. Gnaser, M. Grieser, O. Heber,M. Lange, O. Novotný, S. Novotny, H. B. Pedersen,A. Petrignani, R. Repnow, H. Rubinstein, D. Shafir,A. Wolf, and D. Zajfman, Structure and Stability of theNegative Hydrogen Molecular Ion, Physical Review Let-ters 107, 193003 (2011).https://doi.org/10.1063/1.3615298https://doi.org/10.1103/PhysRevLett.110.137404https://doi.org/10.1103/PhysRevLett.110.137404https://doi.org/10.1103/PhysRevLett.101.246401https://doi.org/10.1103/PhysRevLett.101.246401https://doi.org/10.1038/nmat4356https://doi.org/10.1126/science.aal2241https://doi.org/10.1038/s41586-018-0013-6https://doi.org/10.1038/s41563-019-0447-8https://doi.org/10.1021/acsphotonics.1c00394https://doi.org/10.1038/s41566-024-01512-0https://doi.org/10.1038/s41566-024-01512-0https://doi.org/10.1103/PhysRevLett.84.2513https://doi.org/10.1038/ncomms13409https://doi.org/10.1038/s41565-019-0520-0https://doi.org/10.1038/s41565-019-0520-0https://doi.org/10.1038/s41563-019-0366-8https://doi.org/10.1088/2053-1583/1/2/025001https://doi.org/10.1140/epjp/s13360-023-03720-zhttps://doi.org/10.1140/epjp/s13360-023-03720-zhttps://doi.org/10.1038/s41467-019-10477-6https://doi.org/10.1038/s41467-019-10477-6https://doi.org/10.1038/s41467-020-14472-0https://doi.org/10.1038/s41467-020-14472-0https://doi.org/10.1103/PhysRevB.105.085302https://doi.org/10.1103/PhysRevB.105.085302https://doi.org/10.1088/2053-1583/aabea3https://doi.org/10.1088/2053-1583/aabea3https://doi.org/10.1103/PhysRevB.94.155425https://doi.org/10.1103/PhysRevB.105.075311https://doi.org/10.1103/PhysRevB.105.075311https://doi.org/10.1103/PhysRevB.96.085302https://doi.org/10.1103/PhysRevB.96.085302https://arxiv.org/abs/1705.02110https://doi.org/10.1063/5.0012721https://doi.org/10.1103/PhysRevA.75.012505https://doi.org/10.1103/PhysRevA.75.012505https://doi.org/10.1103/PhysRevLett.107.193003https://doi.org/10.1103/PhysRevLett.107.19300313[54] P. Nagler, M. V. Ballottin, A. A. Mitioglu, M. V. Durnev,T. Taniguchi, K. Watanabe, A. Chernikov, C. Schüller,M. M. Glazov, P. C. M. Christianen, and T. Korn, Zee-man Splitting and Inverted Polarization of BiexcitonEmission in Monolayer WS2, Physical Review Letters121, 057402 (2018).[55] L. V. Keldysh, Ionization in the field of a strong electro-magnetic wave, J. Exptl. Theoret. Phys. 20, 56 (1965).[56] T. Venanzi, M. Selig, A. Pashkin, S. Winnerl, M. Katzer,H. Arora, A. Erbe, A. Patanè, Z. R. Kudrynskyi, Z. D.Kovalyuk, L. Baldassarre, A. Knorr, M. Helm, andH. Schneider, Terahertz control of photoluminescenceemission in few-layer InSe, Applied Physics Letters 120,092104 (2022).[57] R. Rosati, K. Wagner, S. Brem, R. Perea-Causín, E. Wi-etek, J. Zipfel, J. D. Ziegler, M. Selig, T. Taniguchi,K. Watanabe, A. Knorr, A. Chernikov, and E. Malic,Temporal Evolution of Low-Temperature Phonon Side-bands in Transition Metal Dichalcogenides, ACS Pho-tonics 7, 2756 (2020).[58] H. H. Fang, B. Han, C. Robert, M. A. Semina,D. Lagarde, E. Courtade, T. Taniguchi, K. Watanabe,T. Amand, B. Urbaszek, M. M. Glazov, and X. Marie,Control of the Exciton Radiative Lifetime in van derWaals Heterostructures, Physical Review Letters 123,067401 (2019).[59] M. Selig, G. Berghäuser, A. Raja, P. Nagler, C. Schüller,T. F. Heinz, T. Korn, A. Chernikov, E. Malic, andA. Knorr, Excitonic linewidth and coherence lifetimein monolayer transition metal dichalcogenides, NatureCommunications 7, 13279 (2016).https://doi.org/10.1103/PhysRevLett.121.057402https://doi.org/10.1103/PhysRevLett.121.057402https://doi.org/10.1063/5.0080784https://doi.org/10.1063/5.0080784https://doi.org/10.1021/acsphotonics.0c00866https://doi.org/10.1021/acsphotonics.0c00866https://doi.org/10.1103/PhysRevLett.123.067401https://doi.org/10.1103/PhysRevLett.123.067401https://doi.org/10.1038/ncomms13279https://doi.org/10.1038/ncomms13279 Terahertz-induced population transfer between exciton complexes in monolayer WSe2 Abstract Introduction Experimental details THz-induced transitions between excitonic states Temperature-dependent dynamics Conclusion Acknowledgments THz spectrum and spot size Trion and charged biexciton transients PL spectra before and after the THz pulse Dissociation dynamics for different THz photon energies Adaptation of the trion dissociation model to charged biexcitons Temperature dependent measurements: complete data set References