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Jiaojian Shi, Ya-Qing Bie, Alfred Zong, Shiang Fang, Wei Chen, Jinchi Han, Zhaolong Cao, Yong Zhang, [Takashi Taniguchi](https://orcid.org/0000-0002-1467-3105), [Kenji Watanabe](https://orcid.org/0000-0003-3701-8119), Xuewen Fu, Vladimir Bulović, Efthimios Kaxiras, Edoardo Baldini, Pablo Jarillo-Herrero, Keith A. Nelson

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[Intrinsic 1$${T}^{{\prime} }$$ phase induced in atomically thin 2H-MoTe2 by a single terahertz pulse](https://mdr.nims.go.jp/datasets/18a7137b-4230-444f-ba3b-2d12790eddd8)

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Intrinsic 1T′ phase induced in atomically thin 2H-MoTe2 by a single terahertz pulseArticle https://doi.org/10.1038/s41467-023-41291-wIntrinsic 1T 0 phase induced in atomically thin2H-MoTe2 by a single terahertz pulseJiaojian Shi 1,15, Ya-Qing Bie 2,3,15 , Alfred Zong 2,4,15, Shiang Fang 5,6,14,Wei Chen 5, Jinchi Han 7,8, Zhaolong Cao3, Yong Zhang9,Takashi Taniguchi 10, Kenji Watanabe 11, Xuewen Fu 12, Vladimir Bulović 7,Efthimios Kaxiras 5, Edoardo Baldini 13, Pablo Jarillo-Herrero 2 &Keith A. Nelson 1The polymorphic transition from 2H to 1T 0-MoTe2, which was thought to beinduced by high-energy photon irradiation among many other means, hasbeen intensely studied for its technological relevance in nanoscale transistorsdue to the remarkable improvement in electrical performance. However, itremains controversial whether a crystalline 1T 0 phase is produced becauseoptical signatures of this putative transition are found to be associated withthe formation of tellurium clusters instead. Here we demonstrate the creationof an intrinsic 1T 0 lattice after irradiating amono- or few-layer 2H-MoTe2 with asingle field-enhanced terahertz pulse. Unlike optical pulses, the low terahertzphoton energy limits possible structural damages.We further develop a single-shot terahertz-pump-second-harmonic-probe technique and reveal a transi-tion out of the 2H-phase within 10 ns after photoexcitation. Our results notonly provide important insights to resolve the long-standing debate over thelight-induced polymorphic transition in MoTe2 but also highlight the uniquecapability of strong-field terahertz pulses in manipulating quantum materials.Tailored, ultrashort pulses of light can be used to manipulatemetastable states1 in functional materials, such as triggering insulator-to-metal transitions2, unveiling hidden states3, or even inducing long-lasting superconducting-like behavior way above the equilibriumcritical temperature4. These photoinduced states have been intenselystudied not only because they yield profound insights into funda-mental light-matter interactions in correlated systems but alsobecause they bring new opportunities to the field of laser fabricationand micro-machining, going beyond traditional laser cutting5,burning6, or stereolithography technologies7. From this applicationReceived: 15 October 2022Accepted: 29 August 2023Check for updates1Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, USA. 2Department of Physics, Massachusetts Institute of Tech-nology, Cambridge, MA 02139, USA. 3State Key Lab of Optoelectronic Materials and Technologies, Guangdong Province Key Laboratory of Display Materialand Technology, School of Electronics and Information Technology, Sun Yat-sen University, Guangzhou 510275, People’s Republic of China. 4Department ofChemistry, University of California, Berkeley, CA 94720, USA. 5Department of Physics, Harvard University, Cambridge, MA 02138, USA. 6Department ofPhysics and Astronomy, Center for Materials Theory, Rutgers University, Piscataway, NJ 08854, USA. 7Department of Electrical Engineering and ComputerScience, Massachusetts Institute of Technology, Cambridge, MA 02139, USA. 8School of Integrated Circuits, Peking University, Beijing 100871, People’sRepublicofChina. 9Center forMaterials Science&Engineering,Massachusetts Instituteof Technology, Cambridge,MA02139,USA. 10International Center forMaterials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan. 11Research Center for Functional Materials,National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan. 12Ultrafast Electron Microscopy Laboratory, The MOE Key Laboratory of Weak-Light Nonlinear Photonics, School of Physics, Nankai University, Tianjin 300071, People’s Republic of China. 13Department of Physics, Center for ComplexQuantum System, The University of Texas at Austin, Austin, TX 78712, USA. 14Present address: Department of Physics, Massachusetts Institute of Technology,Cambridge, MA 02139, USA. 15These authors contributed equally: Jiaojian Shi, Ya-Qing Bie and Alfred Zong. e-mail: bieyq@mail.sysu.edu.cn;pjarillo@mit.edu; kanelson@mit.eduNature Communications |         (2023) 14:5905 11234567890():,;1234567890():,;http://orcid.org/0000-0002-1703-6363http://orcid.org/0000-0002-1703-6363http://orcid.org/0000-0002-1703-6363http://orcid.org/0000-0002-1703-6363http://orcid.org/0000-0002-1703-6363http://orcid.org/0000-0003-2755-9472http://orcid.org/0000-0003-2755-9472http://orcid.org/0000-0003-2755-9472http://orcid.org/0000-0003-2755-9472http://orcid.org/0000-0003-2755-9472http://orcid.org/0000-0003-2047-3801http://orcid.org/0000-0003-2047-3801http://orcid.org/0000-0003-2047-3801http://orcid.org/0000-0003-2047-3801http://orcid.org/0000-0003-2047-3801http://orcid.org/0000-0002-9412-6426http://orcid.org/0000-0002-9412-6426http://orcid.org/0000-0002-9412-6426http://orcid.org/0000-0002-9412-6426http://orcid.org/0000-0002-9412-6426http://orcid.org/0000-0003-3598-2369http://orcid.org/0000-0003-3598-2369http://orcid.org/0000-0003-3598-2369http://orcid.org/0000-0003-3598-2369http://orcid.org/0000-0003-3598-2369http://orcid.org/0000-0002-6102-9734http://orcid.org/0000-0002-6102-9734http://orcid.org/0000-0002-6102-9734http://orcid.org/0000-0002-6102-9734http://orcid.org/0000-0002-6102-9734http://orcid.org/0000-0002-1467-3105http://orcid.org/0000-0002-1467-3105http://orcid.org/0000-0002-1467-3105http://orcid.org/0000-0002-1467-3105http://orcid.org/0000-0002-1467-3105http://orcid.org/0000-0003-3701-8119http://orcid.org/0000-0003-3701-8119http://orcid.org/0000-0003-3701-8119http://orcid.org/0000-0003-3701-8119http://orcid.org/0000-0003-3701-8119http://orcid.org/0000-0001-6116-4680http://orcid.org/0000-0001-6116-4680http://orcid.org/0000-0001-6116-4680http://orcid.org/0000-0001-6116-4680http://orcid.org/0000-0001-6116-4680http://orcid.org/0000-0002-0960-2580http://orcid.org/0000-0002-0960-2580http://orcid.org/0000-0002-0960-2580http://orcid.org/0000-0002-0960-2580http://orcid.org/0000-0002-0960-2580http://orcid.org/0000-0002-4682-0165http://orcid.org/0000-0002-4682-0165http://orcid.org/0000-0002-4682-0165http://orcid.org/0000-0002-4682-0165http://orcid.org/0000-0002-4682-0165http://orcid.org/0000-0002-8131-9974http://orcid.org/0000-0002-8131-9974http://orcid.org/0000-0002-8131-9974http://orcid.org/0000-0002-8131-9974http://orcid.org/0000-0002-8131-9974http://orcid.org/0000-0001-8217-8213http://orcid.org/0000-0001-8217-8213http://orcid.org/0000-0001-8217-8213http://orcid.org/0000-0001-8217-8213http://orcid.org/0000-0001-8217-8213http://orcid.org/0000-0001-7804-5418http://orcid.org/0000-0001-7804-5418http://orcid.org/0000-0001-7804-5418http://orcid.org/0000-0001-7804-5418http://orcid.org/0000-0001-7804-5418http://crossmark.crossref.org/dialog/?doi=10.1038/s41467-023-41291-w&domain=pdfhttp://crossmark.crossref.org/dialog/?doi=10.1038/s41467-023-41291-w&domain=pdfhttp://crossmark.crossref.org/dialog/?doi=10.1038/s41467-023-41291-w&domain=pdfhttp://crossmark.crossref.org/dialog/?doi=10.1038/s41467-023-41291-w&domain=pdfmailto:bieyq@mail.sysu.edu.cnmailto:pjarillo@mit.edumailto:kanelson@mit.eduperspective, the metastable 1T 0 phase of MoTe2 induced in a semi-conducting 2H polymorph is considered an important candidate fortackling the issue of high contact resistance for two-dimensionalelectrical device8. In this polymorphic transition, the hexagonal unitcell of 2H-MoTe2 in its ground state is transformed into themetastable1T 0 phase after going through an intermediate structure (Fig. 1a), aprocess that was believed to be readily instigated by opticalirradiation9–13. Compared to other methods14,15 such as ionic gating16,17and strain application18, the focused laser spot down to the sub-micrometer regime makes an optically driven transition especiallyappealing for precision fabrication of small devices.Despite the high technological impact of this polymorphic tran-sition inMoTe2, recent studies have cast doubt on the existence of thepurported 1T 0 structure upon optical illumination. In particular, themain evidence for the 1T 0 polymorphwas the Ag phonon peaks around120 cm−1 and 140 cm−1 in Raman spectroscopy19,20 (SupplementaryFigs. 1 and 2),which have been shown tooriginate fromnano- ormicro-sized Te clusters. Given the high energy barrier Ebarrier of morethan 0.8 eV per MoTe2 formula unit21 as well as the large Te displace-ment of more than 1 Å required for the transition22 (Fig. 1a), it remainsunclear whether light can initiate the polymorphic change after all.In this work, instead of using visible light, we report the successfultransition from 2H to 1T 0-MoTe2 using single-cycle, field-enhancedterahertz (THz) pulses. The transition can be induced with as few as asingle THz pulse on atomically thin crystals of 2H-MoTe2. Using Ramanspectroscopy, we show that the induced phase reflects an intrinsic 1T 0structure16,17,23, which is free fromany spurious features associatedwithTe clusters. The validity of the metastable 1T 0 phase is further con-firmed by selected area electron diffraction and energy-dispersiveX-ray spectroscopy, which demonstrate a macroscopic structuraltransition without stoichiometric changes in the sample. Using single-shot, time-resolved nonlinear optical spectroscopy, we also unveil thedifferent stages of this irreversible phase transition. These findingsprovide a route of structural manipulation using high-field THz pulses,which crucially avoid large-scale defect generation that often accom-panies intense optical excitation9.ResultsPolymorphic transition in monolayer MoTe2In our experiments, we positioned a flake of 2H-MoTe2 encapsulatedby hexagonal-boron nitride (h-BN) on top of two parallel gold stripsseparated by a gap (Fig. 1b). The h-BN layers were used to isolatemonolayers and bilayerMoTe2 samples from field-induced emission inthe gold layers24 and to prevent their direct exposure to air. Figure 1dshows the optical image of a monolayer sample prepared using a drytransfermethod25. Upon illumination of this structurewith a free-spaceTHz pulse, the field strength can be enhanced by more than 100 timesat certain hot spots and about 20 times at the gap center, as shown inFig. 1c. Since the peak electric field of the THz pulse in free spacereached ~ 300 kV/cm at the focus, the electrical field amplitude can bemore than 5 MV/cm in large parts of the sample26 (see Fig. 1c, Sup-plementary Fig. 3, “Methods” section, and Supplementary Note 1 fordetailed information on THz generation). After irradiating this struc-ture with a free-space THz field of 270 kV/cm, we probed latticechanges via spontaneous Raman scattering. Figure 1e shows theRaman spectra. Before the THz illumination, the spectrum consists ofthe A1g phonon at 171.5 cm−1 and the E2g phonon at 236 cm−1, inagreement with previous studies of the 2H phase22. After excitation 0 0.2 0.4-1  0  1z (µm)x (µm)04080120Enhancement factorbMoxz E // xTeAuh-BNFused silicaSingle/a fewTHz pulsescAu Aua2H phaseMo TeIntermediate EnergyTransition coordinatex-axisy-axisGap1T' phaseEbarrier5 µmd100 150 200 250 300x10 pulsex85 pulsesx81000 pulsesA1gAgE2gRaman intensity (arb. u.)Wavenumber (cm-1)eFig. 1 | Polymorphic transition in MoTe2 induced by high-field THz pulses.a Schematic energy landscape and top view of lattice structures of 2H-MoTe2 (left),the intermediate state (middle), and 1T 0-MoTe2 (right). Ebarrier is the potential bar-rier between the 2H and 1T 0-phase. bCross-sectional schematic illustration of a 2H-MoTe2 crystal spanning an insulating gap between deposited gold strips, whichserve as a THz field enhancement structure. The monolayer 2H-MoTe2 crystal isencapsulated between top and bottom h-BN. THz pulses are incident from thebottom side of the fused silica substrate. c Field strength calculation of the THzenhancement structure. Numerical simulation results showing THz field enhance-ment by a factor of 20–50 in significant regions in and near the gap between thegold strips. The enhancement factor is defined as the ratio between the actual andincident electric field. dOptical micrograph of a monolayer sample, including a 15-nm-thick top h-BN (white dashed line), a monolayer 2H-MoTe2 (green dashed line),and a 5-nm-thick bottom h-BN (blue dashed line), all spanning a 1.8-μm insulatinggap (dark horizontal line) between the top and bottomgold strips. eRaman spectraof monolayer MoTe2 after successive THz pulse irradiation with free-space fieldamplitudes of 270 kV/cm. The Ag mode of the 1T 0 phase sets in after 5 THz pulsesand dominates upon further irradiation. By contrast, the A1g and E2g modes of the2H phase are strongly reduced. The reduction of the overall Raman intensity inmonolayer samples may be related to compromised crystallinity after the irradia-tion. The Raman measurements were conducted with a 1-μm-diameter laser spotand were therefore averaged over regions that had been subjected to differentlyenhanced THz field strengths. The two dashed lines are aligned with the Ag and A1gmode of the 1T 0 and 2H-phase, respectively.Article https://doi.org/10.1038/s41467-023-41291-wNature Communications |         (2023) 14:5905 2with five THz pulses, bothmodes of the 2H phase disappear and a peakat 163.3 cm−1 emerges (see Supplementary Fig. 4 for additional peakwidth analysis). This feature signifies a change of the crystalline sym-metry, and it is considered a specific fingerprint of the 1T 0 phase ofMoTe216,22. The new phonon peak becomes more prominent afterfurther exposure to THz pulses (Fig. 1e), indicating the growth of alarger 1T 0 sample area. Importantly, the spectrum does not show anylocalized modes due to damage-related Te clusters19 even though theresultant 1T 0 phase is not homogeneously produced in the flake sus-pended between the gap of the gold strips (Supplementary Figs. 5 and6). These observations were reproduced in multiple samples, sug-gesting that the action of a strongTHz fieldon ourmonolayerMoTe2 isto induce a phase transition from the 2H to the 1T 0 polymorph.To further investigate this structural transformation, we con-ducted experiments ona series ofmonolayer 2H-MoTe2 samples. Sincethe induced 1T 0 phase is long-lived, eachmeasurement required a freshspecimen that was carefully positioned over the gap between the goldstrips, as indicated in Fig. 1b. First, we studied how the 2H phaseresponded to a sequence of THz pulses with a gradually increasingfield strength. Toprobe the phase transition,we tracked the changes incrystalline symmetry via optical second harmonic generation (SHG), asensitive probe of inversion symmetry breaking that can distinguishbetween the non-centrosymmetric 2H phase and the centrosymmetric1T 0 phase in samples with an odd number of layers27,28 (see “Methods”section). Figure 2a shows three representative real-space images of theSHG intensity before and after THz-field exposures with free-spaceamplitudes of 210 kV/cm and 240 kV/cm; additional SHG intensityimages are shown in Supplementary Fig. 7. A plot of the SHG intensitiesfrom three sample locationswithin thefield-enhancement gap (labeledI, II, III in Fig. 2a) is presented in Fig. 2b.We observe that the SHG signaldrops when the field strength exceeds 183 kV/cm, disappearing com-pletely above 270 kV/cm. As the field enhancement factor generatedby the gold strips is around 20, the threshold needed to quench theSHG intensity is estimated to be approximately 4.0MV/cm. We com-plemented these measurements by recording Raman spectra as afunction of the THz field strength. Figure 2c shows the spectraacquired after irradiation at 200 kV/cm and 270 kV/cm. Consistentwith the picture supported by the SHG results in Fig. 2b, a single-pulseirradiationfirst leads to adrop in the intensity of the 2Hphononswith afree-space field strength of 200 kV/cm, and stronger THz pulses at270 kV/cm subsequently cause a further suppression, leading to anearly complete extinction of the peak after 1000 pulses. Simulta-neously, the Raman mode of the 1T 0 phase grows, eventually dom-inating the spectrum. 0 2 4 2 4 6 8 SHG intensity (arb. u.)0 kV/cm 0 2 4  x'  (µm)210 kV/cm  0 2 40 2 4 6 8y' (µm)240 kV/cm  0 2 4 6 8 10 120 100 200 300SHG intensity (arb. u.)THz Field (kV/cm) 100  150  200  250  3000 kV/cm200 kV/cm 270 kV/cm270 kV/cm 1000 pulsesx1x 1x 3x 20AgA1gMonolayerWavenumber (cm-1)Raman intensity (arb. u.)Raman intensity (arb. u.)0 kV/cm300 kV/cm 1 pulse 100  150  200  250Wavenumber (cm-1)IIIIIIa b cd eE2gMultilayerAgAgE2gBefore After a single shot (1 0 0)(0 1 0)(½ ½ 0)IIIIIIFig. 2 | THz field dependence of the phase transition in monolayer and multi-layer MoTe2. a SHG images ofmonolayer MoTe2 before THz irradiation (0 kV/cm),after irradiation with one THz pulse at 210 kV/cm free-space field amplitude, andafter irradiation with a second THz pulse at 240kV/cm. The dashed lines indicatethe edges of gold strips in the field enhancement structure. Different areas ofmonolayerMoTe2 in the gap are outlined by thewhite dashed boxes and labeled byI, II, and III. The primes in x0 and y0 of the axes are added to differentiate the x–ycoordinates in Fig. 1 due to the rotatedfieldof view in SHGmicroscopy.b SHG fromdifferent areas (I–III) in a measured after irradiation of the sample by single THzpulses with successively increasing field strength. The vertical error bars measurethe deviation of the averaged SHG intensity in the regions of interest from that ofnearby pixels in the image. c Raman spectra of monolayer MoTe2 samples prior toTHz irradiation (blue curve), after THz irradiation with a single pulse at 200kV/cm(violet curve), another single pulse at 270 kV/cm (pink curve), and 1000 pulses at270 kV/cm (red curve). The monolayer MoTe2 sample shows the Ag mode at163.5 cm−1 after irradiation with 1000 pulses at 270 kV/cm. d Electron diffractionpattern of a multilayer ( ~ 10 layers) MoTe2 before and after a single THz pulseirradiation at 300 kV/cm, showing the emergence of superstructure peaks (yellowcircle) that are characteristic of the 1T 0 phase. The scale bars represent 0.1Å−1.e Raman spectra of the multilayer MoTe2 before and after a single THz pulseirradiation at 300 kV/cm, showing the emergence of new Raman peaks that arecharacteristic of the induced 1T 0 phase.Article https://doi.org/10.1038/s41467-023-41291-wNature Communications |         (2023) 14:5905 3Polymorphic transition in multilayer MoTe2The THz-induced transition is not restricted to monolayer samples,and we observed similar phenomenology in bilayer and multilayer(~10 layers) 2H-MoTe2 (see Supplementary Figs. 6c, d and 8a, b).Figure 2d, e show the results obtained on a multilayer 2H-MoTe2,presenting the [001] zone-axis electron diffraction pattern andRaman spectra before and after irradiation with a single THz pulse at300 kV/cm. We find that the diffraction pattern of the unexcitedcrystal exhibits the six-fold symmetry expected for the 2H phase.Consistent with this observation, the Raman spectrum only displaysan E2g phonon mode of bulk 2H-MoTe229. After THz irradiation, newpeaks emerge in the diffraction pattern taken along the [001] zoneaxis of the new unit cell (Fig. 2d), revealing a cell-doublingsuperstructure30–33. On the other hand, our energy-dispersive X-rayspectroscopy mapping of the irradiated sample shows that thestoichiometric ratio of ~1: 2 is maintained and Te clusters are absent(Supplementary Fig. 9). The change in the electron diffraction pat-tern is accompanied by the appearance of Ag phonons (164 cm−1 and270 cm−1) in the Raman spectrum (Fig. 2e)16,19. Both observables arecharacteristic signatures of the 1T 0 lattice, offering additional vali-dation for the creation of an intrinsic 1T 0 phase (see SupplementaryNote 2 and 3, and Supplementary Fig. 10).Temporal evolution of the polymorphic transitionTo gain microscopic insights into the THz-driven transition, wealso examined the photo-induced dynamics to trace out the temporalevolution of the crystalline lattice. This investigation cannot beaccomplished by conventional time-resolved spectroscopy methodsbecause their stroboscopic approach precludes the study of irrever-sible processes. We therefore developed a specialized apparatus forTHz pump-SHG probe single-shot spectroscopy, which is well suitedfor probing the formation dynamics of the long-lived metastable 1T 0state. A schematic illustration of our experiment is presented in Fig. 3a;details of the setup are described in Methods. In our measurements,we irradiated trilayer samples with a single THz pulse, whose fieldstrength was adjusted to exceed the phase transition threshold. Wesubsequently recorded the SHG signal at different pump-probe delays.Since the THz field was above the threshold for generating the 1T 0phase, the experiment was challenging because each THz shot with itspredetermined delay time needed to be conducted on a fresh piece ofsample. An added difficulty is the extremely weak single-shot SHGsignal from a few-layer sample; using a photomultiplier tube, weobtained measurable second harmonic intensity under an incident800-nm pulse fluence of ~ 20 mJ/cm2, which is beyond the opticaldamage threshold. Here, the collection of single-shot SHG signals 1.5#4ab cSingle THz pump pulseSingle 800 nm probe pulse400 nm secondharmonic signalBandpassfilterPhotomultipliertubeMultiple MoTe2 samplesa 0  0.5 1 2 2.5-20 100 101 102 103 104 105Delay t (ps) #1a0  0.5 1 1.5 2 2.5Transition2H(2H*)1.00.0Relative energy (eV)Transition2H 1T'd0.050.10.20.30.40.5Neutral 0.01.02.03.04.05.0Charged1T'#1b#2a#2b#3a#3b#4b#5a#5b#6a#6b#7a#7bItIiIfIiFig. 3 | Dynamics and driving mechanism of the THz-field-induced phasetransition. a Schematic illustration of the setup for single-shot SHG probe with aTHz excitation pulse. Single-shot measurements were conducted by using a freshsample in each shot. b SHG intensities measured from trilayer MoTe2 samples atdifferent delay times t from several picoseconds to hundreds of nanoseconds (bluedots) aswell as around 1minute (reddots). Thedata points on the same dashed lineshow the SHG intensity measured from the same flake shortly after (It) and 1 minafter the THz excitation pulse (If), normalized by the SHG signal intensity prior toTHz excitation (Ii). Due to the destructive natureof single-shot SHG, Ii, It, and If weremeasured at three different spots on the same sample. For this reason, we use #1a,#1b, etc. to emphasize that these data points were taken at different spots (a, b) ofthe same flake (#1). The error analysis is provided in Supplementary Note 4. c Theenergy potential as a function of transition coordinate with nonequilibrium carrierdistribution (charge neutral). Nonequilibrium distribution of carriers is qualita-tively described by the Fermi-smearing method. The free-energy barrier decreasesfrom 1.64 eV to 0.91 eV as the Fermi-smearing width increases from 0.05 eV to0.5 eV. The figure legend has a unit of eV. d The energy potential as a function oftransition coordinate upon charge dopings. As the added charge density increasesto 1.0 e/MoTe2, which corresponds to 9 × 1014 cm−2, the activation energy decreasesfrom 1.66 eV to 1.19 eV. The figure legend has a unit of e/MoTe2. The atomicstructures of transition states are shown in Supplementary Figs. 18 and 19.Article https://doi.org/10.1038/s41467-023-41291-wNature Communications |         (2023) 14:5905 4relies on the concept of “probe before destruction”34,35, which is wellapplicable toMoTe2 (see Supplementary Note 4). The small beam spotfor SHG (1μm) relative to the sample size (≥10μm) and good spatialuniformity of the trilayer crystals (Supplementary Fig. 11) allow acomparison among three locally destructive single-shot SHG mea-surements at different sample locations: one prior to THz excitation(Ii), the second at the selected delay time following THz excitation (It),and the third at a long delay time at ~1min (If).Figure 3b shows the time evolution of the SHG signal relative tothe initial signal followingTHz excitation (It/Ii, blue dots). The responsefirst increases, reaching about twice the initial value at 20 ps. It thenremains higher than Ii for several hundred picoseconds, before drop-ping to ~0.2Ii at 12.5 ns. Finally, at long pump-probe delays, the SHGsignal disappears completely, indicating the stabilization of a meta-stable phase. Themetastability is further confirmed by the value of thefinal signals after 1min of photoexcitation, If/Ii, shownas the reddots inFig. 3b. We were able to reproduce these complex dynamics on dif-ferent samples, with the initial enhancement of the SHG occurringeven below the phase transition threshold (Supplementary Fig. 12).From our single-shot spectroscopy data, we identify a characteristictimescale on the order of 10 ns that is needed to completely switchMoTe2 out of the 2H polymorph. We attribute the increase in SHGsignal observed at shorter times to a possible nucleation of a transientstructure with a lower crystalline symmetry than 2H. One such possi-bility is the distorted trigonal prismatic 2H* phase12,14 (SupplementaryFig. 18), which is metastable along the transition pathway and canfurther prolong the transition timescale. This scenario at short delaytimes requires additional characterizations by single-shot time-resolved techniques beyond SHG, such as diffraction or visible-IRreflectivity to probe transient lattice and electronic responses, whichare beyond the scope of our present study.DiscussionThe set of experimental data from multiple techniques allows us totheoretically explore different effects that can account for the 2H-to-1T 0 transition. Based on the nanosecond phase transition timescale, anatural hypothesis involves a THz field-driven carrier excitationmechanism.Microscopically, an intense electric field at THz frequencycangenerate high carrier densities throughmechanisms such as Poole-Frenkel ionization, which liberate carriers and accelerate them tomulti-eV energies (see Supplementary Note 5). These processes lead toimpact ionization, liberating more carriers and evolving as a cascadeevent36. Such electron-hole generation can lead to either (i) a neutralcarrier redistribution when excited carriers remain quasi-free, or (ii)charge doping when carriers localize at the substrate/h-BN interfacialstates or spatially separate under the drive of intense THz electricfields especially in the presence of trap states37 (see SupplementaryNote 5). To account for both scenarios, we numerically investigatedthe effects of neutral aswell as charged carrier excitationon the energylandscape of MoTe2.The results of our calculations are shown in Fig. 3c,d (seeMethodsand Supplementary Note 6 for details of the first-principles calcula-tions). In equilibrium, themetastable 1T 0 phase lies about 0.1 eV higherin energy than the 2H phase (blue horizontal lines); here, energy valuesare quoted per two formula units ofMoTe2 due to unit cell doubling inthis transition. The 2H-to-1T 0 activation barrier and the reverse acti-vation barrier at charge neutrality in equilibrium is 1.66 eV and 1.56 eV,respectively, thus protecting the 2H and 1T 0 phases against thermalfluctuations. Upon excitation of a neutral electron-hole redistribution(Fig. 3c) or charge doping (Fig. 3d), two effects occur. First, the acti-vation barrier lowers substantially (violet-to-red horizontal lines) andsecond, the 1T 0 energy decreases, both effects favoring the occurrenceof the 2H-to-1T 0 transition.Our joint experimental and theoretical studies establish intenseTHz fields as a viable route to steer the polymorphic transition inatomically thin MoTe2. Crucially, this transition cannot be induced byphotons at higher energies9–11,38, as evidenced by our experimentsperformed with mid-infrared and near-infrared pulses centeredaround 225meV and 1.55 eV, respectively (Supplementary Figs. 1 and2). The key difference betweenmid- or near-infrared and THz pulses isthat the former usually leads to mobile carriers at very high-energyconduction bands, and these high-energy mobile carriers will result insignificant lattice heating — hence structural damage including Tecluster formation — through intraband relaxation. On the other hand,strong THz fields can create mobile carriers at the conduction bandedge without significant lattice heating effect (see SupplementaryNote 5). We also noticed that another mechanism involving THz-excited phonons was proposed to drive the 2H-1T 0 phase transition inMoTe239, which could also contribute to our observation uniquelydriven by THz pulses.The large structural distortion also leads to a significant electronicstructure reconstruction involving band inversion around the Γ point,driving a topologically trivial 2H phase of few-layer MoTe2 into aquantum spin Hall insulator state in the 1T 0 phase40 (see Supplemen-taryNote 7). The capability demonstrated in this workhenceopens thetantalizing prospect in the search for phases of matter with non-trivialband topology in addition to highlighting the unique capability ofhigh-field THz pulses in shaping material properties in a complex,multi-phase energy landscape.MethodsFabrication of THz field-enhancement structure on fused silicasubstrateThe fabrication of the metal microslit array was based on a standardphotolithography and lift-off process. Image reversal photoresistAZ5214 was spin-coated on a fused silica substrate at 3000 rpm for30 s, soft baked at 110 °C for 50 s on a hotplate, UV exposed by amaskless alignerMLA 150with a dose of 24mJ/cm2, and post-exposurebaked at 120 °C for 2min followedbyfloodexposure anddevelopmentin AZ422. A thin filmof 2-nmCrwas deposited onto the substrate as anadhesion layer by thermal evaporation followed by a 98-nm-thick Authin film. The samplewas soaked in acetone and PG remover for lift-offto complete fabrication of the field enhancement structure.Layered MoTe2 integration with the field enhancementstructureThemonolayer, few-layer MoTe2, and layered h-BN were exfoliated onSiO2/Si substrate from bulk MoTe2 (HQ graphene) or h-BN crystals.Monolayer and bilayer MoTe2 were identified by optical contrast andRaman spectroscopy. The layered materials were picked up by atransfer slide composed of a stack of glass, a polydimethylsiloxane(PDMS)filmand apolycarbonate (PC)film41. The resulting stacks of toph-BN layer, MoTe2 monolayer or bilayer, and bottom h-BN layer werethen placed on top of the field enhancement structure with the help ofa transfer setup under an optical microscope24. For trilayer or thickerMoTe2 samples used in the SHG and electron microscopy measure-ments, h-BN encapsulation was not used.High-field THz pulse generationHigh-field THz pulses were generated in a Mg:LiNbO3 crystal by tiltingthe optical pulse front of an 800-nm pump pulse to achieve phasematching42. By using a three-parabolic-mirror THz imaging system, theimage of the THz beam spot on the sample was focused to its dif-fraction limit of around 500 μm in diameter. The incident THz pulsetemporal profile was measured in the time domain using electro-opticsamplingwith a 100-μm-thick 110-orientedGaPcrystal.Whenpumpingwith the 4 W output from an amplified Ti:sapphire laser system(repetition rate 1 kHz, central wavelength 800 nm, pulse duration100 fs), the peakelectricfieldof the THzpulses reached ~ 300 kV/cmatthe focus, with a spectrum centered at around 0.5 THz (seeArticle https://doi.org/10.1038/s41467-023-41291-wNature Communications |         (2023) 14:5905 5Supplementary Figs. 13 and 14). The repetition rate of the laser outputwas down-counted from 1 kHz to 125 Hz by three successive phase-locked choppers, providing sufficient temporal separation betweenpulses for a mechanical shutter to isolate single THz pulses.Spontaneous Raman scattering measurementsSpontaneous Raman scattering was performed on samples before andafter THz irradiation using a commercial Raman spectrometer (HoribaLabRAM)with aHeNe laser (λ = 632.8 nm). The laser beamwas focusedon the samples by a 100× objective into a spot with a diameter ofapproximately 1μm.Transmission electron diffraction microscopyTo prepare the samples for transmission electron microscopy (TEM)characterizationbefore and after THz excitation, we designed a specialTEM grid with a THz field enhancement structure. The enhancementpattern made of a 50-nm-thick gold film is deposited on a siliconnitride TEM window. The enhancement structure is a 300-μm-long, 2-μm-wide air gap within the gold film, as shown in SupplementaryFig. 10.We transferred amultilayerMoTe2flake on topof the gapusinga PDMS dry-transfer method43 and measured the diffraction patternbefore and after the single THz pulse excitation. The transmissionelectron diffractionmeasurement was performed using a transmissionelectron microscope (FEI Tecnai Multipurpose Digital TEM) operatedat 60 keV at room temperature.SHG mappingAs shown in Supplementary Fig. 14, the SHG fundamental pulses wereprovided by a mode-locked Ti:sapphire oscillator centered at 800nm.The laser pulse duration was around 35 fs at an 80MHz repetition rate.The pulse was linearly polarized by an achromatic polarizer(400–800 nm) and the polarization at the samplewas controlled by anachromatichalf-waveplate inamotorized rotational stage. SHGsignalsfrom the sample were collected by the same objective for focusing thefundamental light, and they transmitted through the same half-waveplate and polarizer, which ensured that the SHG components detectedwere parallel to the polarization of the fundamental field. A photo-multiplier tube (PMT, Hamamatsu Photonics H10721) was used toanalyze the SHG signal. In the fast mapping mode, the laser beam onthe sample was scanned using two-axis Galvo mirrors (Thorlabs,GVS412) to acquire in situ SHG images. We also use the SHG mappingmethod to look for trilayer samples which have uniform response asshown in Supplementary Fig. 11 for the single-shot THz pump-SHGprobe measurements.Single-shot THz pump-SHG probe microscopyThe THz pump arm was combined with SHG pulses from a second,temporally synchronized 12 W amplified Ti:sapphire laser (repetitionrate 1 kHz, central wavelength 800 nm, pulse duration 35 fs). SHG lightwas focused to anear-diffraction-limited size (1μm)at the samplewith a50×objective. The SHG light was collected by the same objective anddetected by a PMT with a confocal microscope to selectively probe thearea at the focuswith single-optical-pulse irradiation. Thepower level ofthe SHG excitation pulse was well above the damage threshold ofMoTe2, but the ultrafast nature of the pulse enabled us to obtain areliable SHG signal before the sample was damaged. The temporaloverlap of the counter-propagating THz field and SHGoptical pulsewasdetermined by THz field-induced second harmonic signal from a 30-μm-thick LiNbO3 slab. All of the optical measurements were conductedon a single-shot basis under ambient conditions. More details are givenin Supplementary Note 4 and Supplementary Figs. 11d, 15, and 16.First-principles calculationsWe performed density functional theory (DFT) calculations using theVASP code44,45 within the general gradient approximation (GGA) —Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional46. Thekinetic energy cutoff was 350 eV for the plane-wave basis sets. Thelattice constants of 2H and 1T 0 phases in the neutral state wereobtained via structural optimization with a convergence threshold offorceon each atomof0.01 eV/Å, with a vacuumregionmore than 36-Å-thick to decouple the neighboring slabs. A Γ-centered 11 × 17 × 1k-pointmesh was used to sample the Brillouin zone. The climbing imagenudged elastic band (CI-NEB)method47with 5 to 7 intermediate imageswas used to determine the activation barrier of the phase transition.Besides converging the electronic ground states with charge neu-trality, we also considered higher temperature smearing effects andcharge doping. More information can be found in SupplementaryNote 6 and Supplementary Figs. 17–20.In these barrier calculations, the in-plane lattice constants were allconstrained to the value of the neutral state 2H phase, even for thecharged state conditions in which the lattice tends to expand.We notethat the treatment of constant area was mainly due to the difficulty ofconvergence when the lattice vectors were allowed to relax during CI-NEB calculations of charged slabs. It simulates the situation where thelattice of monolayer MoTe2 is strongly constrained by its interactionwith substrates.Based on the converged electronic ground state with spin-orbitcoupling in a given crystal structure, we further constructed theWannier electronic model using theWannier90 code48,49 by projectingthe Mo d-orbitals and Te p-orbitals near the Fermi level. Besides cap-turing the electronic band structure, the Wannier models also allowedus to investigate the topological properties by evaluating the Z2topological index from the non-Abelian Berry connections along theWilson loop50 and the Fu-Kane parity formulation51 (when inversionsymmetry is present). More information can be found in Supplemen-tary Figs. 21–23.Data availabilityRelevant data supporting the key findings of this study are availablewithin the article and the Supplementary Information file. All raw datagenerated during the current study are available from the corre-sponding authors upon request.References1. Sie, E. J., Nyby,C.M., Pemmaraju,C.D., Park, S. J. &Shen, X. et al. Anultrafast symmetry switch in a Weyl semimetal. Nature 565,61–66 (2019).2. Liu, M., Hwang, H. Y., Tao, H., Strikwerda, A. C. & Fan, K. et al.Terahertz-field-induced insulator-to-metal transition in vanadiumdioxide metamaterial. Nature 487, 345–348 (2012).3. 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L., Bernevig, A., Fang, Z. &Dai, X. Equivalent expressionof Z2 topological invariant for band insulators using the non-Abelian Berry connection. Phys. Rev. B 84, 075119 (2011).51. Fu, L. & Kane, C. L. Topological insulators with inversion symmetry.Phys. Rev. B 76, 045302 (2007).AcknowledgementsThe authors acknowledge discussions and technical supports from Z.F.Ren, X. Wang, A. Sood, Y. Yoon, B. Pein, P.-C. Shen, A. Maznev, T. Mah-ony, F. Gao, and Y. Chen. J.S. and K.A.N. acknowledge support from theU.S. Department of Energy, Office of Basic Energy Sciences, underAward No. DE-SC0019126. P.J.-H. acknowledges support from GordonandBettyMoore Foundation’s EPiQS Initiative throughgrantGBMF4693.A.Z. acknowledges support from theMiller Institute for Basic Research inScience. W.C., S.F. and E.K. acknowledge support by ARO MURI awardW911NF-14-0247. S.F. is also supported by a Rutgers Center for MaterialTheory Distinguished Postdoctoral Fellowship. J.-C.H. and V.B.acknowledge support from the Center for Energy Efficient ElectronicsScience (NSFAward0939514). Y.-Q.B. andZ.C.L. acknowledges supportfrom National Natural Science Foundation of China (Grant No.’s61974167 and 91963205), the National Key R&D Program of China (No.2019YFA0210203) and No. 2019QN01X113. Y.-Q.B. further acknowl-edges the Open Project of Guangdong Province Key Lab of DisplayMaterial and Technology (No. 2020B1212060030). K.W. and T.T.acknowledge support from the Elemental Strategy Initiative conductedby the MEXT, Japan (Grant No. JPMXP0112101001) and JSPS KAKENHI(Grant No.’s 19H05790 and JP20H00354). E.B. was supported by theARL-UT Austin Cooperative Agreement W911NF-21-2-0185. X.F.acknowledges support from National Natural Science Foundation ofChina (Grant No.’s 11974191 and 2217830).Author contributionsY.-Q.B., J.S. and A.Z. conceived the experiments, performed the mea-surements, and analyzed the data. W.C., S.F. and E.K. provided theore-tical calculations. Y.-Q.B., J.H., and V.B. fabricated field enhancementstructures. Y.Z. helped with the electron diffraction measurements. Y.-Q.B. and X.F. performed energy-dispersive X-ray spectroscopy. Z.C.performed the field enhancement calculations. T.T. and K.W. grew thecrystals of hexagonal boron nitride. A.Z., Y.-Q.B., J.S. and E.B. led themanuscript preparation with input from all authors. The project wassupervised by Y.-Q.B., P.J.-H. and K.A.N.Competing interestsThe authors declare no competing interests.Additional informationSupplementary information The online version containssupplementary material available athttps://doi.org/10.1038/s41467-023-41291-w.Correspondence and requests formaterials should be addressed to Ya-Qing Bie, Pablo Jarillo-Herrero or Keith A. Nelson.Peer review information Nature Communications thanks Burak Guzel-turk and the other, anonymous, reviewer(s) for their contribution to thepeer review of this work.Reprints and permissions information is available athttp://www.nature.com/reprintsPublisher’s note Springer Nature remains neutral with regard to jur-isdictional claims in published maps and institutional affiliations.Open Access This article is licensed under a Creative CommonsAttribution 4.0 International License, which permits use, sharing,adaptation, distribution and reproduction in any medium or format, aslong as you give appropriate credit to the original author(s) and thesource, provide a link to the Creative Commons license, and indicate ifchanges were made. The images or other third party material in thisarticle are included in the article’s Creative Commons license, unlessindicated otherwise in a credit line to the material. 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To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.© The Author(s) 2023Article https://doi.org/10.1038/s41467-023-41291-wNature Communications |         (2023) 14:5905 8https://doi.org/10.1038/s41467-023-41291-whttp://www.nature.com/reprintshttp://creativecommons.org/licenses/by/4.0/http://creativecommons.org/licenses/by/4.0/ Intrinsic 1T′ phase induced in atomically thin 2H-MoTe2 by a single terahertz pulse Results Polymorphic transition in monolayer MoTe2 Polymorphic transition in multilayer MoTe2 Temporal evolution of the polymorphic transition Discussion Methods Fabrication of THz field-enhancement structure on fused silica substrate Layered MoTe2 integration with the field enhancement structure High-field THz pulse generation Spontaneous Raman scattering measurements Transmission electron diffraction microscopy SHG mapping Single-shot THz pump-SHG probe microscopy First-principles calculations Data availability References Acknowledgements Author contributions Competing interests Additional information