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[Kenji Setoura](https://orcid.org/0000-0002-4610-7407), [Tomoya Oshikiri](https://orcid.org/0000-0002-1268-0256), [Mamoru Tamura](https://orcid.org/0000-0002-8445-5613), Ken Morita, Hideki Fujiwara, [Satoshi Ishii](https://orcid.org/0000-0003-0731-8428), Yusuke Fujii, [Yasutaka Matsuo](https://orcid.org/0000-0002-5071-0284), [Takuya Iida](https://orcid.org/0000-0003-1313-7025), [Kohei Imura](https://orcid.org/0000-0002-7180-9339)

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[Chiral Plasmonic Surface Temperature Switching by Several Tens of Kelvins in Titanium Nitride Nanostructures](https://mdr.nims.go.jp/datasets/3ae71bce-d605-4816-95c5-3758e71d3d25)

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Chiral Plasmonic Surface Temperature Switching by Several Tens of Kelvins in Titanium Nitride NanostructuresChiral Plasmonic Surface Temperature Switching by Several Tens ofKelvins in Titanium Nitride NanostructuresKenji Setoura,* Tomoya Oshikiri, Mamoru Tamura, Ken Morita, Hideki Fujiwara, Satoshi Ishii,Yusuke Fujii, Yasutaka Matsuo, Takuya Iida, and Kohei ImuraCite This: Nano Lett. 2026, 26, 351−357 Read OnlineACCESS Metrics & More Article Recommendations *sı Supporting InformationABSTRACT: The strong asymmetric optical response of plasmonic metal nanostruc-tures to right- and left-handed circularly polarized light has attracted great interest innanotechnology. However, when considering heat generation in these structures, thesurface temperature distribution becomes nearly isothermal regardless of whichhandedness of circularly polarized light is used. This is because of the high thermalconductivity of noble metals and the diffusive nature of heat transfer. In this study, weexperimentally show that the surface temperature patterns of chiral plasmonicnanostructures made from titanium nitride, which has a thermal conductivity lessthan 10% that of gold, become clearly different under right- and left-circularly polarizedlight, with the temperature contrast reaching several tens of kelvins. This temperature switching allows nanoscale spatial control ofphotothermal chemical reactions. Our findings suggest a significant potential for shaping nanoscale temperature distributions in thefield of thermoplasmonics.KEYWORDS: Thermoplasmonics, plasmonic heating, refractory plasmonics, chiral plasmonics, circular dichroismMetal nanostructures that exhibit localized surface plasmonresonance (LSPR) show two main functions whenilluminated by light: the optical antenna effect1 and the role asnanoheaters.2 The latter has been widely studied in the field ofthermoplasmonics, with reported applications to hyperthermiatherapy,3 catalytic reactions,4 solar vapor generation,5,6 con-vective assembly,7,8 macroscopic photothermal reactions,9−11and thermophoretic manipulation.12−14 A recent hot topic inthis field is “chirality in plasmonic heating.” Examples includethe circular dichroism in absorption and heat generation of Γ-shaped silver nanostructures,15 temperature measurements ofchiral gold nanohelicoid particles under right- and left-handedcircularly polarized light,16,17 and photothermal imaging ofchiral nanostructures using the thermal lens effect of solvents.18This trend corresponds to the growing interest in the chiraloptical responses of metallic and dielectric nanostructures innanophotonics and nanotechnology.19−21 However, chirality inthermoplasmonics has a fundamental limitation: the surfacetemperature patterns on nanostructures become nearlyisothermal regardless of the handedness of the circularpolarization. For example, in simulations of a Γ-shaped silvernanostructure with a length of 350 nm under right-handedcircularly polarized illumination at an excitation wavelength of860 nm, the maximum temperature rise (ΔTmax(RCP)) was≈16.9 K and the minimum (ΔTmin(RCP)) was ≈15.7 K on thenanostructure surface. The ratioΔTmin/ΔTmax is 93%, indicatingthat the surface temperature distribution across the nanostruc-ture is nearly uniform.15 A similar ratio was obtained when thenanostructure was illuminated with left-handed circularlypolarized light under the same conditions. A very recent studyreported the direct measurement of the surface temperaturepatterns of a single 200 nm gold nanohelicoid illuminated withcircularly polarized light using atomic force microscopy (AFM)with a sensitivity of 0.1 K.17 The surface temperature imagingshowed that when the overall temperature rise of thenanohelicoid was about 1 K, a nonuniform temperaturedistribution on the order of the detection sensitivity wasobserved. This slightly nonuniform surface temperature patternwas clearly found to switch depending on the handedness of thecircularly polarized light. The surface temperature differencemeasured by AFM is estimated to be comparable to that of theΓ-shaped silver nanoparticles mentioned above. These resultsindicate that, due to the diffusive nature of heat transport, thesurfaces of small structures tend to be nearly isothermal.22In this study, to achieve large surface temperature differencesthrough chiral plasmonic switching, we focused on titaniumnitride (TiN), a material commonly used in refractoryplasmonics.23 Titanium nitride shows LSPR in a wavelengthrange close to that of gold and has an exceptionally high bulkmelting point of about 3200 K, making it an excellent opticalmaterial.24 On the other hand, an important point is that itsReceived: October 17, 2025Revised: December 1, 2025Accepted: December 17, 2025Published: December 22, 2025Letterpubs.acs.org/NanoLett© 2025 The Authors. Published byAmerican Chemical Society351https://doi.org/10.1021/acs.nanolett.5c05212Nano Lett. 2026, 26, 351−357This article is licensed under CC-BY 4.0Downloaded via NATL INST FOR MATLS SCIENCE (NIMS) on February 9, 2026 at 04:19:28 (UTC).See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Kenji+Setoura"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Tomoya+Oshikiri"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Mamoru+Tamura"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Ken+Morita"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Hideki+Fujiwara"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Satoshi+Ishii"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Yusuke+Fujii"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Yusuke+Fujii"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Yasutaka+Matsuo"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Takuya+Iida"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Kohei+Imura"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/showCitFormats?doi=10.1021/acs.nanolett.5c05212&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c05212?ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c05212?goto=articleMetrics&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c05212?goto=recommendations&?ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c05212?goto=supporting-info&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c05212?fig=tgr1&ref=pdfhttps://pubs.acs.org/toc/nalefd/26/1?ref=pdfhttps://pubs.acs.org/toc/nalefd/26/1?ref=pdfhttps://pubs.acs.org/toc/nalefd/26/1?ref=pdfhttps://pubs.acs.org/toc/nalefd/26/1?ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://doi.org/10.1021/acs.nanolett.5c05212?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://pubs.acs.org/NanoLett?ref=pdfhttps://pubs.acs.org/NanoLett?ref=pdfhttps://creativecommons.org/licenses/by/4.0/thermal conductivity is 29 W m−1K−1,25 which is less than 10%of that of gold (314 W m−1K−1). In our previous numericalsimulations, we found that using such a low-conductivity opticalmaterial allows the spatial characteristics of plasmon modes tobe clearly imprinted on the temperature distribution, leading tosurface temperature differences exceeding 100 K.26,27 In thisLetter, based on this concept, we designed an S-shaped TiNnanostructure that shows chiral photothermal responses,fabricated it by electron-beam lithography, and performedlaser irradiation experiments. We demonstrate that circularlypolarized illumination can induce chiral plasmonic switching ofhighly nonuniform surface temperature patterns: the ratio ofminimum to maximum surface temperatures reaches 56%, andthe absolute temperature difference is remarkably large, on theorder of several tens of kelvins.Here, we first designed the photothermal response of TiN S-shaped nanostructures by the finite element method (FEM),and then present experimental results of circularly polarizedlaser irradiation on the fabricated nanostructures. Figure 1ashows the geometry of the system used in the FEM calculations.The S-shaped TiN nanostructure was placed on a sapphiresubstrate, with water as the superstrate. The sapphire substrateacts as a heat sink for the S-shaped nanostructures because of itshigh thermal conductivity. This role as a heat sink is laterevaluated quantitatively by comparing it with a more commonlyused glass substrate. The fabrication process of the S-shapedstructure will be described later, but its dimensions are asfollows: as shown in the scanning electron microscope (SEM)image in Figure 1b, the overall length is about 770 nm, the linewidth is 100 nm, and the thickness is 40 nm. We consider thecase where the S-shaped nanostructure is illuminated from thetop (−z direction) by a monochromatic circularly polarizedplane wave. The calculation procedure is briefly described here,while details are given in our previous works.26,27 The governingequations are the frequency-domain Maxwell equations fornonmagnetic materials and the steady-state heat conductionequation. First, the optical response of the nanostructure tocircularly polarized plane waves was obtained by solvingMaxwell’s equations, giving the spatial distribution of theelectric field. Once the plasmonic polarization pattern isdetermined, polarization currents induce the Joule heatingeffect, which generates localized heat inside the nanostructure.28By solving steady-state heat conduction with this spatiallyinhomogeneous Joule heat as the source, the spatial pattern ofthe “chiral plasmon modes” is imprinted on the surfacetemperature of the S-shaped nanostructure. As boundaryconditions, the outer region for Maxwell’s equations wassurrounded by perfectly matched layers, while the outerboundary for steady-state heat conduction was set at roomtemperature (293 K), as shown in Figure 1a. The constantrefractive indices of water and sapphire substrate were set to 1.33and 1.77, respectively. The dielectric function of TiN was takenfrom the measured values of the thin film we deposited bysputtering (Supporting Information S1). The thermal con-ductivities of water, sapphire, and TiNwere set to 0.6, 42, and 29W m−1 K−1, respectively. Since the experiments employed CWlaser irradiation on the S-shaped nanostructures, the calculationsalso treated steady-state solutions.Figure 1c shows the calculated absorption cross-sectionspectra when the S-shaped nanostructure is illuminated withright- and left-handed circularly polarized (RCP and LCP) lightat various wavelengths. The rotation directions of RCP and LCPare shown in Figure 1a. The structure exhibits slightly differentabsorption for the two circular polarizations, reflecting its chiralplasmonic response. Because the overall length of thenanostructure is relatively large (770 nm), a broad resonanceband appears around wavelengths of 1200−4000 nm. Fromthese absorption spectra, we calculated the g-factor using theexpression (σLCP − σRCP)/((σLCP + σRCP)/2), and the results areshown as a dashed line in Figure 1c. The g-factor of this S-shapednanostructure reaches at most 0.03. In comparison, Γ-shapedsilver nanostructures of similar size with a back reflector,designed for chiral photothermal conversion, exhibit g-factorsranging from 0.1 to 0.4,15 indicating that the optical anisotropyof the present S-shaped structure is relatively small. Never-theless, due to the low thermal conductivity of TiN, the surfacetemperature of the nanostructure changes dramatically undercircularly polarized light, as will be shown in detail later. Fromthis broad absorption band, we focused on the wavelength of1550 nm and carried out more detailed calculations of chiraltemperature switching of plasmons. There are two reasons forusing the wavelength of 1550 nm: (i) the absorption crosssections for RCP and LCP are almost identical, allowingrigorous verification of surface temperature switching under thesame laser power, and (ii) we already have a continuous-waveFigure 1. (a) Geometry of the simulation, where a TiN S-shapednanostructure with a total length of 770 nm is illuminated by circularlypolarized light. (b) SEM image showing the dimensions of the S-shapednanostructure. Scale bar: 200 nm. (c) Calculated absorption cross-section spectra of the S-shaped nanostructure under RCP and LCPillumination. The g factor is shown by the dashed line on the rightvertical axis.Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.5c05212Nano Lett. 2026, 26, 351−357352https://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.5c05212/suppl_file/nl5c05212_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c05212?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c05212?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c05212?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c05212?fig=fig1&ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.5c05212?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-aslaser at this wavelength for optothermal manipulation, whichmade the experiments easier to perform.Figure 2 shows the electric field normalized by the incidentfield E0, surface charge, Joule heat from polarization currents,and steady-state temperature distribution of the S-shapednanostructure and its surroundings, obtained from thepreviously described calculations of the electric field andsteady-state heat conduction. The upper panels correspond toRCP, and the lower panels to LCP. All the mappings in Figure 2represent cross sections at the TiN−water interface (z = 40 nm)in the x−y plane, viewed from the water side. The light intensitywas 1.0 × 1010Wm−2. From the electric fields in Figure 2a and e,it is seen that under RCP the field enhancement is approximately2-fold along the edges of the S-shaped nanostructure, whereasunder LCP the enhancement is localized at both ends of the S-shape and reaches about 5. The corresponding surface chargedensities were calculated from Gauss’s law27 and are shown inFigure 2b and f. These results indicate that different plasmonmodes are excited depending on the handedness of the circularpolarization. A more detailed assignment of the plasmon modeswas carried out using other finite element solvers, CST StudioSuite (https://www.3ds.com/products/simulia/cst-studio-suite) and FreeFEM++, for mode analysis and eigenfunctioncalculations, as described in Supporting Information S2.29,30As reported in earlier simulations of plasmonic heating, thelocation of local heat generation strongly depends on theplasmon mode.28 In Figure 2c (RCP), heat is generated mainlyat the center of the S-shape, while in Figure 2g (LCP), it appearsat both ends of the S-arms. Since the steady-state temperaturedistribution essentially follows the mapping of the heat sources,the same trend is observed in Figure 2d and h: under RCP onlythe center of the S-shape is selectively heated, while under LCPonly the ends of the arms show a temperature rise. Remarkably,the surface temperature of the S-shaped nanostructure iscompletely switched between RCP and LCP, and in each casethe maximum temperature difference on the nanostructuresurface reaches asmuch as 30 K.Here, we quantitatively evaluatethe surface temperature differences of the nanostructures. UnderRCP illumination, the maximum temperature rise occurred atpoint P1 (0, 0, 40 nm) in Figure 2d, while the minimumtemperature rise appeared at both ends of the S-shaped arms.Please note that the origin of the x−y coordinates corresponds topoint P1 at the center of the S-shaped nanostructure (Figure2d), and that z = 0 refers to the water−sapphire interface, asalready shown in the calculation geometry in Figure 1a. UnderLCP illumination, the maximum temperature rise occurred atpoint P2 (90 nm, 260 nm, 40 nm) in Figure 2h, and thetemperature rise at P1 was the smallest. Therefore, we calculatedthe surface temperature ratio by dividing the temperature rise atP2 by that at P1 for RCP, and by dividing the temperature rise atP1 by that at P2 for LCP. The resulting ratios wereΔTP2/ΔTP1 =63% for RCP and ΔTP1/ΔTP2 = 56% for LCP. These surfacetemperature ratios remain constant regardless of the incidentlight intensity. In water, individual nanostructures under steady-state CW laser heating do not induce bubble formation untilabout 220 °C,31 so with higher light intensity the surfacetemperature difference in TiN S-shaped nanostructures couldreach 100 K or higher, based on the above estimation of thesurface temperature ratios. Such a large temperature differencecannot be achieved in noble metal nanostructures with highthermal conductivity.22,32 Up to this point, we have mainlydiscussed the calculation results at 1550 nm. For comparison,the steady-state temperature distributions at a longer wavelengthof 2400 nm and a shorter wavelength of 800 nm are shown inSupporting Information S3. Briefly, excitation at 2400 nmprovided a clearer contrast in chiral temperature switching thanat 1550 nm. In contrast, when the structure was illuminated at800 nm, higher-order plasmon modes were excited, causing theheat generation sites to split into multiple locations and leadingto a nearly uniform surface temperature of the S-shapednanostructure.We next performed heat-conduction calculations in which thethermal conductivity of the S-shaped nanostructure (kmetal) andthat of the substrate (ksub) were treated as variables. The purposewas to quantitatively examine the advantage of TiN over Au inshaping the surface temperature distribution, as well as the roleof the substrate as a heat sink. In the following calculations, thedielectric function of the S-shaped nanostructure was fixed tothat of TiN shown in Supporting Information S1, and theexcitation wavelength was set to 1550 nm. Using the dielectricfunction of Au or changing the excitation wavelength would bepossible options, but such changes would significantly alter theelectric-field distribution and plasmon polarization patternsshown in Figure 2. Therefore, we kept the plasmonic mode fixedat 1550 nm circularly polarized excitation (as in Figure 2), andvaried only kmetal and ksub in the steady-state heat-conductionsimulations. The refractive index of the substrate was also fixedat that of sapphire, n = 1.77, and the superstrate remained water.Figure 2. (a, e) Electric field distributions normalized by the incident field E0. (b, f) Surface charge density distributions calculated using Gauss’s law.(c, g) Joule heat generated from polarization currents inside the nanostructure. (d, h) Steady-state temperature distributions on and around the surfaceof the S-shaped nanostructure. All figures represent x−y cross sections at z = 40 nm from the sapphire surface. The upper panels correspond to RCPillumination, and the lower panels correspond to LCP illumination. The light intensity was set to 1.0 × 1010 Wm−2, and the excitation wavelength was1550 nm.Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.5c05212Nano Lett. 2026, 26, 351−357353https://www.3ds.com/products/simulia/cst-studio-suitehttps://www.3ds.com/products/simulia/cst-studio-suitehttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.5c05212/suppl_file/nl5c05212_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.5c05212/suppl_file/nl5c05212_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c05212?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c05212?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c05212?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c05212?fig=fig2&ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.5c05212?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asFor the kmetal, we used 314Wm−1 K−1 for Au and 29Wm−1 K−1for TiN. For the ksub, we used 1.0 Wm−1 K−1 for glass and 42 Wm−1 K−1 for sapphire. The resulting steady-state temperaturedistributions are shown in Figure 3. These two-dimensionaltemperature maps correspond to a cross-section at z = 40 nm.The incident intensity was adjusted so that the maximumtemperature rise on the nanostructure surface becameapproximately 53 K (the exact value is noted in the figurecaption).We first focus on Figure 3a and d, where the thermalconductivities of the nanostructure and substrate were Au andglass, respectively. Under both RCP and LCP illumination, thesurface of the nanostructure became nearly isothermal. Incontrast, when a TiN nanostructure was placed on a glasssubstrate (Figure 3b and e), the surface temperature becameclearly nonuniform for both RCP and LCP illumination. Mostnotably, even when the nanostructure was made of highlyconductive Au, a clear temperature contrast was obtained if itwas placed on a sapphire substrate (Figure 3c and f). These threecombinations of thermal conductivities demonstrate that, forthin two-dimensional nanostructures on substrates, the heat-sink function of a high-thermal-conductivity substrate plays asignificant role. To quantitatively evaluate the surface temper-ature patterns, we computed the surface temperature ratiosΔTP2/ΔTP1 for RCP illumination and ΔTP1/ΔTP2 for LCPillumination using the same procedure as before, andsummarized the values in Table 1. For comparison, the valuesfor a TiN nanostructure on a sapphire substrate (Figure 2) arealso included.We now discuss the values in Table 1. For Au nanostructureson a glass substrate, the surface temperature ratios were 95−97%for both RCP and LCP illumination, close to values reported forAg nanostructures.15 For TiN nanostructures on glass, the ratiowas roughly 80%, which is low enough for the heat-conductionsimulations to show a clearly nonuniform surface temperaturepattern. On the other hand, even with a highly conductive Aunanostructure, placing a thin two-dimensional structure on agood heat sink yielded a distinctly nonuniform temperaturedistribution. While sapphire is widely used as a transparentsubstrate with high thermal conductivity, silicon carbide (SiC) isanother option; it is a wide-bandgap semiconductor with athermal conductivity of about 280 W m−1 K−1 and exhibitstransparency in the visible range depending on the dopantspecies. Previous studies have reported that SiC can serve as aneffective heat sink for optical nanoheating.33 The mostremarkable values in Table 1 are the 63% and 56% obtainedwhen TiN nanostructures were placed on a sapphire substrate.These ratios indicate that, for example, when the maximumsurface temperature of the nanostructure is 100 °C, the colderregion is about 60 °C. Such a large temperature difference isexpected to enable spatial control of thermal reactions.To perform experiments under conditions comparable to thecalculations in Figure 2, S-shaped TiN nanostructures werefabricated by electron-beam lithography. The procedurefollowed literatures.34,35 Briefly, a 40 nm TiN film was depositedon a sapphire substrate (SS-2SC-2525, 25.4 mm × 25.4 mm ×0.5 mm, C plane, double sides polished, ALLIANCEBiosystems) by RF sputtering using a Ti target under Ar andN2 flow at 600 °C (JEC-SP360M, Jeol). The dielectric functionof this TiN film was measured by a spectroscopic ellipsometer(FE-5000, Otsuka Electronics Co., Ltd., see SupportingInformation S1), and used in the FEM calculations describedabove. On top of the TiN film, an electron-beam resist(ZEP520A) was spin-coated, and the S-shaped patterns werepatterned by an electron beam lithography system (ELS−F125-U, ELIONIX). After developing the resist, a 50 nm Cr layer wasdeposited as a hard mask. The resist was lifted off, and the TiNfilm was dry-etched using Ar and Cl gas (RIE-101iPH,Samco),36 followed by wet etching to remove the Cr. Thisprocess yielded the TiN nanostructures shown in Figure 1b. Allelectron microscopy images were taken using a JSM-7610F(Jeol) after sputtering Pt onto the substrate with a fine coater(JEC-3000FC, Jeol). The S-shaped nanostructures werearranged in a square lattice with a pitch of 9 μm on the sapphiresubstrate. The optical setup used for the laser irradiationexperiments is briefly described as follows. Figure 4a shows aninverted dark-field optical microscope (IX-73, Olympus), and animage of the array of S-shaped nanostructures observed with thissystem is presented in Figure 4b. In this setup, a CW laser at1550 nm (FLH-1550−35-PM-B, Civil Laser) was converted toRCP or LCP using a quarter-wave plate (CP1R and CP1L,Thorlabs), and then focused onto individual S-shapednanostructures by an objective lens (LCPLN50XIR, NA =0.65, Olympus). The sapphire substrate, which can exhibitbirefringence depending on the incident laser conditions, wasplaced on the upper side of the sample chamber as describedlater, shown in Figure 4a. The diameter of the focused laser spotwas measured with a near-infrared camera (Cat# 56−567,Edmund Optics) to be 2.8 μm (as full width half-maximum:Figure 3. Calculated steady-state temperature distributions on andaround the surface of the S-shaped nanostructure. All figures representx−y cross sections at z = 40 nm from the substrate surface. The upperpanels correspond to RCP illumination, and the lower panelscorrespond to LCP illumination. The excitation wavelength was 1550nm. (a, d) The thermal conductivities of kmetal = 314Wm−1 K−1 and ksub= 1.0 W m−1 K−1 were used, and the light intensity was set to 7.4 × 108W m−2. (b, e) The thermal conductivities of kmetal = 29 W m−1 K−1 andksub = 1.0 W m−1 K−1 were used, and the light intensity was set to 7.4 ×108 Wm−2. (c, f) The thermal conductivities of kmetal = 314 Wm−1 K−1and ksub = 42Wm−1 K−1 were used, and the light intensity was set to 1.5× 1010 W m−2.Table 1. Surface Temperature RatiosΔTP2/ΔTP1 under RCP(%)ΔTP1/ΔTP2 under LCP(%)kmetal = 314: ksub = 1.0 95 97kmetal = 29: ksub = 1.0 79 86kmetal = 314: ksub = 42 73 77kmetal = 29: ksub = 42 63 56Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.5c05212Nano Lett. 2026, 26, 351−357354https://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.5c05212/suppl_file/nl5c05212_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.5c05212/suppl_file/nl5c05212_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c05212?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c05212?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c05212?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c05212?fig=fig3&ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.5c05212?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asfwhm), which is sufficiently larger than the size of the S-shapednanostructures.The nanoscale temperature distributions shown in Figure 2dand h are difficult to image directly with an optical microscopebecause of the diffraction limit. To address this, we attempted tovisualize these unique distributions by inducing a thermalreaction through optical heating of the nanostructures and thenobserving the products with SEM. A suitable process for thispurpose is the hydrothermal synthesis of ZnO. It is known thatbulk gold films and individual gold nanostructures can locallyheat and trigger ZnO deposition from precursors in aqueoussolution.37,38 In this study, we prepared precursor solutions byusing deionized water as the solvent, mixing equal volumes of 75mM zinc nitrate hexahydrate aqueous solution and 75 mMhexamethylenetetramine aqueous solution. A 40 μL droplet ofthis precursor solution was placed on a sapphire substrate. Usinga 200 μm-thick silicone rubber spacer, the droplet wassandwiched with a borosilicate glass coverslip (C024321,Matsunami Glass Ind. LTD) to form the sample chambershown in Figure 4a. Circularly polarized laser irradiation wasapplied to individual S-shaped nanostructures for 3 s. Theirradiance in the laser spot was 1.0 × 1010 W m−2, which is thesame as the incident light intensity used in Figure 2. Duringirradiation, dark-field observation showed an increase in lightscattering associated with ZnO formation (see SupportingInformation Video S1).SEM images of the S-shaped nanostructures after irradiationwith RCP and LCP are shown in Figure 4c and d. With RCP, thethermal products were selectively deposited at the center of theS-shaped structure, whereas with LCP, they appeared at bothends of the arms. This striking result contrasts sharply with thecase of gold nanostructures, where ZnO tends to coat the entirestructure.38 To demonstrate the reproducibility of ZnOdeposition under identical laser irradiation conditions, weincluded more than ten SEM images each for RCP and LCP inSupporting Information S4. Since several SEM images for bothRCP and LCP showed good agreement with the calculatednonuniform temperature patterns, we consider the reproduci-bility of this experiment to be sufficient. Under these irradiationconditions, the maximum ΔT values in Figure 2d and h wereabout 50 K, which is consistent with the literature on ZnOhydrothermal synthesis induced by gold nanostructure heat-ing,38 and thus reasonable. Elemental mapping was performedusing the energy-dispersive X-ray spectroscopy (EDS) duringSEM observation. The resulting 2D Zn map confirmed that theZn signal was localized to the product on the S-shapednanostructures, suggesting the deposits are ZnO (SupportingInformation S5).Before presenting the conclusion, several remarks should benoted in this Letter. First, because water exhibits moderateabsorption at a wavelength of 1550 nm,39 the backgroundheating of water cannot be ignored. However, under theirradiation conditions shown in Figure 4, we applied the laser tothe precursor solution without nanostructures, and no ZnOformation was observed by either optical microscopy or SEM.On the other hand, weak natural convection driven by waterabsorption was observed under the optical microscope. If thebackground water temperature were sufficiently high, one mightbe concerned that ZnO microcrystals formed in the bulksolution could be transported by convection and deposited ontothe TiN nanostructures. However, as noted above, ZnOformation does not occur in the background solution at thislaser intensity. Therefore, we consider the contribution ofnatural convection to be minor in this study.Finally, we discuss the size of the S-shaped nanostructuresused in this Letter. Simulations similar to those in Figure 2showed that chiral temperature switching also occurs in TiN S-shaped nanostructures with a total length of 400 nm and a linewidth of 50 nm. However, given our limited access to electron-beam lithography, fabricating such small structures withsufficient resolution was challenging. In contrast, when thetotal length exceeds 1 μm, the contribution of localizedplasmons to light absorption decreases, making it difficult toachieve a clear temperature contrast that depends on thewavelength or polarization. Supplementary calculations regard-ing these size effects are provided in Supporting Information S6.■ CONCLUSIONSIn summary, we experimentally demonstrated nanoscale spatialcontrol of photothermal chemical reactions on TiN S-shapednanostructures that exhibit chiral temperature switching undercircularly polarized light. The design of these nanostructures wasfirst optimized using finite element method simulations, whichrevealed completely different surface temperature distributionsunder right- and left-circularly polarized light. We thenfabricated the designed S-shaped nanostructures by electron-beam lithography and related nanofabrication techniques. Tovisualize the predicted chiral temperature distributions ofindividual nanostructures, we conducted hydrothermal syn-Figure 4. (a) Schematic of laser irradiation on an individual S-shapednanostructure under dark-field optical microscopy. (b) Dark-fieldimage of individual S-shaped nanostructures arranged with a 9 μmpitch. (c, d) SEM images of S-shaped nanostructures irradiated for 3 swith RCP or LCP laser light at a wavelength of 1550 nm. In both cases,the irradiance was 1.0 × 1010 W m−2. Scale bar: 200 nm.Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.5c05212Nano Lett. 2026, 26, 351−357355https://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.5c05212/suppl_file/nl5c05212_si_002.mp4https://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.5c05212/suppl_file/nl5c05212_si_002.mp4https://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.5c05212/suppl_file/nl5c05212_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.5c05212/suppl_file/nl5c05212_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.5c05212/suppl_file/nl5c05212_si_001.pdfhttps://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.5c05212/suppl_file/nl5c05212_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c05212?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c05212?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c05212?fig=fig4&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c05212?fig=fig4&ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.5c05212?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asthesis of ZnO under optical microscopy while irradiating themwith either right- or left-circularly polarized lasers. SEM imagesof the reaction products confirmed that the S-shapednanostructures exhibit the chiral temperature switching aspredicted by the simulations. This large temperature switchingof several tens of kelvins cannot be achieved with noble-metal-based thermoplasmonic systems. Therefore, the findings of thisLetter provide a useful basis for achieving more precisespatiotemporal control of plasmonic optofluidics and thermo-plasmonic reactions.■ ASSOCIATED CONTENT*sı Supporting InformationThe Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acs.nanolett.5c05212.Dielectric function spectra of TiN (S1), detailedcharacterization of the plasmon modes at 1550 nm(S2), steady-state temperature distributions of the S-shaped nanostructure at excitation wavelengths of 800and 2400 nm (S3), supplementary SEM images of ZnOhydrothermal synthesis under RCP and LCP (S4), 2DEDS mapping of the products of the hydrothermalsynthesis (S5), and the effect of size on chiral plasmonictemperature switching (S6) (PDF)Temporal increase of light scattering associated with ZnOformation on the S-shaped nanostructure (MP4)■ AUTHOR INFORMATIONCorresponding AuthorKenji Setoura − Department of Electrical Materials andEngineering, Graduate School of Engineering, University ofHyogo, Himeji, Hyogo 671-2280, Japan; orcid.org/0000-0002-4610-7407; Email: setoura@eng.u-hyogo.ac.jpAuthorsTomoya Oshikiri − Institute of Multidisciplinary Research forAdvanced Materials, Tohoku University, Sendai, Miyagi 980-8577, Japan; Research Institute for Electronic Science,Hokkaido University, Sapporo, Hokkaido 001-0021, Japan;orcid.org/0000-0002-1268-0256Mamoru Tamura − School of Science, Kwansei GakuinUniversity, Sanda, Hyogo 669-1330, Japan; Research Institutefor Light-induced Acceleration System (RILACS), OsakaMetropolitan University, Sakai, Osaka 599-8570, Japan;orcid.org/0000-0002-8445-5613Ken Morita − Department of Chemistry and Biochemistry,School of Advanced Science and Engineering, WasedaUniversity, Shinjuku, Tokyo 169-8555, JapanHideki Fujiwara − Faculty of Engineering, Hokkai-GakuenUniversity, Sapporo 064-0926, JapanSatoshi Ishii − International Center for MaterialsNanoarchitectonics (MANA), National Institute for MaterialsScience (NIMS), Tsukuba, Ibaraki 305-0044, Japan;Graduate School of Science and Technology, University ofTsukuba, Tsukuba, Ibaraki 305-8577, Japan; orcid.org/0000-0003-0731-8428Yusuke Fujii − Graduate School of Chemical Sciences andEngineering, Hokkaido University, Sapporo 060-8628, JapanYasutaka Matsuo − Research Institute for Electronic Science,Hokkaido University, Sapporo, Hokkaido 001-0021, Japan;orcid.org/0000-0002-5071-0284Takuya Iida − Research Institute for Light-induced AccelerationSystem (RILACS), Osaka Metropolitan University, Sakai,Osaka 599-8570, Japan; Department of Physics, GraduateSchool of Science, Osaka Metropolitan University, Sakai,Osaka 599-8570, Japan; orcid.org/0000-0003-1313-7025Kohei Imura − Department of Chemistry and Biochemistry,School of Advanced Science and Engineering, WasedaUniversity, Shinjuku, Tokyo 169-8555, Japan; orcid.org/0000-0002-7180-9339Complete contact information is available at:https://pubs.acs.org/10.1021/acs.nanolett.5c05212Author ContributionsK.S. coordinated the project. K.S., T.O., S.I., Y.F., and Y.M.fabricated and characterized the TiN nanostructures. K.S., M.T.,K.M., T.I., and K.I. conducted numerical simulations. K.S. andH.F. performed laser irradiation experiments under an opticalmicroscope. The manuscript was written through contributionsof all authors. All authors have given approval to the final versionof the manuscript.NotesThe authors declare no competing financial interest.■ ACKNOWLEDGMENTSThis work was supported by JSPS KAKENHI Grant NumbersJP23K04561, JP25H01633, JP22H05131 (a Grant-in-Aid forTransformative Research Areas “Evolution of Chiral MaterialsScience using Helical Light Fields”), JP22H05136,JP23H01916, JP25K22238, JP25H00828, JP24K08282,JP25H01627, JP23H01825, JP24K21723, JP25H00421,JP23H01927, JP25H01637, and also by the Japan Science andTechnology Agency (JST) JPMJFR2139, and JPMJMI21G1. Apart of this work was performed under the Cooperative ResearchProgram of “Network Joint Research Center for Materials andDevices (MEXT).” A part of this work was supported by“Advanced Research Infrastructure for Materials and Nano-technology in Japan (ARIM)” of the Ministry of Education,Culture, Sports, Science and Technology (MEXT), GrantNumber JPMXP1225HK0024 (Hokkaido University).■ REFERENCES(1) Stockman, M. I.; Kneipp, K.; Bozhevolnyi, S. I.; Saha, S.; Dutta, A.;Ndukaife, J.; Kinsey, N.; Reddy, H.; Guler, U.; Shalaev, V. M.;Boltasseva, A.; Gholipour, B.; Krishnamoorthy, H. N. S.; MacDonald,K. F.; Soci, C.; Zheludev, N. I.; Savinov, V.; Singh, R.; Groß, P.; Lienau,C.; Vadai, M.; Solomon, M. L.; Barton, D. R.; Lawrence, M.; Dionne, J.A.; Boriskina, S. V.; Esteban, R.; Aizpurua, J.; Zhang, X.; Yang, S.;Wang,D.; Wang, W.; Odom, T. W.; Accanto, N.; de Roque, P. M.; Hancu, I.M.; Piatkowski, L.; van Hulst, N. F.; Kling, M. F. Roadmap onPlasmonics. J. 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