Letter pubs.acs.org/NanoLett
Terahertz-Triggered Phase Transition and Hysteresis Narrowing in a Nanoantenna Patterned Vanadium Dioxide Film Zachary J. Thompson,† Andrew Stickel,† Young-Gyun Jeong,‡ Sanghoon Han,¶ Byung Hee Son,§ Michael J. Paul,† Byounghwak Lee,† Ali Mousavian,† Giwan Seo,∥,⊥ Hyun-Tak Kim,∥,⊥ Yun-Shik Lee,*,† and Dai-Sik Kim*,‡ †
Department of Physics, Oregon State University, Corvallis, Oregon 97331-6507, United States Department of Physics and Astronomy and Center for Atomic Scale Electromagnetism, Seoul National University, Seoul 151-747, Republic of Korea ¶ Photonic Systems Laboratory, School of EECS, Seoul National University, Seoul 151-744, Republic of Korea § Department of Physics and Department of Energy Systems Research, Ajou University, Suwon 443-749, Republic of Korea ∥ Creative Research Center of Metal−Insulator Transition, Electronics and Telecommunications Research Institute, Daejeon 305-700, Republic of Korea ⊥ School of Advanced Device Technology, University of Science & Technology, Daejeon 305-333, Republic of Korea
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‡
ABSTRACT: We demonstrate that high-field terahertz (THz) pulses trigger transient insulator-to-metal transition in a nanoantenna patterned vanadium dioxide thin film. THz transmission of vanadium dioxide instantaneously decreases in the presence of strong THz fields. The transient THz absorption indicates that strong THz fields induce electronic insulator-to-metal transition without causing a structural transformation. The transient phase transition is activated on the subcycle time scale during which the THz pulse drives the electron distribution of vanadium dioxide far from equilibrium and disturb the electron correlation. The strong THz fields lower the activation energy in the insulating phase. The THz-triggered insulator-to-metal transition gives rise to hysteresis loop narrowing, while lowering the transition temperature both for heating and cooling sequences. THz nanoantennas enhance the field-induced phase transition by intensifying the field strength and improve the detection sensitivity via antenna resonance. The experimental results demonstrate a potential that plasmonic nanostructures incorporating vanadium dioxide can be the basis for ultrafast, energy-efficient electronic and photonic devices. KEYWORDS: vanadium dioxide, terahertz, high-field, phase transition, nanoantenna, plasmonics electric fields, where Coulomb interactions are screened by the electrically injected carriers. The phase transition occurs under nonequilibrium conditions in the high field regime, which cannot be described by a simple electrothermal model.16−18 Ultrafast dynamics of terahertz (THz)-field induced IMT has been demonstrated in VO2 metamaterial and in a VO2 thin film at room temperature.19,20 The field-induced phase transition in VO2 has huge potential for new types of electrical and optical switches. Terahertz-Field-Induced Absorption. Our study demonstrates that high-field THz pulses trigger transient IMT in a nanoantenna patterned VO2 thin film in a wide temperature range. THz spectroscopy is a powerful method to study the dynamics of field-induced phase transition. Pure field effects
P
hotonic integrated circuits based on ultrafast phase transitions in nanostructured vanadium oxide can lead to breakthroughs in high-speed, low-energy communication and data-storage technologies.1,2 Vanadium dioxide (VO2), a correlated oxide compound, is one of the few materials that undergo insulator-to-metal transition (IMT) near room temperature (Tc = 67 °C).3,4 The phase transition in VO2 is unique because the transition mechanism involves both lattice distortion and electronic correlations,5 yet their underlying processes are fundamentally different.6−8 Recent studies have tapped into the origin of IMT in VO2,9,10 yet its microscopic mechanism is not fully understood. An intriguing property of VO2 is that the phase transition can be triggered in a very short time scale. Femtosecond pump−probe studies observed structural and electronic dynamics in VO2, showing that the photoinduced phase transition was faster than the internal thermalization in the nonequilibrium regime where the insulating electronic state was melted yet no structural transformation occurred.11−15 IMT can also be triggered by © XXXX American Chemical Society
Received: May 19, 2015 Revised: August 17, 2015
A
DOI: 10.1021/acs.nanolett.5b01970 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
room temperature as shown in the transmission spectrum in Figure 1a.21 The spectrum shows that the antenna is resonant at 0.9 THz with insulating phase at room temperature (blue), whereas the resonant transmission disappears with metal phase at high temperature (red). The total array area, 2 mm by 2 mm, is larger than the incident THz beam size at the focal plane. THz electric fields undergo 20−40-fold enhancement at the antenna gap with an insulating substrate. The near field at the antenna gap is uniformly distributed, existing only in the gap regions.26 We obtain the near-field amplitude (Enear) from the field enhancement factor α of the nanoantenna structure
can be resolved, not disturbed by parasitic hot carriers, because of its low photon energies (4.1 meV at 1 THz, which is much lower than the VO2 band gap energy, 0.65 eV). THz nanoantennas provide a promising platform of active photonic devices which can exploit subwavelength confinement and field enhancement of light.21−24 The nanoantenna patterned VO2 film used in this study is illustrated in Figure 1a. The sample
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α=
Enear 1 = E inc β
T τr
(1)
where T is the THz transmission of the sample, β is the antenna coverage ratio (β = 1/350 for this sample), and τr is the ratio of the pulse duration of transmitted radiation to that of incident radiation.27,28 THz transmission T of the sample ranges from 0.01 to 0.03 and the pulse duration ratio τr varies between 1.1 and 2.5; therefore, THz fields at the antenna gap reaches 20− 30 MV/cm for the highest incident field of 0.8 MV/cm. We performed THz transmission measurements to investigate field-induced IMT in VO2 using strong, broadband THz pulses (central frequency, 1 THz; bandwidth, 1 THz). The THz pulses were generated by optical rectification of femtosecond laser pulses with tilted pulse fronts (pulse energy, 1 mJ; pulse duration, 90 fs; repetition rate, 1 kHz) in a LiNbO3 crystal (THz field amplitude reaches 0.9 MV/cm at an optical pulse energy of 0.8 mJ).29,30 We detected the THz pulses using a L-He cooled Si:Bolometer to acquire spectrally integrated total THz transmitted power and THz transmission spectra via Michelson interferometry. We also measured transmitted THz waveforms using electro-optic sampling with a 1 mm thick ZnTe crystal. Figure 1b shows the transmission of broadband THz pulses through the nanoantenna patterned VO2 film as a function of incident THz peak-field amplitude Einc at sample temperatures of 45, 59, 62, and 65 °C (we define the transmission as the ratio of transmitted THz power to incident one). Strong THz pulses make no permanent change in transmission and there is no hysteresis for a complete cycle of increasing and decreasing field amplitude. The temperature was gradually increased from room temperature. The intense THz fields generate remarkably strong nonlinear effects in the VO2 film: THz transmission undergoes huge reduction as field strength increases, for example, the transmission drops 75% at 65 °C as the field strength is raised from 100 kV/cm to 800 kV/cm. These nonlinear THz effects are greatly enhanced by the nanoantennas. As shown in the inset of Figure 1b, THz transmission of a bare VO2 film without nanoantennas exhibits no change at low temperature (45 °C) and only a slight decrease near the transition temperature (71 °C) in the high field regime. The huge reduction of THz transmission in the nanoantenna patterned VO2 film corresponds to a large increase of conductivity, that is, VO2 becomes more metallic in the presence of higher THz fields. A possible mechanism for the conductivity increase is that the THz fields strongly disturb the electron correlation in VO2 and transiently give rise to IMT by collapsing the bandgap (Figure 1c).8 We have observed that the field-induced phase transition is transient, and the experimental results will be shown in the last section. In the following section, we will show that intense THz fields activate IMT in
Figure 1. Intensity dependent THz transmission of nanoantenna patterned VO2 film. (a) THz nanoantenna array (antenna width, 200 nm; length, 60 μm) is patterned on a 100 nm thick VO2 thin film deposited on a sapphire substrate. A transmission spectrum shows the antenna resonance at 0.9 THz with insulating phase at room temperature (blue), whereas the resonant transmission disappears with metal phase at high temperature (red). (b) THz transmission of the nanoantenna patterned VO2 film as a function of peak field amplitude Einc at the sample temperatures of 45, 59, 62, and 65 °C (red lines are to guide the eyes). Temperature was gradually raised from 30 to 80 °C. The inset shows THz transmission through a bare VO2 film without nanoantennas at 45 and 71 °C. (c) Schematic energy diagram for THz-driven insulator-to-metal transition in VO2, resulting in bandgap collapse.
consists of a 100 nm thick VO2 film deposited on a 430-μmthick c-plane sapphire substrate via reactive rf-magnetron sputtering.25 The typical grain size is about 100 nm. The antenna array fabricated on the VO2 film consists of slot antennas that are 60 μm in length and 0.2 μm (= 200 nm) in width. The slot antenna has a simple structure that local THz fields are uniform and exist only in the gap regions. The antenna periodicity is 70 μm in the length direction and 60 μm in the width direction. Each antenna is resonant at 0.9 THz at B
DOI: 10.1021/acs.nanolett.5b01970 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters
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VO2, demonstrating field-induced changes in hysteresis curves of the nanoantenna patterned VO2 film. Hysteresis and Activation Energy. Figure 2 shows temperature dependent THz transmission through the nano-
Figure 2. THz modulated hysteresis curves. Temperature dependent transmission of the nanoantenna patterned VO2 film shows hysteresis for a heating and cooling cycle. The incident THz field amplitudes are 90, 300, 560, and 790 kV/cm. The inset shows hysteresis curves of a bare VO2 film at Einc = 70 and 680 kV/cm.
antenna patterned VO2 sample for incident THz peak-field strengths of 90, 300, 560, and 790 kV/cm. The VO2 sample exhibits hysteresis for a complete heating and cooling cycle. For comparison, hysteresis curves of a bare VO2 film at 70 and 680 kV/cm are also shown in the inset. THz fields have no effect on the hysteresis curves of the bare VO2 sample until the field amplitude reaches 300−400 kV/cm. When the field amplitude is raised to 680 kV/cm, the transmission curve (red dots in the inset) shows a small decrease near the transition temperature for the heating sequence, whereas there is no noticeable change during cooling. This result is consistent with the THz intensity dependent transmission shown in the inset of Figure 1b. On the contrary, the hysteresis curves of the nanoantenna patterned VO2 film exhibit huge deformation, indicating that the nanoantenna sample is highly sensitive to the field-induced changes in VO2. The THz transmission undergoes substantial reduction in the entire temperature range as the field strength increases. At the same time, the hysteresis curves shift toward the lower temperature side, that is, the THz fields effectively lower the phase transition temperature. These results clearly demonstrate that THz fields induce insulator-to-metal transition in VO2. In order to understand the underlying mechanism of fieldinduced IMT, it is crucial to determine how THz fields modulate electrical resistivity in VO2. We obtain the resistivity of the VO2 samples by analyzing THz transmission measurements.31 Thin-film Fresnel equations lead to the THz transmission of a thin film as a function of conductivity TR =
Figure 3. Resistivity and THz transmission. (a) Normalized transmission vs conductivity for 100 nm thick VO2 film and nanoantenna patterned VO2. (b) Resistivity hysteresis curves of nanoantenna patterned VO2 at Einc = 90, 300, 560, and 790 kV/cm.
depicts the conductivity-dependent transmission of a bare VO2 film. We apply this curve to the temperature-dependent transmission data of the bare VO2 sample at a low THz field (300 kV/cm) falls below 0.3 eV. The fieldinduced reduction of activation energy demonstrates that intense THz fields actively modulate the band structure and cause the bandgap to shrink in the insulating phase. A simple model based on the Poole−Frenkel effect predicts that the activation energy attenuation is proportional to the square of the applied field strength, ΔEa = −(e3/πϵ)1/2E1/2, where E is the D
DOI: 10.1021/acs.nanolett.5b01970 Nano Lett. XXXX, XXX, XXX−XXX
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ization. This observation implies that strong THz fields transiently modulate electron distribution and induce an insulator−metal transition without causing a structural phase transition on the subpicosecond time scale. It is challenging but important to figure out how the transient IMT occurs in the microscopic scale. The Poole−Frenkel effect is a possible mechanism, but the threshold behavior of the activation energy shown in Figure 4a indicates that more complicated mechanisms may be involved in the process.5,20 Summary. In summary, our experimental study demonstrates that strong THz pulses transiently induce insulator-tometal transition in VO2 thin films. Strong THz fields reduce activation energy in an insulating phase and lower phase transition temperature both for heating and cooling sequences. The nonlinear THz effects are greatly enhanced by THz nanoantennas. The field-driven phase transition occurs on the subpicosecond time scale, and hence, the dynamical IMT is a nonequilibrium phenomenon. The transience of phase transition indicates that the field-induced IMT is a nonthermal process and a structural phase transition makes little contribution to the conductivity changes at the initial stage. This preliminary study demonstrates that nanoantenna patterned VO2 devices have a potential to be an excellent platform for ultrafast, energy-efficient electronic and photonic devices.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: [email protected]. Phone: +1 541-7375057. Fax: +1 541-737-1683. *E-mail: [email protected]. Author Contributions
D.-S.K. and Y.-S.L. conceived the experiments, Y.-S.L. conducted theoretical analysis and wrote the manuscript, Z.T. and A.S. conducted the experiments and analyzed the data, Y.G.J. and B.H.S. performed e-beam lithography for nanoantenna fabrication, S.H. and G.S. fabricated VO2 thin films under the supervision of H.-T.K., M.P., B.L., and A.M. conducted supplemental experiments. All authors reviewed the manuscript. Notes
Figure 5. THz triggered Insulator−metal transition is transient: (a) THz waveforms transmitted through the nanoantenna VO2 sample at different temperatures, 45, 65, and 67 °C during heating for a low THz field (Einc = 150 kV/cm). The corresponding spectra are shown in the inset on the right. The gray line indicates the incident pulse spectrum. The incident waveform is shown in the inset on the left. (b), (c) THzintensity dependent waveforms for the incident peak THz fields of 150, 300, 630, and 850 kV/cm at (b) 45 °C and (c) 65 °C during heating. The corresponding spectra are shown in the insets. (d) Fielddriven (red) and temperature-driven (blue) changes in THz waveform. The yellow shaded area indicates the waveform of Einc = 150 kV/cm at 65 °C. The red and blue lines present the waveforms of Einc = 850 kV/ cm at 65 °C and of Einc = 150 kV/cm at 67 °C, respectively.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The OSU work is supported by the National Science Foundation (NSF) (1063632). The SNU work is supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (2005-0093838, 2008-00580, 2008-0061906, 2015031768). The ETRI group acknowledges Creative Research Project of ETRI.
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REFERENCES
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DOI: 10.1021/acs.nanolett.5b01970 Nano Lett. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.nanolett.5b01970 Nano Lett. XXXX, XXX, XXX−XXX
Terahertz-Triggered Phase Transition and Hysteresis Narrowing in a Nanoantenna Patterned Vanadium Dioxide Film Zachary J. Thompson,† Andrew Stickel,† Young-Gyun Jeong,‡ Sanghoon Han,¶ Byung Hee Son,§ Michael J. Paul,† Byounghwak Lee,† Ali Mousavian,† Giwan Seo,∥,⊥ Hyun-Tak Kim,∥,⊥ Yun-Shik Lee,*,† and Dai-Sik Kim*,‡ †
Department of Physics, Oregon State University, Corvallis, Oregon 97331-6507, United States Department of Physics and Astronomy and Center for Atomic Scale Electromagnetism, Seoul National University, Seoul 151-747, Republic of Korea ¶ Photonic Systems Laboratory, School of EECS, Seoul National University, Seoul 151-744, Republic of Korea § Department of Physics and Department of Energy Systems Research, Ajou University, Suwon 443-749, Republic of Korea ∥ Creative Research Center of Metal−Insulator Transition, Electronics and Telecommunications Research Institute, Daejeon 305-700, Republic of Korea ⊥ School of Advanced Device Technology, University of Science & Technology, Daejeon 305-333, Republic of Korea
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‡
ABSTRACT: We demonstrate that high-field terahertz (THz) pulses trigger transient insulator-to-metal transition in a nanoantenna patterned vanadium dioxide thin film. THz transmission of vanadium dioxide instantaneously decreases in the presence of strong THz fields. The transient THz absorption indicates that strong THz fields induce electronic insulator-to-metal transition without causing a structural transformation. The transient phase transition is activated on the subcycle time scale during which the THz pulse drives the electron distribution of vanadium dioxide far from equilibrium and disturb the electron correlation. The strong THz fields lower the activation energy in the insulating phase. The THz-triggered insulator-to-metal transition gives rise to hysteresis loop narrowing, while lowering the transition temperature both for heating and cooling sequences. THz nanoantennas enhance the field-induced phase transition by intensifying the field strength and improve the detection sensitivity via antenna resonance. The experimental results demonstrate a potential that plasmonic nanostructures incorporating vanadium dioxide can be the basis for ultrafast, energy-efficient electronic and photonic devices. KEYWORDS: vanadium dioxide, terahertz, high-field, phase transition, nanoantenna, plasmonics electric fields, where Coulomb interactions are screened by the electrically injected carriers. The phase transition occurs under nonequilibrium conditions in the high field regime, which cannot be described by a simple electrothermal model.16−18 Ultrafast dynamics of terahertz (THz)-field induced IMT has been demonstrated in VO2 metamaterial and in a VO2 thin film at room temperature.19,20 The field-induced phase transition in VO2 has huge potential for new types of electrical and optical switches. Terahertz-Field-Induced Absorption. Our study demonstrates that high-field THz pulses trigger transient IMT in a nanoantenna patterned VO2 thin film in a wide temperature range. THz spectroscopy is a powerful method to study the dynamics of field-induced phase transition. Pure field effects
P
hotonic integrated circuits based on ultrafast phase transitions in nanostructured vanadium oxide can lead to breakthroughs in high-speed, low-energy communication and data-storage technologies.1,2 Vanadium dioxide (VO2), a correlated oxide compound, is one of the few materials that undergo insulator-to-metal transition (IMT) near room temperature (Tc = 67 °C).3,4 The phase transition in VO2 is unique because the transition mechanism involves both lattice distortion and electronic correlations,5 yet their underlying processes are fundamentally different.6−8 Recent studies have tapped into the origin of IMT in VO2,9,10 yet its microscopic mechanism is not fully understood. An intriguing property of VO2 is that the phase transition can be triggered in a very short time scale. Femtosecond pump−probe studies observed structural and electronic dynamics in VO2, showing that the photoinduced phase transition was faster than the internal thermalization in the nonequilibrium regime where the insulating electronic state was melted yet no structural transformation occurred.11−15 IMT can also be triggered by © XXXX American Chemical Society
Received: May 19, 2015 Revised: August 17, 2015
A
DOI: 10.1021/acs.nanolett.5b01970 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
room temperature as shown in the transmission spectrum in Figure 1a.21 The spectrum shows that the antenna is resonant at 0.9 THz with insulating phase at room temperature (blue), whereas the resonant transmission disappears with metal phase at high temperature (red). The total array area, 2 mm by 2 mm, is larger than the incident THz beam size at the focal plane. THz electric fields undergo 20−40-fold enhancement at the antenna gap with an insulating substrate. The near field at the antenna gap is uniformly distributed, existing only in the gap regions.26 We obtain the near-field amplitude (Enear) from the field enhancement factor α of the nanoantenna structure
can be resolved, not disturbed by parasitic hot carriers, because of its low photon energies (4.1 meV at 1 THz, which is much lower than the VO2 band gap energy, 0.65 eV). THz nanoantennas provide a promising platform of active photonic devices which can exploit subwavelength confinement and field enhancement of light.21−24 The nanoantenna patterned VO2 film used in this study is illustrated in Figure 1a. The sample
Downloaded by GEORGETOWN UNIV on August 28, 2015 | http://pubs.acs.org Publication Date (Web): August 28, 2015 | doi: 10.1021/acs.nanolett.5b01970
α=
Enear 1 = E inc β
T τr
(1)
where T is the THz transmission of the sample, β is the antenna coverage ratio (β = 1/350 for this sample), and τr is the ratio of the pulse duration of transmitted radiation to that of incident radiation.27,28 THz transmission T of the sample ranges from 0.01 to 0.03 and the pulse duration ratio τr varies between 1.1 and 2.5; therefore, THz fields at the antenna gap reaches 20− 30 MV/cm for the highest incident field of 0.8 MV/cm. We performed THz transmission measurements to investigate field-induced IMT in VO2 using strong, broadband THz pulses (central frequency, 1 THz; bandwidth, 1 THz). The THz pulses were generated by optical rectification of femtosecond laser pulses with tilted pulse fronts (pulse energy, 1 mJ; pulse duration, 90 fs; repetition rate, 1 kHz) in a LiNbO3 crystal (THz field amplitude reaches 0.9 MV/cm at an optical pulse energy of 0.8 mJ).29,30 We detected the THz pulses using a L-He cooled Si:Bolometer to acquire spectrally integrated total THz transmitted power and THz transmission spectra via Michelson interferometry. We also measured transmitted THz waveforms using electro-optic sampling with a 1 mm thick ZnTe crystal. Figure 1b shows the transmission of broadband THz pulses through the nanoantenna patterned VO2 film as a function of incident THz peak-field amplitude Einc at sample temperatures of 45, 59, 62, and 65 °C (we define the transmission as the ratio of transmitted THz power to incident one). Strong THz pulses make no permanent change in transmission and there is no hysteresis for a complete cycle of increasing and decreasing field amplitude. The temperature was gradually increased from room temperature. The intense THz fields generate remarkably strong nonlinear effects in the VO2 film: THz transmission undergoes huge reduction as field strength increases, for example, the transmission drops 75% at 65 °C as the field strength is raised from 100 kV/cm to 800 kV/cm. These nonlinear THz effects are greatly enhanced by the nanoantennas. As shown in the inset of Figure 1b, THz transmission of a bare VO2 film without nanoantennas exhibits no change at low temperature (45 °C) and only a slight decrease near the transition temperature (71 °C) in the high field regime. The huge reduction of THz transmission in the nanoantenna patterned VO2 film corresponds to a large increase of conductivity, that is, VO2 becomes more metallic in the presence of higher THz fields. A possible mechanism for the conductivity increase is that the THz fields strongly disturb the electron correlation in VO2 and transiently give rise to IMT by collapsing the bandgap (Figure 1c).8 We have observed that the field-induced phase transition is transient, and the experimental results will be shown in the last section. In the following section, we will show that intense THz fields activate IMT in
Figure 1. Intensity dependent THz transmission of nanoantenna patterned VO2 film. (a) THz nanoantenna array (antenna width, 200 nm; length, 60 μm) is patterned on a 100 nm thick VO2 thin film deposited on a sapphire substrate. A transmission spectrum shows the antenna resonance at 0.9 THz with insulating phase at room temperature (blue), whereas the resonant transmission disappears with metal phase at high temperature (red). (b) THz transmission of the nanoantenna patterned VO2 film as a function of peak field amplitude Einc at the sample temperatures of 45, 59, 62, and 65 °C (red lines are to guide the eyes). Temperature was gradually raised from 30 to 80 °C. The inset shows THz transmission through a bare VO2 film without nanoantennas at 45 and 71 °C. (c) Schematic energy diagram for THz-driven insulator-to-metal transition in VO2, resulting in bandgap collapse.
consists of a 100 nm thick VO2 film deposited on a 430-μmthick c-plane sapphire substrate via reactive rf-magnetron sputtering.25 The typical grain size is about 100 nm. The antenna array fabricated on the VO2 film consists of slot antennas that are 60 μm in length and 0.2 μm (= 200 nm) in width. The slot antenna has a simple structure that local THz fields are uniform and exist only in the gap regions. The antenna periodicity is 70 μm in the length direction and 60 μm in the width direction. Each antenna is resonant at 0.9 THz at B
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VO2, demonstrating field-induced changes in hysteresis curves of the nanoantenna patterned VO2 film. Hysteresis and Activation Energy. Figure 2 shows temperature dependent THz transmission through the nano-
Figure 2. THz modulated hysteresis curves. Temperature dependent transmission of the nanoantenna patterned VO2 film shows hysteresis for a heating and cooling cycle. The incident THz field amplitudes are 90, 300, 560, and 790 kV/cm. The inset shows hysteresis curves of a bare VO2 film at Einc = 70 and 680 kV/cm.
antenna patterned VO2 sample for incident THz peak-field strengths of 90, 300, 560, and 790 kV/cm. The VO2 sample exhibits hysteresis for a complete heating and cooling cycle. For comparison, hysteresis curves of a bare VO2 film at 70 and 680 kV/cm are also shown in the inset. THz fields have no effect on the hysteresis curves of the bare VO2 sample until the field amplitude reaches 300−400 kV/cm. When the field amplitude is raised to 680 kV/cm, the transmission curve (red dots in the inset) shows a small decrease near the transition temperature for the heating sequence, whereas there is no noticeable change during cooling. This result is consistent with the THz intensity dependent transmission shown in the inset of Figure 1b. On the contrary, the hysteresis curves of the nanoantenna patterned VO2 film exhibit huge deformation, indicating that the nanoantenna sample is highly sensitive to the field-induced changes in VO2. The THz transmission undergoes substantial reduction in the entire temperature range as the field strength increases. At the same time, the hysteresis curves shift toward the lower temperature side, that is, the THz fields effectively lower the phase transition temperature. These results clearly demonstrate that THz fields induce insulator-to-metal transition in VO2. In order to understand the underlying mechanism of fieldinduced IMT, it is crucial to determine how THz fields modulate electrical resistivity in VO2. We obtain the resistivity of the VO2 samples by analyzing THz transmission measurements.31 Thin-film Fresnel equations lead to the THz transmission of a thin film as a function of conductivity TR =
Figure 3. Resistivity and THz transmission. (a) Normalized transmission vs conductivity for 100 nm thick VO2 film and nanoantenna patterned VO2. (b) Resistivity hysteresis curves of nanoantenna patterned VO2 at Einc = 90, 300, 560, and 790 kV/cm.
depicts the conductivity-dependent transmission of a bare VO2 film. We apply this curve to the temperature-dependent transmission data of the bare VO2 sample at a low THz field (300 kV/cm) falls below 0.3 eV. The fieldinduced reduction of activation energy demonstrates that intense THz fields actively modulate the band structure and cause the bandgap to shrink in the insulating phase. A simple model based on the Poole−Frenkel effect predicts that the activation energy attenuation is proportional to the square of the applied field strength, ΔEa = −(e3/πϵ)1/2E1/2, where E is the D
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Nano Letters
ization. This observation implies that strong THz fields transiently modulate electron distribution and induce an insulator−metal transition without causing a structural phase transition on the subpicosecond time scale. It is challenging but important to figure out how the transient IMT occurs in the microscopic scale. The Poole−Frenkel effect is a possible mechanism, but the threshold behavior of the activation energy shown in Figure 4a indicates that more complicated mechanisms may be involved in the process.5,20 Summary. In summary, our experimental study demonstrates that strong THz pulses transiently induce insulator-tometal transition in VO2 thin films. Strong THz fields reduce activation energy in an insulating phase and lower phase transition temperature both for heating and cooling sequences. The nonlinear THz effects are greatly enhanced by THz nanoantennas. The field-driven phase transition occurs on the subpicosecond time scale, and hence, the dynamical IMT is a nonequilibrium phenomenon. The transience of phase transition indicates that the field-induced IMT is a nonthermal process and a structural phase transition makes little contribution to the conductivity changes at the initial stage. This preliminary study demonstrates that nanoantenna patterned VO2 devices have a potential to be an excellent platform for ultrafast, energy-efficient electronic and photonic devices.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: [email protected]. Phone: +1 541-7375057. Fax: +1 541-737-1683. *E-mail: [email protected]. Author Contributions
D.-S.K. and Y.-S.L. conceived the experiments, Y.-S.L. conducted theoretical analysis and wrote the manuscript, Z.T. and A.S. conducted the experiments and analyzed the data, Y.G.J. and B.H.S. performed e-beam lithography for nanoantenna fabrication, S.H. and G.S. fabricated VO2 thin films under the supervision of H.-T.K., M.P., B.L., and A.M. conducted supplemental experiments. All authors reviewed the manuscript. Notes
Figure 5. THz triggered Insulator−metal transition is transient: (a) THz waveforms transmitted through the nanoantenna VO2 sample at different temperatures, 45, 65, and 67 °C during heating for a low THz field (Einc = 150 kV/cm). The corresponding spectra are shown in the inset on the right. The gray line indicates the incident pulse spectrum. The incident waveform is shown in the inset on the left. (b), (c) THzintensity dependent waveforms for the incident peak THz fields of 150, 300, 630, and 850 kV/cm at (b) 45 °C and (c) 65 °C during heating. The corresponding spectra are shown in the insets. (d) Fielddriven (red) and temperature-driven (blue) changes in THz waveform. The yellow shaded area indicates the waveform of Einc = 150 kV/cm at 65 °C. The red and blue lines present the waveforms of Einc = 850 kV/ cm at 65 °C and of Einc = 150 kV/cm at 67 °C, respectively.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The OSU work is supported by the National Science Foundation (NSF) (1063632). The SNU work is supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (2005-0093838, 2008-00580, 2008-0061906, 2015031768). The ETRI group acknowledges Creative Research Project of ETRI.
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REFERENCES
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