This paper describes research on the optics of functional materials, which can change their dielectric properties according to their function. Vanadium dioxide is a good example of such a material where the insulator-to-metal phase transition offers the possibility to control dielectric properties and to use them as a triggering element for photonic applications in the wide spectral range from optical to terahertz frequencies. We observed emission of terahertz (THz) radiation from films in insulating and conductive phase states under femtosecond pulse irradiation. We found that the efficiency of THz emission increases up to 30 times after the insulator-to-metal phase transition. This process occurs in thin films while it is fundamentally forbidden in the bulk material, and polarization analysis of the emitted radiation reveals the crucial importance of interface contributions. The properties of the THz radiation emitted by are determined by displacement photocurrents at the –air and –substrate interfaces induced by the incident laser light. In each phase state the contributions of the two boundaries are different. Properties of the effective dielectric susceptibility tensor for the insulating phase were defined. In demonstrating the conversion of optical into THz radiation in films, we found that fundamental symmetry restrictions are not applicable to problems of nonlinear optics of thin films.
© 2015 Optical Society of America
Vanadium dioxide () is a substance that exhibits phase transition from insulating to conductive state at a temperature of 68°C, which leads to an increase in conductivity of up to four orders of magnitude . During this first-order phase transition from monoclinic (low-temperature) to tetragonal (high-temperature) phase , the measurable optical and terahertz (THz) properties are drastically changed. The index of refraction for optical radiation decreases from 2.67 to 2.26 , while for THz radiation the film becomes almost reflective . Therefore, vanadium dioxide films can be used in various controllable elements for optical and THz radiation (optical modulators, saturable absorbers, ultrafast switches, and metamaterials). High transmission contrast between insulating and conductive phase for THz radiation makes this material attractive for producing active beam guiding elements: controllable attenuators, amplitude and phase screens, mirrors, polarizers, etc. [5–8].
Thin films can be deposited by the magnetron sputtering method [9,10], the sol-gel method , metal organic chemical vapor deposition (MOCVD) , and pulsed laser deposition (PLD) [13–16]. The vanadium oxide films grown on sapphire substrates are attractive for practical applications because of the high transparency of sapphire for both THz and optical radiation and the best conditions for epitaxial growth of unstrained films with narrow hysteresis and high conductivity contrast. The best transition parameters of the films can be achieved on the R-cut sapphire substrates due to the best match of crystalline lattice dimensions of the and substrate , which ensures the epitaxial mode of growth of the films. Nevertheless, these substrates are birefringent for both THz and optical radiation, which restricts these films from phase and polarization-sensitive applications.
The phase transition can be induced either by heating  or by optical pumping , applied voltage , and pressurizing . These factors can be applied together to reach the most convenient conditions for phase transition for different purposes .
Sharp contrast in the conductivity of the two phases of vanadium dioxide leads to a substantial difference in nonlinear optical properties. Thus, third harmonic generation , z-scanning , and degenerate four-wave mixing experiments [23,24] show an increase of up to one order of magnitude in the nonlinear response of the films above the phase transition temperature.
Both metallic and insulating phases of belong to the centrosymmetric class of materials (semiconductor monoclinic phase has monoclinic structure and space group is , and metallic phase has rutile structure and space group is ); thus second-order processes are not expected in the bulk material . Nevertheless, in this work we find that thin (up to 300 nm) V films can generate broadband THz radiation as a result of interaction with femtosecond optical pulses from a Ti:sapphire regenerative amplifier. The THz emission efficiency shows a step-like increase at the phase transition temperature.
During the phase transition, both metallic and insulating phases can coexist, signaling the percolative nature of the phase transition similar to that observed in the ultrathin Au films at insulator–metal phase transition. However, for vanadium dioxide films the phase transition is much more convenient to achieve and investigate. THz emission is observed in thin metal films in both metallic [25–28] and transition state , and the emission was attributed to both the nonlinear response of the surface and the free charge carriers. In our study, we use two types of sapphire substrates (R- and C-cut), which influence the orientation of the film to show that the THz emission process is sensitive to the film orientation and the –substrate interface. Another peculiarity of the films is the very short mean-free path of the charge carriers, which is comparable to the lattice parameter , which makes us consider the conductive phase of as “strongly correlated” metal.
The aim of this work is to investigate THz emission in thin films grown on sapphire substrates having two different symmetries (C- and R-cut) before and after the insulator–metal phase transition. We focus our attention on polarization of the THz radiation since it gives the possibility to define the underlying mechanism of THz emission.
2. EXPERIMENTAL SETUP
For investigation of the polarization of the THz pulses generated in thin films we used an experimental setup shown schematically in Fig. 1. The laser pulses from a Ti:sapphire regenerative amplifier (SpectraPhysics Spitfire) having pulse energy 0.5 mJ, duration 50 fs, and 1 kHz repetition rate were used for the generation of THz radiation. A vanadium dioxide film on a sapphire substrate was irradiated with a collimated Gaussian laser beam at normal incidence. The radiation intensity in our studies reached , which is still below the threshold value for the light-induced phase transition . The film acted as a source of THz radiation. THz radiation from the film was collimated with an off-axis parabolic mirror. Optical radiation was blocked with a 0.5-mm-thick polyethylene plate and a 0.35-mm-thick silicon filter. The polyethylene plate was used to scatter the optical beam and avoid parasitic THz generation in the silicon filter. Another off-axis parabolic mirror was used to focus THz radiation into the detector. We used electro-optical sampling based on a 4-mm-thick ZnTe crystal and balanced photodiodes, which allowed us to investigate the THz radiation in the spectral range from 0 to 2.5 THz. This part of the spectrum does not include any phonon properties of crystals and is governed mostly by the free-carrier conductivity [4,31].
For polarization analysis of THz radiation we placed a wire-grid THz polarizer into a collimated beam between two parabolic mirrors. Since electro-optical detection is itself polarization sensitive, we kept the polarization of the detection beam, the orientation of ZnTe crystal, and the analyzer fixed throughout the experiment, but rotated the polarization of the pump beam and the sample azimuth angle using two motorized rotation stages. In a single measurement series, we rotated the sample and pump polarization by 360° synchronously with 20° steps, so that the angle between the sample and the pump optical field remained constant, and recorded THz waveforms for each sample orientation. After that, we changed the pump polarization by 15° with respect to the sample and repeated a full 360° rotation once again. For each studied sample, we carried out measurements at room temperature and above the phase transition temperature .
In our work, we used both MOCVD and PLD methods for depositing the vanadium oxide films. MOCVD growth was performed in the hot-wall tubular vertical reactor using reaction between (SuperOx, 98%) and water vapors in inert atmosphere. The argon flow rate regulated by the mass flow controller was 3.6 l/h, and the resulting pressure in the system was 0.01 Torr. Water vapor was added into the system by injection of liquid water into the hot zone (100°C) with rate of 30 mcl/min and frequency of 0.5 Hz. The temperature of the sublimator was 100°C. The temperature of the reactor was 400°C. Film deposition was carried out on the single crystal one-side polished substrates of cleaned by acetone and dried on air. After deposition at 400°C the samples were annealed in the reactor at 600°C for 1 h under low pressure (0.01 Torr, Ar flow 3.6 l/h).
For the PLD method, we used R- and C-cut sapphire substrates having thickness of 0.33 mm. The films were grown using laser power density from 5.2 to . 99.9% vanadium was used as a target for deposition. For uniform depletion of the target it was rotating with 1 Hz frequency. An excimer KrF laser with 10 Hz repetition rate was used for target ablation. Mechanical separation of evaporated particles was used for droplet-free deposition. backing pressure was achieved using turbomolecular and cryogenic pumps. The substrate was positioned at 70 mm from the target. Deposition was performed at 20 mTorr pressure and 630°C substrate temperature.
The results of x-ray analysis (-scanning) of indicate the high in-plane texture of the phase in two (100) and orientations. In MOCVD films we observed no impurities of other phases or orientations [Fig. 2(a)], while for PLD films some inclusions of and a metastable phase were detected. Analysis of the pole figure of reflection at [Fig. 2(b)] and its comparison with the -scan for (0006) [Fig. 2(c)] reflection confirmed the epitaxial growth in the two orientations with the following relationship with the substrate: , and , in agreement with . Mutual directions of sapphire and (200) axes in their two phase states are shown schematically in Fig. 2(d). For the sample grown on the C-cut sapphire we found (001) oriented texture: , which cannot be distinguished from (010) texture, which has a similar angle (see [32,33]).
4. EXPERIMENTAL RESULTS
For initial investigation of the conductivity change during the phase transition, we performed THz absorption measurements in a conventional THz spectrometer in the transmission scheme. For most of the applications of vanadium dioxide in the THz range the transmission contrast between conducting and insulating states is the most important property of the film. It depends on the film conductivity in the THz range and shows the uniformity of the film. Higher conductivity contrast corresponds to the most uniform oxidation level of vanadium in the film.
The decrease in transmitted THz amplitude is shown in Fig. 3(a) with a 5°C wide hysteresis clearly visible. In the frequency range available for our spectrometer (from 0.1 to 2.5 THz) all films showed no resonance features [see Fig. 3(b)] in the transmission spectrum since all phonon frequencies are above 4.4 THz . After the phase transition, the transmission coefficient decreased uniformly within this frequency range. The rotation of the azimuth angle of the sample for all films did not reveal any absorption anisotropy both in the insulating and in the conductive phase of .
The change in the transmission coefficient revealed nonmonotonic dependence on the film thickness (see Table 1). The highest contrast (first-order decrease of transmitted amplitude) was achieved for the sample grown on R-sapphire by the MOCVD method; for laser-deposited films, the best result obtained was a four times decrease of transmission for 100 nm film on R-sapphire. This is consistent with the results of x-ray phase analysis: the laser-deposited samples showed the presence of several phases of vanadium oxide, which led to a decrease in crystallite size and total contrast in resistivity. The absorption contrast for the 34-nm-thick film grown on C-cut sapphire was lower than for films on the R-cut sapphire substrates and reached only 0.64.
The irradiation of films with the femtosecond pulses from a Ti:sapphire regenerative amplifier having energies up to 0.5 mJ led to the generation of THz pulses in the films. THz emission was observed in all investigated films, but the emitted field amplitude in some of the samples was rather small. The THz pulses had a broadband spectrum and showed no visible resonance features in the frequency range available for our detector (0.1–2.5 THz). A typical waveform and spectrum are shown in Fig. 4.
The detected THz amplitude increased drastically above the insulator–metal phase transition temperature. The waveform and emitted spectrum remained identical for all samples, and the highest amplitude obtained was about 1% of that observed from the LT-GaAs surface with the same beam used for THz generation. Quantitatively the increase in amplitude showed no correlation either with the change in the sample transmittance or with the thickness of the film (see Table 1).
We also performed measurements of the THz field amplitude dependence on pump beam energy. The THz amplitude showed linear dependence on the pump fluence above the phase transition temperature for all samples (see Fig. 5). Below the phase transition temperature, the THz amplitude dependence on pump fluence was linear up to . The observed emission of the THz radiation can be treated in terms of the phenomenological three-wave mixing, or optical rectification: . The THz energy dependences fit perfectly to this model for both conductive and insulating states. At fluences above the THz amplitude growth rate increased. This increase results from the light-induced phase transition that is expected at about .
The polarization of the emitted THz pulses was investigated by synchronous rotation of the sample and pump pulse polarization and the fixed orientation of the THz polarizer, the detector crystal, and the probe beam. We registered the THz waveforms of the radiation passing through the THz analyzer for each orientation of the film. After the measurement, we changed the angle of the pump pulse polarization with respect to the film axis.
In the rutile phase of , polarization of the THz beam was perfectly linear [Fig. 6(b)] for all films, which was confirmed by identical shapes of the THz waveform at any angle between the film and the THz analyzer. For low-temperature monoclinic phase we observed slight discrepancies from the linear state of polarization, but this ellipticity was on the order of the noise level [Fig. 6(a)].
We observed a drastic discrepancy between the polarization of THz radiation from films in semiconductor and metallic state. In the former case, the THz intensity and polarization were dependent on the angle between the pump pulse polarization and the film orientation. The polarization direction β of the THz pulse in the films grown on the R-cut sapphire reached 15°–18° from the  axis, and its intensity changed by 80% as the pump polarization direction was rotated [see Fig 7(a)]. In contrast, for rutile phase of the film the THz intensity and polarization dependence on the pump beam polarization were very weak [Fig. 7(b)]. Emitted THz polarization was polarized along the  axis [which is essentially parallel to the  direction in the phase state; see Fig. 2(d)] with almost constant intensity as the pump beam polarization direction α was changed over 100°.
The film grown on C-cut sapphire substrate also generated THz radiation in both the conductive and the insulating phase. Its intensity was small in relation to that of the samples on R-cut sapphire, and not high enough to thoroughly investigate its polarization state. The change in THz emission intensity after the phase transition was also small (see Table 1).
We also performed test experiments on four-wave interaction in a manner similar to THz wave generation in gases. For this we applied second harmonic of the laser radiation in addition to the fundamental radiation in the excitation beam. We used a 0.1 mm I-type BBO crystal placed into a collimated laser beam. The second harmonic radiation was approximately two orders of magnitude less intensive than that of fundamental radiation. No THz radiation from the films was detected when the film was irradiated with a pure SH beam for any polarization of the second harmonic radiation. For the case of both first and second harmonics present in the pump beam, the generated THz signal was equal to the sum of the THz radiation generated by the fundamental beam and the weak THz signal originating from the BBO crystal. Thus we did not observe any evidence of a four-wave rectification process as it happens in gaseous media.
Finally, we performed measurements of THz generation efficiency for variable incidence angles of the optical radiation for monoclinic phase of the film. The increase of emission efficiency at angles different from normal incidence would suggest the mechanism of THz emission due to photocurrent directed perpendicularly to the film surface as it happens for the surface of the LT-GaAs  or at metal films above the percolation threshold . The observed emission efficiency decreased as the incidence angle changed from 0° to 70°.
Since the films in both semiconductor and metallic phase state belong to centrosymmetric space groups, the second-order nonlinear susceptibility tensor and optical gyrotropy should vanish for the bulk material. Nevertheless, second-order nonlinear processes are not forbidden at the sharp boundaries between conductive film and nonconductive media such as air and sapphire substrate . This happens due to excitation of the displacement current at the surface of the conductive layer, which accounts for the emission of electromagnetic radiation at combinative frequencies. For this mechanism to take place, it is necessary to have a nonzero electric field component of incident optical radiation along the surface normal. This occurs for nonzero incidence angles or, as in our case, for limited dimensions of the optical beam. The symmetry of the surface of the medium is in general different from that of the bulk medium, and it defines the polarization of emitted radiation at mixing frequencies (in our case, “zero” frequency, which accounts for THz emission).
film in conductive phase possesses high symmetry; its bulk has symmetry, and its surface exposed to air has p1g1 (rectangular 2D space group). Also, the R-sapphire substrate surface also has symmetry that corresponds to the rectangular class. In both cases, the two orthogonal in-plane directions are not equivalent. Therefore, the nonlinear-optical mixing process that occurs at the –sapphire and -air boundary has a preferred polarization axis linked to the substrate (and film) orientation, exactly as it happens in the experiment. In contrast, the C-cut sapphire surface has high hexagonal 3m symmetry and does not possess a preferential direction. For these substrates, we observed only very weak THz emission almost at the noise level. We should add here that the sapphire surfaces without films did not emit THz radiation. The observed THz signal is a property of the and -air boundary layers.
At the same time, the linear-optical properties and bulk conductivity of the film in the THz frequency range are isotropic, as follows from our THz absorption measurements.
For the low-temperature monoclinic phase of the nonlinear response is influenced by optically induced conductivity. The interaction of fundamental radiation with the low-temperature phase of the leads to generation of the Frenkel exciton, which decays rapidly into a Wannier–Mott exciton . If the optical pump fluence exceeds the threshold value of about , a new charge-transfer vibronic exciton phase is formed leading to charge transfer in the crystalline lattice, free-electron decay, and the formation of a conductive phase. Since in our work we use the pump values just below the threshold value (except the above-mentioned experiment on the intensity dependence of the THz amplitude), conductivity appears due to tunneling of electrons from Frenkel or from Wannier–Mott exciton states into unoccupied excited states in the conductive phase (see ). The surface symmetry of the is not necessarily the same as that for the conductive phase of , which influences the emitted THz pulse polarization through the direction of the surface displacement current. In addition, the ratio between contributions of the -air and surfaces is different from that of the conductive phase, which accounts for the difference in THz polarization in the two phase states of film. In the three-wave mixing formalism, we can introduce from the experimental data the effective tensor of nonlinear susceptibility of the low-temperature phase as7(a) along with experimental data obtained for two different samples.
In this work we have discovered the THz field emission in films undergoing insulator-to-metal phase transition after irradiation with femtosecond laser pulses with high intensity. The THz range used in our study (0.1–2.5 THz) restricts our attention to the carrier dynamics in both metallic and optically excited semiconductor phases of vanadium dioxide. The THz emission amplitude for the metallic phase of follows a linear dependence on the pump laser intensity. In the semiconducting phase, the amplitude dependence of the THz pulse on the pump fluence is also linear up to , after which it increases due to light-induced phase transition. The THz emission efficiency in the metallic phase of the film is up to 30 times higher than that for the semiconducting phase. Polarization studies of THz radiation revealed linear polarization for both monoclinic and rutile phase of grown on R-cut sapphire substrates. In the high-temperature phase, THz polarization is defined by anisotropic photocurrent influenced by intrinsic electrostatic field, which is directed along the preferred  direction. Thus the emitted field polarization is linear and almost independent from pump pulse polarization. THz absorption studies show no anisotropy of in-plane conductivity of the film. In both the insulating and the conductive phase states, the THz radiation is governed by the contribution of the surface displacement current induced by the incident optical beam on the and -air boundaries. The parameters of the effective susceptibility tensor for the semiconductor phase were defined.
Army Research Office (ARO) (W911NF111029); Laboratory of Bio-photonics of Tomsk State University; M.V. Lomonosov Moscow State University Program of Development; Program of Introducing Talents to Universities on Photonics and Optoelectronics Science & Technology (111 plan B07038); Russian Academy of Sciences (RAS) (1.II.2); Russian Foundation for Basic Research (RFBR) (12-03-31377, 14-02-00979, 14-22-01098, 15-29-01171, 15-32-20961); U.S. Army Research Laboratory (ARL).
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