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The indices of refraction, extinction constants and complex conductivities of the GaN film for frequencies ranging from 0.2 to 2.5 THz are obtained using THz time-domain spectroscopy. The results correspond well with the Kohlrausch stretched exponential model. Using the Kohlrausch model fit not only provides the mobility of the free carriers in the...
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... measurements were taken at room temperature. Figure 2 shows a schematic of the experimental setup for THz-TDS, similar to the conventional THz system. A mode-locked amplified Ti:sapphire laser (λ = 800 nm) with 0.6 W average output power generated ~130 fs pulses at a repetition rate of 1 kHz. ...
Citations
... Terahertz time-domain spectroscopy (THz-TDS) [10, 11] is a widely applied method to measure the refractive index of materials at THz frequencies, which can be converted into various parameters of the materials, such as complex conductivity and dielectric constants [12,13]. The THz-TDS provides the opportunity to discuss those anisotropic properties and also other physical parameters such as mobility, scattering time, DC conductivity, and so on [14][15][16][17], with simple free carrier models: the Drude and Drude-Smith models [18,19]. Blumenschein et al. have discussed weak anisotropic properties at low temperature on UID (unintentionally doped) and Sn-doped ( 201 ) β-Ga 2 O 3 , and the temperature dependence of UID β-Ga 2 O 3 properties [20]. ...
Anisotropic conductivity and its temperature dependence of unintentionally doped and Fe-doped bulk (010) β-Ga2O3 were studied by using terahertz time-domain spectroscopy (THz-TDS) with the THz polarization being parallel to the [100] and the [001] directions. The responses to the THz waves were different along the a-axis and the c-axis, where the Drude and the Drude-Smith model were applied for fitting the data of these two axes, respectively. This suggests that the carriers travel along the a-axis as free carriers while those are highly localized along the c-axis. The unintentionally doped sample showed a unique temperature dependence, in particular, a decreasing trend of the real part of the complex conductivity with increasing temperature. This is attributed to carriers being completely thermally excited from shallow impurity levels, and afterward, the scattering behavior has been enhanced, causing the conductivity to decrease. Conversely, the Fe-doped sample showed little temperature dependence, indicating this sample to be thermally stable. Moreover, the mobility along the a-axis in the Fe-doped sample was estimated to be much higher than that along the c-axis.
... It is a non-destructive, contactless technique suitable even for examining highly fragile materials like thin films [8], biological tissues [9], etc. Many semiconductors such as ZnO [10], GaN [11,12], and Graphene [13][14][15] have been characterized extensively using THz-TDS. However, there are only a few studies on the THz optical properties of the doped and undoped β-Ga 2 O 3 [16][17][18][19][20]. ...
We report the terahertz optical properties of vanadium doped (100) β-Ga2O3 using terahertz time-domain spectroscopy (THz-TDS). The V-doped β-Ga2O3 crystal shows strong birefringence in the 0.2-2.4 THz range. Further, phase retardation by the V-doped β-Ga2O3 has been measured over the whole THz range by terahertz time-domain polarimetry (THz-TDP). It is observed that the V-doped β-Ga2O3 crystal behaves both as a quarter waveplate (QWP) at 0.38, 1.08, 1.71, 2.28 THz, and a half waveplate (HWP) at 0.74 and 1.94 THz, respectively.
... Note that d jjj is written in tensor notation which via matrix notation can be written by d jj . On the other hand, pz coefficients presented in Eq. (9) can be found by analysing the frequency and refractive index dependence in GaN crystals [26][27][28][29][30]. ...
... The fundamental lattice vibrations in GaN in the IR limit have been rigorously explored by analysing the Raman and IR spectra [29]. Piezoelectric coefficient of GaN has been precisely determined by direct electro optic measurements and laser optical interferometer methods. ...
... The value d 31 , d 32 and d 33 was measured to be − 1.7 × 10 − 12 , − 1.7 × 10 − 12 and 3.1 × 10 − 12 (m/V) respectively [30][31][32][33][34][35][36]. The high-frequency dielectric constants, resonance frequencies, the damping constants, and the oscillator strengths are precisely obtained by analysing the IR reflection spectra of GaN using Kramers-Kronig relations [25][26][27][28][29]. The material density ρ of GaN crystal is 6.65 (g/cm 3 ), and its ion's valence number p is four. ...
At optical frequency range, lattice vibrations in piezoelectric crystal are associated with ionic polarizations. A relationship is formulated, based on classical dispersion theory, involving the dielectric constant (εjj) and the piezoelectric coefficient (djj) of wurtzite GaN. The relation is utilized to investigate the piezoelectric lattice vibrations in GaN at optical frequency, and a close agreement between the computed and the experimental value of djj is found. The study reveals that applications of piezoelectric nature of GaN could be extended to optical frequencies to generate phase coherent acoustic phonons to modulate thermal conduction. At optical frequencies there are no direct experimental methods exist at present for measurement of djj; however, the proposed relation can help to determine djj from infrared reflection measurements.
... There was no reflection from the three antifouling paint layers and also from two anticorrosive paint layers because the RIs are same for the layers with same materials. The oscillation after the strong reflection from the coating/steel interface was caused by multiple reflections inside the sample (Tsai et al., 2006). ...
... The plasma frequency of the 2DEG can be then defined by ω p N s e 2 ∕ε 0 m 0.5 , where m 0.22m 0 is the effective electron mass in GaN, and m 0 is the free-electron mass. The complex conductivity of the Drude model is given by σω ε 0 ω 2 p τ 0 ∕1 − iωτ 0 , and the frequency-dependent dielectric constant is defined as εω ε d iσ∕ωε 0 n r in i 2 , where ε d is the complex contribution of the dielectric and n r and n i are the real and imaginary indices of refraction, respectively [31]. ...
Engineering metamaterial-based devices such as terahertz bandpass filters (BPFs) play a definitive role in advancement of terahertz technology. In this article, we propose a design procedure to obtain a considerably broadband terahertz BPF at a normal incidence; it shows promising filtering characteristics, including a wide passband of ∼ 1.34 THz at a central frequency of 1.17 THz, a flat top in a broad band, and high transmission, compared to previous reports. Then, exploiting the voltage-dependent carrier density control in an AlGaN/GaN heterostructure with a Schottky gate configuration, we investigate the tuning of the transmission properties in a narrow-band terahertz filter. A combination of the ultra-wide, flat-top BPF in series with the tunable, narrow band filter designed in the current study offers the ability to tune the desired resonance frequency along with high out-of-band rejection and the suppression of unwanted resonances in a large spectral range. The proposed structure exhibits a frequency tunability of 103 GHz for a voltage change between − 8 and 2 V, and a transmission amplitude change of ∼ 0.51 . This scheme may open up a route for the improved design of terahertz filters and modulators.
... From the transmitted THz pulse field sam ( ) E passing through the sample and the reference electric field ref ( ) E passing through the substrate, the complex transmission ( ) t is given as [45] ...
We employ optical pump-terahertz probe spectroscopy to investigate the composition-dependent photoconductivity in ternary CdSx
Se1−x
nanobelts. The observed carrier dynamics of CdS nanobelts display much shorter lifetime than those of ternary CdSx
Se1−x
nanobelts. This indicates the implementation of CdS nanobelts as ultrafast switching devices with a switching speed potentially up to 46.7 GHz. Surprisingly, ternary CdSx
Se1−x
nanobelts are found to exhibit much higher photoconductivity than binary CdS and CdSe. This is attributed to the higher photocarrier densities in ternary compounds. In addition, the presence of Se in samples resulted in prominent CdSe-like transverse optical (TO) phonon modes due to electron-phonon interactions. The strength of this mode shows a large drop upon photoexcitation but recovers gradually with time. These results demonstrated that growth of ternary nanostructures can be utilized to alleviate the high surface defect density in nanostructures and improve their photoconductivity.
... Figure 2 presents the real and imaginary parts of the refractive indices for the three GaN samples with different carrier concentrations extracted from the THz-TDS numerical fitting and those of GaN samples previously reported. [10][11][12][13] At 1 THz, the refractive index is determined to be 3.14 for u-GaN (N ¼ 2.7 Â 10 17 cm À3 ), 6.48 and 7.62 for n-GaN at N ¼ 1.8 Â 10 18 cm À3 and N ¼ 3.3 Â 10 18 cm À3 , respectively. The index measured by Zhang et al. 10 for a sample with a smaller carrier concentration (N ¼ 0.9 Â 10 16 cm À3 ) is lower at $2.8, while Fang et al., 13 Tsai et al., 11 and Guo et al. 12 provide intermediate values ranging from $3.6 to $7 for carrier concentrations between N ¼ 6.1 Â 10 17 cm À3 and N ¼ 2.0 Â 10 18 cm À3 . ...
... [10][11][12][13] At 1 THz, the refractive index is determined to be 3.14 for u-GaN (N ¼ 2.7 Â 10 17 cm À3 ), 6.48 and 7.62 for n-GaN at N ¼ 1.8 Â 10 18 cm À3 and N ¼ 3.3 Â 10 18 cm À3 , respectively. The index measured by Zhang et al. 10 for a sample with a smaller carrier concentration (N ¼ 0.9 Â 10 16 cm À3 ) is lower at $2.8, while Fang et al., 13 Tsai et al., 11 and Guo et al. 12 provide intermediate values ranging from $3.6 to $7 for carrier concentrations between N ¼ 6.1 Â 10 17 cm À3 and N ¼ 2.0 Â 10 18 cm À3 . At frequency higher than 2 THz, the difference of refractive indices for different doping concentrations in GaN sample becomes smaller, while at frequency lower than 1 THz, the refractive index of the highly doped GaN sample increases quickly due to the stronger plasma effect. ...
In this letter, we report the characterization of the refractive indices and complex conductivities of a set of GaN films with different carrier concentrations, InN film, and InxGa1−xN films with indium content varying from x = 0.07 to x = 0.14 grown by metalorganic chemical vapor deposition for frequencies ranging from 0.3 to 3 THz using terahertz time-domain spectroscopy (THz-TDS). The refractive indices of InxGa1−xN films at THz range are reported. The carrier density and mobility determined using THz-TDS method show good agreement with four-probe Hall measurements.
... Similar phenomena were found in n-type polycrystalline ZnO thin films and nanostructures 15 and n-type GaN epilayer. 18 The refractive index of the sample 1 is slightly higher than that of sample 2, due to the higher free carrier concentration in sample 1. The measured refractive index is in reasonable agreement with Drude model. ...
Using terahertz time-domain spectroscopy, we measured the frequency dependent complex dielectric response and conductivity of n -type single-crystal ZnO epilayers with different carrier concentrations over the frequency range from 0.1 to 3.0 THz. The measured complex dielectric response and conductivity are analyzed using Drude model.
Active control of the optical parameters in strontium titanate (SrTiO3, STO) thin films is highly desirable for tunable terahertz (THz) integrated devices such as filters, phase modulators, and electro-optical devices. In this work, optically tuned dielectric parameters of a STO thin film epitaxially grown on a silicon wafer were characterized in the THz region with an 800 nm laser pumpTHz detection system. The refractive index, extinction coefficient, and complex dielectric constant of the STO thin film were calculated using thin-film parameter extraction. Owing to carrier transportation and soft-mode oscillation, the above optical parameters change notably with the pump power of the external laser. This study is of great significance for rapid and non-contact THz phase-modulation technology and may serve as a powerful tool to tune the dielectric properties of the STO thin films.
