Frequency-angular distributions of optical signals generated via spontaneous parametric downconversion with the Stokes idler frequency shifts 0.2–5 THz are studied simultaneously with the same distributions in the anti-Stokes range while the nonlinear Mg:LiNbO3 crystal is cooled from 300 K to 4.2 K. The temperature dependencies of the angular distributions at fixed idler frequencies are analyzed using a theoretical Klyshko–Kirchhoff approach with account of thermal field fluctuations and inherent crystal absorption at terahertz frequencies. Although all the measurements are performed in the optical range, we demonstrate how such analysis enables to predict the temperature behavior of the total number of spontaneous parametric downconversion-generated idler photons in the terahertz range, the temperature variation of the optical-terahertz biphoton function, and to study the contributions of classical thermal and pure quantum field fluctuations to parameters of biphotons. It is shown that the temperature-induced growth of the numbers of signal and idler photons and their non-normalized correlation function is provided by overwhelming increase in classical fluctuations, while the pure quantum contributions to these parameters are not so sensitive to the crystal temperature.
The terahertz wave generation by spiral photoconductive antennas fabricated on low-temperature In0.5Ga0.5As films and In0.5Ga0.5As/In0.5Al0.5As superlattices is studied by the terahertz time-domain spectroscopy method. The structures were obtained by molecular beam epitaxy on GaAs and InP substrates with surface crystallographic orientations of (100) and (111)A. The pump-probe measurements in the transmission geometry and Hall effect measurements are used to characterize the properties of LT-InGaAs and LT-InGaAs/InAlAs structures. It is found that the terahertz radiation power is almost four times higher for LT-InGaAs samples with the (111)A substrate orientation as compared to (100). Adding of LT-InAlAs layers into the structure with (111)A substrate orientation results in two orders of magnitude increase of the structure resistivity. The possibility of creating LT-InGaAs/InAlAs-based photoconductive antennas with high dark resistance without compensating Be doping is demonstrated.
Statistical distributions of the analog readings of an antenna-coupled THz superconducting bolometer were measured and analyzed under a special type of irradiation by low-energy fluxes of THz photons with Poisson photon statistics and controllable mean photon numbers. The photons were generated via low-gain parametric down-conversion in pulse-pumped Mg:LiNbO3 crystal placed to a cooled cryostat together with the bolometer NbN film. Results of theoretical approximation of experimental histograms reveal the discrete nature of THz detection by superconducting bolometers and open a way for studying their quantum characteristics. It is shown that bolometer readings per pulse consist of discrete counts (“single charges”), with the mean number linearly dependent on the number of input photons. Contributions of single counts to a total analog reading are statistically distributed according to the normal law, with average values slightly depending on the number of counts in each reading. A general formula is proposed to describe the relationship between continuous statistical distribution of the bolometer readings and discrete quantum statistics of the incident photons.
The terahertz (THz) wave generation by the spiral photoconductive antennas fabricated on the low-temperature and high-temperature grown undoped and Si-doped In0.5Ga0.5As films is studied by the terahertz time-domain spectroscopy method. The In0.5Ga0.5As layers were grown by molecular beam epitaxy on GaAs substrates with (1 0 0) and (1 1 1)A crystallographic orientations utilizing step-graded InxGa1−xAs metamorphic buffer. The antennas are excited by radiation of Er3+-fiber laser at 1.56 μm wavelength in two regimes: with pulse durations of 2.5 ps or 100 fs. It is found that the THz wave generation is 3–4 times more effective in the case of InGaAs-based antennas on (1 1 1)A GaAs substrates as compared to the (1 0 0) substrates. Power-voltage characteristic of the LT-InGaAs antenna up to and beyond threshold breakdown voltage are reported.
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