ИСТИНА

Various methods of nonlinear optics provide promising techniques for generation and detection of terahertz frequency (THz) radiation. The progress is stimulated by important applications of THz electromagnetic waves in communication, spectroscopy, biomedicine, medical tomography and other areas [1]. But there is a lack of works concerning quantum optical properties of the terahertz fields, in particular, on the optical-terahertz biphotons which can be generated via parametric down-conversion process in the strongly frequency non-degenerate regime. In order to investigate angular-frequency spectra of SPDC in the signal channel we used the standard SPDC spectrograph scheme improved for detection at the smallest Stokes shifts [2,3]. The optical scheme included an Ar-laser as a pump source at 514.5 nm, Glan-Thomson prisms as the polarizer and analyzer, a lens system, an iodine cell, lithium niobate (LN) crystal and a spectrograph supplied by a CCD camera. The iodine vapor cell heated up to 80 degrees Celsius was used for suppressing the pump radiation after the crystal. Because the absorption line of the molecular iodine is very narrow, it was necessary to reduce the bandwidth of the pump radiation. For this purpose, a Fabry-Perot etalon was placed into the laser cavity for selecting a single longitudinal mode. The scattered radiation was focused by a lens system on the input slit of our spectrograph. Two-dimensional wavelength-angular spectra were registered at the output of the spectrograph by means of a CCD-camera (Fig. 1). We have observed the almost complete suppression of an anti-Stokes branch at a crystal temperature 4.8 K. For detection of THz idler photons we used a hot electron bolometer (HEB). A very low noiseequivalent power achievable for these detectors in the range of 0.5-5 THz in combination with a small time constant (smaller than 100 ps) allowed us to detect THz pulses with quite low energies. The detector was based on a thin NbN film. The superconducting transition temperature was about 9 K, and a working temperature of the detector was 4.8 K. The optical scheme with HEB included Q-switched Nd3+:YLF laser as a pump source at 1046.7 nm with 10 ns pulse duration, and LN crystal integrated with silicon prism in the Cherenkov geometry. The HEB-signal was registered from THz idler photons emitted from LN crystal cooled to 4.8 K. The nonlinear character of this dependence indicates that the high-gain parametric down-conversion regime is obtained at high pump peak intensity. This may find application in the study of correlation properties of optical-terahertz biphotonic fields [4]. Correlation characteristics of biphotons – a noise reduction factor (NRF) and a second-order correlation function – were calculated using the generalized Kirchhoff law proposed by D N Klyshko [5]. References [1] S S Dhillon, M S Vitiello, E H Linfield, et al., J. Phys. D 50, 043001 (2017) [2] K A Kuznetsov, S P Kovalev, G K Kitaeva, T D Wang, Y Y Lin, Y C Huang, I I Naumova and A N Penin, Appl. Phys. B 101, 811 (2010) [3] K A Kuznetsov, G Kh Kitaeva, S P Kovalev, S A Germansky, A M Buryakov, A N Tuchak and A N Penin, Appl. Phys. B 122, 223 (2016) [4] V V Kornienko, S A Germanskiy, G Kh Kitaeva and A N Penin, Int. J. Quantum Inf. 12, 1560023 (2014) [5] D N Klyshko, Photons and Nonlinear Optics (Gordon and Breach, New York, 1988)