V. V. Kornienko

Scattering Effects in Terahertz Wave Spectroscopy of Granulated Solids

Applications of terahertz spectroscopy methods to granulated solids are substantially constrained by the effects of scattering of the incident terahertz waves by spatial heterogeneities of a sample. The experimental results obtained by means of terahertz time-domain spectroscopy are reported for the samples of granulated 2, 4-dinitrotoluene and alpha-lactose without any special filling. We show that in samples of the same substance, but prepared in a different manner, the overall increase of the background extinction with frequency and appearance of additional extinction maxima can obscure the true absorption features of the material. In samples with large grain size, of order of 400 μm, a complicated irregular spectral structure is observed instead of a fingerprint maxima at 1.37 THz for alpha-lactose and at 1.08 THz for 2,4-dinitrotoluene. At higher frequencies no true spectral fingerprints are clearly seen in the raw experimental spectra of all the samples with the grain sizes of 130 μm and more. The influence of grain size, air-filling factor and the dispersion properties of the materials is studied. Theoretical analysis is made with the use of Mie theory in simplifying approximations and taking into account interference between the waves transmitted through a porous sample. It has been shown that the scattering-induced masking effects can be properly simulated even for the air-filled objects with initially unknown spatial and dispersion parameters. The procedure of eliminating the influence of scattering effects from the raw spectral data is proposed. Detection of true high-frequency spectral fingerprints is demonstrated using this procedure for various grain sizes up to 400 μm.

Evaluating the spectral sensitivity of the nonlinear-optical terahertz wave radiation detecors via spontaneous parametric down-conversion spectra

Principal parameters of the nonlinear-optical crystals are analyzed, which govern the spectral sensitivity of the terahertz wave detectors based on the frequency conversion from terahertz to optical range. A method is developed and experimentally demonstrated for characterizing these parameters. The method is founded on the spontaneous parametric down-conversion spectra observation without external terahertz wave radiation.

Towards terahertz detection and calibration through spontaneous parametric down-conversion in the terahertz idler-frequency range generated by a 795 nm diode laser system

We study a calibration scheme for terahertz wave nonlinear-optical detectors based on spontaneous parametric down-conversion. Contrary to the usual low wavelength pump in the green, we report here on the observation of spontaneous parametric down-conversion originating from an in-growth poled lithium niobate crystal pumped with a continuous wave 50 mW, 795 nm diode laser system, phase-matched to a terahertz frequency idler wave. Such a system is more compact and allows for longer poling periods as well as lower losses in the crystal. Filtering the pump radiation by a rubidium-87 vapor cell allowed the frequency-angular spectra to be obtained down to ∼0.5 THz or ∼1 nm shift from the pump radiation line. The presence of an amplified spontaneous emission “pedestal” in the diode laser radiation spectrum significantly hampers the observation of spontaneous parametric down-conversion spectra, in contrast to conventional narrowband gas lasers. Benefits of switching to longer pump wavelengths are pointed out, suc...

Terahertz continuous wave nonlinear-optical detection without phase-locking between a source and the detector

We demonstrate the possibility of nonlinear-optical detection of terahertz (THz) wave radiation without phase-locking between a source and a detector. Spectrally resolved room-temperature incoherent nonlinear-optical detection is demonstrated for 0.22-THz continuous wave (CW) radiation by upconversion using a 100-mW CW laser in a 15-mm-long Mg:LiNbO3 crystal.

Strongly nondegenerate spontaneous parametric down-conversion for calibration of terahertz-wave detectors

The impact of spatial limitation of the nonlinear interaction volume on the spontaneous parametric down-conversion spectra in a strongly nondegenerate regime is analyzed from the point of absolute calibration of the terahertz-wave nonlinear-optical detectors. We show that the idler wave angular resolution of the calibration method can be inherently low (about 1 to 10 degrees in lithium niobate crystals), and to fill all the input modes with external idler radiation, one has to take special care on geometry of the nonlinear interaction. Angular sensitivity distribution function is constructed, and a consecutive description is provided for both the parametric down-conversion and the external idler radiation detection processes.

Terahertz-radiation generation in low-temperature InGaAs epitaxial films on (100) and (411) InP substrates

The spectrum and waveforms of broadband terahertz-radiation pulses generated by low-temperature In0.53Ga0.47As epitaxial films under femtosecond laser pumping are investigated by terahertz time-resolved spectroscopy. The In0.53Ga0.47As films are fabricated by molecular-beam epitaxy at a temperature of 200°C under different arsenic pressures on (100)-oriented InP substrates and, for the first time, on (411)A InP substrates. The surface morphology of the samples is studied by atomic-force microscopy and the structural quality is established by high-resolution X-ray diffraction analysis. It is found that the amplitude of terahertz radiation from the LT-InGaAs layers on the (411)A InP substrates exceeds that from similar layers formed on the (100) InP substrates by a factor of 3–5.

Generation of THz radiation in epitaxial InGaAs films on InP substates of various crystallographic orientations

We study the THz wave generation by the time-domain spectroscopy method in low-temperature grown InGaAs layers on InP substrates with crystallographic orientations (100) and (411) It was found that the THz wave generation is 3-5 times more effective in the case of (411)A InP substrates as compared to the (100) substrates. In samples grown at high pressure of As4 generation of THz waves is more effective at low-frequency range less than 200 GHz.

A method for nonlinear-optical calibration of the terahertz wave spectral brightness

Experimental results are presented for the detection of 0.22 THz radiation from a frequency-doubled impact ionization avalanche transit-time (IMPATT) diode. A procedure is discussed for standard-less measurement of the terahertz wave spectral brightness. The method is based on the use of spontaneous parametric down-conversion of light under the nonlinear-optical detection of terahertz wave radiation.