R. A. Akhmedzhanov

Quasi-phase-matched probe-energy electro-optic sampling as a method of narrowband terahertz detection

Implementation of free-space quasi-phase-matched electro-optic detection is reported, based on measuring the energy variation of a femtosecond laser pulse induced by a terahertz field. Narrowband probe-energy type of detection is demonstrated using wide-aperture periodically poled Mg:Y:LiNbO3 crystals with as-grown domain gratings. Spectra of air plasma generation measured by this method are compared with the spectrum measured by the conventional ZnTe-based probe-phase ellipsometry scheme. Besides, the conventional method was performed for both types of sources, for the air plasma generation, and for the quasi-phase-matched optical rectification in the same periodically poled Mg:Y:LiNbO3 crystals.

Terahertz emission from a metallic surface induced by a femtosecond optic pulse

Results of experimental and theoretical investigations on generation of terahertz radiation at the interaction of femtosecond laser pulses with a metal surface are presented. Investigations are performed with the laser pulse intensities higher compared with that used in papers [Opt. Lett.29, 2674 (2004); Opt. Lett.30, 1402 (2005)]. The most effective generation is observed for p-polarized optical pulses with incidence angles in the range 5°-10° (from the surface), depending on the kind of metal. For the copper, the exponential growth of terahertz pulse energy with the increase of optical pulse energy was registered. Theoretical interpretation for some of the experimental results is proposed based on the model of free electrons in metal.

Generation of Terahertz Radiation by the Optical Breakdown Induced by a Bichromatic Laser Pulse

Experimental results are presented on generation of terahertz radiation by the air breakdown induced by a high-intensity laser pulse having not only a fundamental component, but also a second-harmonic one. A theoretical explanation of the experimental data is proposed, based on a model of field ionization of a gas by a bichromatic laser.

Terahertz time-domain electro-optic measurements by femtosecond laser pulses with an edge-cut spectrum.

Balanced electro-optic detection techniques of terahertz wave radiation are proposed based on variations of the energy and ellipticity of laser pulses with an edge-cut spectrum. The techniques are compared with the standard electro-optic detection scheme utilizing laser pulses with Gaussian spectrum shape. Our calculations and measurements show that the studied schemes have a much better response to the terahertz wave radiation at high frequencies compared with the standard one.

Terahertz radiation in In0.38Ga0.62As grown on a GaAs wafer with a metamorphic buffer layer under femtosecond laser excitation

The results of time-domain spectroscopy of the terahertz (THz) generation in a structure with an In0.38Ga0.62As photoconductive layer are presented. This structure grown by molecular-beam epitaxy on a GaAs substrate using a metamorphic buffer layer allows THz generation with a wide frequency spectrum (to 6 THz). This is due to the additional contribution of the photo-Dember effect to THz generation. The measured optical-to-terahertz conversion efficiency in this structure is 10–5 at a rather low optical fluence of ~40 μJ/cm2, which is higher than that in low-temperature grown GaAs by almost two orders of magnitude.

Laser pulse amplitude changes induced by terahertz waves under linear electro-optic effect

Changes in the amplitude of femtosecond laser pulses and in the energy of terahertz wave radiation induced during their co-propagation in ZnTe and GaP crystals are studied theoretically and experimentally. The results show that variation of the optical field amplitude leads to changes in the laser pulse energy and spectrum shift. We investigate the quantitative correlations between variations of the optical pulse energy, spectrum, phase and terahertz radiation energy. The values of laser pulse energy change and spectrum shift are proportional to the first time derivative of the magnitude of terahertz electric field, which enables coherent electro-optic detection. A simple and convenient calibration technique for terahertz energy detectors based on the correlation between laser and terahertz energy changes is proposed and tested.

Cavity-assisted atomic frequency comb memory in an isotopically pure 143Nd3+ :YLiF4 crystal

In this work we present an implementation of cavity-assisted atomic frequency comb (AFC) memory protocol in an isotopically pure 143Nd3+ :YLiF4 crystal. We use a tunable confocal Fabry–Perot cavity that is placed inside the cryostat. For a 1 mm thick sample with optical depth of 0.2 we obtain total storage efficiency of 3%, which is a 15-fold enhancement compared to the no cavity case. The memory bandwidth is limited by the inhomogeneous broadening of the optical transition and allows us to store short 30 ns pulses.

Atomic frequency comb memory in an isotopically pure 143Nd3+:Y7LiF4 crystal

We implemented the atomic frequency comb protocol for optical quantum memory in an isotopically pure crystal of Y7LiF4 doped by 143Nd3+ ions. Echo signals were observed on the 4I9/2(1)–4F3/2(1) transition, which had inhomogeneous broadening much smaller than the hyperfine splitting of the ground and excited states. We performed hole-burning spectroscopy measurements on several transitions, obtaining information about the hyperfine state lifetimes. An intrinsic hole structure was found on some of the transitions, which allowed us to prepare a comb structure with two clearly defined periods and to observe echo pulses with different time delays.

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.

Generation of terahertz radiation by the axicon focusing of ionizing laser pulses

Experimental results on the generation of terahertz radiation in the gas breakdown by high-power quasi-monochromatic laser pulses focused by an axicon lens are presented. A theoretical model developed for the terahertzradiation generation includes the nonlinear interaction of a femtosecond pulse with an ionized gas and the excitation of transverse oscillations of the plasma column. The results of the theoretical analysis are compared with the experimental data.