G. Kh. Kitaeva

Terahertz generation in Czochralski-grown periodically poled Mg:Y:LiNbO3 by optical rectification

Using a canonical pump-probe experimental technique, we studied terahertz (THz) wave generation and detection by optical rectification and mixing in Czochralski-grown periodically poled Mg:Y:LiNbO3 (PPLN) crystals. THz waves with frequencies at 1.37 and 0.68 THz as well as 1.8 THz were obtained for PPLN with nonlinear grating periods of 30 and 60 µm, respectively. A general theoretical model was developed by considering the dispersion and damping of a low-frequency phonon-polariton mode. Our results show that THz waves are generated in forward and backward directions by pumping pulse rectification. The generated THz waves depend on the spectral shape of the laser pulses, quasi-phase mismatches, and dispersion characteristics of a crystal.

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.

Second Harmonic Generation in a Strontium Barium Niobate Crystal with a Random Domain Structure

The phenomenon of the second-harmonic (SH) generation in a strontium barium niobate (SBN) crystal in directions not obeying the conditions of phase matching for spatially homogeneous media has been studied. The appearance of the SH signal is interpreted as resulting from the quasi-phase-matching nonlinear wave interaction due to the random domain structure of the SBN crystal containing acicular ferroelectric microdomains. A theoretical model describing the SH generation in such two-dimensional, spatially inhomogeneous nonlinear optical structures is proposed. An analysis of the second-harmonic emission directivity patterns shows that the main contribution to the noncollinear SH generation in SBN is due to near-surface domains extending to a depth of up to several dozen microns.

Spontaneous parametric down-conversion

Spontaneous parametric down-conversion (SPDC) in media with no inversion center and the use of this phenomenon in the spectroscopy of natural oscillation states of a crystal lattice (i.e., optical phonons) are retrospectively described. We think that the SPDC spectroscopy method is estimated inappropriately and hope to again attract the attention of readers to one of the most interesting quantum phenomena of nonlinear optics that has no classical analog. The capabilities of SPDC spectroscopy will certainly be used in both fundamental science and technology of new materials.

Two alternative approaches to electro-optical detection of terahertz pulses

The electro-optical detection of terahertz radiation is analyzed theoretically. While a conventional detection scheme of this type is based on the measurement of the phase shift of an optical pulse arising from its interaction with the terahertz field in a nonlinear medium, it is shown that both the phase and amplitude of the optical pulse vary because of this interaction. The amplitude modulation carries information on the dynamics of the amplitude and phase of the terahertz field and can be used to measure these parameters. With this energy-sensitive detection scheme, avoiding all restrictions on the symmetry type of the nonlinear medium is much simpler than with the phase scheme. Variants of the electro-optical detection technique using periodically poled crystals with the lithium-niobate symmetry and crystals with the zincblende structure are considered. It is shown that spectral sensitivities of the methods based on measurements of the phase and amplitude modulations are related as the frequencies of the laser and terahertz fields.

Mapping of periodically poled crystals via spontaneous parametric down-conversion

A new method for characterization of periodically poled crystals is developed based on spontaneous parametric down-conversion. The method is demonstrated on crystals of Y:LiNbO3, Mg:Y:LiNbO3 with non-uniform periodically poled structures, obtained directly under the Czochralski growth procedure and designed for application of 1.064 μmpumped OPO in the mid-infrared range. Infrared dispersion of refractive index, effective working periods and wavelengths of OPO were determined by special treatment of frequency-angular spectra of spontaneous parametric down-conversion in the visible range. Two-dimensional mapping via spontaneous parametric down-conversion is proposed for characterizing spatial distribution of bulk quasi-phase matching efficiency across the input window of a periodically poled sample.

Parametric Frequency Conversion in Layered Nonlinear Media

Frequency-angular distributions of signal wave intensity are calculated for spontaneous parametric down-conversion and parametric frequency conversion in spatially nonuniform nonlinear media. Wave reflection from interfaces is taken into account, and both regular and irregular nonuniform distributions of second-order nonlinear susceptibility are considered. A unified approach using a scattering matrix and a generalized Kirchhoff law is applied in calculations of spontaneous and stimulated processes in dissipative nonlinear media. Interference of electromagnetic zero-point fluctuations of the vacuum, nonlinear interference, and nonlinear diffraction are examined for media with various absorptive properties. Theoretical foundations are developed for diagnostics of nonuniform distributions of the second-order susceptibility, based on measurement of the line profiles of nonlinear signals.

Generation and detection of terahertz radiation by means of periodically and aperiodically poled crystals

The nonlinear optical methods for generating and detecting terahertz radiation using periodically and aperiodically poled crystals in the quasi-phase-matching mode are analyzed. Two radically different versions of optical pumping are considered: a quasi-continuous mode and pumping by Fourier-limited femto-second pulses. General relations are obtained, which determine the spectra of generated and detected waves through the optical pumping parameters and the crystal generator and crystal detector nonlinear transfer functions. It is shown that, by choosing the optimal spatial modulation of the crystal nonlinear susceptibility, one can control the shape of the generation and detection spectra. An optical scheme is proposed and the optimal parameters of domain structures for quasi-phase-matched generation and frequency-matched detection of forward and backward terahertz waves under quasi-continuous-wave optical pumping are reported.

Compact 1.64 THz source based on a dual-wavelength diode end-pumped Nd:YLF laser with a nearly semiconfocal cavity

We describe a compact dual-wavelength (1.047 and 1.053 μm) diode end-pumped Q-switched Nd:YLE laser source which has a number of applications in demand. In order to achieve its dual-wavelength operation it is suggested for the first time to use essentially nonmonotonous dependences of the threshold pump powers at these wavelengths on the cavity length in the region of the cavity semiconfocal configuration under a radius of the pump beam smaller than the radius of the zero Gaussian mode. Here we demonstrate one of the most interesting applications for this laser: difference frequency generation in a GaSe crystal at a frequency of 1.64 THz. A superconducting hot-electron bolometer is used to detect the THz power generated and to measure its pulse characteristics.

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.