S. A. Kozlov

Methods of generating superbroadband terahertz pulses with femtosecond lasers

This paper presents experimental results and an analysis of methods of generating terahertz (THz) radiation, using femtosecond laser sources: generation by photoconductive semiconductor antennas, nonlinear-optical generation of the difference frequency or optical rectification, and generation using optical breakdown in gases under the action of femtosecond pulses. An undoped semiconductor crystal of indium arsenide (InAs) located in a magnetic field, an electrooptic ZnTe crystal, an organic DAST crystal, and an optical spark in air were used as generators of THz radiation

Secure polarization-independent subcarrier quantum key distribution in optical fiber channel using BB84 protocol with a strong reference.

A quantum key distribution system based on the subcarrier wave modulation method has been demonstrated which employs the BB84 protocol with a strong reference to generate secure bits at a rate of 16.5 kbit/s with an error of 0.5% over an optical channel of 10 dB loss, and 18 bits/s with an error of 0.75% over 25 dB of channel loss. To the best of our knowledge, these results represent the highest channel loss reported for secure quantum key distribution using the subcarrier wave approach. A passive unidirectional scheme has been used to compensate for the polarization dependence of the phase modulators in the receiver module, which resulted in a high visibility of 98.8%. The system is thus fully insensitive to polarization fluctuations and robust to environmental changes, making the approach promising for use in optical telecommunication networks. Further improvements in secure key rate and transmission distance can be achieved by implementing the decoy states protocol or by optimizing the mean photon number used in line with experimental parameters.

Formation of a sequence of ultrashort signals in a collision of pulses consisting of a small number of oscillations of the light field in nonlinear optical media

Collisions of pulses consisting of a small number of oscillations and propagating in a nonlinear optical medium unidirectionally but with different group velocities because of a difference in the spectral composition are considered. It is shown that, at intensities of radiation ensuring the superbroadening of its spectrum, the interaction of two pulses can result in the formation of a quasi-discrete spectral supercontinuum whose temporal structure is described by a sequence of ultrashort signals with a nearly rectangular shape.

Generation of ultrabroadband terahertz radiation under optical breakdown of air by two femtosecond pulses of different frequencies

Experimental results on the generation of terahertz (THz) electromagnetic radiation under ionization of air by femtosecond pulses at fundamental and second-harmonic frequencies of a Ti-sapphire laser are presented. The mean power of the generated THz radiation as a function of the time delay between two pulses of different frequencies is found to be quasi-periodical. Theoretically, we show that efficient generation of THz radiation is governed by the inertial part of the nonlinear response of the medium, which is determined by the dynamics of population of highly excited states with subsequent emission of electrons.

Quantum key distribution in an optical fiber at distances of up to 200 km and a bit rate of 180 bit/s

An experimental demonstration of a subcarrier-wave quantum cryptography system with superconducting single-photon detectors (SSPDs) that distributes a secure key in a single-mode fiber at distance of 25 km with a bit rate of 800 kbit/s, a distance of 100 km with a bit rate of 19 kbit/s, and a distance of 200 km with a bit rate of 0.18 kbit/s is described.

Dynamics of the fields of counterpropagating light pulses of a few oscillations in nonlinear dielectric media

Evolution equations have been derived for the fields of counterpropagating light pulses of a few oscillations interacting with dielectric media with nonresonant dispersion and inertialess electron nonlinearity. Changes in the field structure and spectrum of a low-intensity pulse colliding with a high-intensity counterpropagating light pulse are illustrated. It has been shown that such a collision can be accompanied by the generation of radiation at multiple frequencies with a duration smaller than the durations of the initial pulses.

Self-action of femtosecond pulses with continuum spectrum

On the basis of the analysis of the wave equations for an electrical field of radiation and without use of slowly varying envelope approximation the self-action of femtosecond light pulses in transparent media is investigated. The results of numerical simulation of spectral supercontinuum evolution, accompanying temporary broadening of intensive pulses with a spectrum in the range of normal group dispersion of medium both with only electronic nonlinearity, and with simultaneous electronic and electronic-vibrational nonlinearities are presented. The opportunity of compression of pulses with supercontinuum spectrum in light formations consisting of one cycle of an electric field is predicted. It is shown that spectral superbroadening of the elliptically polarized radiation is accompanied by nonuniform rotation of a polarization ellipse.

The transmission of 45 bits of information by a pair of interfering femtosecond pulses with superwide spectra

It has been experimentally demonstrated that 45 bits of information can be optically recorded and transmitted by one quasi-discrete spectral supercontinuum, formed when two femtosecond pulses with superwide spectra interfere with a time delay between them less than the width of each pulse.

Formation of a Sequence of Femtosecond Optical Pulses with a Terahertz Repetition Rate

We have shown experimentally that, upon interference of two frequency-modulated femtosecond pulses, a sequence of ultrashort subpulses is formed to which a quasi-discrete spectral continuum corresponds in the spectral range. We have shown that it is possible to control the duration and repetition rate of subpulses in the sequence by varying the delay between the interfering pulses.

Characteristic features of generation of few-cycle pulses in infrared and terahertz spectral ranges upon interaction of two femtosecond pulses of different frequencies in air

The results of a theoretical analysis of the generation of broadband radiation in the infrared and terahertz spectral ranges upon the excitation of plasma in air by two femtosecond pulses at the fundamental and second-harmonic frequencies of a Ti-sapphire laser are presented. It is found that the appearance of long-wavelength radiation in a strong field of pulses of different frequencies can be described in terms of strongly anharmonic oscillations of optical electrons, whereby electrons are pulled far away from their atoms; these oscillations are accompanied by cascade transitions of electrons from their ground state to a bound excited state, followed by a transition to the continuum. It is shown that the generated infrared and terahertz radiation appears in the form of pulses containing a few oscillations of the light field. The efficiency of terahertz generation varies periodically with an increase in the interaction length of the femtosecond pulses of different frequencies.