Yu. A. Tolmachev

Generation of unipolar optical pulses in a Raman-active medium

Response of a Raman-active media (RAM) to the excitation by a series of ultrashort (few-cycle) optical pulses propagating at a superluminal velocity is studied theoretically. It is shown that under certain conditions rectangular unipolar pulses (video-pulses) can be generated as the RAM response. The duration, shape and amplitude of these video-pulses can be widely tuned by modifying the pump pulse parameters.

Generation of unipolar pulses in nonlinear media

Methods recently proposed for generating unipolar pulses in nonlinear media in terahertz and optical electromagnetic ranges are reviewed. Such pulses have nonzero “electric area” (time integral of the field strength over the entire duration of a pulse) and, correspondingly, a significant component of the field with zero frequency, thus exhibiting quasistatic properties. Effective generation of unipolar pulses would allow, e.g., transferring mechanical momentum to charged particles and, thereby, controlling the motion of wave packets of matter, which can be useful for compact accelerators of charged particles and for other applications.

All-optical control of unipolar pulse generation in a resonant medium with nonlinear field coupling

We study the optical response of a resonant medium possessing nonlinear coupling to an external field driven by a few-cycle pump pulse sequence. We demonstrate the possibility of directly producing unipolar half-cycle pulses from the medium possessing an arbitrary nonlinearity, by choosing the proper pulse-to-pulse distance of the pump pulses in the sequence. We examine various ways of shaping the medium response using different geometrical configurations of nonlinear oscillators and different wavefront shapes for the excitation pulse sequence. Our approach defines a general framework to produce unipolar pulses of controllable form.

Diffraction of an ultrashort pulse by an aperture

The scalar problem of diffraction of an infinitely short pulse by a plane screen is solved within Kirchhoff’s approximation. The response of an infinitely small aperture is calculated, and the explicit solution is found for the case of a circular aperture.

Generation of unipolar pulses in a circular Raman-active medium excited by few-cycle optical pulses

We study theoretically a new possibility of unipolar pulse generation in a Raman-active medium excited by a series of few-cycle optical pulses. We consider the case when the Raman-active particles are uniformly distributed along a circle or helix, and demonstrate the possibility of obtaining rectangular unipolar pulses with an arbitrarily long duration above the minimum value equal to the natural period of the low-frequency vibrations in the Raman-active medium.

Emission of a spatially modulated resonant medium excited with superluminal velocity

Specific characteristics of the radiation of a resonant medium excited by an ultrashort light pulse propagating through the medium with a superluminal velocity are considered. The medium is assumed to consist of identical linear harmonic oscillators with a spatial density periodically modulated along the direction of propagation of the superluminal excitation. The field of radiation of the resonant medium under these conditions is calculated. It is shown that, under the superluminal excitation, the radiation spectrum of the medium shows, along with the fundamental frequency of the oscillators, new frequencies that depend on the spatial frequency of the distribution of oscillators and on the angle of observation. Possible application of the effect is discussed.

Diffraction of an ultrashort pulse by a slit

An analysis of a specific feature of the interaction of an ultrashort pulse with a slit is made. Analytical expressions for a monochromatic wave in the form of an ultrashort pulse diffracted by an infinite slit are obtained.

Fresnel lens for the generator of encoded sequences of ultrashort pulses. The spectral evidence of series of pulses formation

Fresnel lens is considered as the effective instrument for the transformation of a single femtosecond pulse into a series of pulses corresponding to some digital code. The theory describing transformation of spectrum of a finite series of equidistant pulses is developed and compared with experimental data received for two Fresnel lens patterns. Accordance of the measured and calculated spectra confirmed the basic idea of pulse transformation method.

Controlling the Radiation Parameters of a Resonant Medium Excited by a Sequence of Ultrashort Superluminal Pulses

We investigate the possibility of controlling the radiation parameters of a spatially periodic one-dimensional medium consisting of classical harmonic oscillators by means of a sequence of ultrashort pulses that propagate through the medium with a superluminal velocity. We show that, in the spectrum of the transient process, in addition to the radiation at a resonant frequency of oscillators, new frequencies arise that depend on the period of the spatial distribution of the oscillator density, the excitation velocity, and the angle of observation. We have examined in detail the case of excitation of the medium by a periodic sequence of ultrashort pulses that travel with a superluminal velocity. We show that it is possible to excite oscillations of complex shapes and to control the radiation parameters of the resonant medium by changing the relationship between the pulse repetition rate, the medium resonant frequency, and the new frequency.

Specific features of the spatiotemporal structure of a broadband signal in the vicinity of the focal point of a converging spherical wave

The propagation and spatiotemporal evolution of a spherical delta wave are investigated. It is shown that the pulse response of a circular aperture at the symmetry axis of the system can be represented as the sum of two delta pulses that have opposite polarities and come close together as the focal point is approached. The delta signal at the focal point is described by the derivative of the delta function. The results of the analysis performed in this paper are confirmed by computer simulation of the propagation of a short spherical Gaussian pulse and agree well with the data obtained in the framework of the diffraction theory of monochromatic waves.