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Laser Physics Letters

Generation of unipolar optical pulses in a Raman-active medium

Rostislav M. Arkhipov, Mikhail V. Arkhipov, P. Belov, Yu. A. Tolmachev, Ihar Babushkin

Published Mar 15, 2016• 44 citations
Publisher

Abstract

Abstract 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.

DOI
10.1088/1612-2011/13/4/046001

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Abstract 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.
We theoretically demonstrate the possibility to produce unipolar pulses of varying temporal profile in an extended layer of a resonant medium with the nonlinear coupling to the driving electric field. The proposed approach relies on the coherent control of the low-frequency medium oscillations by a sequence of ultrashort excitation pulses as well as the collective emission of multiple resonant centers along an optically thick medium layer. In particular, we show that creating an inhomogeneous spatial profile of the density of resonant centers allows obtaining unipolar pulses of adjustable waveform, such as rectangular or triangular ones. Such optical response could be realized in Raman-active media, e.g., in molecular crystals with well-resolved vibrational terahertz resonances.
Abstract Recent progress in generation of optical pulses of durations comparable to one optical cycle has presented great opportunities for studies of the fundamental processes in matter as well as time-resolved spectroscopy of ultrafast processes in nonlinear media. It opened up a new area of research in modern ultrafast nonlinear optics and led to appearance of the attosecond science. In parallel, a new research area related to emission from resonant media excited by superluminally propagating ultrashort bursts of electromagnetic radiation has been actively developed over the last few years. In this paper, we review our recent results on theoretical analysis of the Cherenkov-type radiation of a resonant medium excited by few-cycle optical pulses propagating at superluminal velocity. This situation can be realized when an electromagnetic pulse with a plane wavefront incidents on a straight string of resonant atoms or a spot of light rotates at very large angular frequency and excites a distant circular string of resonant dipoles. Theoretical analysis revealed some unusual and remarkable features of the Cherenkov radiation generated in this case. This radiation arises in a transient regime which leads to the occurrence of new frequencies in the radiation spectrum. Analysis of the characteristics of this radiation can be used for the study of the resonant structure properties. In addition, a nonlinear resonant medium excited at superluminal velocity can emit unipolar optical pulses, which can be important in ultrafast control of wave-packet dynamics of matter. Specifics of the few-cycle pulse-driven optical response of a resonant medium composed of linear and nonlinear oscillators is discussed.
The possibility of generation of quasi-unipolar extremely short pulses (with a duration of several periods of field oscillations) is theoretically analyzed for the excitation of a circular, nonlinear, and resonant medium with a light spot. The light spot rotates along a circle with a velocity that exceeds the speed of light in vacuum, while particles of the medium are distributed uniformly along the circle. It is shown that, by introducing a delay in the form of a phase plate into the system, one can control the shape, the duration, and the amplitude of generated pulses.
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.