R. M. Arkhipov

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.

Transient Cherenkov radiation from an inhomogeneous string excited by an ultrashort laser pulse at superluminal velocity

The optical response of a one-dimensional string made of dipoles with a periodically varying density excited by a spot of light moving along the string at superluminal (subluminal) velocity is studied theoretically. The Cherenkov radiation in such a system is rather unusual, possessing both transient and resonant character. We show that, under certain conditions, in addition to the resonant Cherenkov peak, another Doppler-like frequency appears in the radiation spectrum. Both linear (small-signal) and nonlinear regimes as well as different string topologies are considered.

On coherent mode-locking in a two-section laser

Currently existing lasers with passive mode-locking are two-section systems including an amplifier section and a saturable absorber section. The mechanism of generation of short pulses in them is based on the effects of saturation of the gain in the amplifier and absorption in the absorber, which prevent obtaining pulses with durations shorter than the relaxation time of the polarization in the amplifying and absorbing media. In this work, the new possibility of the generation of ultrashort pulses in a laser with passive mode-locking owing to the coherent character of the interaction of light with matter in the amplifying and absorbing media in ring and linear cavities has been theoretically analyzed. A practically interesting case where the amplifying and absorbing media are separated in the space of the cavity, rather than being homogeneously mixed in its volume, as was previously considered for such type of lasers, has been studied. It has been shown that the width of the spectrum of generated pulses can be much larger than the width of the amplification line and can vary depending on the parameters of the amplifying and absorbing media of the laser. The existence of nonsoliton scenarios of the regime of coherent passive mode synchronization, as well as the possibility of generation without injection of an auxiliary pulse, has been demonstrated.

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.

Mode-locking in a laser with a coherent absorber

Mode-locking in a cw narrowband tunable dye laser whose cavity contains absorbing cells with molecular iodine vapor has been studied experimentally. When the wavelength of radiation of the dye laser is tuned to absorption lines of molecular iodine vapor in a cell in the cavity, short pulses following with a period equal to or multiple of the round-trip time of light around the cavity appear. The duration of the generated pulses is about 1 ns, which is much shorter than the phase memory time in iodine vapor. In such a situation, modelocking occurs owing to the coherent character of the interaction of laser radiation with narrow absorption resonances in molecular iodine vapor.

Ultrafast creation and control of population density gratings via ultraslow polarization waves

In the regime of resonant coherent light-matter interaction, light pulses may interact with each other indirectly via a polarization wave created by the other pulse. We show that such interaction allows fast creation and erasing of high-contrast dynamic population density gratings, as well as control of their period in a few-cycle regime. Our scheme uses counter-propagating optical pulses, which do not cross each other in the medium. The mechanism is able to work with pulse durations up to the single-cycle limit. Somewhat surprisingly, ultrafast grating wave vector control requires the generation of polarization waves with the phase velocity much smaller than that of light.

Few-cycle pulse-driven excitation response of resonant medium with nonlinear field coupling

We study theoretically the optical response of the resonant medium on the few-cycle pulse-driven excitation for different types of the coupling strength to the field. It is demonstrated that the medium can exhibit a specific excitation response depending on the field coupling nonlinearity. Remarkably, the analysis of this response revealed the possibility of the controllable generation and shaping of unipolar optical pulses, which can be important in real-time observation and time-domain control of ultrafast processes in matter.

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.

Formation and erasure of population difference gratings in the coherent interaction of a resonant medium with extremely short optical pulses

In the regime of coherent interaction of short optical pulses with a resonant medium, which is implemented with a pulse duration shorter than the relaxation times in the medium, the formation of population gratings can occur without overlapping the pulses therein. In this case, there are new possibilities for controlling optical pulses, which are especially pronounced for extremely short pulses. It is shown that, with the proper choice of the parameters of a sequence of extremely short optical pulses, not only the formation of population gratings, but also their erasure are possible. It is demonstrated that this effect can be used for the creation of an ultrahigh-speed optical deflector.

Effects of inversionless lasing in two-level media from the point of view of specificities of the spatiotemporal propagation dynamics of radiation

We report the results of computer simulation of the emission of radiation by an extended two-level medium in a ring cavity. The cases of using strong external monochromatic, quasi-monochromatic, and biharmonic radiation for pumping the two-level medium are analyzed. It is shown that the emission of radiation with spectral content different from that of the pump radiation, which is interpreted as the inversionless oscillation, is the result of the spatiotemporal dynamics of light propagation in an extended two-level medium imbedded in a cavity. The appearance of this radiation is not related to known resonances of amplification of a weak probe field in a thin layer of the two-level system (the effect of inversionless oscillation) induced by strong resonance monochromatic or biharmonic field, as was thought before.