Unipolar light: existence, generation, propagation, and impact on microobjects

The most typical way to optically control population of atomic and molecular systems is to illuminate them with radiation, resonant to the relevant transitions. Here we consider a possibility to control populations with the subcycle and even unipolar pulses, containing less than one oscillation of electric field. Despite the spectrum of such pulses covers several levels at once, we show that it is possible to selectively excite the levels of our choice by varying the driving pulse shape, duration or time delay between consecutive pulses. The pulses which are not unipolar, but have a peak of electric field of one polarity much higher (and shorter) than of the opposite one, are also capable for such control.

Selective ultrafast control of multi-level quantum systems by subcycle and unipolar pulses

Our recent studies of methods for obtaining ultrashort light pulses and analysis of the impact of ultrashort optical pulses on classical and quantum micro-objects with the identification of the determining role of the degree of unipolarity of pulses (maximum for strictly unipolar pulses) in the efficiency of direct laser acceleration of charged particles and excitation of atoms and molecules have been reviewed. Special attention has been paid to the coherent mode locking regime in lasers, which is implemented when the laser pulse duration is less than the relaxation time of the working transition in media with resonance amplification and absorption (laser analog of the phenomenon of self-induced transparency). Experimental data on coherent mode locking in a titanium-sapphire laser with a cell with rubidium vapor at self-induced transparency in rubidium have been presented. The interaction of ultrashort pulses in resonant media has also been analyzed.

Ultrashort Optical Pulses and Their Generation in Resonant Media (Scientific Summary)

Up to now, full tunability of waveforms was possible only in electronics, up to radio-frequencies. Here we propose a new concept of producing few-cycle terahertz (THz) pulses with widely tunable waveforms. It is based on control of the phase delay between different parts of the THz wavefront using linear diffractive optical elements. Suitable subcycle THz wavefronts can be generated via coherent excitation of nonlinear low-frequency oscillators by few-cycle optical pulses. Using this approach it is possible to shape the electric field rather than the slow pulse envelope, obtaining, for instance, rectangular or triangular waveforms in the THz range. The method is upscalable to the optical range if the attosecond pump pulses are used.

/pdf/unusual-terahertz-waveforms-from-a-resonant-medium-hi35lg69s3.pdf

Unusual terahertz waveforms from a resonant medium controlled by diffractive optical elements

Our recent theoretical studies of generation of ultrashort pulses based on collective spontaneous emission from a thin resonant medium layer excited by a pair of high-frequency (compared to the transition frequency of the medium) pulses are reviewed. The source of generated radiation is a resonant polarization pulse in the medium (“stopped polarization pulse”), which is excited and switched off by a pair of light pump pulses. This method allows obtaining attosecond pulses in the optical and ultraviolet ranges, as well as femtosecond terahertz pulses. The effect of the superradiance in generation is discussed. The use of coherent and collective processes makes it possible to achieve a high efficiency of the generation of terahertz radiation. The proposed method is an alternative method of obtaining ultrashort pulses compared to the currently well-developed methods such as the high-harmonic generation method.

Generation of Ultrashort Attosecond and Terahertz Pulses Based on the Collective Spontaneous Emission from a Thin Resonant Medium (Brief Review)

Laser beam deflector based generation of few-cycle electromagnetic pulses in a circular nonlinear medium

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Laser beam deflector based generation of few-cycle electromagnetic pulses in a circular nonlinear medium