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We propose a novel scheme to generate single-cycle terahertz (THz) pulses via reflection of an optical femtosecond pulse train from a thin layer of nonlinear resonant medium. Our method is based on a coherent control of low-frequency oscillations and free induction decay in the medium. The specific single-cycle shape of generated THz pulses requires a plane wavefront and detection in the near field. Our theoretical results pave the way to a new, simple and high-efficiency way to generate single-cycle waveshape-tunable terahertz pulses. more...

In a Ti:Sa laser with an absorbing with Rb vapor cell stable self-starting passive mode-locking is demonstrated. We show that the mode-locking appears due to self-induced transparency (SIT) in the Rb cell, that is, the pulse in the Rb cell is a 2pi SIT pulse. For the best of our knowledge, in these experiments we present the first time demonstration of SIT mode-locking in laser systems, which was discussed only theoretical before. more...

It is believed that passive mode locking is virtually impossible in quantum cascade lasers (QCLs) because of too fast carrier relaxation time. Here, we revisit this possibility and theoretically show that stable mode locking and pulse durations in the few cycle regime at terahertz (THz) frequencies are possible in suitably engineered bound-to-continuum QCLs. We achieve this by utilizing a multi-section cavity geometry with alternating gain and absorber sections. The critical ingredients are the very strong coupling of the absorber to both field and environment as well as a fast absorber carrier recovery dynamics. Under these conditions, even if the gain relaxation time is several times faster than the cavity round trip time, generation of few-cycle pulses is feasible. We investigate three different approaches for ultrashort pulse generation via THz quantum cascade lasers, namely passive, hybrid and colliding pulse mode locking. more...

Named Entity Recognition (NER) is one of the most common tasks of the natural language processing. The purpose of NER is to find and classify tokens in text documents into predefined categories called tags, such as person names, quantity expressions, percentage expressions, names of locations, organizations, as well as expression of time, currency and others. Although there is a number of approaches have been proposed for this task in Russian language, it still has a substantial potential for the better solutions. In this work, we studied several deep neural network models starting from vanilla Bi-directional Long Short-Term Memory (Bi-LSTM) then supplementing it with Conditional Random Fields (CRF) as well as highway networks and finally adding external word embeddings. All models were evaluated across three datasets: Gareev's dataset, Person-1000, FactRuEval-2016. We found that extension of Bi-LSTM model with CRF significantly increased the quality of predictions. Encoding input tokens with external word embeddings reduced training time and allowed to achieve state of the art for the Russian NER task., Comment: Artificial Intelligence and Natural Language Conference (AINL 2017) more...

We present a significantly different reflection process from an optically thin flat metallic or dielectric layer and propose a strikingly simple method to form approximately unipolar half-cycle optical pulses via reflection of a single-cycle optical pulse. Unipolar pulses in reflection arise due to specifics of effectively one-dimensional pulse propagation. Namely, we show that in considered system the field emitted by a flat medium layer is proportional to the velocity of oscillating medium charges instead of their acceleration as it is usually the case. When the single-cycle pulse interacts with linear optical medium, the oscillation velocity of medium charges can be then forced to keep constant sign throughout the pulse duration. Our results essentially differ from the direct mirror reflection and suggest a possibility of unusual transformations of the few-cycle light pulses in linear optical systems. more...

We study optical response of a resonant medium possessing nonlinear coupling to external field driven by a few-cycle pump pulse sequence. We demonstrate the possibility to directly produce 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 the various ways of the shaping of 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., Comment: 9 pages, 9 figures more...

We study theoretically a new possibility of unipolar pulses generation in 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 the circle, and demonstrate a possibility to obtain a unipolar rectangular video pulses with an arbitrarily long duration, ranging from a minimum value equal to the natural period of the low frequency vibrations in the Raman-active medium. more...

We demonstrate that the resonant medium with essentially nonlinear field coupling can exhibit specific response when excited by the few-cycle optical pulse. We provide an effective theoretical background that allows relating the type of the field coupling function with the medium oscillators output on the few-cycle pulse-driven excitation. Possible applications of such the optical response are discussed for the controllable generation and shaping of the unipolar video pulses. more...

Self-induced transparency mode-locking (or coherent mode-locking, CML) which is based on intracavity self-induced transparency soliton dynamics, allows potentially to achieve nearly single cycle intracavity pulse durations, much below the phase relaxation time $T_2$ in a laser, which, despite of great promise, has not yet been realized experimentally. We develop a diagram technique which allows to predict the main features of CML regimes in a generic two-section laser. We show that CML can arise directly at the first laser threshold if the phase relaxation time is large enough. Furthermore, CML regimes can be unconditionally stable. We also predict the existence of ``super-CML regimes``, with a pulse coupled to several Rabi oscillations in the nonlinear medium. more...