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We report on unusual regimes of operation of a laser with a gain medium with a large Raman scattering cross-section, which is often inherent in new types of gain media such as colloidal and epitaxial quantum dots and perovskite materials. These media are characterized by a strong electron-phonon coupling. Using the Fröhlich Hamiltonian to describe the electron-phonon coupling in such media, we analyze the operation of the system above the lasing threshold. We show that below a critical value of the Fröhlich constant, the laser can only operate in the conventional regime: namely, there are coherent cavity photons but no coherent phonons. Above the critical value, a new pump rate threshold appears. Above this threshold, either joint self-oscillations of coherent phonons in the gain medium and photons in a cavity or a chaotic regime are established. We also find a range of the values of the Fröhlich constant, the pump rate, and the resonator eigenfrequency, in which more than one dynamical regime of the system is stable. In this case the laser dynamics is determined by the initial values of the resonator field, the active medium polarization, the population inversion, and phonon amplitude., (© 2022. The Author(s).)

We demonstrate a new type of non-Hermitian phase transition in open systems far from thermal equilibrium, which can have place in the absence of an exceptional point. This transition takes place in coupled systems interacting with reservoirs at different temperatures. We show that the spectrum of energy flow through the system caused by the temperature gradient is determined by the [Formula: see text]-potential. Meanwhile, the frequency of the maximum in the spectrum plays the role of the order parameter. The phase transition manifests itself in the frequency splitting of the spectrum of energy flow at a critical point, the value of which is determined by the relaxation rates and the coupling strength. Near the critical point, fluctuations of the order parameter diverge according to a power law with the critical exponent that depends only on the ratio of reservoirs temperatures. The phase transition at the critical point has the non-equilibrium nature and leads to the change in the energy flow between the reservoirs. Our results pave the way to manipulate the heat energy transfer in the coupled out-of-equilibrium systems., (© 2021. The Author(s).)

In this Letter, we give a new, to the best of our knowledge, perspective on the origin of light coherence in lasers. We demonstrate that a coherence appears below the lasing threshold and manifests itself as long-range correlations between polarizations of active medium atoms. These correlations contribute to the formation of a collective state of atomic polarizations and electromagnetic field modes, which interacts more effectively with the active medium and lases when pumping exceeds the lasing threshold. We demonstrate that inhibiting these atomic correlations leads to the destruction of the collective state and suppression of lasing. The obtained results open up new ways to control coherence.

Usually, the cavity is considered an intrinsic part of laser design to enable coherent emission. For different types of cavities, it is assumed that the light coherence is achieved by different ways. We show that regardless of the type of cavity, the lasing condition is universal and is determined by the ratio of the width of the atomic spectrum to the product of the number of atoms and the spontaneous radiation rate in the laser structure. We demonstrate that cavity does not play a crucial role in lasing since it merely decreases the threshold by increasing the photon emission rate thanks to the Purcell effect. A threshold reduction can be achieved in a cavity-free structure by tuning the local density of states of the electromagnetic field. This paves the way for the design of laser devices based on cavity-free systems.

We suggest a quantum description of Rayleigh light scattering on atoms. We show that an entangled state of the excited atom and the incident photon is formed during the scattering. Due to entanglement, a photon is never completely absorbed by the atom. The formation of the scattering spectrum is considered as a relaxation of incident photons to the reservoir of free space modes that are in thermal equilibrium. Additional excitations of the reservoir modes occurring during scattering are treated as scattered light. We show that even if the frequency of incident photons is incommensurate with an atomic transition frequency, the scattered light spectrum has a maximum at the frequency of incident photons. In addition, the linewidth of the scattered light is much smaller than that of the spontaneous emission of a single atom. Therefore, the process can be considered as elastic.

We develop a theory of lasing of a collection of pumped active atoms without a resonator (either regular or random). Due to spontaneous emission into free space, phases of free space electromagnetic modes fluctuate. These phase fluctuations can be reduced to frequency fluctuations. The closer the frequency of fluctuation to the transition frequency of the active atoms, the higher lifetime of the fluctuation. We show that because of this, the average frequency of modes pulls toward the transition frequency. This leads to a maximum in the density of states of the electromagnetic field and a decrease of the mode group velocity. Consequently, the coupling of modes with atoms as well as the lifetime of fluctuations increase. Thus, mode pulling provides positive feedback. When the pump rate exceeds a certain threshold, the lifetime of one of the realized fluctuations diverges, and radiation becomes coherent.

We study the second-order coherence function of a plasmonic nanoantenna fed by near-field of a single-photon source incoherently pumped in the continuous wave regime. We consider the case of a strong Purcell effect, when the single-photon source radiates almost entirely in the mode of a nanoantenna. We show that when the energy of thermal fluctuations, kT, of the nanoantenna is much smaller than the interaction energy between the electromagnetic field of the nanoantenna mode and the single-photon source, ℏΩ R , the statistics of the emission is close to that of thermal radiation. In the opposite limit, ℏΩ R >>kT, the nanoantenna radiates single photons. In the last case, we demonstrate the possibility of overcoming the radiation intensity of an individual single-photon source. This result opens the possibility of creating a high-intensity single-photon source.

We propose a method for increasing Raman scattering from an ensemble of molecules by up to 4 orders of magnitude. Our method requires an additional coherent source of IR radiation with the half-frequency of the Stokes shift. This radiation excites the molecule electronic subsystem that in turn, via Fröhlich coupling, parametrically excites nuclear oscillations at a resonant frequency. This motion is coherent and leads to a boost of the Raman signal in comparison to the spontaneous signal because its intensity is proportional to the squared number of molecules in the illuminated volume.

Properties of light sources based on amplified spontaneous emission (ASE) are similar to the properties of lasers in many regards. However, even though ASE has been widely studied, its photon statistics have not been settled. There are no reliable theoretical estimates or unambiguous experimental data for the second-order coherence function of photons that characterizes the coherence properties of a light source. Our computer simulation clearly establishes that, independently of pump power, the light produced by ASE is similar to that of a thermal source. This result lays bare the fundamental difference between ASE radiation and laser radiation.

We study the evolution of an open quantum system described by a dynamical semigroup having the Lindblad superoperator as a generator. This generator may have an eigenfunction with a zero eigenvalue referred to as a constant of motion (COM). An open quantum system has a unique stationary state if and only if it has no COMs. A system with multiple stationary states has a basis of COMs, any COM of the system is a linear combination of the basis COMs. The basis divides the space of system states into subspaces. In each subspace, its own stationary state is formed, and any stationary state of the system is a linear combination of these states. Usually, neither the basis of COMs nor even their number is known. We demonstrate that finding the stationary state of the system does not require looking for COMs. Instead, one can construct a set of "invariant" subspaces. If the system evolution begins from one of these subspaces, the system will remain in it, arriving at a stationary state independent of evolution in other subspaces. We suggest a direct way of finding the invariant subspaces by studying the evolution of the system. We show that the sets of invariant subspaces and subspaces generated by the basis of COMs are equivalent. A stationary state of the system is a weighted sum of stationary states in each invariant subspace; weighting factors are determined by the initial state of the system.

Surface plasmon polaritons are commonly believed to be a future basis for the next generation of optoelectronic and all-optical devices. To achieve this, it is critical that the surface plasmon polariton modes be strongly confined to the surface and have a sufficiently long propagation length and a nanosize wavelength. As of today, in the visible part of the spectrum, these conditions are not satisfied for any type of surface plasmon polaritons. In this paper, we demonstrate that in the ultraviolet range, surface plasmon polaritons propagating along a periodically nanostructured aluminum-dielectric interface have all these properties. Both the confinement length and the wavelength of the mode considered are smaller than the period of the structure, which can be as small as 10 nm. At the same time, the propagation length of new surface plasmon-polaritons can reach dozens of its wavelengths. These plasmon polaritons can be observed in materials that are uncommon in plasmonics such as aluminum. The suggested modes can be used for miniaturization of optical devices.

We show that a two-level atom with a high transition frequency ω SO can be inverted via non-radiative interaction with a cluster of excited low-frequency two-level atoms or quantum oscillators whose transition frequencies are smaller than ω SO . This phenomenon occurs due to the Förster resonant energy transfer arising during a train of quantum superoscillation of low-frequency two-level atoms. The suggested model could explain the mechanism of biophoton emission.

In 1954, Dicke predicted that a system of quantum emitters confined to a subwavelength volume would produce a superradiant burst. For such a burst to occur, the emitters must be in the special Dicke state with zero dipole moment. We show that a superradiant burst may also arise for non-Dicke initial states with a nonzero dipole moment. Both for Dicke and non-Dicke initial states, superradiance arises due to a decrease in the dispersion of the quantum phase of the emitter state. For non-Dicke states, the quantum phase is related to the phase of long-period envelopes which modulate the oscillations of the dipole moments. A decrease in the dispersion of the quantum phase causes a decrease in the dispersion of envelope phases that results in constructive interference of the envelopes and the superradiant burst.

We suggest a mechanism by which a superradiant burst emerges from a subwavelength array of nonlinear classical emitters that are not initially synchronized. The emitters interact via the field of their common radiative response. We show that only if the distribution of initial phases is not uniform does a non-zero field of radiative response arise, leading to a superradiant burst. Although this field cannot synchronize the emitters, it engenders long period envelopes for their fast oscillations. Constructive interference in the envelopes of several emitters creates a large fluctuation in dipole moments that results in a superradiant pulse. The intensity of this pulse is proportional to the square of the number of emitters participating in the fluctuation.

At the plasmon resonance, high Joule losses in a metal nanoparticle of a spaser result in its low Q-factor. Due to the latter, to achieve the spasing regime, in which the number of coherent plasmons exceeds the number of incoherent plasmons, unsustainably high pump rates may be required. We show that under the condition of loss compensation by a spaser driven by an external optical wave, the number of coherent plasmons increases dramatically, and the quantum noise is suppressed. Since the compensation of losses of the driving wave may occur even near the spasing threshold, the number of coherent plasmons may exceed the number of spontaneously excited plasmons at achievable pump rates.

We study counterintuitive dynamics of a two-level system (TLS) interacting with electric field superoscillating in time. We show that a TLS may be excited by an external light pulse whose spectral components are below the absorption line of the TLS. We attribute this unique dynamics to the Rabi oscillations of the TLS in a superoscillating driving field.

We show that due to near-field interaction of plasmonic particles via gain particles, a two-dimensional array of incoherently pumped spasers can be self-synchronized so that the dipole moments of all the plasmonic particles oscillate in phase and in parallel to the array plane. The synchronized state is established as a result of competition with the other possible modes having different wavenumbers and it is not destroyed by radiation of leaking waves, retardation effects, and small disorder. Such an array produces a narrow beam of coherent light due to continuous-wave superradiance. Thus, spasers, which mainly generate near-fields, become an efficient source of far-field radiation when the interaction between them is sufficiently strong.

We present an electrodynamical model of a quantum plasmonic device--the magneto-optical (MO) spaser. It is shown that a spherical gain nanoparticle coated with a metallic MO shell can operate as a spaser amplifying circularly polarized surface plasmons. The MO spaser may be used in design of an optical isolator in plasmonic transmission lines as well as in spaser spectrometry of chiral molecules.

We show that in plasmonic systems, exact loss compensation can be achieved with the help of spasers pumped over a wide range of pumping values both below and above the spasing threshold. We demonstrate that the difference between spaser operation below and above the spasing threshold vanishes, when the spaser is synchronized by an external field. As the spasing threshold loses its significance, a new pumping threshold, the threshold of loss compensation, arises. Below this threshold, which is smaller than the spasing threshold, compensation is impossible at any frequency of the external field.

We propose an exactly solvable electrodynamical model for surface plasmon amplification by stimulated emission of radiation (spaser). The gain medium is described in terms of the nonlinear permittivity with negative losses. The model demonstrates the main feature of a spaser: a self-oscillating state (spasing) arising without an external driving field if the pumping exceeds some threshold value. In addition, it properly describes synchronization of a spaser by an external field within the Arnold tongue and the possibility of compensating for Joule losses when the pumping is below threshold. The model also gives correct qualitative dependencies of spaser characteristics on pumping.

In the review, peculiarities of spectra of one-dimensional photonic crystals made of anisotropic and/or magnetooptic materials are considered. The attention is focused on band gaps of a special type-the so called degenerate band gaps which are degenerate with respect to polarization. Mechanisms of formation and properties of these band gaps are analyzed. Peculiarities of spectra of photonic crystals that arise due to the linkage between band gaps are discussed. Particularly, it is shown that formation of a frozen mode is caused by linkage between Brillouin and degenerate band gaps. Also, existence of the optical Borrmann effect at the boundaries of degenerate band gaps and optical Tamm states at the frequencies of degenerate band gaps are analyzed.

We demonstrate that when the frequency of the external field differs from the lasing frequency of an autonomous spaser, the spaser exhibits stochastic oscillations at low field intensity. The plasmon oscillations lock to the frequency of the external field only when the field amplitude exceeds a threshold value. We find a region of values of the external field amplitude and the frequency detuning (the Arnold tongue) for which the spaser synchronizes with the external wave.

We find the conditions upon the amplitude and frequency of an external electromagnetic field at which the dipole moment of a Bergman-Stockman spaser oscillates in antiphase with the field. For these values of the amplitude and frequency the loss in metal nanoparticles is exactly compensated by the gain. This shows that spasers may be used as inclusions in designing lossless metamaterials.

Modified boundary conditions for composite material are suggested. The modified RT-retrieval procedure yields bulk values of effective impedance and refractive index, which are independent of system size and boundary realization, whereas the conductivities of the excess surface currents depend on the property of the interface. Simultaneous treatment of all the possible realizations of the system removes the dependence. The accuracy of the latter procedure is the same as the usage of static effective parameters, namely k(eff)d.

We demonstrate the existence of a spectrally narrow localized surface state, the so-called optical Tamm state, at the interface between one-dimensional magnetophotonic and nonmagnetic photonic crystals. The state is spectrally located inside the photonic band gaps of each of the photonic crystals comprising this magnetophotonic structure. This state is associated with a sharp transmission peak through the sample and is responsible for the substantial enhancement of the Faraday rotation for the corresponding wavelength. The experimental results are in excellent agreement with the theoretical predictions.

The impact of the concentration of a base electrolyte (0.57; 0.85, 2.28% NaCl) on the pH-metric study of ionic equilibrium was studied in theerythrocytic suspensions obtained from the peripheral blood of healthy individuals, patients with various diseases and in the latter after their treatment. The use of the isotonic concentration of the base electrolyte at an ionic force of micro = 0.15 (0.85% NaCl) is optimal in terms of the accuracy of establishment of the most important parameters of denaturation erythrograms at an elevated temperature (58 degrees C), by continuously recording the pH value of a native red blood cell suspension.

We have considered a simple, square, two-dimensional (2D) PC built of a magneto-optic matrix with square holes. It is shown that using such a magnetophotonic crystal it is possible to deflect a light beam at very large angles by applying a nonzero external magnetic field. The effect is called the giant photonic Hall effect (GPHE) or the magnetic superprism effect. The GPHE is based on magneto-optical properties, as is the photonic Hall effect [B. A. van Tiggelen and G. L. J. A. Rikken, in, edited by V. M. Shalaev (Springer-Verlag, Berlin, 2002), p. 275]; however GPHE is not caused by asymmetrical light scattering but rather by the influence of an external magnetic field on the photonic band structure.

A simple approach to the problem of light localization in one-dimensional system is presented. The role of the Bragg reflection in one-dimensional localization of light is discussed. Contrary to the existent viewpoint, we show that the origin of band gaps of regular crystals and the localization due to disorder have a common nature, that is, the Bragg reflection. We expand the concept of band structure to random systems of finite thickness L and relate the Anderson localization of light with the total band gap growth, which is observed in our computer simulation of disordered system, as L increases.

A scaling theory for homogenization of the Maxwell equations is developed upon the representation of any field as a sum of its dipole, quadrupole, and magnetic dipole moments. This representation is exact and is connected neither with multipole expansion nor with the Helmholtz theorem. A chain of hierarchical equations is derived to calculate the moments. It is shown that the resulting macroscopic fields are governed by the homogenized Maxwell equations. Generally, these fields differ from the mean values of microscopic fields.

A method of differentiated measurements of pH is suggested to be used in study of the time course of ionic equilibrium in red cell suspensions exposed to heating. The described method yields more accurate and complete data on the type and distribution of red cells as regards their membrane characteristics.