A. A. Pukhov

Dipole response of spaser on an external optical 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.

Forced synchronization of spaser by an external optical wave

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.

Loss compensation by spasers in plasmonic systems

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.

Spaser operation below threshold: autonomous vs. driven spasers

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.

Superradiance of non-Dicke states

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.

Optimum gain for plasmonic distributed feedback lasers

Although nanolasers typically have low Q-factors and high lasing thresholds, they have been successfully implemented with various gain media. Intuitively, it seems that an increase in the gain coefficient would improve of characteristics of nanolasers. For a plasmonic distributed feedback (DFB) laser, we propose a self-consistent model that takes into account both spontaneous emission and the multimode character of laser generation to show that for given pumping strength, the gain coefficient has an optimal value at which the radiation intensity is at a maximum and the radiation linewidth is at a minimum.

Superradiance of a subwavelength array of classical nonlinear emitters.

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.

Spontaneous radiation of a two-level atom into multipole modes of a plasmonic nanoparticle

Abstract We consider the relaxation of an excited two-level system (TLS) positioned near a spherical plasmonic nanoparticle (NP). The transition frequency of the TLS is assumed to coincide with the frequency of the condensation point of NP plasmonic resonances. We show that the relaxation of the TLS excitation is a two-step process. Following an initial exponential decay, the TLS breaks in to Rabi oscillations. Depending upon the distance between the TLS and NP, the probability of the TLS being in the excited state exhibits either chaotic or nearly regular oscillations. In the latter case, the eigenfrequency of the TLS-NP system coincides with one of NP multipole modes.

Inversion of a two-level atom by quantum superoscillations

We show that a two-level atom with a high transition frequency ωSO 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 ωSO. 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.