Picosecond spectroscopy of polaritons in anthracene crystals II. Luminescence

The low-temperature luminescence spectrum of anthracene crystals is investigated by applying simultaneous time and frequency resolution. The complicated kinetics of the emission in the polariton bottleneck region reflects directly the evolution and relaxation of the polariton distribution in the crystal. Three distinct relaxation stages are distinguished: (1) the ultrafast decay of initial vibronic excitations, mediated by optical phonons and resulting in a broad distribution of polaritons near the band bottom; (2) the formation of a narrow distribution of polaritons with a characteristic time of 30 ps, which is caused by scattering on acoustic phonons; (3) relaxation through the bottleneck region on a subnanosecond time scale. It is suggested that the polaritons immediately below the resonance frequency are responsible for the observed excitonic energy transfer in anthracene crystals.     

基于厚金属狭缝阵列的表面等离子激元光刻

提出一种利用厚金属狭缝阵列耦合激发表面等离子激元制作非周期图形的纳米光刻模型.采用时域有限差分电磁场模拟仿真软件研究了厚金属狭缝阵列中表面等离子激元的激发、模式选择以及光刻胶中的光场分布.结果表明,通过优化厚金属狭缝阵列结构参量和匹配介质参量可有效抑制表面等离子激元在光栅狭缝出口处的发散,增加表面等离子激元的穿透深度,...     

基于厚金属狭缝阵列的表面等离子激元光刻

提出一种利用厚金属狭缝阵列耦合激发表面等离子激元制作非周期图形的纳米光刻模型.采用时域有限差分电磁场模拟仿真软件研究了厚金属狭缝阵列中表面等离子激元的激发、模式选择以及光刻胶中的光场分布.结果表明,通过优化厚金属狭缝阵列结构参量和匹配介质参量可有效抑制表面等离子激元在光栅狭缝出口处的发散,增加表面等离子激元的穿透深度,可获得高分辨率的较大曝光深度的周期和非周期纳米图形,可为纳米激光直写技术提供有益的借鉴.     

On the equilibrium spectrum of surface polaritons thermally excited by solids

The spectral energy densities of surface plasmon and phonon polaritons are calculated in case of thermodynamic equilibrium. It is demonstrated that the spectral distribution of surface polaritons in equilibrium is quite different from the Planck?s law for photons. Temperature dependence of the surface density of energy for two-dimensional fluctuations is different from the temperature dependence of volume density of energy for photons in equilibrium.     

Surface plasmon interference excited by tightly focused laser beams

We show that interfering surface plasmon polaritons can be excited with a focused laser beam at normal incidence to a plane metal film. No protrusions or holes are needed in this excitation scheme. Depending on the axial position of the focus, the intensity distribution on the metal surface is either dominated by interferences between counterpropagating plasmons or by a two-lobe pattern characteristic of localized surface plasmon excitation. Our experiments can be accurately explained by use of the angular spectrum representation and provide a simple means for locally exciting standing surface plasmon polaritons.     

Pseudosurface dispersion polaritons and their resonance excitation

The conditions for the transformation of surface polaritons (surface electromagnetic waves) into pseudosurface polaritons under the influence of perturbations are investigated. The transformation occurs because of the appearance of a weak energy loss flow typical of pseudosurface polaritons, which carries energy away from the surface of the solid. The dispersion polaritons existing at negative values of the dielectric constant are considered. The domain of existence of the pseudosurface polaritons is studied; their main characteristics and the parameters of the resonance excitation of these polaritons by a pumping wave are investigated.     

Relaxation kinetics for temperature and degeneracy of microcavity polaritons

We apply a semi-classical Boltzmann kinetics for a gas of laser-pulse excited microcavity polaritons taking into account their mutual interaction and their interaction with acoustic phonons. Fitting the temporally evolving polariton distribution above the ground state with a Bose–Einstein distribution, we find the evolution of the temperature and the degeneracy parameter, i.e. the ratio of the chemical potential to the thermal energy. Studying the relaxation in particular for GaAs microcavities we compare our results with recent measurements by Deng et al. In agreement with the experiment we find that the lattice temperature can be reached and that the degeneracy of the condensed gas holds up to 60–80 ps provided a detuning of the cavity mode is applied which increases the exciton component of the lower-branch polaritons and thus their scattering rates.     

Scattering of surface polaritons in the vicinity of phase transition points

In the region of phase transitions the intensity of dielectric permeability fluctuations increases due to fluctuations of the order parameter. This can lead to the decrease in the length of propagation of surface polaritons and also it can influence the angular distribution of light reflected from the medium surface. In this paper the scattering of surface polaritons by the order parameter fluctuations is considered in conditions when the surface polariton is propagating along the surface of the metal coated with a thin dielectric film near the phase transition point in this film and for the case when the film is absent and the phase transition occurs in the whole substrate volume.     

Light scattering by extraordinarily polarized polaritons

The objective of this work is to investigate how the anisotropy of the interaction between dipole-active vibrations of a crystal lattice and infrared electromagnetic waves is manifested in the spontaneous parametric light scattering spectra of polaritons (Raman scattering by small angles). The case where scattering occurs by extraordinarily polarized polaritons—quasiparticles formed as a result of the coupling of the wave polarized in the symmetry plane of a biaxial crystal simultaneously with two phonons possessing orthogonal dipolar moments—is studied. A series of spectra of equilibrium fluctuations of the electromagnetic (infrared) field, each of which represents an intensity distribution in frequency-wave number coordinates for a fixed direction of the wave vector, are constructed on the basis of a scattering model that takes account of the tensor character of the permittivity and the quadratic and cubic susceptibilities of the crystal. Analysis of the computed spectra identified the basic laws and dependences which are determined by the anisotropy of the electromagnetic susceptibilities of various orders and made it possible to explain previous experimental results which cannot be interpreted on the basis of the generally accepted model of transversely polarized polaritons. A method is proposed for determining the contributions of the dipole-active vibrations of the crystal lattice to the permittivity and the quadratic and cubic susceptibilities, as well as the absorption of the material from the spectra of the extraordinarily polarized polaritons.     

Nonlinear Dynamics of Parametric Oscillations of Polaritons in a Microcavity

Optics and Spectroscopy - The dynamics of polaritons in a microcavity in the parametric oscillator mode, when two pump polaritons turn into the signal and idler polaritons and vice versa, has been...     

Diamagnetism of microcavity polaritons induced by spin-dependent polariton–polariton interactions

Diamagnetism of condensed microcavity polaritons in a vertically applied magnetic field is theoretically studied by using the density of free energy of polaritons. The magnetic dependence of polariton–polariton interactions and spin polarization degree of polaritons are derived, and are used to show the diamagnetic behavior of the polariton spin polarization, which is discussed for GaAs-based microcavities. We show that for strong magnetic field the spin polarization of the polaritons is paramagnetic as usual, while around positive exciton–photon detuning and special Rabi splitting, the spin polarization of the polaritons could be diamagnetic. In addition, weak magnetic field and high polariton density are beneficial to observe the polariton diamagnetism.     

Controlled spin pattern formation in multistable cavity–polariton systems

Theoretical studies are performed of planar cavity–polariton systems under resonant optical excitation. We show that if the cavity is spatially anisotropic, the polariton spin is highly sensitive to the pump polarization direction, which can be used to modulate the circular polarization of the output light. In particular, when the right- and left-circular components of the incident wave have equal intensities and mutually opposite angular momenta, the pump has strictly linear yet angle-dependent polarization and as such brings about a periodic angular variation of the polariton spin. Free motion of polaritons is the other factor determining the shape of the cavity-field distribution. Such externally driven and highly tunable spin patterns represent a counterpart of spin shaping in nonresonantly excited Bose–Einstein condensates of cavity polaritons.     

The effect of the surface enhanced polariton field on the tunneling current of a STM

The influence of surface polaritons (SPs), excited on the external (fast mode) and internal (slow mode) surfaces of a gold film, on the tunneling current of a STM is investigated. The role of the surface polaritons is directly demonstrated by the angular excitation spectra of the induced tunneling current. Possible mechanisms which might give rise to the SP induced change in the tunneling current are discussed. An electromagnetic field of SPs within a tunneling gap is estimated to be surface enhanced in order to explain the value of the induced tunneling current. Images of the polariton induced signal distribution over the sample surface taken by means of different SP modes are compared in order to obtain information on the structure of the internal surface of the film.     

Spatial condensation of trapped polaritons in graphene and semiconductor structures

The theoretical and experimental status of the Bose–Einstein Condensation (BEC) of trapped quantum well (QW) polaritons in a microcavity is presented. The results of recent experiments that have shown the possibility to create an in-plane harmonic potential trap for a two-dimensional (2D) exciton polaritons in a cavity are discussed. We report the theory of BEC and of the trapped QW exciton polaritons in a microcavity. In addition, we study the BEC of trapped magnetoexciton polaritons in a graphene layer (GL) embedded in an optical microcavity in high magnetic field. In both cases the polaritons are considered to be in a harmonic potential trap. We compare the theoretical results with the existing experiments and discuss the experimental observation of predicted phenomena.     

Excitonic surface polaritons in ZnTe

Excitonic surface polaritons are excited resonantly in ZnTe utilizing attenuated total reflection (ATR). Lineshape analyses of ATR and normal-incidence-reflection spectra yield the eigenenergies of excitonic surface and bulk polaritons. Comparison of ATR and reflection spectra with luminescence spectra invalidates a recently published attribution of luminescence maxima to excitonic surface polaritons.     

Energy relaxation of excitonlike polaritons in semiconductor microcavities: Effect on the parametric scattering of polaritons

Polariton emission in GaAs-based microcavities has been studied under variable conditions, which made it possible to excite (a) polaritons from the upper polariton branch and hot free polaritons and electrons, (b) polaritons from the lower polariton branch (LPB) and localized excitons, and (c) the mixed system. Variation of the excitation conditions leads to substantial differences in the energy distributions of polaritons and in the temperature dependences of polariton emission. It is established that the energy relaxation of resonantly excited LPB polaritons via polariton and localized exciton states at liquid helium temperatures is ineffective. Instead, the relaxation bottleneck effect is suppressed with increasing temperature by means of exciton delocalization (due to thermal excitation by phonons). The most effective mechanism of relaxation to the LPB bottom is via scattering of delocalized excitons on hot free carriers. It is found that the slow energy relaxation of polaritons excited below the free exciton energy can be significantly accelerated at low temperatures by means of additional weak generation of hot excitons and, especially, hot electrons. This acceleration of the energy relaxation of polaritons by means of additional overbarrier photoexcitation sharply decreases the barrier for stimulated parametric scattering of polaritons excited at an LPB inflection point. Therefore, additional illumination can be used to control the polariton-polariton scattering.     

Dispersionless polaritons at symmetrically oriented surfaces of biaxial crystals

A set of dispersion relations is derived for surface polaritons in optically biaxial crystals at the surfaces parallel to the symmetry planes of the permittivity tensor ?. The domains of existence, as well as the sectors of the propagation directions of dispersionless surface polaritons which arise at positive components of the tensor ?, are analyzed. Three nonoverlapping domains of the dielectric-anisotropy parameters where dispersionless polaritons can exist are found for weakly anisotropic crystals. In each of these domains, polaritons exist at two different mutually orthogonal surfaces of the crystal. In optically biaxial crystals, in contrast to optically uniaxial media, polaritons arise not only in positive but also in negative crystals. The evolution of the optical-axis configuration is traced as the anisotropy parameters vary in the domains of existence of polaritons.     

Dynamics of a parametric exciton-polariton oscillator in a microcavity

We study the dynamics of parametric oscillations of polaritons in a microcavity that consists of a periodic conversion of a pair of pump polaritons into polaritons of signal and idle modes and vice versa. The period and amplitude of oscillations considerably depend on the initial polariton density, the initial phase difference, and the resonance detuning. We show that there is a possibility of phase controlling the polariton dynamics in the microcavity.     

Cavity plasmon polaritons in monolayer graphene

Plasmon polaritons in a new system, a monolayer doped graphene embedded in optical microcavity, are studied here. The dispersion law for lower and upper cavity plasmon polaritons is obtained. Peculiarities of Rabi splitting for the system are analyzed; particularly, role of Dirac-like spinor (envelope) wave functions in graphene and corresponding angle factors are considered. Typical Rabi frequencies for maximal (acceptable for Dirac-like electron spectra) Fermi energy and frequencies of polaritons near polariton gap are estimated. The plasmon polaritons in considered system can be used for high-speed information transfer in the THz region.     

周期地嵌入电介质球壳的金属表层的表面等离子激元及其与电介质腔体模式的耦合

通过观察金属底板中周期地嵌入电介质球壳的体系的光学吸收性质，研究了表面等离子激元 以及与其他电磁模式的耦合特性.在这种周期结构的金属表面，发现存在两种响应频率，分 别对应于表面等离子激元模式和金属中的电介质腔体模式.在这些响应频率上，可观察到与 它们相对应的吸收峰.由于金属的表面模式不能与平面入射光直接耦合，而腔体模式与平面 入射光和表面等离子激元模式的耦合一般较弱，因而通常情况下这些吸收峰的峰值有限.然 而，通过调整体系中的某些参数，可以使腔体模式和表面模式的频率非常接近，这时二者之 间的耦合强度将大大提高.此时，在相应的频率附近可观察到极强的吸收峰.详细地研究了介 质球壳的物理和几何参数对此共振吸收的影响. 关键词： 腔体模式 表面等离子体模式 共振吸收