Virtual cavity in distributed Bragg reflectors
We show theoretically and experimentally that distributed Bragg reflector (DBR)
supports a surface electromagnetic wave exhibiting evanescent decay in the air and oscillatory
decay in the DBR. The wave exists in TM polarization only. The field extension in the air
may reach several wavelengths of light. Once gain medium is introduced into the DBR a
novel class of diode lasers, semiconductor optical amplifiers, light-emitting diodes, etc. can
be developed allowing a new type of in-plane or near-field light outcoupling. To improve the
wavelength stability of the laser diode, a resonant cavity structure can be coupled to the DBR,
allowing a coupled state of the cavity mode and the near-field mode. A GaAlAs-based
epitaxial structure of a vertical-cavity surface-emitting laser (VCSEL) having an
antiwaveguiding cavity and multiple GaInAs quantum wells as an active region was grown
and processed as an in–plane Fabry-Pérot resonator with cleaved facets. Windows in the top
stripe contact were made to facilitate monitoring of the optical modes. Three types of the
optical modes were observed in electroluminescence (EL) studies under high current densities
> 1 kA/cm2. Mode A with the longest wavelength is a VCSEL–like mode emitting normal to
the surface. Mode B has a shorter wavelength, emitting light at two symmetric lobes tilted
with respect to the normal to the surface in the direction parallel to the stripe. Mode C has the
shortest wavelength and shifts with a temperature at a rate 0.06 nm/K. Polarization studies
reveal predominantly TE emission for modes A and B and purely TM for mode C in
agreement with the theory. Spectral position, thermal shift and polarization of mode C
confirm it to be a coupled state of the cavity mode and near-field DBR surface-trapped mode.
We propose the use of dipolaritons-quantum well excitons with a large dipole moment, coupled to a planar microcavity-for generating terahertz (THz) radiation. This is achieved by exciting the system with two THz detuned lasers that leads to dipole moment oscillations of the exciton polariton at the detuning frequency, thus generating a THz emission. We have optimized the structural parameters of a system with microcavity embedded AlGaAs double quantum wells and shown that the THz emission intensity is maximized if both of the laser frequencies match different dipolariton states. The influence of the electronic tunnel coupling between the wells on the frequency and intensity of the THz radiation is also investigated, demonstrating a trade-off between the polariton dipole moment and the Rabi splitting.
The possibility to observe a macroscopically coherent state in a gas of two-dimensional direct excitons at temperatures up to tens of Kelvin is described. The dramatic increase of the exciton lifetime allowing effective thermalization is predicted for the o -resonant cavities that strongly suppress exciton recombination. The material systems considered are single GaAs quantum wells of di erent thicknesses and a transition metal dichalcogenide monolayer, embedded in a layered medium with subwavelength period. The quantum hydrodynamic approach combined with the Bogoliubov description yield the one-body density matrix of the system. Employing the Kosterlitz-Thouless \dielectric screening" problem to account for vortices, we obtain the superfluid and the condensate densities and the critical temperature of the Berezinskii-Kosterlitz-Thouless crossover, for all geometries in consideration. Experimentally observable manyfold increase of the photoluminescence intensity from the structure as it is cooled below the critical temperature is predicted.
In the reported experiment, a picosecond strain pulse induces a sharp transition between the steady states in a bistable cavity-polariton system. The strain pulse of 10-ps duration, generated in the GaAs substrate and injected into a high-Q GaAs/AlAs microcavity, modulates the exciton resonance energies of the embedded quantum wells and correspondingly of the polariton resonances. When the microcavity is pumped by a laser with the photon energy slightly above the lower-polariton resonance, the strain-induced energy shift triggers the irreversible switching of the bistable polariton system from the lower to the upper state. This transition is accompanied by an instant increase of the optical emission from the microcavity by more than an order of magnitude.
We performed a numerical study of a surrounding medium influence on coupling efficiency between a microdisk resonator supporting optical whispering gallery modes and a straight optical waveguide. Quality factors of the modes and relative optical power coupled to the waveguide were calculated using COMSOL Multiphysics environment. It was shown that the most efficient coupling takes place when propagation constants of the modes of the microdisk and the waveguide match. The coupling can be significantly strengthened by increasing the index of the surrounding medium.
The anisotropic superfluidity in a weakly interacting two‐dimensional Bose gas of photons in a dye‐filled optical microcavity is investigated, taking into account the dependence of the photon effective mass on the in‐plane coordinate. With the use of the generalized Gross–Pitaevskii equation and the Bogoliubov approach, it is shown that the modulation of the microcavity width leads to an effective periodic potential and the periodicity of the condensate wave function, and both the condensate energy and the spectrum of elementary excitations depend on the direction of motion. The anisotropic character of the dynamical and superfluid properties, such as helicity modulus, superfluid density, and sound velocity, as well as experimentally observable manifestations of their anisotropy are described.
Optical bistability of exciton polaritons in semiconductor microcavities is a promising platform for digital optical devices. Steady states of coherently driven polaritons can be toggled back and forth in tens of picoseconds under short external pulses of appropriate amplitude and phase. We have analyzed the switching behavior depending on the pulse amplitude, phase, and duration. The switches are found to change dramatically when the inverse pulse duration becomes comparable to the frequency detuning between the driving field and polariton resonance. If the detuning is large compared to the polariton linewidth, the system becomes extremely sensitive to initial conditions and thus responds unpredictably.
The dynamics of a two-component Davydov-Scott (DS) soliton with a small mismatch of the initial location or velocity of the high-frequency (HF) component was investigated within the framework of the Zakharov-type system of two coupled equations for the HF and low-frequency (LF) fields. In this system, the HF field is described by the linear Schrödinger equation with the potential generated by the LF component varying in time and space. The LF component in this system is described by the Korteweg-de Vries equation with a term of quadratic influence of the HF field on the LF field. The frequency of the DS soliton`s component oscillation was found analytically using the balance equation. The perturbed DS soliton was shown to be stable. The analytical results were confirmed by numerical simulations.
Radiation conditions are described for various space regions, radiation-induced effects in spacecraft materials and equipment components are considered and information on theoretical, computational, and experimental methods for studying radiation effects are presented. The peculiarities of radiation effects on nanostructures and some problems related to modeling and radiation testing of such structures are considered.
This volume presents new results in the study and optimization of information transmission models in telecommunication networks using different approaches, mainly based on theiries of queueing systems and queueing networks .
The paper provides a number of proposed draft operational guidelines for technology measurement and includes a number of tentative technology definitions to be used for statistical purposes, principles for identification and classification of potentially growing technology areas, suggestions on the survey strategies and indicators. These are the key components of an internationally harmonized framework for collecting and interpreting technology data that would need to be further developed through a broader consultation process. A summary of definitions of technology already available in OECD manuals and the stocktaking results are provided in the Annex section.