Microcavity Research Articles

Fermi Edge Polaritons in a Microcavity Containing a High Density Two-Dimensional Electron Gas

Sharp, near band gap lines are observed in the reflection and photoluminescence spectra of GaAs/AlGaAs structures consisting of a modulation doped quantum well (MDQW) that contains a high density two-dimensional electron gas (2DEG) and is embedded in a microcavity (MC). The energy dependence of these lines on the MC-confined photon energy shows level anticrossings and Rabi splittings very similar to those observed in systems of undoped QW's embedded in a MC. The spectra are analyzed by calculating the optical susceptibility of the MDQW in the near band gap spectral range and using it within the transfer matrix method. The calculated reflection spectra indicate that the sharp spectral lines are due to k{ parallel}=0 cavity polaritons that are composed of e-h pair excitations just above the 2DEG Fermi edge and are strongly coupled to the MC-confined photons.

High-Resolution Measurement of Resonant Wave Patterns by Perturbing the Evanescent Field Using a Nanosized Probe in a Transmission Scanning Near-Field Optical Microscopy Configuration

In order to model transmission scanning near-field optical microscopy (T-SNOM) experiments, we study the interaction between a nanosized atomic-force-microscopy-type probe and the optical field in a microcavity (MC) at or near resonance. Using a 2-D cross-sectional model of an experimentally studied photonic crystal MC, we have simulated the T-SNOM method by scanning a probe over the surface while monitoring the transmitted and reflected power. The simulations were performed for two probe materials: silicon and silicon nitride. From the probe-induced change in the transmission and reflection spectra, a wavelength shift was extracted. A shift almost proportional to the local field intensity was found if the resonator was excited just below a resonance wavelength. However, at the spots of highest interaction, we observed that besides the desired resonance wavelength shift, there was an increase in scattering. Furthermore, by moving the probe at such a spot in the vertical direction to a height of approximately 0.5, a 5% increase in transmission can be established because the antiresonant condition is satisfied. Finally, a 2-D top view simulation is presented of the experimentally studied T-SNOM method, which shows a remarkably good correspondence in intensity profile, except for the exact location of the high-interaction spots.

Angular dependent luminescence behavior in one-dimensional Si-based microcavities

One-dimensional all solid-state microcavities (MCs) based on amorphous silicon-nitride films have been fabricated in plasma enhanced chemical vapor deposition system. Spectral narrowing and enhancement of room temperature photoluminescence due to the modulation effect in MCs were observed. The modulated luminescence behavior in MCs was further studied by angular dependent luminescence measurements. The distinct cavity mode energy shifting and variation of luminescence intensity with the detect angle were observed, which reflected the directionality of emission properties in the planar MCs. The deviation of the measured angular dependent energy shifting from the theory prediction was discussed as being related to the structure properties of the MCs.

Porous silicon microcavities fabricated using ion irradiation

By employing microcavity (MC) techniques, the broad spectral band and wide emission angle of photoluminescence from porous silicon (PSi) can be narrowed, directed and tuned in PSi microcavities. The microcavity structure is formed with a porous silicon active layer sandwiched between two Bragg reflectors. We have characterised the optical properties of the porous silicon multilayer structures. A number of Bragg reflectors and Fabry–Perot optical microcavities are fabricated with tuning of emission wavelengths over a wide range in heavily doped p-type silicon. The optical properties of these microstructures were investigated using reflectivity and photoluminescence measurements at different temperatures. This paper reports the ability of focussed 2 MeV proton beam irradiation to controllably blue shift the resonant wavelength of porous silicon microcavities in heavily doped p-type wafers. Using this process wafers are patterned on a micrometer lateral scale with microcavities tuned to different resonant wavelengths, giving rise to high-resolution full-colour reflection images over the full visible spectrum.

Impact of disorder on high quality factor III-V nitride microcavities

The authors report on the micron scale characterization of a monolithic GaN microcavity (MC) with lattice matched AlInN∕GaN distributed Bragg reflectors by means of a microtransmission setup. This technique allows extracting very high quality factors (Q up to 2800), in accordance with theoretical predictions, contrary to what was previously reported for nitride based MCs. Furthermore, two-dimensional mappings of the MC transmission spectrum allow probing the disorder in this MC. The direct relationship between an increased disorder and a reduction in the Q factor is clearly observed.

SAW-driven optical microcavities for device applications

The optical properties of microcavity (MC) polaritons parametrically driven by a surface acoustic wave (SAW) is analysed. In this case, the resonant acousto-optic nonlinearity gives rise to a very short ‘light–acoustic wave’ interaction length even for rather modest acoustic intensities. This can effectively be used for possible device applications. Two schemes ‘SAW pumping–optical probing’ are proposed for semiconductor MCs: one deals with the use of bulk incoming/outgoing photons, which resonantly couple with MC polaritons through a top distributed Bragg reflector, and another involves in-plane light delivered to and collected from the lateral surfaces of a MC chip.

Impact of inhomogeneous excitonic broadening on the strong exciton-photon coupling in quantum well nitride microcavities

GaN/AlGaN and InGaN/GaN quantum wells (QWs) are investigated as the active region for room-temperature strong exciton-photon coupling in high-quality AlInN/(Al)GaN microcavities (MCs). Angular resolved photoluminescence (PL) measurements performed on an AlInN/AlGaN MC with GaN/AlGaN QWs reveal cavity polariton dispersion curves. A vacuum-field Rabi splitting of 30 meV is observed, from which an exciton oscillator strength of 4.8x10(13) cm(-2) per QW is deduced, a value ten times larger than for InGaAs QWs. In contrast, the PL spectra of an InGaN/GaN QW MC do not exhibit such an anticrossing behavior as should be expected from the strong-coupling regime (SCR). By modeling cavity absorption and PL spectra at the resonance, the conditions, in terms of inhomogeneous excitonic broadening, for the observation of the SCR in these nitride MCs are determined.

Exciton–polariton spectra and limiting factors for the room-temperature photoluminescence efficiency in ZnO

Static and dynamic responses of excitons in state-of-the-art bulk and epitaxial ZnO are reviewed to support the possible realization of polariton lasers, which are coherent and monochromatic light sources due to Bose condensation of exciton–polaritons in semiconductor microcavities (MCs). To grasp the current problems and to pave the way for obtaining ZnO epilayers of improved quality, the following four principal subjects are treated: (i) polarized optical reflectance (OR), photoreflectance (PR) and photoluminescence (PL) spectra of the bulk and epitaxial ZnO were recorded at 8 K. Energies of PR resonances corresponded to those of upper and lower exciton–polariton branches, where A-, B- and C-excitons couple simultaneously to an electromagnetic wave. PL peaks due to the corresponding polariton branches were observed. Longitudinal–transverse splittings (ωLT) of the corresponding excitons were 1.5, 11.1 and 13.1 meV, respectively. The latter two values are more than two orders of magnitude greater than that of GaAs being 0.08 meV. (ii) Using these values and material parameters, corresponding vacuum-field Rabi splitting of exciton–polaritons coupled to a model MC mode was calculated to be 191 meV, which is the highest value ever reported for semiconductor MCs and satisfies the requirements to observe the strong exciton–light coupling regime necessary for polariton lasing above room temperature. (iii) Polarized OR and PR spectra of an out-plane nonpolar ZnO epilayer grown by laser-assisted molecular beam epitaxy (L-MBE) were measured, since ZnO quantum wells (QWs) grown in nonpolar orientations are expected to show higher emission efficiencies due to the elimination of spontaneous and piezoelectric polarization fields normal to the QW plane. They exhibited in-plane anisotropic exciton resonances according to the polarization selection rules for anisotropically-strained wurzite material. (iv) Impacts of point defects on the nonradiative processes in L-MBE ZnO were studied using time-resolved PL making a connection with the results of positron annihilation measurement. Free excitonic PL intensity at room temperature naturally increased with the increase in nonradiative lifetime (τnr). The value of τnr increased and density or size of Zn vacancies (VZn) decreased with increasing growth temperature (Tg) in heteroepitaxial films grown on a ScAlMgO4 substrate, and the use of homoepitaxial substrates further reduced VZn density. The value of τnr was shown to increase with the decrease in gross density of positively and negatively charged and neutral point defects including complexes rather than with the decrease in VZn density. The results indicate that the nonradiative recombination process is governed not by single point defects, but by certain defects introduced with the incorporation of VZn, such as VZn-defect complexes. As a result of defect elimination by growing the films at high Tg followed by subsequent post-growth in situ annealing, combined with the use of high-temperature-annealed ZnO self-buffer layer, a record long τnr for spontaneous emission of 3.8 ns was obtained at room temperature. By using progressively improving epitaxial growth methods, the polariton laser effect is expected to be observed at room temperature in the near future.

Bistability vs stimulated scattering in semiconductor microcavities

We report an analysis of polariton scattering in semiconductor microcavity (MC) under a strong cw resonant excitation. We show that near the threshold of the parametric scattering accounting for the pump induced blue shift results in an qualitative modification of scattering scenario. Namely the maximumof the scattered signal above the threshold of stimulated parametric scattering does not shift along the microcavity lower polariton branch with the change of pump detuning or angle of incidence but is fixed at normal direction. We show that such a behavior can be modelled numerically by a system of Maxwell and nonlinear Schrodinger equations for cavity polaritons and explained via the competition between the bistability of a driven nonlinear MC polariton and the instabilities of parametric polariton-polariton scattering. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Two-photon fluorescence excitation using an integrated optical microcavity: a promising tool for biosensing of natural chromophores

Application of an integrated optics (IO) microcavity (MC) for evanescent excitation of two-photon excited fluorescence (TPF) is demonstrated. The MC provides a high local intensity, which is required for the TPF, because of resonant enhancement of the intracavity power and a strong two-dimensional confinement of the guided mode. Numerical estimations show a large increase, by more than a factor of 10 4 of the TPF intensity at the MC compared to a conventional straight waveguide. This will lead to a significant improvement of the detection limits of UV-absorbing chromophores (down to 10 −8 M) when using the MC as a biosensor. Feasibility of TPF excitation using an IO MC is confirmed experimentally for the first time.

Time-resolved and cw photoluminescence from strongly coupled organic microcavities

The optical properties of microcavities (MCs) are strongly dependent on both polarization of incident and emitted light and its angle of observation. Here we report the measurements of cw- and time-resolved photoluminescence (PL) observed at negative detuning and at resonance for s- and p-polarization in the strong coupling regime of a planar MC containing J-aggregates of a cyanine dye. Following non-resonant excitation, the emission spectra consist of three types of features: direct J-aggregate exciton emission, polariton emission, and uncoupled monomer emission through the transmission maxima of the distributed Bragg reflector beyond the stop-band. We compare our experimental results with a transfer-matrix calculation of the transmission for s- and p-polarization and explain the different positions of the polariton branches, the stop-band width, and the high- and low energy transmission maxima of the MC. Time-resolved PL experiments show an increase in the decay lifetime of the exciton-like mode when it is positioned far from the cavity mode. Close to resonance, the lower polariton branch decays with the natural lifetime of the J-aggregates.

Condensation kinetics of cavity polaritons interacting with a thermal phonon bath

The kinetics of the boson condensation for micro-cavity (MC) polaritons interacting with thermal acoustic phonons is studied within rate equations, taking into account the finite MC cross-section. For a smoothly switched on cw excitation and finite polariton lifetimes we find a build-up of a large population in the lowest state of the polariton spectrum for sufficiently large deformation potential coupling at sufficiently high densities, which are still below the saturation density. The critical influence of the value of the heavy-hole deformation potential is illustrated, as well as the favorable influence of positive values of the detuning. This spontaneous polariton condensation would result in a perpendicular emission out of the micro-cavity.

Bistable optical response in quantum well semiconductor microcavity

We report an experimental study of the optical bistability in a semiconductor microcavity (MC), consisting of a wedged λ GaAs spacer containing a single 8.5 nm In0.04Ga0.96As quantum well (QW). Increasing the excitation power, a transition between the normal coupling and the weak coupling regime of the MC modes is observed. At intermediate excitation power the reflectivity spectrum shows the simultaneous presence of three resonances, with a third peak spectrally lying between the two normal modes. Monitoring the reflected signal for different laser energy as a function of the excitation power, we found that optical bistability is observed only at the energy of the third peak. The experimental findings do not agree with transfer matrix calculations based on exciton bleaching, suggesting that more complex nonlinear mechanisms are relevant in the observed bistable behaviour.

Anisotropic polariton migration and its effect on photoluminescence amplification in a GaAs/AlAs microcavity containing a two-dimensional electron gas

Photoluminescence (PL) image size and angular resolved spectra were studied in a GaAs/AlAs planar microcavity (MC), with an embedded GaAs quantum well (QW) that contains a photogenerated, variable density, two-dimensional electron gas (2DEG). The PL was measured at T = 7 K, under nonresonant, cw excitation. The in-plane polariton migration from the photoexcitation spot (whose radius is ∼30 μm) is deduced from the PL image size. It is observed to be anisotropic in the MC plane, and for negative detuning energy values (δ = E C - E X ) it extends over in-plane distances of ∼300 μm. Introducing a 2DEG into the QW while maintaining the strong coupling between the excitons and the MC photon mode, reduces the migration range. On the other hand, the angular resolved PL spectra show that the emission from the k∥ ∼ 0 polariton states is enhanced as the 2DEG density is increased. From these observations we propose that the polaritons migrate while they occupy the long-lived states above the relaxation bottleneck in the lower polariton (LP) branch. The efficient electron-polariton scattering processes transfer the polaritons through the bottleneck into the radiative trap (k∥ ∼ 0) in the LP branch. These scattering processes occur at the point of photoexcitation in the MC plane, and therefore they decrease the migration range.

Radiative corrections to the excitonic molecule state in GaAs microcavities

The optical properties of excitonic molecules (XXs) in GaAs-based quantum well microcavities (MCs) are studied, both theoretically and experimentally. We show that the radiative corrections to the XX state, the Lamb shift $\Delta^{\rm MC}_{\rm XX}$ and radiative width $\Gamma^{\rm MC}_{\rm XX}$, are large, about $10-30 %$ of the molecule binding energy $\epsilon_{\rm XX}$, and definitely cannot be neglected. The optics of excitonic molecules is dominated by the in-plane resonant dissociation of the molecules into outgoing 1$\lambda$-mode and 0$\lambda$-mode cavity polaritons. The later decay channel, ``excitonic molecule $\to$ 0$\lambda$-mode polariton + 0$\lambda$-mode polariton'', deals with the short-wavelength MC polaritons invisible in standard optical experiments, i.e., refers to ``hidden'' optics of microcavities. By using transient four-wave mixing and pump-probe spectroscopies, we infer that the radiative width, associated with excitonic molecules of the binding energy $\epsilon_{\rm XX} \simeq 0.9-1.1$ meV, is $\Gamma^{\rm MC}_{\rm XX} \simeq 0.2-0.3$ meV in the microcavities and $\Gamma^{\rm QW}_{\rm XX} \simeq 0.1$ meV in a reference GaAs single quantum well (QW). We show that for our high-quality quasi-two-dimensional nanostructures the $T_2 = 2 T_1$ limit, relevant to the XX states, holds at temperatures below 10 K, and that the bipolariton model of excitonic molecules explains quantitatively and self-consistently the measured XX radiative widths. We also find and characterize two critical points in the dependence of the radiative corrections against the microcavity detuning, and propose to use the critical points for high-precision measurements of the molecule bindingenergy and microcavity Rabi splitting.

Silicon-based electrically driven microcavity LED

Introduction: The realisation of Si-based, electrically driven light emitters is a key requirement for the implementation of low-cost Si-based optoelectronics [1]. Realising Si-based light sources in a process technology compatible with mainstream microelectronics technology is one of the big challenges of semiconductor technology. Because of its indirect bandgap Si has a low radiative recombination rate, leading to efficiencies of the bandgap electroluminescence (EL) in the range of 10 � 6 [2, 3]. Recently, different approaches have led to an increase of the power efficiency of the bandgap EL by more than three orders of magnitude up to values of 0.1‐1% [4‐6] .T hese approaches are based on pn-diodes, where either the non-radiative lifetime is increased by using high-purity floatzone Si, combined with surface texturing to improve the outcoupling efficiency [5] ,o r where specific defects introduced by ion implantation enhance the radiative recombination rate through carrier confinement effects [4, 6]. However, the spectral width and temporal response of these devices still constrains their practical application. One possible route for a further enhancement of the efficiency of these devices is a photonic confinement of the emitting layer. III‐V semiconductor based LEDs gained significantly in performance by incorporation into microcavities (MCs) [7]. Planar MCs enhance the brightness, efficiency and directionality of the emission from a high-index material and lead to more than an order of magnitude increase in the spectral power

Resonant acousto-optics of microcavity polaritons

We propose and analyse theoretically a resonant acousto-optic Stark effect for microcavity (MC) polaritons parametrically driven by a surface acoustic wave. For GaAs-based microcavities our scheme ‘acoustic pumping–optical probing’ deals with surface acoustic waves of frequency νSAW ≃ 0.5–3 GHz and intensity ISAW ≃ 0.1–10 mW mm−1. The acoustically-induced stop gaps in the MC polariton spectrum drastically change the optical response of MC polaritons. Because an acoustically pumped intrinsic semiconductor microcavity remains in its ground electronic state, no many-body effects screen and weaken the resonant acousto-optic Stark effect. In the meantime, this allows us to work out an exactly-solvable model for resonant acousto-optics of MC polaritons which deals with giant acousto-optical nonlinearities. Finally, we discuss possible applications of the proposed resonant acoustic Stark effect for optical modulation and switching and describe an acousto-optic device based on a (GaAs) microcavity driven by a surface acoustic wave.

Polaritons in microcavities containing a two-dimensional electron gas

We review the spectroscopic and dynamic properties of cavity polaritons in a λ-wide GaAs/GaAlAs microcavity (MC) structure having a single GaAs/AlAs quantum well in which a variable density two-dimensional electron gas (2DEG) is photoexcited, with a density in the range 0 < ne < 5 × 1011 cm−2. The main observed effects that will be discussed here are: (a) photoexcitation of negatively charged polaritons. These polaritons result of the coupling of the MC photon and the negatively charged excitons. They appear at low temperatures (T <25 K), in the range of low 2DEG densities (ne < 5 × 1010 cm−2), and their coupling strength increases as √ne. (b) The effect of electron–polariton scattering on the polariton spectral linewidth, its complex dependence on the detuning energy, and the electron-induced transition from strong to weak coupling regimes, as ne increases. (c) The efficient polariton transfer across the relaxation bottleneck in the lower branch induced by electron–polariton scattering, and the resulting non-linear dependence of the polariton photoluminescence intensity on ne.

Non-linear resonant Rayleigh scattering from microcavity

A strong superlinear dependence of the resonant Rayleigh scattering (RRS) from a semiconductor microcavity (MC) after cw excitation is observed. The effect is associated with transition from normal mode coupling (NMC) to weak coupling regime of the MC modes due to the exciton bleaching. Since the MC-RRS mainly arises from the scattering of the photonic part of the MC polariton and in NMC regime the RRS is quenched by the excitonic absorption, the loss of exciton oscillator strength produces the strong RRS nonlinearity.

Performance of integrated optical microcavities for refractive index and fluorescence sensing

The performance of a novel sensor based on an integrated optical (IO) disk microcavity (MC) is theoretically studied. The MC is a resonant waveguide structure in which multiple interference of a guided mode occurs. At resonance, the MC is extremely sensitive to refractive index changes and it sustains locally enhanced optical field. Therefore, the MC is naturally suitable as an extremely responsive sensor for measuring minute changes in refractive index and/or fluorescence of an analyte. Combination of multiple interference and extremely small sampling volume (few femtoliters) provides unique sensitivity of the MC device. A factor of 9 increase in fluorescence sensitivity versus waveguide sensor is anticipated. Provided shot-noise limited detection, the refractive index resolution down to 10 −9 is feasible with the MC sensor, which exceeds that of the straight waveguide interferometer by the order of magnitude. MC-sensors can be combined into arrays providing high throughput detection of both labeled and non-fluorescent biological species.