Microcavity Samples Research Articles

Electrically Controlling Vortices in a Neutral Exciton-Polariton Condensate at Room Temperature.

Manipulating bosonic condensates with electric fields is very challenging as the electric fields do not directly interact with the neutral particles of the condensate. Here we demonstrate a simple electric method to tune the vorticity of exciton-polariton condensates in a strong coupling liquid crystal (LC) microcavity with CsPbBr_{3} microplates as active material at room temperature. In such a microcavity, the LC molecular director can be electrically modulated giving control over the polariton condensation in different modes. For isotropic nonresonant optical pumping we demonstrate the spontaneous formation of vortices with topological charges of +1, +2, -2, and -1. The topological vortex charge is controlled by a voltage in the range of 1 to 10V applied to the microcavity sample. This control is achieved by the interplay of a built-in potential gradient, the anisotropy of the optically active perovskite microplates, and the electrically controllable LC molecular director in our system with intentionally broken rotational symmetry. Besides the fundamental interest in the achieved electric polariton vortex control at room temperature, our work paves the way to micron-sized emitters with electric control over the emitted light's phase profile and quantized orbital angular momentum for information processing and integration into photonic circuits.

Inverse-phase Rabi oscillations in semiconductor microcavities

We study experimentally the oscillations of a non stationary transient signal of a semiconductor microcavity with embedded InGaAs quantum wells. The oscillations occur as a result of quantum beats between the upper and lower polariton modes due to the strong exciton-photon coupling in the microcavity sample (Rabi oscillations). The registration of spectrally resolved signal has allowed for separate observation of oscillations at the eigenfrequencies of two polariton modes. Surprisingly, the observed oscillations measured at the lower and upper polariton modes have opposite phases. We demonstrate theoretically that the opposite-phase oscillations are caused by the pump-induced modification of polariton Hopfield coefficients, which govern the ratio of exciton and photon components in each of the polartion modes. Such a behaviour is a fundamental feature of the quantum beats of coupled light-matter states. In contrast, the reference pump-probe experiment performed for the pure excitonic states in a quantum well heterostructure with no microcavity revealed the in-phase oscillations of the pump-probe signals measured at different excitonic levels.

Nuclear spin relaxation inn-GaAs: From insulating to metallic regime

Nuclear spin relaxation is studied in n-GaAs thick layers and microcavity samples with different electron densities. We reveal that both in metallic samples where electrons are free and mobile, and in insulating samples, where electrons are localized, nuclear spin relaxation is strongly enhanced at low magnetic field. The origin of this effect could reside in the quadrupole interaction between nuclei and fluctuating electron charges, that has been proposed to drive nuclear spin dynamics at low magnetic fields in the insulating samples. The characteristic values of these magnetic fields are given by dipole-dipole interaction between nuclei in bulk samples, and are greatly enhanced in microcavities, presumably due to additional strain, inherent to micro and nanostructures.

Observation of a hybrid state of Tamm plasmons and microcavity exciton polaritons.

We present evidence for the existence of a hybrid state of Tamm plasmons and microcavity exciton polaritons in a II-VI material based microcavity sample covered with an Ag metal layer. The bare cavity mode shows a characteristic anticrossing with the Tamm-plasmon mode, when microreflectivity measurements are performed for different detunings between the Tamm plasmon and the cavity mode. When the Tamm-plasmon mode is in resonance with the cavity polariton four hybrid eigenstates are observed due to the coupling of the cavity-photon mode, the Tamm-plasmon mode, and the heavy- and light-hole excitons. If the bare Tamm-plasmon mode is tuned, these resonances will exhibit three anticrossings. Experimental results are in good agreement with calculations based on the transfer matrix method as well as on the coupled-oscillators model. The lowest hybrid eigenstate is observed to be red shifted by about 13 meV with respect to the lower cavity polariton state when the Tamm plasmon is resonantly coupled with the cavity polariton. This spectral shift which is caused by the metal layer can be used to create a trapping potential channel for the polaritons. Such channels can guide the polariton propagation similar to one-dimensional polariton wires.

Dynamics of the energy relaxation in a parabolic quantum well laser

We explore two parabolic quantum well (PQW) samples, with and without Bragg mirrors, in order to optimise the building blocks of a Bosonic Cascade Laser. The photoluminescence spectra of a PQW microcavity sample is compared against that of a conventional microcavity with embedded quantum wells (QWs) to demonstrate that the weak coupling lasing in a PQW sample can be achieved. The relaxation dynamics in a conventional QW microcavity and in the PQW microcavity was studied by a non-resonant pump-pump excitation method. Strong difference in the relaxation characteristics between the two samples was found. The semi-classical Boltzmann equations were adapted to reproduce the evolution of excitonic populations within the PQW as a function of the pump power and the output intensity evolution as a function of the pump-pump pulse delay. Fitting the PQW data confirms the anticipated cascade relaxation, paving the way for such a system to produce terahertz radiation.

Tailoring the optical properties of wide-bandgap based microcavities via metal films

We report on the tuning of the optical properties of II-VI-material-based microcavity samples, which is achieved by depositing Ag films on top of the structures. The micro-reflectivity spectra show a spectral shift of the sample resonance dependent on the metal layer thickness. By comparison of the experimental findings with the theoretical calculations applying the transfer matrix method on a metal-dielectric mirror structure, the influence of the metal layer particularly with regard to its partial oxidation was explored. Tamm plasmon modes are created at the interface between an open cavity with three ZnSe quantum wells and a metal layer on top. When tuning the excitonic emission relative to the mode by changing the sample temperature, an anticrossing of the resonances was observed. This is a clear indication that the strong coupling regime has been achieved in that sample configuration yielding a Rabi splitting of 18.5 meV. These results are promising for the realization of polariton-based optical devices with a rather simple sample configuration.

Algebraic order and the Berezinskii-Kosterlitz-Thouless transition in an exciton-polariton gas

We observe quasi-long range coherence in a two-dimensional condensate of exciton-polaritons. Our measurements are the first to confirm that the spatial correlation algebraically decays with a slow power-law, whose exponent quantitatively behaves as predicted by the Berezinskii-Kosterlitz-Thouless theory. The exciton-polaritons are created by non-resonant optical pumping of a micro-cavity sample with embedded GaAs quantum-wells at liquid helium temperature. Michelson interference is used to measure the coherence of the photons emitted by decaying exciton-polaritons.

Room temperature Bloch surface wave polaritons

Polaritons are hybrid light-matter quasi-particles that have gathered a significant attention for their capability of showing room temperature and out-of-equilibrium Bose-Einstein condensation. More recently, a novel class of ultrafast optical devices have been realized by using flows of polariton fluids, such as switches, interferometers, and logical gates. However, polariton lifetimes and propagation distances are strongly limited by photon losses and accessible in-plane momenta in normal microcavity samples. In this work, we show experimental evidence of the formation of room temperature propagating polariton states arising from the strong coupling between organic excitons and a Bloch surface wave. This result, which was only recently predicted, paves the way for the realization of polariton devices that could allow lossless propagation up to macroscopic distances.

Adjustable YAG : Ce3+ photoluminescence from photonic crystal microcavity

Four different photonic bandgap (PBG) structures embedding a YAG : Ce3+ layer inside two three-period Bragg mirrors were prepared by sol–gel processing, forming Fabry–Perot microcavities whose defect peaks moved from red to green. Under irradiation of blue Ar+ laser light, the typical broad YAG : Ce3+ photoluminescence (PL) emission band was highly narrowed in these four samples, with the new position of the modified PL peaks corresponding to the resonance wavelength of each microcavity sample, while the simultaneous colour changes could be easily observed by the human eye. The adjustable range demonstrated here was wide enough to generate white light with colour temperatures from warm white (∼2700 K) to daylight white (∼5600 K), by mixing the modified PL with light from any usual blue LED excitation source. This result provides a novel technique to solve the red-deficiency problem in the white LED industry: instead of relying on the development of new phosphors, the well-known PL of YAG : Ce3+ can be conveniently adjusted by 1D PBG structures.

Coherence signatures and density-dependent interaction in a dynamical exciton-polariton condensate

We report on pump-power-dependent emission features of a nonthermalized and interacting dynamical condensate of exciton polaritons. The system based on a planar AlAs/AlGaAs microcavity sample with twelve GaAs quantum wells in the active region is investigated comprehensively by measuring the energy-momentum dispersion characteristics, the spatial coherence, and the photon statistics under resonant, fs-pulsed optical excitation at high momentum. We observe a significant polariton-population-dependent modification of the emission signatures above the quantum degeneracy. The nonequilibrium polariton condensate is confirmed by its polaritonic energy-momentum dispersion as well as excitation-power-dependent coherence properties and its photon statistics, being different from that of an ideal coherent state. The polaritonic condensate is characterized by a spatial coherence length of up to 4 \ensuremath{\mu}m and a super-Poissonian photon statistic of the emitted light well above threshold. Results obtained in second-order photon autocorrelation measurements in momentum-space resolved spectroscopy indicate polariton repulsive interaction throughout the condensate and a spatial coherence length being shorter than the condensate extension of 20--30 \ensuremath{\mu}m.

Penrose-Onsager Criterion Validation in a One-Dimensional Polariton Condensate

We perform quantum tomography on one-dimensional polariton condensates, spontaneously occurring in linear disorder valleys in a CdTe planar microcavity sample. By the use of optical interferometric techniques, we determine the first-order coherence function and the amplitude and phase of the order parameter of the condensate, providing a full reconstruction of the single particle density matrix for the polariton system. The experimental data are used as input to theoretically test the consistency of the Penrose-Onsager criterion for Bose-Einstein condensation in the framework of nonequilibrium polariton condensates. The results confirm the pertinence and validity of the criterion for a nonequilibrium condensed gas.

Photon lasing inGaAsmicrocavity: Similarities with a polariton condensate

We study experimentally the lasing regime of a $\mathrm{GaAs}$ based microcavity sample under strong optical pumping. The very same sample exhibits the strong coupling regime at low excitation power with a Rabi splitting as large as $15\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. We show that some features which may be considered as experimental evidence of polariton Bose-Einstein condensation are also observed in the weak coupling regime when the cavity is behaving as a regular photon laser. In particular, the emission pattern in the lasing regime displays a sharp peak near the energy minimum followed by a Boltzmann distribution at higher energies.

On the way to InGaN quantum dots embedded into monolithic nitride cavities

AbstractWe present photoluminescence measurements on single InGaN quantum dots (QDs) grown by metalorganic vapor phase epitaxy, and on monolithicly grown GaN‐based quantum well airpost pillar microcavities. The observed sharp emission lines of the quantum dots are characterized by excitation density dependent measurements. The photoluminescence of individual quantum dots can easily be detected for temperatures up to 150 K. The micro‐photoluminescence measurements on microcavities reveal three‐dimensional confined optical modes which are not seen in the luminescence of the simply planar cavity. The realization of rather temperature stable QDs as well as of nitride based microcavity samples are promising with respect to the intended implementation of QD layers into microcavities. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Observation of the Purcell effect in high-index-contrast micropillars

The authors have fabricated pillar microcavity samples with Bragg mirrors consisting of alternate layers of GaAs and aluminum oxide. Compared to the more widely studied GaAs∕AlAs micropillars, these mirrors can achieve higher reflectivities with fewer layer repeats and reduce the mode volume. The authors have studied a number of samples containing a low density of InGaAs∕GaAs self-assembled quantum dots in a cavity and here report observation of a threefold enhancement in the radiative lifetime of a quantum dot exciton state due to the Purcell effect.

Conformal coverage for two-dimensional arrays of microcavites with quasi-three dimensional confinement by distributed Bragg reflectors

Different from conventional three-dimensional confined microcavity fabrication method in which micropillar microcavities were obtained through the etching of planar semicoductor microcavities, we adopted the conformal coverage to fabricate two-dimensional arrays of quasi three-dimensional confined optical microcavities providing both vertical and lateral optical confinement by the distributed Bragg reflectors (DBRs). Our microcavity samples were directly deposited on the patterned substrates with two-dimensional arrays of air holes. The SEM and cross-section TEM images show that the periodicity of the patterned substrate was still kept after deposition while the growth of DBRs along the sidewalls occurred simultaneously, which provided the transverse optical confinement. In order to probe the optical modes of this kind of microcavities, room temperature photoluminescence signals from prepared microcavities were detected. Three resonant modes were presented and exhibited obvious angular dependence. We attributed these phenomena to quantization of the in-plane wave vector components confined by lateral DBRs.

Dynamics of microcavity polaritons in the presence of an electron gas

The dynamics of microcavity polaritons in the presence of an electron gas is experimentally studied through time-resolved photoluminescence under nonresonant excitation. Electrons are shown to accelerate the emission dynamics of the whole system, both in the polariton trap close to the center of the Brillouin zone $(k=0)$ and in the reservoir of large in-plane wave-vector excitonic states. By studying both the bare quantum well and the microcavity sample, the observed features are explained by the interplay between the physics of excitons and that of polariton relaxation.

Spontaneous phase condensation of CdTe exciton‐polaritons

AbstractWe have studied CdTe/CdMgTe microcavity samples containing 4 quantum wells and characterized by a vacuum field Rabi‐splitting of 13.2 meV. Electron‐hole pairs were excited at 7.3K by linearly‐polarized light at 1.74 eV provided by a Ti:sapphire laser operating in the pulsed mode (at 80 MHz rate). Spectroscopic imaging in reciprocal space as well as near‐field spectroscopy show that, above a critical stimulation threshold, polaritons in our microcavity undergo a spontaneous transition to a condensed phase characterized by the massive occupation of the polariton ground state. The coherence of this quantum state is evidenced by the spectral and angular narrowing of its emission. We point out the conditions which should be fulfilled to achieve BEC of microcavity polaritons. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Surface plasmon coupling in hexagonal textured metallic microcavity

The coupling of surface plasmons to the photonic modes in hexagonal textured metallic microcavity was studied. Two types of sample structures (symmetric and asymmetric) were investigated. Both angle-resolved transmission and photoluminescent measurements showed the presence of surface plasmon modes for the textured metallic microcavity samples. For the asymmetric structured sample, the bandgap was observed at the Brillouin zone edge. The simulation of the photonic band structure using decoupled approximation for the standing wave modes agrees with the experimental result. We have derived the surface modes due to various interfaces in the microcavity. It was found that the surface plasmon modes from metal/air interface are most dominant, and was observed to couple strongly with both the transverse electric and transverse magnetic modes. Light extraction enhancement due to surface plasmon coupling was achieved with directional enhancement as much as 12 times compared with planar microcavity for the symmetric structured sample.

On-demand single-photon source for 1.3μm telecom fiber

We demonstrate an on-demand single-photon source that is compatible with standard telecom optical fiber. Through careful control of the critical strains of InAs∕GaAs self-assembled quantum dots, we produce a microcavity sample with a low density of large dots emitting into the fiber-optic transmission band at 1.3μm. The second-order correlation function of the source reveals a strong suppression in the rate of multiphoton pulses at both 5K and above 30K. The source may be useful for fiber-optic-based single-photon applications, such as quantum metrology, quantum communications, and distributed quantum computing.

Active textured metallic microcavity

Electron-beam lithography was used to create a two-dimensional wavelength-scale periodic texture on one of the cavity mirrors with nanometer precision. In the microcavity, Bragg scattering is expected to provide a full continuous optical band gap and flat dispersion of optical waveguide modes in the visible regime. When the luminescing material is placed inside the 2D patterned microcavity, the coupling of the luminescing matter to the photonic eigenmodes of the metallic microcavity is observed. We found that planar microcavity transmission result is in good agreement with the theoretical calculation. For the laterally structured metallic microcavity samples, we found that the dispersion of optical modes inside the microcavity is significantly altered compared with the planar metallic microcavity.