Ayan Das

Electrically driven polarized single-photon emission from an InGaN quantum dot in a GaN nanowire

In a classical light source, such as a laser, the photon number follows a Poissonian distribution. For quantum information processing and metrology applications, a non-classical emitter of single photons is required. A single quantum dot is an ideal source of single photons and such single-photon sources in the visible spectral range have been demonstrated with III-nitride and II-VI-based single quantum dots. It has been suggested that short-wavelength blue single-photon emitters would be useful for free-space quantum cryptography, with the availability of high-speed single-photon detectors in this spectral region. Here we demonstrate blue single-photon emission with electrical injection from an In0.25Ga0.75N quantum dot in a single nanowire. The emitted single photons are linearly polarized along the c axis of the nanowire with a degree of linear polarization of ~70%.

Blue single photon emission up to 200 K from an InGaN quantum dot in AlGaN nanowire

We demonstrate polarized blue single photon emission up to 200u2009K from an In0.2Ga0.8N quantum dot in a single Al0.1Ga0.9N nanowire. The InGaN/AlGaN dot-in-nanowire heterostructure was grown on (111) silicon by plasma assisted molecular beam epitaxy. Nanowires dispersed on a silicon substrate show sharp exciton and biexciton transitions in the micro-photoluminescence spectra. Second-order correlation measurements performed under pulsed excitation at the biexciton wavelength confirm single photon emission, with a g(2)(0) of 0.43 at 200u2009K. The emitted photons have a short radiative lifetime of 0.7u2009ns and are linearly polarized along the c-axis of the nanowire with a degree of polarization of 78%.

Polariton Bose–Einstein condensate at room temperature in an Al(Ga)N nanowire–dielectric microcavity with a spatial potential trap

A spatial potential trap is formed in a 6.0-μm Al(Ga)N nanowire by varying the Al composition along its length during epitaxial growth. The polariton emission characteristics of a dielectric microcavity with the single nanowire embedded in-plane have been studied at room temperature. Excitation is provided at the Al(Ga)N end of the nanowire, and polariton emission is observed from the lowest bandgap GaN region within the potential trap. Comparison of the results with those measured in an identical microcavity with a uniform GaN nanowire and having an identical exciton–photon detuning suggests evaporative cooling of the polaritons as they are transported into the trap in the Al(Ga)N nanowire. Measurement of the spectral characteristics of the polariton emission, their momentum distribution, first-order spatial coherence, and time-resolved measurements of polariton cooling provides strong evidence of the formation of a near-equilibrium Bose–Einstein condensate in the GaN region of the nanowire at room temperature. In contrast, the condensate formed in the uniform GaN nanowire–dielectric microcavity without the spatial potential trap is only in self-equilibrium.

Quantum dot polarized light sources

The design, operation and performance of quantum dot spin-polarized vertical cavity surface emitting lasers (VCSELs) and single-photon sources are described and discussed. The effects of spin-induced gain anisotropy on output polarization and threshold current reduction have been studied along with the high-frequency response in a spin-polarized VCSEL. While the output circular polarization in a VCSEL follows the out-of-plane magnetization characteristics of the ferromagnetic spin injector, the output polarization of the spin-polarized single-photon source shows a switching behavior which is explained by invoking the exciton fine structure in the quantum dots and the effects of electron–hole exchange splitting due to in-plane quantum dot rotational asymmetry.

Room temperature strong coupling effects from single ZnO nanowire microcavity

Strong coupling effects in a dielectric microcavity with a single ZnO nanowire embedded in it have been investigated at room temperature. A large Rabi splitting of ~100 meV is obtained from the polariton dispersion and a non-linearity in the polariton emission characteristics is observed at room temperature with a low threshold of 1.63 μJ/cm(2), which corresponds to a polariton density an order of magnitude smaller than that for the Mott transition. The momentum distribution of the lower polaritons shows evidence of dynamic condensation and the absence of a relaxation bottleneck. The polariton relaxation dynamics were investigated by time-resolved measurements, which showed a progressive decrease in the polariton relaxation time with increase in polariton density.

An electrically injected quantum dot spin polarized single photon source

The characteristics of an electrically injected spin polarized single photon source have been investigated. The GaAs-based microcavity diode consists of a single InAs/GaAs self-organized quantum dot as the single photon source and a MnAs/Al0.1Ga0.9As Schottky tunnel barrier for the ferromagnetic contact to inject spin polarized electrons. The measured output circular polarization of the biexciton emission at λ∼1130u2002nm exhibits a switching behavior as a function of magnetic field, in the Faraday geometry, the value remaining near-zero for B<1u2002T and ∼6%–8% for B≥1u2002T. The linear polarization shows a complementary trend. The results are explained in the framework of the exciton fine structure in the quantum dot and the effects of electron-hole exchange splitting, due to in-plane quantum dot anisotropy, and Zeeman splitting on the spin eigenstates and their coupling to the photon field.

Polariton emission characteristics of a modulation-doped multiquantum-well microcavity diode

The role of polariton-electron scattering on the performance characteristics of an electrically injected GaAs-based quantum well (QW) microcavity diode in the strong coupling regime has been investigated. An electron gas is introduced in the quantum wells by modulation doping with silicon dopants. It is observed that polariton-electron scattering suppresses the relaxation bottleneck in the lower polariton branch. However, it is not adequate to produce a degenerate coherent condensate at k∥u2009∼u20090 and coherent emission.

Room temperature polariton lasing from a single GaN nanowire microcavity

The paper demonstrates an ultra-low threshold polariton laser by using a single GaN nanowire as the active medium embedded in a dielectric microcavity. We have also studied the effect of detuning on the lasing characteristics.

GaAs electrically injected exciton-polariton laser

Electrically injected polariton lasing from a GaAs-based modulation-doped microcavity diode has been demonstrated under an applied magnetic field of 7 Tesla at 30 K. Polariton lasing and condensation characteristics have been measured and analyzed.

Electrically injected polariton lasing from a GaAs-based microcavity under magnetic field

Suppression of relaxation bottleneck and subsequent polariton lasing is observed in a GaAs-based microcavity under the application of a magnetic field. The threshold injection current density is 0.32 A/cm2 at 7 Tesla.