High-occupancy effects and stimulation phenomena in semiconductor microcavities

This paper describes recent work on high-occupancy effects in semiconductor microcavities, with emphasis on the variety of new physics and the potential for applications that has been demonstrated recently. It is shown that the ability to manipulate both exciton and photon properties, and how they interact together to form strongly coupled exciton-photon coupled modes, exciton polaritons, leads to a number of very interesting phenomena, which are either difficult or impossible to achieve in bulk semiconductors or quantum wells. The very low polariton density of states enables state occupancies greater than one to be easily achieved, and hence stimulation phenomena to be realized under conditions of resonant excitation. The particular form of the lower polariton dispersion curve in microcavities allows energy and momentum conserving polariton-polariton scattering under resonant excitation. Stimulated scattering of the bosonic quasi-particles occurs to the emitting state at the center of the Brillouin zone, and to a companion state at high wave vector. The stimulation phenomena lead to the formation of highly occupied states with macroscopic coherence in two specific regions of k space. The results are contrasted with phenomena that occur under conditions of nonresonant excitation. Prospects to achieve "polariton lasing" under nonresonant excitation, and high-gain, room-temperature ultrafast amplifiers and low-threshold optical parametric oscillator under resonant excitation conditions are discussed

Control of spontaneous emission from InP single quantum dots in GaInP photonic crystal nanocavities

We demonstrate semiconductor quantum dots coupled to photonic crystal cavity modes operating in the visible spectrum. We present the design, fabrication, and characterization of two dimensional photonic crystal cavities in GaInP and measure quality factors in excess of 7500 at 680 nm. We demonstrate full control over the spontaneous emission rate of InP quantum dots and by spectrally tuning the exciton emission energy into resonance with the fundamental cavity mode we observe a Purcell enhancement of similar to 8. (C) 2010 American Institute of Physics. [doi:10.1063/1.3510469

Spin-order-dependent magneto-elastic coupling in two dimensional antiferromagnetic MnPSe3 observed through Raman spectroscopy

Layered antiferromagnetic materials have recently emerged as an intriguing subset of the two-dimensional family providing a highly accessible regime with prospects for layer-number-dependent magnetism. Furthermore, transition metal phosphorus trichalcogenides, MPX3 (M = transition metal; X = chalcogen) provide a platform on which to investigate fundamental interactions between magnetic and lattice degrees of freedom and further explore the developing fields of spintronics and magnonics. Here, we use a combination of temperature dependent Raman spectroscopy and density functional theory to explore magnetic-ordering-dependent interactions between the manganese spin degree of freedom and lattice vibrations of the non-magnetic sub-lattice via a Kramers-Anderson super-exchange pathway in both bulk, and few-layer, manganese phosphorus triselenide (MnPSe3). We observe a nonlinear temperature-dependent shift of phonon modes predominantly associated with the non-magnetic sub-lattice, revealing their non-trivial spin-phonon coupling below the Néel temperature at 74 K, allowing us to extract mode-specific spin-phonon coupling constants

Dynamic nuclear polarization in InGaAs/GaAs and GaAs/AlGaAs quantum dots under nonresonant ultralow-power optical excitation

We study experimentally the dependence of dynamic nuclear spin polarization on the power of nonresonant optical excitation in two types of individual neutral semiconductor quantum dots: InGaAs/GaAs and GaAs/AlGaAs. We show that the mechanism of nuclear spin pumping via second-order recombination of optically forbidden (“dark”) exciton states recently reported in InP/GaInP quantum dots [E. A. Chekhovich et al., Phys. Rev. B 83, 125318 (2011)] is relevant for material systems considered in this work. In the InGaAs/GaAs dots this nuclear spin polarization mechanism is particularly pronounced, resulting in Overhauser shifts up to ∼80 μeV achieved at ultralow optical excitation power, ∼1000 times smaller than the power required to saturate ground state excitons. The Overhauser shifts observed at ultralow power pumping in the interface GaAs/AlGaAs dots are generally found to be smaller (up to ∼40 μeV). Furthermore in GaAs/AlGaAs we observe dot-to-dot variation and even sign reversal of the Overhauser shift which is attributed to the dark-bright exciton mixing originating from electron-hole exchange interaction in dots with reduced symmetry. Nuclear spin polarization degrees reported in this work under ultralow-power optical pumping are comparable to those achieved by techniques such as resonant optical pumping or above-gap pumping with high-power circularly polarized light. Dynamic nuclear polarization via second-order recombination of “dark” excitons may become a useful tool in single quantum dot applications, where manipulation of the nuclear spin environment or electron spin is required

Anomalous Stark Shifts in Single Vertically Coupled Pairs of InGaAs Quantum Dots

Vertically coupled Stranski Krastanow QDs are predicted to exhibit strong tunnelling interactions that lead to the formation of hybridised states. We report the results of investigations into single pairs of coupled QDs in the presence of an electric field that is able to bring individual carrier levels into resonance and to investigate the Stark shift properties of the excitons present. Pronounced changes in the Stark shift behaviour of exciton features are identified and attributed to the significant redistribution of the carrier wavefunctions as resonance between two QDs is achieved. At low electric fields coherent tunnelling between the two QD ground states is identified from the change in sign of the permanent dipole moment and dramatic increase of the electron polarisability, and at higher electric fields a distortion of the Stark shift is attributed to a coherent tunnelling effect between the ground state of the upper QD and the excited state of the lower QD.Comment: Conference paper for QD2004 3 figure

Characterization and quantification of oxidative stress induced particle debris from polypropylene surgical mesh

Explanted polypropylene (PP) surgical mesh has frequently been reported to show surface alterations, such as cracks and flaking. However, to date the consequence of PP mesh degradation is not clearly understood, particularly its potential to influence the biological host response of surrounding tissues. Of particular concern is a possible host reaction to polypropylene particles released through degradation of surgical mesh in vivo. This concern is driven by previous studies which have postulated that an oxidative stress environment has the potential to etch away particles from the surface of a PP fibers. The release of such particles is of considerable significance as particles in the nano- to micro range have been shown to have the capacity to irritate cells and stimulate the immune system. The authors are not aware of any previous studies that have attempted to characterize, quantify or identify any particles released from PP mesh after exposure to an oxidative stress environment. Characterization of the PP mesh, post oxidative stress exposure, including identification of particles was achieved through application of a range of techniques: low voltage-scanning electron microscopy (LV-SEM), pyrolysis gas chromatography mass spectrometry (Pyr-GCMS), nano-Fourier transform infrared spectroscopy (nano-FTIR), scattering-type, scanning near-field optical microscopy (s-SNOM), atomic force microscopy (AFM), attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) and uniaxial tensile testing. The findings of this study indicate that oxidative stress alone is a major factor in the production of PP particle debris. PP debris identified within solution, using Pyr-GCMS, was shown to be in order of the micron scale

Electrically pumped single-defect light emitters in WSe2_2

Recent developments in fabrication of van der Waals heterostructures enable new type of devices assembled by stacking atomically thin layers of two-dimensional materials. Using this approach, we fabricate light-emitting devices based on a monolayer WSe$_2$, and also comprising boron nitride tunnelling barriers and graphene electrodes, and observe sharp luminescence spectra from individual defects in WSe$_2$ under both optical and electrical excitation. This paves the way towards the realization of electrically-pumped quantum emitters in atomically thin semiconductors. In addition we demonstrate tuning by more than 1 meV of the emission energy of the defect luminescence by applying a vertical electric field. This provides an estimate of the permanent electric dipole created by the corresponding electron-hole pair. The light-emitting devices investigated in our work can be assembled on a variety of substrates enabling a route to integration of electrically pumped single quantum emitters with existing technologies in nano-photonics and optoelectronics

Ultrafast exciton and trion dynamics in high-quality encapsulated MoS2 monolayers

The extreme confinement and reduced screening in monolayer transition metal dichalcogenides (TMDs) leads to the appearance of tightly bound excitons which can also couple to free charges, forming trions, owing to strong Coulomb interactions. Low temperatures and encapsulation in hexagonal boron nitride (hBN) can narrow the excitonic linewidth, approaching the regime of homogeneous broadening, mostly dominated by the radiative decay. Ultrafast spectroscopy is a perfect tool to study exciton formation and relaxation dynamics in TMD monolayers. However, high-quality hBN-encapsulated structures have usually lateral sizes of the order of a few micrometers, calling for the combination of high spatial and temporal resolution in pump–probe experiments. Herein, a custom broadband pump–probe optical microscope is used to measure the ultrafast dynamics of neutral and charged excitons in high-quality hBN-encapsulated monolayer MoS2 at 8 K. Neutral excitons exhibit a narrow linewidth of 7.5 meV, approaching the homogeneous limit, which is related to the fast recombination time of ≈130 fs measured in pump–probe. Moreover, markedly different dynamics of the trions over the neutral ones are observed. The results provide novel insights on the exciton recombination processes in TMD monolayers, paving the way for exploring the ultrafast behavior of excitons and their many-body complexes in TMD heterostructures

On-chip waveguide coupling of a layered semiconductor single photon source

Fully integrated quantum technology based on photons is in the focus of current research, because of its immense potential concerning performance and scalability. Ideally, the single-photon sources, the processing units, and the photon detectors are all combined on a single chip. Impressive progress has been made for on-chip quantum circuits and on-chip single-photon detection. In contrast, nonclassical light is commonly coupled onto the photonic chip from the outside, because presently only few integrated single-photon sources exist. Here, we present waveguide-coupled single-photon emitters in the layered semiconductor gallium selenide as promising on-chip sources. GaSe crystals with a thickness below 100 nm are placed on Si3N4 rib or slot waveguides, resulting in a modified mode structure efficient for light coupling. Using optical excitation from within the Si3N4 waveguide, we find nonclassicality of generated photons routed on the photonic chip. Thus, our work provides an easy-to-implement and robust light source for integrated quantum technology

Single-photon emitters in GaSe

Single-photon sources are important building blocks for quantum information technology. Emitters based on solid-state systems provide a viable route to integration in photonic devices. Here, we report on single-photon emitters in the layered semiconductor GaSe. We identify the exciton and biexciton transition of the quantum emitters with power-dependent photoluminescence and photon statistics measurements. We find evidence that the localization of the excitons is related to deformations of the GaSe crystal, caused by nanoscale selenium inclusions, which are incorporated in the crystal. These deformations give rise to local strain fields, which induce confinement potentials for the excitons. This mechanism lights the way for the controlled positioning of single-photon emitters in GaSe on the nanoscale