Bright Photons Research Articles

Electrically-driven ultrafast out-of-equilibrium light emission from hot electrons in suspended graphene/hBN heterostructures

Nanoscale light sources with high speed of electrical modulation and low energy consumption are key components for nanophotonics and optoelectronics. The record-high carrier mobility and ultrafast carrier dynamics of graphene make it promising as an atomically thin light emitter, which can be further integrated into arbitrary platforms by van der Waals forces. However, due to the zero bandgap, graphene is difficult to emit light through the interband recombination of carriers like conventional semiconductors. Here, we demonstrate ultrafast thermal light emitters based on suspended graphene/hexagonal boron nitride (Gr/hBN) heterostructures. Electrons in biased graphene are significantly heated up to 2800 K at modest electric fields, emitting bright photons from the near-infrared to the visible spectral range. By eliminating the heat dissipation channel of the substrate, the radiation efficiency of the suspended Gr/hBN device is about two orders of magnitude greater than that of graphene devices supported on SiO2 or hBN. We further demonstrate that hot electrons and low-energy acoustic phonons in graphene are weakly coupled to each other and are not in full thermal equilibrium. Direct cooling of high-temperature hot electrons to low-temperature acoustic phonons is enabled by the significant near-field heat transfer at the highly localized Gr/hBN interface, resulting in ultrafast thermal emission with up to 1 GHz bandwidth under electrical excitation. It is found that suspending the Gr/hBN heterostructures on the SiO2 trenches significantly modifies the light emission due to the formation of the optical cavity and showed a ∼440% enhancement in intensity at the peak wavelength of 940 nm compared to the black-body thermal radiation. The demonstration of electrically driven ultrafast light emission from suspended Gr/hBN heterostructures sheds the light on applications of graphene heterostructures in photonic integrated circuits, such as broadband light sources and ultrafast thermo-optic phase modulators.

(Invited) Quantum Defects as Sensors

Organic color-centers are molecularly tunable quantum defects that can be synthetically introduced into semiconducting carbon nanotube hosts. Charge carriers in the form of electron-hole pairs (or excitons) are efficiently harvested at these atomic defect sites to emit as bright shortwave-infrared photons that carry local chemical information. These intriguing properties open new possibilities of pushing the limits of sensing and imaging. In this talk, we will discuss recent advances as well as the challenges and opportunities at this emerging frontier.

High-spectral-purity photon generation from a dual-interferometer-coupled silicon microring

We experimentally demonstrate a high-spectral-purity photon source by designing a dual-Mach–Zehnder-interferometer-coupled silicon ring resonator, wherein the linewidths of pump and signal (idler) resonances can be engineered independently. A high spectral purity of 95 % ± 1.5 % is obtained via a time-integrated g ( 2 ) correlation measurement, which exceeds the theoretical 93% bound of a traditional ring’s spontaneous four-wave-mixing photon source. This source also possesses high performance in other metrics including a measured coincidence of 9599 pairs/s and a preparation heralding efficiency of 52.4% at a relatively low pump power of 61 µW as well as high drop-to-through suppression of 20.2 dB. By overcoming the trade-off between spectral purity and brightness in the post-filtering way, such a method guarantees bright pure photons and will pave the way for development of on-chip quantum information processing with improved operation fidelity and efficiency.

Constraints on alternative theories of gravity with observations of the Galactic Center

To evaluate a potential usually one analyzes trajectories of test particles. For the Galactic Center case astronomers use bright stars or photons, so there are two basic observational techniques to investigate a gravitational potential, namely, (a) monitoring the orbits of bright stars near the Galactic Center as it is going on with 10m Keck twin and four 8m VLT telescopes equipped with adaptive optics facilities (in addition, recently the IR interferometer GRAVITY started to operate with VLT); (b) measuring the size and shape of shadows around black hole with VLBI-technique using telescopes operating in mm-band. At the moment, one can use a small relativistic correction approach for stellar orbit analysis, however, in the future the approximation will not be precise enough due to enormous progress of observational facilities and recently the GRAVITY team found that the first post-Newtonian correction has to be taken into account for the gravitational redshift in the S2 star orbit case. Meanwhile for smallest structure analysis in VLBI observations one really needs a strong gravitational field approximation. We discuss results of observations and their interpretations. In spite of great efforts there is a very slow progress to resolve dark matter (DM) and dark energy (DE) puzzles and in these circumstances in last years a number of alternative theories of gravity have been proposed. Parameters of these theories could be effectively constrained with of observations of the Galactic Center. We show some cases of alternative theories of gravity where their parameters are constrained with observations, in particular, we consider massive theory of gravity. We choose the alternative theory of gravity since there is a significant activity in this field and in the last years theorists demonstrated an opportunity to create such theories without ghosts, on the other hand, recently, the joint LIGO & Virgo team presented an upper limit on graviton mass such as mg< 1:2 × 10-22eV [1] analyzing gravitational wave signal in their first paper where they reported about the discovery of gravitational waves from binary black holes as it was suggested by C. Will [2]. So, the authors concluded that their observational data do not indicate a significant deviation from classical general relativity. We show that an analysis of bright star trajectories could estimate a graviton mass with a commensurable accuracy in comparison with an approach used in gravitational wave observations and the estimates obtained with these two approaches are consistent. Therefore, such an analysis gives an opportunity to treat observations of bright stars near the Galactic Center as a useful tool to obtain constraints on the fundamental gravity law. We showed that in the future graviton mass estimates obtained with analysis of trajectories of bright stars would be better than current LIGO bounds on the value, therefore, based on a potential reconstruction at the Galactic Center we obtain bounds on a graviton mass and these bounds are comparable with LIGO constraints. Analyzing size of shadows around the supermassive black hole at the Galactic Center (or/and in the center of M87) one could constrain parameters of different alternative theories of gravity as well.

Quantum interference between autonomous single-photon sources from Doppler-broadened atomic ensembles

To realize a quantum network based on quantum entanglement swapping, bright and completely autonomous sources are essentially required. Here, we experimentally demonstrate Hong-Ou-Mandel (HOM) quantum interference between two independent bright photon pairs generated via the spontaneous four-wave mixing in Doppler-broadened ladder-type 87Rb atoms. Bright autonomous heralded single photons are operated in a continuous-wave (CW) mode with no synchronization or supplemental filters. The four-fold photon coincidence counts per hour correspond to 5000 events for a temporal range of 10 ns, one order higher than those of previously reported for autonomous single photons. We observe an HOM dip with 83% visibility with the two autonomous heralded single photons for an effective measurement time of 900 s. The achievement of the HOM interference between the two bright autonomous photon-pair sources has made an important step towards a practical scalable quantum network.

Single shot, double differential spectral measurements of inverse Compton scattering in the nonlinear regime

Inverse Compton scattering (ICS) is a unique mechanism for producing fast pulses---picosecond and below---of bright photons, ranging from x to $\ensuremath{\gamma}$ rays. These nominally narrow spectral bandwidth electromagnetic radiation pulses are efficiently produced in the interaction between intense, well-focused electron and laser beams. The spectral characteristics of such sources are affected by many experimental parameters, with intense laser effects often dominant. A laser field capable of inducing relativistic oscillatory motion may give rise to harmonic generation and, importantly for the present work, nonlinear redshifting, both of which dilute the spectral brightness of the radiation. As the applications enabled by this source often depend sensitively on its spectra, it is critical to resolve the details of the wavelength and angular distribution obtained from ICS collisions. With this motivation, we present an experimental study that greatly improves on previous spectral measurement methods based on x-ray $K$-edge filters, by implementing a multilayer bent-crystal x-ray spectrometer. In tandem with a collimating slit, this method reveals a projection of the double differential angular-wavelength spectrum of the ICS radiation in a single shot. The measurements enabled by this diagnostic illustrate the combined off-axis and nonlinear-field-induced redshifting in the ICS emission process. The spectra obtained illustrate in detail the strength of the normalized laser vector potential, and provide a nondestructive measure of the temporal and spatial electron-laser beam overlap.

Highly bright photon-pair generation in Doppler-broadened ladder-type atomic system.

We report a bright photon-pair source with a coincidence counting rate per input power (cps/mW) of tens of thousands, obtained via spontaneous four-wave mixing from a Doppler-broadened atomic ensemble of the 5S1/2-5P3/2-5D5/2 transition of 87Rb. The photon-pair generation rate is enhanced by the two-photon coherence contributions from almost all the atomic velocity groups in the Doppler-broadened ladder-type atomic system. We obtained the violation of the Cauchy-Schwarz inequality by a factor of 2370 ± 150. We believe that our scheme for highly bright paired photons is important as a useful quantum light source for quantum entanglement swapping between completely autonomous sources.

Heralding single photons from a high-Q silicon microdisk

Integrated quantum photonics has recently attracted considerable attention due to the promise of realizing chip-scale quantum information processing with unprecedented capability and complexity. Their implementation relies essentially on a high-quality chip-scale photon source to support diverse quantum functionalities. Microresonator-based photon sources are a promising solution for generating bright, pure, and single-mode photons with excellent power efficiencies. However, their low Klyshko efficiency, typically around a few percentages, is a major bottleneck restricting this type of device from practical quantum applications. In this paper, we improve the Klyshko efficiency of a telecom-band heralded single-photon source from a high-Q silicon microdisk to as high as 48%. We characterize the photon antibunching properties at the same time, with a conditional self-correlation below 0.01 at a detected photon pair flux up to 0.002 counts per 5 ns gate at a repetition rate of 3 MHz. At an optical peak power of 73 μW, the photon source has a large photon flux of 0.01 counts per gate, a high Klyshko efficiency of 46%, and a strong photon antibunching with a conditional self-correlation smaller than 0.05. In particular, we find a relation between the Klyshko efficiency and high-order correlations for the first time to our knowledge. This relation contributes to the understanding of photon statistics in the heralding process and also provides a method to verify the Klyshko efficiency. The improved heralding efficiency, together with the great photon antibunching property and power efficiency, renders the microresonator-based photon source promising for diverse quantum applications, including linear-optical quantum computing and quantum key distribution.

Production of bright entangled photons from moving optical boundaries

We discuss a mechanism of generating two separable beams of light with high degree of entanglement in momentum using a fast and sharp optical boundary. Three regimes of light generation are identified depending on the number of resonant interactions between the optical perturbation and the electromagnetic field. The intensity of the process is discussed in terms of the relevant physical parameters: variation of refractive index and apparent velocity of the optical boundary. Our results suggest a different class of generation entangled light robust against thermal degradation by exciting zero point fluctuations using parametric resonant optical modulations.

Indirect generation of quadrupole polaritons from dark excitons inCu2O

The momentum-dependent electron-hole exchange interaction splits the triply degenerate orthoexcitons into the bright and dark states in ${\mathrm{Cu}}_{2}\mathrm{O}$, depending on the direction of $\mathbf{k}$ inside the crystal. Employing resonant two-photon excitation along a (100) direction at $2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, we selectively create the dark orthoexcitons that do not couple to the radiation field. However, these excitons can radiatively recombine due to subsequent conversion into the bright states. By measuring the polarization dependence, we find that the two bright states are equally populated due to cross relaxation. We also find that this bright orthoexciton luminescence is highly directional and well aligned with the initial laser propagating direction. This strongly indicates that propagating quadrupole orthoexciton-polaritons, which are the quantum superposition of bright orthoexcitons and photons, are indirectly generated. We find that the dark-to-bright relaxation mechanism can be significantly affected in the presence of hot excitons due to density-dependent scattering.

Bose-Einstein Condensation in Semiconductors: The Key Role of Dark Excitons

Bose-Einstein condensation in semiconductors is controlled by the nonelementary-boson nature of excitons. Pauli exclusion between the fermionic components of composite excitons produces dramatic exchange couplings between bright and dark states. In microcavities, where bright excitons and photons form polaritons, they force the condensate to be linearly polarized, as observed. In bulk, they also force linear polarization, but of dark states, due to interband Coulomb scatterings. To evidence this dark condensate, indirect processes are thus needed.