Adrien Delga

Dielectric GaAs antenna ensuring an efficient broadband coupling between an InAs quantum dot and a Gaussian optical beam.

We introduce the photonic trumpet, a dielectric structure which ensures a nearly perfect coupling between an embedded quantum light source and a Gaussian free-space beam. A photonic trumpet exploits both the broadband spontaneous emission control provided by a single-mode photonic wire and the adiabatic expansion of this mode within a conical taper. Numerical simulations highlight the outstanding performance and robustness of this concept. As a first application in the field of quantum optics, we report the realisation of an ultra-bright single-photon source. The device, a GaAs photonic trumpet containing few InAs quantum dots, demonstrates a first-lens external efficiency of 0.75 ± 0.1.

Highly directive and Gaussian far-field emission from “giant” photonic trumpets

Photonic trumpets are broadband dielectric antennas that efficiently funnel the emission of a point-like quantum emitter—such as a semiconductor quantum dot—into a Gaussian free-space beam. After describing guidelines for the taper design, we present a “giant” photonic trumpet. The device features a bottom diameter of 210 nm and a 5 μm wide top facet. Using Fourier microscopy, we show that 95% of the emitted beam is intercepted by a modest numerical aperture of 0.35. Furthermore, far-field measurements reveal a highly Gaussian angular profile, in agreement with the predicted overlap to a Gaussian beam Mg=0.98. Future application prospects include the direct coupling of these devices to a cleaved single-mode optical fiber. The calculated transmission from the taper base to the fiber already reaches 0.59, and we discuss strategies to further improve this figure of merit.

Johnson and shot noises in intersubband detectors

Johnson and shot noises are usually considered as independent in intersubband detectors. In this paper, we discuss some simple ideas showing that they are actually the equilibrium and far from equilibrium limits of a single phenomenon. We present an intuitive framework to consistently understand and model these noises in unipolar detectors, in order to enlarge the toolbox of quantum designers.

Quantum dot spontaneous emission control in a ridge waveguide

We investigate the spontaneous emission (SE) of self-assembled InAs quantum dots (QDs) embedded in GaAs ridge waveguides that lay on a low index substrate. In thin enough waveguides, the coupling to the fundamental guided mode is vanishingly small. A pronounced anisotropy in the coupling to non-guided modes is then directly evidenced by normal-incidence photoluminescence polarization measurements. In this regime, a measurement of the QD decay rate reveals a SE inhibition by a factor up to 4. In larger wires, which ensure an optimal transverse confinement of the fundamental guided mode, the decay rate approaches the bulk value. Building on the good agreement with theoretical predictions, we infer from calculations the fraction β of SE coupled to the fundamental guided mode for some important QD excitonic complexes. For a charged exciton (isotropic in plane optical dipole), β reaches 0.61 at maximum for an on-axis QD. In the case of a purely transverse linear optical dipole, β increases up to 0.91. This optimal configuration is achievable through the selective excitation of one of the bright neutral excitons.

Harvesting, coupling and control of single exciton coherences in photonic waveguide antennas

We perform coherent nonlinear spectroscopy of individual excitons strongly confined in single InAs quantum dots (QDs). The retrieval of their intrinsically weak four-wave mixing (FWM) response is enabled by a one-dimensional dielectric waveguide antenna. Compared to a similar QD embedded in bulk media, the FWM detection sensitivity is enhanced by up to 4 orders of magnitude, over a broad operation bandwidth. Three-beam FWM is employed to investigate coherence and population dynamics within individual QD transitions. We retrieve their homogenous dephasing in a presence of low-frequency spectral wandering. Two-dimensional FWM reveals off-resonant Förster coupling between a pair of distinct QDs embedded in the antenna. We also detect a higher order QD nonlinearity (six-wave mixing) and use it to coherently control the FWM transient. Waveguide antennas enable us to conceive multicolor coherent manipulation schemes of individual emitters.

Predictive circuit model for noise in quantum cascade detectors

Electronic noise in quantum cascade structures is investigated theoretically and experimentally under dark conditions. A model based on a unified and insightful vision of noise generating mechanisms is proposed and describes both thermal and shot noise behaviors. Dark measurements of quantum cascade detectors operating at 8 μm and 15 μm are retrieved with good quantitative agreement. This model is expected to be applicable to other quantum structures and under illumination.

Reduction of noise in very long wave infrared quantum cascade detectors

Noise in quantum cascade detectors is studied experimentally and theoretically. Measurements were performed in dark conditions on a quantum cascade detector operating at 14.5 μm, in the very long wave infrared range. To investigate the signal-to-noise contributions of each intersubband transition involved in the transport, a model of noise has been developed. It is based on a noise equivalent electrical circuit of the quantum cascade detector. Non-radiative diagonal transitions (fundamental state to levels of the cascade structure) are identified as dominant contributions to the dark current and noise in the measured device. Based on these theoretical considerations, new optimized structures for the very long wave-infrared range are designed and exhibit a noise reduction down to a factor three at optimum responsivity.

Intersubband impact ionization in THz QWIPs: shaping band structure reorganizations to design novel detectors

Electronic transport in AlGaAs/GaAs THz Quantum Wells Intersubband Photodetectors (QWIPs) exhibits two different regimes separated by huge discontinuities (up to five orders of magnitude) in the resistivity. They are interpreted in terms of band structure reorganizations triggered by intersubband impact ionization. We will analyze and model their in uence on the electronic transport. The magnitude of the transport modifications is explained by the small transition energy and the sharpness of the electrons distribution at stake in THz QWIPs. Measurements under magnetic field or temperature show that the broadening of the electron distribution damps the effects of impact ionization. Some experimental features of the electronic transport of shorter wavelength detectors are then reproduced. The use of intersubband impact ionization in THz QWIPs to design high gain and fast novel detectors is discussed.

The photonic nanowire: an emerging platform for highly efficient single-photon sources for quantum information applications

Efficient coupling between a localized quantum emitter and a well defined optical channel represents a powerful route to realize single-photon sources and spin-photon interfaces. The tailored fiber-like photonic nanowire embedding a single quantum dot has recently demonstrated an appealing potential. However, the device requires a delicate, sharp needle-like taper with performance sensitive to minute geometrical details. To overcome this limitation we demonstrate the photonic trumpet, exploiting an opposite tapering strategy. The trumpet features a strongly Gaussian far-field emission. A first implementation of this strategy has lead to an ultra-bright single-photon source with a first-lens external efficiency of 0.75 ± 0.1 and a predicted coupling to a Gaussian beam of 0.61 ± 0.08.

A photonic nanowire trumpet for interfacing a quantum dot and a Gaussian free-space mode

Efficient coupling between a localized quantum emitter and a well defined optical channel represents a powerful route to realize single-photon sources and spin-photon interfaces. The tailored fiber-like photonic nanowire embedding a single quantum dot has recently demonstrated an appealing potential. However, the device requires a delicate, sharp needle-like taper with performance sensitive to minute geometrical details. To overcome this limitation we demonstrate the photonic trumpet, exploiting an opposite tapering strategy. The trumpet features a strongly Gaussian far-field emission. A first implementation of this strategy has lead to an ultra-bright single-photon source with a first-lens external efficiency of 0.75 ± 0.1 and a predicted coupling to a Gaussian beam of 0.61 ± 0.08.