Emmanuel Dupuy

Strain-mediated coupling in a quantum dot–mechanical oscillator hybrid system

Recent progress in nanotechnology has allowed the fabrication of new hybrid systems in which a single two-level system is coupled to a mechanical nanoresonator. In such systems the quantum nature of a macroscopic degree of freedom can be revealed and manipulated. This opens up appealing perspectives for quantum information technologies, and for the exploration of the quantum-classical boundary. Here we present the experimental realization of a monolithic solid-state hybrid system governed by material strain: a quantum dot is embedded within a nanowire that features discrete mechanical resonances corresponding to flexural vibration modes. Mechanical vibrations result in a time-varying strain field that modulates the quantum dot transition energy. This approach simultaneously offers a large light-extraction efficiency and a large exciton-phonon coupling strength g0. By means of optical and mechanical spectroscopy, we find that g0/2 π is nearly as large as the mechanical frequency, a criterion that defines the ultrastrong coupling regime.

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.

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.

Surface effects in a semiconductor photonic nanowire and spectral stability of an embedded single quantum dot

We evidence the influence of surface effects for InAs quantum dots embedded into GaAs photonic nanowires used as efficient single photon sources. We observe a continuous temporal drift of the emission energy that is an obstacle to resonant quantum optics experiments at the single photon level. We attribute the drift to the sticking of oxygen molecules onto the wire, which modifies the surface charge and hence the electric field seen by the quantum dot. The influence of temperature and excitation laser power on this phenomenon is studied. Most importantly, we demonstrate a proper treatment of the nanowire surface to suppress the drift.

Strain-Gradient Position Mapping of Semiconductor Quantum Dots

We introduce a nondestructive method to determine the position of randomly distributed semiconductor quantum dots (QDs) integrated in a solid photonic structure. By setting the structure in an oscillating motion, we generate a large stress gradient across the QDs plane. We then exploit the fact that the QDs emission frequency is highly sensitive to the local material stress to map the position of QDs deeply embedded in a photonic wire antenna with an accuracy ranging from ±35  nm down to ±1  nm. In the context of fast developing quantum technologies, this technique can be generalized to different photonic nanostructures embedding any stress-sensitive quantum emitters.

Giant nonlinear interaction between two optical beams via a quantum dot embedded in a photonic wire

Optical non-linearities usually appear for large intensities, but discrete transitions allow for giant non-linearities operating at the single photon level. This has been demonstrated in the last decade for a single optical mode with cold atomic gases, or single two-level systems coupled to light via a tailored photonic environment. Here we demonstrate a two-modes giant non-linearity by using a three-level structure in a single semiconductor quantum dot (QD) embedded in a photonic wire antenna. The large coupling efficiency and the broad operation bandwidth of the photonic wire enable us to have two different laser beams interacting with the QD in order to control the reflectivity of a laser beam with the other one using as few as 10 photons per QD lifetime. We discuss the possibilities offered by this easily integrable system for ultra-low power logical gates and optical quantum gates.

Spectral analysis of the line-width and line-edge roughness transfer during self-aligned double patterning approach

We report a 20 nm half-pitch self-aligned double patterning (SADPP) process based on a resist-core approach. Line/space 20/20 nm features in silicon are successfully obtained with CDvariation, LWR and LER of 0.7 nm, 2.4 nm and 2.3 nm respectively. The LWR and LER are characterized at each technological step of the process using a power spectral density fitting method, which allows a spectral analysis of the roughness and the determination of unbiased roughness values. Although the SADP concept generates two asymmetric populations of lines, the final LLWR and LER are similar. We show that this SADP process allows to decrease significantly the LWR and the LER of about 62% and 48% compared to the initial photoresist patterns. This study also demonstrates that SADP is a very powerful concept to decrease CD uniformity and LWR especially in its low-frequency components to reach sub-20 nm node requirements. However, LER low-frequency components are still high and remain a key issue tot address for an optimized integration.

All-optical switching of a microcavity repeated at terahertz clock rates

We exploit the electronic Kerr effect that enables switching of the cavity faster than all relevant time scales. We employ the virtually instantaneous electronic Kerr effect to repeatedly and reproducibly switch a GaAs-AlAs planar microcavity made by MBE growth that operates in the telecom O-band (λ=1300 nm).

Ultra-low power optical transistor using a single quantum dot embedded in a photonic wire

Optical logic down to the single photon level holds the promise of data processing with a better energy efficiency than electronic devices [1]. In addition, preservation of quantum coherence in such logical components would enable optical quantum logical gates [2-8]. Optical logic requires optical non-linearities to allow for photon-photon interactions. Non-linearities usually appear for large intensities, but discrete transitions in a well coupled single two-level system allow for giant non-linearities operating at the single photon level.

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.