Community Research and Development Information Service - CORDIS

Final Report Summary - OMSIQUD (Coupling Confined Optical and Mechanical Modes to a Single Quantum Dot)

The OMSiQuD project aims at bridging the gap between two yet distinct research fields: optomechanics of deformable cavities (cavity optomechanics) and cavity quantum electrodynamics (cavity QED). This project explores a hybrid interface between these two domains, using semiconductor nanostructures. In particular, we propose the use of a single quantum dot (QD) coupled to a pillar microcavity to study photon-phonon interactions. It has been demonstrated that an optimized GaAs/AlAs acoustic resonator is automatically an optimized optical resonator. This results in a novel platform for optomechanics based on GaAs/AlAs optical micropillars and planar microcavities with optomechanical coupling with the potential to reach 80 THz/nm, working frequencies of the order of 20-200 GHz, and an operating optical wavelength in the NIR range. The confined modes of the pillar microcavity are of very high quality factor (reaching 1 million), and enable a coupling of the photons both to the ~20 GHz mechanical modes of the pillar and to a single InAs QD inserted in the center of the microcavity. In this novel field of research proposed by the project, the longterm objective is to control a coupled tri-partite system: a cavity photon interacts with a coherent quantum emitter (two-level atom) and with a single mechanical mode.

The general objectives of the OMSiQuD project are:
- To demonstrate a new solid-state platform for optomechanics based on micropillar cavities
- To control the phononic density of states, and thus the decoherence of a single quantum emitter
- To control an optomechanical system with a single quantum emitter

The work performed during the project was centered on the following main tasks:

- The conception and setup of a Brownian motion measurement experiment, able to work in the 15-20 GHz range, working at wavelengths compatible with the emission of quantum dots. Part of the time was devoted to the optimization of the detection and amplification chain, and the mechanical stabilization of the setup. The optimization of the coupling between the incident beam and the micropillar optical mode, and the minimization of the optical losses of the setup represented an important fraction of the spent time.
- The engineering, optimization and fabrication of optomechanical micropillars deterministically coupled to a single QD.
- The study of novel phononic confinement strategies based on topological design. By using a transfer matrix method we explored novel structructures presenting acoustic confinement in the 20-400 GHz range based on spacerless cavities. The calculations also allowed us to optimize the expected Raman signals in the structures. We used combined acoustic and optical cavities that have the potential to enhance the signals by 6 orders of magnitude.
In order to study the spacerless cavities, a high resolution Raman scattering experiment was set up. A ‘topologically’ engineered acoustic cavity was optimized and fabricated to confine longitudinal acoustic phonons in the 200-300 GHz range. This acoustic cavity was embedded in an optical cavity to perform double optical resonant optical enhancement of the signals (enhancement of both the incident laser and scattered fields).

Among the most remarkable results of this project we can highlight:

Coupled QD-optomechanical microcavities structures.
Taking advantage of the in-situ lithography technique developed in the host institution, and based on the acquired know-how, we fabricated a sample with an optimized localization of a single quantum dot inside a micropillar structure to maximize simultaneously the coupling with the electromagnetic and mechanical strain fields. During the project we set up a Brownian motion measurement experiment to put in evidence QD-assisted optomechanical dynamical back-action phenomena. The first round of experiments did not provide positive results due to the weak optomechanical signals. Alternative experiments were proposed and are being implemented after the finalization of the OMSiQuD project. Parallel strategies to access the optomechanical behaviour of the devices were explored including high resolution Raman scattering and coherent phonon generation experiments.

Engineering of micropillar optomechanical resonators.
In collaboration with Fainstein’s team, we experimentally demonstrated 3-dimensional mechanical modes in the 20-100 GHz range in GaAs/AlAs micropillar optomechanical cavities, with quality factors of 1000, and operating in the near infrared range. The effects of 3-dimensional confinement was evidenced by an increase in the signal intensity when reducing the micropillar radius. We developed a mechanical model using a commercial finite elements tool that allowed us to precisely describe the nanomechanical eigenstates of the semiconductor micropillars and the optomechanical couplings. We determined the role of the Poisson ratio in the nanomechanical response of the system, unveiling a complex behaviour and coupling between displacement fields along different directions. The micropillars proved to be a suitable platform for ultrahigh frequency optomechanics.

Design and demonstration of novel acoustic cavities.
Based on previously proposed concepts in optics we designed and optimized planar (1-dimensional) spacerless acoustic cavities with the potential of improved optomechanical couplings. We installed and set up a high resolution Raman scattering experiment that allowed us to measure these confined modes in the 200-300 GHz range. We were able to account for the experimental results using a simple photoelastic model. The explored concepts open the road to the study of more complex structures based on the engineering of topological properties of the distributed Bragg reflectors.

By the end of the project, the Brownian motion setup is operative and new optimized samples are being studied, the high resolution Raman scattering setup was incorporated to the host laboratory as a nanophononic characterization technique and a pump probe experiment is being set up. The finite element models for the optical and mechanical studies of micropillars were implemented and the determination of optomechanical couplings are now part of the computational toolbox of the host group.

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Life Sciences
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