Tilted-potential photonic crystal cavities for integrated quantum photonics

We propose and investigate a new type of photonic crystal (PhC) cavity for integrated quantum photonics, which provides tailored optical modes with both confined and extended spatial components. The structures consist of elongated PhC cavities in which the effective index of refraction is varied quasi-linearly along their axis, implemented by systematic lateral shifts of the PhC holes. The confined modes have approximately Airy-function envelopes, exhibiting single peaks and extended tails, which is useful for optimizing single photon extraction and transmission in integrated quantum photonic devices. The measured spectrally resolved near-field patterns of such devices show the expected spatial and resonance wavelength behavior, in agreement with numerical simulations of the Airy-Bloch modes. The effects of fabrication-induced disorder on the mode features are also analyzed and discussed. Selective excitation of specific Airy-Bloch modes using integrated, site-controlled quantum dots as localized light sources is demonstrated. Based on the tilted-potential cavity, multiple-QD single photon emitters exploiting wavelength division multiplexing are proposed. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Functional Elements for Quantum-Dot-Based Integrated Quantum Photonics

Antoine Maxime Delgoffe

Quantum Integrated Photonics (QIP) harnesses quantum-states of light on tiny chips, from generation to processing and eventual detection. Within this context, this thesis explores functional QIP elements resulting from the monolithic integration of semiconductor quantum dots (QDs) in tailored photonic crystal systems of coupled cavities and waveguides. QDs present atom-like optical and electronic properties, which make them ideal for a deterministic, on-demand generation of single photons. Such QD-based devices can be expected to provide solutions for applications of QIP in quantum computing, communication and metrology. Here, we exploit the pyramidal site-controlled QD technology to reproducibly fabricate quantum photonic elements in which an arbitrary number of QD quantum emitters are embedded in photonic crystal (PhC) structures with alignment precision better than 50nm. Systematic investigation of reference structures with dense arrays of QDs to observe optical mode structures, and identical structures with only a few QDs for selective excitation and coupling to prescribed modes, is conducted using spatially-resolved micro-photoluminescence spectroscopy. Observations are consolidated by quantitative comparisons to 2D FDM numerical simulations. First, defect-potential PhC cavities are made by precise transverse shifts of selected holes in a PhC defect waveguide. In these structures, defect modes confined by a local mismatch in the photonic dispersion are observed, with peculiar polarization signatures. The fundamental defect modes display quality factors and mode volumes on par with conventional Ln=3 cavities. Defect modes can be selectively excited, and weak coupling of a single QD to cavity modes is demonstrated using thermal tuning. Next, tilted-potential PhC cavities supporting Airy-Bloch photonic modes are realized by staggered shifts of holes from 0 to smax along the axis. Such modes are suitable for local interaction and extraction of single photons from site-controlled QDs and their transmission to remote locations. Selective excitation of several modes with properly positioned QDs is demonstrated. Weak coupling of a QD to a single mode is achieved by thermal tuning, showing features of Purcell effect. The quasi linear energy-position dispersion of the Airy-Bloch modes maxima provides a mean for constructing an efficient wavelength-multiplexed single-photon injector. Finally, a monolithic model structure, consisting of two PhC waveguides side-coupled to a center Ln=3 cavity, with four ports connected to suspended nanobeams and grating couplers for light extraction/injection, is fabricated and tested. Five QDs are embedded in specific positions; four can act as on-chip single photon sources while the fifth, embedded in the center cavity, could be used for photon processing functions. The quasi-resonant excitation of this fifth QD via its p-shell is demonstrated using a pump laser beam that is injected into a waveguide through the grating coupler. Part of the s-shell emission is coupled to another targeted optical channel, demonstrating a complete photonic connectivity of all elements, a fundamental requirement towards on-chip processing. The results of this thesis consolidate the assets of the site-controlled pyramidal QD technology and tailored PhC confinement towards the realization of functional elements for quantum integrated photonics circuits.

EPFL2020

Étude expérimentale des propriétés optiques des cristaux photoniques bidimensionnels et de leur accordabillité

Barbara Wild

Photonic crystals are periodic dielectric structures, where the periodicity varies in one, two or three dimensions. Analogous to the periodic potential of electrons in semiconductors, the periodic variation of the dielectric constant influences the electromagnetic properties. The energy of the light is separated in bandgaps, energy ranges in which the propagation of the light is forbidden for certain directions and energies. These properties suggest that photonic crystals may be suitable for fabrication of the components needed for integrated optics. Since the fabrication of three dimensional photonic crystals is still limited by complex fabrication problems we have studied two dimensional photonic crystals. These photonic crystals can be fabricated by standard microelectronic technology. The photonic crystals studied in this thesis consist of GaAs and InP based low index vertical waveguides with a matrix of holes etched into it. The fabrication of photonic crystals with good optical properties is an important factor for photonic crystal devices in integrated optics. In the first part we studied the optical properties of photonic crystal slabs and of Fabry-Pérot cavities. These structures were measured by the internal light source technique, which allows quantitative normalized transmission measurements. The photonic crystals were etched by three different dry etching technologies. Out-of plane scattering is described by a 2D-FDTD fit of the transmission spectra and by a semi-analytical model. The finite hole depth and the conical shape of the holes are important structural parameters. The detailed study of their effects yields an important feedback for the critical parameters of the fabrication process. Photonic crystal waveguides are another group of components in integrated optics. We measured two types of straight photonic crystal waveguides by the endfire technique. Here the light is coupled by optical fibers and ridge waveguides in the photonic crystal waveguides. The transmission spectra reveal fringes which are due to multiple interference of the light in the sample. This interference contains information about propagation losses of the waveguides and an analysis based on the Fourier transform of the transmission spectra enabled us to deduce the propagation losses of the photonic crystal waveguides. The necessity of tuning or trimming the optical properties of photonic crystals is an important issue either compensating fabrication imperfections (trimming) or controlling the optical properties on demand (tuning) for devices like filters. We have shown that it is possible to tune the optical properties of planar photonic crystal cavities by temperature. The experimental results were validated by theoretical calculations. Also we have shown that is possible to tune the optical properties of photonic crystals by infiltrating liquid crystals in the holes. Liquid crystals are a birefringent material whose optical axis can be tuned by external fields (electric, temperature, photonic source, etc.). Phase transitions exhibiting abrupt changes in the refractive index are important characteristics of liquid crystals. The holes of the photonic crystal are infiltrated by a reversible and reliable infiltration process. The infiltration efficiency and the orientation of the molecules in the holes were determined by polarization resolved internal light source measurements. We have shown that the frequency of the Fabry-Pérot resonance changes at the phase transitions of the liquid crystal.

EPFL2006

Functional Elements for Quantum-Dot-Based Integrated Quantum Photonics

Antoine Maxime Delgoffe

EPFL2020

Étude expérimentale des propriétés optiques des cristaux photoniques bidimensionnels et de leur accordabillité

Barbara Wild

EPFL2006