Monolithic Silicon-Based Nanobeam Cavities for Integrated Nonlinear and Quantum Photonics

Photonic resonators that allow an electromagnetic field to be confined in an ultrasmall volume with a long decay time are crucial to a number of applications requiring enhanced nonlinear effects. For applications to integrated photonic devices on chip, compactness and optimized in-plane transmission become relevant figures of merit as well. Here we optimize an encapsulated Si/SiO2 photonic-crystal nanobeam cavity at telecom wavelengths by means of a global optimization procedure, where only the first few holes surrounding the cavity are varied to decrease its radiative losses. This strategy allows us to theoretically achieve intrinsic quality factors close to 10 million, sub-diffraction-limited mode volumes, and in-plane transmission above 65%, in a structure with a very small footprint of about 8 mu m(2). We address and quantitatively assess the dependence of the main figures of merit on the nanobeam length and on fabrication disorder. Finally, we study a system of two optimized and laterally coupled nanobeam cavities with the goal of demonstrating an unconventional photon blockade at room temperature in a monolithic passive device. We estimate the single-photon nonlinearity of this device and discuss the relevant figures of merit, which lead to sub-Poissonian photon statistics of the transmitted signal. Our results hold promise for prospective experiments in low-power nonlinear and quantum photonics.

Optimal antibunching in passive photonic devices based on coupled nonlinear resonators

Vincenzo Savona

We propose the use of weakly nonlinear passive materials for prospective applications in integrated quantum photonics. It is shown that strong enhancement of native optical nonlinearities by electromagnetic field confinement in photonic crystal resonators can lead to single-photon generation only exploiting the quantum interference of two coupled modes and the effect of photon blockade under resonant coherent driving. For realistic system parameters in state of the art microcavities, the efficiency of such a single-photon source is theoretically characterized by means of the second-order correlation function at zero-time delay as the main figure of merit, where major sources of loss and decoherence are taken into account within a standard master equation treatment. These results could stimulate the realization of integrated quantum photonic devices based on non-resonant material media, fully integrable with current semiconductor technology and matching the relevant telecom band operational wavelengths, as an alternative to single-photon nonlinear devices based on cavity quantum electrodynamics with artificial atoms or single atomic-like emitters.

Institute of Physics (IoP) and Deutsche Physikalische Gesellschaft2013

Light Confinement and Nonlinear Light-Matter Interaction in Semiconductor Photonic Crystal Cavities

Mohamed Sabry Abdel-Aliem Mohamed

The ability to control and manipulate light is a fundamental aspect that is at the very core of the development of integrated photonic circuits. It is desirable to achieve such control down to a scale that is comparable to or smaller than the wavelength of light, to enable highly efficient interaction with matter, which can eventually go down to single photon levels. Photonic crystals are particularly appealing in that respect, by providing optical bandgaps and facilitating dispersion shaping, through the precise patterning of layers. This is to be accomplished ideally in compact designs that are well-suited for fabrication with current technology and by utilizing material that has the capacity for integration. By exploiting existing light-matter interaction mechanisms, the available toolset for component design can be expanded, allowing to build optical chips with a wide range of functionality. In the current work, a platform for integrated III-nitride photonic components on silicon is designed and implemented. A refined fabrication process for III-nitride micro- and nanostructures is developed, leading to the realization of suspended optical devices with both high performance and structural stability. Thorough optical and material characterization of the semiconductor layers is conducted to identify potential limitations, particularly with epitaxial GaN on Si technology, with respect to current crystal growth techniques. AlN layers based on a sputter-deposition process are proposed as an alternative for passive optical devices. Two-dimensional photonic crystals are designed in gallium nitride and aluminum nitride, targeting operational wavelengths in the near-infrared telecom range. Optimized photonic crystal cavities are considered for maximizing light confinement in the semiconductor layers. By utilizing resonance enhancement, second and third harmonic generation are demonstrated in gallium nitride photonic crystal cavities featuring record-high quality factor values. This is achieved through a far-field coupling approach under low-power continuous-wave operation. High conversion efficiencies are attained in gallium nitride, exceeding previous demonstrations in the material. Propagation of the harmonic signals in the semiconductor layer is analyzed using finite-element time-domain modeling for the feasibility of an extraction mechanism. Moreover, slow light coupled cavity waveguides are developed in a silicon platform, relying on optimized cavity designs that feature wideband operation in the near-infrared telecom range. A record-high value for group-index bandwidth product is achieved in the devices through improved design implementation. An end-fire based Fourier-space imaging technique is applied for the characterization of the optical structures, through which spatially-selective reconstruction of dispersion maps enabled accurate extraction of slow light properties. The influence of finite-size effects on slow light behavior is elucidated by implementing extended cavity chains. Furthermore, light transport regimes across the dispersion band are identified including the transition towards diffusive light transport and subsequent localization. The performance of coupled-cavity waveguides in the presence of disorder is quantified, revealing constraints on slow light transport, considering state-of-the-art fabrication.

EPFL2018

Coherence properties of microcavity polaritons

Stefan Kundermann

This thesis presents the coherence properties of polaritons in semiconductor microcavities. Semiconductor microcavities are microstructures in which the exciton ground state of a semiconductor quantum well is coupled to a photonic mode of a microresonator. The strong coupling mixes the character of excitons and photons, giving rise to the lower and upper polariton branches, quasiparticles with an unusual energetic dispersion relation due to the extreme mass difference between exciton and photon. Particularly special is the dispersion of the lower polariton, which forms a dip in the 2-dimensional k-space around the lowest energy state with zero in-plane momentum. In this dip, which can be seen as a trap in momentum space, the polaritons are efficiently isolated from dephasing mechanisms involving phonons. Polaritons can be resonantly excited at desired points on the polariton dispersion by shining on the microcavity laser light at the appropriate angle and wavelength. Polaritons can interact and scatter pairwise with each other conserving energy and in plane momentum k, a process similar to parametric scattering of photons in a nonlinear crystal. One polariton from a pump reservoir scatters down to the signal state at k = 0 (corresponding to normal incidence) and a second takes away the excess energy and momentum of the first and scatters up to the idler position ({kP, kP} → {0, 2kP }). This process can be stimulated by a small amount of signal polaritons injected with a probe laser beam at normal incidence. Here the coherence properties of the polariton parametric scattering have been investigated using spectroscopy techniques sensitive to the optical phase, for example coherent control with phase-locked femtosecond probe pulses. Just above the threshold for the stimulated parametric scattering, the parametric amplification process is given by the linear superposition of the individual amplification processes of each probe pulse. The emission of signal, pump, and idler can be controlled by tuning the relative phase of the 150fs-long probe pulses, which are separated by a few picoseconds in time. Experiments are presented that deal with the real-time dynamics of the parametric scattering in the spontaneous and the stimulated regime. It is shown, that in the spontaneous regime the scattering is started by a small amount of polaritons which have relaxed to the band bottom by emitting phonons. In the regime where polariton scattering is stimulated by an external probe, the rise of the signal intensity is delayed with respect to the arrival time of both pump and probe, a feature that can be attributed to the complex phase-matching mechanism for the parametric scattering. In the second part of the thesis, the spontaneous build up of a macroscopic coherence in a CdTe microcavity under non-resonant laser excitation is analysed. The build up of a long-range spatial coherence easily exceeding the thermal wavelength of the polaritons is shown. This is the hallmark of Bose-Einstein condensation and the proof of a macroscopic wavefunction. Experimental data on the statistical distribution of the polaritons in time, the polarisation of the non-linear emission, and the quantum transition from a thermal to a coherent state1 confirm that Bose-Einstein condensation of microcavity polaritons has been observed. We regard these observations as the first bullet-proof evidence for spontaneous Bose-Einstein condensation in a solid state system, a phenomenon that has been the subject to many investigations and controversies during the past four decades. ------------------------------ 1 The data about the statistical distribution, the polarisation, and the transition from a thermal to a coherent state is by courtesy of Jacek Kasprzak of the University of Grenoble.