Electromechanical tuning of photonic crystal cavities

Photonic crystal cavities (PCCs) are electromagnetic resonators obtained introducing defects in periodic dielectric structures. They have been widely used in semiconductor nanophotonics devices to realize low-threshold lasers, filters and switches operating at telecommunication wavelengths. Moreover, when coupled to quantum emitters such as quantum dots, PCC are used to enhance their spontaneous emission rate according to Fermi’s golden rule. Such a coupled cavitydot system provides a small-scale integrated implementation of a single photon source, a device which plays a fundamental role for quantum information processing. However, fabrication imperfections and ageing make the resonant wavelength of PCCs non-reproducible and tuning methods are needed to compensate the spectral mismatch between a cavity and a quantum dot during experiments. Moreover, for quantum information processing it is important to electrically tune many cavities independently over a range of several nanometers at low temperatures. In this thesis work I explored novel devices for the spectral control and reconfiguration of PCCs using nano electromechanical systems (NEMS). The fundamental idea, described in the first chapter, consists in fabricating the photonic crystal on two, closely spaced, parallel slabs to form a coupled system and modifying their distance electro-mechanically to alter the coupling strength. The displacement is obtained using doped layers to form a p-i-n junction across the air gap between the membranes and operating it under reverse bias to exert an attractive electrostatic pressure on each slab. A simple model to describe the coupled cavity system and the electrostatic actuator is proposed. This model forms the basis for the device design and the estimation of the tuning range. The latter is limited only by pull-in, an electrostatic instability which occurs whenever the membranes are displaced more than one third of the distance at rest. In chapter 1, an introduction to the physics of photonic crystals, quantum dots and cavity quantum electrodynamics is also provided along with a detailed review of the cavity tuning methods which have been already proposed in the literature. The second chapter discusses the fabrication of double-slab photonic crystals and their integration with the electrostatic actuator. The chapter addresses the problem of stiction (or static friction) between the membranes due to the strong capillary forces involved during the sample drying. A novel fabrication procedure which reduces stiction by increasing the total stiffness of the system with a dielectric layer is described. Chapter 2 also includes an overview of the experimental setups used for the electro-optical characterization of PCCs and quantum dots. The chapter ends with a discussion on the device design based on the model from chapter 1 and the practical fabrication limits due to capillary forces. In the third chapter, the experimental results on the electromechanical tuning of InGaAsP at room temperature are reported. The simultaneous blue- and redshift of the coupled normal modes is observed. A maximum tuning of 10 nm has been measured with a reverse bias of 5.8 V beyond which, the pull-in phenomenon occurs. Using a periodic signal as a driving force and measuring the spectral response, the signature of mechanical resonances has been observed and the corresponding frequencies have been compared to simulations. All these results provide a conclusive demonstration of the mechanical origin of the tuning. The fourth chapter describes the tuning of GaAs devices at low temperatures for the spectral alignment of cavity modes to single quantum dots. A PCC resonance has been shifted over 13 nm to match the emission of a far-detuned excitonic line. The enhancement of spontaneous emission rate has been confirmed with timeresolved photoluminescence measurements, a technique which allows measuring the emitter’s lifetime. A four-fold enhancement has been obtained between the dot on-resonance and the dots in the homogeneous (or bulk) medium, indicating that PCC can be used to enhance the rate of single photon emission from single quantum dots. The fifth chapter describes a slightly different tunable photonic crystal based on two, vertically-coupled, nanobeams. The device, realized on GaAs, is realized with an original fabrication method which prevents adhesion of these nanostructures under capillary forces. A new design is also introduced to mount the nanobeams on flexible frames to enhance the tunability. A tuning range of 15.6 nm has been measured, which is the current record for electromechanical tuning on doublemembrane NEMS. The sixth chapter contains several new ideas and perspectives on the integration of double membranes in photonic circuits and on the extension of the tuning range. The coupling to composite ridge waveguides and an original method to fabricate them on double slabs is discussed. The first experimental results have shown the possibility to observe Fabry-Pérot modes in a photonic crystal waveguide from the cleaved facet of a ridge waveguide, located 1 mm away from the source. The overall transmission, however, still requires optimization. The double membrane can also be integrated with the wavelength tuning of quantum dots (via Stark effect) using a third contact layer, opening up new perspectives on the generation of indistinguishable photons. The chapter ends with a proposed structure to realize a pull-in free device, thereby extending the total tuning range beyond the current record values. Finally, the last chapter summarizes the most relevant results of this thesis work and the open issues which set the basis for future research activities

Electromechanical wavelength tuning of double-membrane photonic crystal cavities

We present a method for tuning the resonant wavelength of photonic crystal cavities (PCCs) around 1.55 um. Large tuning of the PCC mode is enabled by electromechanically controlling the separation between two parallel InGaAsP membranes. A fabrication method to avoid sticking between the membranes is discussed. Reversible red/blue shifting of the symmetric/anti-symmetric modes has been observed, which provides clear evidence of the electromechanical tuning, and a maximum shift of 10 nm with < 6 V applied bias has been obtained.Comment: 9 pages, 3 figure

Electromechanical tuning of vertically-coupled photonic crystal nanobeams

We present the design, the fabrication and the characterization of a tunable one-dimensional (1D) photonic crystal cavity (PCC) etched on two vertically-coupled GaAs nanobeams. A novel fabrication method which prevents their adhesion under capillary forces is introduced. We discuss a design to increase the flexibility of the structure and we demonstrate a large reversible and controllable electromechanical wavelength tuning (> 15 nm) of the cavity modes.Comment: 11 pages, 4 figure

Electro-optic routing of photons from single quantum dots in photonic integrated circuits

Recent breakthroughs in solid-state photonic quantum technologies enable generating and detecting single photons with near-unity efficiency as required for a range of photonic quantum technologies. The lack of methods to simultaneously generate and control photons within the same chip, however, has formed a main obstacle to achieving efficient multi-qubit gates and to harness the advantages of chip-scale quantum photonics. Here we propose and demonstrate an integrated voltage-controlled phase shifter based on the electro-optic effect in suspended photonic waveguides with embedded quantum emitters. The phase control allows building a compact Mach-Zehnder interferometer with two orthogonal arms, taking advantage of the anisotropic electro-optic response in gallium arsenide. Photons emitted by single self-assembled quantum dots can be actively routed into the two outputs of the interferometer. These results, together with the observed sub-microsecond response time, constitute a significant step towards chip-scale single-photon-source de-multiplexing, fiber-loop boson sampling, and linear optical quantum computing.Comment: 7 pages, 4 figues + supplementary informatio

Single-photon nonlinear optics with a quantum dot in a waveguide

Strong nonlinear interactions between photons enable logic operations for both classical and quantum-information technology. Unfortunately, nonlinear interactions are usually feeble and therefore all-optical logic gates tend to be inefficient. A quantum emitter deterministically coupled to a propagating mode fundamentally changes the situation, since each photon inevitably interacts with the emitter, and highly correlated many-photon states may be created . Here we show that a single quantum dot in a photonic-crystal waveguide can be utilized as a giant nonlinearity sensitive at the single-photon level. The nonlinear response is revealed from the intensity and quantum statistics of the scattered photons, and contains contributions from an entangled photon-photon bound state. The quantum nonlinearity will find immediate applications for deterministic Bell-state measurements and single-photon transistors and paves the way to scalable waveguide-based photonic quantum-computing architectures

From Parental Bonding to Problematic Gaming: The Mediating Role of Adult Attachment Styles

Background: Research has found that parental failures of care during childhood and insecure attachment styles are positively associated with problematic gaming. From a developmental framework, it is possible to hypothesize that attachment styles mediate the relationship between parental bonding and problematic gaming. Methods: This hypothesis was tested in a sample of 598 videogame players (410 males, 68.56%) aged between 18 and 61 years old (M = 26.68, SD = 7.23). Participants were recruited through an online survey. Self-report instruments were administered to assess problematic gaming, parental bonding, and adult attachment styles. Results: Positive paternal care was a direct and negative predictor of problematic gaming, whereas maternal overprotection indirectly predicted problematic gaming through preoccupied attachment. Conclusions: These findings suggest that positive paternal care represents a protective factor for problematic gaming; in contrast, maternal overprotection might foster a negative view of the self in the child, which increases the risk to excessively use videogames, perhaps as a maladaptive coping strategy to regulate negative feelings. Prevention programs might be aimed to improve the responsiveness of parents towards the child’s emotional needs, to prevent the development of problematic gaming. Also, clinical intervention with problematic gamers might foster their feelings of security toward relationships, to promote both a healthier use of videogames and a better quality of life

Ultrafast nonlocal control of spontaneous emission

Solid-state cavity quantum electrodynamics systems will form scalable nodes of future quantum networks, allowing the storage, processing and retrieval of quantum bits, where a real-time control of the radiative interaction in the cavity is required to achieve high efficiency. We demonstrate here the dynamic molding of the vacuum field in a coupled-cavity system to achieve the ultrafast nonlocal modulation of spontaneous emission of quantum dots in photonic crystal cavities, on a timescale of ~200 ps, much faster than their natural radiative lifetimes. This opens the way to the ultrafast control of semiconductor-based cavity quantum electrodynamics systems for application in quantum interfaces and to a new class of ultrafast lasers based on nano-photonic cavities.Comment: 15 pages, 4 figure

Waveguide-coupled Electrically-tunable Cavity-Emitter System

In scalable quantum photonic integrated circuits it is imperative to spectrally tune both cavities and emitters independently, in order to overcome their intrinsic energy mismatch and generate indistinguishable single-photons on a chip. Here we present the first fully-controllable cavity-emitter system in the solid state and discuss its coupling to ridge waveguides

Nano-Opto-Electro-Mechanical Systems

A new class of hybrid systems that couple optical, electrical and mechanical degrees of freedom in nanoscale devices is under development in laboratories worldwide. These nano-opto-electro-mechanical systems (NOEMS) offer unprecedented opportunities to dynamically control the flow of light in nanophotonic structures, at high speed and low power consumption. Drawing on conceptual and technological advances from cavity optomechanics, they also bear the potential for highly efficient, low-noise transducers between microwave and optical signals, both in the classical and quantum domains. This Progress Article discusses the fundamental physical limits of NOEMS, reviews the recent progress in their implementation, and suggests potential avenues for further developments in this field.Comment: 27 pages, 3 figures, 2 boxe