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

Individual fitness is decoupled from coarse‐scale probability of occurrence in North American trees

Habitat suitability estimated with probability of occurrence in species distribution models (SDMs) is used in conservation to identify geographic areas that are most likely to harbor individuals of interest. In theory, probability of occurrence is coupled with individual fitness so that individuals have higher fitness at the centre of their species environmental niche than at the edges, which we here define as 'fitness‐centre' hypothesis. However, such relationship is uncertain and has been rarely tested across multiple species. Here, we quantified the relationship between coarse‐scale probability of occurrence projected with SDMs and individual fitness in 66 tree species native of North America. We used 1) field data of individuals' growth rate (height and diameter standardized by age) available from the United States Forest Inventory Analysis plots; and 2) common garden data collected from 23 studies reporting individual growth rate, survival, height and diameter of individuals originated from different provenances in United States and Canada. We show 'fitness–centre' relationships are rare, with only 12% and 11% of cases showing a significant positive correlation for field and common garden data, respectively. Furthermore, we found the 'fitness–centre' relationship is not affected by the precision of the SDMs and it does not depend upon dispersal ability and climatic breath of the species. Thus, although the 'fitness–centre' relationship is supported by theory, it does not hold true in nearly any species. Because individual fitness plays a relevant role in buffering local extinction and range contraction following climatic changes and biotic invasions, our results encourage conservationists not to assume the 'fitness–centre' relationship when modelling species distribution

Immunoregulatory properties of bone marrow mesenchymal stromal cell-derived extracellular vesicles

Mesenchymal stromal cells (MSCs) are adult stem cells of mesodermal origin that can be isolated from various tissues, including bone marrow (BM), adipose tissue and amniotic fluid. MSCs express mesenchymal markers, i.e. CD73, CD90, and CD105, and lack expression of hematopoietic markers, such as CD45, CD34, CD11b and CD14. In addition to their tri-lineage differentiation towards adipocytes, chondrocytes and osteoblasts, MSCs modulate the immune response. In fact, MSCs can regulate the proliferation and activation of different immune effector cells (IECs), including T, B and NK cells. The biological effects of MSCs are not exclusively related to their close interaction with target cells by cell-to-cell contact, but can be mediated by molecule release. For instance, MSC immunomodulation may occur through paracrine mechanisms, including indolamine 2,3 dioxygenase, prostaglandin E2, heme-oxygenese-1, and TGF-\u3b2. In the last decade, a key mechanism of cell-to-cell communication of MSCs through extracellular vesicles (EVs) has been clarified. The potential therapeutic role of MSC-derived EVs has been described in different diseases, including cardiovascular disease, acute kidney injury, and lung injury. EVs are molecular shuttles consisting of a phospholipid bilayer containing different molecules, including proteins and different types of RNAs (mRNA and miRNA). EVs are a family of different shedding vesicles, including exosomes (EXs, 50-100 nm), microvesicles (MVs, 100-1000 nm), and apoptotic bodies (ABs, 50-500 nm). EXs originate by multivesicular body and express specific markers, such as CD63, CD9 and Alix. MVs result from the plasmatic membrane and express specific proteins of the cells of origin. To understand whether the MSC immunomodulatory effects are mediated by EV release, we characterized the protein content and immunomodulatory functions towards different immune effector cells of EVs derived from BM-MSCs. In addition, we evaluated the capability of unfractionated PBMCs to internalize MSC-derived EVs. We observed that the rate of EV internalization was higher in B cells and correlated with their capability to reduce B cell proliferation. By using a reproducible and standardized method we showed a new mechanism of MSC-mediated immunosuppression, thus characterizing better the biological function of MSC-derived EVs and paving the way to a possible clinical application of EVs as alternative cell-free therapy

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

Global patterns of intraspecific leaf trait responses to elevation

Elevational gradients are often used to quantify how traits of plant species respond to abiotic and biotic environmental variations. Yet, such analyses are frequently restricted spatially and applied along single slopes or mountain ranges. Since we know little on the response of intraspecific leaf traits to elevation across the globe, we here perform a global meta-analysis of leaf traits in 109 plant species located in 4 continents and reported in 71 studies published between 1983 and 2018. We quantified the intraspecific change in seven morpho-ecophysiological leaf traits along global elevational gradients: specific leaf area (SLA), leaf mass per area (LMA), leaf area (LA), nitrogen concentration per unit of area (Narea), nitrogen concentration per unit mass (Nmass), phosphorous concentration per unit mass (Pmass) and carbon isotope composition (delta C-13). We found LMA, Narea, Nmass and delta C-13 to significantly increase and SLA to decrease with increasing elevation. Conversely, LA and Pmass showed no significant pattern with elevation worldwide. We found significantly larger increase in Narea, Nmass, Pmass and delta C-13 with elevation in warmer regions. Larger responses to increasing elevation were apparent for SLA of herbaceous compared to woody species, but not for the other traits. Finally, we also detected evidences of covariation across morphological and physiological traits within the same elevational gradient. In sum, we demonstrate that there are common cross-species patterns of intraspecific leaf trait variation across elevational gradients worldwide. Irrespective of whether such variation is genetically determined via local adaptation or attributed to phenotypic plasticity, the leaf trait patterns quantified here suggest that plant species are adapted to live on a range of temperature conditions. Since the distribution of mountain biota is predominantly shifting upslope in response to changes in environmental conditions, our results are important to further our understanding of how plants species of mountain ecosystems adapt to global environmental change

Nanomechanical single-photon routing

The merger between integrated photonics and quantum optics promises new opportunities within photonic quantum technology with the very significant progress on excellent photon-emitter interfaces and advanced optical circuits. A key missing functionality is rapid circuitry reconfigurability that ultimately does not introduce loss or emitter decoherence, and operating at a speed matching the photon generation and quantum memory storage time of the on-chip quantum emitter. This ambitious goal requires entirely new active quantum-photonic devices by extending the traditional approaches to reconfigurability. Here, by merging nano-optomechanics and deterministic photon-emitter interfaces we demonstrate on-chip single-photon routing with low loss, small device footprint, and an intrinsic time response approaching the spin coherence time of solid-state quantum emitters. The device is an essential building block for constructing advanced quantum photonic architectures on-chip, towards, e.g., coherent multi-photon sources, deterministic photon-photon quantum gates, quantum repeater nodes, or scalable quantum networks.Comment: 7 pages, 3 figures, supplementary informatio

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

Enhanced spontaneous emission from quantum dots in short photonic crystal waveguides

We report a study of the quantum dot emission in short photonic crystal waveguides. We observe that the quantum dot photoluminescence intensity and decay rate are strongly enhanced when the emission energy is in resonance with Fabry-Perot cavity modes in the slow-light regime of the dispersion curve. The experimental results are in agreement with previous theoretical predictions and further supported by three-dimensional finite element simulation. Our results show that the combination of slow group velocity and Fabry-Perot cavity resonance provides an avenue to efficiently channel photons from quantum dots into waveguides for integrated quantum photonic applications.Comment: 12 pages, 4 figure