Classical and fluctuation-induced electromagnetic interactions in micronscale systems: designer bonding, antibonding, and Casimir forces

Whether intentionally introduced to exert control over particles and macroscopic objects, such as for trapping or cooling, or whether arising from the quantum and thermal fluctuations of charges in otherwise neutral bodies, leading to unwanted stiction between nearby mechanical parts, electromagnetic interactions play a fundamental role in many naturally occurring processes and technologies. In this review, we survey recent progress in the understanding and experimental observation of optomechanical and quantum-fluctuation forces. Although both of these effects arise from exchange of electromagnetic momentum, their dramatically different origins, involving either real or virtual photons, lead to different physical manifestations and design principles. Specifically, we describe recent predictions and measurements of attractive and repulsive optomechanical forces, based on the bonding and antibonding interactions of evanescent waves, as well as predictions of modified and even repulsive Casimir forces between nanostructured bodies. Finally, we discuss the potential impact and interplay of these forces in emerging experimental regimes of micromechanical devices.Comment: Review to appear on the topical issue "Quantum and Hybrid Mechanical Systems" in Annalen der Physi

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

MEMS for Photonic Integrated Circuits

The field of microelectromechanical Systems (MEMS) for photonic integrated circuits (PICs) is reviewed. This field leverages mechanics at the nanometer to micrometer scale to improve existing components and introduce novel functionalities in PICs. This review covers the MEMS actuation principles and the mechanical tuning mechanisms for integrated photonics. The state of the art of MEMS tunable components in PICs is quantitatively reviewed and critically assessed with respect to suitability for large-scale integration in existing PIC technology platforms. MEMS provide a powerful approach to overcome current limitations in PIC technologies and to enable a new design dimension with a wide range of applications

Miniaturization of optical spectrometers

Spectroscopic analysis is one of the most widely used analytical tools across both scientific research and industry. Whilst laboratory bench-top spectrometer systems offer superlative resolution and spectral range, their miniaturization is crucial for applications where portability is paramount, or in-situ measurements must be made. Advancement in this field over the last three decades is now yielding microspectrometers with performance and footprint near those viable for lab-on-a-chip systems, smartphones and other consumer technologies. In this review, we briefly summarize the technologies that have emerged toward achieving these aims - including miniaturized dispersive optics, narrowband filter systems, Fourier transform interferometers and reconstructive microspectrometers - and discuss the challenges associated with improving spectral resolution while device dimensions shrink ever further.EPSRC: EP/L016087/1 National Natural Science Foundation of China (51706141, 51976122

On-chip infrared sensors: redefining the benefits of scaling

Infrared (IR) spectroscopy is widely recognized as a gold standard technique for chemical and biological analysis. Traditional IR spectroscopy relies on fragile bench-top instruments located in dedicated laboratory settings, and is thus not suitable for emerging field-deployed applications such as in-line industrial process control, environmental monitoring, and point-of-care diagnosis. Recent strides in photonic integration technologies provide a promising route towards enabling miniaturized, rugged platforms for IR spectroscopic analysis. It is therefore attempting to simply replace the bulky discrete optical elements used in conventional IR spectroscopy with their on-chip counterparts. This size down-scaling approach, however, cripples the system performance as both the sensitivity of spectroscopic sensors and spectral resolution of spectrometers scale with optical path length. In light of this challenge, we will discuss two novel photonic device designs uniquely capable of reaping performance benefits from microphotonic scaling. We leverage strong optical and thermal confinement in judiciously designed micro-cavities to circumvent the thermal diffusion and optical diffraction limits in conventional photothermal sensors and achieve a record 104 photothermal sensitivity enhancement. In the second example, an on-chip spectrometer design with the Fellgett's advantage is analyzed. The design enables sub-nm spectral resolution on a millimeter-sized, fully packaged chip without moving parts.National Science Foundation (U.S.) (Award 1506605)United States. Department of Energy (Grant DE-NA0002509

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

Near-infrared spectroscopic cathodoluminescence imaging polarimetry on silicon photonic crystal waveguides

We measure polarization- and wavelength-resolved spectra and spatial emission intensity distributions from silicon photonic crystal waveguides in the near-infrared spectral range using spectroscopic cathodoluminescence imaging polarimetry. A 30 keV electron beam, incident along the surface normal of the sample, acts as an ultrabroadband and deeply subwavelength excitation source. For photonic crystal waveguides with a broad range of design parameters, we observe a dominant emission intensity distribution that is strongly confined to the waveguide. For a period of 420 nm and a hole radius of 120 nm, this occurs at a wavelength of 1425 nm. The polarization-resolved measurements demonstrate that this feature is fully linearly polarized along the waveguide axis. Comparing the modal pattern and polarization to calculations of the electric field profiles confirms that we measure the odd TE waveguide mode of the system. This result demonstrates that the electron beam can couple to modes dominated by in-plane field components in addition to the more commonly observed modes dominated by out-of-plane field components. From the emission directionality, we conclude that we sample a leaky portion of the odd waveguide mode

Reconfigurable photonic crystal

Tunability and programmability are highly demanded for silicon photonic integrated circuits (PICs) to expand their applications in the next-generation photonics. The main objective of this thesis is to develop several reconfigurable and programmable photonic crystal (PC) devices. In Chapter 2, we developed a relatively general nanofabrication process for integrating PC devices with movable mechanical components on silicon-on-insulator (SOI) wafers. We also investigated grating coupling technology, to facilitate coupling lights into and out of PC devices. In Chapter 3, we developed an all-optical programmable PC device that integrates digital micromirror device (DMD), photo-responsive LC, and PC technologies. We demonstrated the functionality and programmability of the device, by forming a point-defect cavity, a straight waveguide, and a waveguide bend on the single device. In Chapter 4, we developed two types of reconfigurable PC devices by leveraging the strengths of optical nanobeam and nano-electro-mechanical systems (NEMS) technologies. The first device consists of an array of movable nanobeams. Each nanobeam is an electrostatically tunable photonic element in a PC waveguide. We demonstrated the capability of the device to engineer different photonic bandgaps, by tuning one unit in group of two neighboring nanobeam units, tuning one or two in group of three units, and forming two reconfigurable PCs, on the single device. To achieve a higher-level integration, we also theoretically studied another reconfigurable PC integrating an array of mechanical tunable nanobeams with an array of fixed pillars into the top silicon layer of a SOI wafer. In Chapter 5, we developed two tunable photonic crystal-cantilever cavity (PC3) resonators. The first device has an NEMS cantilever embedded into a L6 cavity in a PC slab. The second device has a similar cantilever to insert into a nanobeam-base waveguide. We studied bending characteristics of the cantilever and optical characteristics of these two devices at different applied voltages. In Chapter 6, we conducted theoretical investigation on a nano-opto-mechanical reconfigurable PIC device consisting of an array of silicon plugs and a 2D PC slab. We theoretically demonstrated that a point-defect cavity, a line-defect waveguide, and a waveguide bend can be configured in the PC slab, by inserting different plugs into an air hole, a straight line of holes, and an L-shape line of holes

Design of a quasi-2D photonic crystal optomechanical cavity with tunable, large x^2-coupling

We present the optical and mechanical design of a mechanically compliant quasi-two-dimensional photonic crystal cavity formed from thin-film silicon in which a pair of linear nanoscale slots are used to create two coupled high-Q optical resonances. The optical cavity supermodes, whose frequencies are designed to lie in the 1500 nm wavelength band, are shown to interact strongly with mechanical resonances of the structure whose frequencies range from a few MHz to a few GHz. Depending upon the symmetry of the mechanical modes and the symmetry of the slot sizes, we show that the optomechanical coupling between the optical supermodes can be either linear or quadratic in the mechanical displacement amplitude. Tuning of the nanoscale slot size is also shown to adjust the magnitude and sign of the cavity supermode splitting 2J, enabling near-resonant motional scattering between the two optical supermodes and greatly enhancing the x^2-coupling strength. Specifically, for the fundamental flexural mode of the central nanobeam of the structure at 10 MHz the per-phonon linear cross-mode coupling rate is calculated to be be g+−/2π=1MHz, corresponding to a per-phonon x^2-coupling rate of g′/2π=1kHz for a mode splitting 2J/2π = 1 GHz which is greater than the radiation-limited supermode linewidths

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