We present an athermal resonant metasurface created by periodically coupled subwavelength structures in an engineered membrane. In this approach, we propose a bilayer metasurface structure with opposite thermo-optic coefficients to compensate for the undesired spectral shift caused by thermal effects. By creating a bilayer metastructure with different thermo-optic coefficients, thermally-induced spectral shifts could be minimized, thus allowing for a nearly temperatureinsensitive resonant metasurface. The working principle of the proposed metasurface is based on the epitaxial engineering of the coupled subwavelength structures in the metasurface to cancel the temperature-dependent resonant wavelength shift. To demonstrate the proposed thermal compensation concept, Fano-type guided mode resonance will be used as an optical probe in a TiO2/Si composite film. The proposed athermal resonant metasurface offers great potential for many applications, such as free-space optical communications, metasurface-based optical sensing, imaging, filters, and modulators.
In this talk, we present our recent findings on compound semiconductor-based nonlinear metasurfaces for all-optical signal processing. Nonlinear metasurfaces have revolutionized the field of nonlinear optics by enabling a radically different way to control light-matter interactions at the subwavelength scale. In this approach, nonlinear optical processes can be maximized by carefully choosing the shape, orientation, and arrangement of subwavelength-scale artificial atoms, called meta-atoms. By introducing Kerr nonlinearity from compound semiconductor materials, such as AlGaAs/GaAs, into a high-quality resonant metasurface, power requirement to achieve optical bistability can be greatly reduced. Optical bistability can has been actively studied due to its potential applications for all-optical switching and optical logic gates. In our research, we will utilize intensity-dependent refractive index in a semiconductor metasurface to realize refractive bistability for all-optical signal processing. Different design strategies will be discussed to excite quasi-bound waves with a high-quality factor and a small mode volume.
Unidirectional transmission is a fundamental function in signal processing. In electronic systems, simple semiconductor p–n diodes provide one-way transmission for electrical signals. In the optical domain, however, achieving one-way transmission is difficult because it requires breaking the time-reversal symmetry of light–matter interaction. Previously, magneto-optic effects have been successfully utilized to break the reciprocity of light–matter interaction. Here, we propose a meta-optic diode that supports nonreciprocal light transmission through excitation of asymmetric quasi-bound waves. The meta-optic diode consists of an ultrathin dielectric slab, patterned with two types of subwavelength resonator arrays. The proposed approach is based on the lifetime engineering of the resonant modes in the arrays and the inherent Kerr nonlinearity of the dielectric slab.
We report light trapping and guiding properties in resonant dielectric metastructures for chip-scale photonic integrated circuit applications. Recently, several optical phenomena in all-dielectric structures have shown a new way to tightly confine light through the engineering of resonant scattering. The engineered resonant light scattering in dielectric artificial subwavelength structures can be utilized as an efficient light coupling platform between free space and integrated photonic devices, and strongly tailor light-matter interactions for a variety of metasurface applications. In this paper, we present the design and numerical modeling of high-index subwavelength asymmetric resonant structures that can guide light for integrated photonic circuits. The metastructures reported here consist of all-dielectric two-dimensional optical antenna arrays patterned on a slab waveguide. Light coupling can be achieved by synchronizing the phase of the resonantly scattered light by the subwavelength antennas to that of the guided modes in the waveguide. Resonant light scattering by high-index subwavelength resonators can lead to light trapping through the excitation of quasi photonic bound states in the continuum. This unique feature can be used to selectively launch a guided mode into a photonic waveguide at a predetermined spectral band.
We present numerical modeling and experimental characterization of the photonic bound states in high-contrast Si-based subwavelength grating waveguide structures. The resonant modes in the grating waveguides show some of the unique features of the photonic bound states in the continuum: continuous narrowing of the resonance linewidth and cancellation of radiative waves. The calculated field distributions show strong internal field buildup around resonances. To verify our simulation results, a Si-based subwavelength grating waveguide was fabricated and experimentally characterized. The measured reflection spectra show two resonance peaks around λ0 = 1490 nm and λ0 = 1505 nm. According to the simulated results, these two peaks are located near a BIC condition. The captured infrared microscope images in the reflection measurement reveal the dynamical interaction between the incident light and the subwavelength grating waveguide. The demonstrated Si grating waveguides has potential to be used as highly efficient frequency-selective couplers between free-space optical waves to in-plane guided optical waves in existing Si integrated photonic circuits.
We report our in-house R&D efforts of designing and developing key integrated photonic devices and technologies for a chip-scale optical oscillator and/or clock. This would provide precision sources to RF-photonic systems. It could also be the basic building block for a photonic technology to provide positioning, navigation, and timing as well as 5G networks. Recently, optical frequency comb (OFC)-based timing systems have been demonstrated for ultra-precision time transfer. Our goal is to develop a semiconductor-based, integrated photonic chip to reduce the size, weight, and power consumption, and cost of these systems. Our approach is to use a self-referenced interferometric locking circuit to provide short-term stabilization to a micro-resonator-based OFC. For long-term stabilization, we use an epsilon-near-zero (ENZ) metamaterial to design an environment-insensitive cavity/resonator, thereby enabling a chip-scale optical long-holdover clock.
Remote sensing of the refractive index of a target surface has important application for detection and classification tasks. In this paper, we introduce a modified pBRDF model with a diffuse scattering term and apply it in the development of a method to jointly estimate the refractive index, slope variance roughness and diffuse scattering parameter from a series of Stokes parameter measurements of a surface. This work is a significant extension of prior work involving degree-of-polarization (DOP) measurements. Numerical trials are presented that indicate diffuse scattering can significantly affect the surface parameter estimation and estimation accuracy improves considerably by including the diffuse component in the scattering model. We also illustrate that using Stokes measurements for the estimation provides more accuracy than using DOP measurements for a limited number of observations. The estimation approach is applied to Stokes and DOP measurements of several material surfaces published previously in the literature and the parameter estimation produces curves that have excellent correspondence with the measurement values.
Optical polarimetry is an approach that shows promise for refractive index estimation from scattering off a target’s surface, which is task of pivotal importance for remote sensing and computer graphics applications. However, the estimation often relies on a microfacet polarimetric bidirectional reflectance distribution function (pBRDF) that is limited to specular targets involving single surface scattering. In this paper, we develop an analytic model for the degree of polarization (DOP) reflected from a rough surface that includes a multiplicative factor for the effect of diffuse scattering. Evaluation of the model indicates that diffuse scattering can significantly affect the DOP values, and the biased DOP values can further lead to inaccurate estimation of the surface refractive index.
A high-fill factor/high-SNR CMOS readout integrated circuit (ROIC) array is designed for high dynamic range infrared
imaging systems. The designed ROIC array uses a single reference photodiode that is routed to each unit cell in the array
to subtract the dark current for high SNR and high fill factor. The achieved average SNR is 80 dB and the fill factor is
28% with a 25 x 25 μm2 unit cell size. With this new unit cell and routing approach, the size of the unit cell is reduced by
300% compared to other high SNR circuits. The maximum power consumption per unit pixel is 500nW.
The integration of planar optical components at the chip scale with electronic and microfluidic systems is enabling an
increased penetration of optical functionality into systems, and in particular, into highly portable systems. Planar
lightwave integrated circuits can enable functions such as optical signal interfaces, optical signal distribution, and optical
sensing in a planar platform that leverages microelectronic manufacturing technologies. Key to the realization of these
chip scale systems are the design and implementation of thin film optoelectronic materials and devices, heterogeneous
integration of components, planar polymer waveguides and waveguiding structures, and optical sensors. The integration
of photonic components and subsystems with electronic systems and microfluidics systems are also topics of intense
investigation for chip scale system with increasing levels of function. Application areas that are emerging include optical
signal interfaces and distribution, as well as chip scale integrated optical sensing systems with applications in medicine
and the environment.
Miniaturized, portable sensing systems for medical and environmental diagnostics and monitoring are an excellent
application area for microresonator sensors. Polymer microresonators are attractive components for chip scale integrated sensing because they can be integrated in a planar format using standard semiconductor manufacturing technologies. Vertically coupled microresonators, where the waveguides lie below or above the microresonator, can be fabricated using standard photolithography, enabling low cost integrated sensor systems. Microresonators can be surface customized for discrimination in, for example, chemical sensing applications, or the surface can be functionalized for biological sensing applications. To create chip scale integrated sensing systems, microresonators can be integrated with planar optical system components, such as polymer waveguides and thin film photodetectors, onto silicon using heterogeneous integration. Heterogeneous integration can also be used to integrate optical sources with sensors onto host substrates such as silicon.
Practical, packaged photodetectors (PDs) must be interfaced to bias and transmission lines, which introduce parasitics. These parasitics (resistance, capacitance and inductance) can be used to shape the temporal and frequency response of packaged photodetectors. Thus, the bias circuitry, external passives, and high speed interconnections must be carefully designed to produce the desired response in a packaged photodetector. Applications dictate the desired PD characteristics, which are generally either a flat frequency response, or a fast, ring-free impulse response. In this paper, the effects of the parasitic resistance, capacitance, and inductance are studied to affect the intrinsic response of photodetectors for a flat frequency response or a fast ring-free impulse response. For the optical transmission of microwave and millimeter wave RF signals, such as remote antennas or radar arrays, a flat frequency response is critical. A flat frequency response can be obtained from controlled ringing in the temporal domain. This paper explores the control of ringing in the temporal domain using varied external loads. A fast fall time, ring-free pulse is useful for digital communications applications where ringing can degrade the bit error rate. Fourier transforms show that a ring-free impulse response has a characteristic fall-off at high frequencies. However, this fall-off is detrimental for frequency domain applications, so the optimization condition for the inductance and capacitance is different for these applications. This paper explores the suppression of the impulse response tail by varying the external loads.
High speed optical interconnections offer an attractive alternative to electrical interconnections, particularly when they can be integrated into electrical systems. In particular, waveguide signal distribution and optical to electrical (O/E) conversion are critical to the integration of optical signals into electrical systems. The integration and interfaces between waveguides and O/E devices is a topic under intensive study. One approach to the integration of optical interconnections into electrical systems is to use fully embedded thin film optoelectronic (OE) devices in planar lightwave components on electrical interconnection substrates. In this approach, the propagating optical signal from the optical waveguide can be evanescently or directly coupled into the embedded thin film OE devices based on the embedded structure. Efficient and high speed optical signal distribution and O/E conversion, such as those using planar channel polymer waveguides with embedded thin film photodetectors, are examples of optical interconnection critical functions that are optimally implemented in electrical systems. In this paper, a 1 by 4 thin film metal semiconductor metal (MSM) photodetector (PD) array is embedded in a 1 by 4 photoimageable polymer multimode interference (MMI) coupler. This optical distribution and E/O system was fabricated and experimentally characterized at a wavelength of 1.3 μm. The measured overall loss, including the propagation loss and splitting loss of the MMI coupler was -0.18 dB at λ = 1.3 μm.
The integration of active optoelectronic devices, passive optical devices, and electronics into planar lightwave integrated circuits (PLICs) at the chip level, and planar lightwave integrated systems (PLIS) at the substrate or package level, have applications in optical interconnection, optical signal distribution and processing, and in integrated optical sensing. Heterogeneous integration of thin film devices is an effective method of creating PLICs and PLIS. Thin film InP-based edge emitters and thin film photodetectors have been integrated with polymer waveguides to create planar lightwave systems.
Optical sensing, and the integration of sensors and electronics into Sensor on a Chip and Sensor on a Package systems are an approach to the creation of miniaturized, portable, customizable, low cost sensor systems for rapid health diagnostics, medical research, environmental monitoring, and security monitoring. To integrate optical sensing systems that are autonomous, it is essential to integrate the sensor, light source, and light detection into a single substrate or chip. The integration of this optical system with signal control and processing electronics enable discrimination with individually customized sensors in sensor arrays, and high sensitivity levels. Thin film optoelectronic active device integration with planar optical passive devices is a heterogeneous integration method for fabricating planar lightwave integrated circuits at the chip level and planar lightwave integrated systems at the substrate and package level.
As an alternative approach to current electrical interconnection technology, optical interconnections at high speeds offer several potential advantages including small footprint, simple system design (in comparison to transmission lines), and immunity to electromagnetic interference. There are a number of approaches to integrating optical signal paths in electrical interconnection substrates such as backplanes, boards, and modules. One approach utilizes the heterogeneous integration of thin film optoelectronic (OE) devices embedded in waveguides. Optical signals can be coupled in from external fibers or from thin film lasers integrated onto the substrate, propagated, distributed, and processed in a planar waveguide format, and then coupled from the waveguide to an embedded thin film photodetector by evanescent field or direct coupling. This approach achieves alignment through assembly and successive masking layers and thus minimizes alignment issues. In addition, the integrated optical signal distribution system can be integrated onto the electrical interconnection substrate after the substrate has been fabricated using post processing, thus, the board facility is not impacted through the integration of the optical links.
In this paper, a discussion of the fabrication processes as well as coupling efficiency and speed measurement results for thin film InGaAs PDs embedded in polymer waveguides integrated onto Si substrates is included. These results are compared to theoretical estimates of the coupling efficiency, which was estimated using the finite difference beam propagation method.
As optoelectronic devices increase in speed, the measurement system used to characterize these devices must have sufficient bandwidth and minimum parasitic loading during test to accurately determine the intrinsic performance of the device under test. Conventional electrical measurement systems have an intrinsic bandwidth due to the available components for test and have parasitic loading due to direct electrical contact to the device under the test. Electro-optic sampling is an excellent measurement technique for characterizing ultra-fast devices because it has high bandwidth, is non-contact, is non-destructive, and relatively non-invasive. In this paper, an optical fiber-based electro-optic sampling system is designed and used for characterizing high speed InGaAs thin film MSM photodetectors. A fiber laser which is operating at 1556 nm wavelength was used for the sampling and excitation beam. Optical fibers were used to connect each component in the system for flexibility. InGaAs thin film MSM photodetectors were fabricated and characterized. InGaAs thin film MSM photodetectors were bonded onto a coplanar strip line deposited on a benzocyclobutene (BCB)-coated glass substrate for characterization. These thin film photodetectors show high speed operation combined with high responsivity and large detection area compared to P-I-N photodetectors operating at similar speeds
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