The periodic patterning of the optical medium achieved through photonic crystal membranes (PCMs) can be
employed for controlling the resonant coupling of external radiation continuum to above-the-light-line flat edges
of the folded band structure in strongly corrugated waveguides, resulting in high reflectivity for an efficient quasi-3D light harnessing. Recently, vertical-cavity surface-emitting lasers (VCSELs) emitting in C-band using a double set of one-dimensional Si/SiO2 photonic crystals as compact, flexible, and power efficient mirrors have been realized within a mass-scale fabrication paradigm by employing standard 200-mm microelectronics pilot lines. Conceived as the basic building block for photonics-on-silicon back-end integration of group III-V laser microsources, the extreme flexibility of the novel photonic architecture enables to perform a tailored modal selection of the optical cavity, including polarization and far-field control. It also offers a wide range of functionality, such as on-chip optical routing and a variety of efficient wavelength tuning-trimming schemes. Device compactness ensures a considerable reduction in the device footprint, power consumption, and parasitics. Furthermore, high fabrication yields obtained thanks to the state-of-the-art molecular wafer bonding of III-V alloys on silicon conjugate excellent device performances with cost-effective high-throughput production, indicating strong perspective industrial potential.
We generalize the theoretical modelling of high contrast gratings (HCGs) to arbitrary incidence angle. We unveil the
HCG band diagram with HCG supermode formulism, and present the intriguing connections between HCG and 1D
photonic crystal (PhC). We show, for the first time, HCG and PhC can be unified in the same theoretical analysis
architecture. They differentiate themselves by operating at different regimes in the band diagram. The HCG band
diagram not only provides a powerful tool to study both HCG and PhC, but also to design various optical components
with different functionalities on the HCG-based optoelectronics platform.
We have developed a new type of Si-based 3D cage-like high-contrast metastructure waveguide with both “slow-light”
and low-loss properties, which has applications in providing a long time-delay line or a high Q cavity in chip-scale optoelectronic integrated circuits (OEIC). Traditional semiconductor optical waveguides always have high loss when used in a high dispersion (slow-light) region. A preliminary computational model has predicted that there is a slow-light and low propagation loss region within cage-like hollow-core waveguide formed by 4 high-contrast-gratings walls/claddings. Using our new processing technique, we fabricated several such waveguides on a Si wafer with different core sizes/shapes and different HCGs for 1550 operation wavelength. We have conducted experimental waveguide delay test measurements using a short optical pulse which indicate that the group velocity of these metastructure waveguides are in the range of 20- 30% of the speed of the light. Using a waveguide “cut-back” method, we have experimentally determined the propagation loss of these waveguides are in the range of 2-5dB/cm. We are also developing this type of high-contrast metastructure hollow-core waveguide for different operating wavelength/frequency such as THz for different applications.
In recent years, various approaches to improve the speed of directly modulated vertical-cavity surface-emitting lasers
(VCSELs) have been reported and demonstrated good improvement. In this paper, we propose and numerically investigate a new possibility of using high-index-contrast grating (HCG) as mirror for VCSELs. By changing the grating
design, one can control the reflection delay of the grating mirror, enabling the control of cavity photon lifetime. On the other hand, short energy penetration depth of the HCG results in smaller modal volume, compared to DBR VCSELs. An example structure shows that the HCG VCSEL has a 30-% higher 3-dB bandwidth than the DBR VCSEL.
Novel VCSEL structure with a high-index-contrast grating (HCG) mirror that can selectively choose one of two LP01 and four LP11 modes is proposed and numerically investigated. This can be achieved by designing the spatial reflectivity profile of the HCG so that the HCG gives a LP mode of interest the highest modal reflectivity among six LP modes. This approach may considerably miniaturize the light source module for mode-division multiplexing.
High Contrast Gratings (HCG) have become a hot research topic, because of their new functionalities at very small
volumes. However no efficient 3D VCSEL model capable to account for HCG has been reported so far. HCG design is
therefore mainly based on 1D simulations. For realistic structures usually FDTD is the most popular approach, with its
well known cumbersome computation drawbacks. VELM code,1 the well established VCSEL electromagnetic solver developed in the last ten years in the Torino group, has now been upgraded to rigorously handle HCG layers. The efficiency of the tool is preserved, and an entire set of HCG VCSEL modes can be computed in minutes on an ordinary desktop. A full set of design tools and guidelines, starting from 1D HCG properties up to 3D simulations which include HCG in the VCSEL design, will be presented and applied to the design of a structure that is in fabrication.
GaN-based high contrast grating surface-emitting lasers (HCG SELs) with AlN/GaN distributed Bragg reflectors were reported. The device exhibited a low threshold pumping energy density of about 0.56 mJ/cm2 and the lasing wavelength was at 393.6 nm with a high degree of polarization of 73% at room temperature. The specific lasing mode and polarization characteristics agreed well with the theoretical modeling. The low threshold characteristics of our GaNbased HCG SELs utilized by the Fano resonance can be potential for development of blue surface emitting laser sources
High contrast sub-wavelength gratings (HCGs) possess unique diffraction properties such as steep angle diffraction, high diffraction efficiency with large spectral bandwidth. Here, we discuss the application of diffraction optimized HCG's as security tags with color shifting properties. Simulations of the gratings carried out using rigorous coupled wave analysis clearly show distinguishable color features for different angles of viewing. The designed security tags find application in documents which are prone to risk of counterfeiting.
A beam-steering device has been a key element for various sensing and imaging applications. We proposed a novel beam steering device based on a Bragg reflector waveguide. A steering angle of over 60 degrees and a number of resolution points over 1,000 can be expected for a few mm long device with a low loss high-contrast Bragg reflector waveguide. One-? thick core is sandwiched by two high-contrast quarter wavelength stack mirrors. A slow-light mode can propagate laterally in such a structure with a low propagation loss thanks to high-contrast DBRs. We could see a large angular dispersion of over 1°/nm for radiated light from the surface. Thanks to the large angular dispersion, we are able to realize beam-steering of the radiated light by tuning the wavelength of incident light. We could obtain a continuous steering angle of more than 60° by wavelength tuning of 40nm. The divergence angle is below 0.04° for all wavelengths. Considering the steering range and sharp beam divergence, we successfully achieved a number of resolution points of over 1,000. It is the highest number among all other beam steering devices without a mechanical scanner. We will discuss super-high resolution beam steering from an ultra-low loss Bragg reflector waveguide with high-contrast metastructures. Also, high contrast sub-wavelength grating offers additional freedom to control the inplane phase and reflectivity in Bragg reflector waveguides, which may enable us to control the far-field pattern profile for higher resolutions.
We present a single crystalline silicon optical phased array using high-contrast-gratings (HCG) for fast two dimensional
beamforming and beamsteering at 0.5 MHz. Since there are various applications for beamforming and beamsteering
such as 3D imaging, optical communications, and light detection and ranging (LIDAR), it is great interest to develop
ultrafast optical phased arrays. However, the beamsteering speed of optical phased arrays using liquid crystal and
electro-wetting are typically limited to tens of milliseconds. Optical phased arrays using micro-electro-mechanical
systems (MEMS) technologies can operate in the submegahertz range, but generally require metal coatings. The metal
coating unfortunately cause bending of mirrors due to thermally induced stress.
The novel MEMS-based optical phased array presented here consists of electrostatically driven 8 × 8 HCG pixels
fabricated on a silicon-on-insulator (SOI) wafer. The HCG mirror is designed to have 99.9% reflectivity at 1550 nm
wavelength without any reflective coating. The size of the HCG mirror is 20 × 20 μm2 and the mass is only 140 pg,
much lighter than traditional MEMS mirrors. Our 8 × 8 optical phased array has a total field of view of ±10° × 10° and a
beam width of 2°. The maximum phase shift regarding the actuation gap defined by a 2 μm buried oxide layer of a SOI
wafer is 1.7π at 20 V.
A novel 8x8 optical phased array based on high-contrast grating (HCG) all-pass filters (APFs) is experimentally demonstrated with high speed beam steering. Highly efficient phase tuning is achieved by micro-electro-mechanical
actuation of the HCG to tune the cavity length of the APFs. Using APF phase-shifters allows a large phase shift with an
actuation range of only tens of nanometers. The ultrathin HCG further ensures a high tuning speed (0.626 MHz). Both one-dimensional and two-dimensional HCGs are demonstrated as the actuation mirrors of the APF arrays with high beam steering performance.
Experiments in the field of high-precision optical metrology are crucially limited by thermal noise of the optical components such as mirrors or beam splitters. Amorphous coatings stacks are found to be a main source for these thermal fluctuations. In this contribution we present approaches to realize coating free optical components based on resonant high contrast gratings (HCGs) made of crystalline silicon. It is shown that beside classical cavity mirrors the concept of HCGs can also be used for reflective cavity couplers. We compare the advantages and challenges of these HCG reflectors with distributed Bragg reflectors made of crystalline coatings for applications in optical metrology.
Subwavelength diffraction gratings patterned into a silicon nitride membrane offer a novel new platform for cavity
optomechanics. The monolithic device combines high reflectivity, high mechanical quality factor, and low mass.
Here we survey results we have obtained using such a device as one mirror of a Fabry-Perot cavity. With a cavity
finesse of F ≈ 2000, we are able to optically cool hundreds of mechanical modes of the membrane. The lowest
effective temperature we achieve, by detuning a laser to the red side of an optical resonance, is approximately
Teff = 1 K. The cooling is accompanied by an optically-induced shift of the mechanical frequency, as expected; both the degree of cooling and frequency shift are proportional to the power of the cooling laser. When we
detune the laser to the blue side of the resonance, the resulting optical “antidamping” causes the dynamics of the
mechanical system to change from thermal to oscillatory, with a well-defined phase. Finally, we computationally
investigate the feasibility of a proposal to realize radiation pressure optomechanics without a cavity, by use of a
subwavelength grating with a rapid variation of reflectivity with wavelength.
In this paper, we report the theoretical and experimental possibility of achieving a quarter-wave plate regime
by using high-contrast gratings, which are binary, vertical, periodic, near-wavelength, and two-dimensional high
refractive index gratings. Here, we investigate the characteristics of two distinct designs, the first one being composed
of silicon-dioxide and silicon, and the second one being composed of silicon and sapphire. The suggested quarter-wave plate regime is achieved by the simultaneous optimization of the transverse electric and transverse magnetic transmission coefficients, TTE and TTM, respectively, and the phase difference between these transmission coefficients, such that |TTM| ≃ |TTE| and ∠TTM − ∠TTE ≃ π/2. As a result, a unity circular polarization conversion efficiency is achieved at λ0 = 1.55 μm for both designs. For the first design, we show the obtaining of unity conversion efficiency by using a theoretical approach, which is inspired by the periodic waveguide interpretation, and rigorous coupled-wave analysis (RCWA). For the second design, we demonstrate the unity conversion efficiency by using the results of finite-difference time-domain (FDTD) simulations. Furthermore, the FDTD simulations, where material dispersion is taken into account, suggest that an operation percent bandwidth of 51% can be achieved for the first design, where the experimental results for the second design yield a bandwidth of 33%. In this context, we define the operation regime as the wavelength band for which the circular conversion efficiency is larger than 0.9.
In this paper, we review our work on efficient, broadband and polarization independent interfaces between a silicon-on-insulator photonic IC and a single-mode optical fiber based on grating structures. The high alignment tolerance and the fact that the optical fiber interface is out-of-plane provide opportunities for easy packaging and wafer-scale testing of the photonic IC. Next to fiber-chip interfaces we will discuss the use of silicon grating structures in III-V on silicon optoelectronic components such as integrated photodetectors and microlasers.
The optical interconnect is the key technology to support the large bandwidth demand of the super computers and data
centers. As the tremendous number of optical links being implemented in the system, wavelength-division multiplexing
(WDM) is the solution to reduce the use of optical fibers. In this work, we propose to use the vertical coupler based on
the high contrast metastructure to enable the multiplexing and input-output coupling on the photonics chip. The coupling
efficiency can reach 90% for 35 nm 1 dB bandwidth by multiplexing four on-chip channels into the optical fiber.
A cavity-resonator-integrated guided-mode-resonance filter (CRIGF) consisting of a grating coupler (GC) and a pair of
distributed-Bragg-reflectors (DBRs) on a thin-film dielectric waveguide is reviewed. The CRIGF has been recently
proposed by the authors to provide a narrow-band reflection spectrum for an incident wave of a small beam width from the free space. A newly developed analysis model for device design with performance simulation is introduced. Curved gratings are utilized to construct a resonator for a small-aperture CRIGF. Design, fabrication and characterization of CRIGFs of 10 μm aperture are described with a resonance wavelength of 850 nm. A Ge:SiO2 guiding core layer was deposited on a SiO2 glass substrate, and GC and DBRs were formed by the electron-beam direct writing lithography. A normal polarization-dependent CRIGF is shown with a obtained narrowband reflection spectrum of 0.2 nm full width at half maximum. A crossed-CRIGF is also discussed to eliminate the polarization dependence. It is successfully demonstrated that measured reflection spectra for TE and TM incident beams were well coincident with each other.
The angular sensitivity of guided mode resonant filters (GMRF) is well known. While at times useful for angle tuning of
the response, this sensitivity can also be a major detriment as angular changes of tenths of a degree can shift the
wavelength response in a narrow bandwidth device by an amount greater than the width of the resonance peak. We
identify geometries where the resonance is more angularly stable, demonstrating high reflectivity at the design
wavelength for several degrees in both azimuth and inclination angular directions with virtually no change in lineshape of the response. The investigation of GMRFs in both classical and conical mounts through simulation using rigorous coupled wave analysis reveals that there are preferred mounts for greater angular tolerance. We simulate a grating at telecom wavelengths using a design that we have previously fabricated. The identical grating placed in different mounts can exhibit angular tolerances that differ by well over an order of magnitude (60x). The most commonly used classical mount has a much more sensitive angular tolerance than does the conical mount. The lineshape of the resonant response shows only negligible changes across the angular band. The angular band for the sample grating is simulated to be several degrees in the conical mount as opposed to a tenth of a degree in the classical mount. We could thus expand the application space for narrow-band GMRFs into areas where angular tolerance cannot be controlled to the degree that we have believed required in the past.
In this paper, we propose a broadband-tunable resonant-cavity-enhanced photodetector (RCE-PD) structure with double high-index-contrast grating (HCG) mirrors and numerically investigate its characteristics. The detector is designed to operate at 1550-nm wavelength. The detector structure consists of a top InP HCG mirror, a p-i-n photodiode embedding multiple quantum wells, and a Si HCG mirror formed in the Si layer of a silicon-on-insulator wafer. The detection wavelength can be changed by moving the top InP HCG mirror suspended in the air. High reflectivity and small penetration length of HCGs lead to a narrow absorption linewidth of 0.38 nm and a broad tuning range of 111 nm. The peak absorption efficiency is 76-84% within the tuning range. This broadband-tunable and narrow-absorption-linewidth RCE-PD is desirable for applications where selective wavelength demultiplexing is required. Furthermore, the fact that it can be fabricated on a silicon platform offers us a possibility of integration with electronics.
We present novel filter elements with an asymmetric angle dependent transmission based on high-contrast gratings.
Asymmetric means a different efficiency for positive and negative incidence angles. Our approach provides the realization of asymmetric direction selective filters by using blaze-like grating structures combined with subwavelength
high contrast gratings respectively grating periods in the resonance domain. We also discuss the influence of the effective medium theory on the transmission function depending on the angle of the incident light. For realization of those high contrast gratings Silicon is chosen as material with high refractive index and adequate compatibility with semiconductor fabrication.
We report broadband reflectance in the long-wavelength infrared (LWIR, 8-12 μm) utilizing suspended-Si, high-index-contrast subwavelength gratings (HCGs). Iterative design optimization using finite element analysis software has been performed accounting for silicon’s wavelength-dependent index of refraction and extinction coefficient. Grating arrays were fabricated using commercial silicon-on-insulator (SOI) substrates, photolithography and reactive ion etching; subsequent selective wet etching of SiO2 was used to provide suspended Si/air gratings. Fourier transform infrared (FTIR) spectroscopy demonstrates broadband, polarization–dependent reflectance between 8.5 and 12 μm, which agrees with the simulated response.