Recently the developments of high contrast optics, such as high contrast grating (HCG), have attracted much attention.
Much of the existing work has been focused on structures that can be characterized as ‘Cartesian’, i.e., which are easily
described by functions that are separable in the Cartesian coordinates. Yet optical fields with cylindrical rather than
Cartesian symmetries, such as Laguerre-Gaussian (LG) modes and their relatives including both the scalar LG modes
and cylindrical vectorial (CV) modes, can be more efficiently manipulated by high contrast structures that have the same
kind of cylindrical symmetries, hence best described in a polar or cylindrical coordinate. An example of such a structure
is the angular grating based silicon photonics micro-ring optical vortex emitter device we reported.
An efficient treatment of cylindrical high contrast structures requires the decomposition of Fourier components of both
the field and the structure in the cylindrical coordinates, so that the coupling process between the field Fourier
components via the structure can be calculated. We have implemented a semi-analytical model that fully describes the
3D vectorial coupling process using a transverse spatial Fourier analysis in the cylindrical space. This model can deal
with HCG structures in cylindrical coordinates with high precision and fast speed, enabling rapid yet accurate simulation
of the coupling of planar waveguide modes with optical vortex modes carrying photonic orbital angular momenta and
allowing optimization of the emission coefficient and emitted beam quality. The details of the method and optimized
silicon photonics integrated OAM emitter devices will be presented.
This paper presents results of computer simulation of 1D monolithic high refractive index contrast grating (MHCG) reflector also called surface grating reflector (SGR). We analyzed optical properties of the GaAs reflector designed for 980 nm wavelength with respect to the grating parameters variation. We also determined the electric field patterns after reflection from the structure in several cases of parameters variation. We show that thanks to the scalability and design simplicity, proposed design is a promising candidate for simple, next generation vertical cavity surface emitting lasers emitting from ultra-violet to infrared.
We designed and fabricated a suspended SiC-based membrane high contrast grating (HCG) reflectors. The rigorous
coupled-wave analysis (RCWA) was employed to verify the structural parameters including grating periods, grating
height, filling factors and air-gap height. From the optimized simulation results, the designed SiC-based membrane HCG
has a wide reflection stopband (reflectivity (R) <90%) of 135 nm for the TE polarization, which centered at 480 nm. The
suspended SiC-based membrane HCG reflectors were fabricated by nanoimprint lithography and two-step etching
technique. The corresponding reflectivity was measured by using a micro-reflectivity spectrometer. The experimental
results show a high reflectivity (R<90%), which is in good agreement with simulation results. This achievement should
have an impact on numerous III-N based photonic devices operating in the blue wavelength or even ultraviolet region.
We present a unique heterogeneous integration approach for VCSELs on silicon using eutectic bonding. An electrically pumped III-V – silicon heterogeneous VCSEL is demonstrated using a high-contrast grating (HCG) reflector on silicon. CW output power >1.5 mW, thermal resistance of 1.46 K/mW, and 5 Gb/s direct modulation is demonstrated. We also explore the possibility of an all-HCG VCSEL structure that would benefit from stronger thermal performance, larger tuning efficiency, and higher direct modulation speeds.
Sub-wavelength high contrast gratings offer the exciting possibility of “membrane-in-the-middle” optomechanics with a low-mass, highly reflective membrane. Theoretical treatments of this system have, to date, employed the model of a zero-thickness polarizable slab. The validity of this model is, however, limited, since in general highly reflective subwavelength gratings do not have an optical thickness that is much smaller than the wavelength of the light employed. In this work, we show that this model in fact makes incorrect predictions concerning the field modes in an optical cavity with a subwavelength grating at the exact center. It predicts that the modes can be classified in doublets, one member of which has an antisymmetric spatial profile and no absorption, the other of which has a symmetric spatial profile and absorptive losses. The situation for a subwavelength grating, however, is quite different: Both modes have absorptive loss, but the mode with the antisymmetric spatial profile has greater loss. In addition, the frequencies of the modes are interchanged: In the case of the zero-thickness slab, the antisymmetric mode has the lower frequency, while in the case of the subwavelength grating, it is the symmetric mode that is the low-frequency member of the doublet. These considerations will be important for a correct interpretation of experimental data as the performance of such sytems continues to improve.
In this paper, nonpolar a-plane GaN-based photonic crystals (PCs) with different defect cavities have been
demonstrated. By using a micro-photoluminescence (μ-PL) system operated at 77 K, the dominant resonant modes of the
GaN-based PC defect cavities show high quality factor (Q) values in the light emission performance which can be up to
4.3×103. Moreover, the degree of polarization (DOP) of the light emission from the nonpolar GaN-based PC defect
cavities was measured to achieve around 64 % along the m crystalline direction.
For GaN-based microcavity light emitters, such as vertical-cavity surface-emitting lasers (VCSELs) and resonant cavity light emitting diodes (RCLEDs) in the blue-green wavelength regime, achieving a high reflectivity wide bandwidth feedback mirror is truly challenging. The material properties of the III-nitride alloys are hardly compatible with the conventional distributed Bragg reflectors (DBRs) and the newly proposed high-contrast gratings (HCGs). Alternatively, at least for the top outcoupling mirror, dielectric materials offer more suitable material combinations not only for the DBRs but also for the HCGs. HCGs may offer advantages such as transverse mode and polarization control, a broader reflectivity spectrum than epitaxially grown DBRs, and the possibility to set the resonance wavelength after epitaxial growth by the grating parameters. In this work we have realized an air-suspended TiO2 grating with the help of a SiO2 sacrificial layer. The deposition processes for the dielectric layers were fine-tuned to minimize the residual stress. To achieve an accurate control of the grating duty cycle, a newly developed lift-off process, using hydrogen silesquioxan (HSQ) and sacrificial polymethyl-methacrylate (PMMA) resists, was applied to deposit the hard mask, providing sub-10 nm resolution. The finally obtained TiO2/air HCGs were characterized in a micro-reflectance measurement setup. A peak power reflectivity in excess of 95% was achieved for TM polarization at the center wavelength of 435 nm, with a reflectivity stopband width of about 80 nm (FWHM). The measured HCG reflectance spectra were compared to corresponding simulations obtained from rigorous coupled-wave analysis and very good agreement was found.
We review a new type of dispersion elements based on a Bragg reflector waveguide, which provides a large angular dispersion of 1~2°/nm. The device functions as sub-wavelength virtually imaged phased array grating. We obtain a number of resolution-points (possible channel-count in demultiplexing) over 1,000. We demonstrate a large-scale wavelength switch based on a Bragg reflector waveguides array. The waveguides array has a small footprint of 2×4 mm2, but provides both ultra-large numbers (>100) of output-ports and wavelength-channels at the same time. Prospects for further increase in the wavelength channel count and output ports will be discussed.
Experiments in the field of high precision metrology such as the detection of gravitational waves are crucially limited by the thermal fluctuations of the optical components. In this contribution we present the current state of knowledge of high contrast gratings (HCGs) as low-noise elements for gravitational wave interferometers. We discuss how the properties of HCGs can be tailored such that beside highly reflective mirrors also diffractive beam splitters can be realized. Further, we show the impact of such gratings on the sensitivity of future gravitational wave detectors which can pave the way for the new field of gravitational wave astronomy.
In this communication, we report about the design, fabrication, and testing of Silicon-based photonic integrated circuits
(Si-PICs) including low-loss flat-band slow-light high-contrast-gratings (HCGs) waveguides at 1.31 μm. The light slowdown
is achieved in 300-nm-thick silicon-on-insulator (SOI) rib waveguides by patterning adiabatically-tapered highcontrast
gratings, capable of providing slow-light propagation with extremely low optical losses, back-scattering, and
Fabry-Pérot noise. In detail, the one-dimensional (1-D) grating architecture is capable to provide band-edge group
indices ng ~ 25, characterized by overall propagation losses equivalent to those of the index-like propagation regime (~
1-2 dB/cm). Such photonic band-edge slow-light regime at low propagation losses is made possible by the adiabatic
apodization of such 1-D HCGs, thus resulting in a win-win approach where light slow-down regime is reached without
additional optical losses penalty. As well as that, a tailored apodization optimized via genetic algorithms allows the
flattening of slow-light regime over the wavelength window of interest, therefore suiting well needs for group index
stability for modulation purposes and non-linear effects generation. In conclusion, such architectures provide key
features suitable for power-efficient high-speed modulators in silicon as well as an extremely low-loss building block for
non-linear optics (NLO) which is now available in the Si photonics toolbox.
We report efficient wave plates with different retardations and orientations of fast axes realized using transmitarrays
composed of a periodic arrangement of amorphous silicon elliptical cylinders on glass. We show that novel polarization
devices which locally rotate the polarization by different angles while preserving the wavefront can be demonstrated
using such a high contrast transmitarray. We present design, fabrication and experimental characterization results for
near infrared transmissive wave retarders with efficiencies in excess of 90%, and discuss the potential applications of atwill
local polarization control enabled by this technology.
We have designed new THz metastructure waveguides on Si wafers, aimed for low propagation loss and integration with
Si-based integrated circuits. The waveguide has a round cross-sectional hollow-core, surrounded by high reflectioncladding-
walls formed by high-contrast metastructure gratings. We developed a new fabrication technique to fabricate
such a 3D metastructure cage waveguide structure. The waveguide is built using the entire wafer thickness which
involves deep Si etching of periodically spaced holes and using isotropic undercut etching to create a connecting a line
of etched spheres in the middle of the wafer to form the waveguide’s hollow core, then deep etch the high-contrast
grating through the entire wafer thickness to form the cladding for the waveguide. We have successfully modeled and
fabricated such a waveguide structure. The next step is to experimentally test and characterize the waveguide in the THz
The ability to actively control the perceived color of objects is highly desirable for a variety of applications, such as camouflage, sensing, and displays. Such a phenomenon can be readily found in nature - the chameleon is an excellent example. However, the capability to change color at-will has yet to be reproduced by humans. Ultra-thin dielectric high contrast metastructures (HCMs) have been shown to exhibit unique versatility to manipulate light. In this work, we report a completely new flexible HCM structure whose color can be varied by stretching the membrane. This is accomplished with a novel HCM design that annihilates the 0th order diffraction in a grating while enhancing the -1st order. The color perception of the HCM, determined by the -1st diffraction order, is thus easily changed with the variation of its period. The ultra-thin HCM is patterned on a silicon-on-insulator wafer and transferred onto a flexible membrane. We measure more than 15 times stronger intensity in the -1st order diffraction than the 0th order, in excellent agreement with theoretical results. We experimentally demonstrate brilliant colors and change the color of a 1 cm×1 cm sample from green to orange (39 nm wavelength change) with a stretch of 4.9% (25 nm period change). The same effect can be used for steering a laser beam. We demonstrate more than 40 resolvable beam spots.
This work is devoted to the design of high contrast grating mirrors taking into account the technological constraints and tolerance of fabrication. First, a global optimization algorithm has been combined to a numerical analysis of grating structures (RCWA) to automatically design HCG mirrors. Then, the tolerances of the grating dimensions have been precisely studied to develop a robust optimization algorithm with which high contrast gratings, exhibiting not only a high efficiency but also large tolerance values, could be designed. Finally, several structures integrating previously designed HCGs has been simulated to validate and illustrate the interest of such gratings.
We propose a broadband free-space on-chip spectrometer based on an array of integrated narrowband filters consisting of Fabry-Perot resonators formed by two high-contrast grating (HCG) based reflectors separated by a low-index thin layer with a fixed cavity thickness. Using numerical simulations, broadband tunability of resonance wavelengths was achieved only by changing the in-plane grating parameters such as period or duty cycle of HCGs while the substrate geometry was kept fixed. Experimentally, the HCG reflectors were fabricated on silicon on insulator (SOI) substrates and high reflectivity was measured, fabrication process for the proposed double HCG-based narrowband filter array was developed. The filtering function that can be spanned over a wide range of wavelengths was measured.
We present design, fabrication, and characterization results of high numerical aperture (NA) micro-lenses based on a
high contrast transmitarray platform. The high contrast transmitarray is created by periodic arrangement of amorphous
silicon posts with different diameters on a fused silica substrate. We report near infrared high NA micro-lenses with spot
sizes as small as 0.57λ and focusing efficiencies in excess of 80%. We demonstrate a trade-off relation between NA and
efficiency of high contrast array flat micro-lenses, and attribute it to the spatial discretization of their phase profiles.
Mid-infrared Vertical cavity surface emitting lasers (MIR-VCSEL) are very attractive compact sources for spectroscopic measurements above 2μm, relevant for molecules sensing in various application domains. A long-standing issue for long wavelength VCSEL is the large structure thickness affecting the laser properties, added for the MIR to the tricky technological implementation of the antimonide alloys system. In this paper, we propose a new geometry for MIR-VCSEL including both a lateral confinement by an oxide aperture, and a high-contrast sub-wavelength grating mirror (HCG mirror) formed by the high contrast combination AIOx/GaAs in place of GaSb/A│AsSb top Bragg reflector. In addition to drastically simplifying the vertical stack, HCG mirror allows to control through its design the beam properties. The robust design of the HCG has been ensured by an original method of optimization based on particle swarm optimization algorithm combined with an anti-optimization one, thus allowing large error tolerance for the nano-fabrication. Oxide-based electro-optical confinement has been adapted to mid-infrared lasers, byusing a metamorphic approach with (Al) GaAs layer directly epitaxially grown on the GaSb-based VCSEL bottom structure. This approach combines the advantages of the will-controlled oxidation of AlAs layer and the efficient gain media of Sb-based for mid-infrared emission. We finally present the results obtained on electrically pumped mid-IR-VCSELs structures, for which we included oxide aperturing for lateral confinement and HCG as high reflectivity output mirrors, both based on AlxOy/GaAs heterostructures.
Antireflection (AR) layers at the tips of optical fibers are indispensable in order to reduce propagation loss and optical
noise. Conventional thin-film AR layers have problems about cost due to vacuum apparatus usage in the fabrication and
requirement of many thin-film layers to obtain excellent AR characteristics. Thus, easy AR coating methods are needed
to reduce Fresnel reflection. AR structures consisting of subwavelength gratings (SWGs), which have periodic structures
with the periods smaller than operating wavelengths, have been extensively investigated. Desired refractive index to
realize the ideal AR condition can be obtained by SWGs. Nano imprint lithography (NIL) is known as the low cost
fabrication technology of SWGs. However, it is difficult to carry out an NIL process on the tips of flexible and long
optical fibers. In this study, we developed a dedicated UV-NIL system for optical fiber end-faces. An SWG with a period
of 700 nm, a width of 560 nm, and a height of 250 nm was successfully fabricated at the tip of a single-mode optical
fiber for optical communications system. We evaluated that reflectance decreased by using the SWG over measured
spectral range. For example, reflectance decreased to 0.2% at a wavelength of 1550 nm.
We present a GaAs-based VCSEL structure, BCB bonded to a Si3N4 waveguide circuit, where one DBR is substituted by
a free-standing Si3N4 high-contrast-grating (HCG) reflector realized in the Si3N4 waveguide layer. This design enables
solutions for on-chip spectroscopic sensing, and the dense integration of 850-nm WDM data communication transmitters
where individual channel wavelengths are set by varying the HCG parameters. RCWA shows that a 300nm-thick Si3N4
HCG with 800nm period and 40% duty cycle reflects strongly (<99%) over a 75nm wavelength range around 850nm. A
design with a standing-optical-field minimum at the III-V/airgap interface maximizes the HCG’s influence on the
VCSEL wavelength, allowing for a 15-nm-wide wavelength setting range with low threshold gain (<1000 cm-1).
The comprehension and manipulation of the spectral characteristics of photonic structures is of great interest for a vast bunch of applications, in particular for energy efficiency. In this paper we focus on a perturbation model capable of providing insight and control on the resonances that are supported by high index contrast gratings.
We present reflective phase shifters based on high contrast gratings resting on a low-index spacer backed by a metallic mirror. The guided resonance of the grating combined with the reflection from the metallic mirror leads to an all-pass filter with 2π phase shift variation and unity reflectivity across the resonance. We present simulations, fabrication and measurement of passive devices fabricated in silicon over gold using a polymer as the spacer layer. Active control at high modulation speeds can be achieved by shifting the guided resonance wavelength using carrier injection or thermo-optic effect in silicon.