We fabricate all-inorganic, high refractive index optics, including metalenses, waveguides, and diffractive optical elements via nanoimprint lithography with TiO2 nanoparticle dispersion inks and report full-wafer fabrication of visible wavelength metalenses with absolute focusing efficiencies greater than 80% (>95% of design efficiency). 3-D metal oxide log-piles are possible via direct NIL using sequential imprint, planarization, imprint cycles followed by removal of the sacrificial planarization layers. 3-D metal log-piles are possible via metallization of imprinted 3-D sacrificial templates. Several examples will be discussed.
We introduce high-resolution speckle-free holograms, created using polarization manipulating metasurfaces. The holograms enable clear projection of 2D and 3D images, in contrast to conventional holograms with significant speckle noise. The approach uses a simple non-iterative algorithm at a low computational cost. As a proof of concept, we demonstrate a reflective hologram composed of silicon nitride nanoposts on an aluminum layer that projects a high-resolution grayscale image. By implementing three holograms for primary colors, full-color far-field and 3D holograms become feasible, with potential applications in anticounterfeiting tags.
We report a holographic alignment system for the precise alignment of 3D semiconductor devices and compound flat optics. The system uses cascaded metasurfaces to project two holographic patterns, and by interfering the patterns in the far field, small misalignments can be measured without the need for a high-resolution microscope. Operating at 850nm, the technique achieves lateral and axial accuracies of 1 nm and 1 µm, respectively, surpassing the lateral diffraction-limit accuracies of microscopic imaging methods by two orders of magnitude. The technique has potential applications in high-precision alignment detection and registration of multilayer patterns and separate samples and wafers.
We fabricate all-inorganic, high refractive index optics, including metalenses, waveguides, and diffractive optical elements via nanoimprint lithography with TiO2 nanoparticle dispersion inks and report full-wafer, high-throughput fabrication of waveguides and visible wavelength metalenses lenses with absolute efficiencies greater than 75% (>90% of design efficiency). We employ atomic layer deposition (ALD) as a post-imprint treatment that enables tuning of the refractive index from 1.9 to 2.25 using less than 20 cycles, which improves lens efficiency. Tuning RI of the imprinted optics to match that of the substrates removes concerns about residual layer thickness, resolving a critical issue for some applications. Additional cycles of ALD enable precise tuning of feature dimensions and feature spacings. Finally, we demonstrate the excellent optical and material stabilities of the all-inorganic imprinted optics.
We present fully-reconfigurable photonic integrated circuits (PICs) based on liquid-crystal-cladded slab waveguides. Our design utilizes a single-mode slab waveguide with a thin nematic liquid crystal (LC) cladding layer. We achieve local effective index modulation by controlling the LC’s director’s direction through electrodes integrated beneath the slab. As a proof of concept, we demonstrate a straight waveguide at 1550nm with voltage-controlled activation. The reconfiguration time and the excess loss due to LC were measured as 0.1 ms and 7.3 dB/cm, respectively. The low-temperature fabrication process enables cost-effective PICs on CMOS chips for 3D sensing, telecommunications, and data processing applications.
Sophisticated techniques to accurately align patterns have been developed that predominantly address the overlay of proximal layers in layer-by-layer processes that are separated by short, micron-scale distances. However, in some applications such as in flat optics, accurate, 3D alignment of workpieces whose patterns may be separated by large distances is desirable. Here, we describe an alignment scheme based on the far-field of a cascade of metasurface elements suitable for accurately aligning patterns separated by millimeter-scale distances. By using an 850nm laser illumination, we demonstrate registration accuracies of 10nm laterally and 50nm axially for patterns separated by mm-scale distances.
On-chip integration of metasurfaces with optoelectronic components enables the manufacture of reliable and low-cost systems that do not require post-fabrication alignment and assembly. Here we present an on-chip miniature beam scanning metasystem by integrating a large 2D array of VCSELs and a metalens. The beam scanner measures 4 mm*4 mm*3 mm and scans a ~1 mW laser beam at 940 nm with <1 degree divergence angle over a 140 degree*140 degree field of view with <30 mW of power consumption. The scanner switches between two angles in <1 µs, enabling the realization of fully-integrated miniature imaging lidar systems.
I will present different techniques developed to design and fabricate visible and near-infrared metasurface with large dimensions. In particular, I will present metasurface design using adjoint optimization and introduce a novel technique for designing arbitrarily large metasurfaces using optimized smaller metasurfaces. I will also discuss metasurface design using grating averaging and nonlinear optimization. Scalable and low-cost fabrication of large diameter visible metasurfaces using different nanoimprint techniques will also be presented.
We describe a high throughput approach to all-inorganic metalens manufacturing using a single step nanoimprint lithography process and titania nanoparticle-based inks. The process yields a refractive index of 1.9, lenses with critical dimensions below 60 nm, feature aspect ratios greater than 8, and efficiencies greater than 55% and consistent device performance across 15 lenses printed within 30 minutes. We further describe pathways to fabricating all-inorganic lenses with RI of 2.1.
With high feature density and subwavelength dimensions, visible
spectrum metalenses are challenging to scalably
manufacture. Electron beam lithography and short-wavelength
photolithography capable of patterning metalenses for the visible
do so at high cost per wafer. Here, we present a low-cost and
scalable fabrication process based on nanoimprint lithography, and
use it to demonstrate metalenses designed for 550 nm light with 4
mm diameter and NA=0.2. Our metalenses are formed of silicon
nitride nanoposts with critical dimensions smaller than 100 nm. In
this presentation we report focusing efficiencies above 50%,
share holographic characterization data, and demonstrate
imaging.
When uncorrected, metalenses exhibit significant chromatic dispersion that limits them to narrowband operation. To address this, several metalens singlets corrected for chromatic aberration recently have been demonstrated. These metalenses mainly rely on engineered metaatom dispersion, an approach that limits them to small diameter and NA. Here, we validate an alternative approach by experimentally demonstrating a metalens doublet that directs light along trajectories of appropriate length to produce the desired achromatic behavior. Our lens is corrected over the 800-900 nm spectral region, collects light incident over a 2 sq. mm area, and has an NA of 0.2.
We present a technique for designing efficient and robust metasurfaces that optimizes the metasurface design curves instead of individual metasurface elements and thus does not suffer from the size limitations of conventional optimization techniques. Spatially varying design curves are parametrized and optimized using the grating averaging technique. We present simulation and experimental results of highly efficient metasurface beam deflectors and lenses that are robust to fabrication errors. In particular, we present an 80° beam deflector with absolute efficiency of 75% and a metalens with NA of 0.8 and an efficiency of 86% that is robust to fabrication errors.
We present a technique for designing efficient and robust metasurfaces that optimizes the metasurface design curves instead of individual metasurface elements and thus does not suffer from the size limitations of conventional optimization techniques. Spatially varying design curves are parametrized and optimized using the grating averaging technique. We present simulation and experimental results of highly efficient metasurface beam deflectors and lenses that are robust to fabrication errors. In particular, we present an 80° beam deflector with absolute efficiency of 75% and a metalens with NA of 0.8 and an efficiency of 86% that is robust to fabrication errors.
We present a new class of grating-integrated microdisk resonators that directly and efficiently couple to free space and can be excited by top illumination. We discuss the theory and design of such devices and present characterization results of 1530-nm-resonators with 0.8 µm to 1.2 µm radii, which are fabricated using amorphous silicon on glass. A 1.2-µm-radius resonator has a measured Q of ~16,000 and is efficiently excited by top illumination as evidenced by an observed thermally-induced bistability threshold of 0.7 mW. The small footprint and ease of coupling enable dense resonator arrays for applications in free space and flat optics.
Hyperspectral imaging divides a scene into many spectral channels with narrow spectral width. Here we present a compact hyperspectral imaging system based on dielectric metasurfaces. Our system has nine channels spanning 795 nm to 970 nm, which are arranged in a rectangular array and acquired in a single snapshot, in contrast to many commercial systems. The system's narrowband filters, necessary for hyperspectral operation, also reduce chromatic aberration, a common problem in metasurface imaging systems. The small footprint of the device (2.5 mm × 2.5 mm × 1.5 mm) facilitates its potential integration into a handheld system (e.g., a mobile phone).
The recent development of efficient dielectric metasurfaces has enabled practical optical components and systems composed of multiple cascaded metasurfaces. In this talk, I present an overview of our work on modeling, design, and implementation of cascaded metasurface components and systems. In particular, I present accurate system-level models for metasurfaces, techniques for designing efficient metasurfaces, multifunctional cascaded metasurfaces, and bilayer birefringent metasurfaces that provide the ultimate control over the wavefront and polarization of light. Furthermore, I will introduce a novel technique for engineering chromatic dispersion by cascading and briefly discuss a single-snapshot hyperspectral imager enabled by cascading multiple metasurfaces.
We present system-level models for ideal and realistic metasurfaces that can be used in designing metasurface optical systems. We show that ideal gradient metasurfaces have spatially varying field transmission amplitudes that can exceed one and discuss its effect on the modulation transfer function of metalenses. We introduce a general model for non-ideal metasurfaces based on the discrete-space impulse response concept. The new model takes into account reflections and undesired diffractions from metasurfaces and enables accurate black-box models that can be incorporated into design tools. We also present examples of its applications in analyzing optical systems composed of cascaded metasurface components.
Standard designs for dielectric metasurfaces suffer from significant chromatic dispersion, impeding their use in broadband systems. We present a fundamental relation between ray trajectories in an optical system and its chromatic dispersion, and describe an associated design procedure to create cascaded optical systems with arbitrary dispersion. We use this procedure to design cascaded metasurface systems with various dispersive characteristics, including an achromatic metalens exploiting the orbital angular momentum of light. As experimental validation, we demonstrate beam deflectors exhibiting several different chromatic dispersions.
Metasurfaces are attractive options for the realization of on-chip optical systems because of their flat form factor and their ability to modify the wavefront, amplitude, and polarization of light with high efficiency. Several metasurface platforms have been reported that provide different levels of control over the polarization and phase of light, and it has been shown that a single layer birefringent metasurface can implement symmetric and unitary Jones matrices. Optical components with such Jones matrices can convert any arbitrary input polarization to any desired output polarization or perform independent wavefront transformations for two orthogonal polarizations while changing their handedness. However, the Jones matrices that describe the most general polarization and phase transformations are not symmetric, and this limits the range of possible devices that single layer birefringent metasurfaces can implement. For example, a single layer birefringent metasurface cannot impart two different phase shifts to x- and y-polarized light while simultaneously converting their polarizations to right- and left-handed circularly polarized. Here we show that bi-layer birefringent metasurfaces do not suffer from such limitations and can implement the most general form of Jones matrices that describe loss-less and reciprocal optical components. By using the Poincare sphere representation and closed-form relations, we identify the degrees of freedom in the design and present a procedure that allows for the design of large-scale devices based on bi-layer metasurfaces. As a proof-of-concept, we demonstrate a chiral bi-layer metasurface that focuses left- and right-handed polarized waves to two different points without changing their polarizations.
Optical metasurfaces are subwavelength-thick arrays of meta-atoms that have attracted significant attention due to their superior capabilities compared with conventional optical devices. Designing metasurfaces for practical applications requires system-level models that accurately predict their responses. The conventional approach for modeling metasurfaces is to ignore the coupling among the meta-atoms and to model metasurfaces as phase, amplitude, or polarization masks that are independent of the incident light’s wavefront, which is an inaccurate assumption for large incident angles. In this talk, we will introduce a novel technique for the modeling and design of metasurfaces based on the discrete-space impulse response (DSIR) concept. Because the waves propagating in free space are spatially band-limited, the incident, the transmitted, and the reflected waves can be represented using discrete-space signals that are obtained by sampling these waves at the Nyquist rate (at half a wavelength intervals). As a result, we can define discrete-space impulse responses for metasurfaces that relate the transmitted/reflected waves to the incident waves. We show that such impulse responses are local, accurately model the interactions among neighboring meta-atoms, and completely characterize the metasurfaces’ response to any incident waves. We also introduce a new approach for designing metasurfaces using the DSIR concept. As a proof-of-concept, we present the characterization results of a high numerical aperture meta-lens that is designed using the DSIR technique and show that its focusing efficiency is higher than that of a similar meta-lens designed using the conventional technique.
Polarization is an important degree of freedom of light carrying information that is usually missing in other degrees of freedom. Polarimetric imaging is the process of measuring the state of polarization of light over an extended scene. It has several applications ranging from remote sensing to biological and medical imaging because it provides various pieces of information about the light source or the objects with which the light has interacted. So far polarization cameras have been made using polarization filters, and therefore suffer from two major drawbacks. First, there is a theoretical 50% upper limit on the efficiency of devices based on polarization filters. Second, to fully determine the state of polarization, multiple layers should be integrated in order to make polarization filters for circular or elliptical polarization states. Here, we present a polarization camera made using dielectric metasurfaces that operates based on separating and focusing orthogonal polarization states instead of polarization filtering. This allows for overcoming both drawbacks of current polarization camera designs. At the core of the design lies the capability of dielectric metasurfaces to fully control the polarization and phase of light. This enables designing and fabricating superpixels that separate and focus orthogonal polarization states of light on adjacent pixels on an image sensor over a single metasurface layer. Using this technique we have demonstrated full-Stokes polarization cameras with experimental efficiencies surpassing 60%, and superpixel dimensions reaching 4.8 µm×7.2 µm. We have also used this camera to form polarization images of custom-designed polarization targets.
Multifunctional metasurfaces perform different functions depending on the wavelength, polarization, or wavefront of the incident light. Designing such metasurfaces require more degrees of freedom (DOF) than what is available in a single layer metasurface, and stacking metasurface layers is one of the approaches for achieving the required DOF for realizing multifunctional metasurfaces. In the conventional metasurface design technique used for designing single layer metasurfaces, the couplings among the meta-atoms are ignored; however, the meta-atoms in multi-layer metasurfaces exhibit significant mutual couplings and multiple scattering phenomena are not negligible. As a result, multi-layer metasurfaces designed using the conventional techniques have low efficiencies. In this talk, we will present an inverse design technique that is suitable for designing efficient large-scale multi-layer metasurfaces. The method is based on a combination of the gradient descent optimization and the adjoint sensitivity techniques and is used to design efficient parametrized multifunctional metasurfaces. The design of multifunctional metasurfaces is cast as a multi-objective optimization problem and the optimal values of meta-atom geometrical parameters are found through an iterative approach. The sensitivities of the objective function and the metasurface response are computed using full-wave simulations; therefore, the mutual interactions and the multiple scattering effects are accurately considered. To demonstrate the effectiveness of the method, we present a bi-layer double-wavelength metasurface composed of more than 2,000 amorphous silicon nano-posts that are embedded in silicon dioxide and arranged in two stacked layers. The bi-layer metasurface projects two different patterns with more than 65% efficiency when illuminated with two different wavelengths.
Characteristic modes (c-modes) are a complete and orthogonal set of modes that can be used as a basis for the expansion of the waves scattered from electromagnetic and optical scatterers. In this talk, we present an introduction to the c-modes and discuss their applications in the analysis and design of 2D and 3D meta-structures. We present an equation for finding the c-modes and demonstrate that the c-modes and the natural (quasi-normal) resonances of meta-structures are related to each other. The relation between the c-modes and the natural mode leads to closed-form solutions for the transmission and reflection spectra of non-diffractive meta-structures that only depend on the complex-valued frequencies of the natural modes. As an example, we show that the wide bandwidth and high reflectivity of 1D high contrast gratings can be attributed to the alignment of two resonances associated with two different c-modes. In addition, using the c-modes concept, we present accurate expressions for the field enhancement and the Purcell factor in the presence of meta-structures and establish an upper bound on the number of degrees of freedom of meta-structures.
Two-photon microscopy is a key imaging technique in biological sciences because of its superior deep tissue imaging capabilities in addition to high transverse and axial resolution. In recent years, development of low-weight miniature two-photon microscopes has been of great interest for in vivo imaging of brain activity. Limited by these mechanical constraints, most of the developed miniature two-photon microscopes utilize graded index objective lenses that usually have inferior optical characteristics compared to conventional refractive objective lenses.
Dielectric metasurfaces, a recent category of diffractive optical elements with enhanced capabilities, have proven versatile in various applications ranging from lensing to holography and polarization control. Their ultrathin form factor and potentially extremely low-weight make them very attractive for applications with stringent size and weight constraints. However, despite their success in various types of microscopy and imaging applications, they have not been previously utilized for multi-photon fluorescence microscopy. The main barrier for using metasurface lenses in multi-photon microscopy arises from their large chromatic dispersion that effectively makes them single-wavelength. Here we will present a double-wavelength metasurface lens especially designed to have the same focal length at 820 and 605 nm, corresponding to the excitation and emission wavelengths of a certain fluorophore. After characterizing the poly-silicon metasurface lens at both wavelengths, we used it in a two-photon microscopy setup and demonstrated its capability to capture two-photon images qualitatively similar to images taken with a conventional objective lens. We will also discuss the effects of chromatic dispersion of the metasurface lens on its two-photon imaging performance.
Miniaturized optical systems with planar form factors and low power consumption have many applications in wearable and mobile electronics, health monitoring devices, and as integral parts of medical and industrial equipment. Flat optical devices based on dielectric metasurfaces introduce a new approach for realization of such systems at low cost using conventional nanofabrication techniques. In this talk, I will present a summary of our recent work on dielectric metasurfaces that enable precise control of both polarization and phase with large transmission and high spatial resolution. Optical metasurface components such as high numerical aperture lenses, efficient wave plates, components with novel functionalities, and their potential applications will be discussed. I will also present the results of our efforts on optimizing and increasing the diffraction efficiency of metasurfaces. Furthermore, by using metasurface cameras and planar retroreflectors as examples, I will discuss a vertical on-chip integration platform that introduces a new architecture for the on-chip integration of conventional and novel optical systems and enables their low-cost manufacturing.
KEYWORDS: Spatial resolution, Current controlled current source, Collimators, Retroreflectors, Diffractive optical elements, Diffraction gratings, Diffraction, Optical components, Polarization, Control systems
Metasurfaces are two-dimensional arrangements of nano-scatterers that enable control of phase, amplitude, and polarization of light with high efficiency and subwavelength resolution. They have enabled diffractive optical elements with enhanced functionalities and performance. Nevertheless, metasurface diffractive optical elements share many of the properties of regular diffractive optical elements. One of these properties is the response of diffractive elements to changing the angle of illumination: if the beam incident on a grating is rotated by an angle, all diffraction orders will rotate by corresponding angles in the same direction. More precisely, because of the constant grating momentum, the change in the sine of all diffraction angles will be equal to the change in the sine of the illumination angle.
Many optical devices of interest, however, do not require this type of behavior, which makes their implementation using metasurfaces very challenging. For instance retroreflectors, which reflect light incident from any angle to the same direction, or collimators, that deflect light coming from any angle to a single given direction, do not follow the regular diffractive optics angular response. We investigate properties of single-layer metasurfaces that enable devices like retroreflectors and collimators. We show that such metasurfaces should have the ability to control the phase, as well as the derivative of phase with respect to angle. We demonstrate designs that provide such control, and use them to show devices that defy the regular response of diffractive optical devices to changes in the illumination angle.
Miniaturized optical systems with planar form factors and low power consumption have many applications in wearable and mobile electronics, health monitoring devices, and as integral parts of medical and industrial equipment. Flat optical devices based on dielectric metasurfaces introduce a new approach for realization of such systems at low cost using conventional nanofabrication techniques. In this talk, I will present a summary of our recent work on dielectric metasurfaces that enable precise control of both polarization and phase with large transmission and high spatial resolution. Optical metasurface components such as high numerical aperture lenses, efficient wave plates, components with novel functionalities, and their potential applications will be discussed. I will also present the results of our efforts on developing multi-wavelength and dispersion engineered metasurfaces, as well as conformal, flexible, and tunable metasurfaces. Furthermore, by using metasurface cameras and planar retroreflectors as examples, I will introduce a vertical on-chip integration platform enabled by vertical stacking of multiple metasurfaces and active optoelectronic components. This vertical integration scheme introduces a new architecture for the on-chip integration of conventional and novel optical systems and enables their low-cost manufacturing.
Diffractive optical devices based on dielectric metasurfaces have recently attracted significant attention. Small size, low weight, planar form factor, and potential for low-cost manufacturing using semiconductor fabrication techniques are some of the main features that make metasurfaces ideal candidates for implementation of low-cost miniaturized optical systems. However, to become competitive for practical applications, metasurfaces should also offer specifications (e.g. efficiency, bandwidth, and wavefront error) comparable to their refractive counterparts. We have recently demonstrated diffraction-limited metasurface lenses with high efficiency using high refractive index nano-posts. Low numerical aperture (NA) metasurface lenses have more than 90% focusing efficiency, but the efficiency of the lenses with NA>0.5 decreases with increasing NA and drops to ~40% for NA=0.9, thus resulting in a trade-off between the NA and efficiency. Here we identify the main physical origin of this trade-off as the low transmission of large diameter nano-posts for transverse-magnetic (TM) polarized light incident at large angles, and show that the low transmission is caused by the excitation of undesired high order modes in these nano-posts. To overcome this issues, we present a novel approach for evaluating different metasurface designs in implementation of high NA metasurface components. The approach is based on adiabatic approximation of aperiodic metasurfaces by periodic gratings, and considers the effect of large deflection angles. Using the proposed design approach, we experimentally demonstrate more than 75% focusing efficiency for metasurface lenses with NA=0.7, and more than 70% deflection efficiency for 50-degree beam deflectors for unpolarized light at 915 nm.
Diffractive optical devices have many applications in various fields of optics. A fundamental property of all diffractive devices is their negative chromatic dispersion: a diffractive grating always disperses light in the opposite order compared to a refractive prism made of a material with positive (normal) dispersion. Unlike refractive devices, chromatic dispersion in diffractive devices stems from geometrical features, and cannot be controlled via the intrinsic material dispersion. In addition to the always negative sign, the amplitude of diffractive chromatic dispersion is set only by the function of the device. For instance, the angular dispersion of a grating is always given by dθ/dλ=tan(θ)/λ (where θ is the deflection angle and λ is wavelength), or the focal distance dispersion of a diffractive lens is given by df/dλ=-f/λ. Therefore, the chromatic dispersion of diffractive devices has always been set by their function (e.g. by the deflection angle for a grating or the focal distance for a lens), and could not be controlled separately. Here, we present our work on breaking this fundamental relation between the function and chromatic dispersion of diffractive devices using metasurfaces providing independent control over phase and group delays. We use a reflective dielectric metasurface to experimentally demonstrate gratings and lenses that have positive, zero, and extraordinary negative chromatic dispersion. Apart from its fundamental scientific value, this concept expands the applications of diffractive devices as it enables various types of chromatic dispersions. For instance, a special case would be a dispersionless lens operating over a wide bandwidth with the same focal distance.
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.
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 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 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.
Simulation results for an etched air hole photonic crystal (PhC) vertical cavity surface emitting laser (VCSEL) structure
with various thicknesses of metal deposited inside the holes are presented. The higher-order modes of the structure are
more spread out than the fundamental mode, and penetrate into the metal-filled holes. Due to the lossy nature of the
metal, these higher-order modes experience a greater loss than the fundamental mode, resulting in an enhanced side
mode suppression ratio (SMSR). A figure of merit for determining which metals would have the greatest impact on the
SMSR is derived and validated using a transmission matrix method calculation. A full three-dimensional simulation of
the PhC VCSEL structure is performed using the plane wave admittance method, and SMSRs are calculated for
increasing metal thicknesses. Of the metals simulated, chromium provided the greatest SMSR enhancement with more
than a 4 dB improvement with 500 nm of metal for an operating current of 12 times threshold.
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