This PDF file contains the front matter associated with SPIE Proceedings Volume 8995, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.

High index contrast / Photonic Crystal membrane (HCG/PCM) resonators can be exploited to perform an arbitrarily adjustable molding of light at the wavelength scale: they can process free-space as well as wave-guided optical modes along a variety of addressing configurations and transfer functions, where the spectral, spatial, polarization, phase, group delay… characteristics can be resolved accurately and adjusted at will. The physics of HCG resonators will be revisited based on a simple analytical approach and intuitive arguments, thus providing direct routes for design rules. Specifically, such desired functionalities as wavelength tuning and beam steering will be emphasized. Practical implementation of these functionalities will be presented in the case of VCSEL devices, where silicon HCG/PCM resonators are used as reflectors and are heterogeneously integrated with III-V semiconductor gain material, along a CMOS compatible technological approach.

Fabrication of high performance gratings may significantly benefit from the use of high index materials such as Ta₂O₅, TiO ₂ or Al₂O₃. However, these materials can typically not be patterned with the required quality by common etching processes. To overcome this limitation we developed novel grating fabrication technologies based on a combination of conventional lithography with Atomic-Layer-Deposition. For that the basic structure of the grating is first realized in a fused-silica substrate or a SiO₂-layer. This template is then functionalized by an ALD-coating in a specific pre-defined manner. The new approach opens up a huge variety of new options for the realization of gratings whose fabrication would otherwise not be possible.

We demonstrate wavelength-tunable VCSELs using high contrast gratings (HCGs) as the top output mirror on VCSELs, operating at 1550 nm. Tunable HCG VCSELs with a ~25 nm mechanical tuning range as well as VCSELs with 2 mW output power were realized. Error-free operation of an optical link using directly-modulated tunable HCG VCSELs transmitting at 1.25 Gbps over 18 channels spaced by 100 GHz and transmitted over 20 km of single mode fiber is demonstrated, showing the suitability of the HCG tunable VCSEL as a low cost source for WDM communications systems.

In this contribution we discuss configurations of stacked silicon high contrast gratings (HCGs) which are separated by a thin silicon dioxide grating such that they are coupled via their near-fields. For a given configuration altering incidence angle allows to either benefit from the optical performance of two separated HCGs or one single grating with enhanced thickness. This effect can serve to realize filters with tailored optical properties and for diffractive cavity couplers. We experimentally demonstrate the coupling effect on a stack of two HCGs for a wavelength of 1550nm and transverse-magnetic polarization. The investigated structure provides a nearly angular independent high reflectance.

In this paper we present results of computer optical simulations of VCSEL with modified high refractive index contrast grating (HCG) as a top mirror. We consider the HCG of two different designs which determine the lateral aperture. Such HCG mirror provides selective guiding effect. We show that proper design of aperture of HCG results in almost sixfold increase in cavity Q-factor for zero order mode and a discrimination of higher order modes.

Silicon photonics is increasingly considered as the most promising way-out to the relentless growth of data traffic in today's telecommunications infrastructures, driving an increase in transmission rates and computing capabilities. This is in fact challenging the intrinsic limit of copper-based, short-reach interconnects and microelectronic circuits in data centers and server architectures to offer enough modulation bandwidth at reasonable power dissipation. In the context of the heterogeneous integration of III-V direct-bandgap materials on silicon, optics with high-contrast metastructures enables the efficient implementation of optical functions such as laser feedback, input/output (I/O) to active/passive components, and optical filtering, while heterogeneous integration of III-V layers provides sufficient optical gain, resulting in silicon-integrated laser sources. The latest ensure reduced packaging costs and reduced footprint for the optical transceivers, a key point for the short reach communications. The invited talk will introduce the audience to the latest breakthroughs concerning the use of high-contrast gratings (HCGs) for the integration of III-V-on-Si verticalcavity surface-emitting lasers (VCSELs) as well as Fabry-Perot edge-emitters (EELs) in the main telecom band around 1.55 μm. The strong near-field mode overlap within HCG mirrors can be exploited to implement unique optical functions such as dense wavelength division multiplexing (DWDM): a 16-λ100-GHz-spaced channels VCSEL array is demonstrated. On the other hand, high fabrication yields obtained via molecular wafer bonding of III-V alloys on silicon-on-insulator (SOI) conjugate excellent device performances with cost-effective high-throughput production, supporting industrial needs for a rapid research-to-market transfer.

GaN-based surface-emitting lasers (SELs) using high contrast grating (HCG) with AlN/GaN distributed Bragg reflectors were reported. The laser device achieved a threshold 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 resonant mode and polarization characteristics matched to the theoretical prediction. GaN-based SELs using HCG supported by the Fano resonance can be potential for development of blue surface emitting laser sources

We report here recent progresses made in the area of Fano resonance based single layer air hole photonic crystal membrane structures for surface-normal photonic devices. Ultra-compact membrane reflector surface emitters have been demonstrated on silicon substrate, with optical and electrical pumping. High Q Fano resonance optical filters were also realized with single and multi-layer stacking of crystalline nanomembranes. These structures can further be integrated for 3D multi-functional integrated photonic systems.

We present our recent activity on highly angular-dependent high contrast grating (HCG) for the transverse mode control of VCSELs. The modeling and the experiment show the design flexibility of HCG to manage the angular dependence of HCG. The optimized angular dependent HCG functions as a spatial frequency filter. We are able to use the engineered angular dependence of HCG for the transverse-mode control of VCSELs by filtering out high-order transverse-modes. We fabricated and characterized amorphous Si HCG mirrors, which clearly show the large angular dependence. We demonstrated single-mode 980nm VCSELs with a HCG mirror functioning as a spatial frequency filter.

This paper presents a novel tunable infrared filter applying a subwavelength grating that substitutes the distributed Bragg reflectors (DBRs) in tunable Fabry-Perot (FP) filters to reduce cost and fabrication effort. It consists of uniformly arranged disc resonators which are made of 100 nm thick aluminum at a 200 nm Si3N4 membrane carrier that stands freely after fabrication. The dimensions of the subwavelength structures were optimized based on finite difference time domain (FDTD) analysis. The fabrication sequence consists of silicon MEMS technology steps like deposition and patterning of electrodes and of isolation layers, silicon etching, and wafer bonding, and it includes nano imprint lithography for forming the subwavelength structures at wafer level. The samples have an aperture of 2 mm and are mechanically tuned by electrostatic forces with tuning voltages up to 80 V. They show the typical characteristics of FP filters but with high peak transmittance within a remarkably large wavelength range (T < 50% @ 2.5 μm … 6.5 μm) spanning over 5 interference orders of the optical resonator. The optical performance was measured by Fourier transform infrared spectrometer and compared to the simulation results. It shows a widely good agreement between calculation and measurement.

We report on the focusing performance of reflective 2D high contrast grating lenses based on silicon. The combination of their subwavelength nature and their high refractive index contrast make it possible to create highly tolerant and planar microlenses. We used a rigorous mathematical code to design the lenses and verified their performance with finite element simulations. We also investigated the effects of grating thickness, angle and wavelength of incidence in these simulations. Experimentally, we show the evolution of the beam profile along the optical axis for a lens with a high (0.37) numerical aperture. We have explored a wide range of numerical apertures (0.1 – 0.93) and focal lengths (5 μm – 140 μm) and show that the lenses behave as expected across the full range. Our analyses demonstrate the large design flexibility with which these lenses can be made along with ease of fabrication and potential for a number of applications in micro-optics.

High-contrast gratings fabricated in free-standing membranes of silicon nitride are a remarkable new platform for optomechanics, as they combine high reflectivity, low mass, and a high mechanical quality factor in a single device. In an effort to further improve on our earlier designs, we are now fabricating high-contrast gratings from stoichiometric silicon nitride. The new gratings have a diameter of 80 μm, a thickness of 250 μm, and are patterned in square membranes from 100 μm to 500 μm on a side. We find reflectivities R < 0.994 for these devices, and fundamental mechanical resonance frequencies above 1.5 MHz. In addition, we have incorporated HCGs fabricated from low-stress silicon nitride into a “membrane-in-the-middle” setup, and observe that the cavity transmission spectrum is distorted from a constant free spectral range of 3 GHz to one characterized by anticrossings separated by 72 ± 2 MHz.

We report an optical phased array (OPA) for two-dimensional free-space beam steering. The array is composed of tunable MEMS all-pass filters (APFs) based on polysilicon high contrast grating (HCG) mirrors. The cavity length of each APF is voltage controlled via an electrostatically-actuated HCG top mirror and a fixed DBR bottom mirror. The HCG mirrors are composed of only a single layer of polysilicon, achieving >99% reflectivity through the use of a subwavelength grating patterned into the polysilicon surface. Conventional metal-coated MEMS mirrors must be thick (1-50 μm) to prevent warpage arising from thermal and residual stress. The single material construction used here results in a high degree of flatness even in a thin 350 nm HCG mirror. Relative to beamsteering systems based on a single rotating MEMS mirror, which are typically limited to bandwidths below 50 kHz, the MEMS OPA described here has the advantage of greatly reduced mass and therefore achieves a bandwidth over 500 kHz. The APF structure affords large (~2π) phase shift at a small displacement (< 50 nm), an order-of-magnitude smaller than the displacement required in a single-mirror phase-shifter design. Precise control of each all-pass-filter is achieved through an interferometric phase measurement system, and beam steering is demonstrated using binary phase patterns.

We demonstrated a super-high resolution beam scanner based on a Bragg reflector waveguide. In this device, radiation profile is wavelength-dependent. However, for specific applications, it is important to optimize the radiation direction. We propose a solution for this by introducing a high-contrast sub-wavelength grating (HCG). Numerical simulations using finite-difference time-domain method (FDTD) and rigorous coupled wave analysis (RCWA) were carried out. We found that, by designing the thickness, period and duty cycle of HCG, the output phase and intensity can be changed. As a result, it is possible to shift the output direction of the beam profile. We discussed their dependences on HCG parameters. On the other hand, the thicknesses (numbers of pairs) of the top- and bottom- distributed Bragg reflectors (DBRs) mirrors are influential to the results. A discussion on the thickness dependence was carried out. We found that, HCG has stronger influence to thinner mirrors. Because HCG can provide high reflectivity, thin mirrors are not a problem in such slow-light waveguides. We believe this proposal can offer us a method to obtain desirable output beam direction of Bragg reflector waveguides deflectors.

High contrast structures with a sub-wavelength pitch, small enough to suppress diffraction, exhibit extraordinary optical properties: depending on the design they may behave as perfect mirrors, anti-reflective interfaces, homogenous materials with controllable refractive index, or strongly dispersive materials. Here we discuss on the design possibilities such structures offer in planar waveguide devices in silicon-on-insulator. We briefly review the application of sub-wavelength structures in a variety of waveguide devices. We then focus on some of the latest advances in the design ultra-compact and ultra-wideband multimode interference couplers based on dispersion engineered sub-wavelength structures.

Guided-mode resonances in structures having periodicity along at least one dimension were widely employed in the last decade in various optical devices. Initially it was shown that at frequencies close to the second order band gap periodic structures can feature total reflection of light due to the guided modes propagating along the surface of the grating. As an application, this allows to substitute a thick multilayer Bragg mirror in VCSELs by a thin grating-based mirror. Most devices utilizing guided-mode resonances were theoretically and numerically investigated with the idealized model of an infinite periodic structure illuminated by a plane wave. To see how grating-based components can perform in real life we take into account two critical factors: the finite size of the grating and the Gaussian shape of the light source replacing a plane wave. These factors can significantly change and impair the performance of filters, mirrors, sensors and other devices operating by the guided-mode resonance effect. We also show experimentally that for some kinds of gratings guided-mode resonances can vanish if the grating is illuminated by extended source, i.e. heated plate in our case, focused on the sample.

We present here the design of a robust broadband high efficiency surface-normal vertical to in-plane optical coupler using fourth-order gratings. The fourth-order gratings can be designed such that the zero-order diffraction is suppressed while the diffraction efficiencies of the higher orders are enhanced for the in-plane coupling. We numerically demonstrated the surface normal incidence light coupling efficiency of 88.5% at 1,535 nm with a 3 dB bandwidth of 42 nm and 1dB bandwidth of 28 nm. Large fabrication tolerance of the fourth-order grating is also assessed.

Recent progress in the area of hyperbolic metamaterials (HMMs) has sparked interest in transparent conducting oxides (TCOs) that behave as plasmonic media in the near-IR and at optical frequencies for imaging and sensing applications. It has been shown that by depositing alternating layers of negative-epsilon/positive-epsilon materials, a medium can be created with unusual index values such as near zero. HMMs support high-k waves corresponding to a diverging photonic density of states (PDOS), the quantity determining phenomena such as spontaneous and thermal emission. Also, modeling such structures allows evanescent fields containing sub-wavelength information to be coupled to propagating radiation. We investigate the optical, electronic, and physical properties of radio frequency plasma-assisted molecular beam epitaxial (RF-MBE) growth of alternating layers of ZnO and TCO of uniform thickness for HMM applications. Preliminary work creating HMMs with ZnO and Al-doped ZnO (AZO) has shown a negative real part of the permittivity at near-IR whose modulus is proportional to the number density of Al dopant. However, increasing the Al content of the AZO increases the transmission losses to unacceptable levels for device applications at industry standard wavelengths. A TCO with conductivity and physical structure superior to that of AZO is gallium-doped ZnO (GZO). Uniformly grown GZO has been demonstrated to possess improved crystal quality over AZO due to the higher diffusivity of Al in the ZnO. AZO and GZO HMM structures grown by RF-MBE are characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), Hall effect, four-point probing, deeplevel transient spectroscopy (DLTS), ellipsometry, visible and ultraviolet spectroscopy (UV-VIS) and in-situ reflection high energy electron diffraction (RHEED).

Based on the metallic-dielectric high contrast gratings, we proposed two new different schemes to achieve the unidirectional coupling of incident free-space light into the plasmonic waveguides. Using the cascaded metal-air subgratings (structure 1) or metallic-dielectric subwavelength periodic gratings (structure 2), the ultra-broadband and wideangular SPPs excitation could be both achieved. The operation principle and performance of the structures are analyzed and discussed, respectively.

A giant field enhancement, respect to the amplitude of the incident wave is achieved in a thin layer lattice with low contrast dielectric, is demonstrated. The key mechanism is a careful control of the parameters, which allows a stabilization of the coupling resonances.