Low Index Silicon Research Articles

Brillouin and Kerr nonlinearities of a low-index silicon oxynitride platform

Nonlinear optical effects including stimulated Brillouin scattering (SBS) and four-wave mixing (FWM) play an important role in microwave photonics, optical frequency combs, and quantum photonics. Harnessing SBS and FWM in a low-loss and versatile integrated platform would open the path to build large-scale Brillouin/Kerr-based photonic integrated circuits. In this letter, we investigate the Brillouin and Kerr properties of a low-index (n = 1.513 @ 1550 nm) silicon oxynitride (SiON) platform. We observed, for the first time, backward Brillouin scattering in SiON waveguides with a Brillouin gain coefficient of 0.3 m−1 W−1, which can potentially be increased to 0.95 m−1 W−1 by just tailoring the waveguide cross section. We also performed FWM experiments in SiON rings and obtained the nonlinear parameter γ of 0.02 m−1 W−1. Our results point to a low-loss and low-index photonic integrated platform that is both Brillouin and Kerr active.

Low Index Silicon Photonic Directional Coupler: Design and Characterization

Low Index Silicon Photonic Directional Coupler: Design and Characterization

Mid-infrared quasi-BIC resonances with sub-wavelength slot mode profiles in germanium-based coupled guided-mode resonance structures

We experimentally demonstrate a novel quasi-bound state in the continuum (BIC) resonance in the mid-infrared wavelength region with the resonant electric field confined as a slot mode within a low-refractive-index medium sandwiched between high-index layers. The structures studied here comprise coupled amorphous germanium guided-mode resonance (GMR) structures with a top one-dimensional grating layer and bottom uniform layer separated by a low-index silicon nitride layer. The slot-mode profile within the silicon nitride layer with mode field confinement > 30 % is achieved as a solution to the electromagnetic wave propagation through the coupled GMR structure with the dominant field component being perpendicular to the layers. The quasi-BIC resonance in symmetric 1D grating structures can be observed even at normal incidence when considering a realistic excitation beam with finite angular spread. The measured transmission peak is found to redshift (remain almost unchanged) under classical (full-conical) mounting conditions. The highest quality factor of ∼ 400 is experimentally extracted at normal incidence under a classical mounting condition with a resonance peak at 3.41 μm wavelength. Such slot-mode GMR structures with appropriately chosen low-index intermediate layers can find applications in resonantly enhanced sensing and active photonic devices.

Numerical investigations of an ultra-compact polarization beam splitter based on augmented low-index guiding and subwavelength grating structures

We report the design of an ultra-compact polarization beam splitter with high performance that is based on augmented low-index guiding and subwavelength grating (SWG) structures. The transverse-electric (TE) and transverse-magnetic (TM) modes are confined in high-index silicon (Si) and low-index silicon nitride (Si3N4), respectively. They are separated by using, respectively, a gradually curved Si waveguide and a Si3N4 SWG structure with optimal grating-element. The footprint of the proposed polarization beam splitters (PBS) is 2.9 × 2.25 μm2. The device offers high polarization extinction ratios (PERs) of ~18 dB for the two polarizations, with low insertion losses of ~0.22 dB (~0.71 dB) for the TE (TM) mode at the wavelength of 1550 nm. Over the broad band from λ = 1500–1650 nm, the PERs of the TE and TM modes are above 17 and 16 dB, respectively. By narrowing the operating band to the range from λ = 1500 to 1600 nm, the proposed PBS provides PERs of >17 dB for both polarizations. Finally, the fabrication tolerance of the designed PBS is also addressed and discussed in detail.

Compact and broadband polarization beam splitter based on a silicon nitride augmented low-index guiding structure.

We propose a compact polarization beam splitter (PBS) based on a silicon nitride enhanced silicon-on-insulator platform using an augmented low-index guiding (ALIG) waveguide structure. In the ALIG structure, the TM mode is mostly confined in the low-index silicon nitride, while the TE mode is confined in the high-index silicon. Since the two modes are confined in two separate layers, their properties can be controlled independently. The PBS is formed using an asymmetric multimode interference (MMI) section. The TM mode is directed to an output port by the ALIG waveguide, while the TE mode is coupled to the other output port via the multimode interferometer. Such a PBS has a very small footprint, low insertion loss, high polarization extinction ratio, and broadband response.

Low-index nanopatterned barrier for hybrid oxide-free III-V silicon conductive bonding.

Oxide-free bonding of a III-V active stack emitting at 1300-1600 nm to a silicon-on-insulator wafer offers the capability to electrically inject lasers from the silicon side. However, a typical 500-nm-thick silicon layer notably attracts the fundamental guided mode of the silicon + III-V stack, a detrimental feature compared to established III-V Separate-Confinement Heterostructure (SCH) stacks. We experimentally probe with photoluminescence as an internal light source the guiding behavior for oxide-free bonding to a nanopatterned silicon wafer that acts as a low-index barrier. We use a sub-wavelength square array of small holes as an effective "low-index silicon" medium. It is weakly modulated along one dimension (superperiodic array) to outcouple the resulting guided modes to free space, where we use an angle-resolved spectroscopy study. Analysis of experimental branches confirms the capability to operate with a fundamental mode well localized in the III-V heterostructures.

Silicon-Based Thermo-Optically Tunable Photonic Crystal Lens

We report an extremely compact (30 ?m × 7 ?m) silicon-based 2-D thermo-optically tunable photonic crystal (PhC) lens operated at around telecommunication wavelength (1.55 ?m ) with transverse-magnetic-like polarization light. A honeycomb lattice array of high index silicon rods with 340 nm in thickness, 234 nm in diameter, and 338 nm in lattice constant were embedded in 3-?m-thick low index silicon dioxide. A 150-nm-thick NiCr micro-heater was placed directly on top of the PhC structure to provide localized heating to the silicon rod array. The localized heating causes refractive index change in silicon due to thermo-optic effect which results in change of the focal length of the PhC lens. The device was characterized with a tunable laser light source in the wavelength range of 1500 ~ 1580 nm. Tuning of focal length in this device was experimentally demonstrated by applying different current through the heater. Such experimental results showed good agreement with the simulation results.

Nanolines on silicon surfaces

Although a wide range of periodic surface nets can be grown on low index silicon surfaces, only a few of these have quasi-one dimensional symmetry. If high index silicon surfaces, such as (553) and (557), are used instead, the surface unit cell contains steps. It is possible to fabricate a number of quasi-one dimensional nanoline systems on the terraces and some of these have nested energy bands near the Fermi level. These nano-scale systems may support exotic many-electron states produced by enhanced electron correlations and a reduction in electron screening in one spatial dimension. In this paper, our groups' experimental and theoretical studies of nanolines phases, grown on both low index and vicinal silicon surfaces are reviewed. These studies give us insight into the electronic properties of artificial nanoline structures.

Thermal Oxide Grown on High Index Silicon Wafers

We characterized the oxide thickness of MOS capacitors fabricated on the high index silicon surfaces (1 1 4) and (5 5 12). The low index silicon wafers (0 0 1), (1 1 1), and (1 1 0) were used as a reference. Four fabrication processes were conducted, two with polysilicon gate (600Aå oxide thickness), and two using aluminum gate (600Aå and 150 Aå oxide thickness). Our results show that the oxide thickness increases as the surface goes from (0 0 1) to (1 1 1).

Strong enhancement of second-harmonic generation in all-dielectric resonant waveguide grating

The authors demonstrate that a dielectric resonant waveguide grating can enhance optical second-harmonic generation by a factor of 550 compared to a similar flat surface. Their structure, which consists of purely dielectric and thereby transparent materials, has a low index silicon dioxide grating covered by a high index titanium dioxide layer and it is optimized for the fundamental wavelength of 1064nm. Polarization dependence of the second-harmonic response suggests that the enhancement arises from the favorable interaction of the resonant waveguide mode and its strong local field with the inherent surface nonlinearity of the dielectric materials.

Inhibited coarsening of a spray-formed and extruded hypereutectic aluminum-silicon alloy in the semisolid state

The microstructural evolution of a spray-formed and extruded hypereutectic aluminum-30 pct silicon-5 pct copper-2 pct magnesium alloy heated into the semisolid state has been investigated. Liquid is formed initially by a quaternary eutectic reaction and then by a ternary melt reaction. These reactions occur relatively quickly. However, the binary Al-Si eutectic melt reaction takes a significant time—around several hours depending on the temperature. The coarsening rate constants (K) for the growth of the silicon particles are approximately three to four orders of magnitude lower than those for the majority of metal spray-formed alloys. This may be associated with difficulties in addition or removal of atoms from the low index silicon facets. Where growth does occur, agglomeration of silicon particles may play a large role, especially at higher liquid contents. Electron backscatter diffraction (EBSD) gives evidence of agglomeration, and furthermore shows that high-angle silicon-silicon boundaries are not wetted with liquid.

Alkali metal promoted oxidation of the Si(113) surface

We have investigated the sodium-promoted oxidation of the Si(113) surface by using X-ray photoelectron spectroscopy (Mg K α , 1253.6 eV). O 1s, Si 2p and Na 1s core level spectra for the O 2/Na/Si(113) surface were measured with increasing oxygen exposure and at several annealing temperatures. It was observed that sodium enhanced oxygen adsorption by about three times. When the 1.4 monolayer Na/Si(113) surface was exposed to oxygen at 150 K, the sodium overlayer was oxidized but the silicon substrate was not. Subsequent annealing transferred oxygen atoms into the silicon substrate and desorbed sodium from the Si(113) surface. This oxidation process resulted in thin silicon dioxide films at about 670 K. The results are consistent with those from low index silicon surfaces, Si(100) and Si(111), and are interpreted within a previously suggested sponge model for the role of alkali metal in the promoted oxidation.

HIGH RESOLUTION SYNCHROTRON RADIATION PHOTOEMISSION SPECTROSCOPY

Photoemission spectroscopy studies with very-high-energetic resolution at third generation synchrotron radiation facilities open up new perspectives in surface science. To illustrate this, recent advances achieved from valence band studies will be described, i.e. investigation of the quantized electronic states of a two-dimensional electron gas at the InAs(110) surface. New insights gained from core level spectroscopy in the most promising soft X-ray regime will also be presented, i.e. evidence of new spectral features in Si 2p studies of low index silicon surfaces and identification of surface core level shifts in III–V semiconductors.

Computer studies of RHEED patterns from low index silicon surfaces

Results of convergent beam reflection high energy electron diffraction (CB-RHEED) calculations are presented for the silicon (111) and (001) surfaces. These double rocking calculations, obtained using a dynamical scattering approach, provide a more stringent comparison between calculation and experiment than conventional RHEED. In particular, symmetry effects in these two dimensional patterns are more easily recognised than in the one-dimensional line scan conventional RHEED case. Preliminary calculations for the Si(001)2 × 1 structure are used as a basis for discussion of experimental reflection diffraction patterns and images.

Computer studies of RHEED patterns from low index silicon surfaces

Abstract Results of convergent beam reflection high energy electron diffraction (CB-RHEED) calculations are presented for the silicon (111) and (001) surfaces. These double rocking calculations, obtained using a dynamical scattering approach, provide a more stringent comparison between calculation and experiment than conventional RHEED. In particular, symmetry effects in these two dimensional patterns are more easily recognised than in the one-dimensional line scan conventional RHEED case. Preliminary calculations for the Si(001)2 × 1 structure are used as a basis for discussion of experimental reflection diffraction patterns and images.

Optical design for the NSLS crystallography beam line

The design goal of this beam line is to produce a photon beam well suited to accurate single-crystal diffraction measurements, with particular attention to the possibilities of the use of anomalous dispersion. These goals lead to the following criteria: (1) a beam uniform in intensity to 1% over an area 0.3×0.3 mm 2, (2) ΔE E ≈ 3×10 −4 , and (3) tunable over the energy range 5–21 keV accessible from a NSLS bending magnet. These criteria lead to a number of design decisions that have been applied to this and other NSLS beam lines: 1. (1) A mirror is placed as the first optical element to better match the vertical opening angle of the photon beam to the typical rocking curves of low-index silicon Bragg peaks, reducing ΔE E in a nondispersive, two-crystal monochromator. 2. (2) With this mirror in place a two-crystal parallel mode monochromator with constant deviation is chosen. 3. (3) A post-mirror to condense the radiation is placed after the monochromator. The entire optical system has been designed to operate in an ultra-high vacuum environment. The details of the mirror designs, coatings and grazing angles are presented as well as other possible choices for these elements. Ray tracing results are given for the crystallography beam line for comparison with the design goals.