A. Lui

Comparison among various Si/sub 3/N/sub 4/ waveguide geometries grown within a CMOS fabrication pilot line

Low-pressure chemical-vapor deposition (LPCVD) thin-film Si/sub 3/N/sub 4/ waveguides have been fabricated on Si substrate within a complementary metal-oxide-semiconductor (CMOS) fabrication pilot line. Three kinds of geometries (channel, rib, and strip-loaded) have been simulated, fabricated, and optically characterized in order to optimize waveguide performances. The number and optical confinement factors of guided optical modes have been simulated, taking into account sidewall effects caused by the etching processes, which have been studied by scanning electron microscopy. Optical guided modes have been observed with a mode analyzer and compared with simulation expectations to confirm the process parameters. Propagation loss measurements at 780 and 632.8 nm have been performed by both using the cutback technique and measuring the drop of intensity of the top scattered light along the length of the waveguide. Loss coefficients of approximately 0.1 dB/cm have been obtained for channel waveguides. These data are very promising in view of the development of Si-integrated photonics.

Propagation losses of silicon nitride waveguides in the near-infrared range

Si3N4∕SiO2 waveguides have been fabricated by low pressure chemical vapor deposition within a complementary metal–oxide–semiconductor fabrication pilot line. Propagation losses for different waveguide geometries (channel and rib loaded) have been measured in the near infrared as a function of polarization, waveguide width, and light wavelength. A maximum thickness of single Si3N4 of 250 nm is allowed by the large stress between Si3N4 and SiO2. This small thickness turns into significant propagation losses at 1544 nm of about 4.5dB∕cm because of the poor optical mode confinement factor. Strain release and control is possible by using multilayer waveguides by alternating Si3N4 and SiO2 layers. In this way, propagation losses of about 1.5dB∕cm have been demonstrated thanks to an improved optical mode confinement factor and the good quality of the interfaces in the waveguide.

P-type macroporous silicon for two-dimensional photonic crystals

Macroporous silicon with two-dimensional periodicity has been produced by electrochemical etching, using a p-type doped silicon substrate. The structure shows photonic energy gaps in the infrared region, as demonstrated by variable angle reflectance measurements. The agreement between measurement and band calculations confirms the high quality of the samples. The use of an optimized electrolyte allows the fabrication of very high quality samples, with high aspect ratio and low roughness both at the surface and on the pore walls. The best results are obtained with aprotic and protophilic solvents.

Whispering-gallery modes and light emission from a Si-nanocrystal-based single microdisk resonator

We report on visible light emission from Si-nanocrystal based optically active microdisk resonators. The room temperature photoluminescence (PL) from single microdisks shows the characteristic modal structure of whispering-gallery modes. The emission is both TE and TM-polarized in 300 nm thick microdisks, while thinner ones (135 nm) support only TE-like modes. Thinner disks have the advantage to filter out higher order radial mode families, allowing for measuring only the most intense first order modal structure. We reveal subnanometer linewidths and corresponding quality factors as high as 2800, limited by the spectral resolution of the experimental setup. Moreover, we observe a modification of mode linewidth by a factor 13 as a function of pump power. The origin of this effect is attributed to an excited carrier absorption loss mechanism.

Silicon-based near-infrared tunable filters filled with positive or negative dielectric anisotropic liquid crystals

Complementary metal-oxide-semiconductor-compatible tunable Fabry–Perot microcavities filled with liquid crystals (LCs) were realized and studied in the near-infrared region. The microcavities were produced by chip bonding technique, which allows one to infill LC between two [SiO2/Si]3λ/4 (λ=1.5 μm) dielectric Bragg reflectors separated by 950-nm-thick SiO2 posts. Liquid crystals with positive and negative dielectric anisotropy were used, i.e. MerckE7 (Δe=13.8) and Merck-6608 LC (Δe=−4.2). Mirror-integrated electrodes allow an external bias to induce an electric field and to tune the LC properties and, hence, the microcavity resonance. Electric-field-induced shifts of the second-order cavity modes of 127 and 49 nm were obtained for Merck-E7 and Merck-6608 LC, with driving potentials of 5 and 10 V, respectively.

Optical response of one-dimensional (Si/SiO2)m photonic crystals

One-dimensional photonic crystals made of (Si/SiO2)m multilayers with m=2,…8 have been grown on SiO2 4-in. wafers by repeated polysilicon low-pressure chemical vapor deposition, oxidation, and wet etching steps. The poly-Si and SiO2 layers were about 220 and 660 nm thick, respectively, thus realizing λ/4 distributed Bragg reflectors. Spectroscopic ellipsometry in the 1.4–5 eV range was used to determine the dielectric function of poly-Si and the actual layer thicknesses, as well as to check the structural and compositional homogeneity of the structures. In order to measure the photonic crystal properties, specular reflectance and transmittance measurements were performed from 0.2 to 6 eV at different angles of incidence θ⩽50° and for transverse electric and transverse magnetic polarizations. The stop-bands characteristic of Bragg reflector multilayers appear up to the fifth order and become more pronounced with increasing m, reaching almost complete rejection for m=4 periods. The experimental spectra were...

Light emitting porous silicon diode based on a silicon/porous silicon heterojunction

A new structure is proposed to improve the external quantum efficiency of porous silicon (PS) light emitting diodes (LED). It is based on a heterojunction between n-type doped silicon and PS. The heterojunction is formed due to the doping selectivity of the etching process used to form PS. The improvement of the proposed LED structure with respect to usual metal/PS LED is demonstrated. This is thought to be due to a different injection mechanism for which carriers are injected directly into conduction band states. Anodic oxidation experiments show further improvements in the LED efficiency.

Design of an integrated optical switch based on liquid crystal infiltration

We present the design and simulation of a novel Fabry-Pe/spl acute/rot optical switch based on liquid crystal infiltration integrated in a high index-contrast silicon-on-insulator waveguide. Careful optimization of the cavity geometry allows designing a device with a resonance peak transmission of 95% and a resonance peak linewidth of 1.7 nm. This kind of device is designed to be fully CMOS compatible and can be used as a building block of a more complex integrated optical circuit.

Photon recycling in Fabry–Perot micro-cavities based on Si3N4 waveguides

Abstract We present a numerical analysis and preliminary experimental results on one-dimensional Fabry–Perot micro-cavities in Si 3 N 4 waveguides. The Fabry–Perot micro-cavities are formed by two distributed Bragg reflectors separated by a straight portion of a waveguide. The Bragg reflectors are composed of a few air slits produced within the Si 3 N 4 waveguides. In order to increase the quality factor of the micro-cavities, we have minimized, with a multiparametric optimization tool, the insertion loss of the reflectors by varying the length of their first pairs (those facing the cavity). To explain the simulation results, the coupling of the fundamental waveguide mode with radiative modes in the Fabry–Perot micro-cavities is needed. This effect is described as a recycling of radiative modes in the waveguide. To support the modelling, preliminary experimental results of micro-cavities in Si 3 N 4 waveguides realized with the focused ion beam technique are reported.

Silicon based near infrared tunable filters based on liquid crystals

Complementary metal-oxide-semiconductor-compatible tunable Fabry-Perot microcavities filled with liquid crystals (LCs) were realized and studied in the near-infrared region. The microcavities were produced by chip bonding technique, which allows one to infill LC between two [SiO2/Si]n λ/4 (λ = 1.5 μm) Dielectric Bragg Reflectors separated by 950 nm thick SiO2 posts. The Dielectric Bragg reflectors were realized on Si or SiO2 substrates Liquid crystals with positive and negative dielectric anisotropy were used, i.e. MerckE7 (Δε=13.8) and Merck-6608 LC (Δε=-4.2). Mirror-integrated electrodes allow an external bias to induce an electrical field and to tune the LC properties and, hence, the microcavity resonance. Electric-field-induced shifts of the second-order cavity modes of ~120 nm and ~50 nm were obtained for Merck-E7 and Merck-6608 LC, with driving potentials of 5 V and 10 V, respectively. The transmittance at the cavity resonance is typically in the order of 10%. Simulation of cavities allows to identify surface roughness of the Dielectric-Bragg-Reflectors as the major origin of the transmission losses. The switching behavior of microcavities filled with E7 were studied as function of applied fields. Both switch-on ton and switch-off toff times were measured and were found to be lower than 5 ms.