We review our recent results on integrating biomedical optical systems onto a silicon chip. Light collection by integrated waveguides has been investigated. Confocal light delivery and collection by a combination of two arrayed-waveguide gratings has been achieved. Using an arrayed-waveguide grating as an integrated spectrometer, Raman spectroscopy and spectral-domain optical coherence tomography have been demonstrated.

Optical biosensors on silicon platforms have demonstrated their great potential in label-free detection and analysis tool. The major challenge of ring resonator based optical biosensors is their high sensitivity to temperature variations. We have designed a double-ring resonator biosensor using a vertical coupling method. Simulation results show that the double-ring configuration effectively eliminates the temperature and environmental fluctuations by the resonant wavelength shift correction induced from the reference ring. The coupling gap in the vertical coupling method can be tuned by timing the deposition/growth rate of the space layer, without the need of any advanced lithography. The vertical coupling method allows a low fabrication cost.

The opto-mechanical properties of silicon together with its fabrication flexibility make it an excellent candidate for many integrated photonic applications. The recent trend in miniaturizing optical components, while maintaining stringent demands for their performance opens a way for various coupling mechanisms to be used for sensing and switching. However, as the system grows in complexity, the number of possible coupling channels increases. Being able to resolve the specific coupling mechanisms becomes a delicate and challenging task. At the same time, the overall performance of the device is often decided by the collective performance of all of these channels rather than by each of them separately. Because of that, it becomes essential to account for all of these mechanisms, while modelling the system. As an example of such a system, a mechanical displacement sensor is presented, whose principle is based on simultaneous evanescent coupling mechnisms between the TE- and TM-like modes. By using the FEM approach, the coupling mechanisms between the waveguides (and modes) are analyzed separately through 2-dimensional simulation and later combined and investigated globally in a series of 3-dimensional simulations.

Photonic crystal surfaces that can be fabricated inexpensively over large surface areas can be designed to produce optical resonances for any desired wavelength in the optical spectrum from ultraviolet to infrared. Label-free biosensing is obtained by measuring shifts in the resonant wavelength as biomaterial deposits on the photonic crystal, while the intensified electric fields that occur due to coupling of illumination at the resonant wavelength may be used to more effectively excite fluorescence or Raman scattering. Photon emitters, such as quantum dots, fluorescent dye molecules, and Raman scatters can efficiently couple their energy to detection instruments when they are in close proximity to a photonic crystal with a resonance that matches the emission wavelength. Finally, the narrowband optical filtering capability of photonic crystals can be effectively applied for infrared absorption imaging of biological specimens at discrete wavelengths. This talk will summarize recent activities in the Nano Sensors Group at the University of Illinois in which photonic crystals are used to address a variety of problems in biological sensing.

We present an innovative and efficient technique for post-fabrication trimming of silicon photonic integrated circuits (PICs). Our approach exploits the high photosensitivity of chalcogenide glasses (ChGs) to induce local and permanent modifications of the optical properties and spectral responses of ChG-assisted silicon devices. We experimentally demonstrate the potential of this technique on ring resonator filters realized on a silicon-on-insulator platform, for which post-fabrication treatments enable to counteract the strong sensitivity to technological tolerances. Photosensitive ChGassisted silicon waveguides were realized by deposition of a As₂S₃ chalcogenide layer on top of conventional silicon channel waveguides. A resonant wavelength shift of 6.7 nm was achieved, largely exceeding the random resonance spread due to fabrication tolerances. Neither the ChG layer deposition, nor the trimming process introduces appreciable additional losses with respect to the bare silicon core waveguide. Performances of the trimming technique, such as speed and saturation effects, as well as nonlinear behavior and infrared writing issues are investigated and experimentally characterized.

We theoretically examine the optical guiding properties of typical 3-layer oxide waveguides with an additional thin high dielectric cladding applied on one side. Modal and FDTD simulations techniques are performed to determine the field evolution and coupling coefficients for light transitioning between the un-clad and clad regions. Results indicate that such waveguide structures lead to novel integrated optic device designs. We also address the issue of efficiently coupling light from a low dielectric value slab waveguide, into the lowest order mode supported in the clad layer by using waveguide tapers. In particular a waveguide structure suitable for efficiently interfacing glass based and silicon based waveguide structures is presented for the SOI fabrication platform.

We present our studies on slot waveguides clad with atomic layer deposition (ALD). We show reduction of propagation loss in silicon slot waveguides. With an ALD grown titania thin film cladding, propagation loss as low as 7 dB/cm for a deep-UV patterned slot waveguide is achieved.

Subwavelength gratings (SWG) are periodically segmented waveguides with a pitch small enough to suppress diffraction. These waveguides can be engineered to implement almost any refractive between the refractive indices of the material that compose the waveguide, thereby opening novel design possibilities. In this communication we explore the use of SWGs in the design and optimization of a variety of integrated optical devices in the silicon-on-insulator platform: fiber-to-chip grating couplers, polarization splitters and high performance multimode interference couplers. We furthermore show that the dispersion properties of SWGs enable the design of novel filters, and discuss the design of low transitions between SWG waveguides of different characteristics.

Silicon photonic wire waveguides are usually highly birefringent, so they are generally designed to operate for one particular polarization. For commonly used waveguides with a silicon thickness of 220 nm, TE polarization is preferred since TM is only weakly guided. For waveguides with a silicon thickness larger than 250 nm, both TE and TM polarizations have been employed. Overall, the choice of polarization has largely appeared arbitrary. In this presentation we review the pertinent polarization-dependent waveguide properties, including losses, back-reflection, polarization conversion and fabrication tolerances, with the intent to suggest guidelines for choosing the proper polarization. Through experimental evidence, we show that TM polarization has several important advantages and can support high performance resonators with a radius down to 2 μm.

We review our recent work on waveguide grating couplers, including an apodized grating coupler with engineered coupling strength to achieve Gaussian-like output profile, which greatly improves the fiber-chip coupling efficiency. We will also discuss a new class of grating couplers involving the use of sub-wavelength nanostructures to engineer the optical properties. Effective medium theory can be used in the design of sub-wavelength structures, which, when properly engineered, can offer broadband coupling and polarization independence. Other applications of waveguide gratings, for example bi-wavelength two dimensional gratings coupler for (de-)multiplexing two different wavelengths, fiber-waveguide hybrid lasers and mid-infrared grating couplers on silicon-on-sapphire wafer will also be briefly discussed.

Different types of slot waveguide couplers are studied theoretically and experimentally. We present strip-to-slot waveguide couplers with a length as small as 10 μm and with their feature sizes no less than 150 nm, exhibiting efficient coupling into the slot mode and negligible scattering. We demonstrate coupling loss of 0.11 dB for a 20-micron long coupler from a strip waveguide to slot waveguide, fabricated using the 248 nm deep-UV lithography. We also discuss ring resonator couplers, multimode multislot structures for 2x2 couplers, and prism coupling characterization of multislot waveguide structures.

SOI-based trapezoidal waveguide with 45° reflector for non-coplanar light bending is proposed and demonstrated. The proposed structures include 45° micro-reflector and silicon trapezoidal waveguide. Due to the SOI-based trapezoidal waveguide with 45° reflector, light wave can be coupled into silicon waveguide easily and have higher coupling efficiency. All of structures are fabricated using a single-step wet etching process. The RMS roughness of waveguide sidewall and 45° micro-reflector is about 30 nm. The coupling efficiency of proposed structure is -4.51 dB, and misalignment tolerance are 42 μm at horizontal direction and 41 μm at vertical direction. The multi-channel trapezoidal waveguide is also demonstrated. This device can transfer the light wave at the same time, and its cross talk is about -50 dB.

Parallel-coupled dual racetrack micro-resonator structures have potential applications for quadrature amplitude modulation. Fabrication of parallel-coupled dual racetrack silicon micro-resonators was conducted, while overcoming for some barriers to fabrication. Fabrication process limitations and design considerations are discussed. Fabrication results are presented. Some barriers to fabrication include stitching and overdosing in electron beam lithography. A multi-input and output test bed with optical and electrical control was necessary for device characterization. The characterization of the fabricated devices is presented, along with the related procedures. Some of the tests performed are wavelength scans and top surface scans.

We present our recent work on silicon resonance and slow-light based devices for optical signal processing. Waveguide self-coupling and mutual coupling are used to tailor the waveguide spectral and dispersion characteristics. With selfcoupling, optical resonances are generated with unique transmission performances. Electromagnetically induced transparency (EIT)-like effect appears in cascaded self-coupled waveguides. With mutual coupling between ridge and slot waveguides, group velocity experiences a big jump and optical signal can be delayed for a large range with low distortion by using thermo-optic tuning.

In this paper we will describe the fabrication and characterization of passive waveguides which exploit the phenomenon of variable charge state mediation of deep-levels in silicon to vary optical absorption. Silicon waveguides are doped with either thallium or indium and co-doped with phosphorus. Optical absorption is reduced s phosphorus doping is increased. These results suggest a novel method of modulation via charge-state control of the deep-level.

Although the potential of hyperspectral imaging has been demonstrated for several applications, using laboratory setups in research environments, its adoption by industry has so far been limited due to the lack of high speed, low cost and compact hyperspectral cameras. To bridge the gap between research and industry, we present a novel hyperspectral sensor that integrates a wedge filter on top of a standard CMOS sensor. To enable the low-cost processing of a microscopic wedge filter, we have introduced a design that is able to compensate for process variability. The result is a compact and fast hyperspectral camera made with low-cost CMOS process technology. The current prototype camera acquires 100 spectral bands over a spectral range from 560 nm to 1000 nm, with a spectral resolution better than 10 nm and a spatial resolution of 2048 pixels per line. The speed is 180 frames per second at illumination levels as typically used in machine vision. The prototype is a hyperspectral line scanner that acquires 16 lines per spectral band in parallel on a 4 MPixel sensor. The theoretic line rate for this implementation is thus 2880 lines per second.

We present photoluminescence and modal gain measurements in a Ga(NAsP) single-quantum well sample pseudomorphically grown on silicon substrate. The temperature dependence indicates that disorder induced localization effects dominate the low temperature photoluminescence spectra. Nevertheless, using the variable stripe length method, we observe modal gain values up to 15 cm^-1 at room temperature. These values are very promising, demonstrate the high optical quality of the new dilute nitride material Ga(NAsP) and underline its candidacy for electrically pumped lasers on silicon substrate.

The emission spectra of pn-junction and punch-through (PT) carrier injection silicon (Si) CMOS light sources were measured at various current densities and temperatures. In contrast to the narrow-band forward-biased junction spectrum, that peaks around 1.1 μm (1.1 eV), the reverse-bias spectrum was found to extend from about 350 nm (3.4 eV) to about 1.7 μm (0.7 eV) covering the UV, Vis and NIR regions. Since the photon energy decreases with increasing wavelength, the significant NIR radiation implies that the quantum conversion efficiency of Si avalanche light sources is appreciably higher than previously reported. The spectrum of PT light source constitutes a scaled combination of both the forwardand reverse-biased junction spectra. Calculating the photon flux at the emission source within the Si against photon energy allowed the deduction and quantification of the physical light emission processes with respect to silicon's electronic band structure. Intra-conduction-band (c-c) electron (e^-) transitions seem to be the dominant physical mechanism responsible for the wide avalanche spectrum. The effect of current densities up to 10⁶ A/cm² and temperatures between 22 °C and 122 °C on the emission spectrum and consequently the physical light generation mechanism are investigated and quantified.

Mid-infrared group IV photonics is emerging as a field with possible applications ranging from gas sensing to free-space communications. Free-carrier induced electro-absorption and electro-refraction have become the most widely used modulation mechanisms in active near-infrared silicon photonic devices. This work examines the magnitude of this effect in group IV materials at mid-infrared wavelengths. In silicon electro-absorption effects are calculated from experimental absorption coefficient data, and electro-refraction is calculated through numerical Kramers-Kronig analysis of absorption spectra. In germanium the Drude-Lorentz equations are used to estimate both change in absorption and change in refractive index.