Accurate measurement of scattering and absorption loss in microphotonic devices

We present a simple measurement and analysis technique to determine the fraction of optical loss due to both radiation (scattering) and linear absorption in microphotonic components. The method is generally applicable to optical materials in which both nonlinear and linear absorption are present and requires only limited knowledge of absolute optical power levels, material parameters, and the structure geometry. The technique is applied to high-quality-factor (Q=1×10^6 to Q=5×10^6) silicon-on-insulator (SOI) microdisk resonators. It is determined that linear absorption can account for more than half of the total optical loss in the high-Q regime of these devices

Beyond the Rayleigh scattering limit in high-Q silicon microdisks: theory and experiment

Using a combination of resist reflow to form a highly circular etch mask pattern and a low-damage plasma dry etch, high-quality-factor silicon optical microdisk resonators are fabricated out of silicon-on-insulator (SOI) wafers. Quality factors as high as Q = 5×10^6 are measured in these microresonators, corresponding to a propagation loss coefficient as small as α ~ 0.1 dB/cm. The different optical loss mechanisms are identified through a study of the total optical loss, mode coupling, and thermally-induced optical bistability as a function of microdisk radius (5-30 µm). These measurements indicate that optical loss in these high-Q microresonators is limited not by surface roughness, but rather by surface state absorption and bulk free-carrier absorption

Self-induced optical modulation of the transmission through a high-Q silicon microdisk resonator

Direct time-domain observations are reported of a low-power, self-induced modulation of the transmitted optical power through a high-Q silicon microdisk resonator. Above a threshold input power of 60 μW the transmission versus wavelength deviates from a simple optical bistability behavior, and the transmission intensity becomes highly oscillatory in nature. The transmission oscillations are seen to consist of a train of sharp transmission dips of width approximately 100 ns and period close to 1 μs. A model of the system is developed incorporating thermal and free-carrier dynamics, and is compared to the observed behavior. Good agreement is found, and the self-induced optical modulation is attributed to a nonlinear interaction between competing free-carrier and phonon populations within the microdisk

Measuring the role of surface chemistry in silicon microphotonics

Utilizing a high quality factor (Q~1.5×10^6) optical microresonator to provide sensitivity down to a fractional surface optical loss of alphas[prime]~10^–7, we show that the optical loss within Si microphotonic components can be dramatically altered by Si surface preparation, with alphas[prime]~1×10^–5 measured for chemical oxide surfaces as compared to alphas[prime]<=1×10^–6 for hydrogen-terminated Si surfaces. These results indicate that the optical properties of Si surfaces can be significantly and reversibly altered by standard microelectronic treatments, and that stable, high optical quality surface passivation layers will be critical in future Si micro- and nanophotonic systems

Optical fiber taper coupling and high-resolution wavelength tuning of microdisk resonators at cryogenic temperatures

A system for studying microcavity resonators at cryogenic temperatures (~10 K) through evanescent coupling via optical fiber taper waveguides is reported, and efficient fiber coupling to AlGaAs microdisk cavities with embedded quantum dots is demonstrated. As an immediate application of this tool, we study high-resolution tuning of microdisk cavities through nitrogen gas adsorption, as first discussed by Mosor, et al. By proper regulation of the nitrogen gas flow and delivery of the gas to the sample surface, continuous tuning can be achieved with modest gas flows, and overall wavelength shifts as large as 4 nm are achieved.Comment: 4 pages, 4 figure

Photonic crystal defect lasers

Optically thin dielectric slabs, in which a fully etched through two-dimensional patterning is applied, can be used to form high-Q optical cavities with modal volumes approaching the theoretical limit of a cubic half-wavelength. A cavity design strategy based upon simple group theoretical techniques is presented in which emphasis is placed upon a momentum space description of the resonant modes. It is shown that photonic crystal laser cavities can be designed with a particular wavelength, polarization, and radiation pattern using these methods

Single quantum dot spectroscopy using a fiber taper waveguide near-field optic

Photoluminescence spectroscopy of single InAs quantum dots at cryogenic temperatures (~14 K) is performed using a micron-scale optical fiber taper waveguide as a near-field optic. The measured collection efficiency of quantum dot spontaneous emission into the fundamental guided mode of the fiber taper is estimated at 0.1%, and spatially-resolved measurements with ~600 nm resolution are obtained by varying the taper position with respect to the sample and using the fiber taper for both the pump and collection channels.Comment: 4 pages, 3 figure

Rayleigh scattering, mode coupling, and optical loss in silicon microdisks

High refractive index contrast optical microdisk resonators fabricated from silicon-on-insulator wafers are studied using an external silica fiber taper waveguide as a wafer-scale optical probe. Measurements performed in the 1500 nm wavelength band show that these silicon microdisks can support whispering-gallery modes with quality factors as high as 5.2 x 10^5, limited by Rayleigh scattering from fabrication induced surface roughness. Microdisks with radii as small as 2.5 microns are studied, with measured quality factors as high as 4.7 x 10^5 for an optical mode volume of 5.3 cubic wavelengths in the material.Comment: 4 pages, 2 figures; contains minor correction to doublet splitting theor

Surface Encapsulation for Low-Loss Silicon Photonics

Encapsulation layers are explored for passivating the surfaces of silicon to reduce optical absorption in the 1500-nm wavelength band. Surface-sensitive test structures consisting of microdisk resonators are fabricated for this purpose. Based on previous work in silicon photovoltaics, coatings of SiNx and SiO2 are applied under varying deposition and annealing conditions. A short dry thermal oxidation followed by a long high-temperature N2 anneal is found to be most effective at long-term encapsulation and reduction of interface absorption. Minimization of the optical loss is attributed to simultaneous reduction in sub-bandgap silicon surface states and hydrogen in the capping material.Comment: 4 pages, 3 figure