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

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

Efficient input and output coupling between planar photonic crystal waveguides and fiber tapers

Highly efficient (>94%) contradirectional coupling into and out of a photonic crystal waveguide is measured using a fiber taper probe as both a source and a collector

Highly efficient coupling to photonic crystal waveguides from optical fiber tapers

Nearly-complete mode-selective coupling from an optical fiber taper to a photonic crystal defect waveguide is demonstrated experimentally. We observe 95% power transfer with a 20 nm bandwidth and a coupling length less then 65 µm

Probing the dispersive and spatial properties of planar photonic crystal waveguide modes via highly efficient coupling from optical fiber tapers

The demonstration of an optical fiber based probe for efficiently exciting the waveguide modes of high-index contrast planar photonic crystal (PC) slabs is presented. Utilizing the dispersion of the PC, fiber taper waveguides formed from standard silica single-mode optical fibers are used to evanescently couple light into the guided modes of a patterned silicon membrane. A coupling efficiency of approximately 95% is obtained between the fiber taper and a PC waveguide mode suitably designed for integration with a previously studied ultra-small mode volume high-Q PC resonant cavity [1]. The micron-scale lateral extent and dispersion of the fiber taper is also used as a near-field spatial and spectral probe to study the profile and dispersion of PC waveguide modes. The mode selectivity of this wafer-scale probing technique, together with its high efficiency, suggests that it will be useful in future quantum and non-linear optics experiments employing planar PCs

An optical fiber-based probe for photonic crystal microcavities

We review a novel method for characterizing both the spectral and spatial properties of resonant cavities within two-dimensional photonic crystals (PCs). An optical fiber taper serves as an external waveguide probe whose micron-scale field is used to source and couple light from the cavity modes, which appear as resonant features in the taper's wavelength-dependent transmission spectrum when it is placed within the cavity's near field. Studying the linewidth and depth of these resonances as a function of the taper's position with respect to the resonator produces quantitative measurements of the quality factor Q and modal volume Veff of the resonant cavity modes. Polarization information about the cavity modes can be obtained by studying their depths of coupling when the cavity is probed along different axes by the taper. This fiber-based technique has been used to measure Q ~ 40,000 and Veff ~ 0.9 cubic wavelengths in a graded square lattice PC microcavity fabricated in silicon. The speed and versatility of this fiber-based probe is highlighted, and a discussion of its applicability to other wavelength-scale resonant elements is given.Comment: Submitted to special section on photonic crystals from the PECS-V conference in IEEE Journal on Selected Areas in Communications (J-SAC), Nanotechnologies for Communications issu

Optical fiber-based measurement of ultra-small mode volume and a high quality factor in a photonic crystal microcavity

Using an optical fiber taper that simultaneously probes the spectral and spatial properties of resonant cavity modes, two-dimensional Si photonic crystal microcavities with a quality factor Q~40,000 and modal volume V_eff ~0.9(λ/n)^3 are experimentally demonstrated

Cavity Q, mode volume, and lasing threshold in small diameter AlGaAs microdisks with embedded quantum dots

The quality factor (Q), mode volume (Veff), and room-temperature lasing threshold of microdisk cavities with embedded quantum dots (QDs) are investigated. Finite element method simulations of standing wave modes within the microdisk reveal that Veff can be as small as 2(lambda/n)^3 while maintaining radiation-limited Qs in excess of 10^5. Microdisks of diameter D=2 microns are fabricated in an AlGaAs material containing a single layer of InAs QDs with peak emission at lambda = 1317 nm. For devices with Veff ~2 (lambda/n)^3, Qs as high as 1.2 x 10^5 are measured passively in the 1.4 micron band, using an optical fiber taper waveguide. Optical pumping yields laser emission in the 1.3 micron band, with room temperature, continuous-wave thresholds as low as 1 microWatt of absorbed pump power. Out-coupling of the laser emission is also shown to be significantly enhanced through the use of optical fiber tapers, with laser differential efficiency as high as xi~16% and out-coupling efficiency in excess of 28%.Comment: 6 figure