[Show abstract][Hide abstract] ABSTRACT: 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=1x10(6) to Q=5x10(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.
[Show abstract][Hide abstract] ABSTRACT: 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 figures
[Show abstract][Hide abstract] ABSTRACT: A small depression is created in a straight optical fiber taper to form a local probe suitable for studying closely spaced, planar microphotonic devices. The tension of the "dimpled" taper controls the probe-sample interaction length and the level of noise present during coupling measurements. Practical demonstrations with high-Q silicon microcavities include testing a dense array of undercut microdisks (maximum Q = 3.3 x 10(6)) and a planar microring (Q = 4.8 x 10(6)). (c) 2007 Optical Society of America.
[Show abstract][Hide abstract] ABSTRACT: Fiber coupled silicon nitride microcavities with Q > 3.6 times 10<sup>6</sup> and effective mode volume < 10(lambda/n)<sup>3</sup> at wavelengths resonant with alkali atom transitions are robustly integrated with magnetostatic atom-chips.
Lasers and Electro-Optics, 2006 and 2006 Quantum Electronics and Laser Science Conference. CLEO/QELS 2006. Conference on; 06/2006
[Show abstract][Hide abstract] ABSTRACT: High-quality factor (Q ∼ 1−5×10<sup>6</sup>) microresonators are used to probe the optical properties of silicon-on-insulator surfaces with 0.04% monolayer sensitivity. A rapid and accurate measurement of linear and nonlinear absorption is utilized to assess new surface-passivation techniques.
Lasers and Electro-Optics and 2006 Quantum Electronics and Laser Science Conference. CLEO/QELS 2006. Conference on; 06/2006
[Show abstract][Hide abstract] ABSTRACT: Design considerations for quasi-planar, high-Q silicon-on-insulator microphotonic resonators are presented. A figure of merit for use in comparison between microphotonic designs is presented and applied to compare existing and proposed designs.
[Show abstract][Hide abstract] ABSTRACT: Ultrasmall volume (Veff ~ 2(lambda/n)3), fiber-coupled microdisks with embedded quantum dots are studied. Quality factors > 105, laser thresholds of ~ 1 muW, and differential (out-coupling) efficiencies of 16% (28%) are measured, and a spontaneous emission factor of 16% is estimated.
[Show abstract][Hide abstract] ABSTRACT: 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 microW 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 micros. 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.
[Show abstract][Hide abstract] ABSTRACT: Although the concept of constructing active optical waveguides in crystalline silicon has existed for over twenty years, it is only in the past few years that silicon photonics has been given serious attention as a displacing technology. Fueled by the predicted saturation of "Moore's Law" within the next decade, universities and industries from all over the world are exploring the possibilities of creating truly integrated silicon opto-electronic devices in a cost effective manner. Some of the most promising silicon photonics technologies are chip-to-chip and intra-chip optical interconnects. Now that compact high-speed modulators in silicon have been achieved, the limiting factor in the widespread adoption of optical interconnects is the lack of practical on-chip optical sources. These sources are critical for the generation of the many wavelengths of light necessary for high-speed communication between the logical elements between and within microprocessors. Unfortunately, crystalline silicon is widely known as a poor emitter because of its indirect bandgap. This thesis focuses on the many challenges in generating silicon-based laser sources. As most CMOS compatible gain materials possess at most 1 dB/cm of gain, much of our work has been devoted to minimizing the optical losses in silicon optical microresonators. Silicon microdisk resonators fabricated from silicon-on-insulator wafers were employed to study and minimize the different sources of scattering and absorption present in high-index contrast Si microcavities. These microdisks supported whispering-gallery modes with quality factors as high as 5 x 10^6, close to the bulk limit of lightly doped silicon wafers. An external silica fiber taper probe was developed to test the microcavities in a rapid wafer-scale manner. Analytic theory and numerical simulation aided in the optimization of the cavity design and interpretation of experimental results. After successfully developing surface chemistry treatments and passivation layers, erbium-doped glasses were deposited over undercut microdisks and planar microrings. Single-mode laser oscillation was observed and carefully characterized for heavily oxidized silicon microdisks. Dropped power thresholds of 690 nW, corresponding to 170 nW of absorbed power, were measured from gain-spectra and Light in-Light out curves. In addition, quantum efficiencies for these lasers were as high as 24%, indicating that this technology may be ready for further development into real-world devices.
[Show abstract][Hide abstract] ABSTRACT: The silicon/silicon dioxide (Si/SiO2) interface plays a crucial role in the performance, cost, and reliability of most modern microelectronic devices, from the basic transistor to flash memory, digital cameras, and solar cells. Today the gate oxide thickness of modern transistors is roughly 5 atomic layers, with 8 metal wire layers required to transport all the signals within a microprocessor. In addition to the increasing latency of such reduced-dimension metal wires, further "Moore's Law" scaling of transistor cost and density is predicted to saturate in the next decade. As a result, silicon-based microphotonics is being explored for the routing and generation of high-bandwidth signals. In comparison to the extensive knowledge of the electronic properties of the Si/SiO2 interface, little is known about the optical properties of Si surfaces used in microphotonics. In this Letter, we explore the optical properties of the Si surface in the telecommunication-relevant wavelength band of 1400-1600 nm. Utilizing a high quality factor (Q ~ 1.5x10^6) optical microresonator to provide sensitivity down to a fractional surface optical loss of 10^-7, we show that optical loss within Si microphotonic components can be dramatically altered by Si surface preparation, with fraction loss of 2 x 10^-5 measured for chemical oxide surfaces as compared to <2 x 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 microelectronics treatments, and that stable, high optical quality surface passivation layers will be critical in future Si micro- and nano-photonic systems.
[Show abstract][Hide abstract] ABSTRACT: 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 figures
[Show abstract][Hide abstract] ABSTRACT: Self-generated periodic modulation of the optical properties of a silicon-on-insulator microdisk is described. A mechanism explaining the modulation, involving competing thermooptic and free carrier effects, is proposed and discussed.
[Show abstract][Hide abstract] ABSTRACT: High-quality-factor (Q~3.6x10^5) AlGaAs microdisks containing InAs/InGaAs dots-in-a-well are demonstrated. Free-space photoluminescence reveals room-temperature pulsed lasing with thresholds <17 microWatts, and measurements using a fiber taper to pump and collect light from the cavity are presented.
[Show abstract][Hide abstract] ABSTRACT: Fiber-coupled, high-quality-factor (Q > 10<sup>5</sup>) AlGaAs microdisks with embedded InAs quan tum dots are demonstrated. Microdisk lasers employing evanescent fiber coupling for optical pump ing and collection of emission exhibit up to two orders of magnitude improvement in differential effi-ciency compared to devices probed through normal-incidence, free-space photoluminescence. The fiber taper coupling is also shown to provide an efficient optical channel for studying and manipulating the microcavity-quantum dot system, with in-fiber cavity mode collection efficiences greater than 20% and cavity loading approaching critical coupling. Progress and current challenges in incorporating such fiber coupled devices into low temperature experiments for studying coherent interaction between single quan tum dot and single cavity mode excitations will be discussed.
Lasers and Electro-Optics, 2005. CLEO/Pacific Rim 2005. Pacific Rim Conference on; 09/2005
[Show abstract][Hide abstract] ABSTRACT: High-quality-factor (Q>106) silicon optical microdisks are fabricated out of silicon-on-insulator wafers. Optical loss mechanisms are characterized through fiber-taper-probe measurements. Nonlinear effects including Raman and Brillouin scattering, and thermal and free-carrier driven optical bistability are presented.
[Show abstract][Hide abstract] ABSTRACT: 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 = 5x10(6) are measured in these microresonators, corresponding to a propagation loss coefficient as small as alpha ~ 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 microm). 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.
[Show abstract][Hide abstract] ABSTRACT: Optical characterization of AlGaAs microdisk resonant cavities with a quantum dot active region is presented. Direct passive measurement of the optical loss within AlGaAs microdisk resonant structures embedded with InAs/InGaAs dots-in-a-well (DWELL) is performed using an optical-fiber-based probing technique at a wavelength (lambda~1400 nm) that is red-detuned from the dot emission wavelength (lambda~1200 nm). Measurements in the 1400 nm wavelength band on microdisks of diameter D = 4.5 microns show that these structures support modes with cold-cavity quality factors as high as 360,000. DWELL-containing microdisks are then studied through optical pumping at room temperature. Pulsed lasing at lambda ~ 1200 nm is seen for cavities containing a single layer of InAs dots, with threshold values of ~ 17 microWatts, approaching the estimated material transparency level. Room-temperature continuous wave operation is also observed. Comment: 4 pages, 3 figures
[Show abstract][Hide abstract] ABSTRACT: 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. 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 ∼95% is obtained between the fiber taper and a PC waveguide mode suitably designed for integration with a previously studied ultrasmall mode volume high- Q PC resonant cavity [ Srinivasan etal, Appl. Phys. Lett. 83, 1915 (2003) ]. The micron-scale lateral extent and dispersion of the fiber taper is used as a near-field spatial and spectral probe to study the profile and dispersion of PC waveguide modes.
[Show abstract][Hide abstract] ABSTRACT: 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.
IEEE Journal on Selected Areas in Communications 07/2004; 23(7). DOI:10.1109/JSAC.2005.851212 · 3.45 Impact Factor