Showing papers on "Optical microcavity published in 2007"

Optical frequency comb generation from a monolithic microresonator

Abstract: Optical frequency combs provide equidistant frequency markers in the infrared, visible and ultraviolet, and can be used to link an unknown optical frequency to a radio or microwave frequency reference. Since their inception, frequency combs have triggered substantial advances in optical frequency metrology and precision measurements and in applications such as broadband laser-based gas sensing and molecular fingerprinting. Early work generated frequency combs by intra-cavity phase modulation; subsequently, frequency combs have been generated using the comb-like mode structure of mode-locked lasers, whose repetition rate and carrier envelope phase can be stabilized. Here we report a substantially different approach to comb generation, in which equally spaced frequency markers are produced by the interaction between a continuous-wave pump laser of a known frequency with the modes of a monolithic ultra-high-Q microresonator via the Kerr nonlinearity. The intrinsically broadband nature of parametric gain makes it possible to generate discrete comb modes over a 500-nm-wide span (approximately 70 THz) around 1,550 nm without relying on any external spectral broadening. Optical-heterodyne-based measurements reveal that cascaded parametric interactions give rise to an optical frequency comb, overcoming passive cavity dispersion. The uniformity of the mode spacing has been verified to within a relative experimental precision of 7.3 x 10(-18). In contrast to femtosecond mode-locked lasers, this work represents a step towards a monolithic optical frequency comb generator, allowing considerable reduction in size, complexity and power consumption. Moreover, the approach can operate at previously unattainable repetition rates, exceeding 100 GHz, which are useful in applications where access to individual comb modes is required, such as optical waveform synthesis, high capacity telecommunications or astrophysical spectrometer calibration.

Generation of single optical plasmons in metallic nanowires coupled to quantum dots

Abstract: Control over the interaction between single photons and individual optical emitters is an outstanding problem in quantum science and engineering. It is of interest for ultimate control over light quanta, as well as for potential applications such as efficient photon collection, single-photon switching and transistors, and long-range optical coupling of quantum bits. Recently, substantial advances have been made towards these goals, based on modifying photon fields around an emitter using high-finesse optical cavities. Here we demonstrate a cavity-free, broadband approach for engineering photon-emitter interactions via subwavelength confinement of optical fields near metallic nanostructures. When a single CdSe quantum dot is optically excited in close proximity to a silver nanowire, emission from the quantum dot couples directly to guided surface plasmons in the nanowire, causing the wire's ends to light up. Non-classical photon correlations between the emission from the quantum dot and the ends of the nanowire demonstrate that the latter stems from the generation of single, quantized plasmons. Results from a large number of devices show that efficient coupling is accompanied by more than 2.5-fold enhancement of the quantum dot spontaneous emission, in good agreement with theoretical predictions.

Label-Free, Single-Molecule Detection with Optical Microcavities

Abstract: Current single-molecule detection techniques require labeling the target molecule. We report a highly specific and sensitive optical sensor based on an ultrahigh quality (Q) factor (Q > 10^8) whispering-gallery microcavity. The silica surface is functionalized to bind the target molecule; binding is detected by a resonant wavelength shift. Single-molecule detection is confirmed by observation of single-molecule binding events that shift the resonant frequency, as well as by the statistics for these shifts over many binding events. These shifts result from a thermo-optic mechanism. Additionally, label-free, single-molecule detection of interleukin-2 was demonstrated in serum. These experiments demonstrate a dynamic range of 10^(12) in concentration, establishing the microcavity as a sensitive and versatile detector.

Resonance fluorescence from a coherently driven semiconductor quantum dot in a cavity.

Abstract: We show that resonance fluorescence, i.e., the resonant emission of a coherently driven two-level system, can be realized with a semiconductor quantum dot. The dot is embedded in a planar optical microcavity and excited in a waveguide mode so as to discriminate its emission from residual laser scattering. The transition from the weak to the strong excitation regime is characterized by the emergence of oscillations in the first-order correlation function of the fluorescence, $g(\ensuremath{\tau})$, as measured by interferometry. The measurements correspond to a Mollow triplet with a Rabi splitting of up to $13.3\text{ }\text{ }\ensuremath{\mu}\mathrm{eV}$. Second-order correlation measurements further confirm nonclassical light emission.

Photon statistics of semiconductor microcavity lasers.

Abstract: We present measurements of first- and second-order coherence of quantum-dot micropillar lasers together with a semiconductor laser theory. Our results show a broad threshold region for the observed high-beta microcavities. The intensity jump is accompanied by both pronounced photon intensity fluctuations and strong coherence length changes. The investigations clearly visualize a smooth transition from spontaneous to predominantly stimulated emission which becomes harder to determine for high beta. In our theory, a microscopic approach is used to incorporate the semiconductor nature of quantum dots. The results are in agreement with the experimental intensity traces and the photon statistics measurements.

Cavity Nonlinear Optics at Low Photon Numbers from Collective Atomic Motion

Abstract: We report on Kerr nonlinearity and dispersive optical bistability of a Fabry-Perot optical resonator due to the displacement of ultracold atoms trapped within. In the driven resonator, such collective motion is induced by optical forces acting upon up to 10(5) 87Rb atoms prepared in the lowest band of a one-dimensional intracavity optical lattice. The longevity of atomic motional coherence allows for strongly nonlinear optics at extremely low cavity photon numbers, as demonstrated by the observation of both branches of optical bistability at photon numbers below unity.

Electro-optically tunable microring resonators on lithium niobate.

Abstract: Electro-optical tuning of a microring resonator fabricated on lithium niobate (LiNbO3) is presented. The device structure, including microring resonator and couplers, is designed in detail and is produced by titanium diffusion on the wet-etched LiNbO3 ridge surface. The resonance wavelengths for TM and TE polarizations can be tuned by electro-optic effect. The output characteristics of through port and drop port in the microring resonators are measured, and the effect of applied voltage on the shift of resonant wavelength is discussed. The presented microring resonators have the features of fast tuning speed, high material stability, bidirection wavelength shift, and no heating interference. Realization of such a microring resonator on LiNbO3 makes the utilization of electro-optic tuning and nonlinear effects in the versatile photonic applications of microring resonators achievable.

Enhanced second-harmonic generation in AlGaAs microring resonators.

Abstract: Highly efficient second-harmonic generation can be achieved by harnessing resonance effects in microring resonator structures. We propose an angular quasi-phase-matching scheme based on the position dependence of polarization inside the ring resonator.

Temperature compensation of optical microresonators using a surface layer with negative thermo-optic coefficient.

Abstract: We theoretically investigate the feasibility of using a surface layer with a negative thermo-optic coefficient to compensate the thermal drift of a resonant frequency in an optical microresonator. Taking a fused-silica microsphere as an example, our analysis has shown that the thermal drift of a whisper-gallery mode can be fully compensated by such a surface layer. We analyze and compare the compensation performances by using different materials as the surface layer.

Atom detection and photon production in a scalable, open, optical microcavity.

Abstract: A microfabricated Fabry-Perot optical resonator has been used for atom detection and photon production with less than 1 atom on average in the cavity mode. Our cavity design combines the intrinsic scalability of microfabrication processes with direct coupling of the cavity field to single-mode optical waveguides or fibers. The presence of the atom is seen through changes in both the intensity and the noise characteristics of probe light reflected from the cavity input mirror. An excitation laser passing transversely through the cavity triggers photon emission into the cavity mode and hence into the single-mode fiber. These are first steps toward building an optical microcavity network on an atom chip for applications in quantum information processing.

2D photonic crystal cavity-based WDM multiplexer

Abstract: We designed a multiport two-dimensional (2D) photonic crystal (PhC) multiplexer based on contra-directional coupling and standing-wave resonators. The device is composed of PhC filters with up to 12 cavities in the same micro-patterned structure: adjacent strips with different periods contain a single cavity, and by selecting the proper lattice constants we tune the dropped frequency of each cavity. Each filter has 1.16 nm bandwidth, at a central wavelength 1550 nm, and the channels are 10 nm spaced. The variations in the local lattice period are very small, and the mismatch between two adjacent structures does not affect the propagation of the beam in the bus channel. The PhC structure has been analyzed by using the plane wave expansion method and the device performance has been investigated with the finite-difference time-domain technique. The transmittance of each drop filter is about 74%, with a non-uniformity between channels less than 5%. The overall longitudinal dimension of the multiplexer is less than 20 μm.

Enhanced spontaneous emission at 1.55 μm from colloidal PbSe quantum dots in a Si photonic crystal microcavity

Abstract: The authors report on the design, fabrication, and characterization of 1.55μm Si-based photonic crystal microcavity light emitters utilizing PbSe quantum dots. Efficient coupling of emission from PbSe quantum dots to Si photonic crystal membrane microcavity is achieved by inserting the quantum dots in a central air hole in the microcavity. Enhancement of spontaneous emission with linewidth of ∼2.0nm is observed at 1550nm at room temperature. The Purcell factor and the spontaneous emission coupling factor are estimated to be 35 and 0.04, respectively.

An optical horn structure for single-photon source using quantum dots at telecommunication wavelengtha)

Abstract: We succeeded in efficiently generating single-photon pulses from an InAs/InP quantum dot at a wavelength of 1.5 μm. Our optical structure, named a single photon horn, can propagate over 95% photon pulses in InP substrate. We extracted the photon pulses through an anti-reflection coating on a substrate, and then we injected them into an objective lens. Total extraction efficiency from the quantum dot to the lens reached ∼11%, which was estimated using a photon correlation measurement. Furthermore we directly observed the single-photon pulse width ∼1.6 ns as an exciton lifetime in the quantum dot, which opens up the possibility of operating the single photon horn over 100 MHz.

High-speed all-optical switching in ion-implanted silicon-on-insulator microring resonators

Abstract: We demonstrate high-speed all-optical switching via vertical excitation of an electron-hole plasma in an oxygen-ion implanted silicon-on-insulator microring resonator. Based on the plasma dispersion effect the spectral response of the device is rapidly modulated by photoinjection and subsequent recombination of charge carriers at artificially introduced fast recombination centers. At an implantation dose of 1×1012 cm−2 the carrier lifetime is reduced to 55 ps, which facilitates optical switching of signal light in the 1.55 μm wavelength range at modulation speeds larger than 5 Gbits/s.

Rotation-induced superstructure in slow-light waveguides with mode-degeneracy: optical gyroscopes with exponential sensitivity

Abstract: We study wave propagation in a rotating slow-light structure with mode degeneracy. The rotation, in conjunction with the mode degeneracy, effectively induces superstructure that significantly modifies the structure's dispersion relation. It is shown that a rotation-dependent stop band is formed in the center of the slow-light waveguide transmission curve. A light signal of frequency within this stop band that is excited in a finite-length section of such a waveguide decays exponentially with the rotation speed and with the coupled resonator optical waveguide's total length or total number of degenerate microcavities. This effect can be used for optical gyroscopes with exponential-type sensitivity to rotation.

Fiber-coupled solid state microcavity light emitters

Abstract: Designs of fiber-coupled solid state microcavity light emitters based on microdisk cavities, photonic crystal cavities and other microcavity configurations to provide efficient optical coupling.

Fiber laser using a microsphere resonator as a feedback element.

Abstract: We show that a glass microsphere resonator can be used as a wavelength-selective mirror in fiber lasers. Due to their high quality factor (Q∼108), microsphere resonators possess a narrow reflection bandwidth. This feature enables construction of single-frequency fiber lasers even when the laser cavity is long. Nonlinear effects (such as stimulated Raman lasing) were also observed in our setup at relatively low pump powers.

Optical range microcavities and filters using multiple dielectric layers in metal-insulator-metal structures.

Abstract: We present a technique to adjust omnidirectional resonances in planar metallic microcavity structures. It is demonstrated numerically with realistic material parameters that by changing the thickness of different dielectric layers in metal-dielectric-metal structures, the omnidirectional resonance can be chosen to lie at an arbitrary frequency in the optical range. These ideas can also be applied to design a high-pass filter in the optical frequency range.

Splitting of microcavity degenerate modes in rotating photonic crystals—the miniature optical gyroscopes

Abstract: A manifestation of the Sagnac effect in a rotating photonic crystal that contains a microcavity with degenerate modes is explored. It is shown that generally rotation can cause the resonance frequency to split into M different frequencies, where M is the order of the stationary-system mode degeneracy. The results are derived using a new rotation-induced eigenvalue theory that holds for any two-dimensional or three-dimensional rotating microcavity with mode degeneracy. Comparison with exact numerical simulations of the rotating system is provided. Miniature optical gyroscopes are discussed.

High-rate entanglement source via two-photon emission from semiconductor quantum wells

Abstract: We propose a compact high-intensity room-temperature source of entangled photons based on the efficient second-order process of two-photon spontaneous emission from electrically pumped semiconductor quantum wells in a photonic microcavity. Two-photon emission rate in room-temperature semiconductor devices is determined solely by the carrier density, regardless of the residual one-photon emission. The microcavity selects two-photon emission for a specific signal and idler wavelengths and at a preferred direction without modifying the overall rate. Pair-generation rate in GaAs/AlGaAs quantum well structure is estimated using a 14-band model to be 3 orders of magnitude higher than for traditional broadband parametric down-conversion sources.

Containing analyte in optical cavity structures

Abstract: A device can include both a photosensing component and an optical cavity structure, with the optical cavity structure including a part that can operate as an optical cavity in response to input light, providing laterally varying output light. For example, the optical cavity can be a graded linearly varying filter (LVF) or other inhomogeneous optical cavity, and the photosensing component can have a photosensitive surface that receives its output light without it passing through another optical component, thus avoiding loss of information. The optical cavity part can include a region that can contain analyte. Presence of the analyte affects the optical cavity part's output light, and the photosensing component can respond to the output light, providing sensing results indicating the analyte's optical characteristics.

Two-photon absorption based optical limiting and stabilization by using a CdTe quantum dot solution excited at optical communication wavelength of ∼1300nm

Abstract: This letter presents the results of two-photon study of CdTe quantum dots (QDs) in chloroform. The measured two-photon absorption (2PA) spectrum shows that 2PA coefficient at ∼1300nm is ∼0.02cm∕GW. Based on a 1cm path-length CdTe QD solution sample of 8mg∕ml concentration, irradiated by a focused ∼1300nm laser beam of ∼160fs duration, the nonlinear transmission could be changed from ∼100% to ∼20% when the input pulse energy was varied from ∼50nJto∼10μJ, demonstrating a superior optical limiting performance. The input laser fluctuation was significantly reduced after passing through the sample, indicating a remarkable optical stabilization behavior.

Terahertz microcavity lasers with subwavelength mode volumes and thresholds in the milliampere range

Abstract: The authors demonstrate terahertz microcavity lasers with ultralow current thresholds (Ith≈4mA) and with reduced mode volumes of ≈0.7(λeffective)3, i.e., less than one cubic wavelength. A double metal waveguide with reduced active core thickness (5.82μm) is used to achieve confinement in the vertical direction, without compromising the laser performances. Confinement in the longitudinal direction is obtained using microdisk resonators. The guiding properties of surface plasmons are exploited to guide the mode with the metal contact. This makes the use of a resonator with vertical and smooth sidewalls unnecessary. The emission wavelength is λ≈114μm. The devices lase up to 70K in pulsed mode, and they achieve continuous-wave operation up to 60K.

Biological and chemical microcavity resonant sensors and methods of detecting molecules

Abstract: Resonant sensors and methods of detecting specific molecules with enhanced sensitivity. Optical energy is introduced into a microcavity, such as a silica toroid-shaped microcavity. The microcavity sensor has a functionalized outer surface and a sufficiently high Q value to generate an evanescent optical field with increased intensity. A molecule bound to the functionalized outer surface interacts with the external optical field, thereby heating the microcavity and generating a detectable resonant wavelength shift, which indicates a small number of molecules, even a single molecule, without the use of fluorescent or metal labels. Resonant sensors and methods can also be used to detect specific molecules, even a single molecule, within an environment. One application is detecting very small quantities or a single molecule of heavy water in ordinary water.

Microresonator array for high-resolution spectroscopy.

Abstract: We investigated the properties of an array of spherical microresonators used as a miniaturized high-resolution spectroscopic device. Sixteen spherical microspheres made from polymethyl methacrylate were placed on a microscope slide serving as an optical wave guide. Light of a tunable narrowband laser source was coupled into this slide so that an evanescent wave was excited on the topside of the slide, where the resonators were placed. This evanescent field generated a particular intensity pattern in the array that depended sensitively on the wavelength. After calibration, that pattern was recorded by a CCD camera and used to identify the wavelength with a resolution of R∼λ/Δλ=7×104.

Dielectric microcavity fluorosensors excited with a broadband light source

Abstract: A microresonator sensor apparatus has a microcavity resonator that defines equatorial whispering gallery modes (EWGMs), whose frequencies are separated by the free spectral range (FSR). The EWGMs lie in a plane perpendicular to a microcavity resonator axis. A light source is optically coupled to inject light into the microcavity resonator. The light source produces output light having an output spectrum whose bandwidth is approximately equal to or broader than the FSR of the EGWMs. One or more fluorescent materials are excited using the excitation light coupled into the microcavity resonator. A fluorescent signal arising from fluorescence of the one or more fluorescent materials is then detected.

Subwavelength Microdisk and Microring Terahertz Quantum-Cascade Lasers

Abstract: We report on the emission characteristics of microcavity quantum-cascade lasers emitting in the terahertz frequency range based on circular-shaped microresonators. Strong mode confinement in the growth and in-plane directions are provided by a double-plasmon waveguide and due to the strong impedance mismatch between the gain material and air. This allows laser emission from devices with overall dimensions much smaller than the free-air emission wavelength (lambda > 100 mum). Hence, for the smallest microdisks we achieved a threshold current as low as 13.5 mA (350 A/cm2) in pulsed-mode operation at 5 K and stable single-mode emission up to 95 K in continuous-wave mode operation. We have observed dynamical frequency pulling of the resonator mode on the gigahertz scale, as a consequence of the gain shift due to the quantum-confined Stark effect. Thus, we were able to estimate the peak gain of the material to 27 cm-1. The smallest microcavities exhibited a strong dependence on the exact placement of the bond wire which resulted in single- as well as double-mode emission. Finite-difference time-domain simulations were performed in order to identify the modes of the recorded spectra. They confirm that most of the observed spectral features can be attributed to the lasing emission of whispering-gallery modes.

Integrated waveguide-coupled terahertz microcavity resonators

Abstract: We demonstrate integrated square terahertz microcavity resonators side coupled to waveguides. We present the microcavity transmission spectra for different resonator sizes and coupling strengths. The measured quality factors due to external coupling and cavity loss are found to be between 40 and 90 and between 30 and 40, respectively, for cavity resonance frequencies between 1.077 and 1.331THz.

Emission spectrum of electromagnetic energy stored in a dynamically perturbed optical microcavity

Abstract: An ultrafast pump-probe experiment is performed on wavelength-scale, silicon-based, optical microcavities that confine light in three dimensions with resonant wavelengths near 1.5 µm, and lifetimes on the order of 20 ps. A below-bandgap probe pulse tuned to overlap the cavity resonant frequency is used to inject electromagnetic energy into the cavity, and an above-bandgap pump pulse is used to generate free carriers in the silicon, thus altering the real and imaginary components of the cavity’s refractive index, and hence its resonant frequency and lifetime. When the pump pulse injects a carrier density of ~5×1017 cm-3before the resonant probe pulse strikes the sample, the emitted radiation from the cavity is blue-shifted by 16 times the bare cavity linewidth, and the new linewidth is 3.5 times wider than the original. When the pump pulse injects carriers, and thus suddenly perturbs the cavity properties after the probe pulse has injected energy into the cavity, we show that the emitted radiation is not simply a superposition of Lorentzians centred at the initial and perturbed cavity frequencies. Under these conditions, a simple model and the experimental results show that the power spectrum of radiation emitted by the stored electromagnetic energy when the cavity frequency is perturbed during ring-down consists of a series of coherent oscillations between the original and perturbed cavity frequencies, accompanied by a gradual decrease and broadening of the original cavity line, and the emergence of the new cavity resonance. The modified cavity lifetime is shown to have a significant impact on the evolution of the emission as a function of the pump-probe delay.

Solution processed microcavity structures with embedded quantum dots

Abstract: The authors report the fabrication of a one-dimensional microcavity structure embedded with colloidal CdSe/ZnS core/shell quantum dots using solution processing. The microcavity structures were fabricated by spin coating alternating layers of polymers of different refractive indices (poly-vinylcarbazole—PVK, and poly-acrylic acid—PAA) to form the distributed Bragg reflectors (DBRs). Greater than 90% reflectivity was obtained using ten periods of the structure. The one-dimensional microcavity was formed by sandwiching a λ / n thick defect layer between two such DBRs. The emission of the quantum dots from the microcavity structure demonstrated directionality following the cavity mode dispersion and spectral narrowing. Room temperature time-resolved photoluminescence measurements carried out on this structure showed significant reduction in the photoluminescence decay time which is attributed primarily to nonradiative mechanism originating in the presence of the PVK host matrix. The photoluminescence decay time of the quantum dots was found to be ∼1000 ps while for the quantum dots embedded in the polymer host and the microcavity were 400 and 150 ps, respectively.