Modern nanotechnology allows one to scale down various important devices (sensors, chips, fibers, etc.) and thus opens up new horizons for their applications. The efficiency of most of them is based on fundamental physical phenomena, such as transport of wave excitations and resonances. Short propagation distances make phase-coherent processes of waves important. Often the scattering of waves involves propagation along different paths and, as a consequence, results in interference phenomena, where constructive interference corresponds to resonant enhancement and destructive interference to resonant suppression of the transmission. Recently, a variety of experimental and theoretical work has revealed such patterns in different physical settings. The purpose of this review is to relate resonant scattering to Fano resonances, known from atomic physics. One of the main features of the Fano resonance is its asymmetric line profile. The asymmetry originates from a close coexistence of resonant transmission and resonant reflection and can be reduced to the interaction of a discrete (localized) state with a continuum of propagation modes. The basic concepts of Fano resonances are introduced, their geometrical and/or dynamical origin are explained, and theoretical and experimental studies of light propagation in photonic devices, charge transport through quantum dots, plasmon scattering in Josephson-junction networks, and matter-wave scattering in ultracold atom systems, among others are reviewed.

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Fano resonances in nanoscale structures

The detection and measurement of gas concentrations using the characteristic optical absorption of the gas species is important for both understanding and monitoring a variety of phenomena from industrial processes to environmental change. This study reviews the field, covering several individual gas detection techniques including non-dispersive infrared, spectrophotometry, tunable diode laser spectroscopy and photoacoustic spectroscopy. We present the basis for each technique, recent developments in methods and performance limitations. The technology available to support this field, in terms of key components such as light sources and gas cells, has advanced rapidly in recent years and we discuss these new developments. Finally, we present a performance comparison of different techniques, taking data reported over the preceding decade, and draw conclusions from this benchmarking.

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Optical gas sensing: a review

We present a comprehensive overview of sensor technology exploiting optical whispering gallery mode (WGM) resonances. After a short introduction we begin by detailing the fundamental principles and theory of WGMs in optical microcavities and the transduction mechanisms frequently employed for sensing purposes. Key recent theoretical contributions to the modeling and analysis of WGM systems are highlighted. Subsequently we review the state of the art of WGM sensors by outlining efforts made to date to improve current detection limits. Proposals in this vein are numerous and range, for example, from plasmonic enhancements and active cavities to hybrid optomechanical sensors, which are already working in the shot noise limited regime. In parallel to furthering WGM sensitivity, efforts to improve the time resolution are beginning to emerge. We therefore summarize the techniques being pursued in this vein. Ultimately WGM sensors aim for real-world applications, such as measurements of force and temperature, or alternatively gas and biosensing. Each such application is thus reviewed in turn, and important achievements are discussed. Finally, we adopt a more forward-looking perspective and discuss the outlook of WGM sensors within both a physical and biological context and consider how they may yet push the detection envelope further.

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Whispering gallery mode sensors

Engineered photonic waveguides have provided in the past decade an extremely rich laboratory tool to visualize with optical waves the classic analogues of a wide variety of coherent quantum phenomena encountered in atomic, molecular or condensed-matter physics. As compared to quantum systems, optics offers the rather unique advantage of a direct mapping of the wave function evolution in coordinate space by simple fluorescence imaging or scanning tunneling optical microscopy techniques. In this contribution recent theoretical and experimental advances in the field of quantum-optical analogies are reviewed. Special attention is devoted to some relevant optical analogies based on the use of curved photonic structures, including: coherent destruction of tunneling in driven bistable potentials; coherent population transfer and adiabatic passage in laser-driven multilevel atomic systems; quantum decay control and Zeno dynamics; electronic Bloch oscillations and Zener tunneling, Anderson localization and dynamic localization in crystalline potentials.

Quantum-optical analogies using photonic structures

We provide the first experimental observation of structure tuning of the electromagnetically induced transparencylike spectrum in integrated on-chip optical resonator systems. The system consists of coupled silicon ring resonators with $10\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ diameter on silicon, where the coherent interference between the two coupled resonators is tuned. We measured a transparency-resonance mode with a quality factor of 11 800.

Experimental realization of an on-chip all-optical analogue to electromagnetically induced transparency

Optical behavior analogous to electromagnetically induced transparency and absorption is observed in experiments using coupled fused-silica microspheres. This behavior results from interference between coresonant whispering-gallery modes of the two spheres. Coupled-resonator-induced transparency and absorption are observed. Which effect is seen depends on the strength of coupling of incident light from a tapered fiber into the first sphere and on the strength of coupling between the two spheres. The observed effects can enhance microresonator performance in various applications.

Induced transparency and absorption in coupled whispering-gallery microresonators

Ultralow-threshold continuous-wave lasing is achieved at room temperature in a fused-silica microsphere that is coated with HgTe quantum dots (colloidal nanoparticles). The 830nm pump input and HgTe microlaser output are efficiently coupled into and out of whispering-gallery modes by tapered fibers. Lasing occurs at wavelengths ranging from 1240 to 1780nm, depending on the size and composition of the quantum dots (HgCdTe is also used). A linear fit to the data determines the lowest observed threshold pump power to be 0±2μW.

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Microsphere whispering-gallery-mode laser using HgTe quantum dots

Tunable diode laser absorption spectroscopy using microresonator whispering-gallery modes (WGMs) is demonstrated. WGMs are excited around the circumference of a cylindrical cavity 125 µm in diameter using an adiabatically tapered fiber. The microresonator is very conveniently tuned by stretching, enabling the locking of an individual WGM to the laser. As the laser is scanned in frequency over an atmospheric trace-gas absorption line, changes in the fiber throughput are recorded. The experimental results of cavity-enhanced detection using such a microresonator are centimeter effective absorption pathlengths in a volume of only a few hundred microns cubed. The measured effective absorption pathlengths are in good agreement with theory.

Cavity-enhanced laser absorption spectroscopy using microresonator whispering-gallery modes

A new electro-optic waveguide platform, which provides unprecedented voltage control over optical phase delays (>
2mm), with very low loss (< 0.5 dB/cm) and rapid response time (sub millisecond), will be presented. This technology,
developed by Vescent Photonics, is based upon a unique liquid-crystal waveguide geometry, which exploits the
tremendous electro-optic response of liquid crystals while circumventing their historic limitations. The waveguide
geometry provides nematic relaxation speeds in the 10's of microseconds and LC scattering losses that are reduced by
orders of magnitude from bulk transmissive LC optics. The exceedingly large optical phase delays accessible with this
technology enable the design and construction of a new class of previously unrealizable photonic devices. Examples
include: 2-D analog non-mechanical beamsteerers, chip-scale widely tunable lasers, chip-scale Fourier transform
spectrometer (< 5 nm resolution demonstrated), widely tunable micro-ring resonators, tunable lenses, ultra-low power (<
5 microWatts) optical switches, true optical time delay devices for phased array antennas, and many more. All of these
devices may benefit from established manufacturing technologies and ultimately may be as inexpensive as a calculator
display. Furthermore, this new integrated photonic architecture has applications in a wide array of commercial and
defense markets including: remote sensing, micro-LADAR, OCT, FSO, laser illumination, phased array radar, etc.
Performance attributes of several example devices and application data will be presented. In particular, we will present a
non-mechanical beamsteerer that steers light in both the horizontal and vertical dimensions.

Liquid crystal waveguides: new devices enabled by >1000 waves of optical phase control

Vescent Photonics Inc. and Jet Propulsion Lab are jointly developing an innovative ultra-compact (volume < 10 cm 3 ), ultra-low power (<10 -3 Watt-hours per measurement and zero power consumption when not measuring), completely non-mechanical electro-optic Fourier transform spectrometers (EO-FT S) that will be suitable for a variety of remote-platform, in-situ measurements. These devices are made possible by a novel electro-evanescent waveguide architecture, enabling chip-scale EO-FTS sensors. The potential performance of these EO-FTS sensors include: i) a spectral range throughout 0.4-5 µ m (25000  2000 cm -1 ), ii) high-resolution ( 0.1 nm), iii) high-speed (< 1 ms) measurements, and iv) rugged integrated optical construction. This performance potential enables the detection and quantification of a large number of different atmospheric gases simultaneously in the same air mass and the rugged construction will enable deployment on previously inaccessible platforms. The sensor construction is also amenable for analyzing aqueous samples on remote floating or submerged platforms. To date a proof-of-principle prototype EO-FTS sensor has been demonstrated in the near-IR (range of 1450-1700 nm) with a 5 nm resolution. This performance is in good agreement with theoretical models, which are being used to design and build the next generation of EO-FTS devices.

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George Farca

Papers

Induced transparency and absorption in coupled whispering-gallery microresonators

Microsphere whispering-gallery-mode laser using HgTe quantum dots

Cavity-enhanced laser absorption spectroscopy using microresonator whispering-gallery modes

Liquid crystal waveguides: new devices enabled by >1000 waves of optical phase control

Compact Liquid Crystal Waveguide Based Fourier Transform Spectrometer for In-Situ and Remote Gas and Chemical Sensing