Our understanding of the cellular implementation of systems-level neural processes like action, thought and emotion has been limited by the availability of tools to interrogate specific classes of neural cells within intact, living brain tissue. Here we identify and develop an archaeal light-driven chloride pump (NpHR) from Natronomonas pharaonis for temporally precise optical inhibition of neural activity. NpHR allows either knockout of single action potentials, or sustained blockade of spiking. NpHR is compatible with ChR2, the previous optical excitation technology we have described, in that the two opposing probes operate at similar light powers but with well-separated action spectra. NpHR, like ChR2, functions in mammals without exogenous cofactors, and the two probes can be integrated with calcium imaging in mammalian brain tissue for bidirectional optical modulation and readout of neural activity. Likewise, NpHR and ChR2 can be targeted together to Caenorhabditis elegans muscle and cholinergic motor neurons to control locomotion bidirectionally. NpHR and ChR2 form a complete system for multimodal, high-speed, genetically targeted, all-optical interrogation of living neural circuits.

Multimodal fast optical interrogation of neural circuitry

Manufactured nanoparticles: An overview of their chemistry, interactions and potential environmental implications

Digital information processing in molecular systems.

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

Optical microcavities that confi ne light in high-Q resonance promise all of the capabilities required for a successful nextgeneration microsystem biodetection technology. Label-free detection down to single molecules as well as operation in aqueous environments can be integrated cost-effectively on microchips, together with other photonic components, as well as electronic ones. We provide a comprehensive review of the sensing mechanisms utilized in this emerging fi eld, their physics, engineering and material science aspects, and their application to nanoparticle analysis and biomolecular detection. We survey the most recent developments such as the use of mode splitting for self-referenced measurements, plasmonic nanoantennas for signal enhancements, the use of optical force for nanoparticle manipulation as well as the design of active devices for ultra-sensitive detection. Furthermore, we provide an outlook on the exciting capabilities of functionalized high-Q microcavities in the life sciences.

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Label-free detection with high-Q microcavities: a review of biosensing mechanisms for integrated devices.

We present a simple, intrinsic intensity modulated fiber optic sensor for determining adulteration of petrol and diesel by kerosene. Experimental results for a prototype fabricated in the laboratory are presented. The sensing principle is based on modulation of intensity of light guided in the fiber due to change in the refractive index of the cladding formed by adulterated fuel and the phenomenon of evanescent wave absorption. The sensor is useful due to its simple construction, operation, safety with inflammable fuels and the possibility of making it compact and portable for on-road measurements.

Fiber optic sensor for determining adulteration of petrol and diesel by kerosene

All-optical switching in bacteriorhodopsin based on M state dynamics and its application to photonic logic gates

We show all-optical switching of an input infrared laser beam at 1310 nm by controlling the photoinduced retinal isomerization to tune the resonances in a silica microsphere coated with three bacteriorhodopsin (BR) protein monolayers. The all-optical tunable resonant coupler re-routes the infrared beam between two tapered fibers in 50 μs using a low power (<200 μW) green (532 nm) and blue (405 nm) pump beams. The basic switching configuration has been used to design all-optical computing circuits, namely, half and full adder/subtractor, de-multiplexer, multiplexer, and an arithmetic unit. The design requires 2n−1 switches to realize n bit computation. The designs combine the exceptional sensitivities of BR and high-Q microcavities and the versatile tree architecture for realizing low power circuits and networks (approximately mW power budget). The combined advantages of high Q-factor, tunability, compactness, and low power control signals, with the flexibility of cascading switches to form circuits, and r...

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All-optical switching with bacteriorhodopsin protein coated microcavities and its application to low power computing circuits

We present a theoretical model to analyze all-optical switching by two-photon absorption induced free-carrier injection in silicon 2 × 2 add-drop microring resonators. The theoretical simulations are in good agreement with experimental results. The results have been used to design all-optical ultrafast (i) reconfigurable De-multiplexer/Multiplexer logic circuits using three microring resonator switches and (ii) universal, conservative and reversible Fredkin and Toffoli logic gates with only one and two microring resonator switches respectively. Switching has been optimized for low-power (25 mW) ultrafast (25 ps) operation with high modulation depth (85%) to enable logic operations at 40 Gb/s. The combined advantages of high Q-factor, tunability, compactness, cascadibility, reversibility and reconfigurability make the designs favorable for practical applications. The proposed designs provide a new paradigm for ultrafast CMOS-compatible all-optical reversible computing circuits in silicon.

All-Optical Ultrafast Switching in 2 × 2 Silicon Microring Resonators and its Application to Reconfigurable DEMUX/MUX and Reversible Logic Gates

A detailed theoretical analysis of ultrafast transition from saturable absorption (SA) to reverse saturable absorption (RSA) has been presented in graphene-oxide thin films with femtosecond laser pulses at 800 nm. Increase in pulse intensity leads to switching from SA to RSA with increased contrast due to two-photon absorption induced excited-state absorption. Theoretical results are in good agreement with reported experimental results. Interestingly, it is also shown that increase in concentration results in RSA to SA transition. The switching has been optimized to design parallel all-optical femtosecond NOT, AND, OR, XOR, and the universal NAND and NOR logic gates.

Sukhdev Roy

Papers

Fiber optic sensor for determining adulteration of petrol and diesel by kerosene

All-optical switching in bacteriorhodopsin based on M state dynamics and its application to photonic logic gates

All-optical switching with bacteriorhodopsin protein coated microcavities and its application to low power computing circuits

All-Optical Ultrafast Switching in 2 × 2 Silicon Microring Resonators and its Application to Reconfigurable DEMUX/MUX and Reversible Logic Gates

Femtosecond all-optical parallel logic gates based on tunable saturable to reverse saturable absorption in graphene-oxide thin films