Metal Nanostructures for Optical Sensing and Signaling
Jim Adleman, Demetri Psaltis

Abstract. The aim of this research is to develop devices based upon two dimensional arrays of metallic nanoparticles, with an optical signatures that are tunable and can measure changes their environment. We have synthesized silver nanoparticles of 3-6 nm in diameter. We have measured resonant scattering from solutions and 2D arrays of these particles throughout the visible spectrum. The resonance of these particles is due to the motion of the ‘free’ electrons in the cluster.

We attempt to modify the shape of this resonance by distorting the shape of the electron cloud of the particle with an external field. To study this effect we spin coat silver nanoparticles on to clear conductive substrates in order to apply large fields both along the direction of propagation and the direction of polarization of light that passes through our devices. Non-linear interaction between nanoparticles which can be tuned by applied fields would make it possible to switch electromagnetic energy confined to a nanometer scale at optical frequencies. This would be very useful in the design of optical switches for computing, and arrays of nanoparticle based sensors that could be used to measure chemical or physical changes in a given environment.

We also are attempting electrical tuning of the metal insulator transition in silver nanoparticles. When a lattice of sufficiently identical nanospheres is compressed so that the electron spillout from individual crystals overlap, the electron states become delocalized across the whole lattice. This gives the lattice the characteristics of a thin metal film. We propose to use external fields to re-localize these electrons to single sites in the lattice. This would allow the film to switch between a metallic state with a flat absorption curve and an insulating state with a resonant absorption curve. (full report)

Holographic Time-Resolved Imaging of Plasma Generated by High-Intensity Laser Pulses
Martin Centurion, Demetri Psaltis

Abstract. We study the formation and time-evolution of plasma generated in air by high intensity femtosecond pulses. We recorded holographic images of the plasma filaments on a CCD camera, which allowed us to reconstruct the phase change induced by the plasma on a probe. The distribution of the free electrons in the plasma is derived from the phase change, revealing multiple filaments and their breakup and recombination. We also demonstrated the capability of this holographic technique for capturing the time evolution of the plasma generation process by capturing a sequence of images of the filaments in a single-shot experiment. (full re port)

Athermal Holographic Filters
Hung-Te Hsieh, Demetri Psaltis, Yu-Chong Tai

Abstract. Holographic filters are used as optical sensors and in wavelength division multiplexing (WDM) filtering applications. Temperature dependence is a critical concern for telecommunications. We realize the design of an athermal holographic filter employing a thermally actuated MEMS mirror to compensate for the drift of Bragg wavelength due to changes of temperature. The center wavelength of our holographic filter is shown to remain constant from 21°C to 60°C. (full report)

Volume Holographic Filters for Spectroscopic Identification of Substances
Zhenyu Li, Demetri Psaltis

We use volume holography to create spectrally specific, selective filters for the identification of substances such as toxic or explosive materials. The identification method is spectroscopy (such as IR or Raman spectroscopy) where the identity of molecules is found in the detailed absorption or emission spectra. Volume holographic filters are able to improve the sensitivity and speed of the measurement by detecting multiple absorption (or emission) spectral lines of the given substance simultaneously. The operation is based on the Bragg selectivity and multiplexing ability of volume holograms. It’s well known that within the dynamic range of the holographic recording medium, multiple holograms can be superimposed, or multiplexed, in the same volume, which makes it possible to construct a holographic filter whose wavelength selectivity curve (spectral response curve) is matched precisely to the absorption spectrum of a given substance. In order to achieve this, a special recording exposure schedule must be carefully designed such that the strength and spectral bandwidth of individual hologram are matched precisely to those of the corresponding peak in the spectrum. With multiple peaks detected simultaneously, it’s expected the detection sensitivity and speed will be increased greatly compared with traditional methods, and the required data volume will decrease by several orders of magnitude, which makes it very attractive for remote sensing applications.. (full report)

Holographic Spatial-Mode-Division-Multiplexing for
Fiber Optic Sensors
Eric Ostby, Demetri Psaltis

Abstract. Fiber optic sensors are currently used to measure temperature, pressure, strain, power, chemical concentrations and more [1]. Evanescent fiber optic sensing is the most popular. The evanescent tails of guided modes interact with the surrounding medium. Information about chemicals or perturbations there are obtained by measuring the change in mode power, polarization or delay. Key benefits of fiber optic sensors include its compact size, durability in extreme environments, low power requirements, and low cost.

Currently, fiber optic sensors do not have control over specific modes, only large groups [2]. For instance, it is desirable to launch significant power into higher order modes to increase the sensitivity of the instrument. But, only one-dimensional knowledge is possible with such limited schemes. Each spatial mode has a different fraction of its power traveling outside the fiber core. The penetration depth of each mode is different, and therefore provides two-dimensional accuracy in measurement. By comparing the power loss of several modes, radial information about concentration variations from the core can be calculated.

The goal of this project is to use a novel multiplexing technique to gain exact control over every spatial mode in optical fibers. Mode-division-multiplexing (MDM) uses the spatial modes present in optical fiber as an orthogonal basis. The spatial profiles of multiple modes are stored in a volume hologram. Individual modes are launched and detected with angle-multiplexed holograms. Therefore, accurate information of mode attenuation due to the surrounding medium is known. In addition to sensing applications, addressing the spatial modes of a multimode fiber (MMF) increases the bandwidth of an optical communication system [3]. Multiple modes in the transmission channel provide extra degrees of freedom, and hence greater capacity [4]. Presently, fiber optic communication systems do not use the spatial modes to carry information. Modal dispersion decreases the useable bandwidth of MMF links that do not address the multimode nature of the channel [5]. This project will also implement the MDM scheme to increase the bandwidth, and therefore, the speed in MMF communication systems. (full report)

Nonlinear Femtosecond Pulse Delivery in Optical Fibers
Mankei Tsang, Demetri Psaltis

Abstract. We investigate two methods to compensate for dispersion and nonlinearity in optical fiber ultrashort pulse propagation for applications in biomedical imaging and optical communications. One method makes use of numerical reverse propagation results to preshape an input optical pulse, such that an output pulse of any shape, width and intensity can be produced amidst all the linear and nonlinear distortions. Another method uses midway spectral phase conjugation to compensate for all dispersion and most nonlinearity. (full report)