Naval Research Laboratory Optical Sciences Division

Förster resonance energy transfer (FRET), which involves the nonradiative transfer of excitation energy from an excited donor fluorophore to a proximal ground-state acceptor fluorophore, is a well-characterized photophysical tool. It is very sensitive to nanometer-scale changes in donor-acceptor separation distance and their relative dipole orientations. It has found a wide range of applications in analytical chemistry, protein conformation studies, and biological assays. Luminescent semiconductor nanocrystals (quantum dots, QDs) are inorganic fluorophores with unique optical and spectroscopic properties that could enhance FRET as an analytical tool, due to broad excitation spectra and tunable narrow and symmetric photoemission. Recently, there have been several FRET investigations using luminescent QDs that focused on addressing basic fundamental questions, as well as developing targeted applications with potential use in biology, including sensor design and protein conformation studies. Herein, we provide a critical review of those developments. We discuss some of the basic aspects of FRET applied to QDs as both donors and acceptors, and highlight some of the advantages offered (and limitations encountered) by QDs as energy donors and acceptors compared to conventional dyes. We also review the recent developments made in using QD bioreceptor conjugates to design FRET-based assays.

A technique for the detection of dynamic strain induced wavelength shifts in fibre Bragg grating sensors is reported that is based on an unbalanced-interferometer wavelength discriminator, which is capable of subnanostrain resolution (0.6 ne/√(Hz) at 500 Hz) sensing.

The operation of a fiber Bragg grating strain sensor system that uses interferometric determination of strain-induced wavelength shifts and incorporates a reference channel to compensate for random thermal-induced drift in the output is described. This system is shown to be capable of resolving sub-microstrain changes in the quasi-static strain applied to a grating and has a resolution of ~6 x 10(-3) microstrain/ radicalHz at a strain perturbation frequency of 1 Hz.

A polarisation-insensitive fibre optic Michelson interferometric sensor configuration is demonstrated. The approach is based on the use of birefringence compensation in a retraced fibre path using Faraday rotator mirror elements.

The authors examine the changes in wavelength and attenuation of long period fibre gratings subjected to bends with curvatures from 0 to 4.4 m–1. The wavelength change with curvature is nonlinear, with a projected minimum detectable curvature change of 2 × 10–3 m–1. The magnitude of the bend-induced wavelength shift depends on the rotation of the cylindrical fibre relative to the bending plane.

Charge compensation is utilized in an InGaAs-InP uni-traveling-carrier photodiode to mitigate the space-charge effect. A 20-μm-diameter photodiode achieved a bandwidth of 25 GHz and large-signal 1-dB compression current greater than 90 mA; the output power at 20 GHz was 20 dBm. A smaller /spl sim/100-μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> photodiode exhibited a bandwidth of 50 GHz and large-signal 1-dB compression current greater than 50 mA. The maximum RF output power at 40 GHz was 17 dBm.

Colloidal metal aerogels are composite nanoscale materials that combine the high surface area and porosity of aerogels with the unique optical and physical properties of metal colloids. As such, they are being developed as advanced sensor, catalytic, and electrocatalytic materials. We have prepared colloidal gold−silica aerogels containing gold colloids ranging in size from 5 to 100 nm. The results presented herein focus on 5- and 28-nm Au-containing silica aerogels for the initial characterization of the interaction between the metal colloid and the silica matrix. A blue-shift of the Au plasmon resonance for silica-immobilized Au colloids (relative to the same colloids in a native Au sol) indicates an interaction between the Au colloid and the nanoscale silica network. Transmission electron microscopy measurements have been used to determine the average size and distribution of the colloidal Au particles, as well as to image the nanoscale silica environment supporting an immobilized Au colloid. Small-angle neutron scattering measurements show no significant changes in the three-dimensional structures of either the base- or acid-catalyzed silica aerogels upon incorporation of small amounts (<0.1 vol %) of colloidal Au. However, for base-catalyzed aerogels, nitrogen physisorption measurements reveal that the average pore size (relative to the pure silica aerogel) decreases as the size of the Au colloid is increased above the ca. 10-nm domain size of the silica (which implies that the Au colloid occludes pore space) while it increases for 5-nm colloidal Au−silica aerogel. The accessibility of the Au surface in colloidal Au−silica aerogels to species introduced from solution is demonstrated by direct adsorption of the dye methyl orange to the Au surface.

A class of high sensitivity fibre-optic magnetic sensors has been developed and successfully tested in the laboratory for the first time. The magnetic sensors employ magnetostrictive jacketing materials in conjunction with conventional single-mode optical fibres. An all-fibre Mach-Zender interferometer was used to detect the magnetically induced changes in optical path length which arise because of strains transferred to the fibre from the magnetostrictive jacket.

A differential temperature sensor based on fiber-optic Bragg-grating elements is described. A high sensitivity to thermally induced Bragg wavelength shifts is obtained using an interferometric detection approach. Results presented show a temperature resolution of <0.05 degrees C, corresponding to a Bragg wavelength shift resolution of <6*10/sup -4/ nm.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A variety of stimulated nonlinear optical processes are observed under relatively low intensity cw excitation in droplet spherical microcavities. These include stimulated Raman and Rayleigh-wing scattering as well as four-wave parametric oscillation. The low thresholds for these stimulated processes are accounted for by all waves being in resonance with high-$Q$ droplet modes and by the existence of a significant cavity QED enhancement (&gt; 100 times) in the nonlinear gain coefficients.

We report offset phase-locking of two Nd:YAG nonplanar ring lasers by electronic feedback. The difference frequency is continuously tunable in three bands from 6 to 34 GHz and exhibits a hold-in range and linewidth of 82 MHz and &lt;1 mHz, respectively.

Interaction of guided optical waves with microwave magnetostatic waves in yittrium iron garnet thin films has been demonstrated. TM↔TE mode conversion induced by codirectional (and contradirectional) magnetostatic waves was experimentally observed with conversion efficiencies of up to 4%. Theoretical expressions for this interaction are given and compared with observations. The thin-film geometry demonstrated could make a practical number of optical signal processing devices in the 1–20-GHz range.

A four-element, time-division multiplexed, fiber grating sensor array operating between the wavelengths of 1280 and 1310 nm was constructed and tested. The array consists of a 1300-nm edge-emitting LED that illuminates four gratings spaced five meters apart. Each grating in the array reflects a spectrally narrow-band (0.3-0.5 mn) wavelength region. Measurand-induced changes in the grating period change the wavelength of the light reflected by each grating. The wavelength shifts are converted to phase changes by routing the reflected signal through a nearly path balanced fiber Mach Zehnder interferometer. The minimum detectable strain was measured to be as low as 2 nanostrain//spl radic/(Hz) for frequencies greater than 10 Hz.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The synthesis and characterization of polysiloxane substituted metal-free and lead phthalocyanines are described. These materials form a class of isotropic liquid phthalocyanines that combine a large nonlinear absorption with an exceptionally large nonlinear thermal refractive effect. These materials effectively couple the nonlinear absorption characteristics of the phthalocyanine (Pc) moiety with the thermorefractive properties typical of siloxanes. The room temperature index of these phthalocyanines can be varied by the choice of the siloxane substituent. Furthermore, the siloxane substituents are effective at inhibiting aggregation of the phthalocyanine rings. The observed aggregation constants in toluene solutions are very small and only a small degree of aggregation is observed even in the pure liquid phthalocyanines. Nonlinear transmission, z-scan, transient absorption and degenerate four-wave mixing studies confirm that the Pb substituted materials possess a broad spectral region of induced absorption with nonlinear absorption coefficients similar to that observed from other lead substituted phthalocyanines. The observed ground state molar absorption coefficients of the siloxy phthalocyanines are somewhat smaller than comparable alkyl substituted phthalocyanines. Taken together, these properties make these materials very useful for practical nonlinear optical applications. The coupling of siloxane chains to the Pc rings represents a promising way to achieve some important properties that are difficult to achieve by other means.

Dark soliton stripes are robust but can decay into optical vortex solitons when subjected to a persistent, long-period, transverse modulation. We explore the nonlinear dynamics of this symmetry-breaking process and determine growth rates, vortex densities, and other characteristics by conducting a nonlinear stability analysis that uses numerical techniques for several cases of special interest.

We report on optical limiting of Q-switched Nd:YAG radiation at 532 nm in an f/5 defocusing geometry using liquid solutions of C60 in 1-chloronaphthalene. The nonlinear optical-limiting mechanism is C60 excited-state absorption and enhanced thermal lensing in the solvent. A limiting threshold energy of 65 nJ, corresponding to an incident fluence of ∼0.3 J/cm2 at the focus in the solution, was observed with a device absorption of 25%.

We present a fabricated metal-semiconductor-metal (MSM) photodetector exhibiting an enhanced photocurrent by integrating a nanoscale metallic grating into its contacts. This serves to increase the incident photon flux about the aperture of the device by guiding incident photons as surface plasmon polaritons. High speed time response data shows that the device responsivity may be increased without sacrificing speed. We demonstrate both a photocurrent enhancement and responsivity increase of about 90% at the design wavelength in comparison to otherwise identical MSM photodetectors without integrated nanoscale gratings. The device retains the MSM advantages of simplicity, planarity, and monolithic integrability.

Novel high-power ytterbium YAG lasers are described. These lasers incorporate the principle of anti-Stokes fluorescence cooling to reduce or eliminate detrimental heating. Lasers with net heating and net cooling are demonstrated. By balancing the spontaneous and stimulated emission, we have reduced the net thermal loading to below 0.01% of the laser's average output power. Design, testing, and analysis are reported for lasers up to 500 W average power and pulsed operation up to 30 s. Issues and limitations of this approach are discussed.

An essential capability in many applications, ranging from commercial, surveillance and defense, is to analyze the spectral content of intercepted microwave and millimeter-wave signals over a very wide bandwidth in real-time and with high resolution. A range of photonic schemes have been introduced for the real-time processing of wideband signals to overcome limitations of current conventional electronic frequency measurement approaches. Here, a novel microwave/millimeter-wave channelizer is presented based on a RF photonic front-end employing parametric wavelength multicasting and comb generation. This new technology enables a contiguous bank of channelized coherent I/Q IF signals covering extremely wide RF instantaneous bandwidth. High channel counts and wide RF instantaneous bandwidth are enabled by use of parametrically generated frequency-locked optical combs spanning >4 THz. Full field analysis capabilities of the coherent detection system are demonstrated by frequency domain analysis of 18 contiguous 1.2 GHz IF channels covering 15.5 GHz to 37.1 GHz input frequency range, and time and spectral domain analysis of a 75 GHz harmonically generated input signal. Sensitivity and dynamic range of the system are analyzed and discussed.

This paper describes the design and performance of two wide-bandwidth photodiode structures. The partially depleted absorber photodiode utilizes an absorbing layer consisting of both depleted and undepleted In/sub 0.53/Ga/sub 0.47/As layers. These photodiodes have achieved saturation currents (bandwidths) of >430 mA (300 MHz) and 199 mA (1 GHz) for 100-/spl mu/m-diameter devices and 24 mA (48 GHz) for 100-/spl mu/m/sup 2/ area devices. Charge compensation has also been utilized in a similar, but modified In/sub 0.53/Ga/sub 0.47/As-InP unitraveling-carrier photodiode design to predistort the electric field in the depletion region in order to mitigate space charge effects. For 20-/spl mu/m-diameter photodiodes the large-signal 1-dB compression current and bandwidth were /spl sim/90 mA and 25 GHz, respectively.