Thomas H. Jeys

Sum frequency generation of sodium resonance radiation.

We have generated pulsed, high power, sodium resonance radiation by sum frequency mixing the 1.06 microm and 1.32 microm outputs of two Nd:YAG lasers with an average power conversion efficiency of 30%. The wavelength of the sum radiation was tuned across the full Doppler width of the sodium-vapor D(2) absorption by tuning the wavelength of either Nd:YAG laser with intracavity etalons. The wavelength of the 1.32 microm Nd:YAG laser was also tuned by injection seeding with a GaInAsP/InP diode laser. We have used this sodium resonance radiation for the lidar observation of the earths naturally occurring atomic-sodium layer at 90 km altitude.

Rational design and optimization of plasmonic nanoarrays for surface enhanced infrared spectroscopy

We present an approach for rational design and optimization of plasmonic arrays for ultrasensitive surface enhanced infrared absorption (SEIRA) spectroscopy of specific protein analytes. Motivated by our previous work that demonstrated sub-attomole detection of surface-bound silk fibroin [Proc. Natl. Acad. Sci. U.S.A. 106, 19227 (2009)], we introduce here a general framework that allows for the numerical optimization of metamaterial sensor designs in order to maximize the absorbance signal. A critical feature of our method is the explicit compensation for the perturbative effects of the analytes refractive index which alters the resonance frequency and line-shape of the metamaterial response, thereby leading to spectral distortion in SEIRA signatures. As an example, we leverage our method to optimize the geometry of periodic arrays of plasmonic nanoparticles on both Si and CaF2 substrates. The optimal geometries result in a three-order of magnitude absorbance enhancement compared to an unstructured Au layer, with the CaF2 substrate offering an additional factor of three enhancement in absorbance over a traditional Si substrate. The latter improvement arises from increase of near-field intensity over the Au nanobar surface for the lower index substrate. Finally, we perform sensitivity analysis for our optimized arrays to predict the effects of fabrication imperfections. We find that <20% deviation from the optimized absorbance response is readily achievable over large areas with modern nanofabrication techniques.

Angle-and polarization-dependent collective excitation of plasmonic nanoarrays for surface enhanced infrared spectroscopy

Our recent work has showed that diffractively coupled nanoplasmonic arrays for Fourier transform infrared (FTIR) microspectroscopy can enhance the Amide I protein vibrational stretch by up to 10(5) times as compared to plain substrates. In this work we consider computationally the impact of a microscope objective illumination cone on array performance. We derive an approach for computing angular- and spatially-averaged reflectance for various numerical aperture (NA) objectives. We then use this approach to show that arrays that are perfectly optimized for normal incidence undergo significant response degradation even at modest NAs, whereas arrays that are slightly detuned from the perfect grating condition at normal incidence irradiation exhibit only a slight drop in performance when analyzed with a microscope objective. Our simulation results are in good agreement with microscope measurements of experimentally optimized periodic nanoplasmonic arrays.

Optical Techniques for Detecting and Identifying Biological-Warfare Agents

Rapid and accurate detection and identification of biological agents is an objective of various national security programs. Detection in general is difficult owing to natural clutter and anticipated low concentrations of subject material. Typical detection architectures comprise a nonspecific trigger, a rapid identifier, and a confirming step, often in a laboratory. High-confidence identification must be made prior to taking action, though this must be traded against regrets stemming from delay. Sensing requirements are best established by positing plausible scenarios, two of which are suggested herein. Modern technologies include the use of elastic scatter and ultraviolet laser-induced fluorescence for triggering and standoff detection. Optical and nonoptical techniques are used routinely in analyzing clinical samples used to confirm infection and illness resulting from a biological attack. Today, environmental sensing serves at best as an alert to medical authorities for possible action, which would include sample collection and detailed analysis. This paper surveys the state of the art of sensing at all levels.

Suppression of laser spiking by intracavity second harmonic generation

Laser spiking in a long pulse Nd:YAG laser has been substantially suppressed by intracavity second harmonic generation with very little loss of laser pulse energy.

Development of a UV-LED-based biosensor

We are investigating the utility of UV light emitting diodes (LEDs) as the excitation source for fluorescence-based biological agent detection. These LEDs may enable the development of small and low-cost biological agent detectors. We have designed, and are currently fabricating, a test bed using UV LEDs for detecting biological agent aerosols. Using an experimental apparatus, we have measured the elastic scattering and fluorescence signals from single bacterial spores illuminated by low-power 408-nm laser diode radiation.

Observation of optical pumping of mesospheric sodium.

We have observed a large variation with laser polarization in the amount of laser light resonantly backscattered from the Earths mesospheric sodium layer located at a 90-km altitude. This variation is evidence of optical pumping of mesospheric sodium atoms.

Multipass optical parametric amplifier

A compact, low-threshold, multipass optical parametric amplifier has been developed for the conversion of short-pulse (360-ps) 1064-nm Nd:YAG laser radiation into eye-safe 1572-nm radiation for laser ranging and radar applications. The amplifier had a threshold pump power of as low as 45 microJ, and at three to four times this threshold pump power the amplifier converted 30% of the input 1064-nm radiation into 1572-nm output radiation.

Two-dimensional angular optical scattering patterns of microdroplets in the mid infrared with strong and weak absorption

Two-dimensional angular optical scattering (TAOS) patterns of droplets composed of a mixture of H2O and D2O are detected in the mid infrared. First, a lens is used in the Abbé sine condition to collect a small solid angle of light, where the scattering pattern matches well numerical simulations based on Mie theory. Next, TAOS patterns from droplets spanning a large (approximately 27pi sr) solid angle are captured simultaneously at two wavelengths. The effects of absorption are evident in the patterns and are discernible without the need for curve matching by Mie theory.

Active infrared multispectral imaging of chemicals on surfaces

We investigated the signature phenomenology of long-wave infrared (LWIR) reflectance of contaminated surfaces using a quantum-cascade laser (QCL) that tunes from λ = 9.1 to 9.8 μm and a HgCdTe focal-plane-array (FPA) with custom read-out integrated circuit (ROIC). A liquid chemical, diethyl phthalate (DEP), was applied to a variety of substrates such as diffusely reflecting gold, concrete, asphalt, and sand. Multispectral image-cubes of the scattered radiation were generated over 81 wavelengths in steps of 1 cm-1 at standoff distances ranging from 0.1 to 5 meters. For idealized substrates such as diffusely reflecting gold, the experimentally measured signatures are in good agreement with theoretical calculations. Clear signatures were also obtained for contaminated concrete, asphalt, and sand. These measurements demonstrate the potential of this technique for detecting and classifying chemicals on native outdoor surfaces.