Eugene Edwards

Broadband IR supercontinuum generation using single crystal sapphire fibers.

In this paper, an investigation on broadband IR supercontinuum generation in single crystal sapphire fibers is presented. It is experimentally demonstrated that broadband IR supercontinuum spectrum (up to 3.2microm) can be achieved by launching ultra-short femtosecond laser pulses into single crystal sapphire fiber with a dimension 115microm in diameter and 5cm in length, which covers both the near IR spectral region and the lower end of the mid-IR spectral range. Furthermore, the mechanism of supercontinuum generation in single crystal sapphire fibers is briefly addressed. When the fiber length is shorter than the dispersion length, the self-phase modulation dominates the broadening effect. In this case, the broad supercontinuum spectrum with a smooth profile can be obtained. However, when the fiber length is longer than the dispersion length, the soliton-related dynamics accompanied by the self-phase modulation dominates the broadening effect. There are discrete spikes in the spectrum (corresponding to different order solitons). The above assumption of supercontinuum generation mechanism is quantitatively modeled by the computer simulation program and verified by the experimental results. Thus, one can adjust the spectral profile by properly choosing the length of the sapphire fibers. The broad IR spectral nature of this supercontinuum source can be very useful in a variety of applications such as broadband LADAR, remote sensing, and multi-spectrum free space communications.

Surface enhanced Raman spectroscopy by interfered femtosecond laser created nanostructures

A type of surface enhanced Raman spectroscopy (SERS) by interfered femtosecond laser created nanostructures on Cu metal is presented. It is found out that finer and more uniform nanostructures (with an average feature size 100 nm or smaller) can be created on Cu metal by interfered femtosecond illumination with a phase mask. Significantly enhanced Raman signal (with an enhancement factor around 863) can be realized by using the nanostructured Cu substrate created by the interfered femtosecond laser illumination. The experimentally measured enhancement factor agrees relatively well with the theoretical analyses. Since the nanostructures can be inscribed in real time and at remote locations by the femtosecond laser inscription, the proposed SERS can be particularly useful for the standoff detection of chemicals.

Analysis of terahertz generation via nanostructure enhanced plasmonic excitations

In this paper, we conduct a quantitative study on the physical mechanism of electrons dynamics near the nanostructured metal film surfaces, as well as the efficiency of generated terahertz radiation associated with different types of nanostructures. The simulation results show that although the oscillating motion of emitted electrons outside the metal surface may affect the terahertz generation efficiency to some extent, this efficiency is predominantly determined by the electric field magnitude inside the metal film associated with nanostructure enhanced plasmonic excitations. Due to the field enhancement effect of the nanostructure, an appropriately designed nanostructured surface could greatly enhance the strength of generated terahertz signal via the increased nonlinear interactions between the light and the nanostructures.

Broadband supercontinuum generation covering UV to mid-IR region by using three pumping sources in single crystal sapphire fiber.

In this paper, we demonstrate that the the bandwidth of the supercontinuum spectrum generated in a large mode area sapphire fiber can be enhanced by employing triple pumping sources. Three pumping sources with wavelengths of 784 nm, 1290 nm, and 2000 nm are launched into a single crystal sapphire fiber that is 5 cm in length and has a core diameter of 115 microm. The nonlinear interactions due to self-phase modulation and four-wave mixing form a broadband supercontinuum that covers the UV, visible, near-IR and lower mid-IR regions. Furthermore, we explore the possibility of generating a broadband supercontinuum expanding from the UV to far-IR region by increasing the number of pumping sources with wavelengths in the mid- and far-IR.

Detection of residual traces of explosives by surface enhanced Raman scattering using gold coated substrates produced by nanospheres imprint technique

Explosives detection for national and aviation security has been an area of concern for many years. In order to improve the security in risk areas, much effort has been focused on direct detection of explosive materials in vapor and bulk form. New techniques and highly sensitive detectors have been extensively investigated and developed to detect and identify residual traces that may indicate an individuals recent contact with explosive materials. This paper reports on the use and results of Surface Enhanced Raman Scattering (SERS) technique, to analyze residual traces of explosives in highly diluted solutions by using low-resolution Raman spectroscopy (LRRS). An evaluation of the detection sensitivity of this technique has been accomplished using samples of explosives such as Trinitrotoluene(TNT), Cyclotrimethylenetrinitramine (RDX) and HMX evaluated at different concentrations. Additionally, different SERS substrates have been studied in order to achieve the best enhancement of the Raman spectrum for residual amounts of materials. New substrates produced by gold-coated polystyrene nanospheres have been investigated. Two different sizes of polystyrene nanospheres, 625nm and 992nm, have been used to produce nanopatterns and nanocavities on the surface of a glass slide which has been coated with sputtered gold. Results from homemade substrates have been compared to a commercial gold-coated substrate consisting of an array of resonant cavities that gives the SERS effect. Sample concentration, starting from 1000ppm was gradually diluted to the smallest detectable amount. Raman spectrum was obtained using a portable spectrometer operating at a wavelength of 780nm.

Surface-enhanced Raman spectroscopy scattering from gold-coated ceramic nanopore substrates: effect of nanopore size

Abstract. The dependence of magnitude of the electric near-field on the separation between metal nanoparticles for surface-enhanced Raman spectroscopy (SERS) substrates was experimentally verified. Diameters of gold-coated nanopores in a ceramic alumina substrate were varied to study the charge buildup near interparticle junctions and its effect on the enhancement factor due to SERS. The substrates were characterized by sensing a Rhodamine dye and calculating the associated Raman enhancement factors. Decreasing Au interparticle distance increases the electric near-field and shifts the plasmon resonance peak accordingly.

Terahertz enhancement from terahertz-radiation-assisted large aperture photoconductive antenna

The observation of enhanced terahertz (THz) wave generation from the large aperture photoconductive (PC) antenna excited by both a femtosecond pump beam and a collinearly propagating ZnTe-pregenerated THz wave is reported within this paper. An analysis based on both the calculated and experimental results demonstrated that the superposition acts as the main physical mechanism of this THz enhancement effect due to the dominant contribution from the rapid change in photoexcited carrier density. A prerequisite for the THz enhancement requires that the polarization of the applied bias and the ZnTe-pregenerated THz should be identical in order to have a constructive superposition. Therefore, this observation introduces the possibility of recycling the unused portion of the pump beam to further improve the THz radiation. The enhancement effect could be optimized by changing the thickness of ZnTe, which could affect the photoexcited-free-carrier absorption of THz in the PC antenna and the bandwidth of final enha...

Mid-IR supercontinuum generation and applications: a review

In this paper, a review on mid-IR supercontinuum generation (SCG) and its applications is presented. First, the physical mechanism of the supercontinuum generation in IR crystal fiber is introduced. Second, the recent progress on IR single crystal fiber, in particular ultrathin core double cladding IR single crystal fiber is described. Third, the transmission characteristics of mid-IR crystal fiber is illustrated. Fourth, the mid-IR supercontinuum generation in IR single crystal fiber is presented. Finally, the application of IR supercontinuum for smart target recognition is illustrated

Zinc oxide nanowires for biosensing applications

In this work, we demonstrate the immobilization of carboxylic acid moieties on the surface of ZnO nanowires and its potential for biosensing applications. ZnO nanowires were synthesized via Chemical Vapor Deposition (CVD) technique and appropriately characterized. Vibrational spectroscopic techniques were used to determine nature of bonding and orientation of oleic acid molecule at nanowire surface, which is used as model system. Furthermore, on the basis of photoluminescence data on modified- and unmodified ZnO nanowires with oleic acid, a generic approach to sense biomolecules along with mechanism of sensing is proposed.

Surface Chemistry of ZnO Nanowires for Nanosensor Applications

In this work, surface modification of ZnO nanowires (NWs) with oleic acid is reported. ZnO NWs were synthesized in a customized chemical vapor deposition furnace and were characterized with scanning electron microscopy, X-ray diffraction, and energy dispersive spectroscopy. Raman and infrared spectroscopies were used to determine that oleic acid modifies the ZnO NW surface with the carboxylate group attached to the surface. The study of the surface chemistry of ZnO NWs provides information about the class of compounds that modify ZnO NWs. This information will assist in tailoring the interface of NWs with appropriate compounds for fabricating nanosensors with high selectivity.