### Session X36: Advances in Instrumentation and Measurements

Sponsoring Units: GIMS
Chair: Charles Agosta, Clark University
Room: Morial Convention Center 228

 Friday, March 14, 2008 8:00AM - 8:12AM X36.00001: Newton, Abbe, and the relation between refractive index and dispersion William Karstens , David Y. Smith Chromatic aberration in lenses is corrected by combining glasses with different index/dispersion characteristics. The correction is based on an empirical linear relation between index and dispersion, a rule known qualitatively for many years that was given definitive form by Abbe and Schott in 1886. Despite a long history, its physical basis has remained obscure. Here we show it is a consequence of the spectral shape of the glasses' UV absorption by expanding the Kramers-Kronig relation for the index in a series of absorption-spectrum moments. To first order, the index is determined by the inverse-first moment, the dispersion by the inverse-third moment. For a complex glass, these moments may be rewritten as sums of UV-absorption moments for the glass-former and the glass-modifiers. The total index is then a sum of glass-former index plus a term proportional to the dispersion introduced by the modifier ions. The index \textit{vs}. dispersion plot gives families of straight lines originating at the host-glass index/dispersion point. A line's slope is determined by the inverse-third moment of the modifier absorption, its length by the modifier concentration. Newton's erroneous conclusion that achromatic lenses could not be made was based on the limited range of glasses available to him, all of which fell on a single index-dispersion line. Friday, March 14, 2008 8:12AM - 8:24AM X36.00002: A Non-Perturbative Series Solution for Diffraction Gratings with Arbitrary Profiles Daniel Nkemzi , Prabasaj Paul In this work, we use a boundary continuation technique to obtain an iterative series expansion for the scattering of a plane wave by a diffraction grating with an arbitrary profile function. The method is efficient and is simple to apply. The results of numerical experiments show excellent agreement with the C-method and the coupled-wave approach. Friday, March 14, 2008 8:24AM - 8:36AM X36.00003: Surface-enhanced Raman scattering by silver clusters at planar dielectric interface above the critical angle. Denis Pristinski , Siliu Tan , Henry Du Light refraction at the planar interface of dielectric media prevents light propagation in the optically denser medium at angles above the critical value. This limitation is broken when the evanescent wave is excited at the opposite side of the interface. The polarization and angle dependence of the light emitted above the critical angle has been previously studied for both luminescence and elastic scattering. In this work we demonstrate the possibility to quantitatively characterize the effect using surface-enhanced Raman signal from thiocyanate molecules adsorbed on clustered silver nanoparticles at the water-glass interface. Evanescently excited silver nanoparticles demonstrate stronger polarization dependence for both absorption and emission, and wider range of emittance angles, as compared with the model of a radiating dipole at a similar interface. Friday, March 14, 2008 8:36AM - 8:48AM X36.00004: A novel time resolved resonance Raman technique - applications in correlated systems B. Schulz , I. Mahns , A. Goos , P. Saichu , S. Binder , S.G. Singer , A. Rusydi , M. Ruebhausen , S.-W. Cheong , P. Guptasarma In many modern materials the functionality of a system can be observed by studying the order parameters in the time domain. Here, we present a time resolved pump probe resonance Raman system that allows to study order parameters in an energy range from 5 meV to several eV and in a time domain from 1 ps to several ns. Due to our fully achromatic, all reflective Raman spectrometer, [1] we are able to combine pump and probe beams ranging in energy from the near infrared to the deep ultraviolet. We show exemplary measurements on the melting process in charge order Manganites (La$_{0.5}$Ca$_{0.5}$MnO$_3$) as well as studies on the temporal evolution of the superconducting parameter in high-T$_c$ cuprates.\newline [1] B. Schulz {\it et al}, Rev. Sci. Instrum. {\bf 76}, 73107 (2005). Friday, March 14, 2008 8:48AM - 9:00AM X36.00005: The Physics of Coherent Anti-Stokes Raman Spectroscopy Laszlo Ujj Coherent anti-Stokes Raman Spectroscopy (CARS) has been shown to be one of the most powerful experimental methodologies for obtaining vibrational information from both stable and transient molecular species. The general theoretical and experimental principles associated with CARS, together with specific examples of its applications to important molecular systems, are presented. A concise theoretical formalism of CARS, and its electronically resonant variant, is presented with an emphasis on points relevant to the interpretation of experimental spectra. The instrumental components required for obtaining typical CARS data are illustrated in a brief description of a picoseconds and a nanosecond experimental system used to record CARS spectra. The details of the design and operation of an all solid-state broadband nanosecond CARS system will be presented. Specific examples of picosecond CARS data are taken from measurements of stable and transient intermediates comprising the photoreactions of photoactive proteins. Polarization sensitive CARS spectra taken with the nanosecond system are also presented. Finally, an overview of developments in this field to be anticipated is discussed. Reference: Laszlo Ujj and George H. Atkinson, Coherent Anti-Stokes Raman Spectroscopy, in Handbook of Vibrational Spectroscopy, John Wiley and Sons., Ltd., 2002. Friday, March 14, 2008 9:00AM - 9:12AM X36.00006: High quality optical and mechanical properties of a dispersive optomechanical device Andrew Jayich , Benjamin Zwickl , Cheng Yang , Jack Harris We have characterized the optical and mechanical properties of a high finesse Fabry Perot cavity (F=17,000) dispersively coupled to a micromechanical SiN$_{x}$ membrane. The membrane's fundamental and higher-order vibrational modes show quality factors above 10\^{}6 and a spectrum corresponding to a simple drumhead model. The optical cavity's transverse and longitudinal spectrum is also in agreement with theory. We find that the cavity's full transmission and reflection properties can be modeled quantitatively once the membrane's small optical loss is accounted for. We will discuss the role of avoided crossings between higher-order optical modes in this system, particularly within the context of potential QND measurements of micromechanical devices. Friday, March 14, 2008 9:12AM - 9:24AM X36.00007: Using Multiple FROG Traces to Generate a Single High Dynamic Range Trace Sebastian W. Winkler , William M. Dennis Frequency resolved optical gating (FROG) is a powerful technique that has been used to characterize the complex electromagnetic field of ultrafast pulses for more than a decade. However, FROG relies on detector with a high dynamic range; at least $10^4$. If the detector's dynamic range is insufficient, the FROG phase retrieval code will not work properly. We describe a method to generate a single FROG trace with high dynamic range from multiple FROG traces of the same pulse but using different integration times. We present successfully retrieved fields from data captured using a spectrometer that would otherwise have insufficient dynamic range for FROG trace acquisition. Friday, March 14, 2008 9:24AM - 9:36AM X36.00008: Theoretical Considerations for Surface Thermal Lensing Studies Using Polarized Light Marshall Thomsen Surface Thermal Lensing (STL) is a well-established photothermal technique for locally probing the optical, thermal, and mechanical properties of a surface. A periodically chopped laser beam, the pump beam, irradiates a sample. A portion of the pump beam is absorbed by the sample and converted into thermal energy, resulting in a periodic local thermal expansion of the surface. A second, weaker and broader laser beam, the probe beam, is directed off- normal at the surface, covering the entire heated area. The result is a modulated diffraction pattern embedded in the reflected portion of the probe beam. The addition of polarizers both between the probe laser and the sample and between the sample and the detector opens up the possibility of gaining further information about the surface. In particular, the repeated flexing of the surface of a polymer sample may give rise to local realignment of polymer chains. The resulting asymmetry may be visible through a polarization analysis of STL signals. Friday, March 14, 2008 9:36AM - 9:48AM X36.00009: Intra-valance transitions for uncooled short wave infrared detection A.G. Unil Perera , S.G. Matsik , P.V.V. Jayaweera , H.C. Liu , M. Buchanan An infrared detector based on p-GaAs/AlGaAs heterojunction exhibiting response in the 2-5 $\mu$m range at room temperature is demonstrated. The basic principle of the detector utilizes inter-valance (heavy hole, light hole, and split-off hole) absorption of a highly p-doped GaAs layer (emitter). The dark current is limited by the work function at the interface between the highly doped emitter and the undoped Al$_{x}$Ga$_{1-x}$As barrier. The barrier height can be tailored by varying the Al fraction to obtained the desired operating temperature. The split-off energy of the material determines the split-off threshold and the band offset determines the free carrier threshold for the photo excited carriers. Detector performance can be controlled by varying these two thresholds. A device consisting of 30 periods of 3$\times$10$^{18}$ cm$^{-3}$ p-doped GaAs emitter and Al$_{0.57}$Ga$_{0.43}$As barrier regions between two contact layers shows infrared detection up to 330 K with a peak responsivity of 1.4 A/W and D* of 2.6$\times$10$^{9}$ Jones at 2.5 $\mu$m . Different materials should give rise to different wavelength threshold infrared detectors operating at high temperatures. Friday, March 14, 2008 9:48AM - 10:00AM X36.00010: Room Temperature Terahertz Detection based on Electron Plasma Resonance in an Antenna-Coupled Gallium Arsenide MESFET Sangwoo Kim , Jeramy Zimmerman , Paolo Focardi , Dong Ho Wu , Arthur C. Gossard , Mark S. Sherwin Terahertz detectors utilizing quantum transitions require cryogens since the thermal energy (kT) needs to be smaller than the transition energy (1 THz $\sim$ 4 meV). A bulk 3-D plasmon is a classical excitation, and hence does not saturate with temperature. Plasma absorptions occur at a density-dependent frequency $1/2\pi \sqrt {n_{3D} e^2/m\varepsilon }$. For 1 THz radiation, the corresponding 3-D free electron density is n$_{3D}$ $\sim$ 10$^{16}$ cm$^{-3}$ in GaAs, a density that can be easily achieved. The density of electrons can be made tunable if a device such as a Field Effect Transistor is employed. We utilize these facts in order to realize a room temperature Terahertz detector. Our device consists of twin-slot antennas, coplanar waveguides, and a GaAs Metal-Semiconductor-Field-Effect-Transistor (MESFET). While the sensitivity of the first set of devices is not competitive, we were able to observe the resonance behavior by sweeping bias voltages. This talk will present design, fabrication, recent measurement, and possible future improvement of our detector. Work supported by NSF-DMR 0703925 and Naval Research Lab. Friday, March 14, 2008 10:00AM - 10:12AM X36.00011: Signal Restoration from Atmospheric Degradation in Terahertz Spectroscopy Dong Ho Wu , Seong Kong We presents a method of restoring signals in Terahertz (THz) spectroscopy by removing the distortion from the observed THz signals. The distortion is generated by the absorption and scattering of gas molecules and water vapor in the atmosphere, during the transmission of THz beams through the air from the source to the spectrometer. Such atmospheric degradation causes spurious spectral dips and peaks in the THz spectrum, which often obscure the spectral peaks specific to the material of interest. This fact makes it challenging to measure the THz spectroscopic signatures of objects at a distance in a humid air environment. A THz signal restoration filter based on an artificial neural network model can be very effective in removing noisy absorption peaks caused by atmospheric degradation in THz spectra. Friday, March 14, 2008 10:12AM - 10:24AM X36.00012: Compressed Sensing and its Applications in Imaging Dharmpal Takhar , Ting Sun , Jason Laska , Marco Duarte , Richard Baraniuk , Kevin Kelly Compressed sensing is a new sampling theory which allows reconstructing signals using sub-Nyquist measurements/sampling. This can significantly reduce the computation required for image/video acquisition/encoding, at least at the sensor end. Compressed sensing works on the concept of sparsity of the signal in some known domain, which is incoherent with the measurement domain. We exploit this technique to build a single pixel camera based on an optical modulator and a single photosensor. Random projections of the signal (image) are taken by optical modulator, which has random matrix displayed on it, corresponding to the measurement domain (random noise). This randomly projected signal is collected on the photosensor and later used for reconstructing the signal. In this scheme we are making a tradeoff between the spatial extent of sampling array and a sequential sampling over time with a single detector. In addition to this method, we will also demonstrate a new design which overcomes this shortcoming by parallel collection of many random projections simultaneously. Applications of this technique in hyperspectral and infrared imaging will be discussed. Friday, March 14, 2008 10:24AM - 10:36AM X36.00013: Test of search methods to extract quasi-periodical signals in noise from gravitational wave detectors. Giovanni Santostasi We have tested several techniques to extract quasi-periodical signals in a simulated advanced gravitational wave detector noise. These methods include chi-square , Zn-square , H-test and Bayesian methods. The efficiency and usefulness, in different contexts, of these methods is discussed. Friday, March 14, 2008 10:36AM - 10:48AM X36.00014: High temperature resonant ultrasound spectroscopy methods Guangyan Li , Gary Lamberton , Josh Gladden Resonant ultrasound spectroscopy (RUS) is a technique to obtain the full elastic tensor of single crystal materials by measuring the mechanical resonances of a polished sample. Any direct resonance measurement at high temperatures is limited by the fact that most ultrasound transducers have an upper operational limit of 200-300C. High temperature RUS measurements are made possible by separating the sample, placed in a tube furnace, and the transducers with buffer rods made of low acoustic attenuation materials with good thermal stability such as ceramic alumina or fused quartz. Tests on stainless steel demonstrated that the system has the ability of acquiring resonance signals at temperatures up to 800C. Experimental issues such as additional resonance peaks introduced by the buffer rods and sample loading will be addressed. The apparatus has been used to study high temperature elastic properties of p-zintl thermoelectrics, single crystal quartz, a novel piezoelectric material kepertite, and the glass transition around 400C in bulk metallic glass compounds. Good results from these studies and high temperature test runs of aluminum and stainless steel demonstrate the potential for RUS measurements at elevated temperatures. Friday, March 14, 2008 10:48AM - 11:00AM X36.00015: Development of a low cost, low temperature cryocooler using the Gifford McMahon cycle A. Ramanayaka , R. Mani Although Helium is the second most abundant element, its concentration in the earth's atmosphere is fairly low and constant, as the portion that escapes from the atmosphere is replace by new emission. Historically, Helium was extracted as a byproduct of natural gas production, and stored in gas fields in a National Helium Reserve, in an attempt to conserve this interesting element. National policy has changed and the cost of liquid Helium has increased rapidly in the recent past. These new circumstances have created new interest in alternative eco-friendly methods to realizing and maintaining low temperatures in the laboratory. There have been number of successful attempts at making low temperature closed cycle Helium refrigerators by modifying an existing closed cycle system, and usually the regenerator has been replaced in order to achieve the desired results. Here, we discus our attempt to fabricate a low cost, low temperature closed cycle Helium refrigerator starting from a 15K Gifford McMahon system. We reexamine the barriers to realizing lower temperature here and our attempts at overcoming them.