Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session T44: Optical/Laser & High Frequency Devices & Applications |
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Sponsoring Units: FIAP Room: Mile High Ballroom 4C |
Thursday, March 6, 2014 11:15AM - 11:27AM |
T44.00001: Experimental Observations of Nanoscale Coaxial Waveguides (Nanocoax) at Optical Frequencies Y.M. Calm, F. Ye, J.M. Merlo, A.H. Rose, N.T. Nesbitt, C. Yang, N. Drachman, G. McMahon, M.J. Burns, K. Kempa, M.J. Naughton The localization and transport of optical energy on subwavelength scales is facilitated by using nanostructured, metallic waveguides. The coaxial cable has no cutoff frequency for the fundamental, TEM-like mode, even up to optical frequencies where this mode obtains plasmonic/polaritonic character,\footnote{Y. Peng., X. Wang, {\&} K. Kempa, \textit{Opt}. \textit{Express} \textbf{16}, 1758 (2008)} and is therefore a natural choice for miniaturization. Epitaxially grown Ag nanowires and nanocoaxes were studied by electron- and focused ion beam microscopies, and their transmission of visible frequencies of light was characterized by optical microscopy. Experimental efforts towards lithographically fabricated nanocoaxes are discussed. Finally, an architecture for a nanocoax-based optical microscope,\footnote{K. Kempa \textit{et al}., \textit{Appl}. \textit{Phys}. \textit{Lett}. \textbf{92}, 043114 (2008)} which extracts near-field (evanescent) information and propagates it into the far-field, is presented. [Preview Abstract] |
Thursday, March 6, 2014 11:27AM - 11:39AM |
T44.00002: Calibration of optical traps by dual trapping of one bead Pavel Dutov, Jay Schieber Optical trapping and tracking is a powerful method for many biological and rheological applications. Recent advances in microrheological techniques, like two-point microrheology, allow probing mechanical properties of viscoelastic networks with mesh size bigger than the size of the microbead itself, but require high signal to noise ratio. Noise level in the system can be reduced by removing active elements, like acousto-optical deflectors or galvo-mirrors from the optical train and making the trap fixed. We introduce a method for optical trap calibration that is suitable for viscoelastic material and allows calibration of a fixed trap. The method is designed for use on experimental setups with two optical tweezers and based on pulling a particle with one trap while simultaneously holding it with the other. No piezo-stage is needed and only one optical trap must be movable with galvo-mirrors, piezo-mirrors or acousto-optical deflectors. The method combines advantages of commonly known PSD-fitting and fast-sweeping methods, allowing calibration of a completely fixed trap in a fluid of unknown viscosity/viscoelasticity. [Preview Abstract] |
Thursday, March 6, 2014 11:39AM - 11:51AM |
T44.00003: Enhanced coherent terahertz beam with a photoconductive antenna containing a chaotic shape electrodes Dong Ho Wu, Christopher Kim, Benjamin Graber Photoconductive antenna is one of the most popular methods to produce a broadband terahertz beam. Our recent experiments indicate that a photoconductive antenna containing a pair of parallel micro-strip-line electrodes produces both incoherent and coherent terahertz beam. When we drive the antenna with a low bias voltage and a weak femto-second laser power, it produces mostly coherent terahertz beam. However, as the bias voltage and/or the femto-second laser power increase, the incoherent terahertz beam strength increases exponentially with the bias voltage.[1] When the bias voltage and/or the femto-second laser power exceeds critical values, heat associated with the incoherent beam eventually leads to a catastrophic antenna failure, resulting in a permanent damage on the antenna.[2] In order to improve our photoconductive antenna we have implemented a chaotic geometry in the photoconductive antenna's electrodes. Our experimental results show that the new antenna produces substantially more coherent terahertz beam and much less incoherent terahertz beam. We will present the details of our experimental results and discuss the merits of new antenna design.~We will also examine some theory to understand our experimental results. [Preview Abstract] |
Thursday, March 6, 2014 11:51AM - 12:03PM |
T44.00004: Nonlinear optical field sensors in extreme electromagnetic and acoustic environments Anthony Garzarella, Dong Ho Wu Sensors based on electro-optic (EO) and magneto-optic (MO) crystals measure external electric and magnetic fields through changes in birefringence which the fields induce on the nonlinear crystals. Due to their small size and all-dielectric structure, EO and MO sensors are ideal in environments involving very large electromagnetic powers. Conventional antennas and metallic probes not only present safety hazards, due to their metallic structure and the presence of large currents, but they can also perturb the very fields they intend to measure. In the case of railguns, the large electromagnetic signals are also accompanied by tremendous acoustic noise, which presents a noise background that the sensors must overcome. In this presentation, we describe extensive data obtained from fiber optic EO and MO sensors used in the railgun of the Naval Research Laboratory. Along with the field measurements obtained, we will describe the interactions between the acoustic noise and the nonlinear crystals (most notably, photoelastic effects), the noise equivalent fields they produce, and methods they could be suppressed through the optical and geometrical configurations of the sensor so that the signal to noise ratio can be maximized. [Preview Abstract] |
Thursday, March 6, 2014 12:03PM - 12:15PM |
T44.00005: Beam Quality Deterioration Due to Angular Dispersion Sergiy Mokhov Laser pulses are often manipulated by different optical elements in free space for purposes of filtering, stretching/compression, shaping, and splitting. This is due to the impossibility of using fiber optical components to withstand high energy pulses. The beam quality factor for free-space propagating optical beams, M2, is typically used to characterize the performance of optical elements. Optical element which preserves M2 in the CW regime may in fact worsen M2 for pulses with the same time-averaged power if this optical element exhibits dispersion in the spectral range of the pulse's bandwidth. Basic dispersive effects can be expressed in terms of aberration-free monochromatic beam optics, and they are longitudinal shift of the waist position, transversal shift of the waist center and angular shift of the propagation direction with wavelength tuning. The first two effects are negligible for optical elements much shorter than the Rayleigh length. We have found an analytical expression for the deterioration of M2 from unity due to angular dispersion for a test pulse which has transverse Gaussian beam profile. This expression depends on both the transverse size of the pulse and the mean square variation of the spectral-angular characteristic of the optical element averaged with the spectral weight distribution of the pulse. In particular, with decreasing of beam size, the M2 deteriorates less because the spectral-angular variation of the propagation direction is mitigated by increasing beam divergence due to diffraction. In our judgment, an optical element should be characterized by its angular dispersion properties rather than measurements of M2. [Preview Abstract] |
Thursday, March 6, 2014 12:15PM - 12:27PM |
T44.00006: Nanomaterials with manageable charge of nanoblocks for adaptive sensing Vladimir Mitin, Guillaume Thomain, Andrei Sergeev, Nizami Vagidov, Kimberly Sablon Development and implementation of adaptable nanomaterials will qualitatively improve infrared sensing to meet the requirements of various applications. Adaptive sensing substantially enhances real-time detection, tracking, and identification capabilities and simultaneously provides optimal use of sensing resources. 2D and 3D nanomaterials, such as quantum dots and quantum wells, allow for effective control and management of photoelectron processes via charge redistribution in dots and wells. We designed, fabricated, and tested quantum dot and quantum well structures with complex selective doping and/or various coupling between nanoblocks. The results obtained demonstrate that the electric charge of dots and wells may be controlled by voltage bias, optical bias, and gate voltage. The charge redistribution strongly changes photocarrier lifetime, concentration of thermally excited photocarriers, and coupling to IR radiation. These adaptable parameters provide effective ways for control and tuning of interrelated detector parameters: responsivity, sensitivity, acquisition time, and dynamic range. [Preview Abstract] |
Thursday, March 6, 2014 12:27PM - 12:39PM |
T44.00007: Photoconductive ultrafast low gap materials: pulsed THz emitters and detectors Branko Petrov, Andre Fekecs, Martin Chicoine, Francois Schiettekatte, Richard Ares, Denis Morris Commonly photoconductive (PC) switches used for pulsed THz generation and detection are made on GaAs which works at 800 nm. However, there is a need for PC materials compatible with laser sources emitting at 1550 nm since they are of high interest for fiber-coupled devices to be integrated in THz imaging and spectroscopy systems. We have developed such materials based on low bandgap III-V semiconductors. With our novel approach, based on cold-implantation of heavy ions followed by a rapid thermal annealing (RTA) treatment, it was possible to obtain high resistivity (up to 2500 $\Omega \cdot $cm) and short lifetime (\textless 1ps) materials [1]. THz PC antennas were made on these materials and their characteristics were studied by using a THz time-domain spectroscopy (TDS) setup. The impact of the RTA process and different electrode designs were investigated in order to compare the characteristics of PC antennas in terms of amplitude, bandwidth, and signal to noise ratio. For the emitters, bias-voltage and pump-power dependences are shown. Remarkably high electric field (\textgreater 50 kV/cm) could be applied for increased emission of pulsed THz radiation due to the high resistivity of our materials. Our THz-TDS setup offers measurement capabilities from 0.1 to 3 THz. [1] A. Fekecs et al., Opt. Mater. 1, 7 (2011) [Preview Abstract] |
Thursday, March 6, 2014 12:39PM - 12:51PM |
T44.00008: Broadband THz Spectroscopy of Single Nanoscale Objects Lu Chen, Giriraj Jnawali, Mengchen Huang, Patrick Irvin, Sangwoo Ryu, Chang-Beom Eom, Jeremy Levy Broadband terahertz (around 10 THz) generation and detection at 10 nm scales has recently been demonstrated\footnote{Y. Ma, \textit{et al.}, Nano Lett. \textbf{13}, 2884 (2013)} using LaAlO$_{3}$/SrTiO$_{3}$ nanostructures created by conductive atomic force microscope lithography.\footnote{C. Cen, \textit{et al.}, Nat. Mater. \textbf{7}, 298 (2008)} This unprecedented control of terahertz emission, on a scale four orders of magnitude smaller than the diffraction limit, provides a useful technique to investigate a variety of nanoscale objects. Here we report initial efforts to apply THz spectroscopy to a variety of objects whose dimensions are comparable to our spatial resolution. Systems under investigation include semiconductor quantum dots, Au nanorods and single molecules. [Preview Abstract] |
Thursday, March 6, 2014 12:51PM - 1:03PM |
T44.00009: A high sensitivity terahertz detector tunable over a very large frequency range Christopher Kim, Dong Ho Wu, Rongjia Tao, Benjamin Graber A high sensitivity terahertz detector is one of the key components for a high resolution terahertz spectrometer or imager. Earlier we have demonstrated a terahertz detector that is tunable over the frequency range from 100 GHz to 1.4 THz. Based on a metal-semiconductor field-effect-transistor (MESFET) and a dipole-antenna, the detector had a sensitivity slightly poorer than 10\textasciicircum -9 W/(Hz)\textasciicircum 1/2 in terms of noise-equivalent-power (NEP). In order to increase the sensitivity and the tuning frequency range, we have modified the MESFET structure and also replaced the dipole antenna with a spiral antenna. Our computer simulations show that the new detector can have a sensitivity much better than 10\textasciicircum -9 W/(Hz)\textasciicircum 1/2 and its frequency tuning range can be from 100 GHz to over 3 THz. We will report details of the design parameters, computer simulations, and experimental results. [Preview Abstract] |
Thursday, March 6, 2014 1:03PM - 1:15PM |
T44.00010: Rigidity of the conductance of an anchored dithioazobenzene optomechanical switch Martina Zemanova Dieskova, Ivan Stich, Peter Bokes We have investigated a reversible optomechanical molecular switch based on a single azobenzene molecule suspended via thiolate links between realistic models of gold tips [1]. Using a combination of the transfer-matrix technique and density functional theory, we focus on the conductance of the nanodevice in the two (meta)stable \textit{cis} and \textit{trans}-junction conformations. We find the conductance of both conformations to be broadly similar. In qualitative agreement with related experiments, we find that the same nanodevice with one/two methylene linker group(s) inserted on one/both ends of the azobenzene molecule is driven into the tunneling regime and reduces the conductances by up to 2 orders of magnitude, again, almost uniformly for both conformations. These results clarify the huge differences in switching ratios found previously and indicate that this nanodevice is not particularly suited for use as a molecular switch based on conductance change. \\[4pt] [1] M. Zemanov\'{a} Die\v{s}kov\'{a}, I. \v{S}tich, and P. Bokes, Phys. Rev. B \textbf{87}, 245418 (2013). [Preview Abstract] |
Thursday, March 6, 2014 1:15PM - 1:27PM |
T44.00011: Non-linear optical properties derived from molecular structure via simultaneous refinement of high-resolution X-ray diffraction data and ab initio calculations Jacqueline Cole The simultaneous refinement of experimental data and ab initio calculations is shown to afford information about the molecular origins of optical non-linearity. Specifically, non-linear optical (NLO) properties are derived from a combined experimental charge-density study, X-ray constrained wavefunction refinement, and quantum-mechanical calculations. Three case studies of well-known organic and metal-organic frequency-doubling materials highlight the power of this combined experimental and computational approach. In particular, the results show how one can derive solid-state tensorial components of molecular (hyper)polarizability directly from high-resolution X-ray structural data [1,2]. Comparing such results with those that incorporate X-ray constrained wavefunction fitting [3] demonstrate superior results. Small differences between ab initio (gas-phase) and X-ray constrained wavefunction refinement (solid-state) also reveal insights into crystal-field forces. Finally, the role of this approach in the quantum-tailored molecular design of NLO materials is forecasted.\\[4pt] [1] Cole et al, J. Appl. Phys. 111 (2012) 033512;\\[0pt] [2] Cole et al, Phys. Rev. B 004100 (2013) doi: 10.1103/PhysRevB.00.004100;\\[0pt] [3] Cole et al, J. Chem. Phys. 139 (2013) 064108. [Preview Abstract] |
Thursday, March 6, 2014 1:27PM - 1:39PM |
T44.00012: Analysis of Charge Carrier Transport in Organic Photovoltaic Thin Films and Nanoparticle Assemblies Xu Han, Dimitrios Maroudas We present a systematic analysis of charge carrier transport in organic photovoltaic (OPV) devices based on phenomenological charge carrier transport models. These transient drift-diffusion-reaction models describe electron and hole transport and their trapping, detrapping, and recombination self-consistently with Poisson's equation for the electric field in the active layer. We predict transient currents in devices with active layers composed of P3HT, PCBM, and PBTDV polymers, as well as donor-acceptor blends. The propensity of the material to generate charge, zero-field carrier mobilities, as well as trapping, detrapping, and recombination rate coefficients are determined by fitting the modeling predictions to experimental data of photocurrent evolution. We have investigated effects of material structure and morphology by comparing the fitting outcomes for active layers consisting of both thin films and nanoparticle assemblies. We have also analyzed the effect on charge carrier transport of nanoparticle surface characteristics, as well as of thermal annealing of both thin-film and nanoparticle-assembly active layers. The model predictions provide valuable input toward synthesis of new nanoparticle assemblies that lead to improved OPV device performance. [Preview Abstract] |
Thursday, March 6, 2014 1:39PM - 1:51PM |
T44.00013: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 1:51PM - 2:03PM |
T44.00014: Transient picosecond studies of singlet fission in PDTP-DFBT low band gap polymer Uyen Huynh, Valy Vardeny, Gang Li, Yang Yang We measured picoseconds transient mid-IR photoinduced absorption (PA) spectra in PDTP-DFBT low band-gap polymer. With 800 nm pumping the PA spectrum at t$=$0 in pristine film and isolated polymer chain in polystyrene shows two prominent PA bands: PA1 at 0.4eV and Pa2 at 0.8eV. PA1 is assigned to absorption from singlet excitons (transition from 1B$_{\mathrm{u}}$ to mA$_{\mathrm{g}})$, whereas PA2 is due to a state of triplet-pair, which is formed via singlet fission in the sub-ps time domain. We found that PA2 lifetime strongly depends on the excitation intensity, showing non linear recombination process in both pristine film and in polystyrene. We also found that the triplet-pair recombines with no trace of fusion back to the singlet exciton; we thus conclude that singlet fission is an exothermic process in this polymer. We therefore do not find any magnetic field effect on the transient dynamics of the triplet-pair within our experimental sensitivity (0.2{\%}). [Preview Abstract] |
Thursday, March 6, 2014 2:03PM - 2:15PM |
T44.00015: Excitation Dependence of Photoinduced Absorption (PA) in $\Pi$-Conjugated Polymers Yaxin Zhai, Tek Basel, Z. Valy Vardeny In order to study the process of singlet fission (SF), where a singlet exciton decomposes into a pair of triplets $S_0+S_1\rightarrow T_1+T_1$, we have investigated the excitation dependence of the photoinduced absorption band of triplet exciton (EXPA) and photoluminescence (EXPL) in various luminescent and non-luminescent $\pi$-conjugated polymers. We found that the EXPA spectrum of luminescent polymers is composed of two steps, showing that two different channels are operative for triplet photogeneration. One process starts at the optical gap and has flat response similar to that of the EXPL spectrum. We therefore identify this process as due to intersystem crossing from the lowest lying singlet exciton. Whereas the second process with an onset at $E\approx 2E_T$, where $E_T$ is the triplet energy is due to singlet fission of hot excitons. We also found that the EXPA spectrum of some non luminescent polymers is different from that of the luminescent polymers. [Preview Abstract] |
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