Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session M06: Optical Tools, Techniques, Methods, and Theory |
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Sponsoring Units: DLS Room: 113 |
Wednesday, March 4, 2020 11:15AM - 11:27AM |
M06.00001: Ultrafast THz Probe of Phonon-Assisted Processes in Photoexcited bulk GaAs Eugenio Cinquanta, Lorenzo Gatto, Gabriele Crippa, Caterina Vozzi, Salvatore Stagira Hot carrier dynamics in semiconductors governs the performances of optoelectronic devices. The out-of-equilibrium distribution of photoinjected carriers relaxes through scattering with the lattice phonons. The main relaxation path can be depicted with an initial hot LO phonon distribution that relaxes through the emission of acoustic phonon. These hot phonon population may lead to the enhancement of two-phonon absorption processes involving optical-acoustic and acoustic-acoustic transitions detectable by time-resolved FIR spectroscopy. By means of Optical Pump - THz probe experiments, we investigated the out-of-equilibrium properties of intrinsic bulk GaAs. By tuning the pump-photon energy it is possible to excite the carriers directly in the Γ and in the higher energy L and X valleys. Independently from the excitation wavelength, we observed a rich transient THz response where a broad Drude-Lorentz complex conductivity is dressed with several sharp Lorentzian features in the 0.5-2.5 THz spectral range. This response is highly fluence-dependent, suggesting a carrier density dependence of the observed behavior. Our outcomes may be related to the enhancement of two-photon absorption processes involving the optical and acoustical out-of-equilibrium phonon distribution. |
Wednesday, March 4, 2020 11:27AM - 11:39AM |
M06.00002: Observations of Group Velocity Dispersion, Raman Scattering, Stimulated Raman, and Spectral Broadening for O-waves and E-waves from a Calcite Crystal Using Femtosecond Laser Pulses Shah Faisal Mazhar, Henry Meyer, Robert Alfano Group velocity dispersion (GVD), Raman Spectra, Stimulated Raman Scattering (SRS), and Supercontinuum (SC) are investigated for ordinary (O) waves and extraordinary (E) waves from a 2.7 cm thick Calcite crystal. Using 390 fs pulses (~2 μJ pulse energy) at 517 nm, the O-wave produced a stronger sharp SRS peak at 1086 cm-1 and weak SC spectrum in the visible range than E-wave. To explain the salient observation, the difference between Raman cross-section, index of refraction (walk-off), GVD, and the size of nonlinear susceptibility, χ3 for O-waves and E-waves of Calcite are presented. |
Wednesday, March 4, 2020 11:39AM - 11:51AM |
M06.00003: Multiphoton photoemission spectroscopy of TiO2 and Au/TiO2 surfaces Namitha James, Hrvoje Petek TiO2 is a widely investigated metal oxide due to its photocatalytic and photovoltaic applications. Until recently, the ability to probe the ultrafast electron dynamics of this system has been limited to transient absorption experiments. Understanding the photoexcited electron dynamics in TiO2 is vital to extend its applications through a thorough understanding of its electronic structure. TiO2 is known to catalyze reactions upon UV excitation. Photocatalysis in TiO2 can be extended into visible by coupling it with plasmonic metal nanoparticles, which can be very efficient light absorbers in the visible through excitation of their localized plasmon resonance. Au nanoclusters coupled to TiO2(110) have been shown to enhance photocatalytic properties upon visible excitation. By probing the rutile TiO2(110) and Au/TiO2 interface using multiphoton photoemission (mPP) with a tunable ultrashort (~20 fs) laser pulse, we investigate the dynamics of electrons excited in its conduction band. mPP spectra reveal resonances where TiO2 defect state electrons absorb energy to reveal new optical energy levels and transitions between them. Decoration of the surfaces with Au overlayer introduces new resonances, but does not show a plasmonic enhancement of the optical response of Au/TiO2 surfaces. |
Wednesday, March 4, 2020 11:51AM - 12:03PM |
M06.00004: Exact Theory for Two-Color Laser-Induced Photoemission from a Biased Metal Surface Peng Zhang, Yi Luo, Yang Zhou Laser-induced electron emission is important to ultrafast electron microscopes, tabletop particle accelerators and x-ray sources, and novel quantum nanocircuits [1-3]. Two-color laser-induced photoemission provides great flexibility for the coherent control of ultrafast electron dynamics due to the interference effect [4]. By solving the time-dependent Schrödinger equation exactly [5-7], we found that under a large DC bias, strong modulation persists in the emission current, whose magnitude is also significantly increased. Application of our model to time-resolved photoelectron spectroscopy demonstrates the dynamics of the multi-photon excited states depends strongly on the applied DC field. Our study suggests a practical way to maintain a strong modulation to high current photoemission. [1] P. Zhang, et al., Appl. Phys. Rev. 4, 011304(2017). [2] E. Forati, et al., Nat. Communications 7, 13399 (2016). [3] P. Zhang, and Y. Y. Lau, J. Plasma Phys. 82, 595820505 (2016).[4] M. Forster, et al., Phys. Rev. Lett. 117, 217601 (2016).[5] P. Zhang and Y. Y. Lau, Sci. Rep. 6, 19894 (2016).[6] Y. Luo, and P. Zhang, Phys. Rev. B, 98, 165442 (2018).[7] Y. Luo, and P. Zhang, Phys. Rev. Applied 12, 044056 (2019). |
Wednesday, March 4, 2020 12:03PM - 12:15PM |
M06.00005: Generation and applications of intense terahertz fields produced by multi-color laser pulses interacting with gas plasmas Luc BERGE Terahertz (THz) pulses are very popular because of their numerous applications, for example in security. Located between microwaves and optical waves in the electromagnetic (EM) spectrum, their spectral domain can be exploited for molecular spectroscopy using the THz waves emitted from plasmas created by two-color femtosecond laser pulses ionizing air. Down-conversion of broadband optical spectra in the plasma produces intense EM radiation suitable for the identification of suspect materials, even remotely. The physical mechanisms involved to create terahertz radiation by laser-matter interaction, such as photocurrents, are here reviewed. The new potentialities offered by ultrafast intense lasers allow the acquisition of many spectral signatures within 20-50 THz broad bandwidths associated to high-field single-cycle THz pulses. We report new features obtained in the framework of the French project ALTESSE, the main objective of which consisted in testing the efficiency of a laser-based terahertz time-domain spectroscopy of various materials, using the air-biased coherent detection (ABCD) method. The last part of this presentation will discuss new perspectives in the production of ultra-intense terahertz pulses from laser-wakefield accelerators in relativistic plasmas. |
Wednesday, March 4, 2020 12:15PM - 12:27PM |
M06.00006: Periodic Lensing from a One-Component Plasma Imaged by Ultrafast Transmission Electron Microscopy Omid Zandi, Allan Eugene Sykes, Ryan Dean Cornelius, Francis Marion Alcorn, Bryan Reed, Renske M van der Veen Transmission electron microscopy (TEM) has become a powerful technique to study the structure of materials at nanoscale. The temporal resolution of TEM, however, is typically limited by the maximum frame rate of the detector to the ms regime. Ultrafast TEM combines the high temporal resolution of ultrafast laser spectroscopy with the excellent spatial resolution of electron microscopy. The structural and electronic changes in the material are initiated by fs laser pulses followed by similarly short photoelectron pulses to prob the dynamics by imaging, diffraction, or electron spectroscopy. Here, we will present our ultrafast TEM at UIUC, and demonstrate its first application in observation of periodic plasma lensing. We generated a one-component electron plasma (OCEP) by two-photon photoemission from a Cu grid. We directly imaged the subsequent evolution of the OCEP under the influence of an external uniform magnetic field inside the TEM on the ps time scale. The cyclotron oscillations give rise to a periodic lensing effect which maximizes when the OCEP is refocused along the magnetic field. We analytically described the dynamics of OCEP and its time-dependent focal length to extract the number of electrons and their velocity spread. We verified the model by N-body simulations. |
Wednesday, March 4, 2020 12:27PM - 12:39PM |
M06.00007: Attosecond Pulses with Time-Varying Orbital Angular Momentum: The Self-Torque of Light Quynh L Nguyen, Kevin Dorney, Laura Rego, Nathan Brooks, Chen-Ting Liao, Julio San Roman, David E Couch, Allison Liu, Emilio Pisanty, Maciej Lewenstein, Luis Plaja, Henry Kapteyn, Carlos Hernandez-Garcia, Margaret Mary Murnane Propagating laser beams with defined orbital angular momentum (OAM) have found applications that enable new paradigms in optical metrology and communication, quantum information, photomechanical manipulation, and super-resolution imaging. To date, however, the generation and application of OAM beams has been largely based on a static interpretation of optical OAM, where OAM is time-independent. We show for the first time that propagating light waves can possess dynamic, time-varying OAM. We exploit the highly non-linear and non-perturbative process of high-harmonic generation to imprint time-dependent OAM onto extreme ultraviolet (EUV) beams. This new photonic property, self-torque (an analogy to mechanical systems that exhibit a self-induced time variation of their angular momentum), manifests as a few-to-sub femtosecond variation of optical OAM states along each pulse in the underlying attosecond pulse train. We confirm that optical self-torque endows unique properties to coherent EUV waveforms, which enable the delivery of optical torque on the natural time and length scales of charge and spin ordering in materials. Ability to sculpt the OAM wavefront of EUV beams also offers exciting potential in imaging, metrology and sensing methods for semiconductor and quantum materials. |
Wednesday, March 4, 2020 12:39PM - 12:51PM |
M06.00008: Direct in-situ single-shot measurements of the absolute carrier-envelope phases of ultrashort pulses Duke Debrah, Gihan Basnayake, Wen Li Many important physical processes such as nonlinear optics and coherent control are highly sensitive to the absolute carrier-envelope phase (CEP) of driving ultrashort laser pulses. This makes the measurement of CEP immensely important in relevant fields. Even though relative CEPs can be measured with a few existing technologies, the estimate of the absolute CEP is not straightforward and always requires theoretical inputs. Here, we demonstrate a novel in-situ technique based on angular streaking that can achieve such a goal without complicated calibration procedures. Single-shot measurements of the absolute CEP have been achieved with an estimated precision of 0.19 radians. |
Wednesday, March 4, 2020 12:51PM - 1:03PM |
M06.00009: Photoninduced quantum entanglement between remote electron-phonon systems Kunio Ishida, Hiroaki Matsueda
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Wednesday, March 4, 2020 1:03PM - 1:15PM |
M06.00010: Characterization of an ultrastable sapphire optical cavity operated at 10 K Jose Valencia, Ethan Clements, May Kim, David B Hume, David Leibrandt The frequency stability of optical clocks is often limited by the linewidth of the probe laser, where the highest stability cavity-stabilized lasers are dominated by thermal noise in the cavity mirrors [1]. We present an optical reference cavity with a calculated thermal noise limit near Δf/f = 3x10-18, which is compatible with reaching the ultimate stability limit due to quantum projection noise for clocks based on the 1S0-3P0 transition in 27Al+ (8 mHz natural linewidth at 267 nm) [2]. The cavity is constructed of single-crystal sapphire with GaAs/AlGaAs crystalline mirror coatings and is operated at 10 K using a closed-cycle cryocooler. We measure environmental and technical noise, including temperature fluctuations and vibrations, and characterize their impact on cavity stability. The cavity stability is found to be near 10-16 by comparing to room-temperature ULE cavities. |
Wednesday, March 4, 2020 1:15PM - 1:27PM |
M06.00011: Optical manipulation system for Steady State Microbunching in storage ring. Pohsun Wu, Hao-Wen Luo, Alexander Wu Chao, Ci-Ling Pan, Chuanxiang Tang Steady State Microbunching (SSMB) is one of the schemes for generation of high average power coherent synchrotron radiation(CSR) in a storage ring. As the first proof-of-principle experiment has succeeded at the MLS storage ring in Berlin, it could be a promising candidate of kW level EUV source for lithography with the advantage of narrow linewidth, high power, collimated and clean beam. To satisfy the requirement of SSMB modulation field, the turn by turn phase must be stable and the field need be as high as ~MV/m over several meters of the modulation length. We have studied the effect on microbunch formation and maintenance of turn by turn phase with arbitrary phase jitter with noise spectrum from a commercially available relatively high-power continuous wave laser with a MW level enhancement cavity. The cases of synthesized field by multicolor laser field and adiabatic modulation are also considered. Compare with single color sheme under the same power, the microbunch length of 2-color scheme can be three times smaller while 3-color scheme can be six times smaller. Thus we can generate CSR at shorter wavelength with shorter microbunch length. |
Wednesday, March 4, 2020 1:27PM - 1:39PM |
M06.00012: Investigating High Harmonic Generation Beam Dynamics with Ptychographic Lensless Imaging David Schmidt, Logan Z Ramlet, Alex Wilhelm, David Goldberger, Daniel Adams, Charles G Durfee High Harmonic Generation (HHG) allows for tabletop EUV coherent light sources and attosecond pulses that can be used to probe material properties. While the process is relatively straightforward to implement, the beam quality of the harmonics is difficult to measure and control. A first step to realizing a tunable and fine controlled HHG setup is the ability to measure these quantities without having to change the setup drastically. Ptychography is a computational imaging technique that has already been used with HHG to reconstruct images of unique specimens. In this work, we shift the emphasis in computational imaging from reconstructing the specimen to understanding the details of the EUV light generated through HHG. Reconstructed harmonic beam profiles can then be propagated back to their generation point in the system to examine how they formed. Using this knowledge, we can optimize the factors affecting the HHG process and learn to control them. |
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