Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session A61: Ultrafast Laser SpectroscopyLive
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Sponsoring Units: DLS Chair: ANTHONY BEGLEY, American Physical Society |
Monday, March 15, 2021 8:00AM - 8:12AM Live |
A61.00001: Fast phase cycling in non-collinear optical two-dimensional coherent spectroscopy Maria Munoz, Adam Andrew Medina, Travis M Autry, Galan Moody, Mark Siemens, Alan D Bristow, Steven Thomas Cundiff, Hebin Li The optical two-dimensional coherent spectroscopy (2DCS) has applications in different systems such as atomic vapors, semiconductor quantum wells, perovskites, and so on. The sensitivity of the detection is critical for increasing the signal to noise ratio in 2DCS. In addition to the desired nonlinear signal corresponding to the specific excitation pathway; there will be background signals, which arise from other excitation pathways, fluorescence, and non-resonance scatter of excitation laser beams. To eliminate this background, we developed a fast phase-cycling scheme in a non-collinear optical 2DCS by using liquid crystal phase retarders to modulate the phases of two excitation pulses [1]. We cancel out the background by adding and subtracting a series of interferograms measured with the proper phase configuration. We demonstrated the effectiveness of this method in optical 2DCS measurements of a potassium atomic vapor. This fast phase-cycling technique will broaden the scope of 2DCS applications. |
Monday, March 15, 2021 8:12AM - 8:24AM Live |
A61.00002: Terahertz investigation of strong coupling between metamaterials and superconducting Josephson plasmons in La2-xSrxCuO4 Kelson Kaj, Ian Newton Hammock, Chunxu Chen, Xiaoguang Zhao, Kevin A Cremin, Michael Fogler, Dmitri Basov, Xin Zhang, Richard Averitt We investigate the coupling between an array of metamaterial resonators and the Josephson Plasma Resonance (JPR) in superconducting La2-xSrxCuO4 (LSCO) using terahertz (THz) time domain spectroscopy. This is accomplished by adhering metamaterial “tapes” to the surface of an LSCO single crystal with the c-axis in-plane. We are able to repeatably bring the resonators within ~1um of the LSCO surface. The resulting electrodynamic response is measured as a function of temperature and resonator-surface distance, essentially changing the coupling strength. In the superconducting state, coupling between the metamaterial mode and the JPR renormalizes the JPR frequency, suggestive of a large modification of the superconducting state. The nonlinear response, measured with THz fields up to approximately 100kV/cm is consistent with strong-coupling induced modification of superconductivity as such measurements also exhibit plasma edge renormalization with increasing field strength. |
Monday, March 15, 2021 8:24AM - 8:36AM Live |
A61.00003: Calculation of Photon Echoes for Pr:YSO Quantum Memory Zachary Levine A density matrix formulation of the calculation of photon echoes in Pr:YSO (praesodymium-doped yttrium orthosilicate) using an atomic frequency comb protocol is presented. Before the calculation can be performed, an appropriate Hamiltonian is selected. The formulation takes advantage of the periodic nature of the AFC protocol to enable a rapid calculation of the final density matrix. Photon echoes are found and compared favorably to an experiment performed at the University of Maryland JQI. |
Monday, March 15, 2021 8:36AM - 8:48AM Live |
A61.00004: Terahertz Magnetospectroscopy Evidence of Spin-Magnon Interactions in the Low-Temperature Phase Transition of Erbium Orthoferrites Nicolas Marquez Peraca, Motoaki Bamba, Kenji Hayashida, Xinwei Li, Xiaoxuan Ma, Takuma Makihara, Diego Fallas Padilla, Chien-Lung Huang, Emilia Morosan, Han Pu, Shixun Cao, Junichiro Kono Magnonic excitations in antiferromagnets can couple with degrees of freedom coming from the same magnetic system, such as spins. This interaction resembles that of two-level atoms cooperatively interacting with a single mode of light, where a superradiant phase transition (SRPT) is theoretically predicted to occur as the coupling strength is increased. To date, a SRPT in thermal equilibrium has not been realized experimentally, and it has been shown that charged particles will remain stable against it. However, there remains a strong possibility to observe a phase transition analogous to the SRPT in magnetically ordered systems, where spins instead of charges interact with a common quantum field, which would then provide new insights into the original photon SRPT. In this work, we studied Er1-xYxFeO3 as a function of temperature, magnetic field, and Y composition (x) through terahertz time-domain magnetospectroscopy. Magnon excitations showed drastic spectral changes as the system undergoes a phase transition at low temperatures. We demonstrate that this phase transition can be modeled by an extended Dicke Hamiltonian taking into account long-range Er-Fe and short-range Er-Er interactions on an equal footing. |
Monday, March 15, 2021 8:48AM - 9:00AM Live |
A61.00005: Ultrafast electron dynamics in rutile TiO2(110) using multi-photon photoemission spectroscopy Namitha James, Hrvoje Petek TiO2 is a widely investigated metal oxide due to its photocatalytic properties. Understanding the photoexcited electron dynamics in TiO2 is vital to extend its applications through a thorough understanding of its electronic structure. In our previous study of reduced TiO2 by two-photon photoemission using UV excitation, we found a pair of nearly degenerate unoccupied states, which we attributed to transitions between Ti-3d bands of t2g and eg symmetry. We extended our study to multiphoton photoemission(mPP) spectroscopy in the near-IR-VIS spectral region. I will report on the dynamics of electrons excited from defect states of reduced TiO2(110) through its conduction band by mPP with a tunable ultrashort(~20 fs) laser pulses. mPP spectra reveal resonances where TiO2 defect state electrons absorb energy to reveal new electronic energy levels and transitions between them. We performed interferometric time resolved-mPP experiments on these resonances to study the photoinduced polarization and electron relaxation dynamics. Surprisingly, the experiment reveals measurable coherent polarization and population relaxation dynamics on tens of femtosecond time scales. Our study contributes to a comprehensive knowledge of these transitions relevant to the photocatalytic processes on TiO2. |
Monday, March 15, 2021 9:00AM - 9:12AM Live |
A61.00006: Tuning time and energy resolutions in time- and angle-resolved photoemission spectroscopy Alexandre Gauthier, Jonathan A Sobota, Nicolas Gauthier, Kejun Xu, Heike Pfau, Costel R. Rotundu, Zhixun Shen, Patrick S Kirchmann Time- and angle-resolved photoemission is a powerful probe of nonequilibrium band structures, for which time and energy resolution are two important tuning parameters. We demonstrate tunable time and energy resolutions by adjusting the thickness of nonlinear BaB2O4 crystals generating 6 eV pulses from a 1.5 eV fundamental. We tune the time resolution between 58 and 103 fs and obtain corresponding energy resolutions of 55 to 27 meV, maintaining a time-bandwidth product under 150% of the Fourier limit. Calculations clarify the interactions between laser bandwidth and nonlinear crystal thickness which contribute to determining experimental resolutions. |
Monday, March 15, 2021 9:12AM - 9:24AM Live |
A61.00007: Non-Equilibrium Carrier Dynamics in Bulk Chalcopyrite Crystals Studied with Time-Resolved Terahertz Spectroscopy Rishmali Sooriyagoda, Jack Powers, Herath Piyathilaka, Kevin Zawilski, Peter Schunemann, Alan D Bristow In this study, time-resolved terahertz spectroscopy (TRTS) is employed to investigate the ultrafast carrier dynamics and AC photoconductivity of cadmium germanium diphosphide (CdGeP2), zinc germanium diphosphide (ZnGeP2) and cadmium silicon diphosphide (CdSiP2) chalcopyrite crystals. It is found that the terahertz (THz) transients decrease with increasing temperature due to the increase in thermally generated free carriers. Analysis of the differential THz transient reveals a two-component exponential relaxation. At elevated temperatures, complex AC photoconductivity decreases since conductive electron experiences increased scattering from the lattice phonons. Carrier density and scattering time are extracted by fitting complex AC photoconductivity with the Drude-Smith model. Carrier density decreases with pump-probe delay time since photoexcited carriers recombine or trapped. Carrier mobility decreases with increasing temperature due to increased scattering from phonons and thermally excited carriers at higher temperatures. This detailed understanding of photoexcited charge carrier dynamics and AC photoconductivity is crucial to potential applications of chalcopyrite crystals. |
Monday, March 15, 2021 9:24AM - 9:36AM Live |
A61.00008: Ultra-supercontinuum broadening from anti-Stokes to Stokes arising from fifth- and third-order susceptibilities from self-phase modulation for isotropic liquid, solid, and gaseous media with extremely intense visible femtosecond pump pulses Robert R Alfano, Shah Faisal Mazhar, Mikhail Sharonov, Lingyan Shi Ultra-supercontinuum (USC) broadening has been theoretically stimulated from the Kerr effect self-phase modulation of the fifth- and third-order susceptibilities under the influence of the envelope of an extremely high-intensity femtosecond laser pulse in condensed and gaseous media. The outcome can produce spectral changes extending from anti-Stokes side to UV and X-ray, and Stokes side to IR down to RF/DC. The theoretical results show that a high-intensity pulse as high as on the order of ~1016 W/m2 can reveal the influence of the fourth-order refractive index n4 arising from fifth-order susceptibility large enough that the nonlinear n4I02 term overtakes the n2I0 term to produce the ultra-supercontinuum broadening in the liquids such as CS2 and N2. Different femtosecond laser systems with J and mJ pulse energies have been used to study the USC generation in CS2 and N2 experimentally. |
Monday, March 15, 2021 9:36AM - 9:48AM Live |
A61.00009: Ultrafast signatures of spin and orbital orders in the spin-orbit entangled Mott insulator Sr2CrO4 Min-Cheol Lee, Connor Occhialini, Jiarui Li, Zhihai Zhu, La Moyne Mix, Dmitry Yarotski, Riccardo Comin, Rohit P Prasankumar We used ultrafast optical spectroscopy to study non-equilibrium spin and orbital order dynamics in Sr2CrO4, a 3d antiferromagnetic Mott insulator. Sr2CrO4 possesses multiple collective phases exhibiting stripe-type orbital order (T < 50 K), Néel-type spin order (T < 113 K), and antiferro-type orbital order (T < 140 K), though the details of the antiferro-type orbital order are not fully understood. In our experiments, we varied the pump photon energy to selectively drive intersite spin hopping between neighboring Cr t2g orbitals and charge transfer-type transitions between oxygen 2p and Cr eg orbitals. This revealed temperature-dependent anomalies in the transient reflectivity at 1.55 eV across the Néel temperature at 113 K as well as 50 K and 140 K, which can be linked to spin and orbital order. Our results reveal the distinct relaxation timescales for spin and orbital orders, respectively, and provide experimental evidence for antiferro-type orbital ordering below 140 K. |
Monday, March 15, 2021 9:48AM - 10:00AM Live |
A61.00010: Coherent exciton-polaritons modified by population dynamics and strong coupling Jagannath Paul, Jared Wahlstrand, Alan D Bristow The exciton-polariton response in a monolithic semiconductor microcavity containing an InGaAs quantum-well is explored with 3-pulse multidimensional coherent spectroscopy (MDCS). Rephasing spectra are measured at three values of detuning between the exciton and cavity modes as a function of the population (mixing) time delay between the second and third pulses. Changes in the complex 2D spectrum with population time probe various interactions typically attributed to population dynamics. Our results show commonly seen Raman coherent contributions and energy transfer from the upper- to lower-polariton. Additionally, a change in amplitude and spectral phase shift associated with the lower and upper exciton-polaritons have been observed. Previous studies of population time dependence of excitons in a GaAs quantum well revealed persistent many-body effects arising due to polarization-polarization scattering between photoexcited bright states within the light-cone and dark states outside the light-cone. Finally, the spectral phase shift is also correlated with a slight change in the peak separation of the two polariton branches (at a single detuning), which would indicate that polarization-polarization scattering may affect the strength of the vacuum Rabi splitting. |
Monday, March 15, 2021 10:00AM - 10:12AM Live |
A61.00011: Intermediate Stages of Ultrafast Melting of Gold Thin Films Tadesse Assefa, Yue Cao, Robert Koch, Longlong Wu, Ana Suzana, Xiaojing Huang, Emil Bozin, Hyunjung Kim, Simon L Billinge, Ian Keith Robinson Understanding the structure of materials with complementary non-destructive X-ray techniques becomes essential to design better and more efficient materials. This talk will describe the results of an ultrafast single-shot melting experiment carried out at the PAL-XFEL, combined with synchrotron-based X-ray nanodiffraction to examine the domain formed. Polycrystalline thin films of gold were irradiated with femtosecond optical pulses in the fluence range sufficient to melt the film. Heterogeneous two-phase melting was observed in the form of a split Au (111) diffraction powder ring. The colder, high-Q, peak showed oscillations as a function of pump-probe delay time, consistent with acoustic waves generated in the film. The new peak on the low-Q side was attributed to a compressed, hotter region of gold that absorbs the latent heat during the melting and increases with the melt-front moving in time. To understand the grain size effects of melting, samples were characterized using X-ray nanodiffraction technique and preliminary result will be presented. |
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