Groups | Seminars || Courses | Outreach

Seminars

Seminars will be held held at room S-141 in the Physics and Astronomy Department building on Mondays at 4:00 PM, unless noted otherwise.

Spring/Summer 2018

January 22, 2018 Prof. Ting Yu Stevens Institute of Technology Non-Markovian Dynamics of Open Quantum Systems: Recent Development (Host: Jin Wang)

In this talk I will report our recent work on quantum dynamics of open systems coupled to an external environment. We will study the crossover property between non-Markovian and Markov dynamics induced by a hierarchical environment consisting of a cavity-reservoir system. We show how the non-Markovian character of the system of interest is influenced by the coupling strength between the qubit and cavity and the correlation time of the reservoir. In particular, we found a new phenomenon whereby the qubit Markovian and non-Markovian transition exhibits an anomalous pattern in a parameter space depicted by the coupling strength and the correlation time of the reservoir. In addition, I will report some recent developments of quantum trajectory methods for cavity QED systems.

January  25, 2018 (11am, YITP) Dr. Stephen Libby Lawrence Livermore Nat'l Laboratory Cold Atom Sensing, Gravity, Tomography, and Gyroscopes (Host: George Sterman, YITP)

The ability to use lasers to cool atoms to micro-kelvin temperatures and to subsequently control their quantum mechanical behavior has led to the development of exquisitely precise 'quantum' sensors.2 Applications of these sensors include the measurement of local gravitational anomalies to unprecedented accuracy and very accurate, highly stable gyroscopes. Our LLNL - AOSense, Inc. collaboration is pursuing diverse applications of these sensors that directly exploit their extraordinary scale factor stability, low noise and bias drift characteristics. These applications include shielded threat detection in passing vehicles, emergency response, and treaty verification, all of which require rapid, passive methods to determine hidden mass configurations precisely and/or verify the masses present in containers. Such dense, localized objects can in principle be discovered and accurately measured by their effect on the local gravitational field.   Furthermore, near field measurements of these gravitational perturbations from multiple vantage points allow for a kind of gravitational 'tomography,' leading to the real-time determination of the hidden mass distribution. Additionally, we are interested in the potential of atom interferometer Sagnac gyroscopes to do accurate 'dead reckoning' navigation without the aid of GPS.4 After reviewing the physics of atom interferometry in atomic fountain-Mach-Zehnder and Sagnac configurations, I will describe the development of a 'gravity tomography' signal analysis system for vehicle portals, including the optimal synthesis of the gravitational sensor signals with complementary radiation detection.

February 26, 2018 Prof. Daniel Turner NYU Spectroscopic Observation of Triplet Separation as a Driving Force of Singlet Fission (Host: Tom Allison)

Singlet fission, a multistep molecular process in which one photon generates two triplet excitons, holds great technological promise. Here, by applying a combination of transient transmittance and two-dimensional electronic spectroscopy with 5 fs laser pulses, we resolve the full set of fission steps before the onset of spin dephasing. In addition to its role as a viable singlet fission material, single-crystalline rubrene is selected because its energetics and transition dipole alignment uniquely allow for the unambiguous identification of the various fission steps through their contributions to distinct spectroscopic features. The measurements reveal that the neighboring correlated triplet pair achieves its maximum population within 20 fs. Subsequent growth of the triplet signal on picosecond time scales is attributable to spatial separation of the triplets, proceeding nonadiabatically through weakly coupled but near-resonant states. As such, we provide evidence in crystalline rubrene for a singlet fission step that, until now, has not been convincingly observed.

March 5, 2018 Prof. Carlos Trallero University of Connecticut An attosecond self-referencing interferometer for phase measurement and control of electronic wavepackets (Host: Tom Weinacht)

We use an equivalent to the Young double-slit interferometer for attosecond pulses to control and directly measure the phase of electrons in the continuum. We can perform this with a resolution of 12.5 attoseconds (half the atomic unit of time) and with sub-attosecond precision.

April 23, 2018 Prof. Mengkun Liu Stony Brook University Recent Development in Scattering-type Scanning Near-field Optical Microscope (Host: Tom Bergeman)

In this talk, I will go over the basics of optical near-field techniques and discuss how they can be used to probe frequency and momentum dependent dielectric constants of materials at the nanometer scale.  Specifically, I will discuss the details of modeling scattering-type scanning near-field infrared microscopy and its application in understanding the phase change materials and two-dimensional materials over a broad spectral range (33 cm-1 to 2500 cm-1). I will also discuss the future development of nano-optics probes including the cryogenic capabilities and its coupling to ultrafast pump-probe spectroscopy including both the IR and THz frequency range. These developments set the stage for future spectroscopic investigations to access the fundamental properties of complex materials at the nanoscale. I will also try my best to spice it up with some quantum analogy in it.

June 25, 2018 Stefan Evans University of Arizona On vacuum structure and light propagation (Host: Hal Matcakf)

Nonzero divergences of laser beam fields suggest nonzero charges in a vacuum void of matter, requiring solutions to the paraxial wave equation to be remedied with introduction of a longitudinal polarization. This motivated a search for nonlinear vacuum effects, modifying Maxwell's equations such that a freely propagating laser beam induces charge in the vacuum. We investigate behavior of the Euler-Heisenberg effective potential, first derived in 1936. This effective potential characterizing the behavior of virtual electron-positron pairs in the vacuum is still progressing as a theory today, en route to describing quantum vacuum effects on light propagation.

June 27, 2018 (3:00PM) Dr. Hamed Merdji LIDYL, CEA, CNRS, Universite Paris-Saclay, France Recent progresses in high harmonic generation in 2D and 3D semiconductors (Host: Tom Allison)

Optoelectronic is extending to the highly non-linear regime and petahertz frequencies. A recent impact of this capability of controlling the response of above band gap electrons under strong fields is the emergence of high harmonic generation (HHG) in crystal. An increased number of studies on HHG in crystals has been reported so far in various bulk semiconductors and very recently in 2D materials like graphene [1-6]. Here, I will review our  recent progresses on HHG in semiconductors, from basics to applications in petahertz optoelectronic. Using a 2µm thulium femtosecond fibre laser we have explored various band gap crystals. As a first illustration I will present results on CaCO3 and GaN crystal and show a strong laser polarization dependence that can be used to gate isolated attosecond pulses. I will then report on very recent insight into the non-linear harmonic response of strongly correlated materials. The harmonic signal exhibits signatures of the few optical cycles insulator to metal phase transition [7]. As last material of interest, I will present results on HHG in graphene. We generated up to the 9th harmonic order from tri-layer and monolayer graphene on quartz which tend to be more intense than free standing graphene. Ellipticity measurements show a stronger harmonic signal for a linearly polarized beam than a circularly polarized one. I will explain why this results deviates from the work reported by Yoshikawa et al.6 and how polarization spectroscopy of the harmonics emission allows exploring non-conical section of the graphene band structure [8]. Finally, inspired by recent work [9,10], I will conclude by drawing a perspective about how these various 2D and 3D crystals, with unique properties, can be used to control electronic currents at petahertz frequencies. 1. Ghimire, S. et al. Observation of high-order harmonic generation in a bulk crystal. Nat. Phys. 7, 138–141 (2011). 2. Luu, T. T. et al. Extreme ultraviolet high-harmonic spectroscopy of solids. Nature 521, 498–502 (2015). 3. Ndabashimiye, G. et al. Solid-state harmonics beyond the atomic limit. Nature 534, 520–523 (2016). 4. You, Y. S., et al. Anisotropic high-harmonic generation in bulk crystals. Nat. Phys. 13, 345–349 (2017). 5. Liu H. et al. High-harmonic generation from an atomically thin semiconductor. Nat. Phys. 13, 262–265 (2017). 6. Yoshikawa, N., et al. High-harmonic generation in graphene enhanced by elliptically polarized light excitation. Science, 356, 736-738 (2017). 7. Kholdtsova M. et al. in preparation 8. Kaassamani S. et al. in preparation 9. Hohenleuter, M. et al. Real-time observation of interfering crystal electrons in HHG. Nature 523, 572-575 (2015). 10. Langer, F. et al., Lightwave-driven quasiparticle collisions on a subcycle timescale. Nature 533, 225–229 (12 May 2016).

June 27, 2018 (3:30PM) Dr. Tais Gorkhover SLAC X-ray Fourier holography takes off (Host: Tom Allison)

The advance of X-ray Free Electron Lasers (FELs) with intense and femtosecond-short opens the door for single-shot studies of non-crystalline nanoparticles with high spatial and temporal resolutions.  The samples are injected into the FEL focus and elastically scatter photons from the incoming X-ray pulse.  The diffracted photons form an image containing the structural information of the intact sample. Coherent X-ray diffractive imaging, however, suffers from intrinsic loss of phase. Therefore structure recovery is often complicated and not always uniquely-defined. X-ray holography can help overcome the phase problem, but is inherently hard to implement for randomly injected samples. I will introduce in-flight holography, which can be used for random specimen and reference configurations.

June 29, 2018 (2:00PM) Dr. Simeon Bogdanov Purdue University. Quantum plasmonics with color centers in diamond (Host:Hal Metcalf)

Quantum photonic technologies impose stringent and often contradictory requirements on the design of integrated optical components. In particular, high quality factors offered by dielectric nanostructures impose a limit on the achievable brightness of single-photon sources. Plasmonic materials promise to confer novel properties to integrated quantum devices that are not achievable with dielectric materials, such as ultrafast operation, nanoscale footprint and very strong light-matter interaction. We will review our recent experiments on coupling color centers in diamond such as nitrogen-vacancies (NV) and germanium-vacancies (GeV) to plasmonic nanostructures. We will particularly focus on the realization of an ultrafast and ultrabright single-photon source and on the study of NV’s spin readout in the Purcell regime. We will discuss potential applications of these results for quantum photonics and nanoscale magnetometry.

July 13, 2018 Dr. Leland Aldridge Yale University. On the Question of Cooling without Spontaneous Emission via the Bichromatic Force (Host:Hal Metcalf)

The bichromatic force, a stimulated optical force, has been observed to slow cool atomic and molecular beams on which it acts. The exact mechanism of cooling and the role that spontaneous emission plays are still open questions. I present semi-classical simulations of BCF in an explicitly decay-free ensemble of two-level systems. In these simulations, increases in position-momentum phase space density were observed to occur transiently. This apparent cooling is consistent with exchange of entropy between the external and internal degrees of freedom of the system.

August 15, 2018 (4:00PM) Prof. Jin-Tae Kim Chosun University, South Korea Progress on hyperfine analysis of deeply bound electronic states of the 85Rb-133Cs molecule through short-range photoassociation (Host:Tom Bergeman)

Various hyperfine structures of deeply bound ro-vibronic levels of the strongly perturbed electronic states of ultracold 85-Rb 133-Cs with \Omega=1 through a short range photo-association have been investigated. These hyperfine experimental spectra match well with simulated eigenvalues considering the magnetic hyperfine terms such as nuclear spin--electron orbital angular momentum, Fermi contact, and dipolar electron spin--nuclear spin interactions. Different transition amplitudes of rotational levels between the electronic states with unresolved hyperfine structures and atomic scattering waves make it possible to identify electronic states ^3\Pi_{0^+} and ^3\Pi_{0^-} with different symmetries. A basis transformation between free atomic scattering states and the molecular hyperfine states of deeply bound molecular electronic states (\Omega=1) with resolved hyperfine structures to estimate transition amplitudes will also be presented.

2018