Session V34: Focus Session: Impact of Ultrafast Lasers V: Applications II

Thermalization of photoexcited electrons in bismuth investigated by time-resolved THz spectroscopy and {\it ab initio} calculations

The charge carrier dynamics of photoexicted bismuth generates a Drude response that evolves in time. In contrast to graphite, the plasma frequency of bismuth displays an initial increase and a subsequent decay. We have performed {\it ab initio} calculations of bulk bismuth within the density functional theory, and show that the non-monotonic behaviour of the plasma frequency is due to the presence of local minima in the conduction band: most of the photoexicted electrons first accumulate in these local minima, and reach the L point only 0.6 ps after photoexcitation.   

Spatio-temporal dynamics of plasmons on a metal cone

Localized surface plasmons (LSP) and propagating plasmon modes (PPM) field emit at the singularity of the metallic tip apex upon ultrafast optical excitation. Interferometric measurements, using frequency modulation-demodulation techniques, allow the characterization of the time profile of the optical field emission and the space-time profile of the propagating modes. A pump pulse launches a plasmon that oscillates on the cone and coherently interferes with a time-delayed probe pulse which is cross-polarized with respect to the pump. We present a simple model that accurately describes the experimental results. However, the observations of trapped propagating modes, their lifetime, and their polarization are not predicted by current theories, which describe tip-plasmons as strictly transverse magnetic modes.   

Time-Resolved ARPES Study of Ultrafast Dynamics in Cuprate Superconductor Bi2212

The momentum-dependent ultrafast dynamics in cuprate superconductor Bi2Sr2CaCu2O8 is investigated using ultra high resolution time-resolved angle-resolved photoemission spectroscopy (tr-ARPES). In this talk, we will present our observation and analysis of dynamics of the quasiparticle states in Bi2Sr2CaCu2O8 in nodal and antinodal direction as a function of doping. Work was performed with temporal resolution of 35fs. The consequences of our findings in terms of nodal-antinodal dichotomy will be discussed.   

SFG and SPR Study of Sodium Dodecyl Sulfate Film Assembly on Positively Charged Surfaces

This study uses sum frequency generation (SFG) vibrational spectroscopy and surface plasmon resonance (SPR) sensing to investigate the structure of sodium dodecyl sulfate (SDS) films formed on positively charged and hydrophilic surfaces. The SPR signals show a good surface coverage suggesting that full monolayer coverage is reached at 1 mM. SFG spectra of SDS adsorbed exhibits well resolved CH$_{3}$ peaks and OH peaks. At both 0.2 mM and 1 mM SDS concentration the intensity of both the CH$_{3}$ and OH peaks decreased close to background levels. We found that the loss of SFG signal at 0.2 mM occurs at this concentration independent of surface charge density. It is more likely that the loss of signal is related to structural inhomogeneity induced by a striped phase - stand-up phase transition. This is supported by a distinct change of the relative SFG phase between CH$_{3}$/OH near 0.2 mM. The second intensity minimum might be related to charge compensation effects. We observed a substrate dependence for the high concentration transition. We also observed distinct SFG signal phase changes for water molecules associated with SDS layers at different SDS solution concentrations indicating that the orientation of bound water changed with SDS surface structure.   

Nanoscale sensing with surface-enhanced CARS spectroscopy

Time-resolved coherent anti-Stokes Raman scattering (CARS) is extended to the nanoscale regime where nano-molar amounts of molecules may be investigated. We show that spectral resolution of CARS generated by ultrashort laser pulses may be improved by collecting a sequence of time-resolved spectra. This procedure provides additional linewidth information and allows detecting two species of pyridine molecules in a vicinity of aggregated gold nanoparticles. Surface-enhanced CARS (SE-CARS) signals of the adsorbed pyridine monolayer are detected in the presence of a bulk pyridine background CARS signal. Time-resolved SE-CARS signals are stronger than the conventional CARS, and more sensitive to the surface environment which makes them suitable for nano-surface characterization with high molecular specificity. This technique allows measuring the vibrational dephasing time of molecules on nano-surfaces and characterizing the effects of the surface local environment on the ultrafast molecular dynamics. This technique may be applied to a variety of artificial and biological systems and complex molecular mixtures and has a potential for nanophotonic sensing applications.   

Time-resolved photocurrent generation in graphene $p-n$ junctions; probing $e-h$ relaxation dynamics near the Fermi level

In this contribution we probe the relevant timescales associated with photocurrent generation by optically exciting graphene at a $p-n$ junction defined by electrostatic gates. Time-resolution of the optical and electrical response are obtained by simultaneous collection of transient photocurrent and transient absorption using a two-pulse correlation approach with 160 fs resolution. The transient absorption signal decays biexponentially in the $p-n$ junction at 0.26 and 2.2 ps; timescales similar to those recently established for hot electron and hot optical phonon cooling in graphene sheets. By contrast, we find transient photocurrent decay signal is surprisingly slow, with tails extending for 100s of ps. At 10 K, the rate coefficient for transient photocurrent decay is 2.7 $\pm $ 0.6 ps$^{-1}$n$A^{-1}$, and it increases monotonically tenfold upon warming to room temperature. We attribute the stark difference in the transient photocurrent and absorption kinetics to a photocurrent response that is sensitive to recombination processes that are occurring well below optical excitation energies, near the Fermi level. Combining the measurements, transient photocurrent alongside the well-studied transient absorption allows us to construct a rough timeline of events associated with photocurrent production at a graphene $p-n$ junction.   

Standoff detection of trace compounds enabled by continuum pulse shaping and coherent Raman scattering

Raman spectroscopy has long been pursued as means to detect hazards from a safe distance. This approach promises high chemical specificity, but is limited in sensitivity because of the very small Raman cross-section. We recently demonstrated detection of trace quantities using a non-linear counterpart, coherent anti-Stokes Raman scattering (CARS), which offers large signal enhancement over spontaneous Raman due to coherent signal addition. Utilizing a pulse shaper and the bandwidth inherent in a 5fs laser pulse, CARS spectra were acquired from an explosive simulant dissolved within thin polymer films. Further, the pulse shaper offers total control of the non-linear process, including selective excitation of particular vibrational modes, enabling single channel detection and associated opportunities for rapid chemical imaging. We will present standoff chemical images and associated CARS spectra acquired in a standoff configuration, demonstrating the applicability of the new spectroscopy in a realistic environment.   

Transient Absorption Spectroscopy for the Detection of Multiexciton States in Semiconductor Quantum Dots

It has been shown that excitations of quantum dots at photon energies well above the band gap can produce a superposition of single exciton (SE) and multiple exciton (ME) states. The resulting electron hole dynamics can then be monitored using transient absorption (TA) spectroscopy. We have modeled the TA signal that can be expected when Pb$_{68}$Se$_{68}$ and Si$_{29}$H$_{24}$ are excited, and subsequently probed with attosecond laser pulses. In the proposed experiment a pump pulse serves to excite the sample. The energy of the probe pulse is chosen such that any detected absorption is attributable to the presence of a ME state. Oscillations in the absorption spectrum indicate the existence of a coherent superposition. The period of the oscillation depends on the energy difference between the superimposed states. We explore how dephasing of this signal is affected by the temperature of the system, as well as the number of states involved. We predict what can and cannot be measured experimentally and propose specific attosecond experimental scenarios.   

How much information do ultrafast spectra contain? The case for ultrafast quantum process tomography

In this talk, I will describe the concept of quantum process tomography (QPT). The QPT protocol is a novel way of thinking about ultrafast spectra that allows to design families of experiments for extracting the quantum dynamical map of any system addressable by ultrafast spectroscopy. I will explain how QPT works, and also will describe our progress towards an experimental realization in collaboration with the group of Keith Nelson (MIT).