Session L34: Focus Session: Impact of Ultrafast Lasers IV: Applications I

Adding a dimension to the infrared spectra of interfaces: 2D SFG spectroscopy via mid-IR pulse shaping

Sum-frequency generation spectroscopy provides an infrared spectrum of interfaces and thus has widespread use in the materials and chemical sciences. In this presentation, I will present our recent work in developing a 2D pulse sequence to generate 2D SFG spectra of interfaces, in analogy to 2D infrared spectra used to measure bulk species. To develop this spectroscopy, we have utilized many of the tricks-of-the-trade developed in the 2D IR and 2D Vis communities in the last decade, including mid-IR pulse shaping. With mid-IR pulse shaping, the 2D pulse sequence is manipulated by computer programming in the desired frequency resolution, rotating frame, and signal pathway. We believe that 2D SFG will become an important tool in the interfacial sciences in an analogous way that 2D IR is now being used in many disciplines.   

Ultrafast Broadband Infrared Source for Nonlinear Infrared Spectroscopy

Existing optical parametric amplifiers can generate 200cm-1 to 400 cm-1 of bandwidth at mid-infrared frequencies, which limit the use of these pulses for studying vibrational dynamics. We have developed a new broadband infrared source that can generate pulses with 1000 cm-1 of bandwidth, spanning most of the mid-infrared region of the spectrum. These pulses can allow us to simultaneously probe higher frequency vibrations like the OH stretch and the fingerprint region at lower frequencies. By focusing the first, second and third harmonics of a 25 fs 800 nm pulse in air to create a plasma, we have been able to generate infrared pulses with a broad spectrum ranging from 3 microns to 9 microns and a sub-100 fs pulse duration. This source can thus allow us to study dynamics that involve distinct vibrational transitions on ultrafast timescales. This presentation will discuss our efforts in characterizing the broadband infrared source and the progress we are making towards setting up a pump-probe 2D IR experiment using these broadband infrared pulses.   

Development of a Tunable Ultra-Broadband Mid IR Pulsed Source for Nonlinear Spectroscopy

We generate ultra- broadband mid-IR pulses tunable from 2.5 -- 8 $\mu $m by focusing 800 nm/400 nm pulses into various gas media. The input 800 nm light is doubled to 400 nm in a type I BBO crystal. The two orthogonally polarized $\omega $/2$\omega $ pulses encounter a birefringent calcite crystal for time delay compensation and are subsequently focused in various gas media (air, argon, neon and nitrogen) contained within a 1.2 m gas cell using a 1 m focal length silver mirror. The tunability of the broadband mid-IR pulses arises from different gases, pressure of gases and the amount of incident 800 nm/400 nm light focused into the gas cell at a given pressure. We measure IR energies as high as 0.5 $\mu $J/pulse for an input 800 nm energy of 3 mJ/pulse in 900 Torr of Argon. The mid IR pulses exhibit $\sim $2{\%} long term stability. The ultrabroadband IR pulses have a spectral bandwidth of $\sim $2000 cm$^{-1}$ corresponding to a sub-cycle pulse centered at 4.5 $\mu $m. We will present our preliminary efforts on using the ultrabroadband IR pulses in nonlinear experiments. The broad spectral content of this novel source affords the possibility of probing multiple vibrations in a coherent manner.   

Transient IR Probes of Spinning Molecules in an Optical Centrifuge

A high-power optical centrifuge based on ultrafast laser pulses is used to drive molecules into very high rotational states. The optical centrifuge consists of reversed-chirped laser pulses that generate a linearly polarized electric field that angularly accelerates over the time of the pulse. Molecules trapped in the optical field are spun into high rotor states through sequential Raman transitions. The properties of the spinning molecules and their spatial distribution are interrogated with high-resolution state-resolved IR transient absorption probing.   

Ultrafast Nonlinear Opto-Electronic Spectroscopy

Using a single color of light and rf beam tagging, parametric nonlinear optical mixing processes can be read not just in photons, but also in electrons. The scheme allows ultrafast time-resolved opto-electronics, which we demonstrate at a tunneling junction. Acousto-optic modulators are used to frequency tag ultrashort laser pulse trains, whereby carrier beats generated in nonlinear mixing processes are down shifted by sampling at the repetition rate of the laser (80 MHz). The method down converts optical carrier frequencies (PHz) to baseband (kHz), where direct current readout is possible with single electron sensitivity. Various parametric processes can be identified through their one-to-one map in baseband. Illustrative implementations that will be presented include the determination of the temporal profile of field-emitted ultrashort electron packets and characterization of the highly nonlinear dynamics involved in light-induced tunneling emission.   

Coherence-Modulated Third Harmonic Generation for Second Hyper-Raman Spectroscopy of Molecules at an Interface

We have developed a method of probing the low-frequency (sub-1000 cm$^{-1}$) vibrational modes of molecules at interfaces using third-harmonic generation (THG). The THG process is enhanced at an interface due to the differences in the third-order nonlinear susceptibilities of the materials. We have used this method to collect low-frequency second hyper-Raman spectra from BGO, BaF$_{2}$ and CdWO$_{4}$ crystals. In addition, we have observed coherent second hyper-Raman scattering arising from CCl$_{4}$ molecules at the liquid-glass interface. We are presently extending these techniques to observe resonant second hyper-Raman scattering from dye molecules adsorbed on gold nanoparticles in order to gain surface enhancement effects. We aim to use this method to characterize the environment at interfaces of reverse micelle systems. The development of this method is significant because we can sensitively probe the low-frequency vibrational modes of only those molecules at an interface.   

New generation ultrafast fiber-lasers with automated pulse compression for biomedical imaging

A double-clad Yb-doped all-normal-dispersion fiber laser is demonstrated to produce 22 nJ pulses at 42.5 MHz repetition rate. With a 3 nm intracavity spectral filter, self-similar evolution is formed in the gain segment. The formation of self-similar evolution allows the achievement of both short pulse duration and high pulse energy form a fiber laser oscillator. These pulses are characterized and compressed via multiphoton intrapulse interference phase scan to as short as 42 fs and 10 nJ/pulse. Adaptive compression underlies the achievement of 250-kW peak power, which enables efficient second and third harmonic generation with spectra spanning 30 nm and 20 nm, respectively. Using the newly developed fiber laser, multiphoton imaging on live tissues is demonstrated.   

Ionization and Coulomb explosion of small hydrocarbons exposed to short intense laser pulses

We have performed first principles numerical simulation of high field ionization in small hydrocarbons followed by a Coulomb explosion. The process was driven by 790 nm 27 fs laser pulses of high intensity (of the order of 10$^{15}$ W/cm$^2$), for which case there exists recent experimental data [S. Roither et al., Phys. Rev. Lett. 106, 163001 (2011)]. We have analyzed the spectra of ejected protons and investigated the ionization-fragmentation mechanism that takes place when molecules are subjected to short intense laser pulses. The results of our simulations support the all-at-once (concerted) fragmentation scenario proposed by Roither et al. At the same time we also observed some quantitative differences between the theoretical and experimental spectra.