Session J3: AMO3: Atomic, Molecular and Optical Physics

Toward molecular switches and biochemical detectors employing adaptive femtosecond-scale laser pulses

The following topics will be discussed: (1) Cis to trans and trans to cis photoisomerization of azobenzene, with nuclear motion allowing extra electronic transitions for pulse durations $>$ about 50 fs. (2) Photoinduced ring-opening and ring-closing in a model dithienylethene. (3) Response of dipicolinic acid to femtosecond-scale laser pulses, including excited states and nuclear motion (with Yuri Rostovtsev). Our technique is semiclassical electron-radiation-ion dynamics (SERID, in which the radiation field and nuclear motion are both treated classically, and the ion cores are regarded as inert (with only the valence electrons included in the dynamics). Recall, however, that one still observes ``n photon'' and ``n-phonon'' processes in a semiclassical treatment. Also, the nuclear motion is treated correctly on reasonably short time scales (e.g., picoseconds). Although real applications (such as molecular switches and biochemical detectors) will involve adaptive techniques -- with femtosecond-scale laser pulses whose durations, photon energies, fluences, shapes, etc. are tailored for specific applications -- as well as larger systems, one needs an understanding of the rich interplay of electronic and nuclear dynamics to guide more empirical approaches. This understanding can be obtained through detailed studies of the kind reported here. This work was supported by the Robert A. Welch Foundation.   

Holographic Data Storage with a Digital Micromirror Device

A holographic data system writes bits by recording the interference between a reference beam and an object beam containing data as a diffraction grating onto a photosensitive disc. The purpose of this research is to evaluate current designs and consider improvements such as the use of a digital micromirror device (DMD) as a spatial light modulator. Other factors addressed are multiple incident angles for volume layering and improving bit contrast.   

Mid/Far-Infrared Photoetection via Second-Order Nonlinear-Susceptibility in Semiconductor Heterostructures

Photodetectors in the mid/far-infrared spectral regions have always presented a challenge stemming from the need to use narrow bandgap materials and inevitably high dark current due to thermal excitations that limit the overall performance of the detector. We propose a nonlinear infrared photodetection scheme based on coherent frequency up-conversion in coupled quantum-well heterostructures, which would permit to take advantage of superior properties of GaAs-based and InP-based materials, and at the same time utilize the well developed photodetector technology at the near-infrared and visible wavelengths. Our analysis includes specific structures and device designs, including the expected performance of such detectors. We show the possibility of single-photon detection in the mid-infrared range with high detection efficiency. We also discuss possibility of monolithically integrating up-conversion detectors with near-IR semiconductor pump lasers, which would yield a compact injection-pumped device.   

The Measurement and Simulation of Terahertz Difference Frequency Generation in Quantum Cascade Lasers

Recently the research on Terahertz (THz) source and imaging has attracted significant attention. To achieve the room-temperature operated semiconductor light source in the THz range became one of the main challenges. Quantum Cascade lasers (QCL) are the primary contenders. However, the goal of achieving room-temperature operation in THz QCLs still remains elusive. Combining optical nonlinearities with a mid-infrared QCL or a near-infrared diode laser is an alternative approach. A device integrating two QCL active cores lasing at different mid-infrared wavelengths and giant second order susceptibility for difference frequency generation (DFG) together could be a promising THz light source. In this talk, the measurement of a THz difference frequency generation QCL is presented. The laser works at two mid-infrared wavelengths, around 9um and 10um. The wavelength of the DFG signal is around 60 um. The result of simulations for the DFG spectra is also presented.   

Second harmonic generation in the near-infrared range in high conduction band offset heterostructures

It is well known that asymmetric coupled semiconductor quantum wells possess giant optical nonlinearities associated with resonant intersubband transitions. These systems attracted a lot of interest in the last several years due to their unmatched flexibility in design and possibility of integration with optoelectronic devices. At the same time, the spectral range covered by devices based on intersubband nonlinearities has been limited to mid/far-infrared wavelengths due to low conduction band offset in most popular GaAs/AlGaAs and InGaAs/AlInAs material systems. Here we analyze the potential of high conduction band offset heterostructures for efficient second harmonic generation (SHG) in the near-infrared range 1-1.6 $\mu $m. We concentrate on Ga$_{0.47}$In$_{0.53}$As/AlAs$_{0.56}$Sb$_{0.44}$ heterostructures that are lattice matched to InP. Their conduction band offset in the Gamma-valley is as high as 1.6 eV. Such quantum wells can be grown on InP substrate; they utilize superior thermal and optical qualities of InP and mature InP technology. We find the optimal asymmetric double quantum well design which maximizes the second-order nonlinearity and discuss the nonlinear conversion efficiency for various geometries.   

Broadband coherent light generation in a Raman-active crystal driven by two-color femtosecond laser pulses

We demonstrate broadband light generation by focusing two-color ultrashort laser pulses into a Raman-active crystal, lead tungstate (PbWO$_{4}$). As many as 20 Anti-Stokes and 2 Stokes fields are generated due to strong near-resonant excitation of a Raman transition. The generated spectrum extends from infrared, through the visible region, to ultraviolet, and consists of discrete spatially-separated sidebands. Our measurements confirm good mutual spatial and temporal coherence among the generated fields, and open possibilities for synthesis of subfemtosecond light waveforms.   

Hybrid CARS for Non-Invasive Blood Glucose Monitoring

We develop a spectroscopy technique that combines the advantages of both the frequency-resolved coherent anti-Stokes Raman scattering (CARS) and the time-resolved CARS. We use broadband preparation pulses to get an instantaneous coherent excitation of multiplex molecular vibration levels and subsequent optically shaped time-delayed narrowband probing pulse to detect these vibrations. This technique can suppress the nonresonant background and retrieve the molecular fingerprint signal efficiently and rapidly. We employ this technique to glucose detection, the final goal of which is accurate, non-invasive (i.e. painless) and continuous monitoring of blood glucose concentration in the Diabetes diagnosis to replace the current glucose measurement process, which requires painful fingerpricks and therefore cannot be performed more than a few times a day. We have gotten the CARS spectra of glucose aqueous solution down to 2 mM.   

Using UV Illumination to Mitigate Excess Charge on Optics in Vacuum

We have studied UV illumination techniques to remove excess surface charge from fused silica optics. We commissioned and calibrated a commercial Kelvin probe to measure the surface potential of charged optics in vacuum. Using a Xenon light source and a monochromator, we directed UV light at the sample and were able to remove the excess charge. We determined that the discharging rate scaled linearly with the intensity of the light and the charge density on the surface. By varying the wavelength of the light, we saw a peak discharge rate at 215nm in both uncoated and coated optics.