[show abstract][hide abstract] ABSTRACT: By focusing femtosecond pulses on the front and rear surface of a fused silica coverslip, we desorb 8-nm thick polymer films at submicron scale. To determine the role of the substrate in the desorption process, we measure the threshold for nonlinear absorption in fused silica and compare it to the threshold for desorption, taking into account the enhancement of the field at the dielectric-air interface. The results indicate that absorption of energy only occurs in the film. We then measure the beam radius in situ by the knife-edge technique and characterize the desorption by atomic force microscopy. The radius of the laser desorbed area is determined by the desorption threshold intensity and can be a factor of 5 smaller than the beam waist.
[show abstract][hide abstract] ABSTRACT: We present high harmonic spectra of xenon obtained with a 1.8 μm, 2 cycle laser source. These spectra contain features due to collective multi-electron effects involving inner shell electrons, in particular the giant resonance at 100eV.
[show abstract][hide abstract] ABSTRACT: We report sub-mJ carrier envelope phase (CEP) stable 1.6 cycle pulses at 1.8micron. With those pulses, we have obtained 160eV cut-off in argon at an intensity of 1.4×1014W/cm2 using the process of high harmonic generation.
[show abstract][hide abstract] ABSTRACT: We use high-harmonic spectroscopy to observe the evolution of the electronic structure of molecules undergoing ultrafast photochemical reactions. Using the transient grating technique allows us to determine amplitude and phase of the excited state emission.
[show abstract][hide abstract] ABSTRACT: Shortening of attosecond pulse duration utilizing high harmonic generation (HHG) requires access to few cycle pulses in the infrared spectral range because the cut-off shifts towards higher photon energies proportional to the square of the driving field wavelength. Furthermore, the ability of performing time-resolved molecular orbital tomography of polyatomic molecules will benefit from longer wavelengths compared to 800 nm because of their low ionization potential. A simple scheme for generating 0.4 mJ 11.5 fs pulses at 1.8 mum is presented. Optical parametric amplified pulses were spectrally broadened in a hollow-core fiber and subsequently compressed by utilizing linear propagation through bulk material. The physical origin of the pulse compression scheme will be confirmed with numerical simulations of nonlinear propagation in the hollow-core fiber. Finally, high harmonic generation of noble gas atoms will be reported.
[show abstract][hide abstract] ABSTRACT: Using the unique characteristics of multiphoton ionization with focused femtosecond pulses, we report on a pump and probe metrology to analyze carrier dynamics inside dielectrics. We characterize the sub-picosecond trapping of carriers inside fused SiO2.
[show abstract][hide abstract] ABSTRACT: We show that non-resonant multiphoton ionization of dielectric crystals depends on the alignment of the laser field to the crystal lattice. Through absorption measurements we probe the local symmetry non-invasively, anywhere inside the sample.
[show abstract][hide abstract] ABSTRACT: Focused femtosecond laser light can produce grating structures consisting of self-assembled planar nanocracks inside fused silica. These arrayed nanocracks provide a unique capability for rewritable optical storage and fabricating microchannels and porous capillaries.
[show abstract][hide abstract] ABSTRACT: With the proper choice of laser parameters focused femtosecond laser light creates long-range self-assembled planar nanocracks inside and on the surface of fused silica glass. The orientation of the crack planes is normal to the laser polarization direction and can be precisely controlled. The arrays of cracks when properly oriented and combined with chemical etching produce high aspect ratio micro- and nanofluidic channels. Direct femtosecond laser writing without any chemical etching can be used to fabricate embedded nanoporous capillaries in bulk fused silica for biofiltering and electrophoresis applications. The morphology of the porous structures critically depends on the laser polarization and pulse energy and can be used to control the transmission rates of fluids through the capillaries. Finally high aspect ratio, polarization-dependent, self-ordered periodic nanoslots can be fabricated from nanocracks produced on the surface of fused silica wafers. Control of the surface slot width from 10 to 60 nm is achieved through selective chemical etching. This technique, which may be useful for Surface Enhanced Raman Scattering (SERS) applications, has sub-diffraction limited resolution and features high throughput writing over centimeters.
[show abstract][hide abstract] ABSTRACT: We study analytically the possibility of monitoring electron motion in a molecule using two ultrashort pulses .The first pump pulse prepares a coherent superposition of two electronic molecular states whereas the second (attosecond) probe photoionizes the molecule. We show that direct information about electron dynamics on an attosecond time scale can be obtained from the photoelectron spectra as a function of time delay between the two pulses. In particular asymmetries in photoelectron angukar distributions provide a simple signature of the electron motion within the initial time-dependent coherently coupled two electron molecular states.It is further shown that ``chirped'' pulses can measure attosecond time scale electron dynamics just as effectively as ``transform-limited'' attosecond pulses of the same bandwith .Chirped pulses with frequency dependent phases create interferences in the photionization spectrum which are absent from transform-limited pulses. These interferences contain important information about the chirp rate and the attosecond electron dynamics.  G Yudin,S Chelkowski,A D Bandrauk, P B Corkum,Phys Rev A 72,051401(2005)  G Yudin,A D Bandrauk,P B Corkum, Phys Rev Lett (2006)
[show abstract][hide abstract] ABSTRACT: An ultrashort laser pulse can free an electron from a molecule and drive it back to the molecule. This recollision electron can image the electron dynamics and structure of molecules with Angstrom and attosecond precision.
[show abstract][hide abstract] ABSTRACT: Polarization dependent periodic nanostuctures consisting of long-range arrayed planes of modification are created in fused silica through femtosecond laser irradiation. The periodicity is revealed by ultrahigh spatial resolution chemical etching combined with atomic force microscopy.
[show abstract][hide abstract] ABSTRACT: We study theoretically the ionization and dissociation of muonic molecular ions (e.g., dd mu) in superintense laser fields. We predict that the bond breaks by tunneling of the lightest ion through a bond-softened barrier at intensity I > or =10(21) W/cm(2). Ionization of the muonic atomic fragment occurs at much higher intensity I > or =6 x 10(22) W/cm(2). Since the field controls the ion trajectory after dissociation, it forces recollision of a approximately 10(5)-10(6) eV ion with the muonic atom. Recollision can trigger a nuclear reaction with sub-laser-cycle precision. In general, molecules can serve as precursors for laser control of nuclear processes.
[show abstract][hide abstract] ABSTRACT: Applying an intense laser pulse whose duration is shorter than one vibrational period at the appropriate time, we show that a vibrational wave packet can be accelerated, decelerated, or dissociated.
[show abstract][hide abstract] ABSTRACT: We have generated high harmonics from aligned molecules. The harmonics show a strong dependence on molecular alignment and symmetry of the electron orbital, which enables us to probe the dynamics of electronic structures.
Annals of Physical and Rehabilitation Medicine. 05/2004;
[show abstract][hide abstract] ABSTRACT: We demonstrate that using intense femtosecond laser pulses to optically time ion flight can lead to a miniature time-of-flight mass spectrometer. After laser ionization, the molecular ion is accelerated by a static electric field and detected using a second, delayed laser pulse. The relative positions of the two laser foci determine the ion flight distance while the time separation of the laser pulses fixes the ion flight time. We mass-resolve CS(2) or C(6)H(6) isotopes after a flight distance of 360 microm using either double ionization or Coulomb explosion detection.
[show abstract][hide abstract] ABSTRACT: We identify a laser configuration in which attosecond electron wave packets are ionized, accelerated to multi-MeV energies, and refocused onto their parent ion. Magnetic focusing of the electron wave packet results in return currents comparable with large scale accelerator facilities. This technique opens an avenue towards imaging attosecond dynamics of nuclear processes.
[show abstract][hide abstract] ABSTRACT: We present a theoretical analysis of how intense, few-cycle infrared laser pulses can be used to image the structure of small molecules with nearly 1 fs temporal and sub-Å spatial resolution. We identify and analyse several physical mechanisms responsible for the distortions of the diffraction image and describe a recipe for recovering an un-distorted image from angle and energy-resolved electron spectra. We also identify holographic patterns in the photoelectron spectra and discuss the requirements to enhancing the hologram resolution for imaging the scattering potential.
Journal of Physics B Atomic Molecular and Optical Physics 01/2004; 37. · 2.03 Impact Factor