Biomedical subjects

P B Corkum

Publications and source records attributed to P B Corkum.

At least 19 recordsLinked to original sources

PubMed2006

Generation of 11 fs pulses by using hollow-core gas-filled fibers at a 100 kHz repetition rate.

Using self-phase modulation in a hollow-core fiber filled with xenon, we were able to produce 2.3 microJ laser pulses with a duration of 10.9 fs at a repetition rate of up to 100 kHz. We started with 45 fs, 4.4 microJ, 800 nm pulses generated by a Coherent RegA Ti:sapphire regenerative amplifier system, then spectrally broadened the 30 nm bandwidth to more than 100 nm. Dispersion compensation was achieved with two pairs of chirped mirrors. This is believed to be the first time this type of compression was achieved at a repetition rate as high as 100 kHz. This brings the advantages of few-cycle laser pulses to experiments that require high-repetition-rate, low-energy laser systems, for example, coincidence experiments.

Journal Article↗

PubMed2006

Field-free three-dimensional alignment of polyatomic molecules.

We experimentally demonstrate field-free, three-dimensional alignment (FF3DA) of polyatomic asymmetric top molecules. We achieve FF3DA in sulfur dioxide gas using two time-delayed, orthogonally polarized, nonresonant, femtosecond laser pulses. Our method avoids the use of rotational revivals and is therefore more robust to temperature. The alignment is probed using time-delayed coincidence Coulomb explosion imaging. FF3DA will be important for all molecular imaging, dynamics, or spectroscopy experiments for which random alignment leads to a loss of information.

Journal Article↗

PubMed2006

Chirped attosecond photoelectron spectroscopy.

We study analytically the photoionization of a coherent superposition of electronic states and show that chirped pulses can measure attosecond time scale electron dynamics just as effectively as transform-limited attosecond pulses of the same bandwidth. The chirped pulse with a frequency-dependent phase creates the interfering photoelectron amplitudes that measure the electron dynamics. We show that at a given pump-probe time delay the differential asymmetry oscillates as a function of photoelectron energy. Our results suggest that the important parameter for attosecond science is not the pulse duration, but the bandwidth of phased radiation.

Journal Article↗

PubMed2006

Optically produced arrays of planar nanostructures inside fused silica.

Linearly polarized femtosecond light pulses, focused inside fused silica to an intensity that leads to multiphoton ionization, produce arrayed planes of modified material having their normal parallel to the laser polarization. The planes are < or = 10 nm thick and are spaced at approximately lambda/2 in the medium for free space wavelengths of both 800 and 400 nm. By slowly scanning the sample under a fixed laser focus, order is maintained over macroscopic distances for all angles between the polarization and scan direction. With the laser polarization parallel to the scan direction we produce long-range Bragg-like gratings. We discuss how local field enhancement influences dielectric ionization, describe how this leads to nanoplane growth, why the planes are arrayed, and how long-range order is maintained.

Journal Article↗

PubMed2005

Controlling attosecond double ionization dynamics via molecular alignment.

We investigate the dynamics of double ionization in aligned nitrogen molecules. An ultrashort, weak laser pulse creates an aligned ensemble of molecules that is ionized with a subsequent, strong probe pulse. We find that the two electrons involved in nonsequential double ionization more likely exit the molecule in the same direction if it is parallel to the probe laser polarization, indicating that they are ejected within a few hundred attoseconds of each other. Double ionization is less probable and takes longer for perpendicular molecules.

Journal Article↗

PubMed2005

Polarization-selective etching in femtosecond laser-assisted microfluidic channel fabrication in fused silica.

We fabricate microchannels in fused silica by femtosecond laser irradiation followed by etching in diluted hydrofluoric acid. We show a dramatic dependence of the etch rate on the laser polarization, spanning 2 orders of magnitude. We establish the existence of an energy-per-pulse threshold at which etching of the laser-modified zones becomes highly polarization selective. The enhanced selective etching is due to long-range, periodic, polarization-dependent nanostructures formed in the laser-modified material.

Journal Article↗

PubMed2005

Controlling high harmonic generation with molecular wave packets.

We show that, by controlling the alignment of molecules, we can influence the high harmonic generation process. We observed strong intensity modulation and spectral shaping of high harmonics produced with a rotational wave packet in a low-density gas of N2 or O2. In N2, where the highest occupied molecular orbital (HOMO) has sigma(g) symmetry, the maximum signal occurs when the molecules are aligned along the laser polarization while the minimum occurs when it is perpendicular. In O2, where the HOMO has pi(g) symmetry, the harmonics are enhanced when the molecules are aligned around 45 degrees to the laser polarization. The symmetry of the molecular orbital can be read by harmonics. Molecular wave packets offer a means of shaping attosecond pulses.

Journal Article↗

PubMed2005

Mapping attosecond electron wave packet motion.

Attosecond pulses are produced when an intense infrared laser pulse induces a dipole interaction between a sublaser cycle recollision electron wave packet and the remaining coherently related bound-state population. By solving the time-dependent Schrödinger equation we show that, if the recollision electron is extracted from one or more electronic states that contribute to the bound-state wave packet, then the spectrum of the attosecond pulse is modulated depending on the relative motion of the continuum and bound wave packets. When the internal electron and recollision electron wave packet counterpropagate, the radiation intensity is lower. We show that we can fully characterize the attosecond bound-state wave packet dynamics. We demonstrate that electron motion from a two-level molecule with an energy difference of 14 eV, corresponding to a period of 290 asec, can be resolved.

Journal Article↗

PubMed2005

Shakeup excitation during optical tunnel ionization.

Shakeup of a two-electron system is investigated in the strong infrared laser field limit, both theoretically and experimentally. During tunnel ionization the electron shakes up a second electron to an excited bound state. Theoretically, a complete analytical theory of shakeup in intense laser fields is developed. We predict that shakeup produces one excited sigma(u) D(+)(2) state in approximately 10(5) ionization events. Shakeup is measured experimentally by using the molecular clock provided by the internuclear motion. The number of measured events is found to be in excellent agreement with theory.

Journal Article↗

PubMed2004

Tomographic imaging of molecular orbitals.

Single-electron wavefunctions, or orbitals, are the mathematical constructs used to describe the multi-electron wavefunction of molecules. Because the highest-lying orbitals are responsible for chemical properties, they are of particular interest. To observe these orbitals change as bonds are formed and broken is to observe the essence of chemistry. Yet single orbitals are difficult to observe experimentally, and until now, this has been impossible on the timescale of chemical reactions. Here we demonstrate that the full three-dimensional structure of a single orbital can be imaged by a seemingly unlikely technique, using high harmonics generated from intense femtosecond laser pulses focused on aligned molecules. Applying this approach to a series of molecular alignments, we accomplish a tomographic reconstruction of the highest occupied molecular orbital of N2. The method also allows us to follow the attosecond dynamics of an electron wave packet.

Journal Article↗

PubMed2004

Phase control of rotational wave packets and quantum information.

Lasers can create rotational wave packets in gas-phase molecules which periodically revive as field-free, aligned distributions. We control the wave packet evolution with relatively weak laser pulses at fractional revivals which modify the phase between wave packet components. We demonstrate two phase control effects in oxygen: coherently switching revivals off and on, and doubling the revival frequency. When viewed as a quantum logic system, these effects correspond to a Hadamard and a T operation.

Journal Article↗

PubMed2004

Laser-induced interference, focusing, and diffraction of rescattering molecular photoelectrons.

We solve the time-dependent Schrödinger equation in three dimensions for H+2 in a one-cycle laser pulse of moderate intensity. We consider fixed nuclear positions and Coulomb electron-nuclear interaction potentials. We analyze the field-induced electron interference and diffraction patterns. To extract the ionization dynamics we subtract the excitations to low-lying bound states explicitly. We follow the time evolution of a well-defined wave packet that is formed near the first peak of the laser field. We observe the fragmentation of the wave packet due to molecular focusing. We show how to retrieve a diffraction molecular image by taking the ratio of the momentum distributions in the two lateral directions. The positions of the diffraction peaks are well described by the classical double slit diffraction rule.

Journal Article↗

PubMed2004

Recollision during the High Laser Intensity Ionization of C60.

Through C60, we address the role of electron recollision in the nonresonant, femtosecond laser ionization of large, highly polarizable molecules. We show how the electron trajectories are influenced by the laser field, the laser induced dipole field, and the Coulomb field of the ion core. Working at long wavelengths we observe recollision in C60 through the ellipticity dependence of the fragmentation it produces. The ionizing electron emerges from C60z+ (z = 3, 4) with a lateral velocity of approximately 12 angstroms/fs, approximately half its Fermi velocity. Despite the large lateral velocity and competing forces on the electron, recollision remains relatively probable for this scale of molecule.

Journal Article↗

PubMed2004

Stress in femtosecond-laser-written waveguides in fused silica.

We identify two states of stress induced in waveguides fabricated by femtosecond lasers in fused silica and show how they can be relieved by annealing. In-plane stress and stress concentration are revealed through birefringence and loss measurements. Another kind of laser-induced stress appears in the form of swelling of the glass surface when waveguides are written near the surface and is a manifestation of confined rapid material quenching. By annealing the sample we reduce the losses by approximately 30% (at 633 nm) and decrease the birefringence by a factor of 4 in fused silica.

Journal Article↗

PubMed2004

Fully differential rates for femtosecond multiphoton double ionization of neon.

We have investigated the full three-dimensional momentum correlation between the electrons emitted from strong field double ionization of neon when the recollision energy of the first electron is on the order of the ionization potential. The momentum correlation in the direction perpendicular to the laser field depends on the time difference of the two electrons leaving the ion. Our results are consistent with double ionization proceeding through transient double excited states that field ionize.

Journal Article↗

PubMed2004

Stopping a vibrational wave packet with laser-induced dipole forces.

Intense near-infrared laser pulses can generate laser-induced dipole forces that are strong enough to influence or control vibrational motion of a small molecule. Generally, the force acts to pull the molecule apart. Our numerical simulations show that, by applying the laser-induced dipole force at an appropriate time within one vibrational period, the wave packet motion of H+2 or D+2 can be accelerated or decelerated. Using the wave packet formed by the rapid ionization of H2 or D2, we also show that it is possible to move the vibrational population almost entirely to the v=0 state. Coherent cooling of the molecular vibrational motion can be achieved.

Journal Article↗

PubMed2003

Internal laser-induced dipole force at work in C60 molecule.

We show how the many electron response of a complex molecule to an intense laser field can be incorporated with the single active electron picture. This enables us to introduce an "over-the-barrier" model for Cz+60 ionization, valid for long wavelength light. Using infrared radiation, we confirm the model and also produce stable, highly charged C60 reaching C12+60, the highest charge state ever observed. At high intensities and high charge states the internal laser-induced dipole force and rapid charging lead to stress on the molecule. The interplay between the forces provides control and suggest strategies for reaching even higher charge states.

Journal Article↗

PubMed2003

Electron-electron momentum exchange in strong field double ionization.

We have investigated the momentum balance between the two electrons from strong field double ionization of argon at 780 nm and 1.9 x 10(14) W/cm(2). Experimental data show that perpendicular to the laser polarization direction the electrons emerge preferentially in opposite directions. Results of model calculations are found to agree well with the data and reveal a dominant role of the Coulomb correlation between the two outgoing electrons in this kinematical geometry. Differences between the experimental observations and the theoretical results for the ion momentum distribution indicate the importance of the further effects during the three-body breakup.

Journal Article↗