Samuel E. Schrauth

Bright coherent ultrahigh harmonics in the keV x-ray regime from mid-infrared femtosecond lasers.

From Long to Short When you play a string instrument, you raise the frequency, or pitch, of the note by shortening the vibrating portion of the string: Drop the length in half, and you hear a harmonic at double the frequency. It is possible to do essentially the same thing with light waves by using selective excitation and relaxation processes of the electrons in crystals or high-pressure gases through which the beam of light is directed to produce light harmonics. Over the past decade, researchers have been optimizing the conversion of red light to the far edge of the ultraviolet, which corresponds to tens of harmonics. Popmintchev et al. (p. 1287) now show that mid-infrared light can undergo a process in high-pressure gas to generate ultrahigh harmonics up to orders greater than 5000 in the x-ray regime. An electron excitation process in a high-pressure gas converts infrared light into a well-confined beam of x-rays. High-harmonic generation (HHG) traditionally combines ~100 near-infrared laser photons to generate bright, phase-matched, extreme ultraviolet beams when the emission from many atoms adds constructively. Here, we show that by guiding a mid-infrared femtosecond laser in a high-pressure gas, ultrahigh harmonics can be generated, up to orders greater than 5000, that emerge as a bright supercontinuum that spans the entire electromagnetic spectrum from the ultraviolet to more than 1.6 kilo–electron volts, allowing, in principle, the generation of pulses as short as 2.5 attoseconds. The multiatmosphere gas pressures required for bright, phase-matched emission also support laser beam self-confinement, further enhancing the x-ray yield. Finally, the x-ray beam exhibits high spatial coherence, even though at high gas density the recolliding electrons responsible for HHG encounter other atoms during the emission process.

Filamentation in air with ultrashort mid-infrared pulses

We theoretically investigate filamentation of ultrashort laser pulses in air in the mid-infrared regime under conditions in which the group-velocity dispersion (GVD) is anomalous. When a high-power, ultra-short mid-infrared laser beam centered at 3.1-μm forms a filament, a spatial solitary wave is stabilized by the plasma formation and propagates several times its diffraction length. Compared with temporal self-compression in gases due to plasma formation and pulse splitting in the normal-GVD regime, the minimum achievable pulse duration (∼70 fs) is limited by the bandwidth of the anomalous-GVD region in air. For the relatively high powers, multiple pulse splitting due to the plasma effect and shock formation is observed, which is similar to that which occurs in solids. Our simulations show that the energy reservoir also plays a critical role for longer propagation of the air filament in the anomalous-GVD regime.

Highly-efficient coupling of linearly- and radially-polarized femtosecond pulses in hollow-core photonic band-gap fibers

We demonstrate extremely efficient excitation of linearly-, radially-, and azimuthally-polarized modes in a hollow-core photonic band-gap fiber with femtosecond laser pulses. We achieve coupling efficiencies as high as 98% with linearly polarized input Gaussian beams and with high-power pulses we obtain peak intensities greater than 10(14) W/cm(2) inside and transmitted through the fiber. With radially polarized pulses, we achieve 91% total transmission through the fiber while maintaining the polarization state. Alternatively with azimuthally-polarized pulses, the mode is degraded in the fiber, and the pure polarization state is not maintained.

Pulse splitting in the anomalous group-velocity-dispersion regime

We investigate experimentally the role that the initial temporal profile of ultrashort laser pulses has on the self-focusing dynamics in the anomalous group-velocity dispersion (GVD) regime. We observe that pulse-splitting occurs for super-Gaussian pulses, but not for Gaussian pulses. The splitting does not occur for either pulse shape when the GVD is near-zero. These observations agree with predictions based on the nonlinear Schrödinger equation, and can be understood intuitively using the method of nonlinear geometrical optics.

Dynamics of elliptical beams in the anomalous group-velocity dispersion regime

We investigate 3D spatio-temporal focusing of elliptically-shaped beams in a bulk medium with Kerr nonlinearity and anomalous group-velocity dispersion (GVD). Strong space-time localization of the mode is observed through multi-filamentation with temporal compression by a factor of 3. This behavior is in contrast to the near-zero GVD regime in which minimal pulse temporal compression is observed. Our theoretical simulations qualitatively reproduce the experimental results showing the highly localized spatio-temporal profile in the anomalous-GVD regime, which contrasts to the weakly localized pulse in the normal-GVD regime.

Efficient excitation of polarization vortices in a photonic bandgap fiber with ultrashort laser pulses

We experimentally investigate the excitation of radially and azimuthally polarized modes in a hollow-core photonic bandgap fiber. With radially-polarized ultrashort pulses, we achieve 91% total transmission through the fiber, including coupling losses.

Picosecond ionization dynamics in femtosecond filaments at high pressures

We observe a 3-fold increase in the electron density within 30 picoseconds after the filamentary propagation of femtosecond pulses in 60-bar argon gases. This suggests that electron-impact ionization dominates on this time scale.

Frontiers in extreme nonlinear optics: Attosecond-to-zeptosecond coherent kiloelectronvolt X-rays on a tabletop

Summary form only given. The past three years in a row marked the 50th anniversaries of three significant innovations in optics: the invention of the laser; the discovery of the nonlinear upconversion of laser light in a spectral region where laser light has not been available; and the outlining of phase matching of this upconversion process - a recipe that makes the newly generated laser-like light bright and usable for applications. The same revolution that made it possible to create well directed beams in the visible region of the spectrum is only now happening for X-rays. Large-scale X-ray free electron lasers are promising to capture images of ultrafast dynamics in a single shot. An extreme version of nonlinear optics - high harmonic generation (HHG) - can also generate bright, coherent, beams of X-rays, with very short wavelengths <;7.7 angstroms, in a tabletop-scale setup for the first time [1]. This practically realizes a coherent version of the Roentgen X-ray tube in the soft X-ray region. Improved understanding of the microscopic quantum physics and macroscopic nonlinear optics of high harmonic generation [2-5], as well as the development of novel ultrafast mid-IR lasers [6] have lead to this rapid progress in the past few years, essentially solving the phase matching problem of HHG in the X-ray region. In addition, these kiloelectronvolt HHG X-rays have a supercontinuum structure with the broadest coherent bandwidth (>1.3 keV) that any light source, large or small scale, can generate to date. Such an ultrabroad spectral bandwidth can support X-ray pulses as short as 2.5 attoseconds and is scalable towards zeptosecond pulse durations. These unique, ultrafast, laser-like X-ray beams promise revolutionary new capabilities for understanding and controlling how the nanoworld works on its fundamental time and length scales. This understanding is relevant to the next generation data and energy storage devices, nano-electronics, bioimaging, and future medical diagnostics.

Ultrafast keV X-rays from Tabletop Femtosecond Lasers

X-rays show elemental and chemical specificity by using characteristic elemental X-ray absorption edges. These advantages spurred development of large-scale X-ray free-electron lasers based on accelerator physics, as well as high harmonic generation (HHG) driven by tabletop femtosecond lasers.

Loss of phase in the interaction of two collapsing beams

When two self-focusing beams with the fixed-initial-phase interact, we observe fluctuations in output mode profiles as predicted by simulations. We attribute these fluctuations to the loss of relative phase as the beams collapse.