Paul Arpin

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.

Bright, Coherent, Ultrafast Soft X-Ray Harmonics Spanning the Water Window from a Tabletop Light Source

We demonstrate fully phase-matched high harmonic emission spanning the water window spectral region important for nano- and bioimaging and a breadth of materials and molecular dynamics studies. We also generate the broadest bright coherent bandwidth (≈300  eV) to date from any light source, small or large, that is consistent with a single subfemtosecond burst. The harmonic photon flux at 0.5 keV is 10³ higher than demonstrated previously. This work extends bright, spatially coherent, attosecond pulses into the soft x-ray region for the first time.

Quasi-phase matching of high-order harmonic generation at high photon energies using counterpropagating pulses

We extend all-optical quasi-phase matching of high-order harmonic generation into spectral regions where conventional phase matching is not possible. The high laser intensities required to generate harmonics at energy >130 eV, coupled with the resulting high level of ionization, preclude conventional phase matching in all nonlinear media. Selective enhancement factors between 40 and 150 in the flux of harmonics at photon energies around 140 eV are demonstrated using a train of two counterpropagating pulses.

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.

Carrier-envelope stabilization of high-average-power ultrafast laser amplifier systems

We report results from thorough characterization of a high average-power, cryogenically cooled, carrier-envelope phase (CEP) stabilized ultrafast laser system. We also discuss the effect of signal averaging on measured RMS noise in CEP.

All-optical quasi-phase matching and quantum path selection of high-order harmonic generation at 140 eV using counterpropagating light

We extend all-optical quasi-phase matching of high harmonic generation to 140-150 eV, where conventional phase matching is not possible. We also demonstrate, and present a model for, selective enhancement of a single quantum trajectory.