William P. Putnam

Pulse synthesis in the single-cycle regime from independent mode-locked lasers using attosecond-precision feedback

We report the synthesis of a nearly single-cycle (3.7 fs), ultrafast optical pulse train at 78 MHz from the coherent combination of a passively mode-locked Ti:sapphire laser (6 fs pulses) and a fiber supercontinuum (1-1.4 μm, with 8 fs pulses). The coherent combination is achieved via orthogonal, attosecond-precision synchronization of both pulse envelope timing and carrier envelope phase using balanced optical cross-correlation and balanced homodyne detection, respectively. The resulting pulse envelope, which is only 1.1 optical cycles in duration, is retrieved with two-dimensional spectral shearing interferometry (2DSI). To our knowledge, this work represents the first stable synthesis of few-cycle pulses from independent laser sources.

AXSIS: Exploring the frontiers in attosecond X-ray science, imaging and spectroscopy

X-ray crystallography is one of the main methods to determine atomic-resolution 3D images of the whole spectrum of molecules ranging from small inorganic clusters to large protein complexes consisting of hundred-thousands of atoms that constitute the macromolecular machinery of life. Life is not static, and unravelling the structure and dynamics of the most important reactions in chemistry and biology is essential to uncover their mechanism. Many of these reactions, including photosynthesis which drives our biosphere, are light induced and occur on ultrafast timescales. These have been studied with high time resolution primarily by optical spectroscopy, enabled by ultrafast laser technology, but they reduce the vast complexity of the process to a few reaction coordinates. In the AXSIS project at CFEL in Hamburg, funded by the European Research Council, we develop the new method of attosecond serial X-ray crystallography and spectroscopy, to give a full description of ultrafast processes atomically resolved in real space and on the electronic energy landscape, from co-measurement of X-ray and optical spectra, and X-ray diffraction. This technique will revolutionize our understanding of structure and function at the atomic and molecular level and thereby unravel fundamental processes in chemistry and biology like energy conversion processes. For that purpose, we develop a compact, fully coherent, THz-driven atto-second X-ray source based on coherent inverse Compton scattering off a free-electron crystal, to outrun radiation damage effects due to the necessary high X-ray irradiance required to acquire diffraction signals. This highly synergistic project starts from a completely clean slate rather than conforming to the specifications of a large free-electron laser (FEL) user facility, to optimize the entire instrumentation towards fundamental measurements of the mechanism of light absorption and excitation energy transfer. A multidisciplinary team formed by laser-, accelerator,- X-ray scientists as well as spectroscopists and biochemists optimizes X-ray pulse parameters, in tandem with sample delivery, crystal size, and advanced X-ray detectors. Ultimately, the new capability, attosecond serial X-ray crystallography and spectroscopy, will be applied to one of the most important problems in structural biology, which is to elucidate the dynamics of light reactions, electron transfer and protein structure in photosynthesis.

Generalizing higher-order Bessel-Gauss beams: analytical description and demonstration

We report on a novel class of higher-order Bessel-Gauss beams in which the well-known Bessel-Gauss beam is the fundamental mode and the azimuthally symmetric Laguerre-Gaussian beams are special cases. We find these higher-order Bessel-Gauss beams by superimposing decentered Hermite-Gaussian beams. We show analytically and experimentally that these higher-order Bessel-Gauss beams resemble higher-order eigenmodes of optical resonators consisting of aspheric mirrors. This work is relevant for the many applications of Bessel-Gauss beams in particular the more recently proposed high-intensity Bessel-Gauss enhancement cavities for strong-field physics applications.

Bessel-Gauss beam enhancement cavities for high-intensity applications

We introduce Bessel-Gauss beam enhancement cavities that may circumvent the major obstacles to more efficient cavity-enhanced high-field physics such as high-harmonic generation. The basic properties of Bessel-Gauss beams are reviewed and their transformation properties through simple optical systems (consisting of spherical and conical elements) are presented. A general Bessel-Gauss cavity design strategy is outlined, and a particular geometry, the confocal Bessel-Gauss cavity, is analyzed in detail. We numerically simulate the confocal Bessel-Gauss cavity and present an example cavity with 300 MHz repetition rate supporting an effective waist of 33 μm at the focus and an intensity ratio from the focus to the cavity mirror surfaces of 1.5 × 10(4).

Radially polarized Bessel-Gauss beams: decentered Gaussian beam analysis and experimental verification

We derive solutions for radially polarized Bessel-Gauss beams in free-space by superimposing decentered Gaussian beams with differing polarization states. We numerically show that the analytical result is applicable even for large semi-aperture angles, and we experimentally confirm the analytical expression by employing a fiber-based mode-converter.

High-intensity bessel-gauss beam enhancement cavities

An enhancement cavity design with significant intensity gain from the mirror surfaces to the focus and larger than millimeter sized apertures in the cavity mirrors is presented. A continuous-wave version of the cavity is demonstrated.

Mapping Photoemission and Hot-Electron Emission from Plasmonic Nanoantennas

Understanding plasmon-mediated electron emission and energy transfer on the nanometer length scale is critical to controlling light-matter interactions at nanoscale dimensions. In a high-resolution lithographic material, electron emission and energy transfer lead to chemical transformations. In this work, we employ such chemical transformations in two different high-resolution electron-beam lithography resists, poly(methyl methacrylate) (PMMA) and hydrogen silsesquioxane (HSQ), to map local electron emission and energy transfer with nanometer resolution from plasmonic nanoantennas excited by femtosecond laser pulses. We observe exposure of the electron-beam resists (both PMMA and HSQ) in regions on the surface of nanoantennas where the local field is significantly enhanced. Exposure in these regions is consistent with previously reported optical-field-controlled electron emission from plasmonic hotspots as well as earlier work on low-electron-energy scanning probe lithography. For HSQ, in addition to exposure in hotspots, we observe resist exposure at the centers of rod-shaped nanoantennas in addition to exposure in plasmonic hotspots. Optical field enhancement is minimized at the center of nanorods suggesting that exposure in these regions involves a different mechanism to that in plasmonic hotspots. Our simulations suggest that exposure at the center of nanorods results from the emission of hot electrons produced via plasmon decay in the nanorods. Overall, the results presented in this work provide a means to map both optical-field-controlled electron emission and hot-electron transfer from nanoparticles via chemical transformations produced locally in lithographic materials.

Laser induced annealing dynamics of photo-electron spectra from silicon field emitter arrays

A marked increase in electron yield, an overall spectral red shift, and the formation of a higher energy peak from Si field emitter arrays (FEAs) are observed in photo-electron spectra throughout a laser annealing process.

Observation and analysis of generalized higher-order Bessel-Gauss beams in optical resonators

We propose and experimentally demonstrate new beam solutions in form of azimuthally symmetric higher-order Bessel-Gauss beams. We experimentally observe these beams as higher-order eigenmodes in optical resonators consisting of aspheric mirrors.

Extending cavity-enhanced high-harmonic generation with Bessel-Gauss beams

At high optical intensities, light can modify the optical properties of media and lead to non-linear optical effects, such as harmonic generation. The invention of the laser brought a convenient means to reach such intensities. Indeed, only a year after the construction of the first laser, researchers generated second harmonics by focusing a light beam to an intensity of around 107W/cm2 in a quartz crystal.1 As optical sources have evolved, higher optical intensities have become accessible, and higher-order nonlinear optical effects have been observed and used. Above intensities of around 1013W/cm2, researchers have uncovered a distinct ‘high-intensity’ regime of nonlinear optics with promising applications. In this high-intensity regime, harmonics of the thousandth order and greater can be produced through a nonlinear optical mechanism aptly named high-harmonic generation (HHG). These high harmonics lie in the generally hard-to-reach extremeultraviolet (EUV) or soft x-ray spectral region. Currently, the predominant sources in this portion of the electromagnetic spectrum involve large and expensive synchrotrons or free-electron lasers. Future sources based on HHG might provide compact, table-top alternatives for the numerous research, medical, and industrial applications of EUV and soft x-ray light. To date, complex amplifier chains have been the most popular means for reaching the high intensities necessary for HHG. Commercial amplifiers can routinely produce milliJoule-energy laser pulses with durations of tens of femtoseconds, thereby easily achieving intensities exceeding 1013W/cm2. The downside of these systems, however, is their low repetition rate. Such amplifiers generally output several thousand or fewer laser pulses per second (around kHz repetition rates), while low-energy pulsed lasers conventionally operate in the millions Figure 1. (a) Illustration of a Gaussian beam enhancement cavity. Note that the harmonics (purple pulse) are generated co-linearly with the driving beam. (b) The intra-cavity Gaussian mode intensity on the cavity mirrors in the x-y plane. The dashed white circles indicate roughly where two of the cavity mirrors lie. (c) Illustration of a Bessel-Gauss enhancement cavity. This cavity is rotationally symmetric about the z-axis (as indicated by the red circle). Also, note that the harmonics propagate along the z-axis. (d) Intra-cavity Bessel-Gauss mode intensity on the segmented cavity mirror in the x-y plane. The dashed white circles roughly show the boundaries between the different sections of the segmented mirror.