Demonstration of electron acceleration in a laser-driven dielectric microstructure.

The enormous size and cost of current state-of-the-art accelerators based on conventional radio-frequency technology has spawned great interest in the development of new acceleration concepts that are more compact and economical. Micro-fabricated dielectric laser accelerators (DLAs) are an attractive approach, because such dielectric microstructures can support accelerating fields one to two orders of magnitude higher than can radio-frequency cavity-based accelerators. DLAs use commercial lasers as a power source, which are smaller and less expensive than the radio-frequency klystrons that power today's accelerators. In addition, DLAs are fabricated via low-cost, lithographic techniques that can be used for mass production. However, despite several DLA structures having been proposed recently, no successful demonstration of acceleration in these structures has so far been shown. Here we report high-gradient (beyond 250 MeV m(-1)) acceleration of electrons in a DLA. Relativistic (60-MeV) electrons are energy-modulated over 563 ± 104 optical periods of a fused silica grating structure, powered by a 800-nm-wavelength mode-locked Ti:sapphire laser. The observed results are in agreement with analytical models and electrodynamic simulations. By comparison, conventional modern linear accelerators operate at gradients of 10-30 MeV m(-1), and the first linear radio-frequency cavity accelerator was ten radio-frequency periods (one metre) long with a gradient of approximately 1.6 MeV m(-1) (ref. 5). Our results set the stage for the development of future multi-staged DLA devices composed of integrated on-chip systems. This would enable compact table-top accelerators on the MeV-GeV (10(6)-10(9) eV) scale for security scanners and medical therapy, university-scale X-ray light sources for biological and materials research, and portable medical imaging devices, and would substantially reduce the size and cost of a future collider on the multi-TeV (10(12) eV) scale.

Acceleration of sub-relativistic electrons with an evanescent optical wave at a planar interface.

We report on a theoretical and experimental study of the energy transfer between an optical evanescent wave, propagating in vacuum along the planar boundary of a dielectric material, and a beam of sub-relativistic electrons. The evanescent wave is excited via total internal reflection in the dielectric by an infrared (λ = 2 µm) femtosecond laser pulse. By matching the electron propagation velocity to the phase velocity of the evanescent wave, energy modulation of the electron beam is achieved. A maximum energy gain of 800 eV is observed, corresponding to the absorption of more than 1000 photons by one electron. The maximum observed acceleration gradient is 19 ± 2 MeV/m. The striking advantage of this scheme is that a structuring of the acceleration element's surface is not required, enabling the use of materials with high laser damage thresholds that are difficult to nano-structure, such as SiC, Al2O3 or CaF2.

Diode laser--pumped solid-state lasers.

Diode laser-pumped solid-state lasers are efficient, compact, all solid-state sources of coherent optical radiation. Major advances in solid-state laser technology have historically been preceded by advances in pumping technology. The helical flash lamps used to pump early ruby lasers were superseded by the linear flash lamp and arc lamp now used to pump neodymium-doped yttrium-aluminum-garnet lasers. The latest advance in pumping technology is the diode laser. Diode laser-pumped neodymium lasers have operated at greater than 10 percent electrical to optical efficiency in a single spatial mode and with linewidths of less than 10 kilohertz. The high spectral power brightness of these lasers has allowed frequency extension by harmonic generation in nonlinear crystals, which has led to green and blue sources of coherent radiation. Diode laser pumping has also been used with ions other than neodymium to produce wavelengths from 946 to 2010 nanometers. In addition, Q-switched operation with kilowatt peak powers and mode-locked operation with 10-picosecond pulse widths have been demonstrated. Progress in diode lasers and diode laser arrays promises all solid-state lasers in which the flash lamp is replaced by diode lasers for average power levels in excess of tens of watts and at a price that is competitive with flash lamp-pumped laser systems. Power levels exceeding 1 kilowatt appear possible within the next 5 years. Potential applications of diode laser-pumped solid-state lasers include coherent radar, global sensing from satellites, medical uses, micromachining, and miniature visible sources for digital optical storage.

Quantitative three-dimensional optical tomographic imaging of supersonic flows.

Three-dimensional imaging of the density of nitrogen in a supersonic expansion from a nozzle by means of beam-deflection optical tomography is described. With a very simple apparatus, images could be obtained with high absolute accuracy, high spatial resolution, and wide dynamic range.

Optical gating and streaking of free electrons with sub-optical cycle precision.

The temporal resolution of ultrafast electron diffraction and microscopy experiments is currently limited by the available experimental techniques for the generation and characterization of electron bunches with single femtosecond or attosecond durations. Here, we present proof of principle experiments of an optical gating concept for free electrons via direct time-domain visualization of the sub-optical cycle energy and transverse momentum structure imprinted on the electron beam. We demonstrate a temporal resolution of 1.2±0.3 fs. The scheme is based on the synchronous interaction between electrons and the near-field mode of a dielectric nano-grating excited by a femtosecond laser pulse with an optical period duration of 6.5 fs. The sub-optical cycle resolution demonstrated here is promising for use in laser-driven streak cameras for attosecond temporal characterization of bunched particle beams as well as time-resolved experiments with free-electron beams.

Invited article: advanced drag-free concepts for future space-based interferometers: acceleration noise performance.

Future drag-free missions for space-based experiments in gravitational physics require a Gravitational Reference Sensor with extremely demanding sensing and disturbance reduction requirements. A configuration with two cubical sensors is the current baseline for the Laser Interferometer Space Antenna (LISA) and has reached a high level of maturity. Nevertheless, several promising concepts have been proposed with potential applications beyond LISA and are currently investigated at HEPL, Stanford, and EADS Astrium, Germany. The general motivation is to exploit the possibility of achieving improved disturbance reduction, and ultimately understand how low acceleration noise can be pushed with a realistic design for future mission. In this paper, we discuss disturbance reduction requirements for LISA and beyond, describe four different payload concepts, compare expected strain sensitivities in the "low-frequency" region of the frequency spectrum, dominated by acceleration noise, and ultimately discuss advantages and disadvantages of each of those concepts in achieving disturbance reduction for space-based detectors beyond LISA.

Yb:YAG master oscillator power amplifier for remote wind sensing.

We have demonstrated key advances towards a solid-state laser amplifier at 1.03 microm for global remote wind sensing. We designed end-pumped zig-zag slab amplifiers to achieve high gain. We overcame parasitic oscillation limitations using claddings on the slab's total internal reflection (TIR) and edge surfaces to confine the pump and signal light by TIR and allow leakage of amplified spontaneous emission rays that do not meet the TIR condition. This enables e3, e5, and e8 single-, double-, and quadruple-pass small-signal amplifier gain, respectively. The stored energy density is 15.6 J/cm3, a record for a laser-diode end-pumped Yb:YAG zig-zag slab amplifier.

20 W single-mode Yb3+ -doped phosphate fiber laser.

We report the demonstration of the first, to our knowledge, cladding-pumped continuous-wave Yb(3+)-doped phosphate-glass fiber laser. Phosphate hosts are of interest because they can be much more heavily doped than silica, and because of the possibility that they may have a higher photodarkening threshold. In an 84.6 cm double-clad fiber doped with 12 wt. % of Yb(2)O(3) and laser-diode pumped at 940 nm, nearly 20 W of single-mode 1.07 microm output power was generated with 60.2 W of absorbed pump power. The measured dependence of the output power on pump power is in excellent agreement with simulations.

Visible-laser acceleration of relativistic electrons in a semi-infinite vacuum.

We demonstrate a new particle acceleration mechanism using 800 nm laser radiation to accelerate relativistic electrons in a semi-infinite vacuum. The experimental demonstration is the first of its kind and is a proof of principle for the concept of laser-driven particle acceleration in a structure loaded vacuum. We observed up to 30 keV energy modulation over a distance of 1000 lambda, corresponding to a 40 MeV/m peak gradient. The energy modulation was observed to scale linearly with the laser electric field and showed the expected laser-polarization dependence. Furthermore, as expected, laser acceleration occurred only in the presence of a boundary that limited the laser-electron interaction to a finite distance.

Beam-deflection optical tomography.

We report two-dimensional density measurements in a supersonic expansion using beam-deflection optical tomography. Quantitative, high-resolution images with absolute accuracy to 3.5% and spatial resolution to 50 microm were taken with a helium-neon laser and simple apparatus.

Beam-deflection optical tomography of a flame.

We report three-dimensional index-of-refraction measurements in a methane-air slot diffusion flame and a methane-air slot subsonic jet using beam-deflection optical tomography. The horizontal and vertical spatial resolutions are 0.26 and 0.635 mm, respectively. The peak temperature is calculated from the index-of-refraction map for the flame and compared with thermocouple temperature measurements.

Continuous-wave operation of a room-temperature, diode-laser-pumped, 946-nm Nd:YAG laser.

Single-stripe diode-laser-pumped operation of a continuous-wave 946-nm Nd:YAG laser with less than 10-mW threshold has been demonstrated. A slope efficiency of 16% near threshold was shown with a projected slope efficiency well above a threshold of 34% based on results under Rhodamine 6G dye-laser pumping. Nonlinear crystals for second-harmonic generation of this source were evaluated. KNbO(3) and periodically poled LiNbO(3) appear to be the most promising.

Compact scanning soft-x-ray microscope using a laser-produced plasma source and normal-incidence multilayer mirrors.

We have constructed a scanning soft-x-ray microscope that uses a laser-produced plasma as the soft-x-ray source and normal-incidence multilayer-coated mirrors in a Schwarzschild configuration as the focusing optics. The microscope operates at a wavelength of 14 nm, has a spatial resolution of 0.5 microm, and has a soft-x-ray photon flux through the focus of 10(4)-10(5) sec(-1) when operated with only 170 mW of average laser power. The microscope is compact; the complete system, including the laser, fits on a single optical table.

Continuous-wave mode-locked Nd:glass laser pumped by a laser diode.

We have demonstrated a diode-laser-pumped, cw, mode-locked Nd:glass laser oscillator. With a 0.5% output coupler the pump threshold for mode-locked operation was 22.9 mW. The mode-locked pulse width was shorter than 36.5 psec, which was the response time of the fast photodiode and the sampling oscilloscope. Diode-laser-pumped mode-locked operation was also extended to Nd:YAG.

Three-dimensional beam-deflection optical tomography of a supersonic jet.

We report 3-D imaging of density in a supersonic expansion using beam-deflection optical tomography. Quantitative high-resolution images with absolute accuracy of 3%, dynamic range of 500:1, and spatial resolution to within a factor of 1.7 of the diffraction limit were produced with a He-Ne laser and simple apparatus. Theory shows that the spatial frequency content of beam-deflection measurements is well suited for tomographic reconstruction. The theory for the diffraction-limited resolution for tomography is presented.

Coherent anti-Stokes Raman scattering from small volumes.

We have used coherent anti-Stokes Raman scattering spectroscopy to resolve the Raman Q-branch line [Q (2) at 2987.18 cm(-1)] of dueterium gas contained in a 60-microm-diameter glass sphere. The 60-microm spheres contained only 10(+13) molecules at standard temperature and pressure; this made it possible to study gases that in larger quantities would be too dangerous or expensive to use.

Optical tomography: experimental verification of noise theory.

Optical absorption tomography is used to map the iodine-vapor density in a plane. Two-dimensional images are obtained with 1-cm spatial resolution by using a fan-beam geometry with a 56-cm-diameter source circle. Experimental results confirm a theoretical analysis of noise in the reconstructed image, including the effects of correlated noise, position within the image, and spatial averaging.

Monolithic, unidirectional single-mode Nd:YAG ring laser.

We have built a nonplanar ring oscillator with the resonator contained entirely within a Nd:YAG crystal. When the oscillator was placed in a magnetic field, unidirectional oscillation was obtained with a pump-limited, single-axial-mode output of 163 mW.

Remote single-ended measurements of atmospheric temperature and humidity at 1.77 microm using a continuously tunable source.

Simultaneous remote measurements of temperature and humidity using a narrow-linewidth, continuously tunable, LiNbO(3) optical parametric oscillator as a transmitter source are reported. Relative measurement errors of 1.0 degrees C for temperature and better than 1% for humidity over a 45-sec averaging time are observed. The absolute accuracy is limited by the accuracy of available spectroscopic data.

Multiple-pass Raman gain cell.

The application of a multiple-pass Herriott cell to stimulated Raman scattering (SRS) is evaluated and demonstrated. The cell combines a long optical path length with periodic refocusing to enhance Raman gain. This technique is especially useful for reducing SRS threshold pump power in the IR where the Raman gain is low. This paper presents an analysis and design procedure for a multiple-pass Raman gain cell and gives experimental results for SRS in H(2) gas.