Joel A. Berger

High-power, femtosecond, thermal-lens-shaped Yb:KGW oscillator

Thermal lens shaping for astigmatism compensation is extended to a high-power, diode-pumped, Yb:KGW laser by employing a gain crystal geometry designed for efficient polarized pumping. The 63MHz oscillator is soliton mode-locked with the aid of a saturable Bragg reflector to yield 250fs (347fs) pulses at an output power of 3.5W (5W). Frequency doubling of the 250fs pulses with an intrinsic efficiency >60% provides 1.65W of average green power.

DC photoelectron gun parameters for ultrafast electron microscopy.

We present a characterization of the performance of an ultrashort laser pulse driven DC photoelectron gun based on the thermionic emission gun design of Togawa et al. [Togawa, K., Shintake, T., Inagaki, T., Onoe, K. & Tanaka, T. (2007). Phys Rev Spec Top-AC 10, 020703]. The gun design intrinsically provides adequate optical access and accommodates the generation of approximately 1 mm2 electron beams while contributing negligible divergent effects at the anode aperture. Both single-photon (with up to 20,000 electrons/pulse) and two-photon photoemission are observed from Ta and Cu(100) photocathodes driven by the harmonics (approximately 4 ps pulses at 261 nm and approximately 200 fs pulses at 532 nm, respectively) of a high-power femtosecond Yb:KGW laser. The results, including the dependence of the photoemission efficiency on the polarization state of the drive laser radiation, are consistent with expectations. The implications of these observations and other physical limitations for the development of a dynamic transmission electron microscope with sub-1 nm.ps space-time resolution are discussed.

Semianalytic model of electron pulse propagation: Magnetic lenses and rf pulse compression cavities

The analytical Gaussian electron pulse propagation model of Michalik and Sipe [J. Appl. Phys. 99, 054908 (2006)] is extended to include the action of external forces on the pulse. The resultant ability to simulate efficiently the effect of electron optical elements (e.g., magnetic lenses and radio-frequency cavities) allows for the rapid assessment of electron pulse delivery systems in time-resolved ultrafast electron diffraction and microscopy experiments.

Excited-state thermionic emission in III-antimonides: Low emittance ultrafast photocathodes

The normalized rms transverse emittance of an electron source is shown to be proportional to m*, where m* is the effective mass of the state from which the electron is emitted, by direct observation of the transverse momentum distribution for excited-state thermionic emission from two III-V semiconductor photocathodes, GaSb and InSb, together with a control experiment employing two-photon emission from gold. Simulations of the experiment using an extended analytical Gaussian model of electron pulse propagation are in close agreement with the data.

Thermal effects in semiconductor saturable-absorber mirrors

A semianalytical evaluation of laser-beam-induced thermal deformation of III–V semiconductor-based saturable Bragg reflectors is presented. The thermally induced bowing and resulting optical aberrations for different incident laser spot sizes and saturable Bragg reflector dimensions are calculated. Also discussed are the effects of the major physical and optical SESAM and laser parameters.

High-power, femtosecond, thermal-lens-shaped Yb:KGW Laser

Thermal lens shaping for astigmatism compensation is extended to a 65 MHz, diode-pumped Yb:KGW oscillator mode-locked with the aid of a saturable Bragg reflector to yield 250fs (347fs) pulses at an output power of 3.5 W (5W).