J. P. Roberts

Scaling of terahertz radiation from large-aperture biased photoconductors

We present a theoretical and experimental investigation into the generation of subpicosecond pulses of terahertz radiation from large-aperture biased photoconductors with 1.5-eV photon excitation. A model that describes the far-field radiation from the optically excited, biased photoconductor is developed. The peak of the radiated electric field as well as waveforms are presented as a function of optical excitation fluence and pulse width. The dependence of the terahertz radiation from biased InP and GaAs emitters on the applied bias field and on incident optical fluence for bias fields as high as 12 kV/cm and for optical fluences of 0.01–1.0 mJ/cm2 is presented. For a given level of optical excitation the radiated electric field is predicted by theory to scale linearly with the applied bias field. This prediction is verified experimentally, with radiated-field strengths as high as 1.23 ± 0.13 kV/cm being demonstrated. The radiated electric field also exhibits the monotonic saturation behavior predicted by theory, and saturation fluences of 0.058 ± 0.015 and 0.018 ± 0.008 mJ/cm2 are obtained for InP and GaAs emitters, respectively.

Two-photon absorption at 248 nm in ultraviolet window materials.

Two-photon-absorption coefficients are measured for fused silica, LiF, and the alkali earth halides BaF(2), SrF(2), CaF(2), and MgF(2), using 0.7-psec, 248-nm pulses with intensities in the range (0.5-30) x 10(10) W/cm (2) . The conse-quences of these measurements in the design of high-brightness excimer lasers are discussed.

High-irradiance 248-nm laser system

A laser system that routinely generates irradiances of 8 x 10(17) W cm(-2) is described. Pulses from a mode-locked visible dye laser are heterodyned to 248 nm and amplified in two KrF amplifiers. Output is 25-30 mJ in a 700-fsec pulse that can be focused to 5 microm(2).

High-intensity subpicosecond XeCl laser system.

A terawatt-class laser, based on the amplification of subpicosecond pulses in XeCl discharge amplifiers, is described. The system generates 250-mJ, 335-fsec pulses at a sustained 1-Hz repetition rate. Using f/3.7 optics, focal-spot dimensions of 3.4 microm x 4.1 microm are measured for the fully amplified output beam, thus demonstrating a mean focalvolume intensity of 4.6 x 10(18) W/cm(2). With f/l optics, this system is therefore capable of producing a focused intensity of 6.4 x 10(19) W/cm(2).

Picosecond gain dynamics in KrF amplifiers

The steady‐state and dynamic gain characteristics in a KrF amplifier are studied with the use of 1 ps pulses. A saturation energy density in the range 1.8–2.0±0.2 mJ/cm2 is measured. The recovery of the small‐signal gain coefficient exhibits two recovery processes with time constants of 67±10 ps and 2.0±0.2 ns.

Ultrashort-pulse energy-extraction measurements in XeCl amplifiers

Energy extraction in XeCl discharge amplifiers is examined experimentally and theoretically for 0.16-, 3.0-, and 600-psec pulse durations. Saturation energies of 1.0, 1.0, and 2.5 mJ/cm2, respectively, are measured. When nonsaturable loss is included in the analysis the values of the saturation energies increase to 1.35, 1.35, and 3.0 mJ/cm2, respectively. A significant decrease in the effective small-signal gain is observed as the pulse duration decreases to the bandwidth limit, in agreement with a simple coherent pulse-propagation calculation.

Synchronously pumped subpicosecond dye oscillator amplifier system

A tunable subpicosecond dye oscillator-amplifier system that produces 0.20-mJ, 190-fsec pulses at 620 nm in a nearly diffraction-limited beam is demonstrated. The output pulse is temporally broadened by 10% relative to the input pulse. The system consists of a synchronously pumped subpicosecond dye laser followed by a dye amplifier that is quasi-longitudinally pumped using the frequency-doubled output from a Nd:YAG regenerative amplifier.

Generation of 39-fs pulses at 815 nm with a synchronously pumped mode-locked dye laser.

We describe a synchronously pumped mode-locked dye laser that uses a novel combination of saturable absorber dyes (HITC-I and DTP) to yield satellite-free, 39-fs pulses at 815 nm.

Efficient frequency-doubled long-pulse generation with a Nd:glass/Nd:YAG oscillator–amplifier system

A laser system based on a Nd:glass oscillator followed by a series of Nd:YAG amplifiers is described. The two-beam energy output is 2.2 J in 85 ns at 1.06 microm. The output is efficiently frequency doubled in KD*P to yield 1.04-J, 110-ns, 0.532-microm pulses at a 5-Hz repetition rate.

The absorption characteristics and recovery dynamics of the tricarbocyanine dye DTP

Abstract The absorption recovery characteristics of the tricarbocyanine dye DTP dissolved in propylene carbonate and ethylene glycol are described. DTP has a peak absorption cross section of 1×10 −15 cm 2 at 824 nm. The induced absorption recovers to 50% of its initial value in 31±10 ps.