Measurement of four-photon absorption in GaP and ZnTe semiconductors.

Intensity-dependent effective four-photon absorption (4PA) coefficients in GaP and ZnTe semiconductors were measured by the z-scan method using pump pulses of 1.75 µm wavelength, 135 fs duration, and up to 500 GWcm-2 intensity. A nonlinear pulse propagation model, including linear dispersion and 4PA was used to obtain the 4PA coefficients from measurements. The intensity-dependent effective 4PA coefficients vary from 2.6 × 10-4 to 65 × 10-4 cm5GW-3 in GaP, and from 3.5 × 10-4 to 9.1 × 10-4 cm5GW-3 in ZnTe. The anisotropy in 4PA was shown in GaP. The knowledge of 4PA coefficients is important for the design of semiconductor photonics devices.

110-mJ 225-fs cryogenically cooled Yb:CaF2 multipass amplifier.

We report on a diode-pumped cryogenically cooled bulk Yb:CaF2 12-pass amplifier delivering 110-mJ, 1030-nm pulses at a 50-Hz repetition rate. The pulses have a spectral bandwidth of 13 nm and are compressed to 225 fs pulse duration in a double reflection grating based compressor having a transmission efficiency of >90%. The measured output beam quality is M2<1.1. A key feature of the amplifier design is the 4f relay imaging onto the gain medium with progressive beam magnification for the mitigation of the spatial gain narrowing effect. The number of passes in the amplifier is scalable by increasing the size of imaging mirrors. In order to prevent accumulation of nonlinear phase due to self-phase modulation in air, the amplifier is enclosed into a low-vacuum case.

High-energy terahertz pulses from semiconductors pumped beyond the three-photon absorption edge.

A new route to efficient generation of THz pulses with high-energy was demonstrated using semiconductor materials pumped at an infrared wavelength sufficiently long to suppress both two- and three-photon absorption and associated free-carrier absorption at THz frequencies. For pumping beyond the three-photon absorption edge, the THz generation efficiency for optical rectification of femtosecond laser pulses with tilted intensity front in ZnTe was shown to increase 3.5 times, as compared to pumping below the absorption edge. The four-photon absorption coefficient of ZnTe was estimated to be ß4=(4±1)×10-5 cm5/GW3. THz pulses with 14 µJ energy were generated with as high as 0.7% efficiency in ZnTe pumped at 1.7 µm. It is shown that scaling the THz pulse energy to the mJ level by increasing the pump spot size and pump pulse energy is feasible.

800-fs, 330-µJ pulses from a 100-W regenerative Yb:YAG thin-disk amplifier at 300 kHz and THz generation in LiNbO3.

Yb:YAG thin-disk lasers offer extraordinary output power, but systems delivering femtosecond pulses at a repetition rate of hundreds of kilohertz are scarce, even though this regime is ideal for ultrafast electron diffraction, coincidence imaging, attosecond science, and terahertz (THz) spectroscopy. Here we describe a regenerative Yb:YAG amplifier based on thin-disk technology, producing 800-fs pulses at a repetition rate adjustable between 50 and 400 kHz. The key design elements are a short regenerative cavity and fast-switching Pockels cell. The average output power is 130 W before the compressor and 100 W after compression, which at 300 kHz corresponds to pulse energies of 430 and 330 µJ, respectively. This is sufficient for a wide range of nonlinear conversions and broadening/compression schemes. As a first application, we use optical rectification in LiNbO3 to produce 30-nJ single-cycle THz pulses with 6 W pump power. The electric field exceeds 10 kV/cm at a central frequency of 0.3 THz, suitable for driving structural dynamics or controlling electron beams.

Efficient generation of THz pulses with 0.4 mJ energy.

Efficient generation of THz pulses with high energy was demonstrated by optical rectification of 785-fs laser pulses in lithium niobate using tilted-pulse-front pumping. The enhancement of conversion efficiency by a factor of 2.4 to 2.7 was demonstrated up to 186 µJ THz energy by cryogenic cooling of the generating crystal and using up to 18.5 mJ/cm2 pump fluence. Generation of THz pulses with more than 0.4 mJ energy and 0.77% efficiency was demonstrated even at room temperature by increasing the pump fluence to 186 mJ/cm2. The spectral peak is at about 0.2 THz, suitable for charged-particle manipulation.

Proposal for carrier-envelope-phase stable single-cycle attosecond pulse generation in the extreme-ultraviolet range.

A robust method for producing half-cycle-few-cycle attosecond pulses in the extreme ultraviolet spectral range is proposed. It is based on coherent undulator radiation of relativistic ultrathin electron layers (nanobunches), which are produced by nanobunching of ultrashort electron bunches by a 10-TW power laser in a modulator undulator. Our numerical calculations predict the generation of nanobunches shorter than 10 nm. By using these electron nanobunches the production of carrier-envelope-phase stable attosecond pulses with up to a few tens of nJ energy and down to 10 nm wavelength and 35 as duration is predicted.

Pump pulse width and temperature effects in lithium niobate for efficient THz generation.

We present a study on THz generation in lithium niobate pumped by a powerful and versatile Yb:CaF(2) laser. The unique laser system delivers transform-limited pulses of variable duration (0.38-0.65 ps) with pulse energies up to 15 mJ and center wavelength of 1030 nm. From previous theoretical investigations, it is expected that such laser parameters are ideally suited for efficient THz generation. Here, we present experimental results on both the conversion efficiency and the THz spectral shape for variable pump pulse durations and for different crystal temperatures, down to 25 K. We experimentally verify the optimum pump parameters for the most efficient and broadband THz generation.

Generation of sub-mJ terahertz pulses by optical rectification.

Recent theoretical calculations predicted an order-of-magnitude increase in the efficiency of terahertz pulse generation by optical rectification in lithium niobate when 500 fs long pump pulses are used, rather than the commonly used ~100 fs pulses. Even by using longer than optimal pump pulses of 1.3 ps duration, 2.5× higher THz pulse energy (125 µJ) was measured with 2.5× higher pump-to-THz energy conversion efficiency (0.25%) than reported previously with shorter pulses. These results verify the advantage of longer pump pulses and support the expectation that mJ-level THz pulses will be available by cooling the crystal and using large pumped area.

Design of high-energy terahertz sources based on optical rectification.

Detailed analysis of the tilted-pulse-front pumping scheme used for ultrashort THz pulse generation by optical rectification of femtosecond laser pulses is presented. It is shown that imaging errors in a pulse-front-tilting setup consisting of a grating and a lens can lead to a THz beam with strongly asymmetric intensity profile and strong divergence, thereby limiting applications. Optimized setup parameters are given to reduce such distortions. We also show that semiconductors can offer a promising alternative to LiNbO(3) in high-energy THz pulse generation when pumped at longer wavelengths. This requires tilted-pulse-front pumping, however the small tilt angles allow semiconductors to be easily used in such schemes. Semiconductors can be advantageous for generating THz pulses with high spectral intensity at higher THz frequencies, while LiNbO(3) is better suited to generate THz pulses with very large relative spectral width. By using optimized schemes the upscaling of the energy of ultrashort THz pulses is foreseen.