Generation of radially polarized terahertz pulses via velocity-mismatched optical rectification

We demonstrate the generation of radially polarized terahertz pulses via optical rectification in a Cherenkov geometry exploiting velocity mismatch, contrary to the traditional approach for generating linearly polarized terahertz beams. A compact system is implemented using 001-cut ZnTe pumped by an ultrafast Yb-doped fiber amplifier.     

Generation of high average power, 7.5-fs blue pulses at 5 kHz by adaptive phase control

The spectral width of a 5-kHz Ti:sapphire laser system was broadened by spectral control in a regenerative amplifier consisting of broadband chirped mirrors. The dispersion over the wide spectral range was compensated by a deformable mirror along with a genetic algorithm, resulting in a pulse width of 15 fs. The pulse width is the shortest, to our knowledge, in chirped pulse amplification systems with a regenerative amplifier. The phase distortion of broadband frequency doubling in addition to the Ti:sapphire laser was compensated by using the self-diffraction intensity in sapphire as the feedback signal into the genetic algorithm, resulting in a pulse width of 7.5 fs. The average power of the second harmonic was 1 W with a fundamental input of 7 W.     

Generation of subpicosecond electrical pulses by optical rectification

We describe the generation of subpicosecond electrical pulses by optical rectification of ultrashort optical pulses. The electrical pulses are generated by the second-order nonlinear response of a LiTaO(3) crystal bonded to a coplanar transmission line. A bipolar temporal waveform with a width of 875 fs was measured after a propagation distance of 175mum . This pulse width was limited by the response time of the photoconductive sampler. We observed both broadening and amplitude reduction in the temporal waveform owing to propagation.     

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.     

Generation of 25 ps pulses by self induced mode locking of a single broad area diode laser with 300 mW average output power

Detailed investigations of the spatiotemporal and spectral emission properties of a high power diode laser are presented. The AR coated laser diode with design wavelength of 940 nm is driven in an external resonator. The laser generates up to 340 mW average output power in a train of picosecond pulses with durations of 25 ps and repetition rates of 2.6 GHz. The mechanism of mode locking is discussed as self pulsation because of the strong correlation between round trip time and repetition rate. The double-sided exponential pulses suggest saturable absorber action.     

Picosecond laser amplification system with 93 W high average power

A high average power picosecond laser amplification system with diode-end-pumped Nd:YVO₄ and diode-side-pumped Nd:YAG is described. Laser with power up to 92.7 W, repetition frequency of 73.3 MHz, pulse duration of 26.5 ps, and beam quality of M² < 3.5 is generated in the amplification system. Thermal-birefringence-induced depolarization in the Nd:YAG rod laser head amplifier is measured to be 21.9 W though birefringence compensation is performed.     

41 mW high average power picosecond 177.3 nm laser by second-harmonic generation in KBBF

We report on the generation of high average power, high repetition rate, and picosecond (ps) deep-ultraviolet (DUV) 177.3 nm laser. The DUV laser is produced by second-harmonic generation of a frequency-tripled mode-locked Nd: YVO₄ laser (<15 ps, 80 MHz) with KBBF nonlinear crystal. The influence of different fundamental beam diameters on DUV output power and KBBF-SHG conversion efficiency are investigated. Under the 355 nm pump power of 7.5 W with beam diameter of 145 μm, 41 mW DUV output at 177.3 nm is obtained. To our knowledge, this is the highest average power for the 177.3 nm laser. Our results provide a power scaling by three times with respect to previous best works.     

Toward generation of μJ range sub-ps THz pulses by optical rectification

Recently the possibility of scaling up the energy of sub-ps THz pulses generated in lithium-niobate by tilted pulse front excitation was demonstrated. Using 500 μJ energy pump pulses at 780 nm center wavelength, we achieved THz pulses with energy up to 240 nJ. In this article, results of calculations using a simple model predict the possibility of increasing the THz pulse energy above 1 μJ and the quantum efficiency up to 50% by decreasing the temperature. The dependence of the THz pulse energy and the maximum achievable electric field on the crystal length and the pump pulse duration is also presented. According to the calculations, generation of the maximum THz energy needs a specific pump pulse duration, because of increasing dispersion and absorption with increasing frequency. Not only longer, but also shorter pulses lead to a degradation of the THz energy. Results of calculations for GaSe, GaP and ZnTe are also presented. PACS 07.57.Hm; 42.65.Ky; 42.79.Nv     

Generation of nanosecond terahertz pulses by the optical rectification method

The possibility of the generation of quasi-cw terahertz radiation by the optical rectification method for broad-band Fourier unlimited nanosecond laser pulses has been experimentally demonstrated. The broadband radiation of a LiF dye-center laser is used as a pump source of a nonlinear optical oscillator. The energy efficiency of terahertz optical frequency conversion in a periodically polarized lithium niobate crystal is 4 × 10⁻⁹ at a pump power density of 7 MW/cm².     

Performance of combined 〈1 0 0〉-〈1 1 0〉 ZnTe crystals in an amplified THz time-domain spectrometer

In this work we report on the performance of combined index-matched ZnTe 〈1 1 0〉-〈1 0 0〉 THz emitters and detectors for reduction of etalon effects in a nonlinear crystal-based THz time-domain spectrometer, driven by an amplified femtosecond laser. We demonstrate that the application of a combined electrooptic detector crystal reduces the strength of the first etalon and the average spectral dynamic range oscillations by a factor of two. We also show that even in a simple emitter configuration the emitter-related etalon is significantly reduced due to the transient dielectric effects of carriers created by two-photon absorption in the emitter.     

Tunable THz pulse generation by optical rectification of ultrashort laser pulses with tilted pulse fronts

Optical rectification of ultrashort near-IR laser pulses with tilted pulse fronts and pulse energies of a few J in Mg-doped stoichiometric LiNbO₃ cooled to low temperature is a powerful technique for efficient generation of THz pulses. The pulse energy critically depends on the Mg doping (necessary for preventing photorefractive damage) and can be easily increased by a factor of three if the MgO content is reduced. Pulse energies up to 400 pJ at repetition rates of 200 kHz and 3.4% quantum conversion efficiency are achieved at 77 K. At 10 K, changing the tilt angle of the pump pulse front results in continuous tuning of the frequency across the 1.0–4.4 THz range. The temporal pulse shapes measured by electro-optic sampling are in good agreement with the signal calculated by a simple theory. This model predicts tunability on a considerably broader range and narrower spectra even at room temperature if GaSe is used instead of LiNbO₃. The advantages of the velocity matching technique utilizing tilted pulse fronts are analyzed in comparison with quasi-phase-matching in periodically poled LiNbO₃ crystals. The first method provides a ten times higher pulse energy conversion efficiency. PACS 42.65.Ky; 42.70.Mp; 42.72.Ai     

Acetylene spectra near 2.6 THz

We present high precision frequency measurements of acetylene in the 2.5-2.7 THz range obtained with our most recent multiplier technology.     

High resolution analysis of the rotational levels of the (0 0 0), (0 1 0), (1 0 0), (0 0 1), (0 2 0), (1 1 0) and (0 1 1) vibrational states of SO2

A high resolution (0.0018 cm⁻¹) Fourier transform instrument has been used to record the spectrum of an enriched ³⁴S (95.3%) sample of sulfur dioxide. A thorough analysis of the ν₂, 2ν₂ − ν₂, ν₁, ν₁ + ν₂ − ν₂, ν₃, ν₂ + ν₃ − ν₂, ν₁ + ν₂ and ν₂ + ν₃ bands has been carried out leading to a large set of assigned lines. From these lines ground state combination differences were obtained and fit together with the existing microwave, millimeter, and terahertz rotational lines. An improved set of ground state rotational constants were obtained. Next, the upper state rotational levels were fit. For the (0 1 0), (1 1 0) and (0 1 1) states, a simple Watson-type Hamiltonian sufficed. However, it was necessary to include explicitly interacting terms in the Hamiltonian matrix in order to fit the rotational levels of the (0 2 0), (1 0 0) and (1 0 1) states to within their experimental accuracy. More explicitly, it was necessary to use a ΔK = 2 term to model the Fermi interaction between the (0 2 0) and (1 0 0) levels and a ΔK = 3 term to model the Coriolis interaction between the (1 0 0) and (0 0 1) levels. Precise Hamiltonian constants were derived for the (0 0 0), (0 1 0), (1 0 0), (0 0 1), (0 2 0), (1 1 0) and (0 1 1) vibrational states.     

GaN nucleation on (0 0 0 1)-sapphire via ion-induced nitridation of gallium

The growth of epitaxial GaN films on (0 0 0 1)-sapphire has been investigated using X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED). In order to investigate the mechanism of the growth in detail, we have focused on the nitridation of pre-deposited Ga layers (droplets) using ion beam-assisted molecular beam epitaxy (IBA-MBE). Comparative analysis of XPS core-level spectra and LEED patterns reveals, that nitride films nucleate as epitaxial GaN islands. The wetting of the surface by GaN proceeds via reactive spreading of metallic Ga, supplied from the droplets. The discussed growth model confirms, that excess of metallic Ga is beneficial for GaN nucleation.     

THz generation via optical rectification with ultrashort laser pulse focused to a line

We report on efficient THz pulse generation via optical rectification with femtosecond laser pulses focused to a line by a cylindrical lens. This configuration provides phase-matched conditions in the superluminal regime. 35 pJ THz pulses have been generated with this technique in a stoichiometric LiNbO₃ crystal pumped by 2 μJ femtosecond laser pulses at room temperature. An unusual superquadratic rise of the THz pulse energy with the laser pulse energy has been observed at high laser energies. This extraordinary energy dependence of the THz generation efficiency is explained by self-focusing of the laser beam in the crystal. Z-scan measurements and comparison of the THz pulse spectra created with laser pulses having different energies confirm this interpretation.     

Generation of 1.5-12 ns width-tunable 532 nm pulses by adopting laser-induced plasma shutter technique

The pulse-shaping technique has found widespread applications in nonlinear optics and material processing. Experimental research on laser-induced plasma shutter to control the 532 nm pulse width is conducted. The impacts of the total pulse output energy on pulse compression are investigated, and a useful conclusion can be drawn that there exists an optimal value of pulse energy at which the shortest output pulse of 3.23 ns can be obtained without a device for delay-time. Once the device for delay-time is employed to change the optical differences between two laser paths, the pulse width can be further shortened to 1.51 ns. In short, the 1.5-12 ns width-tunable 532 nm laser pulses have been obtained by adopting the laser-induced plasma shutter technique.     

Growth and morphology of the epitaxial Fe(1 1 0)/MgO(1 1 1)/Fe(1 1 0) Trilayers

Growth and surface morphology of epitaxial Fe(1 1 0)/MgO(1 1 1)/Fe(1 1 0) trilayers constituting a magnetic tunnel junction were investigated by low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). STM reveals a grain-like growth mode of MgO on Fe(1 1 0) resulting in dense MgO(1 1 1) films at room temperature as well as at 250 °C. As observed by STM, initial deposition of MgO leads to a partial oxidation of the Fe(1 1 0) surface which is confirmed by Auger electron spectroscopy. The top Fe layer deposited on MgO(1 1 1) at room temperature is relatively rough consisting of clusters which can be transformed by annealing to an atomically flat epitaxial Fe(1 1 0) film.     

Fourfold in-plane magnetic anisotropy in epitaxial Fe(1 1 0)/Cu(0 0 1) and Fe(1 1 0)/Ni(0 0 1) bilayers

Epitaxial Fe(1 1 0) films with thicknesses of 100-800 nm on Cu(0 0 1) and Ni(0 0 1) buffer layers grown on MgO(0 0 1) substrates have been fabricated. These films contain Fe(1 1 0) crystallites which are in the Pitsch orientation relationship. Magnetization and the fourfold in-plane magnetic anisotropy constants of these films have been determined by torque measurements. All the samples under study are characterized by a fourfold magnetic anisotropy with easy axes parallel to the [1 0 0] and [0 1 0] directions of Cu(0 0 1) and Ni(0 0 1) layers. The measured values of the constant for Fe(1 1 0)/Cu(0 0 1) are found to depend on deposition temperature; a maximum value of (2.5±0.1)×10⁵ erg/cm³ is reached after annealing at 600 °С. The in-plane torque measurements on Fe(1 1 0)/Ni(0 0 1) bilayers obtained at 300 °С, on the other hand, exhibit a constant value of (2.7±0.1)×10⁵ erg/cm³. Assuming an exchange interaction between the Fe(1 1 0) crystallites, which are in the Pitsch orientation relationship, the fourfold in-plane magnetic anisotropy has been calculated as 2.8×10⁵ erg/cm³. The deviations of the experimental values from the predicted one may be explained by the formation of a polycrystalline phase within the Fe(1 1 0) layer and a partial disorientation of the epitaxial crystallites.     

High-flux high harmonic soft X-ray generation up to 10 kHz repetition rate

We report the operation of a Ti:sapphire oscillator-amplifier system with a high, variable repetition rate adjustable between 1 and 15 kHz. The oscillator uses cavity dumping and the multipass amplifier is based on a liquid nitrogen cooled crystal. The system produces pulses with 28 fs duration at 1.1 mJ pulse energy. When pumping the amplifier crystal with 72 W, an average output power of 11 W is obtained at a repetition rate of 10 kHz, resulting in a quantum efficiency of 25%. The output pulses are used to generate high harmonic radiation in argon, neon, and helium, which are detected up to a photon energy of 110 eV, limited by the sensitivity of the toroidal grating employed.     

Generation of long bursts of high-repetition-rate arbitrarily shaped optical pulses using time lens

A technique for the generation of long ultrahigh-speed bursts of optical pulses with arbitrary shapes is proposed. A laser pulse is temporally chirped by a time lens and then passes through a filter with a reconfigurable periodic spectral response, which produces time-delayed replicas of the chirped pulse and recombines them. As a result of the temporal interference between the replicas, the chirped pulse is broken up into short pulses with the shape determined by the chosen filter response. It is demonstrated that the filter acts on a long chirped optical pulse as a temporal modulator with a periodic modulation function. The modulation frequency and bandwidth of the modulator can be much higher than for commercially available high-frequency modulators. The additional advantage of this modulator is the arbitrary shape of the modulation function. A 2.4 ns burst of nearly flat-top pulses with a repetition rate of about 400 GHz is obtained in numerical simulations. In addition, the technique proposed can act as a pulse repetition rate multiplier and a pulse compressor. A repetition rate of 1.589 THz and an individual pulse width of 212 fs are achieved in simulations for a 9.7 ns sinusoidally phase modulated pulse burst.