Abstract:The use of laser pulse sequences to drive the cascaded difference frequency generation of high energy, high peak-power and multi-cycle terahertz pulses in cryogenically cooled periodically poled lithium niobate is proposed. Detailed simulations considering the coupled nonlinear interaction of terahertz and optical waves show that unprecedented opticalto-terahertz energy conversion efficiencies > 5%, peak electric fields of hundred(s) of Mega volts/meter at terahertz pulse durations of hundred(s) of picoseconds can be achieved. The proposed methods are shown to circumvent laser-induced damage at Joule-level pumping by 1µm lasers to enable multi-cycle terahertz sources with pulse energies >> 10 milli-joules. Various pulse sequence formats are proposed and analyzed. Numerical calculations for periodically poled structures accounting for cascaded difference frequency generation, selfphase-modulation, cascaded second harmonic generation and laser induced damage are introduced. Unprecedented studies of the physics governing terahertz generation in this high conversion efficiency regime, limitations and practical considerations are discussed. Varying the poling period along the crystal length and further reduction of absorption is shown to lead to even higher energy conversion efficiencies >>10%. An analytic formulation valid for arbitrary pulse formats and closed-form expressions for important cases are presented. Parameters optimizing conversion efficiency in the 0.1-1 THz range, the corresponding peak electric fields, crystal lengths and terahertz pulse properties are furnished. IntroductionMulti-cycle or narrowband terahertz pulses in the frequency range of 0.1 to 1 THz have garnered interest as drivers of compact particle acceleration [1, 2] , coherent X-ray generation [3] and electron beam diagnostics. An impediment to the widespread deployment of these applications has been the inadequate development of accessible sources of narrowband terahertz radiation (hundred(s) of picoseconds (ps) pulse duration) with simultaneously high pulse energy (> 10 milli-joules (mJ)) and peak powers (> 100 Mega-Volt per meter (MV/m) peak electric fields).Among existing methods, photoconductive switches can be efficient [4] but relatively challenging to scale to high pulse energies, vacuum electronic devices [5] are limited in their frequency of operation and peak powers while free electron lasers [6] are relatively expensive.With the rapid scaling of laser pulse energies, laser driven approaches employing second order nonlinear processes such as difference frequency generation (DFG) or optical rectification (OR) are promising. However, scaling this approach to high terahertz pulse energies will require achieving high optical-to-terahertz energy conversion efficiencies (or conversion efficiency for short) as well as the development of high energy optical lasers. Here, we describe approaches to improve conversion efficiencies for multi-cycle terahertz generation, which are still relatively low at the sub-percent level. This ...

We report on an ytterbium-doped fiber chirped-pulse amplification (CPA) system delivering millijoule level pulse energy at repetition rates above 100 kHz corresponding to an average power of more than 100 W. The compressed pulses are as short as 800 fs. As the main amplifier, an 80 microm core diameter short length photonic crystal fiber is employed, which allows the generation of pulse energies up to 1.45 mJ with a B-integral as low as 7 at a stretched pulse duration of 2 ns. A stretcher-compressor unit consisting of dielectric diffraction gratings is capable of handling the average power without beam and pulse quality distortions. To our knowledge, we present the highest pulse energy ever extracted from fiber based femtosecond laser systems, and a nearly 2 orders of magnitude higher repetition rate than in previously published millijoule-level fiber CPA systems.

We derive an expression describing pre-compensation of pulse-distortion due to saturation effects in short pulse laser-amplifiers. The analytical solution determines the optimum input pulse-shape required to obtain any arbitrary target pulse-shape at the output of the saturated laser-amplifier. The relation is experimentally verified using an all-fiber amplifier chain that is seeded by a directly modulated laser-diode. The method will prove useful in applications of high power, high energy laser-amplifier systems that need particular pulse-shapes to be efficient, e.g. micromachining and scientific laser-matter-interactions.

An efficient and simple approach for converting pulsed near-IR laser radiation into visible and mid-IR light by exploiting degenerate four-wave-mixing in an endlessly single-mode, large-mode-area photonic-crystal fiber is presented. Coupling a 1 MHz, 200 ps, 8 W average power pulsed source emitting at 1064 nm into this fiber results in average powers of 3 W at 673 nm signal wavelength and of 450 mW at 2539 nm idler wavelength, respectively. The excellent pulse energy conversion efficiencies of 35% for the signal and 6% for the idler wavelength are due to the unique combination of characteristics of this type of fiber.

A highly efficient, practical approach to high-energy terahertz (THz) generation based on spectrally cascaded optical parametric amplification (THz-COPA) is introduced. The THz wave initially generated by difference frequency generation between a strong narrowband optical pump and optical seed (0.1-10% of pump energy) kick-starts a repeated or cascaded energy down-conversion of pump photons. This helps to greatly surpass the quantum-defect efficiency and results in exponential growth of THz energy over crystal length. In cryogenically cooled periodically poled lithium niobate, energy conversion efficiencies >8% for 100 ps pulses are predicted. The calculations account for cascading effects, absorption, dispersion and laser-induced damage. Due to the coupled nonlinear interaction of multiple triplets of waves, THz-COPA exhibits physics distinct from conventional three-wave mixing parametric amplifiers. This in turn governs optimal phase-matching conditions, evolution of optical spectra as well as limitations of the nonlinear process. DOIMulti-cycle or narrowband terahertz (THz) frequency sources with simultaneously high pulse energies (>10 mJ) and peak fields (>100 MV/m) in the frequency range between 0.1 and 1 THz are of great interest for compact particle acceleration [1], coherent X-ray generation [3], linear and nonlinear spectroscopy and radar applications. Such a class of THz sources has the potential to achieve considerable reduction in the cost and size of current accelerators to enable unprecedented modalities in biomedical imaging, therapy and protein structure determination [2]. Among existing THz generation methods, photoconductive switches can be efficient [3], but are challenging to scale to high pulse energies.Vacuum electronic devices such as gyrotrons [4] are limited in their frequency of operation or peak powers while free-electron lasers are expensive large-scale facilities [5].Due to the rapid increase in laser pulse energies produced by solid-state laser sources, laser-driven narrowband THz generation methods based on difference-frequency generation (DFG) are attractive. However, an issue that needs to be addressed is the realization of high optical-to-THz energy conversion efficiencies (or conversion efficiencies), particularly at high pump energies. To generate tens of millijoules (mJs) of THz energy from Joule-class lasers, conversion efficiencies >>1% are necessary.Previous work on multi-cycle THz generation demonstrated conversion efficiencies in gallium arsenide (GaAs) of 10 -4 [6,7], in gallium phosphide 10 -6 [8], and organic materials 10 -5 [9]. In lithium niobate (LN), multi-cycle THz generation by interfering chirped and delayed copies of a pulse with tilted-pulse-fronts (TPF) was demonstrated [10]. However, TPFs have limitations due to group-velocity dispersion (GVD) induced by angular dispersion [11]. Such issues were circumvented by optical rectification in cryogenically cooled periodically poled lithium niobate (PPLN) crystals, but the conversion efficiency was only 0.1 % at 0....