Sa Amplifier Research Articles

Versatile Femtosecond Laser Synchronization for Multiple-Timescale Transient Infrared Spectroscopy.

Several ways to electronically synchronize different types of amplified femtosecond laser systems are presented based on a single freely programmable electronics hardware: arbitrary-detuning asynchronous optical sampling (ADASOPS), as well as actively locking two femtosecond laser oscillators, albeit not necessarily to the same round-trip frequency. They allow us to rapidly probe a very wide range of timescales, from picoseconds to potentially seconds, in a single transient absorption experiment without the need to move any delay stage. Experiments become possible that address a largely unexplored aspect of many photochemical reactions, in particular in the context of photo-catalysis as well as photoactive proteins, where an initial femtosecond trigger very often initiates a long-lasting cascade of follow-up processes. The approach is very versatile and allows us to synchronize very different lasers, such as a Ti:Sa amplifier and a 100 kHz Yb-laser system. The jitter of the synchronization, and therewith the time-resolution in the transient experiment, lies in the range from 1 to 3 ps, depending on the method. For illustration, transient IR measurements of the excited state solvation and decay of a metal carbonyl complex as well as the full reaction cycle of bacteriorhodopsin are shown. The pros and cons of the various methods are discussed, with regard to the scientific question one might want to address, and also with regard to the laser systems that might be already existent in a laser lab.

Generation of Joule-level green bursts of nanosecond pulses from a DPSSL amplifier.

A new approach to generation of a burst of high-energy green pulses by placing a high-energy multi-slab Yb:YAG DPSSL amplifier and SHG crystal inside a regenerative cavity is presented. In a proof-of-concept test, stable generation of a burst of six green (515 nm) pulses, each 10 ns in duration and separated by 29.4 ns (34 MHz), with 2.0 J total energy has been demonstrated at 1 Hz from a non-optimized ring cavity design. A maximum individual green pulse energy of 580 mJ was produced from a 1.78 J circulating infrared (1030 nm) pulse (average fluence 0.9 J/cm2), corresponding to a SHG conversion efficiency of 32%. Experimental results have been compared with predicted performance from a simple model. Efficient generation of a burst of high energy green pulses offers an attractive pump source for Ti:Sa amplifiers, providing the potential to reduce the impact of amplified stimulated emission by reducing instantaneous transverse gain.

Ti:Sa Crystal Geometry Variation vs. Final Amplifiers of CPA Laser Systems Parameters

In this paper, the different Ti:Sapphire crystal configurations of the final amplifiers, depending on the Chirped Pulse Amplification laser system parameters, such as the repetition rates and pulse energy, are discussed. Restrictions placed on the final Ti:Sa amplifiers with a high repetition rate are discussed. The repetition rate of these systems is limited due to the crystal overheating, which leads to stress fracturing or significant beam distortion. The heating density threshold leading to stress fracturing was calculated and taken as the limit of the upper level of the possible pump average power. On the basis of these calculations, the highest repetition rates and corresponding thermolens focal distances were estimated for conventional crystal geometry of the most suitable thicknesses. It was demonstrated that conventional crystal shapes, such as a thin disc, can be used for systems with repetition rates below a few hundred Hz if several Joules of the output pulse energy are required. The rectangular thin crystal plate geometry was more suitable for Ti:Sa amplifiers with repetition rates above 1 kHz. Finally, the parameters of rectangular thin crystal plate Ti:Sa laser amplifier with an output energy above 3 J per pulse for a laser system with more than 100 TW pulse power and 1 kHz repetition rates are presented.

Demonstration of a side-pumped cross-seeded thin-slab pre-amplifier for high-power Ti:Sa laser systems

We demonstrate a room-temperature Ti:Sapphire (Ti:Sa) amplifier that uses a cross pump-seed geometry (cross-thin-slab) to generate 30-mJ output pulses at 0.5-kHz repetition rate, and 25 mJ at 1 kHz when pumped by 100-mJ, 515-nm pulses from a diode-pumped Yb:YAG laser. The geometry allows to maintain a crystal temperature of ∼30°C using cooling water at 10°C. The amplifier is an attractive solution for use in the first stages of amplification in high peak and high average power chirped pulse amplification laser systems.

Ti:Sa Crystals in Ultra-High Peak and Average Power Laser Systems

In this paper, Ti:Sa amplifiers with crystals of the different geometries are discussed. Benefits of using this active medium for a thin disk (TD) and slab amplifiers are evaluated numerically and tested experimentally. Thermal management for amplifiers with multi-kW average power and multi-J pulse energy has been demonstrated. The presented numerical simulations revealed the existing limitations for heat extraction in TD geometry in the sub-joule energy regime for higher repetition rate operation. Geometry conversion from TD to thin-slab (TS) and cross-thin-slab (XTS) configurations significantly increases the cooling efficiency with an acceptable crystal temperature for pump average power values up to few kW with room temperature cooling, and up to tens of kW with cryogenic cooling. The abilities to attain 0.3 J output energy and a greater than 50% extraction efficiency were demonstrated with a repetition rate exceeding 10 kHz with room temperature cooling and one order more of a repetition rate with cryogenic conditions with pulsed pumping. Direct diode pumping simulated for CW regimes demonstrated 1.4 kW output power with 34% extraction efficiency using room temperature cooling and more than 10 kW and ~40% efficiency with cryogenic cooling.

Analysis of THz generation using the tilted-pulse-front geometry in the limit of small pulse energies and beam sizes.

Optical rectification in lithium niobate using the tilted-pulse-front geometry is one of the most commonly used techniques for efficient generation of energetic single-cycle THz pulses and the details of this generation scheme are well understood for high pulse energy driving lasers, such as mJ-class, kHz-repetition rate Ti:Sa amplifier systems. However, as modern Yb-based laser systems with ever increasing repetition rate become available, other excitation regimes become relevant. In particular, the use of more moderate pulse energies (in the few µJ to multi-10 µJ regime), available nowadays by laser systems with MHz repetition rates, have never been thoroughly explored. As increasing the repetition rate of THz sources for spectroscopy becomes more relevant in the community, we present a thorough numerical analysis of this regime using a 2+1-D numerical model. Our work allows us to confirm experimental trends observed in this unusual excitation regime and shows that the conversion efficiency is naturally limited by the small pump beam sizes as a consequence of spatial walk-off between the pump and THz beams. Based on our findings, we discuss strategies to overcome the current limitations, which will pave the way for powerful THz sources approaching the watt level with multi-MHz repetition rates.

30 TW and 33 fs pulses delivered by a Ti:Sa amplifier system seeded with a frequency-doubled fiber laser.

We report a full experimental comparison study on the injection of a Ti:Sa multi-terawatt amplifier chain with a standard 15 fs Ti:Sa oscillator and 35 fs frequency-doubled fiber oscillator. The study highlights that the Ti:Sa oscillator, with high performance in terms of pulse duration and spectral width, can be replaced by the frequency-doubled fiber oscillator to seed Ti:Sa amplifier chains almost without any compromise on the output pulse duration and picosecond contrast. Finally, we demonstrate for the first time to our knowledge a 30 TW and 33 fs Ti:Sa amplifier injected by a fiber oscillator.

Development of a few TW Ti:Sa laser system at 100 Hz for proton acceleration

We report the development of a table-top high peak power Titanium:Sapphire (Ti:Sa) CPA laser working at 100 Hz capable of delivering 205 mJ, 55 fs pulses. Every amplification stage is pumped by Nd-doped solid-state lasers and fully powered by diodes. Thermal effects in the Ti:Sa amplifiers are compensated passively with optics. This system is intended to be used for proton acceleration experiments at high repetition rates.

Carrier-envelope phase stability of a polarization-encoded chirped pulse Ti:Sapphire amplifier

The scheme of polarization-encoded chirped pulse amplification (PE-CPA) reduces the gain narrowing effect in Ti:Sapphire (Ti:Sa) amplifiers by utilizing both crystal axes of Ti:Sa. Hence, the carrier-envelope phase (CEP) of a PE-CPA pulse originates from two othogonal polarization directions. The CEP stability of PE-CPA pulses was investigated for various amplification conditions by varying the pump pulse energy. The CEP stability is directly compared to the conventional CPA scheme under the same laser parameters. A quasi-common-path interferometer was realized inside a four-pass amplifier stage, ensuring exceptional geometrical path length stability and high spectral phase sensitivity. The spectral phase noise of PE-CPA pulses showed a minor increase of 4 mrad compared to the conventional scheme, while at unsaturated amplification of a net gain of 13, it revealed a CEP stability better than 63 mrad.

Broadband UV-Vis vibrational coherence spectrometer based on a hollow fiber compressor.

We describe a broadband transient absorption (TA) spectrometer devised to excite and probe, in the blue to UV range, vibrational coherence dynamics in organic molecules in condensed phase. A 800-nm Ti:Sa amplifier and a hollow fiber compressor are used to generate a 6-fs short pulse at 1 kHz. Broadband sum frequency generation with the fundamental pulse is implemented to produce a 400-nm, 8-fs Fourier limited short pulse. A UV-Vis white-light supercontinuum is implemented as a probe with intensity self-referencing to achieve a shot-noise-limited sensitivity. Rapid scanning of the pump-probe delay is shown very efficient in suppressing the noise resulting from low-frequency pump intensity fluctuations. Using either of the 800-nm or 400-nm broadband pulses as the pump for TA spectroscopy of organic molecules in solution, we resolve oscillatory signals down to the 320 nm probing wavelength with a 3200 cm-1 FWHM bandwidth. Their Fourier transformation reveals the corresponding molecular vibrational spectra. Finally, we demonstrate the use of this setup as a vibrational coherence spectrometer for the investigation of the vibrational dynamics accompanying the sub-ps C=C photoisomerization of a retinal-like molecular switch through a conical intersection.

Intense THz source based on BNA organic crystal pumped at Ti:sapphire wavelength.

We report on high-energy terahertz pulses by optical rectification (OR) in the organic crystal N-benzyl-2-methyl-4-nitroaniline (BNA) directly pumped by a conventional Ti:sapphire amplifier. The simple scheme provides an optical-to-terahertz conversion efficiency of 0.25% when pumped by collimated laser pulses with a duration of 50 fs and a central wavelength of 800 nm. The generated radiation spans frequencies between 0.2 and 3 THz. We measured the damage threshold, as well as the dependency of the conversion efficiency on the pump fluence, pump wavelength, and pulse duration.

Design of a thin disk amplifier with extraction during pumping for high peak and average power Ti:Sa systems (EDP-TD).

Combination of the scheme of extraction during pumping (EDP) and the Thin Disk (TD) technology is presented to overcome the limitations associated with thermal cooling of crystal and transverse amplified spontaneous emission in high average power laser systems based on Ti:Sa amplifiers. The optimized design of high repetition rate 1-10 PW Ti:Sapphire EDP-TD power amplifiers are discussed, including their thermal dynamic behavior.

Mechanisms of Multiphoton Bleaching of Red Fluorescent Proteins

Two-photon laser scanning microscopy (TPLSM) has several advantages over one-photon confocal microscopy, including deeper tissue penetration, higher signal-to-background ratio, and less photodamage in the out-of-focus volume. However, due to very high instantaneous light intensities in the focal volume, the probability of further, stepwise resonant photon(s) absorption increases dramatically, leading to very efficient bleaching of a probe. To deal with this challenge one has to understand the underlying mechanisms. Here we measured the power dependence of multiphoton bleaching rates of several red fluorescent proteins expressed in live E. coli cells under two-photon microscope conditions. To clarify the photophysical mechanisms, we also used much lower repetition rate (1 kHz) and different pulse durations in experiments in vitro with Ti:Sa amplifier excitation. Our experimental data supported by quantum mechanical calculations of the chromophore in protein environment are consistent with the mechanism of ultrafast (<150 fs) singlet-singlet stepwise absorption of one or two additional photons following initial simultaneous two-photon absorption. In the DsRed2 protein, the third photon absorption most probably results in an ultrafast electron transfer (ET) from the anionic chromophore to an excited Rydberg state of a nearby positive amino acid residue (e.g. K163+). The transient radical state of the chromophore tends to accept an electron from the deprotonated E215- amino acid, thus promoting the first step of the recently established decarboxylation reaction. In mFruits proteins, the third photon promotes an ET from the chromophore to the 4S Rydberg state of nearby K70+ residue with its subsequent photoionization by the fourth photon.

New routes towards even shorter attosecond soft X‐ray pulses

New routes towards even shorter attosecond soft X‐ray pulses

Compact, simple, and robust cross polarized wave generation source of few-cycle, high-contrast pulses for seeding petawatt-class laser systems

A compact and robust, dual-crystal cross polarized wave generation setup combined with a hollow waveguide filter is implemented to deliver few-cycle, high-contrast laser pulses sourced from a commercial multipass Ti:Sa amplifier. The initial 25-fs pulses with a temporal contrast of 108 are shortened to 10&#xA0;fs with an improved contrast of at least 1010. The single nonlinear stage for spectral broadening and contrast enhancement of a commercial amplifier serves as an ideal injector for petawatt-class laser systems.

Time-resolved photo-electron spectroscopy on mass-selected metal clusters using a regenerative femtosecond amplifier up to 100 Hz

A femtosecond photo-electron experiment is described which provides excellent conditions for measuring time-resolved photo-electron spectra of free, monodispersed cluster anions using repetition rates up to 100 Hz. Cluster anions are synthesized in an electric arc and subsequently cooled in a helium carrier gas. A time-of-flight spectrometer is used for mass separation of the negatively charged clusters. The kinetic energy of the photo-electrons is analyzed by a magnetic-bottle time-of-flight spectrometer, which guarantees a maximum collection efficiency. Femtosecond laser pulses are generated by a seeded regenerative Ti:Sa amplifier, which is externally pumped with the second harmonic of a diode-pumped solid-state Nd:YAG laser. A retroreflector mounted on a computer-controlled translation stage serves as a reproducible time delay of the probe pulse. The high energy output of the laser pulses (∼3 mJ) in combination with the variable repetition rate and the high stability of the amplified pulses provide excellent conditions for recording pump-probe photo-electron spectra of mass-separated cluster anions even at the fairly low ion density of pulsed plasma cluster sources. First results on the electron dynamics of the Pt3- cluster demonstrate the reliability of the whole system. Further experimental investigations will concentrate on electron-relaxation processes in transition- and noble-metal clusters as well as on the nuclear and transition-state dynamics of chemically reacted adsorbate clusters.