10 W CEP-stable few-cycle source at 2 µm with 100 kHz repetition rate.

We developed a high repetition rate optical parametric chirped-pulse amplification (OPCPA) laser system based on fiber-laser-seeded Innoslab to generate few-cycle pulses around 2 µm with passively stable carrier-envelope phase (CEP) by difference frequency generation (DFG). Incorporating a piezo mirror before the DFG stage permits rapid CEP control. The OPCPA system is seeded by a stable supercontinuum generated in bulk material with the picosecond Innoslab pulses. Few-cycle pulses with durations of 17 fs and energies of over 100 µJ were produced in a single OPCPA stage. Three different nonlinear crystals: BBO, BiBO, and LNB were tested in the final parametric amplifier, and their average power related limitations are addressed.

Discrete dispersion scanning as a simple method for broadband femtosecond pulse characterization.

A simple and easy to implement technique for femtosecond pulse characterization is proposed and experimentally verified. It is based on the introduction of a known amount of dispersion (by controlling the number of passes through dispersive material) and subsequent recording of the spectral positions of second harmonic peaks obtained in a non-linear crystal. Such dependence allows for direct retrieval of the pulse spectral phase. The presented pulse characterization method is beneficial especially for broadband pulses, where the second harmonic spectrum exceeds the detection bandwidth of a single spectrometer.

LED based NMR illumination device for mechanistic studies on photochemical reactions--versatile and simple, yet surprisingly powerful.

An LED based illumination device for mechanistic studies on photochemical reactions by means of NMR spectroscopy is presented. The LEDs are directly switched by the NMR spectrometer with the help of a one-stage electronic circuit. This allows for continuous or alternatively pulsed operation of the LEDs. Continuous operation provides direct comparability with conditions in synthetic chemistry, in pulsed operation the short time light power can be enhanced ninefold. The LEDs are efficiently coupled to a 1000 µm core optical fiber guiding the light into the spectrometer by simply bringing it in close contact to the fiber. The tip of the fiber is roughened by sandblasting and thus emits light in a uniform and efficient way over the full length of the receiver coil. The combination of these techniques tremendously increases the amount of light brought into the NMR sample and makes LEDs an easy, versatile and handy light source for the in situ illumination of NMR samples allowing even for single millisecond time resolved Photo-CIDNP spectroscopy.

Approaching the full octave: noncollinear optical parametric chirped pulse amplification with two-color pumping.

We present a new method to broaden the amplification range in optical parametric amplification toward the bandwidth needed for single cycle femtosecond pulses. Two-color pumping of independent stages is used to sequentially amplify the long and short wavelength parts of the ultrabroadband seed pulses. The concept is tested in two related experiments. With multi-mJ pumping pulses with a nearly octave spanning spectrum and an uncompressed energy of 3 mJ are generated at low repetition rate. The spectral phase varies slowly and continuously in the overlap region as shown with 100 kHz repetition rate. This should allow the compression to the Fourier limit of below 5 fs in the high energy system.

Sub-20 fs pulses shaped directly in the UV by an acousto-optic programmable dispersive filter.

Direct pulse shaping in the UV was used to compress and structure pulses throughout the range of 250 - 400 nm. Broadband pulses generated by SHG of a NOPA were used as input to an acousto-optic programmable dispersive filter. As this shaper creates lateral dispersion, aspects of Gaussian and geometric optics had to be considered for the design of the beam path. Special care was taken to produce a homogeneous input beam. We show nearly Fourier-limited pulses as short as 16.8 fs at 320 nm and 19.5 fs at 260 nm. Full control over amplitude and phase is demonstrated by generating arbitrary shapes like square pulses and complex pulse sequences. The subpulses were manipulated individually in intensity, temporal delay, chirp, relative phase and central wavelength.

Octave wide tunable UV-pumped NOPA: pulses down to 20 fs at 0.5 MHz repetition rate.

Femtosecond laser pulses, which are tunable from 440 to 990 nm, are generated at MHz repetition rates by noncollinear parametric amplification (NOPA). The pulses have durations of 20 to 30 fs over the major part of the tuning range and a high energy stability of 1.3% (rms). The NOPA is pumped with ultraviolet pulses from the third harmonic of an ytterbium doped fiber laser system and seeded by a smooth continuum generated in bulk sapphire. The residual second harmonic is used to pump an additional NOPA, which is independently tunable from 620 to 990 nm. Interference experiments show that the two NOPA systems have a precisely locked relative phase, despite of being pumped by different harmonics with a random phase jitter. This demonstrates that the phase of pulses generated by optical parametric amplification does not depend on the pump phase.

Terahertz radiation from bacteriorhodopsin reveals correlated primary electron and proton transfer processes.

The kinetics of electrogenic events associated with the different steps of the light-induced proton pump of bacteriorhodopsin is well studied in a wide range of time scales by direct electric methods. However, the investigation of the fundamental primary charge translocation phenomena taking place in the functional energy conversion process of this protein, and in other biomolecular assemblies using light energy, has remained experimentally unfeasible because of the lack of proper detection technique operating in the 0.1- to 20-THz region. Here, we show that extending the concept of the familiar Hertzian dipole emission into the extreme spatial and temporal range of intramolecular polarization processes provides an alternative way to study ultrafast electrogenic events on naturally ordered biological systems. Applying a relatively simple experimental arrangement based on this idea, we were able to observe light-induced coherent terahertz radiation from bacteriorhodopsin with femtosecond time resolution. The detected terahertz signal was analyzed by numerical simulation in the framework of different models for the elementary polarization processes. It was found that the principal component of the terahertz emission can be well described by excited-state intramolecular electron transfer within the retinal chromophore. An additional slower process is attributed to the earliest phase of the proton pump, probably occurring by the redistribution of a H bond near the retinal. The correlated electron and proton translocation supports the concept, assigning a functional role to the light-induced sudden polarization in retinal proteins.

Ultrasensitive ultraviolet-visible 20 fs absorption spectroscopy of low vapor pressure molecules in the gas phase.

We describe an ultrasensitive pump-probe spectrometer for transient absorption measurements in the gas phase and in solution. The tunable UV pump and the visible (450-740 nm) probe pulses are generated by two independently tunable noncollinear optical parametric amplifiers, providing a temporal resolution of 20 fs. A homebuilt low gain photodetector is used to accommodate strong probe pulses with a shot noise significantly lower than the overall measurement noise. A matched digitizing scheme for single shot analysis of the light pulses at kilohertz repetition rates that minimizes the electronic noise contributions to the transient absorption signal is developed. The data processing scheme is optimized to yield best suppression of the laser excess noise and thereby transient absorbance changes down to 1.1 x 10(-6) can be resolved. A collinear focusing geometry optimized for a 50 mm interaction length combined with a heatable gas cell allows us to perform measurements on substances with low vapor pressures, e.g., on medium sized molecules which are crystalline at room temperature. As an application example highlighting the capability of this instrument, we present the direct time-domain observation of the ultrafast excited state intramolecular proton transfer of 2-(2(')-hydroxyphenyl)benzothiazole in the gas phase. We are able to compare the resulting dynamics in the gas phase and in solution with a temporal precision of better than 5 fs.

Sub-20 fs visible pulses with 750 nJ energy from a 100 kHz noncollinear optical parametric amplifier.

We demonstrate the operation of a 100 kHz noncollinear optical parametric amplifier that is pumped by just a few microjoules of 800 nm pulses with 50 fs duration. The device delivers sub-20 fs pulses tunable from 460 nm to beyond 1 microm and pulse energies up to 750 nJ when it is pumped with 7 microJ of energy. The design of the single-stage amplifier has been carefully optimized, and the design considerations are discussed.

19 fs shaped ultraviolet pulses.

We report the generation of shaped tunable ultrashort ultraviolet pulses with full control over the spectral phase and amplitude. The output of a noncollinearly phase-matched optical parametric amplifier is shaped in phase and amplitude by a liquid-crystal spatial light modulator. The resulting structured visible pulses are transferred into the ultraviolet by sum-frequency mixing with strongly chirped 775 nm pulses. Single, double, and triple pulses at 344 nm with subpulses as short as 19 fs are explicitly demonstrated. The method can easily be adapted to arbitrarily shaped pulses throughout the 295-370 nm range.

Scaling up the energy of THz pulses created by optical rectification.

The possibility for up-scaling the energy of sub-ps THz pulses generated by tilted pulse front excitation is demonstrated. Using 150-fs-long 500 muJ optical pump pulses at 800 nm up to 240 nJ THz pulse energy has been achieved. For a 1.2 mm2 pump spot area, the energy conversion efficiency of pump energy to THz pulse energy had a maximum of 5 x 10-4 at 300 muJ pump pulse energy. The corresponding photon conversion efficiency amounts to 10 %. For comparison, the maximum attainable THz pulse energy was limited to 3.1 nJ if a line focusing excitation geometry was utilized. This limit was reached at 32 muJ pump energy. For the latter configuration the THz energy dropped for larger pump energies. The tilted pulse front excitation allows further up-scaling of the THz pulse energy by using a larger pump spot size and still stronger pump pulses.

Real time observation of the photo-Fries rearrangement.

The photo-Fries rearrangement of 4-tert-butylphenyl acetate dissolved in cyclohexane is investigated by two-color femtosecond pump probe spectroscopy. The spectral transmission changes are characterized in the visible and ultraviolet spectral region and allow for the first time to temporally resolve the primary reaction steps. We find that the photoinduced homolytic cleavage of the CO bond occurs within 2 ps and that the geminate recombination of the generated radical pair to the intermediate substituted cyclohexadienone takes 13 ps. The experimental results support a model in which the initial reaction proceeds from the originally excited pipi(*) state via a barrier to a dissociative pisigma(*) state.

12-fs pulses from a continuous-wave-pumped 200-nJ Ti:sapphire amplifier at a variable repetition rate as high as 4 MHz.

We demonstrate a novel compact femtosecond Ti:sapphire laser system operating at repetition rates from 10 kHz to 4 MHz. The scheme is based on the combination of a broadband cavity-dumped oscillator and a double-pass Ti:sapphire amplifier pumped by a low-noise cw solid-state laser. Amplified pulses with an extremely smooth spectrum, a duration of only 12 fs, and less than 0.25% rms fluctuation are generated in a beam with M2 < 1.2. A maximum pulse energy of 210 nJ and an average output power of as much as 720 mW are achieved. This output energy is sufficient to generate a stable continuum in a sapphire disk.

Phase-coherent generation of tunable visible femtosecond pulses.

Stable interference between the outputs of two noncollinearly phase-matched optical parametric amplifiers seeded by separate white-light continua has been observed. This means that the tunable visible pulses have a well-defined relative phase and that the temporal jitter between them is less than 1 fs. The residual phase variations are due to fluctuations of the pump power.

20-50-fs pulses tunable across the near infrared from a blue-pumped noncollinear parametric amplifier.

A two-stage blue-pumped noncollinearly phase matched optical parametric amplifier was used to generate near-infrared pulses that were continuously tunable from 865 to 1600 nm. The pulse lengths scaled from 20 fs at the shorter wavelengths to below 50 fs at 1600 nm, with a nearly Fourier-transform-limited bandwidth. From 200 muJ of 775-nm pump light at a 1-kHz repetition rate and a 130-fs duration, 7-2.5-muJ pulse energies were generated, corresponding to a typical quantum efficiency of 25% from blue to near-infrared light.

Sub-20-fs pulses tunable across the visible from a blue-pumped single-pass noncollinear parametric converter.

Femtosecond pulses with center wavelengths between 470 and 750 nm are generated in a single-stage type I BBO optical parametric amplifier pumped by a frequency-doubled 1-kHz Ti:sapphire amplifier. A high-quality white-light continuum is used as the seed. Pulse durations as short as 16 fs and pulse energies of as much as 11 microJ are observed. The quantum efficiency is ~25% for both 7- and 40-microJ pump pulses. This unique combination of ultrashort pulse duration and high conversion is made possible by noncollinear phase matching that permits a sufficiently large amplification bandwidth. Simultaneously the group velocities of the signal and the idler are effectively matched. As a result widely tunable sub-20-fs pulses can be generated in a nonlinear crystal as thick as 2 mm.

Generation of 16-fs pulses at 425 nm by extracavity frequency doubling of a mode-locked Ti:sapphire laser.

An efficient doubling scheme capable of producing 16-fs pulses centered at 425 nm with an average power of 40 mW is described. The system uses 15-fs pulses from a continuous-wave mode-locked Ti:sapphire oscillator centered at 850 nm. The pulse characteristics resulting from doubling with beta-barium borate crystals of various lengths are presented. The results compare favorably with previous attempts at intracavity doubling and provide a more convenient route to femtosecond experiments with fully synchronized second-harmonic radiation.