Generation of intense highly coherent femtosecond pulses in the mid infrared

Long seed, short pump: converting Yb-doped laser radiation to multi-µJ few-cycle pulses tunable through 2.5-15 µm

We present a setup for generating broadband (up to 1050 cm^-1) and broadly tunable (2.5-15 µm) mid-infrared pulses using an Yb-doped femtosecond laser as the pump source. Our scheme, comprising two parametric amplifiers and a mixing stage, exploits favorable group velocity matching conditions in GaSe pumped at 2 µm to directly produce sub-70 fs pulses throughout the tuning range without any additional dispersion compensation, while 30-50 fs pulse durations are achieved with simple dispersion compensation by propagation through thin bulk media. The generated pulses have sub-1% short- and long-term energy noise, as well as stable spectral parameters, while delivering 0.5-2 W average mid-IR power. We expect the source to be useful for various spectroscopic applications in the mid-IR.

Mid-infrared pulsed nanosecond difference frequency generation of oxide LGN crystal up to 5.7 µm

We demonstrate the tunable difference frequency generation (DFG) of an oxide La₃Ga_5.5Nb_0.5O₁₄ (LGN) crystal pumped by near-infrared lasers with nanosecond pulses for the first time to our knowledge. The type I and II phase-matching conditions of DFG were calculated in the mid-infrared region. With the processed LGN crystals, tunable lasers in the wavelength range from 4.4 to 5.7 µm and 4.56 to 5.6 µm were achieved under type II and I phase-matching conditions, respectively, with the maximum output energy of 13.1 µJ, which agreed well with the theoretical calculation. This work provides the kind of promising mid-infrared nonlinear crystals for the pumping of nanosecond pulsed lasers as well as a tunable mid-infrared laser source at a wavelength over 5 µm in further photonic applications.

Mid-IR femtosecond pulse generation on the microjoule level up to 5 μm at high repetition rates

We show efficient generation of mid-IR pulses tunable between 1 and 5 μm from 100 kHz class femtosecond systems. The concept can be applied to various sources, particularly based on Ti:sapphire and the newly evolving Yb+ lasers. The mid-IR pulses are generated as the idler of a collinear optical parametric amplifier pumped by the laser fundamental. The seed for this amplifier is the idler of a previous amplification stage pumped with the second harmonic and seeded with a visible continuum. This enhances the energy and allows us to influence the bandwidth of the final output. Pulses with microjoule energy and Fourier limits of 50 fs are achieved.

Time-resolved observation of the Eigen cation in liquid water

Experimental observation and time relaxation measurement of the hydrated proton Eigen form [H(3)O(+)(H(2)O)(3)] are presented here. Vibrational time-resolved spectroscopy is used with an original method of investigating the proton excess in water. The anharmonicity of the time-resolved spectra is characteristic of the Eigen-type proton geometry. Proton relaxation occurs in less than 200 fs. A calculation of the potential energy confirms the experimental result and the Eigen cation lifetime is in good agreement with previous molecular dynamics simulations.

Libration induced stretching mode excitation for pump-probe spectroscopy in pure liquid water

We developed an experimental approach to study pure liquid water in the infrared and avoid thermal effects. This technique is based on libration induced stretching excitation of water molecules. A direct correspondence between frequencies within the libration and OH stretching bands is demonstrated. Energy diffusion is studied in pure liquid water by measuring wave packet dynamics of OH stretching vibrator with infrared femtosecond spectroscopy. Wave packet dynamics reveals ultrafast energy dynamics and reflects 130 fs intermolecular energy transfer between water vibrators. Energy diffusion is almost two orders of magnitude faster than self diffusion in water.

Generation of mid-infrared pulses by X(3) difference frequency generation in CaF(2) and BaF(2)

Tunable mid-IR pulses in the range 1300-4200 cm(-1) (7.7-2.4 microm) are generated through a phase-matched four-wave mixing process in ordinary mid-IR window materials such as CaF(2) and BaF(2) . In this process the difference frequency v(3)=2v(2)-v(1) is generated from pump fields v(1) and v(2) . The process can be phase matched to different frequencies by adjustment of the angle between the pump fields.

Noise suppression in femtosecond mid-infrared light sources

An experimental and theoretical study of intrinsic correlations and noise-suppression mechanisms in two-stage femtosecond mid-IR light sources is presented. The setup, based on parametric amplification in BBO and subsequent difference-frequency mixing in AgGaS(2), delivers approximately 100-fs mid-IR pulses with 1-2-muJ energy. Exceptionally low pulse-energy fluctuations of only 0.2% in the mid-IR (lambda approximately 3-6 mum) are found, which are much smaller than the Ti:sapphire amplifer noise. The noise suppression is analyzed and found to stem from the interplay between dispersion and pump depletion.

Mercury thiogallate mid-infrared femtosecond optical parametric generator pumped at 1.25 microm by a Cr:forsterite regenerative amplifier

We demonstrate a novel traveling-wave-type optical parametric generator based on 1.25-microm pumping of HgGa(2)S(4) that produces tunable, high-power, transform-limited infinity 200-fs pulses in the mid-IR from 5 to 9 microm. Output idler energies on the microjoule level are obtained with maximum conversion efficiency of 11% for the amplifier stage, which is more than two times better than the results obtained with an analogous sample of the widely spread material AgGaS(2).

Motion of hydrogen bonds in diluted HDO/D2O solutions: direct probing with 150 fs resolution

An experiment is described to study temporal variations of the hydrogen bond length in diluted HDO/D2O solutions. The principles of this laser spectroscopic experiment are explained first. The construction of a laser source generating 150 fs pulses in the 2.5-4.5 microns spectral region at a 10 microJ power level is detailed next. The OH stretching band is reproduced for different excitation frequencies and different pump-probe delay times. A theory, based on statistical mechanics of nonlinear optical processes, is proposed to calculate the lowest two spectral moments. An effect is reported, the delay dependent vibrational solvatochromism. It is shown how this effect can be exploited to follow temporal variations of the OH...O bond length directly, in real time. The corresponding time scales are of the order of 700 fs. No bond oscillations are observed.

Femtosecond Traveling-Wave Optical Parametric Amplification in MgO:LiNbO(3)

A mid-infrared femtosecond optical parametric amplifier tunable in the chemically important spectral region between 3.1 and 3.9 mum (at >10-muJ idler pulse energy) has been constructed on the basis of MgO:LiNbO(3) with 7% doping. With femtosecond pumping near 800 nm (Ti:sapphire regenerative amplifier) and narrow-band (long-pulse) seeding, this simple single-stage device provides maximum conversion efficiencies of 40% and exhibits extremely low seed threshold (<10-mW peak seed power for >1-muJ idler output). The generated idler pulses are almost transform limited with <200-fs pulse duration. The pulse-to-pulse fluctuations reproduce the stability of the pump source at 1-kHz repetition rate.

High-power picosecond mid-infrared optical parametric amplifier for infrared Raman spectroscopy

A high-power (50-MW), kilohertz, picosecond, mid-IR optical parametric amplifier that is pumped by an amplified Ti:sapphire laser and also produces a fixed-frequency visible pulse is described. Mid-IR pulse energies of 40-55 microJ with 0.6-0.8-ps durations and 35-cm (-1) bandwidths are reported in the 3650- 2800-cm (-1) range. The combination of picosecond mid-IR and visible pulses is useful for two-color spectroscopies, which require simultaneous time and frequency resolution. To illustrate the above, we present vibrational relaxation data for the polyatomic molecule nitromethane, using time-resolved infrared Raman spectroscopy.