Research and Technological Highlights

Ultra-broad bandwidth Optical Parametric Amplification - Road towards: optical continuum generation and amplification using second order-optical nonlinearity; tunable sub-10 fs pulses

Using an ultrabroadband amplification technique in a beta-barium borate noncollinear optical parametric amplifier, pumped by the third harmonic of a 1 kHz Ti:sapphire laser, we generate tunable femtosecond pulses in the range of 335-480 nm with energies of a few hundred nJ. The developed setup is an amplification source with a bandwidth of more than 200 THz and provides femtosecond pulses in the near-ultraviolet spectral range using coherent amplification. Parts of the amplified white-light continuum spectrum were compressed to 24-35 fs using a prism pair. Further improvements could make it possible to generate tunable ultraviolet pulses as short as 4-5 fs.

Ultrabroadband parametric amplification of a white-light continuum in the near IR (100 THz, 1.2-2.4 µm) is demonstrated in BiB3O6 pumped by 45 fs long pulses at 800 nm at a repetition rate of 1 kHz. The energy obtained with a 5 mm thick crystal reached 50 µJ, corresponding to external conversion efficiency of 20%.

Ultrabroadband optical parametric generation in the near-IR (~135 THz, 1.15-2.4 µm) is demonstrated using bismuth triborate, BiB3O6 (BIBO), in a collinear geometry. The white light continuum energy obtained with a single stage reached 15 ?J (internal conversion efficiency of about 7%). Integral pulse durations as short as 63 fs were derived from the recorded FROG traces, comparable to the 45 fs pulse duration of the 1 kHz Ti:sapphire regenerative amplifier used for pumping at 800 nm.

We review the physical properties, linear and nonlinear optical characteristics, and phase-matching configurations of BiB3O6 (BIBO), the first low-symmetry (monoclinic) inorganic nonlinear crystal that has found broad applications for frequency conversion of laser sources from the UV, across the visible, to the near-IR based on three-wave interactions. We describe in detail the most relevant optical properties that make this material an attractive candidate for nonlinear frequency conversion of laser light in general, and ultrafast femtosecond laser sources in particular. With special focus on ultrafast frequency conversion, characteristics such as group-velocity mismatch and spectral acceptance, parametric gain bandwidth, group-velocity dispersion, as well as angular acceptance and spatial walk-off are evaluated and optimum configurations for the attainment of maximum conversion efficiency, minimum pulse duration, and highest spatial beam quality are identified and compared with the most widely established alternative borate crystal, β-BaB2O4. Experimental results are presented on both parametric up- and down-conversion of femtosecond pulses, from the high-energy, low-repetition-rate (1 kHz) to the low-energy, high-repetition rate (56-76 MHz) regime, demonstrating the unique versatility of BIBO for efficient frequency conversion of femtosecond pulses with broad tunability from 250 nm in the UV, throughout the visible, up to ?3000 nm in the IR.

UV absorption delocalized in DNA

In the study of the DNA damage process leading to random mutations, researchers have largely focused on the charge migration induced in DNA by UV radiation. Exactly how UV rays initially excite the electrons in DNA bases has not been extensively explored. It has been suspected, but not experimentally shown, that excitations can be delocalized across multiple DNA bases. Ivan Buchvarov et al. show that delocalization indeed occurs, by using spectroscopy to observe how the excited-state absorption spectrum of single- and double-stranded DNA oligonucleotides evolves over time scales from 100 fs to several hundred picoseconds. At the short wavelength end of the spectrum, a transient band at 380 nm undergoes an ultrafast decay, and a new band emerges at 330 nm. This phenomenon is independent of length and represents electronic transitions within individual bases. However, the long-wavelength region of the spectrum, 440-480 nm at 5 ps, shows a distinct dependence on the number of bases; the longer the oligonucleotide, the greater its absorption. Therefore, the longer-lived electronic transitions involve cooperation between different color-absorbing parts of the molecules.

Understanding UV absorption in DNA.

We present an extensive investigation of the polarization properties of a femtosecond-laser-induced whitelight (WL) continuum generated in a cubic crystal CaF2. The WL spectrum and threshold energies have been examined for input polarizations with various degrees of ellipticity. For linear input polarization, the WL spectrum shows strong depolarization around the input wavelength, while the preservation of the input polarization is pronounced toward the blue spectral region. The observed depolarization effect has been elucidated in the framework of current models for WL generation.

WL Polarization properties for linearly polarized pump

Periodically poled stoichiometric lithium-tantalate is used for mode locking of a diode-pumped Nd:GdVO4 laser by intracavity second-harmonic generation. Stable and self-starting operation is observed achieving average output powers of up to 5 W at a pulse-repetition rate of 107 MHz. The obtained pulse durations range from 6.5 ps at maximum output power down to 3.2 ps at 1.4 W.

Chi-2 lens formation

Femtosecond transient circular dichroism spectrometer

General Experimental Layout for CD Spectroscopy

Developing a New Methodology: Broadband fs-CD Spectroscopy

A broadband, femtosecond transient circular dichroism (TRCD) spectrometer has been developed and tested in the wavelength range from 350 to 700 nm. The spectrometer uses a femtosecond probe white light with well-defined circular polarization. The latter is modulated by the polarization of a narrowband seed pulse. We have implemented dual-beam probe geometry with phase-locked detection technique to increase the signal-to-noise ratio and to reduce optical artifacts. The spectrometer allows the acquisition of TRCD spectra with sub-picosecond time resolution and typical noise levels of 10?4 absorbance units. The performance of this instrument has been demonstrated on bis(merocyanine) nanorod aggregates in tetrahydrofurane / methylcyclohexane solution. The case study confirmed that this spectrometer is effective for the investigation of chiral properties in various molecular and nanostructural systems that have transient spectra in the UV-visible spectral range.

Femtosecond Broadband Transient Absorption Spectrometer

Principle of Broadband Pump-Probe Experiments

UV-VIS Broadband Probing

Transient Absorption Spectrometer