Ivan Buchvarov

Electronic energy delocalization and dissipation in single- and double-stranded DNA.

The mechanism that nature applies to dissipate excess energy from solar UV light absorption in DNA is fundamental, because its efficiency determines the vulnerability of all genetic material to photodamage and subsequent mutations. Using femtosecond time-resolved broadband spectroscopy, we have traced the electronic excitation in both time and space along the base stack in a series of single-stranded and double-stranded DNA oligonucleotides. The obtained results demonstrate not only the presence of delocalized electronic domains (excitons) as a result of UV light absorption, but also reveal the spatial extent of the excitons.

Base pair motions control the rates and distance dependencies of reductive and oxidative DNA charge transfer.

In 1999, Wan et al. [Proc. Natl. Acad. Sci. USA 96, 6014–6019] published a pioneering paper that established the entanglement between DNA base pair motions and the transfer time of the charge carrier. The DNA assemblies contained an ethidium covalently bound via a flexible alkyl chain to the 5′ hydroxyl group of the DNA backbone. Although covalently attached, the loose way in which the ethidium was linked to DNA allowed for large degrees of conformational freedom and thus raised some concern with respect to conformational inhomogeneity. In this letter, we report studies on a different set of ethidium DNA conjugates. In contrast to the “Caltech systems,” these conjugates contain ethidium tightly incorporated (as a base pair surrogate) into the DNA base stack, opposite to an abasic site analog. Despite the tight binding, we found that charge transfer from the photoexcited ethidium base pair surrogate across two or more base pairs is several orders of magnitude slower than in case of the DNA systems bearing the tethered ethidium. To further broaden the scope of this account, we compared (oxidative) electron hole transfer and (reductive) electron transfer using the same ethidium chromophore as a charge donor in combination with two different charge acceptors. We found that both electron and hole transfer are characterized by similar rates and distance dependencies. The results demonstrate the importance of nuclear motions and conformational flexibility and underline the presence of a base gating mechanism, which appears to be generic to electronic transfer processes through π-stacked nucleic acids.

Ultrabroadband continuum amplification in the near infrared using BiB3O6 nonlinear crystals pumped at 800 nm.

Ultrabroadband parametric amplification of a white-light continuum in the near IR (approximately 100 THz, 1.2-2.4 microm) 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 microJ, corresponding to external conversion efficiency of 20%.

Broadband optical parametric amplification in the near UV–VIS

Abstract An optical parametric amplification scheme yielding a broad bandwidth in a BBO crystal (Type I) pumped by the third harmonic of a femtosecond Ti:Sapphire laser has been investigated theoretically and experimentally. A broad amplification bandwidth of 205 THz between 346 and 453 nm is obtained in a noncollinear geometry of parametric interaction. The bandwidth of the amplification is reduced significantly when the pump–signal angle is changed by only 0.1°. A UV-extended white-light continuum is used as a signal seed. Several materials for white-light generation – such as CaF2,MgF2 and LiF – have been investigated.

High-power picosecond Nd:GdVO4 laser mode locked by SHG in periodically poled stoichiometric lithium tantalate.

Periodically poled stoichiometric lithium-tantalate is used for mode locking of a diode-pumped Nd:GdVO(4) 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.

Passive mode-locking of a diode-pumped Nd:YVO(4) laser by intracavity SHG in PPKTP.

Experimental results on passive mode-locking of a Nd:YVO(4) laser using intracavity frequency doubling in periodically poled KTP (PPKTP) crystal are reported. Both, negative cascaded chi((2)) lensing and frequency doubling nonlinear mirror (FDNLM) are exploited for the laser mode-locking. The FDNLM based on intensity dependent reflection in the laser cavity ensures self-starting and self-sustaining mode-locking while the cascaded chi((2)) lens process contributes to substantial pulse shortening. This hybrid technique enables generation of stable trains of pulses at high-average output power with several picoseconds pulse width. The pulse repetition rate of the laser is 117 MHz with average output power ranging from 0.5 to 3.1 W and pulse duration from 2.9 to 5.2 ps.

Tunable femtosecond pulses in the near-ultraviolet from ultrabroadband parametric amplification

Using an ultrabroadband amplification technique in a β-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.

High-power Femtosecond Optical Parametric Amplification at 1 kHz in BiB(3)O(6) pumped at 800 nm.

Substantial power scaling of a travelling-wave femtosecond optical parametric amplifier, pumped near 800 nm by a 1 kHz Ti:sapphire laser amplifier, is demonstrated using monoclinic BiB(3)O(6) in a two stage scheme with continuum seeding. Total energy output (signal plus idler) exceeding 1 mJ is achieved, corresponding to an intrinsic conversion efficiency of approximately 32% for the second stage. The tunability extends from 1.1 to 2.9 microm. The high parametric gain and broad amplification bandwidth of this crystal allowed the maintenance of the pump pulse duration, leading to pulse lengths less than 140 fs, both for the signal and idler pulses, even at such high output levels.

Ultrabroadband operation of a femtosecond optical parametric generator based on BiB 3 O 6 in the near-IR

Ultrabroadband optical parametric generation in the near-IR (approximately 135 THz, 1.15-2.4 microm) is demonstrated using bismuth triborate, BiB(3)O(6) (BIBO), in a collinear geometry. The white light continuum energy obtained with a single stage reached 15 microJ (internal conversion efficiency of approximately 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.

Experimental demonstration of pulse shaping and shortening by spatial filtering of an induced-phase-modulated probe wave

An asymmetrically induced phase modulation resulting in an intensity-dependent spatial beam deflection and a subsequent spatial filtering have been used for all-optical pulse shaping and shortening. A compression factor of eight and controllable symmetrical super-Gaussian pulses and pulse trains have been observed in a good agreement with previous analyses in a pump-probe beam configuration.