Oleksandr Isaienko

Generation of ultra-broadband pulses in the near-IR by non-collinear optical parametric amplification in potassium titanyl phosphate

Non-collinear optical parametric amplification in potassium-titanyl phosphate (KTP) pumped with 800 nm pulses is reported. Broadband phase matching is achieved with non-collinear geometry and a slightly divergent signal seed. This enables a gain bandwidth up to approximately 2500 cm(-1) in near-IR region. Introducing a chirp into the pump pulse makes it possible to amplify the white light seed in a broad spectral region from approximately 1050 to approximately 1400 nm simultaneously. Pulse compression to sub-40 fs is readily achieved, while the spectrum should support approximately 8.5 fs pulses. Angular dispersion of the broadband output is discussed.

Hydrophobicity of hydroxylated amorphous fused silica surfaces.

Understanding the mechanism of water adsorption on silica is important in many fields of science and technology, such as geo- and atmospheric chemistry. Vibrational IR-visible sum-frequency generation (SFG) spectroscopy of hydroxyls (~3100-3800 cm(-1)) at the amorphous SiO2 surface in contact with air of varying relative humidity provides information about the adsorption sites and orientation of water molecules. The similar magnitudes of the resonant and nonresonant contributions to the interfacial second-order susceptibility, χ((2)), allow the phases of the various hydroxyls (SiOH; HOH), and thus their orientations with respect to the surface, to be determined. The surface silanols (SiOH) appear to interact weakly with adsorbed water as indicated by the persistence of the narrow surface silanol (SiOH) peak at ~3750 cm(-1) as the relative humidity of ambient air increases from <5% to >95%. Adsorbed water molecules are represented by two oppositely oriented hydroxyl modes, at ~3350-3400 and ~3650 cm(-1), respectively. The weakly hydrogen-bonded water hydroxyls (~3650 cm(-1)) are oriented toward the silica substrate and are assigned to water molecules that aggregate over the hydrophobic silica areas with exposed siloxane bridges. We believe that this is the first experimental identification of water molecules in contact with siloxane network whose hydrophobic nature has been predicted by molecular dynamics simulations for tetrahedral (SiO4) surface of kaolinite. The SFG data suggest that, at the molecular level, hydroxylated amorphous fused silica has hydrophobic character.

Ultra-broadband sum-frequency vibrational spectrometer of aqueous interfaces based on a non-collinear optical parametric amplifier.

We describe an ultrabroadband IR-visible sum-frequency (SF) setup that allows simultaneous acquisition of the entire vibrational spectrum of water molecules at mineral surfaces in the OH stretching region without ever tuning the IR laser pulses. Our newly developed 800-nm pumped noncollinear optical parametric amplifier (NOPA) generates broadband mid-IR pulses (~1800-3500 nm, or ~2900 - 6000 cm-1) with bandwidths >600 cm-1 at half-maximum at near 3500 cm-1. Using the ultra-broadband IR NOPA, we constructed a sum-frequency vibrational spectrometer that allowed the acquisition of spectra of the OH stretches of water at hydrophilic and hydrophobic silica surfaces, over the frequency range ~2900 - 3800 cm-1, within 60 s, much shorter than with scanning SFG spectrometers. The ultra-broadband SFG spectrometer reported here can be potentially applied to time-resolved measurements of kinetics at interfaces.

Pulse-front matching of ultrabroadband near-infrared noncollinear optical parametric amplified pulses

Noncollinear optical parametric amplifiers (NOPAs) pumped with ultrashort subpicosecond pulses often suffer from pulse-front tilting, resulting in angular dispersion and noncompressibility of the amplified signal pulses. We show that pulse-front matching (PFM) with a prism-telescope setup corrects for pulse-front tilts in a near-IR NOPA. We discuss the conditions that lead to pulse-front tilt and angular dispersion in NOPA-amplified signal pulses, thus requiring pulse-front mismatch correction. We review the method of PFM and describe the application of PFM to an 800 nm pumped near-IR NOPA based on a 2-mm-thick bulk potassium-titanyl phosphate (KTP) crystal. The introduction of pulse-front matching into the KTP-NOPA reduces the signal pulse angular dispersion significantly over the >35 THz bandwidth, thus nearly removing the pulse-front tilt. Compression of 1200 nm pulses to ~25 fs can be readily achieved with a fused-silica prism pair compressor.

Observation of the Bending Mode of Interfacial Water at Silica Surfaces by Near-Infrared Vibrational Sum-Frequency Generation Spectroscopy of the [Stretch + Bend] Combination Bands.

Vibrational sum-frequency generation (SFG) spectroscopy of interfacial water at mineral/aqueous interfaces is extended to the near-IR range containing the low cross section stretch + bend combination bands (νcomb = νOH + δHOH) of liquid water at silica surfaces near 5000-5300 cm(-1), for the first time. The assignments of SFG spectra are supported by FTIR and Raman spectroscopic measurements of the bulk water νcomb modes. The SFG spectra contain significant contributions from two combinations, [νs + δ] ≈ 5060 cm(-1) and [νas + δ] ≈ 5300 cm(-1). These measurements provide the first, to our knowledge, reported probe of the bending mode of water at buried interfaces. The data suggest that the interfacial water bending mode is blue-shifted from that of bulk water.

High-repetition-rate near-infrared noncollinear ultrabroadband optical parametric amplification in KTiOPO 4

We report efficient noncollinear optical parametric amplification (NOPA) of ultrabroadband near-IR pulses tunable in the 1.1-1.5 μm range at a repetition rate of 250 kHz. Improved generation of smooth near-IR continua (extending over ~1.0-1.6 μm) at 250 kHz was achieved by weakly focusing ~20% of the 1 W driving laser beam into a sapphire plate with longer focal length lenses than previously reported. Using bulk potassium-titanyl phosphate (KTiOPO(4)) pumped at 800 nm, powers as high as 11 mW (14% pump conversion efficiency) and signal pulse durations as short as 23 fs were obtained after a single white-light seeded NOPA stage.

Generation of sub-30-fs microjoule mid-infrared pulses for ultrafast vibrational dynamics at solid/liquid interfaces.

We describe temporal compression of ultrabroadband, few microjoule mid-infrared (mid-IR) pulses from a noncollinear optical parametric amplifier (NOPA) employed in a sum-frequency generation (SFG) vibrational spectroscopic system, operating in total-internal-reflection geometry. The propagation of the mid-IR beam through optical materials results in a significant temporal chirp at the probed interface, which is analyzed and corrected by properly managing the total dispersion of materials introduced into the mid-IR beam path. By employing the simultaneous spatial and temporal focusing of the broadband infrared pulses at the probed interface, we achieve a sub-50-fs full width at half-maximum (FWHM) for the instrument response function, measured via SFG cross correlation of the ultrashort mid-IR pulses with an ultrashort (~30 fs) near-IR pulse from a synchronized, independently tunable NOPA. From the SFG cross-correlation FWHM, we extract a sub-30-fs mid-IR pulse duration, making it a suitable SFG spectroscopic system to investigate vibrational dynamics in hydrogen-bonded systems at interfaces.

Ultra-broadband near-IR non-collinear optical parametric amplification in potassium niobate and lithium niobate

We report on generation of ultra-broadband (≫2900 cm{su−1) near-IR pulses from non-collinear optical parametric amplifier based on congruent lithium niobate and potassium niobate bulk crystals pumped at 800 nm.

Ultra-Broadband Infrared Pulses from a Potassium-Titanyl Phosphate Optical Parametric Amplifier for VIS-IR-SFG Spectroscopy

A non-collinear KTP-OPA to provide ultra-broadband mid-infrared pulses was designed and characterized. With proper pulse-front and phase correction, the system has a potential for high-time resolution vibrational VIS-IR-SFG spectroscopy.

Non-Collinear Optical Parametric Amplification of near-IR Pulses in KTiOPO 4 at a High Repetition Rate

We demonstrate broadband non-collinear optical parametric amplification of near-IR pulses in bulk KTiOPO4 pumped with 800-nm pulses at 250-kHz repetition rate. Conversion efficiencies of ~20% are achieved when employing two subsequent NOPA stages.