Interferometric measurements of nonlinear refractive index in the infrared spectral range.

This study presents the development and application of interferometric technique for the measurement of nonlinear refractive index of optical materials, while directly accounting for experimentally determined laser pulse shape and beam profile. The method was employed in a systematic study of nonlinear refractive index on a series of common optical materials used in near and mid-IR spectral range, where experimental data on nonlinear material properties is still scarce. The values of nonlinear refractive index were determined at 1.03 µm, 2.2 µm, and 3.2 µm. The measurement results are compared to the values determined by previous studies (where available), and the influence of cascaded second-order nonlinearities is discussed.

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.

Singlet fission in naturally-organized carotenoid molecules.

We have investigated the photophysics of aggregated lutein/violaxanthin in daffodil chromoplasts. We reveal the presence of three carotenoid aggregate species, the main one composed of a mixture of lutein/violaxanthin absorbing at 481 nm, and two secondary populations of aggregated carotenoids absorbing circa 500 and 402 nm. The major population exhibits an efficient singlet fission process, generating µs-lived triplet states on an ultrafast timescale. The structural organization of aggregated lutein/violaxanthin in daffodil chromoplasts produces well-defined electronic levels that permit the energetic pathways to be disentangled unequivocally, allowing us to propose a consistent mechanism for singlet fission in carotenoid aggregates. Transient absorption measurements on this system reveal for the first time an entangled triplet signature for carotenoid aggregates, and its evolution into dissociated triplet states. A clear picture of the carotenoid singlet fission pathway is obtained, which is usually blurred due to the intrinsic disorder of carotenoid aggregates.

Exploiting optical nonlinearities for group delay dispersion compensation in femtosecond optical parametric oscillators.

A wavelength tunable femtosecond optical parametric oscillator pumped by the second harmonic of a Yb: KGW solid state oscillator was investigated. The intracavity group delay dispersion was positive, and soliton condition was satisfied by introducing negative nonlinearity from cascaded quadratic nonlinearity (CQN). Two different approaches were investigated - CQN induced by the same amplifying nonlinear crystal or CQN induced by an additional second harmonic generating nonlinear crystal inside the same resonator. The second crystal was shown to correct the resonator misalignment induced by the rotation of the amplifying crystal as the wavelength was tuned in the range of 770-970 nm. It simultaneously compensated positive resonator GDD offsets of +/- 1000 fs2 with +/- 5% SHG power losses, simulating a method for compensation of GDD ripples in a broadband mirror.

Combined Picosecond Time-Resolved UV-Vis and NMR Techniques Used for Investigation of the Excited State Intramolecular Triplet-Triplet Energy Transfer.

The phenomenon of the intramolecular triplet-triplet (T-T) energy transfer observed for spiro[9,10-dihydro-9-oxoanthracene-10,2'-5',6'-benzindan] (AN) molecule was investigated using stationary and time-resolved techniques in the UV/vis spectral region. The rate constant for energy transfer from anthrone chromophore to the triplet state localized on the naphthalene subunit of AN molecule is 2.8 × 1010 s-1. NMR spectroscopy is rarely used for investigation of molecules in the electronically excited states. Here, we propose 1H NMR combined with UV laser irradiation as a useful method for the recognition of an electron spin densities distribution in the excited triplet state that exists for tens of microseconds in the liquid phase. The direct registration of the 1H NMR signals from molecules in the excited triplet state was not possible due to its short lifetime. However, even the short interaction between unpaired electrons and nuclear spins leads to the changes in the NMR spectrum. The analysis of difference NMR spectra delivers information about the electron spin densities distribution over the skeleton of the molecule in the excited triplet state. In order to understand the nature of the excited states involved in the triplet-triplet energy transfer process, quantum chemical calculations were performed.

DNA-Damaging Effect of Different Wavelength (206 and 257 nm) Femtosecond Laser Pulses.

Objective: The purpose of this study was to investigate the possible cytotoxic and genotoxic impact of new-generation 206 nm femtosecond solid-state laser irradiation on murine skin cells in vitro, and to compare the cell and DNA damage caused by different wavelength (206 vs. 257 nm) femtosecond laser pulses. Background data: The first attempts to evaluate the possible genotoxic impact of ultrashort laser pulses on the murine bone marrow cells in vitro revealed the unlooked-for DNA-damaging effect. However, the impact of far-ultraviolet (UV) radiation on genetic material of internal and external organs' cells may differ due to differences in size, structure, and biochemical composition of the cells. Methods: Mouse skin cells were exposed to different doses of 206 and 257 nm wavelength femtosecond laser, and 254 nm UV lamp irradiation. Comet assay in two versions-the standard alkaline and the enzyme-linked-was used for the evaluation of DNA damage. Results: The irradiation determined by different parameters demonstrated intensity-dependent genotoxic impact. The pyrimidine dimers made up the greater part of DNA photodamage, but with rising exposure dose the increase of relative amount of more energy-consuming primary damage-DNA strand breaks-was detected. Conclusions: The 206 nm femtosecond laser irradiation was much more cytotoxic but caused less primary DNA damage than the same pulse duration longer wavelength (257 nm) laser irradiation. DNA-damaging effect of 206 nm femtosecond laser pulses with extremely low penetration force may highly depend on the size, structure, and biochemical composition of the cells of organ or tissue targets.

Quantitative assessment of nonlinearly absorbed energy in fused silica via time-resolved digital holography.

A fraction of incident optical energy nonlinearly absorbed by a solid medium is considered to be the main quantitative parameter of damage-inducing light-matter interaction. However, its reliable experimental evaluation is a non-trivial task. We have addressed this problem using time-resolved digital holography. This well-proven technique enables recording of time-dependent single-shot induced thermal lens in fused silica excited at fluence levels above the damage threshold and constructing a detailed picture of the dissipation of nonlinearly absorbed optical energy. In addition, we explored the dependence between the absorbed laser pulse energy and incident energy. We found that material modification started to occur when the sample absorbed more than 10% of incident energy, while the absorbance above 15% resulted in catastrophic damage. The proposed approach is expected to become a convenient tool for future studies of light-matter interaction in transparent solids.

Ultrafast energy transfer within the photosystem II core complex.

We report 2D electronic spectroscopy on the photosystem II core complex (PSII CC) at 77 K under different polarization conditions. A global analysis of the high time-resolution 2D data shows rapid, sub-100 fs energy transfer within the PSII CC. It also reveals the 2D spectral signatures of slower energy equilibration processes occurring on several to hundreds of picosecond time scales that are consistent with previous work. Using a recent structure-based model of the PSII CC [Y. Shibata, S. Nishi, K. Kawakami, J. R. Shen and T. Renger, J. Am. Chem. Soc., 2013, 135, 6903], we simulate the energy transfer in the PSII CC by calculating auxiliary time-resolved fluorescence spectra. We obtain the observed sub-100 fs evolution, even though the calculated electronic energy shows almost no dynamics at early times. On the other hand, the electronic-vibrational interaction energy increases considerably over the same time period. We conclude that interactions with vibrational degrees of freedom not only induce population transfer between the excitonic states in the PSII CC, but also reshape the energy landscape of the system. We suggest that the experimentally observed ultrafast energy transfer is a signature of excitonic-polaron formation.

Unveiling the excited state energy transfer pathways in peridinin-chlorophyll a-protein by ultrafast multi-pulse transient absorption spectroscopy.

Time-resolved multi-pulse methods were applied to investigate the excited state dynamics, the interstate couplings, and the excited state energy transfer pathways between the light-harvesting pigments in peridinin-chlorophyll a-protein (PCP). The utilized pump-dump-probe techniques are based on perturbation of the regular PCP energy transfer pathway. The PCP complexes were initially excited with an ultrashort pulse, resonant to the S0→S2 transition of the carotenoid peridinin. A portion of the peridinin-based emissive intramolecular charge transfer (ICT) state was then depopulated by applying an ultrashort NIR pulse that perturbed the interaction between S1 and ICT states and the energy flow from the carotenoids to the chlorophylls. The presented data indicate that the peridinin S1 and ICT states are spectrally distinct and coexist in an excited state equilibrium in the PCP complex. Moreover, numeric analysis of the experimental data asserts ICT→Chl-a as the main energy transfer pathway in the photoexcited PCP systems.

Bifurcation in the Ultrafast Dynamics of the Photoactive Yellow Proteins from Leptospira biflexa and Halorhodospira halophila.

We explored the photoisomerization mechanisms in novel homologues of photoactive yellow protein (PYP) from Leptospira biflexa (Lbif) to identify conserved features and functional diversity in the primary photochemistry of this family of photoreceptors. In close agreement with the prototypical PYP from Halorhodospira halophila (Hhal), we observe excited-state absorbance near 375 nm and stimulated emission near 500 nm, with triphasic excited-state decay. While the excited-state decay for Lbif PYP is the slowest among those of known PYPs due to the redistribution of the amplitudes of the three decay components, the quantum yield for productive photocycle entry is very similar to that of Hhal PYP. Pro68 is highly conserved in PYPs and is important for the high photochemical quantum yield in Hhal PYP, but this residue is Ile in wild-type Lbif PYP. The level of photoproduct formation is slightly increased in I68P Lbif PYP, indicating that this residue regulates the photochemical quantum yield in the entire PYP family. Lbif PYP also exhibited a blue-shifted photoproduct previously undiscovered in ultrafast studies of PYP, which we have named pUV. We posit that pUV is a detour in the PYP photocycle with a twisted protonated pCAH configuration. Cryokinetic experiments with Hhal PYP confirmed the presence of pUV, but the population of this state in room-temperature ultrafast experiments is very small. These results resolve the long-standing inconsistency in the literature regarding the existence of a bifurcation in the room-temperature photocycle of PYP.

Investigation of the S1/ICT equilibrium in fucoxanthin by ultrafast pump-dump-probe and femtosecond stimulated Raman scattering spectroscopy.

Time-resolved multi-pulse spectroscopic methods-pump-dump-probe (PDP) and femtosecond stimulated Raman spectroscopy-were used to investigate the excited state photodynamics of the carbonyl group containing carotenoid fucoxanthin (FX). PDP experiments show that S1 and ICT states in FX are strongly coupled and that the interstate equilibrium is rapidly (<5 ps) reestablished after one of the interacting states is deliberately depopulated. Femtosecond stimulated Raman scattering experiments indicate that S1 and ICT are vibrationally distinct species. Identification of the FSRS modes on the S1 and ICT potential energy surfaces allows us to predict a possible coupling channel for the state interaction.

DNA Damage in Bone Marrow Cells Induced by Femtosecond and Nanosecond Ultraviolet Laser Pulses.

OBJECTIVE: The purpose of this study was to investigate the possible genotoxic impact of new generation 205 nm femtosecond solid-state laser irradiation on the DNA of murine bone marrow cells in vitro, and to compare the DNA damage caused by both femtosecond and nanosecond UV laser pulses. BACKGROUND DATA: Recent experiments of corneal stromal ablation in vitro and in vivo applying femtosecond UV pulses showed results comparable with or superior to those obtained using nanosecond UV lasers. However, the possible genotoxic effect of ultrashort laser pulses was not investigated. METHODS: Mouse bone marrow cells were exposed to different doses of 205 nm femtosecond, 213 and 266 nm nanosecond lasers, and 254 nm UV lamp irradiation. The comet assay was used for the evaluation of DNA damage. RESULTS: All types of irradiation demonstrated intensity-dependent genotoxic impact. The DNA damage induced depended mainly upon wavelength rather than on other parameters such as pulse duration, repetition rate, or beam delivery to a target. CONCLUSIONS: Both 205 nm femtosecond and clinically applied 213 nm nanosecond lasers' pulses induced a comparable amount of DNA breakage in cells exposed to the same irradiation dose. To further evaluate the suitability of femtosecond UV laser sources for microsurgery, a separate investigation of the genotoxic and mutagenic effects on corneal cells in vitro and, particularly, in vivo is needed.

High-speed photorefractive keratectomy with femtosecond ultraviolet pulses.

Femtosecond near-infrared lasers are widely used for a number of ophthalmic procedures, with flap cutting in the laser-assisted in situ keratomileusis (LASIK) surgery being the most frequent one. At the same time, lasers of this type, equipped with harmonic generators, have been shown to deliver enough ultraviolet (UV) power for the second stage of the LASIK procedure, the stromal ablation. However, the speed of the ablation reported so far was well below the currently accepted standards. Our purpose was to perform high-speed photorefractive keratectomy (PRK) with femtosecond UV pulses in rabbits and to evaluate its predictability, reproducibility and healing response. The laser source delivered femtosecond 206 nm pulses with a repetition rate of 50 kHz and an average power of 400 mW. Transepithelial PRK was performed using two different ablation protocols, to a total depth of 110 and 150 µm. The surface temperature was monitored during ablation; haze dynamics and histological samples were evaluated to assess outcomes of the PRK procedure. For comparison, analogous excimer ablation was performed. Increase of the ablation speed up to 1.6 s/diopter for a 6 mm optical zone using femtosecond UV pulses did not significantly impact the healing process.

Optically controlled molecular switching of an indolobenzoxazine-type photochromic compound.

Photochromic forward (oxazine ring-opening) and backward (oxazine ring-closing) switching dynamics of an indolobenzoxazine compound were studied by femtosecond pump-repump-probe technique. A UV pulse was used to excite the ring-closed form of the photochromic compound, causing a C-O bond cleavage and the formation of a spectrally red-shifted isomer within a time scale of ca. 100 ps. A successive, temporally delayed near-IR pulse, resonant to the red-most absorption maximum of the ring-opened form, was used to reexcite the molecular system, causing a fast photoinduced oxazine ring closure, thereby "short-circuiting" the normally nanosecond lasting photocycle and returning ∼6% of the molecules to the main molecular ground state. Two possible models, involving the S1 excited state of the terminal photoproduct and its hot ground state, are introduced to explain the pre- and post-reexcitation spectral development and the photoinduced switching back mechanics.

Toward an understanding of the retinal chromophore in rhodopsin mimics.

Recently, a rhodopsin protein mimic was constructed by combining mutants of the cellular retinoic acid binding protein II (CRABPII) with an all-trans retinal chromophore. Here, we present a combined computational quantum mechanics/molecular mechanics (QM/MM) and experimental ultrafast kinetic study of CRABPII. We employ the QM/MM models to study the absorption (λ(a)max), fluorescence (λ(f)max), and reactivity of a CRABPII triple mutant incorporating the all-trans protonated chromophore (PSB-KLE-CRABPII). We also study the spectroscopy of the same mutant incorporating the unprotonated chromophore and of another double mutant incorporating the neutral unbound retinal molecule held inside the pocket. Finally, for PSB-KLE-CRABPII, stationary fluorescence spectroscopy and ultrafast transient absorption spectroscopy resolved two different evolving excited state populations which were computationally assigned to distinct locally excited and charge-transfer species. This last species is shown to evolve along reaction paths describing a facile isomerization of the biologically relevant 11-cis and 13-cis double bonds. This work represents a first exploratory attempt to model and study these artificial protein systems. It also indicates directions for improving the QM/MM models so that they could be more effectively used to assist the bottom-up design of genetically encodable probes and actuators employing the retinal chromophore.

Separating annihilation and excitation energy transfer dynamics in light harvesting systems.

The dependence of excitation energy transfer kinetics on the electronic state of the acceptor (ground vs excited) has been resolved with a novel multipulse prePump-Pump-Probe spectroscopy. The primary energy transfer and annihilation dynamics in two model light-harvesting systems were explored: an artificially synthesized carotenoid-zinc-phthalocyanine dyad and a naturally occurring light-harvesting peridinin-chlorophyll protein complex from Amphidinium carterae. Both systems use carotenoid as the primary excitation energy donor with porphyrin chromophores as the acceptor molecules. The prePump-Pump-Probe transient signals were analyzed with Monte Carlo modeling to explicitly address the underlying step-by-step kinetics involved in both excitation migration and annihilation processes. Both energy transfer and annihilation dynamics were demonstrated to occur with approximately the same rate in both systems, regardless of the excitation status of the acceptor pigments. The possible reasons for these observations are discussed in the framework of the Förster energy transfer model.

Corneal stromal ablation with femtosecond ultraviolet pulses in rabbits.

PURPOSE: To determine the effectiveness of femtosecond ultraviolet (UV) pulses in ablating corneal stroma in a rabbit model and to compare the healing response between eyes treated with femtosecond UV pulses and eyes treated with standard excimer photorefractive keratectomy. SETTING: Laser Research Center, Vilnius University, Vilnius, Lithuania. DESIGN: Experimental study. METHODS: Myopic photoablation using a femtosecond UV solid-state laser system was applied to corneas of pigmented rabbits. Experiments in 16 eyes were performed for optimization of the laser system parameters (fluence, spot size, pulse repetition rate) and calibration of ablation rate. In 7 rabbits, deep femtosecond UV ablation (∼130 µm) in 1 eye and shallow ablation (∼30 µm) in the contralateral eye were performed. Nine rabbits received an approximately 30 µm ablation with femtosecond UV pulses in 1 eye and with a conventional excimer system in the contralateral eye. Two eyes were used as controls. The ablation process and surface-temperature dynamics were monitored and recorded. Surface quality and haze development were evaluated. Rabbits were humanely killed 0 to 6 months after surgery, and eyes were enucleated for histological examination. RESULTS: Rabbit corneas ablated with femtosecond UV pulses or excimer laser radiation were similar in terms of the corneal wound-healing process, surface quality, and histology. CONCLUSIONS: The experiments indicate the feasibility of clinical application of femtosecond UV lasers for stromal ablation. The ability to switch between laser harmonics allows fast changeover from infrared to the UV mode, implying that a wide range of ophthalmic procedures can be performed using a single solid-state laser device. FINANCIAL DISCLOSURE: Ms. Gabryte and Mr. Danielius are paid employees of Light Conversion Ltd. Mr. Danielius is a shareholder of Light Conversion Ltd. No other author has a financial or proprietary interest in any material or method mentioned.

Direct visualization of exciton reequilibration in the LH1 and LH2 complexes of Rhodobacter sphaeroides by multipulse spectroscopy.

The dynamics of the excited states of the light-harvesting complexes LH1 and LH2 of Rhodobacter sphaeroides are governed, mainly, by the excitonic nature of these ring-systems. In a pump-dump-probe experiment, the first pulse promotes LH1 or LH2 to its excited state and the second pulse dumps a portion of the excited state. By selective dumping, we can disentangle the dynamics normally hidden in the excited-state manifold. We find that by using this multiple-excitation technique we can visualize a 400-fs reequilibration reflecting relaxation between the two lowest exciton states that cannot be directly explored by conventional pump-probe. An oscillatory feature is observed within the exciton reequilibration, which is attributed to a coherent motion of a vibrational wavepacket with a period of ∼150 fs. Our disordered exciton model allows a quantitative interpretation of the observed reequilibration processes occurring in these antennas.

Deconstructing the excited-state dynamics of ß-carotene in solution.

The femtosecond to nanosecond dynamics of the all-trans ß-carotene carotenoid dissolved in 3-methylpentane is characterized and dissected with excitation-wavelength and temperature-dependent ultrafast dispersed transient absorption signals. The kinetics measured after red-edge (490 nm) and blue-edge (400 nm) excitation were contrasted under fluid solvent (298 K) and rigid glass (77 K) conditions. In all four measured data sets, the S* population kinetics was resolved prompting the development of a modified multicompartment model. The temperature-dependent and excitation wavelength-dependent S* quantum yield is ascribed to a competition of population surmounting a weak (55 cm(-1)) energy barrier on the S(2) state to favor S(1) generation and rapid internal conversion that favors S* generation. When cooled from room temperature to 77 K, the S* decay time scale shifted significantly from 30 to 400 ps, which is ascribed to small-scale structural relaxation with a 115 cm(-1) energy barrier. For the first time under low-energy excitation conditions, the triplet state is observed and confirmed to not originate from S* or S(1), but from S(2). The interconnectivity of the S* and S(1) populations is discussed, and no observed population flow is resolved between S* and S(1). Comparison of samples obtained from different laboratories with different purity levels demonstrates that sample contamination is not the primary origin of the S* state.