[Show abstract][Hide abstract] ABSTRACT: Distributions of small molecular weight (less than 300 Da) compounds inside biological tissue have been obscure because of the lack of appropriate methods to measure them. Although fluorescence techniques are widely used to characterise the localisation of large biomolecules, they cannot be easily applied to the cases with small molecule compounds. We used CARS spectroscopy to detect and identify a label-free small molecule compound. To facilitate detection in aqueous environment, we utilised time-resolved and phase-sensitive techniques to reduce non-resonant background generated from water. We applied this technique to detect small molecular weight compound, taurine, inside mouse cornea tissue immersed in taurine solution as an initial model experiment. We detected a Raman peak of taurine near wavenumber 1033 cm(-1) inside cornea and successfully characterised its depth profile in the tissue. Our CARS spectra measurement can be a promising method to measure and visualise the distribution of small bio-related compounds in biological background without using any labeling, paving the way for new cell biological analysis in various disciplines.
[Show abstract][Hide abstract] ABSTRACT: Time-resolved X-ray absorption spectroscopy was performed for aqueous ammonium iron(III) oxalate trihydrate solutions using an X-ray free electron laser and a synchronized ultraviolet laser. The spectral and time resolutions of the experiment were 1.3 eV and 200 fs, respectively. A femtosecond 268 nm pulse was employed to excite [Fe(III)(C2O4)3]3− in solution from the high-spin ground electronic state to ligand-to-metal charge transfer state(s), and the subsequent dynamics were studied by observing the time-evolution of the X-ray
spectrum near the Fe K-edge. Upon 268 nm photoexcitation, the Fe K-edge underwent a red-shift by more than 4 eV within 140 fs; however, the magnitude of the redshift subsequently diminished within 3 ps. The Fe K-edge of the photoproduct remained lower in energy than that of [Fe(III)(C2O4)3]3−. The observed red-shift of the Fe K-edge and the spectral feature of the product indicate that Fe(III) is upon excitation immediately photoreduced to Fe(II), followed by ligand dissociation from Fe(II). Based on a comparison of the X-ray absorption spectra with density functional theory calculations, we propose that the dissociation proceeds in two steps, forming first [(CO2
•)Fe(II)(C2O4)2]3− and subsequently [Fe(II)(C2O4)2]2−.
Preview · Article · May 2015 · Structural Dynamics
[Show abstract][Hide abstract] ABSTRACT: We report a broadband Yb-doped fiber laser system with a gain-narrowing compensator comprised of multiple dielectric layers. Utilizing this filter, we obtained broadband pulses over a bandwidth of 1020–1080 nm directly from the amplifier. After the dispersion compensation, the chirped pulse amplification system delivered 65-fs pulses with energies of 100 nJ and a repetition rate of 3 MHz.
[Show abstract][Hide abstract] ABSTRACT: We present femtosecond time-resolved X-ray absorption spectroscopy of aqueous solution using a hard x-ray free electron laser (SACLA) and a synchronized Ti:sapphire laser. The instrumental response time is 200 fs, and the repetition rate of measurement is 10 Hz. A cylindrical liquid beam 100 μm in diameter of aqueous ammonium iron(III) oxalate solution is photoexcited at 400 nm, and the transient X-ray absorption spectra are measured in the K-edge region of iron, 7.10 - 7.26 keV, using a dual X-ray beam dispersive detection method. Each of the dual beams has the pulse energy of 1.4 μJ, and pump-induced absorbance change on the order of 10<sup>-3</sup> is successfully detected. The photoexcited iron complex exhibits a red shifted iron K-edge with the appearance time constant of 260 fs. The X-ray absorption difference spectra, with and without the pump pulses, are independent of time delay after 1.5 ps up to 100 ps, indicating that the photoexcited species is long-lived.
[Show abstract][Hide abstract] ABSTRACT: We developed efficient phase modulation in phase sensitive method for Coherent Anti-Stokes Raman Scattering (CARS) spectroscopy. This method enables us to extract broad vibrational spectra from intense background noise by only 4 spectra with different modulation phases. We demonstrated two-dimensional density maps of small molecules, which difficult to label, by referring their intrinsic vibrational spectra. As a demonstration, we clearly obtained sevoflurane droplets embedded in water. Intense background noise generated from water is selectively suppressed with time-resolved technique.
[Show abstract][Hide abstract] ABSTRACT: We applied polarization pulse shaping to control the trajectory of two-dimensional vibrational motion in α-quartz. Polarization twisted pulses were used to impart peudorotational motion of the degenerate E-symmetry optical phonon mode selectively through impulsive stimulated Raman scattering.
[Show abstract][Hide abstract] ABSTRACT: We have performed the first time-resolved X-ray absorption spectroscopy using the SPring-8 Angstrom Compact Free Electron laser (SACLA) with a dual X-ray beam dispersive detection method. The test compound is aqueous 0.5 Mammonium iron(III) oxalate trihydrate solution. The photoexcited iron complex exhibits a red shifted iron K-edge with a rise time constant of 260 fs. The X-ray absorption difference spectra between the presence and absence of pump pulses are independent of time delay between 1.5 and 100 ps, indicating that the photoexcited species is long-lived.
[Show abstract][Hide abstract] ABSTRACT: We have developed a method of dispersive x-ray absorption spectroscopy with a hard x-ray free electron laser (XFEL), generated by a self-amplified spontaneous emission (SASE) mechanism. A transmission grating was utilized for splitting SASE-XFEL light, which has a relatively large bandwidth (ΔE/E ∼ 5 × 10-3), into several branches. Two primary split beams were introduced into a dispersive spectrometer for measuring signal and reference spectra simultaneously. After normalization, we obtained a Zn K-edge absorption spectrum with a photon-energy range of 210 eV, which is in excellent agreement with that measured by a conventional wavelength-scanning method. From the analysis of the difference spectra, the noise ratio was evaluated to be ∼3 × 10-3, which is sufficiently small to trace minute changes in transient spectra induced by an ultrafast optical laser. This scheme enables us to perform single-shot, high-accuracy x-ray absorption spectroscopy with femtosecond time resolution.
[Show abstract][Hide abstract] ABSTRACT: Polarization shaping of terahertz pulses enables us to manipulate the temporal evolution of the amplitude and direction of electric-field vectors in a prescribed manner. Such arbitrary control of terahertz waves has great potential in expanding the scope of terahertz spectroscopy, the manipulation of terahertz nonlinear phenomena and coherent control. This is analogous to the use of pulse-shaping techniques for optical frequencies that involve light's polarization states as a controllable degree of freedom. Here, we propose and demonstrate a method for generating a prescribed terahertz polarization-shaped waveform by the optical rectification of a laser pulse whose instantaneous polarization state and intensity are controlled by an optical pulse shaper. We have developed a deterministic procedure to derive input parameters for the pulse shaper that are adequate to generate the desired terahertz polarization-shaped waveform, with the benefit of simple polarization selection rules for the rectification process of light waves propagating along the three-fold axis of a nonlinear optical crystal.
No preview · Article · Aug 2013 · Nature Photonics
[Show abstract][Hide abstract] ABSTRACT: The polarization pulse shaping, that is, the manipulation of the direction and timing of electromagnetic fields, has played a pivotal role in handling the interactions between strong pulsed light fields and matter, such as in increasing multiphoton ionization yield, optical gating for higher harmonic generation, and photoassisted asymmetric synthesis of chiral molecules. Recent advances in terahertz technologies have extended the scope of these studies to terahertz frequency ranges. The shaping of the temporal trajectories of terahertz vector fields could substantially increase the degree of freedom of such nonlinear interactions. However, to date, the methods for terahertz polarization pulse shaping have been limited to combining terahertz waves generated by multiple laser pulses, and their flexibility is far behind the Fourier-synthesis based shaping techniques available for optical frequencies which can actively control numerous parameters necessary to generate tailored pulses.
[Show abstract][Hide abstract] ABSTRACT: We effectively suppressed a nonresonant background, which is general issue of femtosecond-laser based multiplex CARS spectroscopy, by applying phase sensitive detection for our rapid phase-modulation CARS method . The modulation phase of each spectral measurement can be extracted from an interference intensity between a narrowband probe and a broadband pulse. We demonstrated the method with a sample of chloroform molecules. By using a phase sensitive detection, signal-to-noise ratio was improved from 2.7 to 9.4 dB.
[Show abstract][Hide abstract] ABSTRACT: Coherent anti-Stokes Raman scattering (CARS) microscopy has been used as label-free imaging due to characteristic frequencies of chemical bonds. However, previous CARS measurement had several drawbacks. First, the conventional CARS microscopy requires two or three laser beams adjusted both spatially and temporally under a microscope. This multi-beam scheme makes these experiments difficult for daily use. Second, simultaneous measurement of multiple vibrational modes is required as an overall view of a specific molecular signature. Finally, the non-resonant background contribution distorts the measured spectral profile. Data processing of background extraction is not straightforward for retrieving the actual Raman spectrum.
[Show abstract][Hide abstract] ABSTRACT: We propose and demonstrate an improved heterodyne coherent anti-Stokes Raman scattering (CARS) measurement with a rapid phase modulation and temporal displacement of the background, to simplify signal extraction and effectively reduce a nonresonant background (NRB). This method is a modification of the single-beam CARS spectroscopy originally proposed by Oron et al. in which a narrowband phase modulation is used to enhance contrast between resonant signals and the NRB through heterodyne detection [Phys. Rev. Lett. 89, 273001 (2002)]. In our scheme, a large delay between the narrow- and broadband components enables us to reduce the NRB while maintaining signal enhancement by heterodyne detection. We develop a frequency-resolved Michelson interferometer in which the narrow- and broadband components are spatially separated and recombined with an arbitrary delay. We show that sharp Raman lines can be obtained from chloroform molecules by the observation of difference spectra and phase sensitive detection. The spectral resolution achieved, which is limited by that of the spectrometer we used, is < 8 cm(-1). This method can potentially be extended to make real-time measurements by further developing a spectrometer that directly accumulates difference spectra.
[Show abstract][Hide abstract] ABSTRACT: Halogenated volatile anesthetics are frequently used for inhaled anesthesia in clinical practice. No appropriate biological method has been available for visualizing their localization in action. Therefore, despite their frequent use, the mechanism of action of these drugs has not been fully investigated. We measured coherent anti-Stokes Raman scattering (CARS) spectra of sevoflurane and isoflurane, two of the most representative volatile anesthetics, and determined the low-frequency vibrational modes without nonresonant background disturbance. Molecular dynamics calculations predict that these modes are associated with multiple halogen atoms. Because halogen atoms rarely appear in biological compounds, the entire spectral landscape of these modes is expected to be a good marker for investigating the spatial localization of these drugs within the intracellular environment. Using live squid giant axons, we could detect the unique CARS spectra of sevoflurane for the first time in a biological setting.
Full-text · Article · Jan 2011 · The Journal of Chemical Physics
[Show abstract][Hide abstract] ABSTRACT: Single-pulse heterodyne CARS (coherent anti-Stokes Raman scattering) detection scheme using shaped femtosecond pulses is one of the most sophisticated approach for managing the problem of non-resonant background disturbance in CARS measurement. However, with the signal processing method conducted in the original report,1-3 we found that background suppression and resonant peak extraction were sometimes difficult and incomplete. We discuss the reason of this unsuccessful signal processing and propose an improved method for signal extraction realizing the better quality of extracted spectra.
No preview · Article · Jun 2010 · Proceedings of SPIE - The International Society for Optical Engineering
[Show abstract][Hide abstract] ABSTRACT: We demonstrate a novel method of phase-contrast CARS spectroscopy by using an optical bandpass filter and a mechanical modulator instead of a 4f pulse shaper used in previous studies. Our system has several advantages over the conventional one; the simplicity of the setup, a larger throughput, and the higher data rate of measurements. We use the known vibrational frequencies of chloroform to verify the efficacy of this method.
[Show abstract][Hide abstract] ABSTRACT: We describe the generation of an ultrahigh-repetition-rate train of ultrashort pulses on the basis of an adiabatic Raman process. We also describe recent progress in studies toward the ultimate regime: realization of an ultrahigh-repetition-rate train of monocycle pulses with control of the absolute phase. We comment on the milestones expected in the near future in terms of the study of such novel light sources and the new field of optical science stimulated by their development.
[Show abstract][Hide abstract] ABSTRACT: We achieved reliable and stable generation of pulses with all possible polarization states by a Mach-Zehnder pulse shaper stabilized using an external laser diode. We generated and measured chiral pulses with twisted polarizing orientation.