Observation of Coherent Perfect Absorption in Oil Film on Water Surface and Sensitive Detection of Refractive Index Anisotropy in the Film.

Sharp reflection dips of 50% were observed when white light was incident from the side of a cell on a 1 µm thick film of silicone oil (polydimethylsiloxane, PDMS, nearly transparent in visible light, with the extinction coefficient κ ≈ 0.0001) above a water surface in the cell so that the total reflection condition was satisfied at the oil-air interface. This is the first observation of a coherent perfect absorption (CPA) phenomenon in liquid. The experimental results can be reproduced by the Fresnel reflectance of the monolayer film, but the wavelength positions at which the dip appears for s-polarized and p-polarized light are reversed if the refractive index of the oil film is assumed to be isotropic. The experimental results were correctly reproduced by assuming that the extraordinary-ray refractive index (light polarized perpendicular to the interface) is 1% larger than the ordinary-ray refractive index (light polarized parallel to the interface). This indicates that the polarization dependence of the CPA phenomenon is extremely sensitive to the difference between the in-plane and out-of-plane refractive indices of the thin film.

Extremely large electro-optic effect of TPPS J-aggregates in the PVA or PVP polymer matrix and aqueous solution.

The molecules of tetra-phenyl porphyrin tetra-sulfonic acid (TPPS) form a J-aggregate via self-organization in aqueous solution. The J-aggregates formed in an aqueous solution with added hydrochloric acid were dispersed in PVP polymer and subjected to electric field modulation spectroscopy. The observed difference in the static polarizability Δαp between the excited and ground states in the TPPS J-aggregates reached a value 2.66 × 104 times and 1.22 × 105 times larger than in the monomer molecules in PVA and PVP polymer matrices, respectively. This large enhancement factor of the electro-optic effect (EOE) may be induced by the higher-order structure of the J-aggregate. The different enhancement factor for each polymer matrix may be introduced by the structural differences between the polymer matrices. Electric field modulation spectroscopy of the J-aggregate in aqueous solution showed an absorbance change of 25% of the magnitude of absorbance peak.

Revealing ultrafast vibronic dynamics of tetracene molecules with sub-8 fs UV impulsive Raman spectroscopy.

Ultrafast dynamics of tetracene molecules in THF solution were investigated using sub-8 fs ultraviolet pulse lasers and ab initio calculations. The time trace of absorbance changes exhibited ultrafast decay with a time constant of 165 ± 10 fs because of the relaxation from a vibronically hot excited state to the potential minimum in the S1 state. From the signals of absorbance changes in the negative time region, we obtained the electronic dephasing time of 31.27 ± 1.63 fs. Inverse Fourier transform of stationary absorption spectra exhibited rapid decay with 2.1 ± 0.08 fs. From these data, we estimated the ratio of total dephasing time to homogenous and inhomogeneous broadening as 6.7% and 93.3%, respectively. Impulsive Raman spectra reflect the wave packet dynamics of vibrational modes. Although inhomogeneous broadening blurred the phase jump across the resonance peak in the spectral range, 1156 and 1680 cm-1 vibrational modes exhibited a phase jump from -π to ∼π and -0.5π to ∼0.5π, respectively. The amplitude profiles of these vibrational modes agree with simulated vibronic progressions of combination bands. Time-frequency analysis revealed coupling dynamics between low- and high-frequency modes, where high-frequency modes are in-plane motions and low-frequency modes are out-of-plane motions. Therefore, these coupling dynamics induce symmetry-breaking of the molecular framework, which fastens the singlet fission process.

Advanced time-resolved absorption spectroscopy with an ultrashort visible/near IR laser and a multi-channel lock-in detector.

Ultrashort visible-near infrared (NIR) pulse generation and its applications to ultrafast spectroscopy are discussed. Femtosecond pulses of around 800 nm from a Ti:sapphire laser are used as a pump of an optical parametric amplifier (OPA) in a non-collinear configuration to generate ultrashort visible (500-780 nm) pulses and deep-ultraviolet (DUV, 259-282 nm) pulses. The visible-NIR pulses and DUV pulses were compressed to 3.9 fs and 10.4 fs, respectively, and used to elucidate various ultrafast dynamics in condensed matter with a sub-10 fs resolution by pump-probe measurements. We have also developed a 128-channel lock-in amplifier. The combined system of the world-shortest visible pulse from the OPA and the lock-in amplifier with the world-largest channel-number can clarify the sub-10 fs-dynamics in condensed matter. This system clarified structural changes in an excited state, reaction intermediate, and a transition state. This is possible even during molecular vibration and reactions via a real-time-resolved vibronic spectrum, which provides molecular structural change information. Also, ultrafast dynamics in exotic materials like carbon nanotubes, topological insulators, and novel solar battery systems have been clarified. Furthermore, the carrier-envelope phase in the ultrashort pulse has been controlled and measured.

Acute Spinal Cord Infarction with Preferential Involvement of Ventral Gray Matter: An Autopsy Report.

Herein, we report abdominal aortic thrombosis as a rare cause of acute spinal cord infarction. A 78-year-old man with multiple vascular risk factors developed acute paraplegia with sensory and urinary disturbances and signs of ischemia in both lower limbs. The post-mortem study done 3 days after the onset of symptoms revealed a large coagulum in the abdominal aorta, distal to the renal arteries and extending to bilateral common iliac arteries; in addition, marked atherosclerosis was present in most large blood vessels. Premature incomplete necrotic foci were seen in the ventral gray matter of the spinal cord from T6 through S5; the surrounding white matter and dorsal gray matter were spared. Considering our autopsy case, spinal cord gray matter may be more vulnerable to ischemia than the white matter.

Femtosecond electronic relaxation and real-time vibrational dynamics in 2'-hydroxychalcone.

Femtosecond ultrafast electronic relaxation and vibrational dynamics in 2'-hydroxychalcone after deep ultraviolet (DUV) excitation were observed by two types of pump-probe spectroscopy experiments, i.e., DUV-pump pulse and visible-broadband-probe pulse (DUV/Vis) experiments and DUV-pump and DUV-probe (DUV/DUV) pulse experiments. Time-dependent density functional theory (TDDFT) calculations were performed to elucidate relaxation dynamics from the third singlet electronic excited state S3. The DUV/Vis experiments and TDDFT calculations have disclosed the ultrafast dynamics of internal conversion from the initial S3 state (τ1 ≈ 35 fs) to the S1 state via a rapid process through the S3/S2 conical intersection and proton transfer [OH: τ2(H) ≈ 93 and OD: τ2(D) ≈ 164 fs] before deactivation through the S1/S0 conical intersection (τ3 ≈ 690 fs). Thanks to the ultrashort pump and probe pulses, real-time observation of vibrational modes coupled to the electronic excitation was realized providing both amplitudes and phases. Spectrogram analyses were performed based on the real-time spectra obtained by the DUV/Vis experiments, in which instantaneous vibrational frequencies reflecting molecular structural change after the impulsive excitation were visualized. The vibrational frequency of central C[double bond, length as m-dash]C bond stretch decreases from ∼1600 cm-1 to ∼1560 cm-1 in about 200-500 fs and recovers in ∼550 fs. Normal mode analyses along the decay path support the observed variation of the C[double bond, length as m-dash]C stretching frequency. The temporal weakening of the central C[double bond, length as m-dash]C bond is connected with the angle of the two aromatic rings which flip back to the initial conformation.

Exploring the ultrafast dynamics of a diarylethene derivative using sub-10 fs laser pulses.

A diarylethene derivative, 1,2-bis(2,4-dimethyl-5-phenyl-3-thienyl)perfluorocyclopentene (DMP), is a photoswitch molecule utilizing a reversible aromatic ring-opening reaction. The quantum yield of the ring-opening reaction is however remarkably low. We investigate the origin of this behaviour by means of ultrafast transient absorption spectroscopy utilizing sub-10 fs pulses, which is an invaluable tool for simultaneously studying both the electronic and the vibrational molecular dynamics. Namely, a noncollinear optical parametric amplifier (NOPA) generating sub-10 fs pulses in the spectral range 605-750 nm is employed. The transient absorption signal is modulated by several vibrational modes, which are compared with experimental and computational Raman spectra and then assigned to the ground or excited electronic state. We observe that the most pronounced vibrational mode - the ethylenic stretching mode at a frequency of 1501 cm-1 - exhibits instantaneous frequency and amplitude modulation. The observed modulations occur due to weak coupling with another 1431 cm-1 stretching mode mediated by a vibrational mode of low frequency, i.e. around 60 cm-1. Fast internal conversion S1 → S0 originates in a relaxation through a conical intersection (found by density-functional theory computations), facilitated by the two aforementioned stretching modes.

Utility of Autonomic Function Tests to Differentiate Dementia with Lewy Bodies and Parkinson Disease with Dementia from Alzheimer Disease.

OBJECTIVE: We studied autonomic disturbance in patients with dementia with Lewy bodies (DLB), Parkinson disease with dementia (PDD), Alzheimer disease (AD), to determine whether autonomic function tests can be used to distinguish these disorders. METHODS: Autonomic function was tested in 56 patients with DLB, 37 patients with PDD, and 59 patients with AD by using the sympathetic skin response, coefficient of variation in R-R interval, the head-up tilt test, serum norepinephrine concentration, and 123I-meta-iodobenzylguanidine cardiac scintigraphy. Symptoms of autonomic dysfunction, such as constipation, urinary symptoms, and orthostatic hypotension, were also noted. RESULTS: The groups did not differ on baseline characteristics other than those associated with Parkinsonism and dementia. All patients with DLB and PDD had some dysautonomia, whereas rates were much lower for patients with AD (19%). Significantly more DLB and PDD patients than AD patients showed abnormalities on autonomic function tests. CONCLUSIONS: Autonomic function tests might be quite useful to distinguish DLB and PDD from AD.

Schiff Base Proton Acceptor Assists Photoisomerization of Retinal Chromophores in Bacteriorhodopsin.

In this study, we investigated the ultrafast dynamics of bacteriorhodopsins (BRs) from Haloquadratum walsbyi (HwBR) and Haloarcula marismortui (HmBRI and HmBRII). First, the ultrafast dynamics were studied for three HwBR samples: wild-type, D93N mutation, and D104N mutation. The residues of the D93 and D104 mutants correspond to the control by the Schiff base proton acceptor and donor of the proton translocation subchannels. Measurements indicated that the negative charge from the Schiff base proton acceptor residue D93 interacts with the ultrafast and substantial change of the electrostatic potential associated with chromophore isomerization. By contrast, the Schiff base proton donor assists the restructuring of the chromophore cavity hydrogen-bond network during the thermalization of the vibrational hot state. Second, the ultrafast dynamics of the wild-types of HwBR, HmBRI, and HmBRII were compared. Measurements demonstrated that the hydrogen-bond network in the extracellular region in HwBR and HmBRII slows the photoisomerization of retinal chromophores, and the negatively charged helices on the cytoplasmic side of HwBR and HmBRII accelerate the thermalization of the vibrational hot state of retinal chromophores. The similarity of the correlation spectra of the wild-type HmBRI and D104N mutant of HwBR indicates that inactivation of the Schiff base proton donor induces a positive charge on the helices of the cytoplasmic side.

Ultrafast dynamics of multi-exciton state coupled to coherent vibration in zinc chlorin aggregates for artificial photosynthesis.

Ultrafast vibronic dynamics induced by the interaction of the Frenkel exciton with the coherent molecular vibrations in a layer-structured zinc chlorin aggregates prepared for artificial photosynthesis have been studied by 7.1 fs real-time vibrational spectroscopy with multi-spectrum detection. The fast decay of 100 ± 5fs is ascribed to the relaxation from the higher multi-exciton state (MES) to the one-exciton state, and the slow one of 863 ± 70fs is assigned to the relaxation from Q-exciton state to the dark nonfluorescent charge-transfer (CT) state, respectively. In addition, the wavelength dependences of the exciton-vibration coupling strength are found to follow the zeroth derivative of the transient absorption spectra of the exciton. It could be explained in term of the transition dipole moment modulated by dynamic intensity borrowing between the B transition and the Q transition through the vibronic interactions.

THz emission from organic cocrystalline salt: 2, 6-diaminopyridinium-4-nitrophenolate-4-nitrophenol.

We report few-cycle THz pulses emission from a novel organic crystal 2,6-diaminopyridinium-4-nitrophenolate-4-nitrophenol (DAP+NP-NP). The observed amplitude of the THz electric field from a DAP+NP-NP crystal is comparable with that from a ZnTe single crystal under the same optical pumping conditions. Both the waveform and spectra of the THz radiation from DAP+NP-NP are similar to those from ZnTe. We conclude that a high quality DAP+NP-NP crystal would be a high potential candidate in THz generation and applications.

Ultrafast dynamics of uracil and thymine studied using a sub-10 fs deep ultraviolet laser.

Single 9.6 fs deep ultraviolet pulses with a spectral range of 255-290 nm are generated by a chirped-pulse four-wave mixing technique for use as pump and probe pulses. The electronic excited state and vibrational dynamics are simultaneously observed for an aqueous solution of uracil and thymine over the full spectral range using a 128-channel lock-in amplifier detector. Two probe photon energy-dependent lifetimes gradually increasing with the probe photon energy are obtained from the decay dynamics data. Ultrafast decay dynamics through the conical intersection is assigned from the first excited ππ* to the final ground state involving the nπ* states. Vibrational modes of the electronic ground state and excited states can be observed, which are strongly coupled to the decay dynamics of the electronic excited state.

Excitonic and vibrational coherence in artificial photosynthetic systems studied by negative-time ultrafast laser spectroscopy.

Quantum coherences between excitonic states are believed to have a substantial impact on excitation energy transfer in photosynthetic systems. Here, the excitonic and vibrational coherence relaxation dynamics of artificially synthetic chlorosomes are studied by a sub 7 fs negative-time-delay laser spectroscopy at room temperature. The results provide direct evidence for the quantum coherence of the excitonic dephasing time of 23 ± 1 fs at physiologically relevant temperatures, which is significant in the initial step of energy transfer in chlorosome or chlorosome-like photosynthetic systems. Meanwhile, coherent molecular vibrations in the excited state are also detected without the effect of wave-packet motion in the ground state, which shows that the excited state wave-packet motion contributes greatly to the vibrational modes of ∼150 and ∼1340 cm(-1) in artificial chlorosome systems.

Observation of an Excitonic Quantum Coherence in CdSe Nanocrystals.

Recent observations of excitonic coherences within photosynthetic complexes suggest that quantum coherences could enhance biological light harvesting efficiencies. Here, we employ optical pump-probe spectroscopy with few-femtosecond pulses to observe an excitonic quantum coherence in CdSe nanocrystals, a prototypical artificial light harvesting system. This coherence, which encodes the high-speed migration of charge over nanometer length scales, is also found to markedly alter the displacement amplitudes of phonons, signaling dynamics in the non-Born-Oppenheimer regime.

Numerical calibration of the spatial overlap for subtraction microscopy.

Recently proposed subtraction microscopy provides a simple and compact approach to enhance the spatial resolution. However, inevitable imperfect overlap of the Gaussian and donut spots could deteriorate the quality of the subtracted images, while this effect is rarely investigated quantitatively. Based on vector diffraction theory, the effects of spatial displacements on the image resolution, signal intensity, and subtraction threshold are studied with several microscope parameters here. The peak-position shift is found to be a useful and convenient parameter to inspect and calibrate the spatial overlap of excitation spots in such microscopes.

Polarization modulation for fluorescence emission difference microscopy.

Effects of the polarization on subtracted images in fluorescence emission difference microscopy are numerically investigated based on vector beam diffraction theory and alteration of Jones matrices. Parameters of resolution, signal loss, and amplitude of negative sidebands after subtraction are discussed along with the polarization of excitation beams and sample sizes. The effects of polarization on the ellipticity and subtraction threshold are also given.

Mechanism study of 2-D laser array generation in a YAG crystal plate.

We have reproduced the process of two-dimensional array generation by using two crossing laser beams in a YAG crystal plate based on numerical simulation considering cross-phase modulation (XPM) and self-focusing. Furthermore, we come to the conclusion that both XPM and the cylindrical symmetry breaking in the initial beam profile contributes to the generation of two-dimensional array. In addition, we have studied the threshold input laser beam power for the two crossing beams splitting in a YAG crystal plate. Our study could be valuable in various applications, such as 2-D all-optical switching devices or multicolor pump-probe experiments.

Simultaneous dual-wavelength imaging of nonfluorescent tissues with 3D subdiffraction photothermal microscopy.

Multi-wavelength microscopic imaging is essential to visualize a variety of nanoscale cellular components with high specificity and high spatial resolution. However, previous techniques are based on fluorescence, and thus cannot visualize nonfluorescent species, which are much less suffered from photodamage or photobleaching and hence are intrinsically useful in wider range of optical microscopy. Here, we show that simultaneous multi-wavelength imaging of nonfluorescent species can be achieved with the use of a photothermal microscope. Dual-wavelength subdiffraction imaging of biological tissues stained with hematoxylin and eosin is demonstrated. Three-dimensional label-free imaging of mouse melanoma tissue section is also presented to demonstrate the effectiveness of the enhanced spatial resolution. Our technique can be implemented using cost-effective and compact laser diodes and is applicable for various types of both fluorescent and nonfluorescent tissues.

"Optimal detection angle in sub-diffraction resolution photothermal microscopy: application for high sensitivity imaging of biological tissues": reply.

The differences between our model and existing models are rationalized in terms of the experimental conditions. The theory in [Opt. Express 22(16), 18833-18842 (2014)] is applicable when the temperature increase is moderate (~1 K) and the spatial extend of refractive index being modulated is comparable to or smaller than the wavelength, which are in accordance with our experiment.

Biological imaging with nonlinear photothermal microscopy using a compact supercontinuum fiber laser source.

Nonlinear photothermal microscopy is applied in the imaging of biological tissues stained with chlorophyll and hematoxylin. Experimental results show that this type of organic molecules, which absorb light but transform dominant part of the absorbed energy into heat, may be ideal probes for photothermal imaging without photochemical toxicity. Picosecond pump and probe pulses, with central wavelengths of 488 and 632 nm, respectively, are spectrally filtered from a compact supercontinuum fiber laser source. Based on the light source, a compact and sensitive super-resolution imaging system is constructed. Further more, the imaging system is much less affected by thermal blurring than photothermal microscopes with continuous-wave light sources. The spatial resolution of nonlinear photothermal microscopy is ~ 188 nm. It is ~ 23% higher than commonly utilized linear photothermal microscopy experimentally and ~43% than conventional optical microscopy theoretically. The nonlinear photothermal imaging technology can be used in the evaluation of biological tissues with high-resolution and contrast.