Sub-nanometer scale depth patterning on sapphire crystal by femtosecond soft x-ray laser pulse irradiation.

Damage thresholds and structures on a metal aluminum and an aluminum oxide crystal induced by the soft x-ray free electron laser irradiations were evaluated. Distinctive differences in damage thresholds and structures were observed for these materials. On the aluminum oxide crystal surface, in particular, a novel, to the best of our knowledge, surface processing, which we suggest defining as "peeling," was recognized. Surface structures formed by peeling had extremely shallow patterning of sub-nanometer depth. For the newly observed peeling process, we proposed a scission of chemical bond, i.e., binding energy model, in the crystal.

Single and sequential double ionization of NO radical in intense laser fields.

We examine the dependences of the single and double ionization probabilities of NO radical on the angle between the NO axis and the laser polarization direction in an intense laser field (790 nm, 100 fs, 1-10 × 1014 W/cm2) and show that the double ionization is enhanced when the NO axis is parallel to the laser polarization direction. We reveal that the angular dependence of the sequential double ionization probability is determined by the shape of the 5σ orbital of NO+ from which the second photoelectron is emitted in the ionization from NO+ to NO2+. We also reveal that the fast oscillation in the probability of the tunnel ionization of NO originating from a coherent superposition of the two spin-orbit components in the electronic ground X2Π state is described well based on the molecular Ammosov-Delone-Krainov (MO-ADK) theory in which the time evolution of the electron density distribution of the 2π orbital is taken into account.

Extremely enhanced N2+ lasing in a filamentary plasma grating in ambient air.

Cavity-free air lasing offers a promising route towards the realization of atmospheric lasers for various applications such as remote sensing and standoff spectroscopy; however, achieving efficient generation and control of air lasing in ambient air is still a challenge. Here we show the experimental realization of a giant lasing enhancement by three to four orders of magnitude in ambient air for the self-seeded N2+ lasing at 428 nm, assigned to the B2Σu+(ν'=0) and X2Σg+(ν''=1) emission, by modulating the spatiotemporal overlap of ultrashort near-infrared control-pump pulses in a filamentary plasma grating; meanwhile, the spontaneous emission from the same transition is only enhanced by three to four times. We find that this enhancement is sensitive to the relative polarization and interference time of the two pulses, and reveal that the formation of the plasma grating induces different population variations in the B2Σu+(ν'=0) and X2Σg+(ν''=1) levels, resulting in an enormous population inversion between the two levels, thereby a higher gain for the giant enhancement of N2+ lasing in ambient air.

Switching Competition between Electron and Energy Transfers in Porphyrin-Fullerene Dyads.

Porphyrin-fullerene dyads were intensively studied as molecular donor-acceptor systems providing efficient photoinduced charge separation (CS). A practical advantage of the dyads is the possibility to tune its CS process by the porphyrin periphery modification, which allows one to optimize the dyad for particular applications. However, this tuning process is typically composed of a series of trial stages involving the development of complex synthetic schemes. To address the issue, we synthesized a series of dyads with properties switching between electron and energy transfer in both polar (benzonitrile) and nonpolar (toluene) media and developed a computation procedure with sufficient reliability by which we can predict the CS properties of the dyad in different media and design new dyads. The dyads photochemistry was established by conducting ultrafast transient absorption studies in toluene, anisole, and benzonitrile. The most crucial step in computational modeling was to establish a procedure for correction of the electronic-state energies obtained by DFT so that the effects of the electron correlation and the long-range interactions are properly incorporated. We also carried out standard electrochemical measurements and show that our computation approach predicts better thermodynamics of the dyads in different solvents.

Giant Enhancement of Air Lasing by Complete Population Inversion in N_{2}

A fine manipulation of population transfer among molecular quantum levels is a key technology for control of molecular processes. When a light field intensity is increased to the TW-PW cm^{-2} level, it becomes possible to transfer a population to specific excited levels through nonlinear light-molecule interaction, but it has been a challenge to control the extent of the population transfer. We deplete the population in the X^{2}Σ_{g}^{+}(v=0) state of N_{2}^{+} almost completely by focusing a dual-color (800 nm and 1.6 µm) intense femtosecond laser pulse in a nitrogen gas, and make the intensity of N_{2}^{+} lasing at 391 nm enhanced by 5-6 orders of magnitude. By solving a time-dependent Schrödinger equation describing the population transfer among the three lowest electronic states of N_{2}^{+}, we reveal that the X^{2}Σ_{g}^{+}(v=0) population is depleted by the vibrational Raman excitation followed by the electronic excitation A^{2}Π_{u}(v=2,3,4)←X^{2}Σ_{g}^{+}(v=1)←X^{2}Σ_{g}^{+}(v=0), resulting in the excessive population inversion between the B^{2}Σ_{u}^{+}(v=0) and X^{2}Σ_{g}^{+}(v=0) states. Our results offer a promising route to efficient population transfer among vibrational and electronic levels of molecules by a precisely designed intense laser field.

Surface processing of PMMA and metal nano-particle resist by sub-micrometer focusing of coherent extreme ultraviolet high-order harmonics pulses.

We demonstrate sub-micrometer processing of two kinds of thin films, polymethyl methacrylate (PMMA) and metal nano-particle resist, by focusing high-order harmonics of near-IR femtosecond laser pulses in the extreme ultraviolet (XUV) wavelength region (27.2-34.3 nm) on the thin film samples using an ellipsoidal focusing mirror. The ablation threshold fluences for the PMMA sample and the metal nano-particle resist per XUV pulse obtained by the accumulation of 200 XUV pulses were determined to be 0.42mJ/cm2 and 0.17mJ/cm2, respectively. The diameters (FWHM) of a hole created by the ablation on the PMMA film at the focus were 0.67 µm and 0.44 µm along the horizontal direction and the vertical direction, respectively. The fluence dependence of the Raman microscope spectra of the processed holes on the PMMA sample showed that the chemical modification, in which C=C double bonds are formed associated with the scission of the PMMA polymer chains, is achieved by the irradiation of the XUV pulses.

Absolute carrier-envelope-phase dependences of single and double ionization of methanol in a near-IR few-cycle laser field.

We investigate the carrier-envelope phase (CEP) dependences of the single and double ionization processes of methanol (CH3OH) in an intense near-IR few-cycle laser field (2.1 × 1014 W/cm2) by the asymmetry in the ejection direction of CH3 + for the non-hydrogen migration channels and CH2 + for the hydrogen migration channels created through the C-O bond breaking after the ionization. Based on the absolute CEP values at the laser-molecule interaction point, calibrated by the method using intense few-cycle circularly polarized laser pulses [Fukahori et al., Phys. Rev. A 95, 053410-1-053410-14 (2017)], we confirm that methanol cations are produced by tunnel ionization and methanol dications are produced by the recollisional double ionization. We obtain the phase offset for the double ionization accompanying no hydrogen migration to be 1.85π as the absolute CEP at which the extent of the asymmetry becomes maximum. We interpret the phase shift of 0.85π from the phase offset of 1.0π for the tunnel ionization, estimated by a tunnel ionization model incorporating the chemical bond asymmetry, as the corresponding time delay associated with the electron recollisional ionization. The positive phase shift of 0.13π for the single ionization in the non-hydrogen migration channel is interpreted as the additional time (165 as) with which a methanol cation can be excited electronically prior to the decomposition. The additional phase shift of 0.22π for the single ionization in the hydrogen migration channel is interpreted as the additional time (280 as) required for a methanol cation to be excited electronically leading to the hydrogen migration prior to the decomposition.

Rotational, Vibrational, and Electronic Modulations in N_{2}

We investigate lasing of a N_{2} gas induced by intense few-cycle near-IR laser pulses. By the pump-probe measurements, we reveal that the intensity of the B^{2}Σ_{u}^{+}-X^{2}Σ_{g}^{+} lasing emission of N_{2}^{+} oscillates at high (0.3-0.5 PHz), medium (50-75 THz), and low (∼3 THz) frequencies, corresponding to the energy separations between the rovibrational levels of the A^{2}Π_{u} and X^{2}Σ_{g}^{+} states. By solving the time-dependent Schrödinger equation, we reproduce the oscillations in the three different frequency ranges and show that the coherent population transfer among the three electronic states of N_{2}^{+} creates the population inversion between the B^{2}Σ_{u}^{+} and X^{2}Σ_{g}^{+} states, resulting in the lasing at 391 nm.

Significant Enhancement of N_{2}

We show that the intensity of self-seeded N_{2}^{+} lasing at 391 nm, assigned to the B^{2}Σ_{u}^{+}(v^{'}=0)→X^{2}Σ_{g}^{+}(v^{''}=0) emission, is enhanced by 2 orders of magnitude by modulating in time the polarization of an intense ultrashort near-IR (40 fs, 800 nm) laser pulse with which N_{2} is irradiated. We find that this dramatic enhancement of the 391 nm lasing is sensitive to the temporal variation of the polarization state within the laser pulse while the intensity of the spontaneous fluorescence emission at 391 nm is kept constant when the polarization state varies. We conclude that a postionization multiple-state coupling, through which the population can be transferred from the X^{2}Σ_{g}^{+} state of N_{2}^{+} to the first electronically excited A^{2}Π_{u} state, leads to the depletion of the population in the X^{2}Σ_{g}^{+} state, and consequently, to the population inversion between the X^{2}Σ_{g}^{+} state and the B^{2}Σ_{u}^{+} state.

Strong-Field Fourier Transform Vibrational Spectroscopy of D_{2}

The photoionization of D_{2} and dissociation of the resultant D_{2}^{+} are monitored by pump-probe measurements using intense near-infrared few-cycle laser pulses. The yields of D_{2}^{+} and D^{+} recorded up to the pump-probe delay time of 527 ps exhibit oscillatory structures reflecting the motion of the created vibrational wave packet of D_{2}^{+}, and the Fourier transform of the data in time domain reveals the vibrational level separations with uncertainties of 0.0002-0.0097 cm^{-1}, showing a potential application of the strong-field pump-probe measurements to high-resolution spectroscopy of molecular ions.

Sub-10-fs population inversion in N2(+) in air lasing through multiple state coupling.

Laser filamentation generated when intense laser pulses propagate in air has been an attractive phenomenon having a variety of potential applications such as detection and spectroscopy of gases at far distant places. It was discovered recently that the filamentation in air induces 'lasing', showing that electronically excited N2(+) is population-inverted, exhibiting marked contrast to the common understanding that molecular ions generated by intense laser fields are prepared mostly in their electronic ground states. Here, to clarify the mechanism of the population inversion, we adopt few-cycle laser pulses, and experimentally demonstrate that the lasing at 391 nm occurs instantaneously after N2(+) is produced. Numerical simulations clarify that the population inversion is realized by the post-ionization couplings among the lowest three electronic states of N2(+). Our results shed light on the controversy over the mechanism of the air lasing, and show that this post-ionization coupling can be a general mechanism of the atmospheric lasing.

Design for sequentially timed all-optical mapping photography with optimum temporal performance.

A recently developed ultrafast burst imaging method known as sequentially timed all-optical mapping photography (STAMP) [Nat. Photonics8, 695 (2014)10.1038/nphoton.2014.163] has been shown effective for studying a diverse range of complex ultrafast phenomena. Its all-optical image separation circumvents mechanical and electronic restrictions that traditional burst imaging methods have long struggled with, hence realizing ultrafast, continuous, burst-type image recording at a fame rate far beyond what is achievable with conventional methods. In this Letter, considering various design parameters and limiting factors, we present an optimum design for STAMP in terms of temporal properties including exposure time and frame rate. Specifically, we first derive master equations that can be used to predict the temporal performance of a STAMP system and then analyze them to realize optimum conditions. This Letter serves as a general guideline for the camera parameters of a STAMP system with optimum temporal performance that is expected to be of use for tackling problems in science that are previously unsolvable with conventional imagers.

A new staging system to predict prognosis of patients with multiple myeloma in an era of novel therapeutic agents.

Various prognostic markers for multiple myeloma (MM) have been identified, and stratification using these markers is considered important to optimize treatment strategies. The international staging system (ISS) is now a widely accepted prognostic staging system for MM patients; however, its validity is controversial in the era of new therapeutic regimens, since ISS had been established before introduction of new agents. We retrospectively reviewed prognostic factors in order to seek out an alternative staging system more suitably applied to MM patients treated with novel agents. We analyzed 178 newly diagnosed MM patients who received either conventional chemotherapy without novel agents (CT; n = 79) or chemotherapy with novel agents (NT; n = 99). Although median overall survival (OS) of patients treated with CT is significantly different depending on stages of ISS, ISS had no effect on OS among patients treated with NT. Meanwhile, we identified hemoglobin (Hb) and plasmacytoma as independent risk factors for OS in patients who received NT. Using these two parameters, we stratified NT patients into three stages; stage 1 (Hb≥10 g/dL and absence of plasmacytoma), stage 2 (not stage 1 or 3), and stage 3 (Hb <10 g/dL and presence of plasmacytoma). We found that there were significant differences in median OS among the three stages (8.13, 5.95, and 2.45 yr for stages 1, 2, and 3, respectively). This preliminary study suggests that this alternative staging system based on Hb and plasmacytoma is a simple and useful way to predict prognosis of MM patients in the novel agent era.

High-speed multispectral videography with a periscope array in a spectral shaper.

We present a simple method for continuous snapshot multispectral imaging or multispectral videography that achieves high-speed spectral video recording without the need for mechanical scanning and much computation for datacube construction. The enabling component of this method is an array of periscopes placed in a prism-based spectral shaper that spectrally separates the image without image deformation. As a proof-of-principle demonstration, we show five-color multispectral video recording in the visible range (200×200 pixels per spectral image frame) at a record high frame rate of at least 2800 frames per second. Our experimental results indicate that this method holds promise for various industrial and biomedical applications such as remote sensing, food inspection, and endoscopy.

High energy proton ejection from hydrocarbon molecules driven by highly efficient field ionization.

We investigated the ejection of energetic protons from a series of polyatomic hydrocarbon molecules exposed to 790 nm 27 fs laser pulses. Using multiparticle coincidence imaging we were able to decompose the observed proton energy spectra into the contributions of individual fragmentation channels. It is shown that the molecules can completely fragment already at relatively low peak intensities of a few 10(14) W/cm(2), and that the protons are ejected in a concerted Coulomb explosion from unexpectedly high charge states. The observations are in agreement with enhanced ionization taking place at many C-H bonds in parallel.

Extreme ultraviolet free electron laser seeded with high-order harmonic of Ti:sapphire laser.

The 13th harmonic of a Ti:sapphire (Ti:S) laser in the plateau region was injected as a seeding source to a 250-MeV free-electron-laser (FEL) amplifier. When the amplification conditions were fulfilled, strong enhancement of the radiation intensity by a factor of 650 was observed. The random and uncontrollable spikes, which appeared in the spectra of the Self-Amplified Spontaneous Emission (SASE) based FEL radiation without the seeding source, were found to be suppressed drastically to form to a narrow-band, single peak profile at 61.2 nm. The properties of the seeded FEL radiation were well reproduced by numerical simulations. We discuss the future precept of the seeded FEL scheme to the shorter wavelength region.

Practical and adequate approach to modeling light propagation in an adult head with low-scattering regions by use of diffusion theory.

A practical and adequate approach to modeling light propagation in an adult head with a low-scattering cerebrospinal fluid (CSF) region by use of diffusion theory was investigated. The diffusion approximation does not hold in a nonscattering or low-scattering regions. The hybrid radiosity-diffusion method was adopted to model the light propagation in the head with a nonscattering region. In the hybrid method the geometry of the nonscattering region is acquired as a priori information. In reality, low-level scattering occurs in the CSF region and may reduce the error caused by the diffusion approximation. The partial optical path length and the spatial sensitivity profile calculated by the finite-element method agree well with those calculated by the Monte Carlo method in the case in which the transport scattering coefficient of the CSF layer is greater than 0.3 mm(-1). Because it is feasible to assume that the transport scattering coefficient of a CSF layer is 0.3 mm(-1), it is practical to adopt diffusion theory to the modeling of light propagation in an adult head as an alternative to the hybrid method.

[Examination of susceptibility artifact in three-dimensional gadolinium-enhanced chest MR angiography].

Signal loss that is sometimes found in the subclavian artery during chest MR angiography is thought to be caused by the susceptibility effect of highly concentrated contrast medium. In our research project, we examined the conditions under which signal loss occurs. We made vessel phantoms (artery phantom, vein phantom) that contained different concentrations of Gd-DTPA water solutions, and placed them in a 0.5 mmol/l Gd-DTPA water solution. We examined signal loss when the vein phantom was parallel to the magnetic field and when it was perpendicular to the magnetic field. We found that there was no signal loss in the artery phantom when the vein phantom was parallel to the magnetic field. In contrast, signal loss occurred in the artery phantom when the vein phantom was perpendicular to the magnetic field. The higher the concentration in the vein phantom, the closer the distance to the vessel phantom, and the longer the echo time (TE), the greater was the signal loss. Thus, the cause of signal loss in the subclavian artery was found to be the perpendicular orientation of the subclavian vein (through which the highly concentrated contrast medium flows) to the magnetic field. With the MRI devices currently in use, perpendicular orientation of the subclavian vein to the magnetic field cannot be avoided. Furthermore, the subclavian vein and subclavian artery are anatomically in close proximity to one another. These factors cause the susceptibility artifact, which is thought to result in signal loss in the subclavian artery.

Evidence for a role of mast cells in the evolution to congestive heart failure.

Mast cells are believed to be involved in the pathophysiology of heart failure, but their precise role in the process is unknown. This study examined the role of mast cells in the progression of heart failure, using mast cell-deficient (WBB6F1-W/W(v)) mice and their congenic controls (wild-type [WT] mice). Systolic pressure overload was produced by banding of the abdominal aorta, and cardiac function was monitored over 15 wk. At 4 wk after aortic constriction, cardiac hypertrophy with preserved left ventricular performance (compensated hypertrophy) was observed in both W/W(v) and WT mice. Thereafter, left ventricular performance gradually decreased in WT mice, and pulmonary congestion became apparent at 15 wk (decompensated hypertrophy). In contrast, decompensation of cardiac function did not occur in W/W(v) mice; left ventricular performance was preserved throughout, and pulmonary congestion was not observed. Perivascular fibrosis and upregulation of mast cell chymase were all less apparent in W/W(v) mice. Treatment with tranilast, a mast cell-stabilizing agent, also prevented the evolution from compensated hypertrophy to heart failure. These observations suggest that mast cells play a critical role in the progression of heart failure. Stabilization of mast cells may represent a new approach in the management of heart failure.

[Examination of MR cholangiopancreatography using the fast recovery single shot fast spin echo sequence].

During this project, we evaluated methods to scan MRCP images with overlapping slice positions during one breath-hold using the FRSSFSE sequence. The FRSSFSE sequence is a technique to arbitrarily change the residual transverse magnetization to longitudinal magnetization. With the SSFSE sequence, the imaging field where the slice position overlaps is subject to substantial influence from the saturation effect of the water component. Therefore, one breath-hold is required for each image. However, the FRSSFSE sequence enabled the deterioration in image quality due to the saturation effect to be minimized even during a short TR. This enabled images of overlapping slice positions to be scanned during one breath-hold. We used a setting of TR=5,000 ms at our hospital, and scanned several images with overlapping slice positions during one breath-hold. The TR=5,000 ms setting was determined from image quality and breath-hold time considerations. The use of the FRSSFSE sequence with MRCP enabled 3-4 images to be scanned at overlapping slice positions during one breath-hold. This method is effective in reducing examination time and the burden on the patient.