Development of a high-performance and cost-effective in-vacuum undulator.

In-vacuum undulators (IVUs), which have become an essential tool in synchrotron radiation facilities, have two technical challenges toward further advancement: one is a strong attractive force between top and bottom magnetic arrays, and the other is a stringent requirement on magnetic materials to avoid demagnetization. The former imposes a complicated design on mechanical and vacuum structures, while the latter limits the possibility of using high-performance permanent magnets. To solve these issues, a number of technical developments have been made, such as force cancellation and modularization of magnetic arrays, and enhancement of resistance against demagnetization by means of a special magnetic circuit. The performance of a new IVU built upon these technologies has revealed their effectiveness for constructing high-performance IVUs in a cost-effective manner.

Green upgrading of SPring-8 to produce stable, ultrabrilliant hard X-ray beams.

SPring-8-II is a major upgrade project of SPring-8 that was inaugurated in October 1997 as a third-generation synchrotron radiation light source. This upgrade project aims to achieve three goals simultaneously: achievement of excellent light source performance, refurbishment of aged systems, and significant reduction in power consumption for the entire facility. A small emittance of 50 pm rad will be achieved by (1) replacing the existing double-bend lattice structure with a five-bend achromat one, (2) lowering the stored beam energy from 8 to 6 GeV, (3) increasing the horizontal damping partition number from 1 to 1.3, and (4) enhancing horizontal radiation damping by installing damping wigglers in long straight sections. The use of short-period in-vacuum undulators allows ultrabrilliant X-rays to be provided while keeping a high-energy spectral range even at the reduced electron-beam energy of 6 GeV. To reduce power consumption, the dedicated, aged injector system has been shut down and the high-performance linear accelerator of SACLA, a compact X-ray free-electron laser (XFEL) facility, is used as the injector of the ring in a time-shared manner. This allows the simultaneous operation of XFEL experiments at SACLA and full/top-up injection of the electron beam into the ring. This paper overviews the concept of the SPring-8-II project, the system design of the light source and the details of the accelerator component design.

Experimental Demonstration to Control the Pulse Length of Coherent Undulator Radiation by Chirped Microbunching.

In seeded free electron lasers (FELs), the temporal profile of FEL pulses usually reflects that of the seed pulse, and, thus, shorter FEL pulses are available with shorter seed pulses. In an extreme condition, however, this correlation is violated; the FEL pulse is stretched by the so-called slippage effect in undulators, when the seed pulse is ultimately short, e.g., few-cycles long. In a previous Letter, we have proposed a scheme to suppress the slippage effect and reduce the pulse length of FELs ultimately down to a single-cycle duration, which is based on "chirped microbunching," or an electron density modulation with a varying modulation period. Toward realization of FELs based on the proposed scheme, experiments have been carried out to demonstrate its fundamental mechanism in the NewSUBARU synchrotron radiation facility, using an ultrashort seed pulse with the pulse length shorter than five cycles. Experimental results of spectral and cross-correlation measurements have been found to be in reasonable agreement with the theoretical predictions, which strongly suggests the successful demonstration of the proposed scheme.

Development of an undulator with a variable magnetic field profile.

An undulator generating a magnetic field whose longitudinal profile is arbitrarily varied has been developed, which is one of the key components in a number of proposed new concepts in free-electron lasers. The undulator is composed of magnet modules, each of which corresponds to a single undulator period, and is driven by a linear actuator to change the magnetic gap independently. To relax the requirement on the actuator, the mechanical load on each module due to magnetic force acting from opponent and adjacent modules is reduced by means of two kinds of spring systems. The performance of the constructed undulator has been successfully demonstrated by magnetic measurement and characterization of synchrotron radiation.

Transient grating in a thin gas target for characterization of extremely short optical pulses.

A transient-grating cross-correlation frequency-resolved optical gating (TG XFROG) with a thin gas target toward characterization of sub-femtosecond optical pulses is discussed. For evaluation of the reliability, sub-10 fs near-infrared pulses are characterized, the results of which are compared with those given by the sum-frequency-generation XFROG. The TG XFROG covers the nanojoule energy range or that for the advanced few-cycle UV pulses recently reported. It is also shown that the TG XFROG fails to characterize and heavily underestimates the durations of intense test pulses. The FROG technique sensitively detects the onset of this anomalous behavior, which represents a serious issue for pulse characterizations.

Interaction mapping of the Sec61 translocon identifies two Sec61α regions interacting with hydrophobic segments in translocating chains.

Many proteins in organelles of the secretory pathway, as well as secretory proteins, are translocated across and inserted into the endoplasmic reticulum membrane by the Sec61 translocon, a protein-conducting channel. The channel consists of 10 transmembrane (TM) segments of the Sec61α subunit and possesses an opening between TM2b and TM7, termed the lateral gate. Structural and biochemical analyses of complexes of Sec61 and its ortholog SecY have revealed that the lateral gate is the exit for signal sequences and TM segments of translocating polypeptides to the lipid bilayer and also involved in the recognition of such hydrophobic sequences. Moreover, even marginally hydrophobic (mH) segments insufficient for membrane integration can be transiently stalled in surrounding Sec61α regions and cross-linked to them, but how the Sec61 translocon accommodates these mH segments remains unclear. Here, we used Cys-scanned variants of human Sec61α expressed in cultured 293-H cells to examine which channel regions associate with mH segments. A TM segment in a ribosome-associated polypeptide was mainly cross-linked to positions at the lateral gate, whereas an mH segment in a nascent chain was cross-linked to the Sec61α pore-interior positions at TM5 and TM10, as well as the lateral gate. Of note, cross-linking at position 180 in TM5 of Sec61α was reduced by an I179A substitution. We therefore conclude that at least two Sec61α regions, the lateral gate and the pore-interior site around TM5, interact with mH segments and are involved in accommodating them.

Attosecond single-cycle undulator light: a review.

Research at modern light sources continues to improve our knowledge of the natural world, from the subtle workings of life to matter under extreme conditions. Free-electron lasers, for instance, have enabled the characterization of biomolecular structures with sub-ångström spatial resolution, and paved the way to controlling the molecular functions. On the other hand, attosecond temporal resolution is necessary to broaden our scope of the ultrafast world. Here we discuss attosecond pulse generation beyond present capabilities. Furthermore, we review three recently proposed methods of generating attosecond x-ray pulses. These novel methods exploit the coherent radiation of microbunched electrons in undulators and the tailoring of the emitted wavefronts. The computed pulse energy outperforms pre-existing technologies by three orders of magnitude. Specifically, our simulations of the proposed Soft X-ray Laser at MAX IV (Lund, Sweden) show that a pulse duration of 50-100 as and a pulse energy up to 5 [Formula: see text]J is feasible with the novel methods. In addition, the methods feature pulse shape control, enable the incorporation of orbital angular momentum, and can be used in combination with modern compact free-electron laser setups.

Mode-locked laser with a repetition rate of 17.6 THz.

In 2001, a concept was proposed to generate mode-locked ultrashort laser pulses with a repetition rate in excess of 10 THz [Phys. Rev. Lett.87, 223901 (2001)PRLTAO0031-900710.1103/PhysRevLett.87.223901], which has not been demonstrated so far. In the present research, the concept is experimentally demonstrated using a dispersion-compensated high-finesse cavity filled with hydrogen gas. Second-order intensity autocorrelation is used for distinguishing two cases with and without mode-locking as well as for characterization of the temporal profile. Mode-locked sub-30-fs pulses with a repetition rate of 17.6 THz are synthesized by continuous-wave laser lines generated via stimulated Raman scattering and four-wave Raman mixing induced in the high-finesse cavity.

Using irregularly spaced current peaks to generate an isolated attosecond X-ray pulse in free-electron lasers.

A method is proposed to generate an isolated attosecond X-ray pulse in free-electron lasers, using irregularly spaced current peaks induced in an electron beam through interaction with an intense short-pulse optical laser. In comparison with a similar scheme proposed in a previous paper, the irregular arrangement of current peaks significantly improves the contrast between the main and satellite pulses, enhances the attainable peak power and simplifies the accelerator layout. Three different methods are proposed for this purpose and achievable performances are computed under realistic conditions. Numerical simulations carried out with the best configuration show that an isolated 7.7 keV X-ray pulse with a peak power of 1.7 TW and pulse length of 70 as can be generated. In this particular example, the contrast is improved by two orders of magnitude and the peak power is enhanced by a factor of three, when compared with the previous scheme.

Resonant and nonresonant multiphoton ionization processes in the mass spectrometry of explosives.

Multiphoton ionization processes were studied for three types of explosives using a line-tunable ultraviolet femtosecond laser. When peroxides such as triacetone triperoxide (TATP) and hexamethylene triperoxide diamine (HMTD) were ionized through a nonresonant two-photon process, a molecular ion was dominantly observed by reducing the excess energy remaining in the ion. However, an aromatic nitro compound such as 2,4,6-trinitrotoluene (TNT) produced large signals arising from molecular and fragment ions by resonant two-photon ionization. In addition, only fragment ions were produced from a nonaromatic nitro compound such as 1,3,5-trinitroperhydro-1,3,5-triazine (RDX), even when a resonant two-photon ionization process was employed, suggesting that a further reduction in excess energy would be necessary if a molecular ion were to be observed.

Generation of intense subcycle optical pulses in a gas.

The generation of intense subcycle laser pulses during the propagation of two-color femtosecond pulses in a gas medium is investigated theoretically and experimentally. Four-wave mixing induced by the laser pulses in a gas medium generates multi-octave laser radiation from the ultraviolet to the infrared, which forms stable subcycle laser pulses after a certain propagation distance in a gas medium with group-velocity dispersion. The intense subcycle laser pulses would allow the coherent control of the waveforms of soft-x-rays generated via high-harmonic generation.

A few positively charged residues slow movement of a polypeptide chain across the endoplasmic reticulum membrane.

Many polypeptide chains are translocated across and integrated into the endoplasmic reticulum membrane through protein-conducting channels. During the process, amino acid sequences of translocating polypeptide chains are scanned by the channels and classified to be retained in the membrane or translocated into the lumen. We established an experimental system with which the kinetic effect of each amino acid residue on the polypeptide chain movement can be analyzed with a time resolution of tens of seconds. Positive charges greatly slow movement; only two lysine residues caused a remarkable slow down, and their effects were additive. The lysine residue was more effective than arginine. In contrast, clusters comprising three residues of each of the other 18 amino acids had little effect on chain movement. We also demonstrated that a four lysine cluster can exert the effect after being fully exposed from the ribosome. We concluded that as few as two to three residues of positively charged amino acids can slow the movement of the nascent polypeptide chain across the endoplasmic reticulum membrane. This effect provides a fundamental basis of the topogenic function of positively charged amino acids.

Frequency-resolved optical gating with two nonlinear optical processes.

A frequency-resolved optical gating (FROG) technique that combines autocorrelation (second-harmonic-generation FROG, SHG-FROG) and cross correlation FROG (XFROG) is reported for simultaneous characterization of two unknown optical pulses. Two SHG-FROG signals and a XFROG signal are acquired in a single measurement using a single diagnostic system. Unlike the conventional combination of SHG-FROG for reference-pulse characterization and XFROG for test-pulse characterization, the ambiguity in the direction of time in SHG-FROG is removed during phase retrieval by simultaneously analyzing the three FROG signals. Furthermore, overall characterization is faster, more robust, and highly convergent than the conventional combination of SHG-FROG and XFROG techniques.

Positive charges on the translocating polypeptide chain arrest movement through the translocon.

Polypeptide chains synthesized by membrane-bound ribosomes are translocated through, and integrated into, the endoplasmic reticulum (ER) membrane by means of the protein translocation channel, the translocon. Positive charges on the nascent chain determine the orientation of the hydrophobic segment as it is inserted into the translocon and enhance the stop-translocation of translocating hydrophobic segments. Here we show that positive charges temporarily arrested ongoing polypeptide chain movement through the ER translocon by electrostatic interaction, even in the absence of a hydrophobic segment. The C-terminus of the polypeptide chain was elongated during the arrest, and then the full-length polypeptide chain moved through the translocon. The translocation-arrested polypeptide was not anchored to the membrane and the charges were on the cytoplasmic side of the membrane. The arrest effect was prevented by negatively charged residues inserted into the positive-charge cluster, and it was also suppressed by high salt conditions. We propose that positive charges are independent translocation regulators that are more active than previously believed.

Phosphatidylserine-binding protein lactadherin inhibits protein translocation across the ER membrane.

Secretory and membrane proteins are translocated across and inserted into the endoplasmic reticulum membrane via translocon channels. To investigate the effect of the negatively-charged phospholipid phosphatidylserine on the translocation of nascent polypeptide chains through the translocon, we used the phosphatidylserine-binding protein lactadherin C2-domain. Lactadherin inhibited targeting of nascent chain to the translocon by signal sequence and the initiation of translocation. Moreover, lactadherin inhibited the movement of the translocating polypeptide chain regardless of the presence or absence of positively-charged residues. Phosphatidylserine might be critically involved in translocon function, but it is not a major determinant for translocation arrest of positively-charged residues.

Tail-extension following the termination codon is critical for release of the nascent chain from membrane-bound ribosomes in a reticulocyte lysate cell-free system.

Nascent chain release from membrane-bound ribosomes by the termination codon was investigated using a cell-free translation system from rabbit supplemented with rough microsomal membrane vesicles. Chain release was extremely slow when mRNA ended with only the termination codon. Tail extension after the termination codon enhanced the release of the nascent chain. Release reached plateau levels with tail extension of 10 bases. This requirement was observed with all termination codons: TAA, TGA and TAG. Rapid release was also achieved by puromycin even in the absence of the extension. Efficient translation termination cannot be achieved in the presence of only a termination codon on the mRNA. Tail extension might be required for correct positioning of the termination codon in the ribosome and/or efficient recognition by release factors.

Pleiotropic effects of membrane cholesterol upon translocation of protein across the endoplasmic reticulum membrane.

Various proteins are translocated through and inserted into the endoplasmic reticulum membrane via translocon channels. The hydrophobic segments of signal sequences initiate translocation, and those on translocating polypeptides interrupt translocation to be inserted into the membrane. Positive charges suppress translocation to regulate the orientation of the signal sequences. Here, we investigated the effect of membrane cholesterol on the translocational behavior of nascent chains in a cell-free system. We found that the three distinct translocation processes were sensitive to membrane cholesterol. Cholesterol inhibited the initiation of translocation by the signal sequence, and the extent of inhibition depended on the signal sequence. Even when initiation was not inhibited, cholesterol impeded the movement of the positively charged residues of the translocating polypeptide chain. In surprising contrast, cholesterol enhanced the translocation of hydrophobic sequences through the translocon. On the basis of these findings, we propose that membrane cholesterol greatly affects partitioning of hydrophobic segments into the membrane and impedes the movement of positive charges.

Ultrafast spectroscopy with sub-10 fs deep-ultraviolet pulses.

Time-resolved transient absorption spectroscopy with sub-9 fs ultrashort laser pulses in the deep-ultraviolet (DUV) region is reported for the first time. Single 8.7 fs DUV 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. Electronic excited state and vibrational dynamics are simultaneously observed for an aqueous solution of thymine over the full spectral range using a 128-channel lock-in detector. Vibrational modes of the electronic ground state and excited states can be observed as well as the decay dynamics of the electronic excited state. Information on the initial phase of the vibrational modes is extracted from the measured difference absorbance trace, which contains oscillatory structures arising from the vibrational modes of the molecule. Along with other techniques such as time-resolved infrared spectroscopy, spectroscopy with sub-9 fs DUV pulses is expected to contribute to a detailed understanding of the photochemical dynamics of biologically significant molecules that absorb in the DUV region such as DNA and amino acids.

Sub-10-fs deep-ultraviolet light source with stable power and spectrum.

In some applications of ultrafast spectroscopy that employ sub-10-fs pulses, the pulse spectrum and power need to be stable for several tens of minutes. In this study, we generate sub-10-fs deep-ultraviolet (DUV) pulses with such stabilities by chirped-pulse four-wave mixing. A power fluctuation of less than 3% rms was realized by employing stabilization schemes that employ a power stabilizer. The pulses generated in this study have been applied to transient absorption spectroscopy in the DUV with a sub-10-fs time resolution [Phys. Chem. Chem. Phys.14, 6200 (2012).10.1039/c2cp23649d]. This sub-10-fs DUV source has a similar performance to widely used noncollinear optical parametric amplifiers.

Simultaneous compression and amplification of a laser pulse in a glass plate.

We demonstrated a novel method of simultaneous compression and amplification of a weak laser pulse in a glass plate using cross-phase modulation in conjunction with four-wave optical parametric amplification that was pumped by an intense femtosecond pulse. A proof-of-principle experiment succeeded in smooth broadening of the weak pulse spectrum by a factor of about three and simultaneously amplifying the pulse energy by more than three times. By using chirped mirrors to compensate the dispersion, the weak pulse was compressed from 22.6 fs to 12.6 fs. Furthermore, the output spectrum of seed pulse could be tuned by varying the delay of the intense pump pulse with respect to the weak seed pulse. This method can also be used for simultaneous spectral broadening of several weak beams with different wavelength at the same time.