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The performance of an externally-pumped photorefractive phase conjugator is greatly improved, especially in stability and response to signal change, by the phase control of incident beams. Using piezoelectrically-driven mirrors in all incident optical paths, the backward pump is made incoherent with the other input beams and the fluctuation of mutual mean phases between forward pump and signal beams is canceled.

The photorefractive effect was observed in He-ion-implanted semi-insulating GaN film in the UV spectral region. The photorefractive grating is induced by the electroabsorption effect near the band-edge. Two beam coupling experiments were performed at a wavelength of 363.8 nm. The experimental results indicate that absorption grating mainly contributes to the beam coupling.

A collinear holographic memory system recording multichannel time sequential signal is proposed and demonstrated. Higher recording density and data transfer rate comparing to the page-oriented system are expected because Run-Length Limited code can be used.

A polarization coded holographic memory is demonstrated. Writing and reading characteristics are examined through numerical calculations and experiments. The polarization state of the readout signal is influenced by the ratio of intensity and polarization gratings.

Efficient second-harmonic generation of 95 fs pulses were achieved by simultaneous group-velocity matching and quasi-phase matching in a 10 mm long periodically poled potassium niobate crystal. The quasi-phase-matched bandwidth was 38 nm and second-harmonic pulse duration was 158 fs with a fundamental input pulse of 95 fs without spectral and temporal distortion in the S-band of telecommunication. Utilizing the larger off-diagonal nonlinear coefficient, d31 of potassium niobate, the conversion efficiency was significantly improved from that of periodically poled MgO-doped lithium niobate. [Appl. Phys. Lett. 82, 3388 (2003)]

Time-resolved absorption bleaching and four-wave mixing techniques have been used for studies of spin relaxation and nonequilibrium carrier dynamics in resonantly excited InGaAs/GaAs multiple quantum well structures. Kinetics of absorption bleaching at 1064 nm provided spin relaxation times of 240–280 ps and carrier lifetimes of 750–1000 ps at low excitations. At higher excitations, nearly three-fold increase of spin-relaxation rate has been measured by the FWM technique, while the decrease of carrier lifetime was much less pronounced. The kinetics of nonequilibrium carrier grating with randomized spins allowed determination of the in-plane bipolar diffusion coefficient in the wells D = 11.5 cm2 s−1 and hole mobility of 230 cm2 V−1 s−1.

Efficient pulse compression was achieved by the use of cascaded quadratic nonlinearities in 10-mm-long periodically poled MgO-doped lithium niobate. An off-diagonal component of the nonlinear coefficient was utilized for group-velocity matched cascaded interactions at the fundamental wavelength of 1560 nm. Simultaneously compressed fundamental and second-harmonic pulses of ∼35 fs duration were obtained from the 110-fs-pump pulses.

合成桁を斜張橋に適用した場合, 連続合成桁同様に負の曲げモーメント部の床版応力が問題となる. 本橋ではこの問題に対処するために, ケーブル張力を調整することによりプレキャストRC床版にプレストレスカを導入する方法を採用した. 合成桁斜張橋にプレキャスト床版を用いた形式, および上記の床版プレストレスの導入法は, わが国で初めての試みであり, 種々の施工試験や現場計測を実施し, 適用した設計法および施工法の妥当性を検証した.

Polychromatic reconstruction(PCR) is a novel nondestructive readout method that utilizes a spectrally broad light source for the probe beam. The stored image can be completely reconstructed even though the probe wavelength is very different from the recording one. The large spectral width of the polychromatic probe beam also causes adverse effect on the storage density. But this can be overcomed if an additional optical component is inserted in the imaging system. Therefore, PCR has a great potential to achieve nondestructive readout and large storage density simultaneously. In addition, PCR enables us to design a simple memory system because its readout tolerance is quite large as compared to the conventional monochromatic readout. Unique and attractive features of the polychromatic reconstruction method was theoretically and experimentally demonstrated.

We demonstrate efficient frequency doubling of 95-fs pulses with a small temporal broadening in a 10-mm-long, periodically poled, 5-mol % MgO-doped lithium niobate crystal. Simultaneous group-velocity matching and quasi phase matching were achieved by the off-diagonal nonlinear optical coefficient d32 at telecommunications range.

The characteristics of the reconstruction of polarization holography are investigated in order to analyze the fundamental performance of polarization holographic memory system. It is found that, in general, the polarization state of the reconstructed signal is different from that of the recorded signal. This fact makes the design of polarization holographic memory system complicated because we have to retrieve the polarization state of original signal from measured data. In this paper we describe preliminary experimental results of photodissociation-type holographic media.We discuss the special material condition that the signal can be faithfully reconstructed.

In volume holographic memory (van Heerden, 1963), the information is stored as a volume hologram and retrieved through the holographic reconstruction process by illuminating the hologram with a readout probe beam whose wavelength, incident angle, and wavefront should be identical to those of the reference beam used in the recording process. This requirement stems from the fact that diffraction from the volume hologram is restricted by Bragg’s law. While such a restriction is responsible for the large storage density of volume holographic memories, it also causes some obstacles for implementing practical memory systems. For example, in rewritable recording media, like photorefractive materials, illumination with a readout probe beam will rewrite the recorded hologram, destroying the stored information. Even in a photopolymer, some of the storage capacity will be wasted during the readout if some monomers still exist in the readout volume. These issues are obviously caused by the destructive probe beam having the ability to expose the recording medium in a similar manner to the recording beam. To avoid such a problem, several nondestructive readout methods have been proposed so far (Gulanyan et al., 1979; Petrov et al., 1979; Kulich, 1987), where the readout is performed at a longer wavelength, outside the sensitive spectral region of the recording material. These methods can successfully reconstruct the stored image at a wavelength different from the recording one, but most of these methods may not be practical for holographic memory systems because the multiplexing capability is considerably lowered. For example, anisotropic diffraction (Petrov et al., 1979) requires a specific recording configuration and thus limits the number of multiplexed pages. A spherical probe beam method (Kulich, 1987) tends to produce severe crosstalk noise from other multiplexed pages, which demands a large angular separation between two adjacent multiplexed holograms, resulting in a small storage density. Recently, we proposed another way to reconstruct an image at a different wavelength (Fujimura et al., 2007). Our method, which we call polychromatic reconstruction (PCR), utilizes a spectrally broad light source for the probe beam, as shown in Fig. 1. Each angular spectral component of the recorded gratings can be Bragg-matched with one particular wavelength within the broadband spectrum of the probe beam. Thus, the whole image can be reconstructed from the volume hologram even though the probe wavelength is very different from the recording one. On the other hand, analogous to the spherical probe beam

Fractionated radiotherapy (RT) is widely used in cancer therapy for its advantages in the preservation of normal tissues. However, repopulation of surviving tumor cells during fractionated RT limits the efficacy of RT. In fact, repopulating tumors often acquire radioresistance and this is the major cause of failure of RT. We have recently demonstrated that human tumor cells acquire radioresistance when exposed to fractionated radiation (FR) of X-rays every 12 hours for 1 month. The acquired radioresistance was associated with overexpression of cyclin D1, a result of a series of molecular changes; constitutive activation of DNA-PK and AKT with concomitant down-regulation of glycogen synthase kinase-3β (GSK3β) which results in suppression of cyclin D1 proteolysis. Aberrant cyclin D1 overexpression in S-phase induced DNA double strand breaks which activated DNA-PK and established the vicious cycle of cycling D1 overexpression. This overexpression of cyclin D1 is responsible for the radioresistance phenotype of long-term FR cells, since this phenotype was completely abrogated by treatment of FR cells by the API-2, an AKT inhibitor or by a Cdk4 inhibitor. Thus, targeting the AKT/GSK3β/cyclin D1/Cdk4 pathway can be an efficient modality to suppress acquired radioresistance of tumor cells. In this article, I overview the newly discovered molecular mechanisms underlying acquired radioresistance of tumor cells induced by FR, and propose a strategy for eradication of tumors using fractionated RT by overcoming tumor radioresistance.

Antiferromagnetic resonance in orthoferrite NdFeO 3 has been investigated using terahertz time-domain spectroscopy. A resonance at 0.5 THz was detected which was assigned to the AF-mode of the antiferromagnetic spin precession.

The intrinsic problems, such as beam fanning and internal beam reflection, in measuring the photorefractive coupling constant are discussed. We propose a new technique, the varying interaction length method, to avoid them. In this method, the coupling constant is obtained by differentiating the coupling strength that is measured with changing the interaction length. We applied this method to BaTiO 3 and obtained a 10–20% larger coupling constant compared with that of the conventional method.

InGaAs/GaAs multiple quantum well (MQW) photorefractive devices that have the sensitivity around the wavelength of 1.06 μm are fabricated and some characteristics are measured. Saturation intensity is 3.6 mW / cm 2 and cut off grating pitch is 1.4 μm . These values show that this device is usable in practical applications using Nd:YAG lasers. A vibration measurement system using two-wave mixing is also demonstrated.

Spin precession was nonthermally induced by an ultrashort laser pulse in orthoferrite DyFeO3 with a pump-probe technique. Both circularly and linearly polarized pulses led to spin precessions; these phenomena are interpreted as the inverse Faraday effect and the inverse Cotton-Mouton effect, respectively. For both cases, the same mode of spin precession was excited; the precession frequencies and polarization were the same, but the phases of oscillations were different. We have shown theoretically and experimentally that the analysis of phases can distinguish between these two mechanisms. We have demonstrated experimentally that in the visible region, the inverse Faraday effect was dominant, whereas the inverse Cotton-Mouton effect became relatively prominent in the near-infrared region., 27 pages, 8 figures

The recording density limit of the collinear holographic memory purely determined by the optical system, that is under the assumption that the recording material is ideal, is estimated. Signal to noise ratio is decreasing with the increase of the number of overlapped hologram pages because of the increase of the off- and degenerated-Bragg diffraction from the gratings in the holograms increases. The signal to noise ratio is calculated with the plane wave model accompanied with statistical considerations. It is shown that 5 Tbyte recording in a compact disc size is possible in ideal case.

Inverse Faraday effect and optical Kerr effect are measured in antiferromegnetic NiO using pump-probe method. The magnetic field is induced by the illumination of 100 fs intense circularly polarized wave, which is detected by the Faraday rotation of probe beam.

The high-density survival (HDS) assay was originally elaborated to assess cancer cell responses to therapeutic agents under the influence of intercellular communication. Here, we simplified the original HDS assay and studied its applicability for the detection of cellular radioresistance. We have recently defined clinically relevant radioresistant (CRR) cells, which continue to proliferate with daily exposure to 2 gray (Gy) of X-rays for more than 30 days in vitro. We established human CRR cell lines, HepG2-8960-R from HepG2, and SAS-R1 and -R2 from SAS, respectively. In an attempt to apply the HDS assay to detect radioresistance with clinical relevance, we simplified the original HDS assay by scoring the total number of surviving cells after exposure to X-rays. The modified HDS assay successfully detected radioresistance with clinical relevance. The modified HDS assay detected CRR phenotype, which is not always detectable by clonogenic assay. Therefore, we believe that the modified HDS assay presented in this study is a powerful tool to predict the effectiveness of fractionated radiotherapy against malignant tumors.

Far-infrared ultrashort-pulse in the wavelength range of 10 −30 μm is generated using an organic nonlinear crystal DAST. The frequency down conversion is performed by the difference frequency generation between the signal and idler waves from an optical parametric amplifier pumped by the 120 fs Ti: sapphire laser pulses. The pulse width is measured using the cross-correlation with the pump. In the present paper, the shortest pulse width of 210 fs was obtained with the spectrum range of 10 – 30 THz.

The amplified signal beam of photorefractive two-wave mixing was stabilized by feedback system. The amplification factor of the two-wave mixing was controlled by changing the phase difference between the interference fringe and the index grating in BaTiO 3 . The interference fringe was shifted by frequency tuning of a laser diode. The short-term fluctuation of the output beam was reduced to 0.8%. We controlled the output intensity within the range from 10 to 4 mW although the change of the input power was 5%.

We propose a method to generate an ultrahigh-repetition-rate pulse by using an array illuminator and present preliminary experimental results to demonstrate the feasibility of its application to optical communication and readout of images.

We present the experimental results of efficient and alignment-free injection locking of a 0.8-μm diode laser with a mutually pumped phase conjugator. After the slave laser is injection locked, the reflectivity of the double conjugate mirror decreases considerably. The injection-locked state continued for more than 20 min in spite of a decrease in reflectivity. When the frequency of the injecting beam is resonant to one of the slave laser's modes that does not oscillate in the free-running state, mode switching is observed; i.e., the oscillating mode jumps to a new mode that matches the injecting beam.

Non-periodic structure of optical pattern can assign distinctive potential landscape to multiple micro particles according to their sizes, and enables optical sorting using only optical gradient force. We have theoretically and experimentally investigated the dependence of sorting success rate as a function of time on beam power in non fluidic flow system. In the experiment, polystyrene spheres suspended in water were used as samples, and the radii were 0.49 μm, 1.03 μm, and 1.61 μm, respectively. We used multiple optical lines in the experiment, and placed it non-periodically with increasing peak intensities. The experimental result qualitatively agreed with the theoretical one, and the sorting success rate was more than 90 % with sufficient beam power. We demonstrated our method in the system with flow by placing optical pattern such that optical gradient force acted on particles orthogonal to the flow. In the experiment with flow, triangle optical pattern was used to lift up micro particles onto the top surface where image of optical pattern was formed and to carry the particles to starting position for sorting. Our sorting system can in principle work in broad range of particle velocity with sufficient beam power.

We numerically show that it is possible to achieve adiabatic compression of femtosecond quadratic solitons in aperiodically poled lithium niobate device. Two-colored solitons of the fundamental wavelength of 1560 nm can be adiabatically shaped by using group-velocity matching schemes available in quasi-phase-matching (QPM) devices. We investigate the performance of the adiabatic compression based on two different group-velocity matching schemes: type-I (e: o + o) collinear QPM geometry and type-0 (e: e + e) non-collinear QPM geometry. Two-colored temporal solitons with pulse duration of 35 fs are generated without visible pedestals from 100-fs fundamental pulse. We also show that walking solitons with shorter pulse durations are adiabatically excited under small group-velocity mismatch condition. The walking solitons experience deceleration or acceleration during compression, depending on the sign of the groupvelocity- mismatch. The demonstrated adiabatic pulse shaping is useful for generation of shorter pulses with clean temporal profiles, efficient femtosecond second harmonic generation and group-velocity control.

Collinear holographic memory is one of the candidates of post Blu-ray disc system. This system has several advantages comparing to other holographic memory systems [1]. For examples, collinear system is quite stable because both reference and signal waves passes through common optical elements. It is a kind of common-path interferometer. Also, recording media of same physical size as CD can be used because this system requires only one objective lens and reflection geometry can be used to write and read holograms which contain the digital page data. The matured servo technologies for conventional optical discs can also be used due to its geometry of the optical head. We have been studying the write and read characteristics of this system, and revealed that the main limiting factors for the recording density is wavelength of the light, numerical aperture (NA) of the objective lens, coding efficiency of the data, and the degree of multiplexing. Among these four factors, theoretical limit of the degree of multiplexing has not been clear yet. It is limited by the dynamic range of the recording media, and decrease of the signal to noise ratio (SNR) due to the intra- and inter- page crosstalk noises. In this paper, we will present the estimation of these noises with our model for the collinear holographic memory system.

Tunable and high power ultra-short pulses in mid- and far-infrared (IR) spectral region are required for pump-probe spectroscopy of molecular vibrational structures and the low-energy excitations in solid state materials. Nonlinear optical frequency down conversion has been demonstrated for mid-IR short-pulse generation [1]. Among inorganic and organic materials, an organic nonlinear optical crystal DAST (4-dimethylamino-N-methyl–4-stilbazolium tosylate), which consists of a stilbazolium cation and a tosylate anion, is quite efficient for the frequency conversion down to THz wave region because of large nonlinear optical coefficients and relatively small absorption [2, 3]. In the present paper, we report the generation of mid-IR short-pulse in spectral range of 10 ∼ 40 µm (7.5 ∼ 30 THz) using DAST crystals.

Single page SNR is obtained with the page data read and write simulation based on the model we proposed before. Shift selectivity is calculated with the simple Fourier Transformation of the reference pixel pattern. Several numerical results are shown and both two factors strongly depend on the reference pattern. Experimental verification of our calculation of the shift selectivity is also presented.

Radiotherapy is one of the major therapeutic modalities for eradicating malignant tumors. However, the existence of radioresistant cells remains one of the most critical obstacles in radiotherapy and radiochemotherapy. Standard radiotherapy for tumor treatment consists of approximately 2 Gy once a day, 5 days a week, over a period of 5-8 weeks. To understand the characteristics of radioresistant cells and to develop more effective radiotherapy, we established a novel radioresistant cell line, HepG2-8960-R with clinical relevance from parental HepG2 cells by long-term fractionated exposure to 2 Gy of X-rays. HepG2-8960-R cells continued to proliferate with daily exposure to 2 Gy X-rays for more than 30 days, while all parental HepG2 cells ceased. After exposure to fractionated 2 Gy X-rays, induction frequencies of micronuclei and remaining foci of gamma-H2AX in HepG2-8960-R were less than those in HepG2. Flow cytometric analysis revealed that the proportion of cells in S- and G2/M-phase of the cell cycle was higher in HepG2-8960-R than in HepG2. These suggest that the response of clinically relevant radioresistant (CRR) cells to fractionated radiation is not merely an accumulated response to each fractionated radiation. This is the first report on the establishment of a CRR cell line from an isogenic parental cell line.

Tunable mid-infrared ultrashort pulses are generated by difference frequency mixing in DAST. The spectrum extends over the wavelength of 10-30 μm. The pulse duration is determined to be 370 fs by the cross-correlation measurement.

We measured the spatial coherernce of a copper vapour laser (CVL) beam with an unstable resonator by the reversal shear interferometer. By this method, we can evaluate the spatial coherence function from a single-shot measurement. The spatial coherence width was 5 mm when an unstable resonator with a magnification factor of 60 was used. Moreover we verified the result by the theoretical calculation on the basis of the passive resonator model.

Linear transformation of lateral speckle-pattern displacements into the transmitted intensity modulation is achieved in a photorefractive GaP crystal using the polarization self-modulation effect. The optical system is very simple, self-referenced, and as sensitive as an interferometer.

We show that a double-random encryption technique can improve the storage capacity of an angular-multiplexed holographic memory system. In the holographic memory system, input binary images are encrypted into white-noise-like images by use of two random phase masks located at the input and the Fourier planes. These encrypted images are stored as holograms in a photorefractive medium by use of angular multiplexing. All the images are encrypted by different sets of random phase masks. Even when the angle separation between adjacent images is small enough to cause cross talk between adjacent images, original binary data can be recovered with the correct phase mask; the other reconstructed images remain white-noise-like images because incorrect masks are used. Therefore the capacity of the proposed system can be larger than that of a conventional holographic memory system without the random phase encryption technique. Numerical evaluation and experimental results are presented to confirm that the capacity of the system with random phase masks is larger than that of the conventional memory system.

A novel secure holographic memory system with polarization encoding is proposed. Two-dimensional original data are encoded as a two-dimensional polarization distribution. The polarization state at each pixel is scrambled by a mask that changes the polarization state into a random state. The mask can rotate the direction of the principal axes of the elliptically polarized light and can change the phase retardation at each pixel. The light with the random polarization state is stored as a hologram that can produce the vector phase-conjugate beam. In the decryption the vector phase-conjugation readout can recover the original polarization state by use of the same mask used in the encryption. Experimental results of encryption and decryption with a bacteriorhodopsin film are presented.

A secure holographic memory system that uses fully phase encryption is presented. Two-dimensional arrays of data are phase encoded. Each array is then transformed into a stationary white-noise-like pattern by use of a random-phase mask located at the input plane and another at the Fourier plane. This encrypted information is then stored holographically in a photorefractive LiNbO(3):Fe crystal. The original phase-encoded data can be recovered, by use of the two random-phase masks, with a phase-conjugate readout beam. This phase information can then be converted back to intensity information with an interferometer. Recording multiple images by use of angular multiplexing is demonstrated. The influence of a limited system bandwidth on the quality of reconstructed data is evaluated numerically. These computer simulation results show that a fully phase-based encryption system generally performs better than an amplitude-based encryption system when the system bandwidth is limited by a moderate amount.

We estimated theoretically the conversion efficiency of the second harmonic generation of a copper vapor laser (CVL) beam. Since the CVL beam is only partially coherent, the theory for the coherent beam is useless for the second harmonic generation of the CVL beam. The focused CVL beam was considered as a set of bundles of rays with propagated in different directions through a crystal and non-collinear interaction between these bundles of rays was investigated. On the basis of this model, we found that the conversion efficiency depends on the product of the beam divergence and the beam diameter, and the length of a nonlinear crystal.

The beam from a copper vapor laser with an unstable resonator was focused in a nonlinear crystal, barium borate, for second harmonic generation. Ultraviolet light of 255.3 nm was generated with an conversion efficiency of 8.9% at an average input power of 2.55 W. The characteristics of conversion were investigated.

The simultaneously group velocity (GV) matched second harmonic generation (SHG) and third harmonic generation (THG) of femtosecond pulse in a single 2D quasi-phase-matching (QPM) device have been demonstrated. The conversion efficiencies of 27% for SHG and 7% for THG with the pulse duration of -100 fs was achieved. The demonstration has proved that 2D QPM devices are of significant importance for compact and powerful ultrafast coherent sources. It was believed that the same concept is promising for other multistep chi(2) cascading applications of ultrashort pulses.

Mid-IR femtosecond pulse generation scheme is developed based on optical parametric amplification in periodically-poled lithium niobate with 800-nm Ti:sapphire laser as the pump. We obtained 620-nm bandwidth mid-IR pulses at center wavelength of 3.5 mum. After the compensation of the second-order dispersion, the pulses are compressed down to 42 fs.

Mathematical simulations of the p53-Mdm2 feedback loop suggest that both proteins will exhibit impulsive expression characteristics in response to high cellular stress levels. However, little quantitative experimental evaluation has been done, particularly of the phosphorylated forms. To evaluate the mathematical models experimentally, we used lysate microarrays from an isogenic pair of γ-ray–irradiated cell lysates from HCT116 (p53+/+ and p53−/−). Both p53 and Mdm2 proteins showed expected pulses in the wild type, whereas no pulses were seen in the knockout. Based on experimental observations, we determined model parameters and generated an in silico “knockout,” reflecting the experimental data, including phosphorylated proteins. [Cancer Res 2007;67(13):6247–52]

In this paper, we propose another way to reconstruct the image from the volume hologram at a different wavelength. Our method utilizes a spectrally broad but spatially coherent light for the probing beam. The paper illustrates the record and readout scheme of our proposed method. In the recording process, the monochromatic signal beam bearing the image information passes through a Fourier transform lens, and records the Fourier hologram in the ordinary way. In the readout process, the collimated probing beam with the broadband spectrum is incident on the crystal at an appropriate angle. Each angular spectral component of the grating can be Bragg-matched at one particular wavelength within the broadband spectrum of the probing beam. Finally the diffracted waves image onto the detector dispersively and reproduce the input image.

We propose and demonstrate group-velocity mismatch compensation in cascaded third-harmonic generation. Second- and third-harmonic generation efficiency of 25% and 7%, respectively, were experimentally obtained for femtosecond pulses by use of two-dimensionally periodically-poled lithium niobate.

We propose a new method to read volume holograms at wavelengths different from that of recording. Whole image can be reconstructed using a spectrally broad but spatially coherent light source in spite of the restriction due to the Bragg condition. This is demonstrated for volume holograms recorded in LiNbO3.

We investigate that the equivalent Kerr self-phase-modulation (SPM) and cross-phase-modulation (XPM) effects introduced in cascaded second-order processes is dependent on the phase mismatch and group velocity mismatch. Adiabatic compression of quadratic solitons and frequency shift is reported.

We fabricated a two-dimensional quasi-phase-matching device. Efficient cascaded 2nd- and 3rd-harmonic generation of a 100-fs pulse at 1570 nm is demonstrated. Group velocity mismatch is compensated by pulse-front tilt technique with non-collinear geometry.