All-optical continuous-tunable image switch based on nonlinear photoinduced anisotropy in bacteriorhodopsin film

Using nonlinear photoinduced anisotropy in bacteriorhodopsin (bR) film, we presented and demonstrated an image switch in which the output can be tuned continuously by the intensity of a pumping beam. A laser with wavelength 532 nm was used as the pumping beam, and a He-Ne laser at wavelength 632.8 nm was used as the probe beam. Without pumping light, a little of polarized probe beam can transmit the crossed polarizers and the output is very low. With the presence of pumping light, owing to photoinduced anisotropy in the bR film, a portion of the probe beam transmits the crossed polarizers, depending on the intensity of the pumping beam. For the low-intensity probe beam (0.44 mW/cm²), the output is dependent on a wide range of pumping beam (2-30 mW/cm²). On the contrary, for the high-intensity probe beam (0.80 mW/cm²), the output is dependent on a narrow range of pumping beam (2-7 mW/cm²).     

KNSBN：Co晶体自泵浦位相共轭特性的研究

本文报道在两块不同尺寸的KNSBN:Co晶体中实现了自泵浦位相共轭输出,通过实验验证了位相共轭反射率与入射角、入射位置以及波长有关,并给出各自的实验曲线.保持激光器工作波长为514.5nm不变,在大块晶体中可得到高达73.0%的位相共轭反射率.文章还给出共轭光的形成时间随系统光强变化关系的实验曲线,并指出自泵浦形成时透射光强和晶体表面反射光强的时间变化特性,解释了自泵浦形成的阈值特性.     

High reflectivity phase conjugation in magnetized diffusion driven semiconductors

Optical phase conjugation via stimulated Brillouin scattering (OPC-SBS) in magnetized diffusion driven semiconductors under the off-resonant transition regime has been investigated theoretically. The model is based upon the coupled-mode approach and incorporates the effect of pump absorption through the first-order induced polarization. The linear dispersion is found not to affect the reflectivity of the phase conjugate Stokes shifted Brillouin mode. The reflectivity of the image radiation is dependent upon the Brillouin susceptibility and can be significantly enhanced through n-type doping of the crystal and the simultaneous application of magnetic field. Moreover, the threshold of the pump intensity required for the occurrence of SBS in the crystal with finite optical attenuation can be considerably diminished through a suitable choice of the excess carrier concentration and the magnetic field. Consequently, OPC-SBS becomes a possible tool in phase-conjugate optics even under not-too-high power laser excitation by using moderately doped n-type semiconductors kept under the influence of magnetic field. Numerical estimates made for n-InSb crystal at 77 K duly irradiated by nanosecond pulsed 10.6 μm CO₂ laser show that high OPC-SBS reflectivity (70%) can be achieved at pump intensities below the optical damage threshold if the crystal is used as an optical waveguide with relatively large interaction length (L ∼5 mm) which proves its potential in practical applications such as fabrication of phase conjugate mirrors.     

Invertible dark-center diffraction of the metal film induced by femtosecond laser

We found reversible dark-center diffraction of the transmitted probe beam passing through the chromium film, which is induced by the pump femtosecond laser. The dark-center diffraction of the transmitted probe beam appears and disappears with and without the pump beam. A view of diffractive optics with binary phase plate is put forward, which explains the reversible dark-center diffractive optical phenomenon. The pre-ablated hole on the metal film can be regarded as a uniform light filed without phase modulation, the surrounding circular part around the pre-ablated hole can be regarded as “phase modulated”. Therefore, this diffraction optic view might be helpful for us to understand the phase change of the metal film introduced by the femtosecond laser pulse.     

SBN:Cr晶体的光折变四波混频相位共轭特性实验研究

从实验上研究了同成分SBN:Cr晶体在He-Ne激光照射下的四波混频相位共轭特性.首先测量了在不同泵浦光与信号光光强比m及不同光束夹角2θ的情况下,晶体的四波混频相位共轭反射率R随泵浦光强比p的变化关系.其次测量了泵浦光束与信号光束耦合方向对SBN:Cr晶体的四波混频相位共轭特性的影响,并就两种不同掺杂浓度的晶体样品进行了对比,所得实验结果与理论分析基本一致.最后,利用SBN:Cr晶体四波混频相位共轭特性进行了图象畸变消除实验.     

Optical phase conjugation and double-exposure phase-conjugate interferometry in dye-doped thin film using He–Ne laser

We have experimentally demonstrated optical phase conjugation and double-exposure phase-conjugate interferometry in a methyl green dye-doped gelatin film via degenerate four-wave mixing using a low-power He–Ne laser for the first time. The origin of phase conjugation associated with this dye-doped film is discussed. A phase-conjugate reflectivity of 0.13% is obtained with a He–Ne laser of total power 35 mW. PACS 42.65.Hw; 42.65.-k; 42.40.-i     

Spatially inhomogeneous carrier concentration dependence of the reflectivity of semiconductors

The excite-and-probe technique was used to study the optically induced charge carrier concentration dependence of the reflectivity of semiconductors, both experimentally and theoretically, in the case of an exponential carrier distribution. The second harmonic of an Nd : YAG laser (pulse duration 25 ps) was used as the exciting beam; the fundamental wave served as the probe beam. At a carrier concentration of about 2.5×10²¹ cm^–3 a minimum reflectivity was obtained if the angle of incidence was greater than 40°. For increasing absorption constants of the semiconductor at the exciting frequency, this minimum value of the reflectivity was shown to increase. The dependence of the reflectivity on the exponentially distributed carrier concentration was studied experimentally for thin amorphous silicon films produced by the glow-discharge technique. For an absorption constant of 6×10⁵ cm^–1 at 532 nm, good agreement was found between the numerical calculations and the experimental results.     

Phase conjugation in potassium vapour using subnanosecond laser pulses

Phase conjugated waves (PCW) were generated for the first time by degenerate four-wave mixing with subnanosecond laser pulses in potassium vapour near the D ₁ transition at 7699 Å. The light source was a long cavity grazing incidence dye laser. The pulses had a duration of 700–800 ps and a linewidth of 0.2 Å. The experiments were performed in a temperature range of 400–550° C. A maximum reflectivity of 57% has been observed together with temporal pulse shortening. PCW reflectivity has been measured as a function of the temperature, and the backward pump intensity.     

Optical transmission and reflection of aluminum film irradiated by nanosecond laser beam and experimental studying of phase-explosion

In this paper we present evidence for a phase explosion during the laser-induced ablation process by studying the optical reflectivity of the ablated plume. The ablation was produced by irradiating thin film aluminum coated on a quartz substrate with a single pulse laser beam in ambient air. The laser pulse was provided by the second harmonic of a Q-switched Nd:YAG laser with ∼10 ns pulse duration. The transmission of a low power He–Ne laser beam through the hot ablated material plume and its reflection (from the front surface, and rear surface of aluminum film) were also monitored during the duration of the ablation event. The results show that the front surface reflectivity is enhanced at an early time of ablation which is described as strong evidence for the creation of a phase explosion in this process.     

Analysis of low power spatial light modulation characteristics of bacteriorhodopsin

We present a detailed and accurate analysis of low power spatial light modulation characteristics of bacteriorhodopsin (bR) based on nonlinear intensity induced absorption. Amplitude modulation of probe laser read beam transmissions at 410 nm and 640 nm, corresponding to the peak absorption of M^II and O states of D96N bR and WT bR respectively, by the modulation laser write beam intensity-induced population changes at 570 nm has been analyzed, considering all intermediate states with both forward and backward transitions in the respective bR photocycles, using the rate equation approach. The SLM characteristics are shown to be sensitive to the normalized small signal absorption coefficient β, rate constants of M^II and O intermediate states and the absorption cross-section of the initial B state at the probe wavelength (σ_Bp). There exists an optimum value of β for which maximum percentage modulation can be achieved. It is shown that for extended M^II state lifetime of 250 s in D96N bR and O state lifetime of 2.2 s in WT bR, with σ_Bp= 0, 100% modulation of read beam transmissions can be achieved, leading to high dynamic range and sensitivity for low laser write beam intensities of 50 μW/cm² and 4 mW/cm² at 570 nm, respectively.     

Lasing characteristics of Rhodamine B and Rhodamine 6G as a sensitizer in sol–gel silica

We report lasing characteristics of Rhodamine B (Rh. B) in sol–gel silica under excitation with frequency-doubled Nd:YAG laser and sensitization with Rhodamine 6G (Rh. 6G). The principle of radiative energy transfer (from Rh. 6G to Rh. B) has been utilized as a longitudinally Rh. 6G laser (at 585 nm)-pumped Rh. B laser process in the same sample. Rh. B offers a high photostable and efficient laser dye in sol–gel silica sensitized with Rh. 6G; 75,000 shots as a laser half-lifetime of the sample and 24% efficiency at pumping intensity 0.1 J/cm² of 532 nm. Wavelength shift occurs from 606 to 630 nm in the Rh. B laser with increasing its concentration from 1×10⁻⁴ to 8×10⁻⁴ M. The measured optical gain for Rh. B sensitized with Rh. 6G in sol–gel silica is higher than that in ethanol. A new effect has been observed; at 1×10⁻⁴ M of Rh. B and 0.5×10⁻⁴ M of Rh. 6G mixture, the emitted color of laser is changed by changing the pump intensity of frequency-doubled Nd:YAG laser.     

Concept of Photorefractive Connection Module by Photorefractive Four-Wave Mixing with Extraordinary Writing Beams and an Ordinary Reading Beam

We propose a new optical network device photorefractive connection module (PRCM) which operates as optical switch, amplifier and signal distributor controlled by parallel optical signals. Simple optical control bus systems can be realized by cascade connection of PRCMs. PRCM branches off a desired channel from the spatial multiplexed optical bus line by appropriate setting of the control beam pattern. PRCM uses cross polarized four wave mixing (CPFWM) with extraordinary polarized writing beams and an ordinary polarized reading beam to achieve a high connection gain to the next PRCM stage. We analyze the phase matching angle of CPFWM in which the optical paths of two pump beams are slightly different. The phase conjugate reflectivity indicating a branching ratio of optical signal is derived and calculated in consideration of the phase mismatching Δk. The optimum pump ratio and the grating vector orientation for the largest phase conjugate reflectivity and signal amplification factor are discussed for optical design of PRCM. Since the measured signal beam power after passing through the BaTiO₃ crystal is three or four times higher than its incident power, PRCM has a sufficient connection gain for optical bus and interconnection systems.     

Stochastic homogenization of the laser intensity to improve the irradiation uniformity of capsules directly driven by thousands laser beams

Illumination uniformity of a spherical capsule directly driven by laser beams has been assessed numerically. Laser facilities characterized by N _D  = 12, 20, 24, 32, 48 and 60 directions of irradiation with associated a single laser beam or a bundle of N _B laser beams have been considered. The laser beam intensity profile is assumed super-Gaussian and the calculations take into account beam imperfections as power imbalance and pointing errors. The optimum laser intensity profile, which minimizes the root-mean-square deviation of the capsule illumination, depends on the values of the beam imperfections. Assuming that the N _B beams are statistically independents is found that they provide a stochastic homogenization of the laser intensity associated to the whole bundle, reducing the errors associated to the whole bundle by the factor \hbox{1/N_B^1/21/N_{B}^{1{/}2}} 1 / N B 1 / 2 , which in turn improves the illumination uniformity of the capsule. Moreover, it is found that the uniformity of the irradiation is almost the same for all facilities and only depends on the total number of laser beams N _tot  = N _D  × N _B .     

High-contrast all-optical switching with Pt:ethynyl complex

We have theoretically analyzed all-optical switching in Pt:ethynyl complex based on nonlinear excited-state absorption. A detailed analysis for Pt:ethynyl complex has been presented based on rate equation approach. It is shown that a pulsed pump laser beam at 355 nm switches the transmission of a cw probe laser beam at 633 nm through a Pt:ethynyl sample. The effect of various parameters, such as pump pulse width, peak pumping intensity, normalized parameter , transition times of S₁→S₀ and S₁→T₁ states and lifetime of triplet state, on switching characteristics has been analyzed in detail. It has been shown that the probe beam can be completely switched off (i.e. 100% modulation) by a pulsed pump laser beam at 50 kW/cm². These results have been used to design all-optical NOT and the universal NOR and NAND logic gates with multiple pump laser pulses.     

Improvement of corrosion resistance of carbon steel using chemical vapor deposition from Cr(CO)6 with an ArF excimer laser

Corrosion resistance of carbon steel coated with thin film deposited from Cr(CO)₆ using an ArF excimer laser (193 nm) has been evaluated by an electrochemical method as a function of laser beam intensity. The carbon steel coated with the film formed at higher beam intensity shows higher corrosion resistance. Microstructure, composition, and thickness of the films have also been investigated. SEM micrographs show that the films consist of small grains which decrease in size with increasing beam intensity. Auger electron spectroscopy (AES) combined with Ar⁺ beam sputtering reveals that the films deposited at higher beam intensity give higher chromium content, and that the thickness at a fixed total irradiation energy increases up to the intensity of 10 MW cm^–2, falling above this intensity. In addition, the change of film thickness by addition of buffer gases (Ar, CO, and H₂O) has been investigated. The thickness is 10 times smaller under the addition of H₂O, and twice smaller under the addition of Ar or CO than without the addition of gases. A deposition mechanism based on photolysis of Cr(CO)₆ in the gas phase is proposed related to the experimental data after the discussion of several possible mechanisms.     

468-W CW operation of a diode-pumped Nd:YAG rod laser with high beam quality and highly efficient concentrator of pump light

A novel design of an efficient, highly reliable, and good beam quality diode-pumped solid-state laser for precise material processing application is presented. Effort has been done to obtain a highly uniform pumping intensity in the active area, which simultaneously reduces the effects of thermal gradient. In this design a novel lens duct configuration which send the light into the center of the rod is used. By this way a uniform power distribution and a maximum absorption of pump power is resulted. Adopting this design, the maximum laser power of 468 W with optical to optical efficiency of 33% in CW mode and M² factor of less than 15 is obtained. Numerical analysis also indicates the superiority of the design to other methods such as direct and diffusive pumping techniques.     

Direct writing of conductive silver micropatterns on flexible polyimide film by laser-induced pyrolysis of silver nanoparticle-dispersed film

This article describes fabrication of Ag micropatterns on a flexible polyimide (PI) film by laser direct writing using an Ag nanoparticle-dispersed film as a precursor. Ag micropatterns are characterized by optical microscopy, atomic force microscopy (AFM), field emission scanning electron microscopy (FE-SEM), surface profilometry, and resistivity measurements. The line width of Ag micropatterns can be effectively controlled by altering the experimental parameters of laser direct writing especially laser intensity, objective lens, and laser beam scanning speed etc. Using an objective lens of 100× and laser intensity of 170.50 kW/cm², Ag micropatterns with a line width of about 6 μm have been achieved. The Ag micropatterns show strong adhesion to polyimide surface as evaluated by Scotch-tape test. The resistivity of the Ag micropatterns is determined to be 4.1 × 10⁻⁶ Ω cm using two-point probe method. This value is comparable with the resistivity of bulk Ag (1.6 × 10⁻⁶ Ω cm).     

Degenerate four-wave mixing experiments in Methyl green dye-doped gelatin film

Degenerate four-wave mixing (DFWM) was performed in Methyl green dye-doped gelatin films using continuous-wave laser radiation () generated by a He-Ne laser of total power 35 mW. Various parameters which influence the phase-conjugate (PC) signal during the DFWM process were studied. The PC signal contributions from induced holographic transmission and reflection gratings were measured. We observed a maximum PC beam reflectivity of 0.13% in these dye-doped gelatin films.     

Effect of the oxygen pressure on the microstructure and optical properties of ZnO films prepared by laser molecular beam epitaxy

A series of ZnO films were prepared on the Si (1 0 0) or glass substrate at 773 K under various oxygen pressures by using a laser molecular beam epitaxy system. The microstructure and optical properties were investigated through the X-ray diffraction, Raman spectrometer, scanning electron microscope, ultraviolet–visible spectrophotometer and spectrofluorophotometer. The results showed that ZnO thin film prepared at 1 Pa oxygen pressure displayed the best crystalinity and all ZnO films formed a columnar structure. Meanwhile, all ZnO films exhibited an abrupt absorption edge near the wavelength of 380 nm in transmission spectra. With increasing the oxygen pressure, the transmission intensity changed non-monotonically and reached a maximum of above 80% at 1 Pa oxygen pressure, based on which the band gaps of all ZnO films were calculated to be about 3.259–3.315 eV. Photoluminescence spectra indicated that there occurred no emission peak at a low oxygen pressure of 10⁻⁵ Pa. With the increment of the oxygen pressure, there occurred a UV emission peak of 378 nm, a weak violet emission peak of 405 nm and a wide green emission band centered at 520 nm. As the oxygen pressure increased further, the position of UV emission peak remained and its intensity changed non-monotonically and reached a maximum at 1 Pa. Meanwhile the intensity of green emission band increased monotonically with increasing the oxygen pressure. In addition, it was also found that the intensity of UV emission peak decreased as the measuring temperature shifted from 80 to 300 K. The analyses indicated that the UV emission peak originated from the combination of free excitons and the green emission band originated from the energy level jump from conduction band to O_Zn defect.     

A miniature electron beam pumped laser

The 1.73 μm XeI laser has been operated in a fully continuous mode using a table-top electron beam pumped laser setup. A 12 keV electron beam sent through a 300 nm thick silicon nitride membrane into a laser gas mixture of typically 600 mbar gas pressure was used for pumping. A low loss cavity was installed, resulting in a very low pumping power of 37 mW to reach laser threshold. The geometrical conditions for the laser setup such as the shape of the beam pumped volume and its overlap with the optical mode volume are discussed. The laser scheme has been clearly identified as a recombination laser scheme by operating the laser in pulsed mode and observing the time structure of the laser pulse.