Switching Light With Light in Chlorophyll-A Molecules Based on Excited-State Absorption

We analyze all-optical switching in chlorophyll-A (Chl-A) molecules for different combinations of pump–probe wavelengths, based on nonlinear intensity-induced excited-state absorption. It is shown that for a pulsed pump beam at 672 nm with peak pump intensity of 5 kW/cm$^{2}$ and Chl-A concentration of 1.5 mM, the transmission of a continuous-wave probe beam at 476 nm can be completely switched off (100% modulation) with switch on–off time of 0.58 and 0.18 $mu$s, respectively. It is also shown that the switching characteristics can be inverted by changing the probe beam wavelength. The effect of various parameters, such as concentration, pump beam intensity, pump pulsewidth, absorption cross section of the ground state, and lifetimes of different states, on the switching characteristics has been analyzed in detail. It is shown that there exists an optimum value of concentration of Chl-A for maximum switching contrast, for the case when the ground state also absorbs the probe beam. The switching characteristics of Chl-A have also been compared with Chl-B and Bchl. Experimental results for all-optical switching in Chl-A with a train of pulses are in good agreement with theoretical results. It is shown that higher contrast and faster switching can be achieved as opposed to what was reported recently in other biomolecules such as archael rhodopsin and phototropin proteins. The results have also been used to design switches and logic gates.      

All-optical switching in Pharaonis phoborhodopsin protein molecules

Low-power all-optical switching with pharaonis phoborhodopsin (ppR) protein is demonstrated based on nonlinear excited-state absorption at different wavelengths. A modulating pulsed 532-nm laser beam is shown to switch the transmission of a continuous-wave signal light beam at: 1) 390 nm; 2) 500 nm; 3) 560 nm; and 4) 600 nm, respectively. Simulations based on the rate equation approach considering all seven states in the ppR photocycle are in good agreement with experimental results. It is shown that the switching characteristics at 560 and 600 nm, respectively, can exhibit negative to positive switching. The switching characteristics at 500 nm can be inverted by increasing the signal beam intensity. The profile of switched signal beam is also sensitive to the modulating pulse frequency and signal beam intensity and wavelength. The switching characteristics are also shown to be sensitive to the lifetimes of$mmb pbf pR_bf M$and$mmb pbf pR_bf O$intermediates. The results show the applicability of ppR as a low-power wavelength tunable all-optical switch.     

All-optical biomolecular parallel logic gates with bacteriorhodopsin

All-optical two input parallel logic gates with bacteriorhodopsin (BR) protein have been designed based on nonlinear intensity-induced excited-state absorption. Amplitude modulation of a continuous wave (CW) probe laser beam transmission at 640 nm corresponding to the peak absorption of O intermediate state through BR, by a modulating CW pump laser beam at 570 nm corresponding to the peak absorption of initial BR state has been analyzed considering all six intermediate states in its photocycle using the rate equation approach. The transmission characteristics have been shown to exhibit a dip, which is sensitive to normalized small-signal absorption coefficient (/spl beta/), rate constants of O and N intermediate states and absorption of the O state at 570 nm. There is an optimum value of /spl beta/ for a given pump intensity range for which maximum modulation can be achieved. It is shown that 100% modulation can be achieved if the initial state of BR does not absorb the probe beam. The results have been used to design low-power all-optical parallel NOT, AND, OR, XNOR, and the universal NAND and NOR logic gates for two cases: 1) only changing the output threshold and 2) considering a common threshold with different /spl beta/ values.     

High-contrast, all-optical switching in bacteriorhodopsin films

We report experiments with nonlinear-absorption-based, high-contrast, all-optical switching in photochromic bacteriorhodopsin (BR) films. The switching action is accomplished by control of the transmission of a weak probe beam through a BR sample with the help of strong pump beam illumination at 532 nm wavelength. We found that the switching properties of BR films depend on several experimentally controllable parameters such as probe wavelength, pump beam intensity, and excitation rate. A comparative study of the switching behavior and other parameters of practical use was carried out at three probe wavelengths (543, 594, and 633 nm) and various beam powers and pump excitation rates. The results are presented for commercially available wild-type and D96N variant BR films.     

All-optical switching properties of poly(methyl methyacrylate) azobenzene composites

In this paper, the all-optical switching polymer composites based on poly(methyl methacrylate) (PMMA) and azobenzene dyes were prepared. The optical switching effects of the composites were measured by using 514 nm pump beam and 632.8 nm probe beam. The composites' photoinduced reversible and repeatable change in the transmittance of the probe beam is attributed to the photoisomerization of the azobenzene dyes. The influencing factors of the optical switching properties, such as the modulation frequency, pump beam intensity and temperature, were studied experimentally. The effects of pump beam power and temperature on the optical switching response were also studied experimentally.     

All-optical 2 x 2 switching with two independent Yb3+ -doped nonlinear optical fibers with a long-period fiber grating pair

We have demonstrated a new type of all-optical 2 x 2 switch by using two independent Yb3+ -doped nonlinear optical fibers with a long-period fiber grating pair and a 3-dB fiber coupler. A 400-Hz square-wave pulse train at approximately 1549.4 nm was fully switched between the two output ports up to 200 Hz by modulated pump signals at 976 nm with a maximum pump power of approximately 35 mW, where the extinction ratio at approximately 1549.4 nm between the on and the off states was approximately 17.5 dB.     

Optical nonlinearities of gallium nitride

Luminescence, induced absorption and degenerate four wave mixing experiments are performed on GaN epilayers grown on a sapphire substrate by MOCVD. We measure the nonlinear behaviour of the luminescence spectra near the excitonic resonance, by using an excitation at 4.026 eV from an excimer laser. At low intensities of excitation, spectra show a saturation of the I₂ line due to the finite donor density in the sample. Higher intensities of excitation induce collision process between photo-created particles. Using a dye laser as a pump beam, we measure the induced variation of absorption of a probe beam as a function of the intensity and of the wavelength of the excitation. With increasing intensities of the pump beam, curves show a red-shift of the absorption edge and of the excitonic resonance. Pulsed degenerate four-wave mixing experiments were performed using the third harmonics of a picosecond Nd-YAG laser at 3.492 eV. A characteristic time of 16 ps has been measured, which is independent of the temperature, of the fringe spacing and of the intensity of the pump beams.     

All-optical switching based on the electromagnetically induced transparency effect of an active photonic crystal microcavity

We report a new all-optical switching in a two-dimensional photonic crystal microcavity made of semiconductor multiple quantum wells and realized based on the electromagnetically induced transparency effect with exciton and two-exciton energy levels. We use the quantum coherence effects to achieve small absorption of the probe field, and the absorption of the probe field can be adjusted by controlling the pump field and decay rate. We turn the control field into pulses of light field so that we can regulate the efficiency of the switch. Through selecting the appropriate control light field intensity, we can obtain a switching efficiency of 85% and a switching time is 10 ps. This result can be used for the development of new types of nanoelectronic devices for realizing the switching process.     

All-optical switching characteristics of ethyl red doped polymer film

The azo dye ethyl red (ER) doped methyl methacrylate (PMMA) polymer films were prepared by the sol–gel process and the repeat-spin-coating method, the all-optical switching characteristics of the film samples were investigated experimentally at different intensities and modulation frequencies of pump beam (532 nm, CW), and the influence of the doping concentrations and the solvents on the all-optical switching effect of the films was discussed. At low pump power (8 mW), the modulation depth of 50% was obtained and the response time was near 1 ms at room temperature. Furthermore, the maximal modulation depth reaches 72%. An excellent switching performance could be obtained by choosing the proper doping concentrations (from 2 wt% to 6 wt%) of the samples, and the switching characteristics of the samples can be improved using cyclohexanone solvent. The experimental results show that ER-doped polymer has potential application in optical switching.     

Investigation of nonlinear dynamics in CdS by time-resolved nondegenerate pump–probe system with phase object

The time-resolved nondegenerate pump–probe system with phase object is employed for investigation of nonlinear absorption and refraction dynamics in CdS. The 532?nm laser beam with 21?ps duration is used as the excitation and the laser beams of 600 and 680?nm with 10?ps duration from optical parametric generation are used for probing. The experimental results at both probe wavelengths show free-carrier absorption and large free-carrier refraction along with two-photon absorption and bound electronic optical Kerr effect. By numerically fitting the experimental data based on the nondegenerate pump–probe theory, the nondegenerate two-photon absorption coefficient, the nondegenerate Kerr coefficient, the free-carrier decay time, the free-carrier absorptive cross-section and free-carrier refractive coefficient at different wavelengths are all determined.     

Performance improvement in a directly 879 nm dual-end-π-polarized-pumped CW and pulsed GdVO₄/Nd:GdVO₄ laser

A continuous-wave and Q-switched 10 kHz GdVO?/Nd:GdVO? laser by 879 nm laser-diode pumping was demonstrated. We combined dual-end-π-polarized-pumping, half-wave plate (HWP) insertion, composite laser crystals, and dual Q-switching techniques to obtain a 1.06 μm nanosecond laser with high temporal and spatial quality. A polarization beam splitter was used to divide the unpolarized pump beam into two linearly polarized beams to increase the absorption efficiency. The output performance was improved obviously by inserting a HWP into the cavity. The pulse duration was compressed and the peak power was increased for a doubly Q-switched laser, in contrast to a single acousto-optical Q-switched laser under the same conditions.     

A generation of 2 μm Q-switched thulium-doped fibre laser based on anatase titanium(IV) oxide film saturable absorber

We demonstrate a Q-switched thulium-doped fibre laser operating at approximately 1935 nm wavelength using anatase titanium(IV) oxide (TiO₂) embedded in polyvinyl alcohol as the passive newly saturable absorber (SA). The film has absorption loss of 3.5 dB and modulation depth of 33%. It is sandwiched between two fibre ferrules in a ring laser cavity to produce self-started pulse train with a repetition rate that is tuned from 30.12 to 36.96 kHz as the 1552-nm pump power is increased from 289 to 485 mW. At maximum pump power, the laser produced a Q-switching pulse train with pulse duration, output power, pulse energy and peak power of 1.91 μs, 11 mW, 0.3 μJ and 146 mW, respectively. These results show that the TiO₂ is a new potential SA material for pulsed laser applications.     

Optimized Photothermal Lens Determination of Nonlinear Absorption

An optimized pump–probe mode-mismatched photothermal lens experiment aimed at determination of nonlinear absorption of an optical sample is reported. The pump beam generates a local thermal gradient or thermal lens that is tested by the probe light. The pump beam is tightly focused, and the probe beam is highly collimated. Changes in the probe light transmission through a small aperture located at some distance from the sample provide the signal. Scanning of the sample around the focal point yields a single-peaked Z-scan signature with a width several times larger than the pump Rayleigh range for linear absorption. If nonlinear absorption is dominant, the width of the peak is significantly smaller and of the order of the Rayleigh range of the pump field. If linear and nonlinear absorption are present simultaneously, a double-peaked Z-scan signature is obtained. In this situation, the linear and nonlinear absorption contributions can be easily separated and compared to each other for calibration purposes. Using the known values of linear absorption, nonlinear absorption coefficients can be estimated with good accuracy. The method is tested by studying nonlinear absorption in nitrobenzene and iron oxide water colloids. The values of the effective nonlinear absorption coefficients are determined. The physical origin of nonlinear absorption in both types of samples is also discussed.     

Passively Q-switched fibre laser utilizing erbium-doped fibre saturable absorber for operation in C-band region

In this paper, a stable and robust all-fibre passively Q-switched erbium-doped fibre laser (EDFL) emitting at 1558?nm is described. The proposed laser utilizes an 11?cm long erbium-doped fibre as saturable absorber (SA). The fibre SA features a linear optical absorption of about 13?dB in the Q-switched EDFL operating regime. By elevating the input pump power from the threshold of 60?mW to the maximum available power of 142?mW, a pulse train with a maximum repetition rate of 86?kHz, minimum pulse width of 3.39?µs, maximum average output power of 10.5?mW, maximum pulse energy of 122?nJ and maximum peak power of 36?mW are obtained. The signal to noise ratio (SNR) of the spectrum is measured to be around 70?dB. This fibre SA is simple, reliable, compact and alignment free. Thus it is suitable for making a portable pulse laser source.     

Nickel oxide nanoparticles thin film saturable absorber for 1-micron pulsed all-fibre lasers operation

A passive Q-switched and mode-locked ytterbium-doped fibre laser (YDFL) pulse generation using a nickel oxide thin film as a saturable absorber is reported. The nickel oxide nanoparticle thin film was fabricated by a simple processing technique, and it has a modulation depth of 39% and saturation intensity of 0.04 MW/cm². The saturable absorber was constructed by inserting a small piece of the film between two fibre ferrules. Then it was integrated in a YDFL cavity. The Q-switching operation started at a threshold pump power of 117.73 mW with an initial wavelength of 1073.5 nm. When the pump power was raised from 117.73 to 133 mW, the repetition rate grew from 9.5 to 15.8 kHz. The pulses had a maximum pulse energy of 478 nJ. Furthermore, a stable self-started mode-locked pulse was also succesfully generated at the threshold pump power of 97.3 mW. The central wavelength and repetition rate of the laser were 1037.72 nm and 23 MHz, respectively. The maximum pulse energy of 0.56 nJ and a peak power of 26.4 W were recorded at a pump power of 137.5 mW.     

Measurement of atomic concentrations in reacting flows through the use of stimulated gain or loss

Apump laser at 226 nm was used to generate a population inversion between the 3p (3)P(2) and 3s (3)S(1) states of the oxygen atom in the postflame gases of a 7-torr H(2)/O(2) flame by means of resonant two-photon absorption from the ground state. The inversion produced an amplified spontaneous emission (ASE) signal at 845 nm. A probe laser tuned to this transition was crossed at a small angle with the pump beam. The probe beam experienced gain at the expense of theASE signal. By analyzing the gain on the probe beam and the loss on the ASE signal, we have determined the density of oxygen atoms in the flame environment.     

Enhancement of the thermal lens signal induced by sample matrix absorption of the probe laser beam

The effect of the absorption of the probe laser beam by the sample matrix on the thermal lens signal of a solute was investigated for aqueous solutions of Tb(III), Yb(III), and Nd(III). The measurements were performed with a thermal lens instrument in which the pump and the probe beam were derived from a tunable Ti:sapphire laser. Thermal lens signals were found to be enhanced in the region where the probe beam overlapped with the absorption band of the sample matrix. The observed enhancement was confirmed further with samples of the same solutes (lanthanide ions) but in D20, which does not absorb in the same spectral region as water. The enhancement may be due to the fact that absorption by the sample matrix led to a change in its refractive index and the production of a temperature gradient. In addition to fundamental importance, the observed enhancement can be used to improve the sensitivity of the thermal lens measurements by judiciously selecting a solvent that absorbs the probe laser beam.     

1.3at%Nd:YAG透明陶瓷的制备及激光性能研究

以高纯氧化物商业粉体为原料, 采用固相反应和真空烧结技术, 制备了高质量的1.3at%Nd:YAG透明陶瓷. 研究了室温下Nd:YAG透明陶瓷的显微结构、光谱及激光性能. 实验结果表明, Nd:YAG透明陶瓷主要以穿晶方式断裂; 平均晶粒尺寸为15μm, 且分布均匀; 晶粒中和晶界处没有检测到杂质和气孔存在, 且成分一致, 无偏析现象. 退火后样品在激光波长1064nm处的透过率高达82.4%; 主吸收峰位于808.6nm处, 峰值吸收系数为4.45cm^-1, 激光波长1064nm处的吸收系数为0.11cm^-1; 主荧光发射峰位于1064nm处, 半高宽为0.82nm, 荧光寿命为258μs. 用LD端面泵浦Nd:YAG陶瓷样品(泵浦源最大输出功率为1000mW), 获得了波长为1064nm的连续激光输出, 激光阈值约530mW, 斜率效率为23.2%, 最大泵浦吸收功率为731mW时, 最大输出功率为45mW.     

Spatial beam shaping of ultrashort laser pulses: theory and experiment

We studied the spatial intensity profile of an ultrashort laser pulse passing through a laser beam shaping system, which uses diffractive optical elements to reshape a Gaussian beam profile into a flat-topped distribution. Both dispersion and nonlinear self-phase modulation are included in the theoretical model. Our calculation shows that this system works well for ultrashort pulses (approximately 100 fs) when the pulse peak intensity is less than 5 x 10(11) W/cm2. Experimental results are presented for 136 fs pulses at 800 nm wavelength from a Ti:sapphire laser with a 6 nJ pulse energy. We also studied the effects of lateral misalignment, beam-size deviation, and defocusing on the energy fluence profile.     

A quantum optical transistor with a single quantum dot in a photonic crystal nanocavity

Laser and strong coupling can coexist in a single quantum dot (QD) coupled to a photonic crystal nanocavity. This provides an important clue towards the realization of a quantum optical transistor. Using experimentally realistic parameters, in this work, theoretical analysis shows that such a quantum optical transistor can be switched on or off by turning on or off the pump laser, which corresponds to attenuation or amplification of the probe laser, respectively. Furthermore, based on this quantum optical transistor, an all-optical measurement of the vacuum Rabi splitting is also presented. The idea of associating a quantum optical transistor with this coupled QD-nanocavity system may achieve images of light controlling light in all-optical logic circuits and quantum computers.