Dynamic diffraction behaviors of bacteriorhodopsin film at 532 nm

To show the relations between dynamic diffraction behaviors of two coupled waves and parameters of bacteriorhodopsin (bR) film, we presented a model of numerical simulation, in which the thermal effect is considered. Using wavelength of 532 nm, the relations between dynamic diffraction behaviors and parameters of bR film are revealed and the numerical results are in good agreement with the experimental ones. For different incident intensities, the higher the intensity, the lower the stable normalized diffraction intensity. For different bR molecular concentrations in the M state, the higher the molecular concentration, the higher the stable value of normalized diffraction intensity. For different thickness of bR film, the larger the thickness, the higher the stable value of normalized diffraction intensity. For different lifetime in the M state, the longer the lifetime of the M state, the lower the stable value of normalized diffraction intensity. The normalized diffraction intensity with low stable value corresponds to its fast rising and decaying rate.     

Surface diffusion of carbon atom and carbon dimer on Si(0 0 1) surface

Carbon (C) atom and carbon dimer (C2) are known to be the main projectiles in the deposition of diamond-like carbon (DLC) films. The adsorption and diffusion of the C adatom and addimer (C2) on the fully relaxed Si(0 0 1)-(2 × 1) surface was studied by a combination of the molecular dynamics (MD) and Monte Carlo (MC) simulation. The adsorption sites of the C and C2 on the surface and the potential barriers between these sites were first determined using the semi-empirical many-body Brenner and Tersoff potential. We then estimated their hopping rates and traced their pathways. It is found that the diffusion of both C and C2 is strongly anisotropic in nature. In addition, the C adatom can diffuse a long distance on the surface while the adsorbed C2 is more likely to be confined in a local region. Thus we can expect that smoother films will be formed on the Si(0 0 1) surface with single C atoms as projectile at moderate temperature, while with C2 the films will grow in two-dimensional islands. In addition, relatively higher kinetic energy of the projectile, say, a few tens of eV, is needed to grow DLC films of higher quality. This is consistent with experimental findings.     

Quantum interferences from cross talk in J = 1/2 ↔ J = 1/2 transitions

We consider the possibility of a control field opening up multiple pathways and thereby leading to new interference and coherence effects. We illustrate the idea by considering the J = 1/2 ↔ J = 1/2 transition. As a result of the additional pathways, we show the possibilities of nonzero refractive index without absorption and gain without inversion. We explain these results in terms of the coherence produced by the opening of an extra pathway.     

The influences of the local impact site and incident energy on the transport behaviors of single copper atom onto Cu (0 0 1) surface

Molecular dynamics (MD) simulation is carried out to study the transport behaviors of a single deposited atom in Cu film homoepitaxy. We consider the normal Cu incident atoms impinging on the Cu (0 0 1) surface at four possible local impact sites (top, bridge, hollow and general). The observed transport behaviors of the deposited atom onto the surface include: direct adsorption (DA), penetration by atomic exchange, and transient penetration (TP), which a deposited atom penetrates the interstitial site and then rapidly migrates to a stable site on the surface. The results show that transport behaviors of the deposited atom are closely related to both the local impact site and the incident energy. The maximum increment of kinetic energy at every impact site approaches to a certain value except for the incident energy below 2.0 eV. Furthermore, as the incident energy is higher than the penetration threshold, TP behavior could be observed again in some energy ranges. This interesting phenomenon, which cannot be explained by the existing theories, is possibly attributed to the dynamical competition between the deposited atom and substrate atoms.     

Quantum interference effects in a multi-driven transition Fg = 3 ↔ Fe = 2

We calculated and studied the quantum coherence effects of a degenerate transition F_g = 3 ↔ F_e = 2 system interacting with a weak linearly polarized (with σ_± components) probe light and a strong linearly polarized (with σ_± components) coupling field. Due to the competition between the drive Rabi frequency and the Zeeman splitting, electromagnetically induced transparency (EIT) and electromagnetically induced absorption (EIA) are appeared at the different values of applied magnetic field in both cases that the Zeeman splitting of excited state Δe is smaller than the Zeeman splitting of ground state Δg (i.e., Δe < Δg) and Δe > Δg. It is shown that the resonance is broader and contrasts are higher for Δe < Δg than that for Δe > Δg at the same Rabi frequencies of probe and coupling fields.     

Absolute frequency measurements and comparisons in iodine at 735 nm and 772 nm

We report frequency measurements at the rovibronic transition P(42)1-14 (772 nm) and R(114)2-11 (735 nm) from the electronic transition of the iodine molecule ¹²⁷I₂ with the help of a frequency comb as a reference. By using Doppler-free saturation spectroscopy a frequency precision in the 7 × 10⁻¹⁰ region is reached and two iodine cells both operated at 550-600 °C are compared. To relate our results to other measurements, the absolute transition frequency of the hyperfine structure line P(148)1-14 a₁ at 780 nm with an already known transition frequency was also determined.     

Theoretical study of Ti and Fe surface alloys on Al(0 0 1) substrate

Accurate density-functional calculations are performed to investigate the formation of Ti and Fe ultrathin alloys on Al(0 0 1) surface. It is demonstrated that a deposition of Ti monolayer on Al(0 0 1) substrate leads to the formation of Al₃Ti surface alloy with Ti atoms arranged according to the L1₂ stacking, distinct from the D0₂₂ structure characteristic of a bulk Al₃Ti compound. A quest for the reason of this distinct atomic arrangement led us to the study of the surface structure of Al₃Ti(0 0 1) compound. It is concluded that even the Al₃Ti(0 0 1) surface is terminated with three layers assuming a L1₂ stacking and hence this stacking fault can be classified as a surface-induced stacking fault. Several possibilities of Fe atoms distributed in the surface region of Al(0 0 1) have been examined. The most stable configuration is the one with the compact Fe monolayer on Al(0 0 1) and covered by one Al monolayer. Lastly, our calculations show that there is no barrier for the penetration of Fe adatoms below the Al(0 0 1) surface; however, such a barrier is present for a Ti-alloyed Al(0 0 1) surface.     

Ytterbium femtosecond fiber laser without dispersion compensation tunable from 1015 nm to 1050 nm

We report on a widely tunable ytterbium fs-fiber laser without dispersion compensation. The all-normal dispersion laser contains a spectral filter for wavelength tuning and for generating additional amplitude modulation to support the nonlinear polarization evolution as mode-locking mechanism. By tilting the interference filter the center wavelength of the laser can be tuned from 1015 nm to 1050 nm with a pulse energy up to 2.0 nJ. The pulses can be dechirped externally to 108 fs.     

Reply to: “Comment on: ‘Effects of including the counterrotating term and virtual photons on the eigenfunctions and eigenvalues of a scalar photon collective emission theory’ [Phys. Lett. A 372 (2008) 2514]” [Phys. Lett. A 372 (2008) 5732]

We compute the emission amplitude for the collective emission from a sphere of identical atoms in the scalar photon theory for both the cases of the complex kernel (i.e. including virtual photons) and real kernel. We explicitly show that the single mode theory based on the real kernel neglects the effects of the different decay rates and frequency shifts associated with the eigenfunctions belonging to the same angular index but with different radial indices. We show that these effects modify, for k₀R?1, both the time dependence of the emission amplitude and its angular distribution, in clear contradiction to the assertions made by the Comment's authors.     

The metamorphosis in the emission angular profile from an inverted two-level atoms system

A rich variety of angular distributions in the cooperative emission from a sphere of inverted N two-level atoms are shown to result from the eigenstructure of the complex kernel of scalar photon theory exp(ik₀R)/(ik₀R). This angular distribution is sensitive both to the size of the sphere and to the instant of observation of the emission.     

Molecular dynamics investigation of deposition and annealing behaviors of Cu atoms onto Cu(0 0 1) substrate

The deposition growth and annealing behaviors of Cu atoms onto Cu(0 0 1) are investigated in atomic scale by molecular dynamics (MD) simulation. The results indicate that the film grows approximately in a layer-island mode as the incident energy is from 1 to 5 eV, while surface intermixing can be significantly observed at 10 eV. The surface roughness of the film decreases with increasing the incident energy, and the film after annealing becomes smoother and more ordered. These phenomena may be attributed to the enhanced atomic mobility for higher incident energy and thermal annealing. It also indicates that atomic mixing is more significant with increasing both the incident energy and substrate temperature. In addition, the peak-to-peak distances of radial distribution function (RDF) clearly indicate that the films before and after annealing are still fcc structure except for that at the melting temperature of 1375.6 K. After annealing, the film at the melting temperature returns to fcc structure instead of amorphous. Moreover, the residual stress and Poisson ratio of the film are remarkably affected by the thermal annealing. Furthermore, the density of thin film is obviously affected by the substrate temperature and annealing process. Therefore, one can conclude that high incident energy, substrate temperature and thermal annealing could help to enhance the surface morphology and promote the microstructure of the film.     

Diode laser spectroscopy of ammonia at 760 nm

A tunable diode laser spectrometer has been employed to examine the unknown overtone absorption lines of NH₃ around (760 nm). The spectrometer sources are commercially available heterostructure AlGaAs tunable diode lasers operating in the “free-running” mode. The detection of the lines has been possible by the use of the wavelength modulation spectroscopy and the second harmonic detection technique. A special algorithm has been used in order to fit the highly modulated absorption lines. The weakest observed resonances have absorption cross sections on the order of ?/molecule or ?/amagat. For some of the more intense lines self-, air-, N₂-, He- and H₂-broadening coefficients have been obtained at room temperature and also some shifting coefficients have been measured.     

Lasing at 1065 nm in bulk Nd-doped telluride-tungstate glass

We have achieved, for the first time to our knowledge, lasing in a new type of telluride-tungstate glass host doped with neodymium: Nd³⁺:(0.8)TeO₂-(0.2)WO₃. Lasing was obtained at 1065 nm with two samples containing 0.5 mol% and 1.0 mol% Nd₂O₃. During gain-switched operation, slope efficiencies of 12% and 10% were obtained with the 0.5 mol% and 1.0 mol% doped samples, respectively, at a pulse repetition rate of 1 kHz. Judd-Ofelt analysis was further employed to determine the emission cross section σ_e at 1065 nm from the absorption spectra and lifetime data. The emission cross section from the Judd-Ofelt analysis came to 3.23 ± 0.09 × 10⁻²⁰ cm², in reasonable agreement with the value of 2.0 ± 0.13 × 10⁻²⁰ cm² obtained from the analysis of laser threshold data.     

Comparison of 885 nm pumping and 808 nm pumping in Nd:CNGG laser operating at 1061 nm and 935 nm

A Nd:CNGG laser operated at 935 nm and 1061 nm pumped at 885 nm and 808 nm, respectively, is demonstrated. The 885 nm direct pumping scheme shows some advantages over the 808 nm traditional pumping scheme. It includes higher slope efficiency, lower threshold, and better beam quality at high output power. With the direct pumping, the slope efficiency increases by 43% and the threshold decreases by 10% compared with traditional pumping in the Nd:CNGG laser operated at 935 nm. When the Nd:CNGG laser operates at 1061 nm, the direct pumping increases the slope efficiency by 14% with a 20% reduction in the oscillation threshold.     

Simultaneous dual-wavelength Q-switched Nd:YAG laser operating at 1.06 μm and 946 nm

Considering the reabsorption loss of the quasi-three level system and the unsaturable loss of the saturable absorber, we obtained the operating condition of a diode-pumped simultaneous dual-wavelength Q-switched Nd:YAG laser operating at 1.06 μm and 946 nm. The dual-wavelength pulsed laser was realized successfully through adaptive coating design of the cavity mirrors. As much as 1.6 W total average output power of the dual-wavelength at 1.06 μm and 946 nm was achieved at the incident pump power of 14.2 W with an optical conversion efficiency of 11.3%.     

Highly efficient THz emission from differently grown InN at 800 nm and 1060 nm excitation

A detailed study on differently molecular-beam epitaxy (MBE) grown InN wavers as THz surface emitters is reported. The samples were excited using 120 fs and 100 fs short laser pulses delivered by a Ti:Sapphire oscillator at 800 nm and a fiber laser amplifier at 1060 nm, respectively. The InN emission properties are compared to a p-type InAs reference sample. At 800 nm, atomically smooth InN with low background electron concentration exhibits slightly stronger THz emission than the well-established p-InAs emitter. This high THz efficiency of InN is reported for the first time. The strong emission of InN is caused by the absence of any intervalley scattering, which in the case of InAs, increases the effective mass of the photogenerated electrons and, thus, reduces the photo-Dember effect, which is most responsible for THz emission. Consequently, InN is a reliable material for strong THz emission.     

Diode-pumped continuous-wave dual-wavelength Yb:CaNb2O6 lasers at 1003 nm and 1038 nm

A diode-pumped high-power continuous-wave (cw) dual-wavelength Yb:CaNb₂O₆ lasers at 1003 nm and 1038 nm is reported. By using an end-pumped structure and employing a 978 nm diode-laser as the pump source. As a result, the total output power of 803 mW dual-wavelength lasers at 1003 nm and 1038 nm is obtained at an incident pump power of 17.8 W. Furthermore, intracavity sum-frequency mixing at 1003 and 1038 nm was then realized in a LBO crystal to reach the green range. We obtained a total cw output power of 94 mW at 510 nm.     

Role of spatial mode function in the presence of two-atom events in a micromaser

In this paper, we discuss the effects of spatial mode function in an one-photon micromaser in the presence of two-atom events. It is shown that two-atom events allow us a possibility to study the effects of different cavity eigenmodes in a micromaser. We find that squeezing properties of the radiation field depend upon the parity (odd or even) and order (lower or higher) of cavity eigenmodes. For example, squeezing can be obtained for odd-order cavity eigenmodes which completely vanishes for even-order modes. Our results also show that effects similar to self-induced transparency are never obtained in the presence of two-atom events. Finally, we consider the effect of pump fluctuations and cavity losses in our system.     

Time-dependent directionality of cooperative emission after short pulse excitation

Cooperative emission from a sphere of N two-level atoms excited by a short pulse is shown to have a time-dependent angular distribution, with the possibility of reversal in the dominant direction of emission. This is a result of the different values of the frequency shifts and decay rates associated with the various collective eigenmodes of the atomic system obtained in a scalar photon model.     

Breakdown limits of optical multimode fibers for the application of nanosecond laser pulses at 532 nm and 1064 nm wavelength

For many applications, optical multimode fibers are used for the transmission of powerful laser radiation. High light throughput and damage resistance are desirable. Laser-induced breakdown at the end faces of fibers can limit their performance. Therefore, the determination of laser-induced damage thresholds (LIDT) at the surface of fibers is essential.Nanosecond (1064 nm and 532 nm wavelength) single-shot LIDT were measured according to the relevant standard on SiO₂ glass preforms (Suprasil F300) as basic materials of the corresponding fibers. For 10 kinds of fused silica fibers (FiberTech) with core diameters between 180 μm and 600 μm, an illumination approach utilizing a stepwise increase of the laser fluence on a single spot was used. For both wavelengths, the LIDT values (0% damage probability) obtained by means of the two methods were compared. The influence of surface preparation (polishing) on damage resistance was investigated. For equal surface finishing, a correlation between drawing speed of the fibers and their surface LIDT values was found. In addition to the surface measurements, bulk LIDT were determined for the preform material.