The near wall velocity measurements in microchannels with different diameter particles by microPIV/PTV. 8th International symposium on PIV

In near wall measurements with microPIV/PTV, whether seeding particles can be effectively used to detect local fluid velocity is a crucial problem. This talk presents our recent measurements in microchannels [1][2]. Based on measured velocity profiles with 200nm and 50nm in pure water, we found that the measured velocity profiles are agreed with the theoretical values in the middle of channel, but large deviations between measured data and theoretical prediction appear close to wall (0.25mm velocity deviation appeared in shear flows.

Synthesis Of Plastic Zr-Based Bulk Metallic Glass Matrix Composites By The Copper-Mould Suction Casting And The Bridgman Solidification

Zr-based bulk metallic glass matrix composites with the composition of Zr56.2Ti13.8Nb5.0Cu6.9Ni5.6Be12.(5) were synthesized by the copper-mould suction casting and the Bridgman solidification. The composite, containing a well-developed flowery beta-Zr dendritic phase, was obtained by the Bridgman solidification with the withdrawal velocity of 0.8 mm/s and the temperature gradient of 45 K/mm, and the ultimate strength of 2050 MPa and fracture plastic strain of 14.6% of the composite were achieved, which was mainly interpreted by the homogeneous dispersion of bcc beta-Zr phase in the glass matrix. Crown Copyright (C) 2008 Published by Elsevier B.V. All rights reserved.

Dispersion analysis and compensation of collinear optical parametric chirped pulse amplification systems

In an optical parametric chirped pulse amplification (OPCPA) laser system, residual phase dispersion should be compensated as much as possible to shorten the amplified pulses and improve the pulse contrast ratio. Expressions of orders of the induced phases in collinear optical parametric amplification (OPA) processes are presented at the central signal wavelength to depict a clear physics picture and to simplify the design of phase compensation. As examples, we simulate two OPCPA systems to compensate for the phases up to the partial fourth-order terms, and obtain flat phase spectra of 200-nm bandwidth at 1064 nm and 90-nm at 800 nm.

Nonlinear copropagation of two optical pulses of different frequencies in photonic crystal fibers

A theoretical investigation of the nonlinear copropagation of two optical pulses of different frequencies in a photonic crystal fiber is presented. Different phenomena are observed depending on whether the wavelength of the signal pulse is located in the normal or the anomalous dispersion region. In particular, it is found that the phenomenon of pulse trapping occurs when the signal wavelength is located in the normal dispersion region while the pump wavelength is located in the anomalous dispersion region. The signal pulse suffers cross-phase modulation by the Raman shifted soliton pulse and it is trapped and copropagates with the Raman soliton pulse along the fiber. As the input peak power of the pump pulse is increased, the red-shift of the Raman soliton is considerably enhanced with the simultaneous further blue-shift of the trapped pulse to satisfy the condition of group velocity matching.

Controllable group velocity of the probe light in a Lambda-type system with two fold levels

The group velocities of the probe laser field are studied in a A-type system where one lower state has two fold levels coupled by a control field. It is found that the interaction of double dark states leads to controllable group velocity of the probe field in this system. It can be easily realized, due to the interacting double dark resonances, that one of the group velocities at transparency positions is much slower than the other by tuning the control field to be off resonance. In particular, when the control field is on resonance. we can obtain two equal slow group velocities with a broader EIT width, which provides potential applications in quantum storage and retrieval of light. (c) 2005 Elsevier B.V. All rights reserved.

Phase-dependent properties for absorption and dispersion in a closed equispaced three-level ladder system

It is shown that in a closed equispaced three-level ladder system, by controlling the relative phase of two applied coherent fields, the conversion from absorption with inversion to lasing without inversion (LWI) can be realized; a large index of the refraction with zero absorption can be gotten; considerable increasing of the spectrum region and value of the LWI gain can be achieved. Our study also reveals that the incoherent pumping will produce a remarkable effect oil the phase-dependent properties of the system. Modifying value of the incoherent pumping can change the property of the system from absorption to amplification and enhance significantly LWI gain. If the incoherent pumping is absent, we cannot get any gain for any value of the relative phase. (c) 2007 Elsevier GmbH. All rights reserved.

Influence of Coulomb potential for photoionization of H atoms in an elliptically polarized laser field: Velocity gauge versus length gauge

We theoretically study the influence of Coulomb potential for photoionization of hydrogen atoms in an intense laser field with elliptical polarization. The total ionization rates, photoelectron energy spectra, and photoelectron angular distributions are calculated with the Coulomb-Volkov wave functions in the velocity gauge and compared with those calculated in the length gauge as well as those calculated with the Volkov wave functions. By comparing the results obtained by the Coulomb-Volkov and Volkov wave functions, we find that for linear polarization the influence of Coulomb potential is obvious for low-energy photoelectrons, and as the photoelectron energy and/or the laser intensity increase, its influence becomes smaller. This trend, however, is not so clear for the case of elliptical polarization. We also find that the twofold symmetry in the photoelectron angular distributions for elliptical polarization is caused by the cooperation of Coulomb potential and interference of multiple transition channels. About the gauge issue, we show that the difference in the photoelectron angular distributions obtained by the velocity and length gauges becomes rather obvious for elliptical polarization, while the difference is generally smaller for linear polarization.

Phase control of gain and dispersion in an open lambda-type inversionless lasing system

The control role of the relative phase between the probe and driving fields on the gain and dispersion in an open Lambda-type inversionless lasing system with spontaneously generated coherence (SGC) is investigated. It is shown that the inversionless gain and dispersion are quite sensitive to variation in the relative phase; by adjusting the value of the relative phase, electromagnetically induced transparency (EIT), a high refractive index with zero absorption and a larger inversionless gain can be realized. It is also shown that, in the contributions to the inversionless gain ( absorption) and dispersion, the contribution from SGC is always much larger than that from the dynamically induced coherence for any value of the relative phase. Our analysis shows that variation in the SGC effect will cause the spectrum regions and values of the inversionless gain and dispersion to vary evidently. We also found that, under the same conditions, the values of the inversionless gain and dispersion in the open system are evidently larger than those in the corresponding closed system; EIT occurs in the open system but cannot occur in the closed system.

Study of noncollinear quasi-phase matching optical parametric amplification with group-velocity matching based on periodically poled crystals

The properties of noncollinear optical parametric amplification (NOPA) based on quasi-phase matching of periodically poled crystals are investigated, under the condition that the group velocity matching (GVM) of the signal and idler pulses is satisfied. Our study focuses on the dependence of the gain spectrum upon the noncollinear angle, crystal temperature, and crystal angle with periodically poled KTiOPO4 (PPKTP), periodically poled LiNbO3 (PPLN), and periodically poled LiTaO3 (PPLT), and the NOPA gain properties of the three crystals are compared. Broad gain bandwidth exists above 85 nm at a signal wavelength of 800 nm with a 532 nm pump pulse, with proper noncollinear angle and grating period at a fixed temperature for GVM. Deviation from the group-velocity-matched noncollinear angle can be compensated by accurately tuning the crystal angle or temperature with a fixed grating period for phase matching. Moreover, there is a large capability of crystal angle tuning.

Phase control of gain and dispersion in an open V-type system

The control role of the relative phase between the probe and driving fields on gain, dispersion and populations in an open V-type three-level system with spontaneously generated coherence is studied. The result shows that by adjusting the value of the relative phase, the transformation between lasing with inversion and lasing without inversion (LWI) can be realized and high dispersion (refractive index) without absorption can be obtained. The shape and value range of the dispersion curve are similar to those of the gain curve, and this similarity is closely related to the relative phase. The effects of the atomic exit and injection rates and the incoherent pump rate on the control role of the relative phase are also analysed. It is found easier to get LWI by adjusting the value of the relative phase using the open system rather than the closed system, and using an incoherent pump rather than without using the incoherent pump. Moreover the open system can give a larger LWI gain than the closed system.

Group velocity of the light pulse in an open V-type system

We investigate the group velocity of the probe light pulse in an open V-type system with spontaneously generated coherence. We find that, not only varying the relative phase between the probe and driving pulses can but varying the atomic exit rate or incoherent pumping rate also can manipulate dramatically the group velocity, even make the pulse propagation switching from subluminal to superluminal; the subliminal propagation can be companied with gain or absorption, but the superluminal propagation is always companied with absorption. (c) 2006 Elsevier GmbH. All rights reserved.