Diffraction properties of opaque disks outside and inside a laser cavity

Diffraction of symmetrical Laguerre-Gauss TEM_p0 beams incident on an opaque disk known as a stop is considered. The near- and far-field patterns are studied. Thanks to zero-field occluding, conversion from TEM₁₀ beam to dark hollow beam can be achieved with better efficiency than from a TEM₀₀ beam. It is shown that the fundamental mode of a laser cavity including a diaphragm and a stop can be TEM₀₀- or TEM₁₀-like in shape depending on their size. This result is interpreted from the new divergence hierarchy, which characterises the diffracted TEM_p0 beams emerging from the stop.     

Real-time observation of in-plane displacements of opaque surfaces

A grating is recorded on a photo-resist coated specimen and re-illuminated with the interference pattern used to form it. ‘Live’ fringes corresponding to displacements can then be observed as the specimen is loaded.     

利用飞秒激光直写实现透明介电材料中三维维微微纳结构的制备与集成

本文综述了利用飞秒激光三维直写技术,在玻璃和晶体等透明介电材料中实现微流体、微光学、微电子学等一系列功能性微纳结构,并进一步构筑新型微纳光子器件的原理、技术与应用。     

Attenuation of surface phonons in opaque materials measured by Brillouin scattering

We have measured the velocity and attenuation of surface phonons in amorphous Si by Brillouin Scattering. Due to the small wavelength of the probed phonons, the elastic properties of the thin films can be determined without substrate interference. Strong differences in the mean free path are found between “amorphized” and sputtered samples. Temperature and frequency dependence suggest relaxational processes rather than scattering by defects to be responsible for the attenuation.     

Inhibition of linear absorption in opaque materials using phase-locked harmonic generation

We theoretically predict and experimentally demonstrate inhibition of linear absorption for phase and group velocity mismatched second- and third-harmonic generation in strongly absorbing materials, GaAs, in particular, at frequencies above the absorption edge. A 100-fs pump pulse tuned to 1300 nm generates 650 and 435 nm second- and third-harmonic pulses that propagate across a 450-microm-thick GaAs substrate without being absorbed. We attribute this to a phase-locking mechanism that causes the pump to trap the harmonics and to impress on them its dispersive properties.     

Direct observation of the temperature field during ablation of materials by multiple femtosecond laser pulses

We report a direct observation of the temperature field on a steel specimen during ablation by multiple femtosecond laser pulses using an infrared thermography technique. From the experimental results and simulation study of the temperature field, we quantified the deposited thermal power into the specimen during the ablation process. We found that more than two thirds of the incident laser power was deposited in the steel specimen when ablated by multiple femtosecond laser pulses. This result provides further understanding of the heating effect in materials processing by ultrashort laser pulses.     

Experimental demonstration of self-collimation inside a three-dimensional photonic crystal

We present our experimental demonstration of self-collimation inside a three-dimensional (3D) simple cubic photonic crystal at microwave frequencies. The photonic crystal was designed with unique dispersion property and fabricated by a high precision computer-controlled machine. The self-collimation modes were excited by a grounded waveguide feeding and detected by a scanning monopole. Self-collimation of electromagnetic waves in the 3D photonic crystal was demonstrated by measuring the 3D field distribution, which was shown as a narrow collimated beam inside the 3D photonic crystal but a diverged beam in the absence of the photonic crystal.     

Direct observation of Josephson capacitance

The effective capacitance has been measured in the split Cooper-pair box (CPB) over its phase-gate bias plane. Our low-frequency reactive measurement scheme allows us to probe purely the capacitive susceptibility due to the CPB band structure. The data are quantitatively explained using parameters determined independently by spectroscopic means. In addition, we show in practice that the method offers an efficient way to do nondemolition readout of the CPB quantum state.     

Direct observation of self-similarity in evolution of transient stimulated Raman scattering in gas-filled photonic crystal fibers

A unique characteristic of transient stimulated Raman scattering, in which the spatiotemporal evolution of the fields and the molecular excitation follow a universal self-similarity law, is observed in gas-filled photonic crystal fibers. As the input laser power is increased, the coupled system "optical fields + molecular excitation" goes through the same phases of time evolution but at a higher rate. Using the self-similarity law we are able to completely reconstruct the evolution of the pump and Stokes fields from one measurement.     

三维圆柱体绕流的发展和演化

结合三阶精度格式求解可压缩NS方程，本文研究了绕过三维圆柱体的流动结构，阐明了流动的演化机理。大攻角下，在三维圆柱体背风区形成了一个脱落涡序列，其截面流态非常类似于二维圆柱绕流，主涡、二次涡以及tertiary涡形成了一个层次结构。前一个主涡脱体后，tertiary涡将演化为其后续的新生主涡，并且合并圆柱体对称面另一侧的二次涡。     

Direct calculation of a three-dimensional diffracted field

We present an approach to calculating the complex amplitude of a three-dimensional (3D) diffracted light field in the paraxial approximation based on a 3D Fourier transform. Starting from the Huygens-Fresnel principle, the method is first developed for the computation of the light distribution around the focus of an apertured spherical wave. The method, with modification, is then extended to treat the 3D diffraction of an aperture with an arbitrary transmittance function.     

Direct probes of linearly polarized gluons inside unpolarized hadrons

We show that linearly polarized gluons inside unpolarized hadrons can be directly probed in jet or heavy quark pair production in electron-hadron collisions. We discuss the simplest cos2? asymmetries and estimate their maximal value, concluding that measurements of the unknown linearly polarized gluon distribution in the proton should be feasible in future Electron-Ion Collider or Large Hadron electron Collider experiments. Analogous asymmetries in hadron-hadron collisions suffer from factorization breaking contributions and would allow us to quantify the importance of initial- and final-state interactions.     

Direct observation of macroscopic charge quantization

The circuit formed by a nanoscale tunnel junction in series with a capacitance and a voltage source is the building block of most multi-junction circuits of single electronics. The state of this single electron box is entirely determined by the numbern of extra electrons on the intermediate island electrode between the junction and the capacitance. We have fabricated such a system and measured the charge on the junction capacitance, which is directly related to the average value ofn, as a function of the bias voltage using a Fulton-Dolan electrometer. At low temperature, the junction charge followed thee-periodic sawtooth function expected from the theory of macroscopic charge quantization. Strikingly,e-periodic variations were also observed when the box was superconducting. The thermal rounding of the sawtooth function is well explained by a simple model, except at the lowest temperatures.     

Direct observation of anomalous Kossel lines

Anomalous Kossel lines corresponding to the yield of X rays from a germanium crystal under the conditions of strongly asymmetric diffraction have been observed in the simplest experimental scheme with the use of characteristic Mo K _α radiation and a germanium crystal analyzer. In agreement with the results of a previous theoretical analysis, such lines are narrow collimated beams of radiation in the form of a ridge in a narrow angular range near a given direction whose intensity is one or two orders of magnitude higher than the background with a divergence of about several seconds of arc in the diffraction plane and of about 10′ along the Kossel line. The possible applications of this effect, in particular, for solving the problems of X-ray holography, are discussed.     

Direct observation of the Aharonov-Casher phase

Ring structures fabricated from HgTe/HgCdTe quantum wells have been used to study Aharonov-Bohm type conductance oscillations as a function of Rashba spin-orbit splitting strength. We observe nonmonotonic phase changes indicating that an additional phase factor modifies the electron wave function. We associate these observations with the Aharonov-Casher effect. This is confirmed by comparison with numerical calculations of the magnetoconductance for a multichannel ring structure within the Landauer-Büttiker formalism.     

Direct observation of the transversal Doppler-shift

An experiment is reported in which the second order Doppler-shift has been determined by the observation of theHα-line emitted by linearly moving hydrogen atoms of velocities 2.53×10⁸ cm/s?9.28×10⁸ cm/s. In contrast to previous experiments the direct transversal observation has been used for the first time. The coefficient of the second order term in the relativistic approximation is found to be 0.52±0.03 which compares good with the theoretical value of 1/2.     

Direct observation of double diffraction patterns

The double diffraction light intensity of two identical gratings placed parallel in the Fresnel region is sensitive to the lateral displacement ΔX, and is also affected by the air gap R between the gratings. In this note, the dependence of the double diffraction light intensity on ΔX and R for each order beam has been obtained directly by observing the intensity pattern on a screen.     

Existence and nature of a freezing transition inside three-dimensional arrays of narrow channels

The density functional for matter confined inside a three-dimensional array of narrow channels is constructed. It is shown that the effect of long range interactions amounts to a three-dimensional quadratic perturbation on an array of one-dimensional hard rods. Hence, it follows that the recently proposed freezing transition of atoms confined in nanotube arrays is continuous. This conclusion is validated beyond mean field theory by Monte Carlo simulations in the NPT ensemble.