Influence of the flow on the anchoring of nematic liquid crystals on Langmuir-Blodgett monolayers studied by optical second-harmonic generation

The influence of capillary flow on the alignment of the nematic liquid crystal 5CB on fatty acid Langmuir-Blodgett monolayers was studied by optical second-harmonic generation (SHG). The surface dipole sensitivity of the technique allows probing the orientation of the first liquid crystal monolayer in the presence of the liquid crystal bulk. It was found that capillary flow causes the first monolayer of liquid crystal molecules in contact with the fatty acid monolayer to be oriented in the flow direction with a large pretilt (78 degrees), resulting in a quasi-planar alignment with splay-bend deformation of the nematic director in the bulk. The large pretilt angle also suggests that the Langmuir-Blodgett film itself is affected by the flow. The quasi-planar flow-induced alignment was found to be metastable. Once the flow ceases, circular domains of homeotropic orientation nucleate in the sample and expand until the whole sample becomes homeotropic. This relaxation process from flow-induced quasi-planar to surface-induced homeotropic alignment was also monitored by SHG. It was found that in the homeotropic state the first nematic layer presents a pretilt of 38 degrees almost isotropically distributed in the plane of the cell, with a slight preference for the direction of the previous flow. Received 8 November 2000 and Received in final form 12 March 2001     

Isotopic ratio measurement of methane in ambient air using mid-infrared cavity leak-out spectroscopy

We report on infrared laser spectroscopic measurements of the isotopic composition of methane (¹²CH₄, ¹³CH₄) in natural air samples with a cavity ring-down technique. A CO overtone sideband laser is utilized to excite a high-finesse cavity which provides an effective optical absorption path length of 3.6 km. We achieved a detection limit of 105 ppt methane in ambient air using an integration time of 20 s. This corresponds to a minimum detectable absorption of 1.9×10^-9 /cm. Rapid determination of the ¹³C/¹²Cisotopic ratio of methane in ambient air without sample preconcentration or gas processing is realized. The present system requires only few minutes for an isotopic ratio measurement with a precision of 11%o . Received: 14 July 2000 / Revised version: 25 October 2000 / Published online: 6 December 2000     

Real-time monitoring of ethane in human breath using mid-infrared cavity leak-out spectroscopy

We report on spectroscopic real-time analysis of ethane traces in exhaled human breath. Ethane is considered the most important volatile marker of free-radical induced lipid peroxidation and cell damage in the human body. Our measurements were carried out by means of mid-infrared cavity leak-out spectroscopy in the 3 μm region, a cw variant of cavity ring-down spectroscopy. The spectrometer is based on a CO overtone laser with tunable microwave sidebands. The resulting system proved to be an unique tool with high sensitivity and selectivity for rapid and precise breath testing. With a 5 s integration time, we achieved a detection limit on the order of 100 parts per trillion ethane in human breath. Thus, sample preconcentration is unnecessary. Time-resolved monitoring of the decaying ethane fraction in breath after smoking a cigarette is demonstrated. Received: 13 March 2001 / Published online: 9 May 2001     

Isochronous Mass Measurements of Hot Exotic Nuclei

A novel method for mass measurements of short-lived exotic nuclides is presented. Exotic nuclides were produced and separated in flight at relativistic energies with the fragment separator (FRS) and were injected into the experimental storage ring (ESR). Operating the ESR in the isochronous mode we performed mass measurements of neutron deficient fragments of ⁸⁴Kr with half-lives larger than 50 ms. However, this experimental technique is applicable in a half-life range down to a few μs. A mass resolving power of 110000 (FWHM) has been achieved. Results are presented for the masses of ⁶⁸As, ^70,71Se and ⁷³Br. This revised version was published online in July 2006 with corrections to the Cover Date.     

The mechanical pre-stressing in ultrasonic piezotransducers

Composed piezotransducers submitted to mechanical pre-stressing present shifts on resonance and anti-resonance frequencies. Changes on characteristic parameters of the ceramic and in the coupling between the parts of the transducer can be the causes for this behavior. In applications where the level of pre-stressing is low (up to 50 MPa) the parameters of the ceramic are not altered, therefore, the shifting on frequencies are attributed to coupling between parts. This paper describes a mathematical model to explain this effect based on difference of effective cross-section between transducers parts under pre-stressing. The results show a proportional relation between pre-stressing and effective coupling of the parts.     

Eigenstates of boson creation operator

We examine the existence of right-hand eigenstates (or eigenkets) of the boson creation operator a and determine their coordinate and their Bargmann representation. The eigenkets of the creation operator are ultrasingular and cannot be considered as a limiting case of normalizable states. Applications of these eigenstates as auxiliary states for purposes of representation of states by path integrals over coherent states are discussed. A completeness relation for coherent states on paths through the complex plane is derived and special examples of its application are considered. Received 9 March 2001 and Received in final form 13 June 2001     

Magnetotransverse scattering of surface plasmon polaritons

We show experimentally that the in-plane scattering of surface plasmon polaritons (SPP) is influenced by a perpendicular magnetic field. The average SPP flux is deflected into the direction perpendicular to both its initial propagation direction and the magnetic field direction. From a phenomenological point of view, this is an analogy to the Hall effect for electrons and a 2D equivalent of the photonic Hall effect.     

Resonator-enhanced optical dipole trap for fermionic lithium atoms

We demonstrate a novel optical dipole trap that is based on enhancement of the optical power density of a Nd:YAG laser beam in a resonator. The trap is particularly suited for experiments with ultracold gases, as it combines a potential depth of the order of 1 mK with storage times of several tens of seconds. We study the interactions in a gas of fermionic lithium atoms in our trap and observe the influence of spin-changing collisions and off-resonant photon scattering. A key element in reaching long storage times is the use of an ultralow-noise laser. The dependence of storage time on laser noise is investigated.     

非球面数字波面检测技术

提出了一种快速检测浅度非球面（非球面度小于0.01 mm）的方法,该方法无需补偿器或其他辅助光学元件进行零位补偿。用移相干涉仪直接测量正轴或离轴的浅度凹非球面,剔除平移、倾斜、失焦等调整误差后,得到实际被测镜面的面形分布数据;根据正轴或离轴的浅度凹非球面矢高方程计算理想非球面的面形分布数据,得到理论波面数据,用实测的面形分布数据减去理论的面形分布数据即可得到被检非球面的剩余波像差,即面形误差。利用该方法测量了一口径为135 mm的双曲面,并用零位补偿法加以验证。两种方法的检测结果精度相当,说明数字波面法可实际应用于正轴或离轴的浅度凹非球面的检测。     

The inclusion of source effect consideration in irradiance determination

In optical radiometry, an accurate realization of spectral irradiance scale depends on the investigation of both optical radiation source’s and detecting unit’s (filter radiometer) features. In the source part; comprehensive theoretical and experimental analysis of effects of lamp filament emissivity and its alterations due to the thermal and geometrical factors on the irradiance were studied. Meanwhile, detailed optical characterizations such as; determination of detecting element responsivity, transmittance of filters and measurements of aperture area were also made for the detecting element part. The inclusion of the source effects and the comprehensive theoretical and experimental investigation resulted in the reduction of the number of estimated parameters to be used in matching the theoretical and experimental data, thereby improving the current uncertainty. Moreover, the method we used in this work to analyze the parameters that may affect the irradiance is considered to bring a new approach to the evaluation of irradiance.