Acousto-optic differential optical absorption spectroscopy for atmospheric measurement of nitrogen dioxide in Hong Kong

Measurement of the atmospheric concentration of nitrogen dioxide (NO(2)) pollutant was demonstrated by differential optical absorption spectroscopy (DOAS) using a visible acousto-optic tunable filter. In a traditional spectral scanning DOAS system for atmospheric concentration monitoring, a highly stable light source is required. When the light intensity fluctuates during scanning, the concentration retrieval will be inaccurate. In order to reduce the error due to intensity fluctuations, a modified DOAS system has been developed by introducing a broadband light intensity monitoring channel. Using the measured intensity of the broadband channel as the intensity of the light source, the spectrum can be de-biased and the residual intensity variation will primarily result from atmospheric extinction. In addition, by employing the lock-in detection technique, the background light interference is also removed in the modified DOAS system. The atmospheric NO(2) concentration measurement was performed at the campus of City University of Hong Kong, and the results were compared with the concentration reported from a nearby monitoring station in Sham Shui Po, operated by the Hong Kong Environmental Protection Department.     

二元光栅照明对光刻系统成象特性的影响

针对光刻系统分辨力和焦深的矛盾,详细研究了二元光栅照明对硅片上光强以及对光刻系统传输频率的影响,结果表明,二元光栅照明可显著提高光强的对比度,改善成象系统的频率传递函数,是一种比较理想的离轴照明技术。     

激光作用下GeS2非晶半导体薄膜的性能及结构变化

采用514.5nm波长的氩离子激光器，结合X射线衍射分析（XRD）、红外光谱分析（IR）、扫描电镜分析（SEM）和透射光谱分析，研究了GeS2非晶半导体薄膜在激光辐照后的性能及结构变化。实验结果发现，经热处理和激光辐照后，薄膜的光学吸收边均移向短波长处，并且随着辐照激光强度和辐照时间的增加而增加，这种平移在退火薄膜中是可逆的。SEM结果分析表明，薄膜在激光辐照后有晶相出现，且随着辐照激光强度的增加，晶相更多。     

Parallel scan spectral surface plasmon resonance imaging

We describe a parallel scan spectral surface plasmon resonance (SPR) imaging technique. We demonstrate experimentally, with a line-shaped light illumination, that an image acquired with an area CCD detector provides both SPR wavelength information and one-dimensional spatial distribution. Thus two-dimensional distribution of the refractive index of the entire sensing plane can be obtained with a one-dimensional optical line parallel scan. The technique offers advantages of both high sensitivity and high throughput, and could have potential applications in biochips analysis.     

Role of light intensification by cracks in optical breakdown on surfaces

The intensity distribution of an initially plane light wave incident on planar and conical surface cracks is calculated numerically by using a wave propagation computer code. The results show that light intensity enhancements caused by interference of internal reflections at the crack and the surface are very sensitive to the light polarization, the beam angle of incidence, and the crack geometry (e.g., crack width and orientation with the surface). The light intensity enhancement factor (LIEF) can locally reach 2 orders of magnitude for conical cracks of ideal shape. The electric field direction relative to the crack surfaces determines the light intensity profile around the crack. For normal-incidence illumination on the output surface, total internal reflection at the crack and the surface can occur and leads to higher LIEFs. For identical geometry and illumination conditions, a crack located on the entrance surface of an optic generates electric field enhancements that are weaker than those on the exit surface. As cracks on polished surfaces are randomly oriented, the probability for large intensity enhancements to occur is high. The model is able to predict quantitatively the magnitude of surface laser-induced damage threshold drop and damage propagation enhancement in dielectric materials that are due to cracks.     

Two-dimensional biosensor arrays based on surface plasmon resonance phase imaging

We present a biosensor design based on capturing the two-dimensional (2D) phase image of surface plasmon resonance (SPR). This 2D SPR imaging technique may enable parallel label-free detection of multiple analytes and is compatible with the microarray chip platform. This system uses our previously reported differential phase measurement approach, in which 2D phase maps obtained from the signal (P) and reference (S) polarizations are compared pixel by pixel. This technique greatly improves detection resolution as the subtraction step can eliminate measurement fluctuations caused by external disturbances as they essentially appear in both channels. Unlike conventional angular SPR systems, in which illumination from a range of angles must be used, phase measurement requires illumination from only one angle, thus making it well suited for 2D measurement. Also, phase-stepping introduced from a moving mirror provides the necessary modulation for accurate detection of the phase. In light of the rapidly increasing need for fast real-time detection, quantification, and identification of a range of proteins for various biomedical applications, our 2D SPR phase imaging technique should hold a promising future in the medical device market.     

基于成像式照度测量的菲涅尔光学助降系统设计

为了解决菲涅尔光学助降系统的检测问题,提出了一种基于成像式照度探测法的菲涅尔灯发光强度测量方案,并研制了成像式照度探测装置。试验结果表明该装置可远距离对大口径、大发光角度的菲涅尔灯进行发光强度的探测,满足菲涅尔光学助降系统检测装置测量的要求。     

激光作用下GeS2非晶半导体薄膜的性能及结构变化

采用514.5石nm波长的氩离子激光器,结合X射线衍射分析（XRD）、红外光谱分析（IR）、扫描电镜分析（SEM）和透射光谱分析,研究了GeS₂非晶半导体薄膜在激光辐照后的性能及结构变化.实验结果发现,经热处理和激光辐照后,薄膜的光学吸收边均移向短波长处,并且随着辐照激光强度和辐照时间的增加而增加,这种平移在退火薄膜中是可逆的.SEM结果分析表明,薄膜在激光辐照后有晶相出现,且随着辐照激光强度的增加,晶相更多.     

Interference of Acoustically Phase-modulated Light Transmitted by a Semi-transparent Film with a Slit Opening

Spatial interference of acoustically phase-modulated light is explained experimentally and theoretically. If the acoustically phase-modulated light is transmitted by a semi-transparent sheet film with a slit opening, the light diffracted by the slit opening is superposed on the undiffracted background light in the Fresnel diffraction region to yield an acoustically modified interference pattern. This pattern is successfully explained as a result of the partially coherent illumination which is due to acoustic wave modulation. An application of this method is described for measurements of the Raman-Nath parameter and the acoustic wavelength from the variational aspect of the interference fringe pattern.     

Application of bispectral speckle imaging to near-diffraction-limited imaging in the presence of unknown aberrations

A laboratory experiment that demonstrates near-diffraction-limited imaging of a detailed object in the presence of unknown fixed aberrations in the imaging system is described. A random-phase plate is introduced in a pupil plane of the imaging system to eliminate the effect of fixed aberrations in the system. We employ a bispectral speckle imaging technique to recover the object from speckled images affected by both the random-phase fluctuations induced by the random-phase plate and the fixed aberrations present in the imaging system. For the case where the random phase is assumed to obey Gaussian statistics an approximate form of the bispectral speckle transfer function is obtained with an asymptotic expansion. This approximate form of the transfer function shows the diffraction-limited nature of bispectral speckle imaging when the standard deviation of the random-phase fluctuations is of the order of a wavelength of light. Experimental results are presented for fixed aberrations associated with lens tilt and defocus in the imaging system.     

Heterodyne wavelength meter for continuous-wave lasers

A wavelength meter based on a heterodyne interferometer is presented. A single-wavelength test laser beam is modulated to two orthogonal linearly polarized components with different frequencies by a pair of acousto-optic modulators. Then the modulated laser beam and a two-wavelength laser beam are sent to a heterodyne interferometer in a common path. The ratio of two laser interference phase shifts in the heterodyne interferometer is equal to the ratio of their wavelengths. The heterodyne technique measures the heterodyne interference phase but not the interference intensity, which means that it could measure a light source whose intensity is not stable. The heterodyne interference signal is an alternating signal that can easily magnify and process the circuit that makes up the heterodyne wavelength meter and could be used to measure the low-intensity light source even when there are environmental disturbances. A tunable diode laser wavelength range of 630-637 nm has been measured to an accuracy of 5 parts in 10(7).     

基于成像式照度探测的光源光强分布测量

基于成像式照度探测的光源光强空间分布特性测量方法以其量值准确、大幅消除背景光干扰等优点,受到人们的广泛关注。本文根据成像式照度探测原理设计了一套光源光强空间分布特性检测装置,通过实验测试了该检测系统,并总结了测量位置与光强空间分布测量误差之间的关系。该系统具有很好的实用价值。     

Real-space mapping of Fano interference in plasmonic metamolecules

An unprecedented control of the spectral response of plasmonic nanoantennas has recently been achieved by designing structures that exhibit Fano resonances. This new insight is paving the way for a variety of applications, such as biochemical sensing and surface-enhanced Raman spectroscopy. Here we use scattering-type near-field optical microscopy to map the spatial field distribution of Fano modes in infrared plasmonic systems. We observe in real space the interference of narrow (dark) and broad (bright) plasmonic resonances, yielding intensity and phase toggling between different portions of the plasmonic metamolecules when either their geometric sizes or the illumination wavelength is varied.     

Two-wavelength method for endoscopic shape measurement by spatial phase-shifting speckle-interferometry

A two-wavelength method for endoscopic topography reconstruction is introduced that can be applied to out-of-plane sensitive electronic-speckle-pattern interferometry systems based on rigid endoscope imaging systems. The surface measurement is performed by detection of the phase-difference distribution affected by a change in the applied laser wavelength. Furthermore, the off-axis endoscopic illumination geometry is taken into account by an approximation. Experimental results of the characterization of the endoscopic surface reconstruction technique and the measurement accuracy obtained are described and discussed. Finally, the applicability of the method is demonstrated with results from the topographic reconstruction of a free-form surface.     

Speckle contrast reduction in a large-core fiber delivering Q-switched pulses for fluid flow measurements

Due to their capability for supporting high-peak powers, large-core hollow optical fibers may be used to deliver high-peak-power nanosecond pulses for the fluid flow measurement technique of particle image velocimetry [Meas. Sci. Technol. 16, 1119 (2005)]. One drawback of using such fibers for fluid flow measurements is that the output suffers from a speckled interference pattern due to the fiber's multimode nature, which can lead to a loss of spatial information and reduced data quality. Presented here is a technique to reduce the speckle contrast from these fibers when delivering nanosecond pulses. Significant smoothing of the output intensity distribution is demonstrated, giving an improved source of illumination for fluid flow measurements and other imaging techniques requiring pulsed laser illumination.     

Radiation force on a nonlinear microsphere by a tightly focused Gaussian beam

We determine the characteristics of the radiation force that is exerted on a nonresonant nonlinear (Kerr-effect) rigid microsphere by a strongly focused Gaussian beam when diffraction and interference effects are significant (sphere radius a < or = illumination wavelength lambda). The average force is calculated from the surface integral of the energy-momentum tensor consisting of incident, scattered, and internal electromagnetic field vectors, which are expressed as multipole spherical-wave expansions. The refractive index of a Kerr microsphere is proportional to the internal field intensity, which is computed iteratively by the Rytov approximation (residual error of solution, 10(-30). The expansion coefficients for the field vectors are calculated from the approximated index value. Compared with that obtained in a dielectric (linear) microsphere in the same illumination conditions, we find that the force magnitude on the Kerr microsphere is larger and increases more rapidly with both a and the numerical aperture of the focusing objective. It also increases nonlinearly with the beam power unlike that of a linear sphere. The Kerr nonlinearity also leads to possible reversals of the force direction. The proposed technique is applicable to other types of weak optical nonlinearity.     

DIC显微术及在大气颗粒物观察中应用的研究

微分干涉相衬(DIC)显微术使用诺马斯基棱镜完成光束的分割、合成,合成光束发生干涉,将样品上各部分折射率、厚度的变化率或表面起伏的不同转化为像面上光强的差别,故像面是物面的微分。DIC与无限远光学系统的结合,减小了棱镜、检偏器的插入引起的负面影响。将该DIC应用于PM2.5大气颗粒物的研究,得到了优于其它显微术的图像。     

The Role of Retro-Reflective Materials in Holography

In taking holograms of solid components, laser light i reflected from the surface of the component to the hologram. In general, the surfaces involved are diffuse, which scatter the reflected light cau ing large intensity los es of the incident illumination and also a depolarization of the highly polarized incident laser light. To illuminate large diffu e surfaced components requires large expensive lasers. The authors have used mall inexpensive la er and retro-reflective coatings on large components, enabling all the incident low intensity laser illumination to be reflected to the hologram. The polarization of the laser light remain unaffected after reflection at the component surface. By retaining the polarization of the incident illumination, high contrast real time fringes are obtained in hologram interferometers. The authors di Cll s the history of retro-reflection referring to British and US Patents and have reviewed the uses of such materials in holography. The main constituent of the latest retro-reflective paints and tapes is an extremely small phere having a core of very high refractive index glass, the outer surface of the sphere being chemically treated to give a lower refractive index: orne retro-reflecli e tape use conventional glass spheres. Such small spheres have been used as imaging lenses with both monochromatic and non-monochromatic component illumination to create 3D images, the images being comparable to those obtained by lenticular or fly’s-eye creen techniques.     

Nonbleaching fluorescence of gold nanoparticles and its applications in cancer cell imaging

In this paper, we investigated the fluorescent properties of gold nanoparticles (GNPs) with several tens of nanometers by ensemble fluorescence spectrometry, fluorescence correlation spectroscopy (FCS), and fluorescence microscopy. We observed that GNPs synthesized by the citrate reduction of chloroauric acid possessed certain fluorescence, narrow full width at half-maximum (17 nm), and with an increase of particle sizes, the emission intensity showed a gradual increase while the emission wavelength remained almost constant (at 610 nm). Especially, the fluorescence of GNPs possessed the excellent behavior of antiphotobleaching under strong light illumination. Despite their low quantum yields, GNPs exhibited strong native fluorescence under relatively high excitation power. The fluorescence of GNPs could be characterized by fluorescence imaging and FCS at the single particle level. On the basis of this excellent antiphotobleaching of GNPs and easy photobleaching of cellular autofluorescence, we developed a new method for imaging of cells using GNPs as fluorescent probes. The principle of this method is that after cells stained with GNPs or GNPs bioconjugates are illuminated by strong light, the cellular autofluorescence are photobleached and the fluorescence of GNPs on cell membrane or inside cells can be collected for cell imaging. On the basis of this principle, we imaged living HeLa cells using GNPs as fluorescent probes and obtained good cell images by photobleaching of cellular autofluorescence. Furthermore, anti-EGFR/GNPs were successfully used as targeted probes for fluorescence imaging of cancer cells. Our preliminary results demonstrated that GNPs possessed excellent behaviors of antiphotobleaching and were good fluorescent probes in cell imaging. Our cellular imaging method described has potential applications in cancer diagnostics, studies, and immunoassays.     

Demonstration of a Fresnel axicon

We design and manufacture a Fresnel axicon (fraxicon) that generates a quasi-diffraction-free/Bessel beam with a large depth of field. The novel optical element is characterized with both coherent and incoherent light, and its behavior is compared with that of a classical axicon. While the fraxicon exhibits a strong interference pattern in the on-axis intensity distribution, it can be smoothed out when using broadband light, partial spatial coherence light, or by period randomization. As inexpensive, compact/lightweight, and low-absorption elements, fraxicons may find applications in imaging, illumination, and situations where low absorption and dispersion are important.