光学相干层析成像技术发展及应用

光学相干层析成像(Optical Coherence Tomography,OCT)在近二十多年来迅速发展,是一种基于相干干涉原理将光、电以及计算机图像处理技术结合为一体,利用样品的后向散射光对样品进行成像的新型成像技术,适用范围广,受到众多科研学者的关注。OCT具有无损伤、非介入、非接触、图像分辨率高且操作简单、便携、易与内窥镜结合等优点,广泛应用于光学检测、工业检测、医学、生物诊断检测、科学研究等领域。由于OCT在用于软组织成像时组织穿透深度有限等原因,导致该技术在生物医学领域尚未实现推广应用。目前OCT技术发展尚不成熟,相关科研工作者致力于增加系统穿透深度、提高分辨率和信噪比、优化系统综合性能等方向的研究。相关技术的突破,能有效改善系统成像质量、扩大OCT技术应用范围、提高医疗检测技术水平,对促进光谱成像技术的发展具有重要意义。     

Optical atomic coherence at the 1-second time scale

Highest-resolution laser spectroscopy has generally been limited to single trapped ion systems because of the rapid decoherence that plagues neutral atom ensembles. Precision spectroscopy of ultracold neutral atoms confined in a trapping potential now shows superior optical coherence without any deleterious effects from motional degrees of freedom, revealing optical resonance linewidths at the hertz level with a good signal-to-noise ratio. The resonance quality factor of 2.4 x 10(14) is the highest ever recovered in any form of coherent spectroscopy. The spectral resolution permits direct observation of the breaking of nuclear spin degeneracy for the 1S0 and 3P0 optical clock states of 87Sr under a small magnetic bias field. This optical approach for excitation of nuclear spin states allows an accurate measurement of the differential Landé g factor between 1S0 and 3P0. The optical atomic coherence demonstrated for collective excitation of a large number of atoms will have a strong impact on quantum measurement and precision frequency metrology.     

光学相干断层扫描仪和角膜内皮细胞计数仪测量中央角膜厚度的结果比较

目的 探讨光学相干断层扫描仪(OCT)与角膜内皮细胞计数仪测量中央角膜厚度(CCT)各自的正常值及相互之间的差异.方法 分别用OCT和角膜内皮细胞计数仪测量45例(90只眼)正常人的CCT并对测量值进行比较,用相关分析方法分析不同仪器测得的角膜厚度的正常值,及不同仪器间测量值的相关性.结果 OCT测得的角膜厚度正常值为(542.43±35.27) μm,角膜内皮细胞计数仪测得的正常值是(521.08±34.20)μm,差异有统计学意义(P＜0.01),两者呈正相关(r=0.954,P＜0.01).结论 不同机器测量得到的CCT正常值不同,医务人员在临床应用时要特别注意,最好每个型号的仪器都设立自己的正常参考值.     

Optical projection tomography as a tool for 3D microscopy and gene expression studies

Current techniques for three-dimensional (3D) optical microscopy (deconvolution, confocal microscopy, and optical coherence tomography) generate 3D data by "optically sectioning" the specimen. This places severe constraints on the maximum thickness of a specimen that can be imaged. We have developed a microscopy technique that uses optical projection tomography (OPT) to produce high-resolution 3D images of both fluorescent and nonfluorescent biological specimens with a thickness of up to 15 millimeters. OPT microscopy allows the rapid mapping of the tissue distribution of RNA and protein expression in intact embryos or organ systems and can therefore be instrumental in studies of developmental biology or gene function.     

Quantum coherence in an optical modulator

Semiconductor quantum well electroabsorption modulators are widely used to modulate near-infrared (NIR) radiation at frequencies below 0.1 terahertz (THz). Here, the NIR absorption of undoped quantum wells was modulated by strong electric fields with frequencies between 1.5 and 3.9 THz. The THz field coupled two excited states (excitons) of the quantum wells, as manifested by a new THz frequency- and power-dependent NIR absorption line. Nonperturbative theory and experiment indicate that the THz field generated a coherent quantum superposition of an absorbing and a nonabsorbing exciton. This quantum coherence may yield new applications for quantum well modulators in optical communications.     

Electron coherence in a melting lead monolayer

We used angle-resolved photoemission spectroscopy to measure the electronic dispersion and single-particle spectral function in a liquid metal. A lead monolayer supported on a copper (111) surface was investigated as the temperature was raised through the melting transition of the film. Electron spectra and momentum distribution maps of the liquid film revealed three key features of the electronic structure of liquids: the persistence of a Fermi surface, the filling of band gaps, and the localization of the wave functions upon melting. Distinct coherence lengths for different sheets of the Fermi surface were found, indicating a strong dependence of the localization lengths on the character of the constituent atomic wave functions.     

Cavity quantum electrodynamics: coherence in context

Modern cavity quantum electrodynamics (cavity QED) illuminates the most fundamental aspects of coherence and decoherence in quantum mechanics. Experiments on atoms in cavities can be described by elementary models but reveal intriguing subtleties of the interplay of coherent dynamics with external couplings. Recent activity in this area has pioneered powerful new approaches to the study of quantum coherence and has fueled the growth of quantum information science. In years to come, the purview of cavity QED will continue to grow as researchers build on a rich infrastructure to attack some of the most pressing open questions in micro- and mesoscopic physics.     

Nuclei-induced frequency focusing of electron spin coherence

The hyperfine interaction of an electron with the nuclei is considered as the primary obstacle to coherent control of the electron spin in semiconductor quantum dots. We show, however, that the nuclei in singly charged quantum dots act constructively by focusing the electron spin precession about a magnetic field into well-defined modes synchronized with a laser pulse protocol. In a dot with a synchronized electron, the light-stimulated fluctuations of the hyperfine nuclear field acting on the electron are suppressed. The information about electron spin precession is imprinted in the nuclei and thereby can be stored for tens of minutes in darkness. The frequency focusing drives an electron spin ensemble into dephasing-free subspaces with the potential to realize single frequency precession of the entire ensemble.     

Spontaneous Bose coherence of excitons and polaritons

In the past decade, there has been an increasing number of experiments on spontaneous Bose coherence of excitons and polaritons. Four major areas of research are reviewed here: three-dimensional excitons in the bulk semiconductor Cu2O, two-dimensional excitons in coupled quantum wells, Coulomb drag experiments in coupled two-dimensional electron gases, and polaritons in semiconductor microcavities. The unifying theory of all these experiments is the effect of spontaneous symmetry breaking in the Bose-Einstein condensation phase transition.     

Optical materials

Optical technologies are becoming increasingly important in areas that were traditionally the domain of electronics. This trend is likely to continue into the foreseeable future with optics and electronics being integral, mutually compatible components of systems for consumer markets, industry, and defense. The basis of this progress is the development of materials that have the required purity, physical properties, and optical quality; glass fibers for optical transmission, semiconductors for lasers and detectors, and nonlinear materials for optical switching are examples. In this article, some of the materials of choice for a variety of applications are described and the frontiers of materials research for new areas of opportunity are discussed. Particular emphasis is placed on optical materials for the transmission and processing of information.     

Electronic confinement and coherence in patterned epitaxial graphene

Ultrathin epitaxial graphite was grown on single-crystal silicon carbide by vacuum graphitization. The material can be patterned using standard nanolithography methods. The transport properties, which are closely related to those of carbon nanotubes, are dominated by the single epitaxial graphene layer at the silicon carbide interface and reveal the Dirac nature of the charge carriers. Patterned structures show quantum confinement of electrons and phase coherence lengths beyond 1 micrometer at 4 kelvin, with mobilities exceeding 2.5 square meters per volt-second. All-graphene electronically coherent devices and device architectures are envisaged.     

Electronic coherence and nonlinear susceptibilities of conjugated polyenes

A dynamic theory that connects electronic motions and the nonlinear optical response of conjugated polyenes is developed by introducing the concept of electronic normal modes. A useful picture for the mechanism of optical nonlinearities is obtained by identifying the few dominant modes. This quasi-particle electron-hole representation establishes a close analogy with small semiconductor particles (quantum dots) and is very different from the traditional approach based on electronic eigenstates. The effective conjugation length (coherence size), which controls the scaling and saturation of the static third-order susceptibility X((3)) with the number of double bonds, is related to the coherence of the relative motion of electron-hole pairs created upon optical excitation.     

A gate-controlled bidirectional spin filter using quantum coherence

We demonstrate a quantum coherent electron spin filter by directly measuring the spin polarization of emitted current. The spin filter consists of an open quantum dot in an in-plane magnetic field; the in-plane field gives the two spin directions different Fermi wavelengths resulting in spin-dependent quantum interference of transport through the device. The gate voltage is used to select the preferentially transmitted spin, thus setting the polarity of the filter. This provides a fully electrical method for the creation and detection of spin-polarized currents. Polarizations of emitted current as high as 70% for both spin directions (either aligned or anti-aligned with the external field) are observed.