Nanoscopic ultrafast space-time-resolved spectroscopy

We propose and analyze a scheme for ultrafast spectroscopy with nanometer spatial and femtosecond temporal resolution. The interaction of polarization-shaped laser pulses with a nanostructure allows us to control the spatial and temporal evolution of the optical near field. Employing a learning algorithm, the field is tailored such that pump and probe excitation occur at different positions and at different times. Both excitations can be restricted to subdiffraction extensions and are separable on a nanometer length scale. This enables the direct spatial probing of nanoscale energy transfer or charge transfer processes.     

High resolution digital holographic microscopy for the study of aggregated natural cellulose nanowhisker fibers

In this paper, digital holographic (DH) microscopy demonstrates its ability to perform a full characterization of nanofibers. The high resolution and magnification of the presented method to study the nanofibers are tested using standard MIL-STD-150A 1951 USAF resolution test target. In this investigation, aggregated natural cellulose nanowhisker fibers are positioned in the front of the microscopic objective using a 3D translation stage in the object arm of DH setup. The recorded off-axis holograms are refocused using the angular spectrum method. The reconstructed complex field is used to calculate optical phase and intensity distributions of the object at different reconstruction depths. A simple algorithm is used to define the focused image with suitable accuracy. The dimensions and orientation of the fibers can be evaluated from the optical field at different depths. Then, the shape and textures along the aggregated natural cellulose nanowhisker fiber can be presented in a 3D space.     

Studies on aperture synthesis in digital Fresnel holography

Aperture synthesis can improve image resolution in digital holography by increasing the numerical aperture of the system. In this paper, we show that both the lateral resolution and image field of view can be enhanced at the same time using a more general Fresnel holography setup and hologram stitching. The impact of aperture synthesis on the lateral resolution is investigated both theoretically and experimentally. In the experiment, the synthesis is executed by moving the compact digital holographic system in two directions. Nine holograms are recorded and stitched into one hologram. The reconstruction results show that expanding aperture improves the lateral resolution. The lensless Fresnel holography used in this paper is demonstrated to have the ability to provide a larger numerical aperture and can compress the object spectrum in recording process.     

纳秒激光单脉冲烧蚀硅表面的超快成像研究

利用ICCD可以在纳秒时间尺度下成像的特点,以飞秒准连续激光产生的超短脉冲光为探测光,对纳秒激光单脉冲烧蚀硅靶表面的演化过程进行动态监测。在能量密度为50J／cm^2时,捕获了纳秒单脉冲激光烧蚀硅靶面过程中等离子体演化的时间分辨图像。图像表明,纳秒激光烧蚀硅靶产生的等离子体开始时密度大,膨胀速度快,当纳秒激光脉冲过后,等离子体不再产生,并且其膨胀速度不再增加,直至完全消失。     

超短脉冲数字全息术中的立体角分复用技术

将角分复用技术应用于超短脉冲数字全息术，可实现对超快过程的动态记录.其中，在角分复用的设计中，由于CCD像素尺寸的限制，一般对物光和参考光的角度要求较为苛刻.为此，提出了一种立体角分复用的方法，并使用连续激光数字全息实验验证了这种方法的有效性.在此基础上，根据超短脉冲数字全息术记录超快过程的特点，设计了能够实现自动连续进行立体角分复用记录的光路.计算分析表明，该光路可在一帧数字全息图上连续记录间隔为纳秒量级的多幅超快过程的动态图像. 关键词： 超短脉冲数字全息术 立体角分复用 超快过程     

Recording and reconstruction of digital Gabor hologram

This paper looks at some as yet undisclosed observations in the recording and reconstruction of digital Gabor hologram. Small particles on the specimen plane diffract the illuminating spherical wave to form the object wave which interferes with the un-diffracted reference to form the in-line hologram. The interference is similar to that between two spherical wavefronts. It is shown that resolution enhancement can be achieved in the recording process through the use of wavefronts with different curvature. Numerical reconstruction of this hologram with a conjugate of the illuminating (reference) wave provides the in-focus real image at the object plane. It is exemplified that the proper choice of the reconstruction wavefront can optically magnify the reconstructed image enabling one to better visualize the resolution enhancement. Both theoretical analysis and experimental confirmation are provided for these observations.     

利用相位模板实现数字全息超分辨成像

为了简化数字全息超分辨记录系统,分别在其物光和参考光部分引入一块相位模板,以获得垂直和倾斜方向照明物体的光束和具有不同载波频率的参考光束.当这些具有不同照射方向的光透过物体后,可以使CCD在位置固定的情况下记录到携带低频和高频信息的物体衍射场,不同载波频率的参考光则保证了高频和低频信息在复合全息图的频谱面上能够相互分离.实验结果证明,通过将记录到的物体高频和低频信息合成,可以获得超出系统衍射极限分辨率的再现像.     

Ultrafast spectroscopy of high-lying excited states via eight-wave mixing

We demonstrate the use of eight-wave mixing (8WM) with femtosecond laser pulses for the detection of the high-lying D state of ethylene (C₂H₄) gas. The D state is reached through three-photon absorption of 427 nm light followed by a delayed single-photon probe interaction with the ionization continuum, resulting in an overall 8WM process. This scheme allows the detection of high-lying states with both spatial and temporal resolution before a thermal grating can form. Alternatively, by using a nanosecond delay between pulses, the induced thermal grating may be studied after the population grating has decayed. We verify the precise 8WM nature of this process and show how to use it to study the excited-state dynamics of ethylene and to measure the profile of an ethylene flow.     

Process of Surface Flashover in Vacuum Under Nanosecond Pulse

In a wide variety of high-power pulsed devices, vacuum surface flashover plays a key role in the system performance. High-speed electrical measurements with temporal resolution in the nanosecond range were employed to measure the flashover characteristics, and high-speed three-frame photography was used to image the flashover channels at different phases along the insulator surface. From these test results, the flashover under nanosecond pulse would experience a process with three typical phases: initial phase, developing phase, and extinguishing phase. Spectra luminosity of the flashover channel is also discussed for better understanding the mechanism of the flashover process.      

基于序列图像的运动目标检测

针对动态环境下运动目标检测中噪声多、目标检测不完整等情况,提出了一种基于金字塔多分辨率模型的运动目标检测方法,在低分辨率下获取目标的区域,在高分辨率下获取目标的细节。对于复杂的环境,还提出了运用高低双阈值替代传统的单阈值进行图像差分运算的方案,阈值可以根据图像自动分析得到。该方法首先将当前帧和背景帧进行尺度变换,得到不同分辨率下的图像组,然后在不同尺度下得到高低阈值差分图像,最后从高层向低层进行有效融合,得到噪声少的完整目标图像。实验表明,该方法提取运动目标的精度比较高,单目标达到0.802,多目标达到0.615,尤其是在复杂的动态环境下,优势比较明显。     

Magnetic microstructures and their dynamics studied by X-ray microscopy

Full-field soft X-ray microscopy in combination with X-ray magnetic circular dichroism as contrast mechanism is a powerful technique to image with elemental specificity magnetic nanostructures and multilayered thin films at high lateral resolution down to 15nm by using Fresnel zone plates as X-ray optical elements. Magnetization reversal phenomena on a microscopic level are studied by recording the images in varying external magnetic fields. Local spin dynamics at a time resolution below 100ps can be addressed by engaging a stroboscopic pump-and-probe scheme taking into account the time pattern of synchrotron storage rings. Characteristic features of magnetic soft X-ray microscopy are reviewed and an outlook into future perspectives with regard to increased lateral and temporal resolution is given.     

利用小尺寸电荷耦合器件实现数字全息高分辨成像

为了在降低数字全息显微成像系统成本的同时实现高分辨成像,对像面数字全息显微术的记录与再现过程进行了理论分析,结合系统的点扩散函数,对该系统的横向分辨率进行了分析. 得到了如下结论：像面数字全息显微系统的横向分辨率对电荷耦合器件（CCD）光敏面尺寸变化不敏感;对于常见的CCD 器件,其像元尺寸的变化对该系统的横向分辨率影响甚微. 此外,对像面数字全息显微系统的成像特点进行了分析,结果表明：利用像面数字全息系统可以实现物体信息的完整记录与再现,其成像分辨率及像质优于预放大数字全息系统. 利用搭建的数字全息实验记录系统,从强度及位相两方面对理论分析结果进行了验证,实验结果表明了理论分析的正确性. 关键词： 像面数字全息术 横向分辨率 点扩散函数 小尺寸电荷耦合器件     

超快中子探测器闪烁体性能计算

超快中子探测器是ICF聚变反应速率测量系统的核心部件。利用蒙特卡罗粒子输运工具包Geant4模拟了一种超快中子探测器——BC-422型闪烁探测器的中子探测过程,计算出了几种厚度的BC-422型闪烁体的探测效率、输出光信号强度和时间分辨力;对比了闪烁体的2种不同反射表面对输出光信号强度和时间分辨力的影响。计算的结果显示:设计适当的BC-422型闪烁探测器能够测量的最低中子产额在108量级,对DT中子的信号时间分辨力好于20 ps,对DD中子的信号时间分辨力达到30 ps,能够用于大型激光装置及其原型的聚变反应速率测量。     

Variable radio frequency proton-electron double-resonance imaging: application to pH mapping of aqueous samples

Proton-electron double-resonance imaging (PEDRI) offers rapid image data collection and high resolution for spatial distribution of paramagnetic probes. Recently we developed the concept of variable field (VF) PEDRI which enables extracting a functional map from a limited number of images acquired at pre-selected EPR excitation fields using specific paramagnetic probes (Khramtsov et al., J. Magn. Reson. 202 (2010) 267-273). In this work, we propose and evaluate a new modality of PEDRI-based functional imaging with enhanced temporal resolution which we term variable radio frequency (VRF) PEDRI. The approach allows for functional mapping (e.g., pH mapping) using specifically designed paramagnetic probes with high quality spatial resolution and short acquisition times. This approach uses a stationary magnetic field but different EPR RFs. The ratio of Overhauser enhancements measured at each pixel at two different excitation frequencies corresponding to the resonances of protonated and deprotonated forms of a pH-sensitive nitroxide is converted to a pH map using a corresponding calibration curve. Elimination of field cycling decreased the acquisition time by exclusion periods of ramping and stabilization of the magnetic field. Improved magnetic field homogeneity and stability allowed for the fast MRI acquisition modalities such as fast spin echo. In total, about 30-fold decrease in EPR irradiation time was achieved for VRF PEDRI (2.4s) compared with VF PEDRI (70s). This is particularly important for in vivo applications enabling one to overcome the limiting stability of paramagnetic probes and sample overheating by reducing RF power deposition.     

基于光场一阶关联的时域成像

与通常利用二阶强度关联测量实现时域鬼成像不同,本文利用时域热光源借助干涉仪通过一阶关联实现时域成像.基于空域光束的近轴衍射和时域窄带脉冲在色散介质中色散之间的空间-时间二象性,在时域脉冲响应函数的基础上得到了表征一阶关联时域成像的强度表达式,分析研究了光源脉冲宽度和相干时间对成像可见度和分辨率的影响.结果一方面表明基于热光场一阶关联的时域成像在不需要额外色散补偿或消除条件下可以实现时域物体信号的再现,另一方面表明当光源脉冲宽度一定时,成像可见度随光源脉冲相干时间的增加而增加,但是成像分辨率逐渐降低,其中当光源脉冲宽度约为100 ps,相干时间约为0.5 ps时,间隔为20 ps,宽度为8 ps的时域矩形波型物体的成像质量(兼顾可见度和分辨率)较好.该结果对于基于热光一阶关联的时域成像在时序信号测量中的应用具有重要意义.     

介质阻挡放电中微放电周期特性的光学测量

采用特殊设计的双水电极介质阻挡放电装置,利用光学方法原位、无干扰地测量了大气压空气介质阻挡放电中微放电的单脉冲特性和周期特性,实验上首次发现在介质阻挡放电斑图模式中,微放电在驱动电压的正负半周的放电时刻并不是固定的,相邻两次放电的时间间隔是长短交替的。根据壁电荷对微放电通道相邻两次放电的不同作用,分析了相邻两次放电时刻的联系,很好地解释了相应的实验现象。     

Digital holography at millimetre wavelengths

In this paper, we describe a method for wavefront reconstruction at millimetre wavelengths using off-axis holography, a frequently used image recording technique at visible wavelengths first demonstrated in the 1960’s. Millimetre radiation has been highlighted recently in the imaging of non-conducting materials and objects, which often show significant transparency at these long wavelengths, even though opaque at visible and infrared wavelengths. Holography provides a method for recording a lens-less image of an object thus reducing loss of spatial frequency information, which is important for obtaining maximum resolution at long wavelengths. An experimental arrangement based on a simple form of Fresnel off-axis holography is described, with the object illumination and reference beams derived using two radiating horn antennas fed by a single coherent source (a Gunn diode oscillator operating at 100 GHz – wavelength 3 mm) via a 3 dB cross-guide coupler. The process is discussed in the context of imaging and in the validation of millimetre wave radiation sources (horns). We show that the entire fields from these and other components can be analysed using computational optics with data derived from a single measurement obtained using inexpensive equipment.     

Image restoration for interlaced scan CCD image with space-variant motion blurs

When the speeds of objects in a scene exceed the temporal resolution of the camera shutter, motion blurs will occur. Since objects are often moving in different directions at different speeds, the degradation of a CCD image is often characterized by space-variant motion blurs. Image restoration algorithms for space-variant motion blurs are available for progressive scan CCD images, but not for interlaced scan images. To address the space-variant image restoration for interlaced scan images, a novel three-step image restoration scheme is proposed. Firstly, one interlaced scan image is divided into odd field and even field images. Secondly, these two field images are further segmented into rectangular blocks and the motion vectors are computed based on these rectangular blocks using an efficient block matching algorithm. Thirdly, image restoration is performed using a blind deconvolution algorithm in the odd or even field image. The final restored image is obtained by combining the restored odd and even field images. The scheme is illustrated by restoring a space-variant blurred moving vehicle image and a synthetic blurred image.     

Enhanced wavefront reconstruction by random phase modulation with a phase diffuser

A phase retrieval technique for enhanced wavefront reconstruction using random phase modulation and a phase diffuser is proposed. The speckle field generated is sampled at multiple axially displaced planes and the speckle patterns are used in an iterative algorithm based on the optical wave propagation in free space. The presentation of this technique is carried out using two setups. In the first setup, a diffuser plate is placed at the image plane of a metallic test object. The benefit of randomizing the phase of the object wave is the enhanced intensity recording due to high dynamic range of the diffusely scattered beam. The use of demagnification optics will also allow the investigations of relatively large objects. In the second setup, a transparent object is illuminated using a wavefront with random phase and constant amplitude by positioning the phase diffuser close to the object. The benefit of phase-only modulation is the increased resolution of the phase structures of the transparent test objects.     

Assessment of laser-induced thermal load on silicon nanostructures based on ion desorption yields

Experimental assessment of the thermal load induced by fast laser pulses on micro- and nanostructures through IR imaging is currently too slow and lacks the spatial resolution to be useful. In this paper, we introduce a method based on measuring the laser-induced yields of ions to compare the thermal loads on nanofabricated silicon structures, when exposed to nanosecond laser pulses. The laser fluences at which the ion yields of, for example, sodiated and potassiated peptides ions are equal for two different structures correspond to equivalent thermal loads. Using alkalinated peptides is a convenient choice because the corresponding ion intensities are easily measured up to the melting point of silicon. As an example, we compare the nanosecond laser heating of silicon nanopost arrays with diverse post diameters and periodicities. Assessment of the thermal load through ion yield measurements can also be used to verify model assumptions for heat transport regimes in nanostructures.