Near-field time-reversal amplification

The spatial resolution of the focused field of a classical time-reversal mirror has a wavelength-order lambda diffraction limit. Previously reported results for subwavelength focus require either the full knowledge of the original source or the evanescent waves in the near field. Here it is shown that subwavelength focusing can be achieved without a priori knowledge of the original probe source. If the field is recorded at a few wavelengths away from the probe source, where the amplitude of the near field is too low for subwavelength focusing, it is shown that the low amplitude near field can be amplified and the spatial resolution improved, using the near-field time reversal (NTR) procedure introduced here. The NTR is performed from the phase of the spatial spectrum of the field recorded on an array around the original probe source using an analytical continuation for the amplitude of the spatial spectrum. Following theory, lambda/20 resolution is experimentally demonstrated with audible acoustic wavefields in the air.     

径向剪切干涉法综合诊断光束质量研究

 为了解决高功率激光装置光束质量诊断系统的自验证问题,研究了基于空间相位调制的径向剪切干涉法综合诊断光束质量的实验应用。该方法理论上只需一台干涉仪且从单幅干涉图中获取光束的近场、相位和远场分布信息。实验结果表明,目前可以得到高分辨率的相位分布,由测量近场和相位恢复出的远场同CCD直接测量得到的远场形态相同,环围能量比曲线相当吻合,不足之处在于只能提取近场较低空间频率成分,还达不到CCD直接测量的空间分辨水平。     

Characterizing individual scattering events by measuring the amplitude and phase of the electric field diffusing through a random medium

We describe observations of the amplitude and phase of an electric field diffusing through a three-dimensional random medium, using terahertz time-domain spectroscopy. These measurements are spatially resolved with a resolution smaller than the speckle spot size and temporally resolved with a resolution better than one optical cycle. By computing correlation functions between fields measured at different positions and with different temporal delays, it is possible to obtain information about individual scattering events experienced by the diffusing field. This represents a new method for characterizing a multiply scattered wave.     

First on-line test of the LINAC superbuncher at nuclear science centre

An on-line test of the LINAC superbuncher at Nuclear Science Centre has been successfully performed. DC O⁷⁺ beam of nominal energy 92 MeV was accelerated through the superbuncher resonator, operating at a field of 4.54 MV/m. The total energy gain of the beam was measured to be 4.5 MeV. For the pulsed beam test a phase locked bunched beam of O⁷⁺ of nominal energy 92 MeV, FWHM 1.3 ns from the pre-tandem multiharmonic buncher was injected into the superbuncher. By properly adjusting the phase and amplitude of the resonator, the best FWHM of the bunched beam was measured to be 185 ps near the entrance of the first LINAC module. Fully depleted cooled surface barrier detector was used for measuring the time width. In a separate experiment the intrinsic time resolution of the same detector was measured to be 134 ps. Consequently the intrinsic time width of the bunched beam, after correcting for the detector resolution, would be 127 ps. Details of the experiment and results are presented.     

Characterization of a chaotic optical field using a high-gain, self-amplified free-electron laser

We report on a characterization of the chaotic optical field from a high-gain, self-amplified spontaneous-emission (SASE) free-electron laser. The temporal structure of the amplitude and phase are measured in a single-shot mode, with a resolution well below the coherence length, and the statistics over multiple pulses is determined. The measurement is in excellent quantitative agreement with the prediction based on analysis of random noise, and further verifies the chaotic nature of the SASE optical field.     

Movable aperture lensless transmission microscopy: a novel phase retrieval algorithm

We propose an iterative phase retrieval method that uses a series of diffraction patterns, measured only in intensity, to solve for both amplitude and phase of the image wave function over a wide field of view and at wavelength-limited resolution. The new technique requires an aperture that is scanned to two or more positions over the object wave function. A simple implementation of the method is modeled and demonstrated, showing how the algorithm uses overlapping data in real space to resolve ambiguities in the solution. The technique opens up the possibility of practical transmission lensless microscopy at subatomic resolution using electrons, x rays, or nuclear particles.     

焦斑二阶矩半径与光束近场分布的关系

为分析光束振幅畸变和相位畸变对其焦斑二阶矩半径的影响,以光波标量衍射理论中的夫琅和费衍射公式为基础,根据光斑二阶矩半径的定义,推导了直接依赖于光束近场复振幅分布的焦斑二阶矩半径的一般表达式,并进一步推导了直接依赖于光束近场强度分布和相位分布的焦斑二阶矩半径和焦斑二阶矩角半径的表达式.结果表明:畸变光束的焦斑二阶矩角半径...     

The application of laser reflective tomography in near-field measurements of ultrasonic transducers

With Laser Reflective Tomography（LRT）,the near fields of ultrasonic transducers were measured and analyzed.The principle of LRT measurement of ultrasonic field distribution was introduced and an experimental system was set up.Acoustic pressure of a multiple element piston transducer was measured by using of a laser vibrometer.Its distribution in amplitude and phase was obtained.The acoustic pressure in the same region was measured with a needle hydrophone to validate the LRT method.Furthermore,through reconstruction of acoustic fields,it indicated that LRT method is suitable for predicting the distribution on transducers＇surface and conditions of active elements.     

An all-optical phase detector by amplitude modulation of the local field in a Rydberg atom-based mixer

Recently, a Rydberg atom-based mixer was developed to measure the phase of a radio frequency (RF) field. The phase of the signal RF (SIG RF) field is down-converted directly to the phase of a beat signal created by the presence of a local RF (LO RF) field. In this study, we propose that the Rydberg atom-based mixer can be converted to an all-optical phase detector by amplitude modulation (AM) of the LO RF field; that is, the phase of the SIG RF field is related to both the amplitude and phase of the beat signal. When the AM frequency of the LO RF field is the same as the frequency of the beat signal, the beat signal will further interfere with the AM of the LO RF field inside the atom, and then the amplitude of the beat signal is related to the phase of the SIG RF field. The amplitude of the beat signal and the phase of the SIG RF field show a linear relationship within the range of 0 to π/2 when the phase of the AM is set with a difference π/4 from the phase of the LO RF field. The minimum phase resolution can be as small as 0.6° by optimizing the experimental conditions according to a simple theoretical model. This study will expand and contribute to the development of RF measurement devices based on Rydberg atoms.     

Propagation of femtosecond surface plasmon polariton pulses on the surface of a nanostructured metallic film: space-time complex amplitude characterization

Ultrashort surface plasmon polariton (SPP) pulses, propagating on the surface of a nanostructured metallic film, are characterized in space and time using time-resolved spatial-heterodyne imaging. Optical pulses are coupled from free space into various surface modes using a 2D array of circular nanoholes, and spatial amplitude and phase characteristics of the scattered surface field are measured with femtosecond-scale time resolution. Demonstrated in-plane focusing of SPP pulse provides additional electromagnetic field localization with possible applications in SPP nanophotonics, nonlinear surface dynamics, biochemical sensing, and ultrafast surface studies.     

畸变光束的焦斑质心位置

 为分析振幅和相位发生畸变情况下光束焦斑质心位置所受的影响,以光波标量衍射理论中的夫琅和费衍射公式为基础,根据光斑强度分布一阶矩质心位置的定义,推导了直接依赖于光束近场复振幅分布的焦斑质心位置的一般表达式。进一步推导了焦斑质心位置对光束近场强度分布和相位斜率分布的依赖关系式,并对该式清晰的物理意义进行了阐述。利用焦斑质心位置表达式,可分析和确定由于振幅调制和相位畸变导致的质心位置偏移。最后,对无相位畸变和存在低阶泽尼克相位畸变情况下的简单畸变光束进行了相应的分析讨论。     

Correlation of ultrasonic scatterer size estimates for the statistical analysis and optimization of angular compounding

Ultrasonic scatterer size estimates generally have large variances due to the inherent noise of spectral estimates used to calculate size. Compounding partially correlated size estimates associated with the same tissue, but produced with data acquired from different angles of incidence, is an effective way to reduce the variance without making dramatic sacrifices in spatial resolution. This work derives theoretical approximations for the correlation between these size estimates, and the coherence between their associated spectral estimates, as functions of ultrasonic system parameters. A Gaussian spatial autocorrelation function is assumed to adequately model scatterer shape. Both approximations compare favorably with simulation results, which consider validation near the focus. Utilization of the correlation/coherence expressions for statistical analysis and optimization is discussed. Approximations, such as the invariance of phase and amplitude terms with angle, are made to obtain closed-form solutions to the derived spectral coherence near the focus and permit analytical optimization analysis. Results indicate that recommended parameter adjustments for performance improvement generally depend upon whether, for the system under consideration, the primary source of change in total coherence with rotation is phase term variation due to the change in the relative position of scattering sites, or field amplitude term variation due to beam movement.     

焦斑二阶矩半径与光束近场分布的关系

为分析光束振幅畸变和相位畸变对其焦斑二阶矩半径的影响,以光波标量衍射理论中的夫琅和费衍射公式为基础,根据光斑二阶矩半径的定义,推导了直接依赖于光束近场复振幅分布的焦斑二阶矩半径的一般表达式,并进一步推导了直接依赖于光束近场强度分布和相位分布的焦斑二阶矩半径和焦斑二阶矩角半径的表达式。结果表明：畸变光束的焦斑二阶矩角半径由光束近场波前相位梯度的强度加权均方根值和对数强度梯度的强度加权均方根值决定。利用焦斑二阶矩半径表达式,可直接分析并计算光束近场分布由于振幅调制和相位畸变导致的焦斑二阶矩半径的变化及焦斑发散角的变化。利用焦斑发散角表达式,直接推出了理想高斯光束和像散高斯光束的远场发散角,得到了与文献一致的结果,验证了表达式的正确性。     

Direct-comb molecular spectroscopy with accurate, resolved comb teeth over 43 THz

We demonstrate a dual-comb spectrometer using stabilized frequency combs spanning 177 to 220 THz (1360 to 1690 nm) in the near infrared. Comb-tooth-resolved measurements of amplitude and phase generate over 4×10(5) individually resolved spectral elements at 100 MHz point spacing and kilohertz-level resolution and accuracy. The signal-to-noise ratio is 100 to 3000 per comb tooth. Doppler-broadened phase and amplitude spectra of CO(2), CH(4), C(2)H(2), and H(2)O in a 30 m multipass cell agree with established spectral parameters, achieving high-resolution measurements with optical bandwidth generally associated with blackbody sources.     

飞秒激光在锥形镀膜探针传输中相位奇异的研究

利用数值模拟的方法对飞秒激光脉冲中光波振荡一个周期内不同时刻的光场进行了计算、比较, 获得了输出端平面内各空间点处光场的最大值及达到最大值时对应的时间, 得到了探针近场的振幅和相位分布. 在研究振幅分布的过程中发现, 在探针输出端的有些区域振幅始终为零, 即在光波振荡的一个周期内这些区域始终为相位奇异点; 通过分析相位分布得到了相位奇异随时间的演化及相位奇异的特点.     

Femtosecond time-resolved interferometric microscopy

We describe optical interferometry with 100 fs time resolution and a spatial resolution of approximately one micrometer. Using a pump-probe scheme and a 2D-Fourier transformation algorithm, we are able to retrieve from the interferograms very small changes in the phase and the amplitude of the reflected probe pulses. The performance of the technique is illustrated by measurements of transient and permanent surface modifications of crystalline GaAs after exposure to intense femtosecond laser pulses with fluences near the ablation threshold. PACS 78.47.+p; 42.65.Re; 42.15.Eq; 68.35.Ja     

Resolution enhancement in noise spectrum by using velocity selective optical pumping in cesium vapor

We demonstrate experimentally that the resolution of amplitude noise spectrum in Cs atomic vapor can be enhanced by narrowing the absorption using velocity selective optical pumping technique. It is found that the steep atomic dispersion accompanied by high absorption leads to more conversion of laser phase noise to amplitude noise, when the field propagates throughout the atoms, and meanwhile the spectral resolution is improved. The effect of optical pumping intensity on the spectrum resolution is experimentally discussed, and a theoretical explanation for this phenomenon is given, which shows that the phase-to-amplitude noise conversion is directly proportional to the dispersion of medium.     

头相关传输函数的低频特性

提出了一种对测量得到头相关传输函数(HRTF)的低频特性进行修正的方法。通过头部的刚球模型对HRTF的低频特性进行分析,证明了无论对远场或近场HRTF,大约在400 Hz以下,其幅度趋于恒定,而相位接近线性。因而只要通过实验测量得到400 Hz以上的HRTF特性,即可采用理论的结果对其进行修正。理论计算和主观实验证明,所提出的方法是切实可行的。     

Apodization‐Assisted Subdiffraction Near‐Field Localization in 2D Phase Diffraction Grating

Conventional phase diffraction gratings can be used to localize the incoming optical radiation in the near‐field region. A new design of the binary phase diffraction grating is proposed with embedded pupil opaque mask inside each stripe. By means of numerical simulations, it is shown that with this masked phase grating the spatial resolution of the near‐field localization can be substantially improved and brought even beyond the solid immersion limit (λ/2n). Moreover, due to anomalous apodization effect, the subdiffraction field localization is accompanied by intensity enhancement as compared to the non‐masked design. The pupil mask rearranges the optical fluxes within the stripes and promotes the Fano resonances excitation in the periodic step lattice. This can be important for advancing the phase grating‐based super‐resolution technologies, including subdiffraction imaging, interferometry, and surface fabrication.     

Direct mid-infrared femtosecond pulse shaping with a calomel acousto-optic programmable dispersive filter

Direct amplitude and phase shaping of mid-infrared femtosecond pulses is realized with a calomel-based acousto-optic programmable dispersive filter transparent between 0.4 and 20 μm. The shaped pulse electric field is fully characterized with high accuracy, using chirped-pulse upconversion and time-encoded arrangement spectral phase interferometry for direct electric field reconstruction techniques. Complex mid-infrared pulse shapes at a center wavelength of 4.9 μm are generated with a spectral resolution of 14 cm(-1), which exceeds by a factor of 5 the reported experimental resolutions of calomel-based filters.