Dielectric resonator-based resonant structure for sensitive ESR measurements at high-hydrostatic pressures

We present a newly developed microwave probe head that accommodates a gasketed sapphire anvil cell (SAC) for performing sensitive electron spin resonance (ESR) measurements under high-hydrostatic pressures. The system was designed around commercially available dielectric resonators (DRs) having the dielectric permittivity of approximately 30. The microwave resonant structure operates in a wide-stretched double-stacked geometry and resonates in the lowest cylindrical quasi TE(011) mode around 9.2 GHz. The most vital parts of the probe's microwave heart were made of plastic materials, thus making the resonant structure transparent to magnetic field modulation at 100 kHz. The overall ESR sensitivity of the probe was demonstrated for a small speck of 2,2-diphenyl-1-picrylhydrazyl radical (DPPH) positioned in the gasket of the SAC, using water as the pressure-transmitting medium. The system was also used for studying pressure-induced changes in spin-relaxation mechanisms of a quasi-1D-conducting polymer, K(1)C(60). For small samples located in the sample hole of the gasket the probe reveals sensitivity that is only approximately 3 times less than that yielded by regular ESR cavities.     

Optical fiber photoacoustic-photothermal probe

We describe the operation of an all-optical probe that provides an alternative means of implementing photoacoustic and photothermal investigative techniques, particularly those used in biomedical applications. The probe is based on a transparent, acoustically and thermally sensitive Fabry-Perot polymer film sensor mounted at the end of an optical fiber. We demonstrate the ability of the system to make photoacoustic and photothermal measurements simultaneously and evaluate its photothermal response, using a nonscattering liquid target of known and adjustable absorption coefficient. The acoustic and thermal noise floors were 2 kPa and 6x10(-3) degrees C , respectively, obtained over a 25-MHz measurement bandwidth and 30 signal averages.     

光纤探头烧结电路设计

为了满足烧结光纤探头的需要,设计了一种基于脉宽调制芯片CA1524的高压放电装置.首先通过推挽功率放大电路,将直流电逆变为高压交流电;然后经倍压整流得到2000多伏的直流电,最后连接电极棒进行高压放电.使用芯片LPC1768控制CA1524输出波形的占空比来改变放电强度,以及放电时间.     

EPR Uniform Field Signal Enhancement by Dielectric Tubes in Cavities

The dielectric tube resonator (DTR) for electron paramagnetic resonance spectroscopy is introduced. It is defined as a metallic cylindrical TE₀₁₁ microwave cavity that contains a dielectric tube centered on the axis of the cylinder. Contour plots of dimensions of the metallic cylinder to achieve resonance at 9.5 GHz are shown for quartz, sapphire, and rutile tubes as a function of wall thickness and average radius. These contour plots were developed using analytical equations and confirmed by finite-element modeling. They can be used in two ways: design of the metallic cylinder for use at 9.5 GHz that incorporates a readily available tube such as a sapphire tube intended for NMR and design of a custom procured tube for optimized performance for specific sample-size constraints. The charts extend to the limiting condition where the dielectric fills the tube. However, the structure at this limit is not a dielectric resonator due to the metal wall and does not radiate. In addition, the uniform field (UF) DTR is introduced. Development of the UF resonator starting with a DTR is shown. The diameter of the tube remains constant along the cavity axis, and the diameter of the cylindrical metallic enclosure increases at the ends of the cavity to satisfy the uniform field condition. This structure has advantages over the previously developed UF TE₀₁₁ resonators: higher resonator efficiency parameter Λ, convenient overall size when using sapphire tubes, and higher quality data for small samples. The DTR and UF DTR structures fill the gap between free space and dielectric resonator limits in a continuous manner.     

Microball lens integrated fiber probe for optical frequency domain imaging

An integrated microball lens fiber catheter probe is demonstrated,which has better lateral resolution and longer working distance than a corresponding bare fiber probe with diverging beam for Fourier domain optical coherence tomography(FDOCT) . Simulation results are shown to gain the effect of the distance between the microball lens and the bare fiber to the focusing plane and beam width. The freedom of modifying the working distance and lateral resolution is shown. This is achieved by changing the gap dis...     

Electroless nickel plating on optical fiber probe

As a component of near-field scanning optical microscope （NSOM）, optical fiber probe is an important factor influncing the equipment resolution. Electroless nickel plating is introduced to metallize the optical fiber probe. The optical fibers are etched by 40% HF with Turner etching method. Through pretreatment, the optical fiber probe is coated with Ni-P film by electroless plating in a constant temperature water tank. Atomic absorption spectrometry （AAS）, scanning electron microscopy （SEM）, and energy dispersive X-ray spectrometry （EDXS） are carried out to characterize the deposition on fiber probe. We have reproducibly fabricated two kinds of fiber probes with a Ni-P film： aperture probe and apertureless probe. In addition, reductive particle transportation on the surface of fiber probe is proposed to explain the cause of these probes.     

Detection properties of an optical fiber probe for laser speckle velocimetry

Temporal power spectra of the speckle signal detected by a simple optical fiber probe are analyzed as a function of the laser spot size at a moving scatter plate, the velocity of the plate, the core diameter of the detecting fiber and the numerical aperture of it. The fiber probe is shown to be useful for velocity measurements of the moving scatter plate.     

Some EPR spin probe and spin label studies of polymer systems

The application of EPR spin probe and spin label technique for solving the two problems of polymer physical chemistry is considered. The first problem is the determination of conformational state and chain sizes in amorphous solid polymers. This determination is based on the analysis of the intramolecular dipolar broadening of EPR spectra of spin labelled macromolecules in glassy solvents or in the bulk of unlabeled polymers at 77 K. The second problem is the determination of molecular mobility and structure of two polymer colloid systems: (a) the complex of colloidal silica and synthetic polycation macromolecule and (b) polymer-surfactant micellar organized systme.     

EPR study of porous silicon

An anisotropic EPR signal was observed in porous Si. According to its symmetry andg value, the EPR signal can be attributed to silicon dangling bonds located on the surface of a porous Si skeleton. The evolution of the EPR signal at room temperature in air was measured. The annealing temperature dependence of the EPR and the PL of porous Si in oxygen and the effects of gamma irradiation on the EPR and the PL spectra of porous Si were studied. The changes of the EPR signal and the PL intensity induced in atmosphere by ethyl alcohol and acetone were discovered. The dangling bond is only one of the factors which affect the PL.This project is supported by the National Natural Science Foundation of China.     

A new positioning approach of fiber probe for near-field scanning optical microscope

A fiber probe-based positioning scan approach was established, whose precision can reach as high as 0.8 μm. And a set of modified microscope system was designed utilizing this approach, in which the scanning probe microscope (SPM) combined with an optical microscope was manipulated. The optical microscope and scanning probe can conveniently be switched through a switch panel. The observation period of samples can significantly be shortened. And more reliable images can be provided using this approach. Our design can effectively solve the inherent disadvantages of SPM technology, which makes SPM scan and image more reliably, conveniently, safely and rapidly.     

光纤探针传输效率的研究

用实验方法测量了光纤探针的传输效率随光纤圆锥角的变化关系,给出了传输效率曲线。通过测定探针传输效率的实验可以看到,只要光纤探针的锥角在30°~55°范围内,就具有高透过率、高分辨率纳米微探针。测量了传输效率与光波波长的关系。     

新型AFM探针的制备及应用

采用熔拉 -腐蚀复合方法 ,将普通单模石英光纤制成直锥形光纤探针。利用自制工具将探针打弯 ,制成悬臂式光纤探针 ,在AFM上取得了较理想的测试结果。将自制光纤探针和商用硅材料探针获得的两种扫描图像进行了对比 ,分析了悬臂式光纤探针的特点     

Diffuse reflectance spectroscopy with a self-calibrating fiber optic probe

Calibration of the diffuse reflectance spectrum for instrument response and time-dependent fluctuation as well as interdevice variations is complicated, time consuming, and potentially inaccurate. We describe a novel fiber optic probe with a real-time self-calibration capability that can be used for tissue optical spectroscopy. The probe was tested in a number of liquid phantoms over a relevant range of tissue optical properties. Absorption and scattering coefficients are extracted with an average absolute error and standard deviation of 6.9%+/-7.2% and 3.5%+/-1.5%, respectively.     

EPR study of deoxygenated high-temperature superconductors

High-T _c superconductors are EPR silent but on a little deoxygenation of the high-T _c materials and their constituents, they yield rich but complex spectra. Spectra of (1) CuO, (2) BaCuO₂, (3) CaCuO₂, (4) Y₂Cu₂O₅, (5) La₂CuO₄, (6) La_2−x M _x CuO₄ (M=Sr, Ba), (7) Y based-123, (8) Bi based-2201, 2212, 2223, (9) Tl based-2223 and (10) Hg based-1212, 1223 have been studied. One thing common to all these materials is the CuO₂ plane which gets fragmented on deoxygenation and the inherent antiferromagnetic coupling is partially destroyed which results in the appearance of the spectra. The spectra recorded have been identified to be due to (1) Cu-monomer, (2) Cu-dimer, (3) Cu-tetramer, (4) Cu-octamer and (5) one signal at very low field which could not be identified because there was no structure in it and may be due to fragments higher than octamers. Very big fragments do not give any spectra because the original AF order probably remains intact in them. It is expected that when the fragments become magnetically isolated from the bulk, they produce EPR spectra. Most of the spectra have been analyzed and their spin-Hamiltonian parameters determined. The spectra of these species vary a little in terms of g-value and fine-structure splitting constant from sample to sample or even in the same sample and this may be attributed to some extra oxygen attachments retained with these species. Most frequently occurring species is the Cu-tetramer, (CuO)₄. As (CuO)₄ represents the unit cell of the all important two-dimensional CuO₂ plane of the high-T _c materials, its spectra have been argued to provide some clue to the mechanism of high-T _c superconductivity. The tetramer (CuO)₄ is a four one-half spin system and is essentially 16-fold degenerate by Heisenberg isotropic exchange, it is split into 6 components: one pentet, three triplets and two singlets. In superconductors the pentet appears to be the ground state and in the non-superconducting constituents the singlets seem to form the ground state as revealed by the temperature variation studies. In the case of La_1.854Sr_0.146CuO₄ we have found the signature of quantum stripe formation. The high-T _c superconductivity theories involving spin bag, antiferromagnetic spin fluctuations and magnons can be explained on the basis of Cu-tetramers.     

Dielectric behavior,conduction and EPR active centres in BiVO4 nanoparticles

Bismuth vanadate (BiVO₄) nanomaterials were synthesized by mechano-chemical ball milling method and complementary investigations were devoted to their structures, nanoparticle morphologies and electronic active centres. The dielectric and conductivity behaviour were analysed systematically in wide temperature and frequency ranges to correlate such physical responses with the peculiarities of the samples. Large interfacial polarisations favoured by high specific surfaces of nanoparticles account for a drastic enhancement of the dielectric function in the quasi-static regime. Exhaustive analyses of the dielectric experiments were achieved and account for the main features of dielectric functions and their related relaxation mechanisms. The electrical conductivity is thermally activated with energies in the range 0.1–0.6 eV depending on the sample features. DC conductivity up to 10^–3 S/cm was obtained in well crystallized nanoparticles. Vanadium ions reduction was revealed by EPR spectroscopy with higher concentrations of the active centres (V⁴⁺) in more agglomerated and amorphous nanopowders. The EPR spectral parameters of V⁴⁺ were determined and correlated with the local environments of reduced vanadium ions and the characteristics of their electronic configurations. An insight is also made on the role of active electronic centres (V⁴⁺) on the conduction mechanism in nanostructured BiVO₄.     

影响近场光纤探针传输效率的若干因素

近场光学技术在高分辨率成像、光谱探测和纳米加工等领域有广泛应用,而光纤探针是其中一个关键部件.如何提高光纤探针的传输效率是近场光学技术应用中的一个重要问题.本文采用三维时域有限差分方法,计算了锥形有孔探针的传输效率,分析了锥角、针尖孔径、波长和金属层厚度等因素对光纤探针传输效率的影响,并比较了裸光纤探针和有金属涂层的光...     

Broad spectral photonic crystal fiber surface enhanced Raman scattering probe

A broad spectral surface enhanced Raman scattering sensor is developed using the solid core holey photonic crystal fiber with silver nanoparticles cluster. This SERS probe offers an operational excitation wavelength range overlaying visible light and near infrared light. The PCF SERS sensing is demonstrated in the detection of the 4-Mercaptobenzoic acid (10⁻⁶ M) solution with 514.5 and 785 nm excitation. In this structure of PCF sensor, the related analysis shows that leakage modes also make an important contribution in the SERS activity not only by the evanescent field way.     

On fiber optic probe hydrophone measurements in a cavitating liquid

The measurement of high-pressure signals is often hampered by cavitation activity. The usage of a fiber optic probe hydrophone possesses advantages over other hydrophones, yet when measuring in a cavitating liquid large variations in the signal amplitude are found; in particular when the pressure signal recovers back to positive values. With shadowgraphy the wave propagation and cavity dynamics are imaged and the important contributions of secondary shock waves emitted from collapsing cavitation bubbles are revealed. Interestingly, just adding a small amount of acidic acid reduces the cavitation activity to a large extent. With this treatment an altered primary pressure profile which does not force the cavitation bubbles close to fiber tip into collapse has been found. Thereby, the shot-to-shot variations are greatly reduced.