Guo Ji-Hua

Shear Force Detection Using Single-Tine Oscillating Tuning Fork for Scanning Near-Field Optical Microscopy

We propose a new method to detect near-field by using a single-tine oscillating tuning fork with mechanically asymmetric excitation that exhibits the sensitivity and stability better than that by using a double-tine oscillating one. Comparison of shear forces for the two methods demonstrate that the single-tine oscillating tuning fork provides a simpler and more sensitive method for near-field measurements. A theoretical analysis is presented for explanation to the greater sensitivity. The method is demonstrated by imaging a sparse-packed layer of micro-spheres in size of 200 nm.

Zeeman-Birefringence He-Ne Dual Frequency Lasers

We demonstrate an approach to obtain the frequency differences from 1 MHz to hundreds of MHz including 3-40 MHz, which was a blank range of the frequency difference in the traditional dual frequency lasers. We employ an intra-cavity stress birefringence element in an He-Ne laser, and at the same time apply a transverse magnetic field to the laser. The intra-cavity stress birefringence element, which is the window plate or the mirror substrate applied by a force, is used to split a frequency into two; i.e. to make a single-frequency laser outputting two frequencies. Moreover, the transverse magnetic field is used to decrease greatly the mode competition between the two frequencies so that they are able to oscillate simultaneously. The minimum value of magnetic field for efficiently eliminating the mode competition to ensure the two frequencies work together is studied experimentally. The power tuning performance of the two frequencies (o-light and e-light) is investigated.

Preparation and Structural Characterization of Superionic Conductor RbAg4I5 Crystalline Grain Film

Superionic conductor RbAg4I5 crystalline grain films were prepared by vacuum thermal evaporation on NaCl crystalline substrates. The surface morphology, microstructure and the electronic energy states of the films were examined by atomic force microscopy, transmission-electron microscopy, x-ray diffraction and x-ray photoelectron spectroscopy. The results show that the obtained RbAg4I5 layer has an epitaxial film of perfect crystalline structure, and the unit cell of crystalline grain RbAg4I5 films belongs to cubic crystal system. The principal x-ray diffraction peaks at d = 3.7447 and 1.8733 A are related to the structure of ternary compound RbAg4I5 films.

Enhancement of optical coherence tomography axial resolution by spectral shaping

We propose a new method of changing the spectrum shape to improve the axial resolution of optical coherence tomography (OCT). Theoretical analysis shows that certain spectral shaping can shorten the coherence length. Comparisons of the simulation and experimental measurements of spectral shape and axial resolution of OCT are given, showing that the axial resolution of OCT is enhanced by a factor of 1.4.

First Overtone Frequency Stimulated Quartz Tuning Fork Used for Shear-Force Scanning Near-Field Optical Microscopy

The conventional 32.768 kHz tuning fork is stimulated at its first overtone resonant frequency of ~190 kHz for shear-force distance control. The time constant is measured to be 0.54 ms and it decreases about 40 times faster than that of the fundamental frequency (20.76 ms). The cross section of a corn root with a height difference of ~3 μm is imaged at a scan speed of 12 μm/s for 256×256 pixels.

Fabrication and application of near-field optical fibre probe

In this paper, the fabrication of a large cone angle near-field optical fibre probe, using the two-step chemical etching method and bent probe, is introduced and the controlling parameters of the coated Cr-Al film at the probe tip are presented. The scanning electron microscopy images display that the tip diameter of the uncoated large cone angle fibre probe obtained is less than 50nm, the cone angle over 90° and the diameter of light aperture at the coated probe tip is less than 100nm. The measured results of the optical transmission efficiency for various probe tips show that the uncoated straight optical fibre probe, film-coated straight probe and film-coated bent probe are 3×10-1, 2×10-3 and 1×10-4 times that of the flat fibre probe, respectively. In addition, the force images and near-field optical images of a standard sample are acquired using a large cone angle and film-coated bent probe.

Properties of the incandescent light emitted from double-walled carbon nanotube filament

We have studied the radiation of a double-walled carbon nanotube (DWNT) filament with a length of 4.5 mm and a diameter of 10 μm by applying an electric current through the filament. The DWNT filament starts emitting incandescent light at voltage U = 6 V. Emission spectra of the DWNT below temperature 1250 K can well be fitted to those of the blackbody radiation. The intensity of the incandescent light shows an exponential dependence on the voltage applied on the DWNT filaments. The resistance of the DWNT filaments is very stable at high temperatures between 900 and 1250 K during the emission of light in the experiments.

Absorption Spectra and Ionic Conductivity of RbxCs1−xAg4I5 Superionic Conductors Thin Films

A series of RbxCs1−xAg4I5 (x = 0–1) thin films were grown by vacuum evaporation on NaCl crystal substrates at 350 K. The absorption spectra of these films were measured at 80 K in the wavelength range from 240 nm to 400 nm. It is shown that superionic conductor thin films of quaternary compound Rb0.5Cs0.5Ag4I5 and ternary compound RbAg4I5 can be obtained at x = 0.5–0.6 and x = 0.7–1, respectively. At x = 0.65, the combined compound film of the mixture of 30 mol% RbAg4I5 and 70 mol% Rb0.5Cs0.5Ag4I5 is presented. Then, based on the spectral positions of the A1 and A2 peaks, we determined that the Rb0.5Cs0.5Ag4I5 exciton coupling energy Rex is 0.21 eV, the forbidden zone width Eg is 3.82 eV and the exciton radius aex is 0.70 nm. Furthermore, the ionic conductivities of superionic conductor thin films of RbAg4I5 and Rb0.5Cs0.5Ag4I5 and their mixture film are investigated, respectively, in the temperature range 303 K–393 K.

Spectral Shaping in Rapid Scanning Optical Delay Line of Optical Coherence Tomography

A small spatial optical filter is put into the rapid-scanning optical delay line (RSOD) to shape the spectrum of the reference beam in optical coherence tomography (OCT). The experimental results show that the longitudinal resolution can be improved by a factor of 81% with this method, while at the same time, the signal-to-noise ratio of the OCT system is not much affected. This method can be used in OCT systems that use RSOD as the reference arm with a light source of superluminescent diodes, femtosecond lasers and crystal fibre as well.

Observation of the in-vivo Reporter of Green Fluorescent Protein in a Plant Root by Scanning Near-Field Optical Microscopy

An in vivo reporter of green fluorescent protein (GFP) in a living plant root has been imaged by scanning near-field optical microscopy (SNOM) in the transmission mode. The exciting light is 488 nm wavelength of the argon ion laser and the bandpass filter (514+/-10) nm is put into the detecting optical pathway. The results indicate that, in the living plant cells, the GFPs gather together and form an area of 2-4 mum, rather than being individually distributed. The transmission coefficient of the eigenfunction is incorporated into Bethes theoretical model modified by Grober, and the near-field excited light intensity is calculated along the fibre probe axis (z-axis) in the air medium and biological medium. Based on this, along the z-axis direction of the GFP detected in the sample, numerous GFPs locate near the epidermal cells wall (in the range of 0-38 nm) in the living root. The experiments show that SNOM has an advantage of optical nanometre-scale resolution along the z-axis.