The precision analysis of the Chinese VLBI Network in probe delay measurement

We propose a VLBI precision evaluation method for probe delay measurement, so as to investigate the error contributions from different components in the Chinese VLBI Network (CVN). This method takes the idea of traditional closure delay analysis for distant radio sources. It focuses on the VLBI closure delay only and therefore excludes the influence of probe orbit determination, which makes it very suitable to evaluate the capability of VLBI probe delay measurement. In this paper, we first introduce the principles of closure delay analysis. Then the statistical results of typical CE5 (Chinese Chang'e 5 lunar exploration mission) and HX1 (Chinese Mars exploration mission) observations are presented, including the comparison of the closure delay precisions between CE5 and HX1 for four closed baseline triangles in CVN. According to the result, we realize that, the precision discrepancy between CE5 and HX1 in the closure delay analysis is less than that of residual delay after orbit determination, which reflects the precision level of the VLBI delay measurement.

In-situ diagnostic of femtosecond laser probe pulses for high resolution ultrafast imaging

Ultrafast imaging is essential in physics and chemistry to investigate the femtosecond dynamics of nonuniform samples or of phenomena with strong spatial variations. It relies on observing the phenomena induced by an ultrashort laser pump pulse using an ultrashort probe pulse at a later time. Recent years have seen the emergence of very successful ultrafast imaging techniques of single non-reproducible events with extremely high frame rate, based on wavelength or spatial frequency encoding. However, further progress in ultrafast imaging towards high spatial resolution is hampered by the lack of characterization of weak probe beams. For pump–probe experiments realized within solids or liquids, because of the difference in group velocities between pump and probe, the determination of the absolute pump–probe delay depends on the sample position. In addition, pulse-front tilt is a widespread issue, unacceptable for ultrafast imaging, but which is conventionally very difficult to evaluate for the low-intensity probe pulses. Here we show that a pump-induced micro-grating generated from the electronic Kerr effect provides a detailed in-situ characterization of a weak probe pulse. It allows solving the two issues of absolute pump–probe delay determination and pulse-front tilt detection. Our approach is valid whatever the transparent medium with non-negligible Kerr index, whatever the probe pulse polarization and wavelength. Because it is nondestructive and fast to perform, this in-situ probe diagnostic can be repeated to calibrate experimental conditions, particularly in the case where complex wavelength, spatial frequency or polarization encoding is used. We anticipate that this technique will enable previously inaccessible spatiotemporal imaging in a number of fields of ultrafast science at the micro- and nanoscale.

Negative photoconductance in 150 GHz transients from irradiated p-type Cz-Silicon

Boron doped (p-type) silicon wafers of the same type are irradiated with gamma, proton and chlorine ion beams. This causes radiation damage in the form of migration of vacancies, traps to photoelectrons. We use time-resolved millimeter wave pump-probe spectroscopy (150 GHz CW probe signal) and 532 nm ultrafast laser as pump source with variable fluence. Upon studying the transient response of the detector probe voltage as function of the pump-probe delay period we note a good positive (absorption) photoconductance peak and soon after recombination of photocarriers there occurs a negative photoconductance (NPC) transient. We consistently find that the NPC lasts for about 36 microseconds and this study points out that the positive to NPC peaks for each laser fluence varies with the type of radiation damaged samples.2 MeV proton beam damage create damage that trap carriers very effectively, and enhances the resistivity of the silicon wafer from 15 Ohms to 150 Ohms. The Chlorine ion damaged silicon responds consistently to the 150 GHz probe beam and correlates strongly with the laser fluence.

Negative photoconductance in 150 GHz transients from irradiated p-type Cz-Silicon

Boron doped (p-type) silicon wafers of the same type are irradiated with gamma, proton and chlorine ion beams. This causes radiation damage in the form of migration of vacancies, traps to photoelectrons. We use time-resolved millimeter wave pump-probe spectroscopy (150 GHz CW probe signal) and 532 nm ultrafast laser as pump source with variable fluence. Upon studying the transient response of the detector probe voltage as function of the pump-probe delay period we note a good positive (absorption) photoconductance peak and soon after recombination of photocarriers there occurs a negative photoconductance (NPC) transient. We consistently find that the NPC lasts for about 36 microseconds and this study points out that the positive to NPC peaks for each laser fluence varies with the type of radiation damaged samples.2 MeV proton beam damage create damage that trap carriers very effectively, and enhances the resistivity of the silicon wafer from 15 Ohms to 150 Ohms. The Chlorine ion damaged silicon responds consistently to the 150 GHz probe beam and correlates strongly with the laser fluence.

Pump-Probe Delay Controlled by Laser-dressed Ionization with Isolated Attosecond Pulses

In a recent work [1], we demonstrated how laser-dressed ionization can be harnessed to control with attosecond accuracy the time delay between an extreme-ultraviolet (XUV) attosecond pulse train and an infrared (IR) femtosecond pulse. In this case, the comb-like photoelectron spectrum obtained by ionizing a gas target with the two superimposed beams exhibits peaks oscillating with the delay. Two of them can be found to oscillate in phase quadrature, allowing an optimal measurement and stabilization of the delay over a large range. Here we expand this technique to isolated attosecond pulses, by taking advantage of the delay-modulation of attosecond streaking traces. Although the photoelectron spectrum contains no peaks in that case, it is possible to reconstruct the pump-probe delay by simply monitoring the mean energy of the spectrum and the amplitude at this energy. In general, we find that active delay stabilization based on laser-dressed ionization is possible as long as the XUV pulses are chirped.

Experimental observation of the liquid-liquid transition in bulk supercooled water under pressure

We prepared bulk samples of supercooled liquid water under pressure by isochoric heating of high-density amorphous ice to temperatures of 205 ± 10 kelvin, using an infrared femtosecond laser. Because the sample density is preserved during the ultrafast heating, we could estimate an initial internal pressure of 2.5 to 3.5 kilobar in the high-density liquid phase. After heating, the sample expanded rapidly, and we captured the resulting decompression process with femtosecond x-ray laser pulses at different pump-probe delay times. A discontinuous structural change occurred in which low-density liquid domains appeared and grew on time scales between 20 nanoseconds to 3 microseconds, whereas crystallization occurs on time scales of 3 to 50 microseconds. The dynamics of the two processes being separated by more than one order of magnitude provides support for a liquid-liquid transition in bulk supercooled water.

Recalling a single object: going beyond the capacity debate

Working memory is now established as a limited capacity system. The debate regarding working memory has been largely between slots and resource based models. The resource model suggests that as the number of items increases, precision of recall decreases because neural resources are dynamically allocate to all the objects needed for task. Slot model on the other hand implies that an item is stored either with the highest precision or not at all. If both these models stand true then quality of memory performance would be near perfect for a single object. However, that may not be the case. In the current work, we investigated recall accuracy for feature(s) of a single object in three successive experiments. In all three experiments, the memory array consisted of a single colored oriented short line presented a short distance away from the center of the display for 1 sec. We probed recall of features from the set of color, location, orientation, and size. In experiment 1 number of recall question varied between 1 – 4 with the order randomized in each trial. In experiment 2 we chose to probe only two feature recall questions, whereas only one recall question was probed for the third experiment. In experiment 3 we varied the delay before the recall probe between 1 and 2 s. The recall response for each feature was mapped on to a continuous variable. Subjects used a color wheel to respond to color, on-screen mouse click to indicate center of the line location, click away from the center to indicate size, and a mouse click to the periphery of centered circle to indicate orientation with the slant of the resulting radial line. We calculated z-scores of errors for each feature for every subject separately. In experiment 1 that for color, location, and size the errors increase significantly with the position of the questions asked. In experiment 2, the errors increased significantly between questions for color and location (but not for orientation and size). In experiment 3, we did not see any significant increase in error with recall probe delay. Overall run-time for each trial was within 10 secs, well within the limits of operation of working memory. This drop in performance poses questions for memory mechanisms proposed by slot and resource models as both would predict near-perfect recall within the time-period for the trials.