Terahertz radiation imaging of supercurrent distribution in vortex-penetrated YBa2Cu3O7−δ thin film strips

Abstract

We have developed a supercurrent distribution imaging system for high T c superconductive thin films and demonstrated the visualization of the supercurrent distribution in the vortex-penetrated YBa 2 Cu 3 O 7Ϫ␦ thin film strips. The terahertz ͑THz͒ radiation and detection system with a scanning femtosecond laser was employed to visualize the distribution. The imaging system utilizes the principle that the femtosecond optical pulses excite THz radiation into the free space by optical supercurrent modulation, and the radiation amplitude is proportional to the local supercurrent density at the optically excited area. Prior to the observation of the supercurrent distribution, we studied optical excitation effects on the vortices trapped in the strips, calibration of the current density from the THz radiation amplitude, temperature dependence of the THz radiation properties, etc. The laser power dependence of the THz radiation in the remanent state revealed that the excitation with powers larger than the relatively weak finite value ͑about 10 mW in the present case͒ strongly affects the vortices trapped in the films. We attributed this behavior to the optically excited depinning effect. We derived a calibration function from the THz radiation images into the supercurrent density distributions by observing the bias-current dependence of the THz radiation, and applied it for the diagnosis of the distributions in the vortex-penetrated strips. The THz radiation images were successfully transferred into the supercurrent density distributions with quantitative agreement. The minimum magnetic flux resolution at the optically excited area was roughly estimated to be 3 0 where 0 is a single flux quantum. The measurement of the laser beam profile indicated that the spatial resolution of the THz radiation images is limited by the laser beam diameter: 25 m in our case. The observed distributions revealed that the vortices easily penetrate into the strip under an external magnetic field B EX of 0.9 mT, and the persistent supercurrent exists only near the strip edges in the remanent state after removal of the field. The calculations of the convolution between the observed laser pattern and the trial functions suggested that the supercurrent distribution width in the remanent state after removal of the field of 0.9 mT is estimated to be less than 1 m. The temperature dependence of the supercurrent distributions revealed that, below 60 K, the thermal activation produces no significant effects on the penetrated vortices at B EX ϭ0.9 mT, whereas, the vortices in the remanent state after removal of the field of 15 mT were strongly affected by the thermal activation. The decreasing rate of the supercurrent density at the edge with increasing temperature was larger than that inside the strip. This suggested that the vortices trapped near the edges exhibit rather different behavior from the ones that penetrated into the inner part of the strip.

Optical magnetic flux generation in superconductor

“…After the removal of the bias current, the supercurrent distribution is visualized by the THz radiation imaging technique at a typical laser power of 2 mW, which is low enough to avoid the optical influence on the generated flux, with a pixel size of 5 mm×5 mm. The details of the THz beam generation and detection system, and the imaging system can be found elsewhere [10,11]. We observe only the x-component of the supercurrent distribution in the present work.…”

Laser terahertz emission microscopy

“…A laser power of 5 mW for the THz radiation imaging is used, so that no significant distortion is induced due to laser exposure. The detail for the experimental condition and data processing procedure has been reported elsewhere [32]. Fig.…”

“…16(b), using a calibration technique as reported in Ref. [32]. Furthermore, one can even quantitatively evaluate the magnetic field induced by either the transport current or trapped vortices (see Ref.…”

“…Thus THz emission spectroscopy, as well as THz time domain spectroscopy (THz-TDS), have become important for exploring material science [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]. When one excites the THz emission from a certain substance and constructs the emission image by scanning the laser beam on it, the system works as a laser-THz emission microscope (LTEM) [23][24][25][26][27][28][29][30][31][32][33]. Since the resolution of the image is limited by the laser beam diameter rather than THz wavelength, sub-micron resolution images would be easily obtainable.…”

Terahertz multistatic reflection imaging

“…2,3 The unique capabilities of the THz time-domain spectrometer have also inspired numerous explorations of new imaging techniques for THz radiation, including time-of-flight reflection imaging and ranging, [4][5][6] emission imaging, 7 nearfield techniques for improved spatial resolution, 8,9 interferometric imaging for improved depth resolution, 10 and near-real-time display-mode imaging. 11 More recently, the availability of fiber-coupled THz transmitters and receivers has created new opportunities for data acquisition and image formation.…”