Supercurrent distribution imaging system for HTS thin films utilizing femtosecond fiber laser M. Misra a, * , A. Moto a , H. Murakami a , M. Hangyo a , M. Tonouchi a,b a Research Center for Superconductor Photonics, Osaka University, 2-1 Yamadaoka, Suita 565-0871, Japan b CREST, JST, Suita 565-0871, Japan Received 27 September 2001; accepted 22 January 2002 Abstract We report the imaging of supercurrent distribution in high-T c superconductor YBa 2 Cu 3 O 7Àd (YBCO) thin film based on the principle of the measurement of THz radiation emission from the current biased YBCO thin film when excited by the femtosecond fiber laser pulses. It is observed that the image obtained by Ti:sapphire laser has better resolution as it shows the clear and sharp supercurrent distribution at the edges of YBCO thin film compared to the image obtained using femtosecond fiber laser source. We attribute this difference to the larger beam spot size in the case of fiber laser source compared to that obtained in the case of Ti:sapphire laser source. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 74.25.Gz; 85.25.Pb Keywords: Supercurrent distribution; THz radiation; HTS thin film 1. Introduction Imaging supercurrent distribution in high-T c superconductors (HTS) provides very important information for their application to electronic de- vices as well as understanding the basic physics of HTS. For the visualization of supercurrent in HTS thin film, our group has developed a technique based on the generation and detection of THz radiation [1,2]. The imaging system utilizes the principle that the femtosecond optical pulses can excite the THz radiation into free space by opti- cally modulating supercurrent in HTS thin film and the THz radiation amplitude is directly pro- portional to the local supercurrent density at op- tically excited region. The two-dimensional THz radiation images obtained by scanning the laser beam on the sample can be directly translated in to the supercurrent distribution. The technique has been proved very useful and used to study the vortex penetration, optical generation of fluxons and supercurrent distribution in HTS thin films under various conditions [2–5]. The laser source used in our earlier work was a mode locked Ti:sapphire laser operating at a repetition rate of 82 MHz producing 50 fs pulses with center wave- length of about 800 nm. Although, the Ti:sapphire laser source is one of the best available laser sources, it is very bulky and costly. With the Physica C 378–381 (2002) 1287–1290 www.elsevier.com/locate/physc * Corresponding author. Tel.: +81-6-6879-7983; fax: +81-6- 6879-7984. E-mail address: mukul@rcsuper.osaka-u.ac.jp (M. Misra). 0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII:S0921-4534(02)01698-2