Scanning Probe Laser Terahertz Emission Microscopy System

Ryotaro Inoue, Naotsugu Uchida, Masayoshi Tonouchi
2006 Japanese Journal of Applied Physics  
The development of a scanning probe laser terahertz emission microscopy system is reported. Femtosecond optical pulse beam scans the sample surface, and terahertz emission from the locally photoexcited area is obtained. Using an optical-fiber probe, we can focus the pulse beam with a beam diameter of 5 mm. The spatial resolution of the system is investigated and the performance of the system is demonstrated by the image of an operational amplifier that is active in an electrical circuit. Recent
more » ... progress in terahertz (THz) technology enables us to image various objects using THz waves. 1-3) Among several imaging techniques using THz waves, laser terahertz emission microscopy (LTEM) system, where we detect THz emission into free space from a small area of the sample surface locally photoexcited by a femtosecond pulse beam, has a unique characteristic as an active microscopy system. We have proposed LTEM for the nondestructive inspection of electrical failures in ICs. 4) Being different from conventional laser-testing techniques such as the optical beam induced current (OBIC) method, a full noncontact inspection is realized in a laser superconducting quantum interference devices (laser SQUID) microscope 5,6) and LTEM system. The spatial resolution of the LTEM system is determined by the diameter of the excited beam and achieved below 3 mm by tight focusing. 7) In this paper, we report the development of the scanning probe LTEM (SP-LTEM) system. Compared with the conventional LTEM system, we can pinpoint the exact location of the photoexcited area by monitoring the sample surface because the excitation beam is irradiated from the vicinity of the surface by an optical-fiber probe. The large numerical aperture of the probe lens inherently provides the tight focusing condition of the excitation beam. Furthermore, the optical-fiber probe improves the operability of the system dramatically, and shows flexibility in terms of integration with scanning near-field optical microscopy (SNOM) system as a future possibility. Spatial resolution is investigated and the performance of the system is demonstrated by observing an operational amplifier that is active in an electrical circuit. Figure 1(a) shows the optical diagram of the SP-LTEM system. A Ti:sapphire laser is used for generating optical pulses (center wavelength, 780 mm; pulse width, 100 fs; repetition rate, 80 MHz). The beam is divided into the pump pulse and trigger pulse, and transmitted by 4-m single-mode optical fibers. The positive group-delay dispersion in the optical fibers is compensated by two compensators that consist of gratings and prisms for retroreflection. The beam of the pump pulse is irradiated to the sample surface by a commercial optical-fiber probe (LWP-LEN-SM, Cascade Microtech, Inc.). We bring the optical-fiber probe to the sample surface at a distance of $5 mm with monitoring of the sample surface by an optical microscope. The average power and beam diameter of the pump pulse are 7 mW and $5 mm, respectively. The THz waves emitted from the sample surface are collimated by an off-axis parabolic reflector (diameter, 25 mm; effective focal length, 50 mm) and focused on the detector by a silicon hyper-hemispherical lens (radius, 6.75 mm; center offset, 3.5 mm). For the detector, we use a bowtie photoconductive (PC) switch antenna fabricated on a low-temperature-grown gallium arsenide (LT-GaAs) substrate. The beam of the trigger pulse is optically delayed and irradiated to the gap of the PC switch with an average power of 5 mW. The amplitudes of the THz wave are measured as a function of time utilizing a lock-in amplifier with an optical/ bias modulation of 2 kHz. The optical-fiber probe and detector are mounted on an automatic stage for the systemscanning imaging with the measured sample fixed, the image of which is shown in Fig. 1(b) . Before estimating system performance, we first investigated the effect of the group-delay dispersion in the optical fibers on the THz emission. In Fig. 2(a) , we show the pulse width as a function of the amount of compensation for various transmitted powers through the optical fibers. Here, we obtained the bare pulse width Át from the correlated pulse-width Át 0 , which is measured by means of an autocorrelator, as Át ¼ Át 0 = ffiffi ffi 2 p assuming that the envelope waveform of the optical pulse is Gaussian. For transmitted power less than 10 mW, we can suppress the pulse width down to $150 fs after the transmission by the 4-m optical fibers. Due to the higher-order dispersion in the optical fibers Delay Sample Detector Compensator 4 m fiber Ti Sapphire LASER 780 nm, 80 fs (a) (b) Fig. 1. (a) Optical diagram and (b) image of SP-LTEM system. Ã
doi:10.1143/jjap.45.l824 fatcat:vvfrfsntvrfi5dm5fiym47kjyi