Tao Yuan, Jingzhou Xu, and Xicheng Zhang

1
Scanning THz Emission Microscope
Tao Yuan, Jingzhou Xu, and Xicheng
Zhang Rensselaer Polytechnic Institute, Troy, New
York
Abstract A THz image system with very high
spatial resolution will provide an important tool
for research of material local property. Here we
report a scanning THz emission microscope. Using
the near field technology, the microscope has a
lateral resolution of 1 nanometer, which
corresponds to a resolution/wave-length ratio of
order of 10-6.
Scanning Experiment Result
Discussion and conclusion
Experiment Set Up
We also put the tip on a 3-D piezo driving stage
so that we can do a 3-D scan to demonstrate the
resolution of the system.
We have built a scanning THz emission Microscope
and demonstrated its nanoscale resolution and 2D
image scanning ability.

• Future work
• Improve scanning speed.
• Improve the position control of the tip.
• Acknowledgment
• This work was supported in part by the Center
  for Subsurface Sensing and Imaging Systems, under
  the Engineering Research Centers Program of the
  National Science Foundation. The project fits in
  level 1 Fundamental Science. R1.

Introduction and State of Art
The resolution of conventional image system is
limited by diffraction ?x ? ?/NA, which is the
so called Rayleigh Criterion. The limitation is
more serious for THz image system because of the
long wavelength used (300 ?m for 1THz).
Sub-wavelength resolution can be achieved by near
field technology shown in figure 1 (b) and (c),
which are sub-wavelength aperture and
aperture-less method. In aperture-less method, a
very sharp metal tip is put very close to the
sample surface so that it scatters the evanescent
wave from the surface to the far field for
detection. In this way, the resolution is
determined by the tip end size. The best
resolution achieved by optical near field
microscope is 1 nm 1.
Fig.2, A schematic graph of the experimental setup
However in these two setups, the signal intensity
or contrast is usually very small. To break
through the bottle neck, we proposed a scanning
THz emission microscope. Figure 2 shows schematic
of our setup. An ultrafast near infrared laser
pulse is shining onto the interface of a STM tip
and semiconductor sample. An AC voltage is
applied between the tip and semiconductor. The AC
voltage produces an electrical field between the
tip and the sample, which modulates the THz pulse
generated by the laser pulse. Detection of this
modulation gives the THz signal of the tip. Now
the interface works as a THz source, which has a
very small size, since it is determined by the
tip end size.
A
Fig. 4, THz 2-D image of a gold grating on p-InAs
wafer.
Figure 4 shows the THz 2-D image of a metal
grating structure on p-InAs. The flat parts
reflect the metal line, which is 6 ?m wide in the
10 ?m grating period. The scanning region is 40 ?
40 ?m with scanning step 0.1 ?m and 0.5 ?m in Y
and X direction respectively.
A

Reference
Fig. 5. The peak value of THz signal as tip
scanning across a metal film edge on p-InAs wafer.
Fig. 3. THz signal and Tip current signal
measured at the same time as tip approaches the
sample.
1 F. Zenhausern, Y. Martin, H.K.
Wickramasinghe, Scanning interferometric
apertureless microscopy Optical imaging at 10
angstrom resolution, Science, 269, 1083
(1995). 2 K.L. Wang, A. Barkan, D.M. Mittleman,
CMP5, The Conderence of Laser and Electro-Optics
(CLEO), (2003). 3 H.-T. Chen, R. Kersting, G.
Cho, Terahertz Imaging with nanometer
resolution, OpAppl. Phys. Lett., Vol. 83, 3009
(2003)
Fig. 1, Rayleigh criterion and Near field method
THz near field microscope have been developed by
several groups. In their setup, the small
particle is the end of a sharp tip. The scattered
signal can be detected2. Another way is to
detect transmitted THz signal and the difference
between incident and transmitted THz signal
resulted from the sum of absorption and
scattering3. Sub micron resolution has been
reported.
Contact zhangxc_at_rpi.edu
Figure 5 shows the THz peak signal as the tip
scanning across the edge of a Cr-Au film (
average 25 nm thick) on a 1 X 1016 cm-3 doping
p-type InAs wafer, The metal film thickness
changes gradually to 0 at the edge. While on the
metal side, there is no tip THz signal. The
transition from the wafer side, which has tip THz
signal, to the metal side is 1 step (1 nm). This
implies a lateral resolution of no more than 1 nm.
Figure 3 shows that the tip current and THz
signal appear exactly the same time as the tip
approaches the sample surface. The 10 to 90
transition range is within 1 nm, demonstrating a
vertical resolution of around 1 nm. This shows a
similar property to STM.