Download Tao Yuan, Jingzhou Xu, and Xicheng Zhang Rensselaer

Scanning THz Emission Microscope
Tao Yuan, Jingzhou Xu, and Xicheng Zhang
Rensselaer Polytechnic Institute, Troy, New York
Abstract
Scanning Experiment Result
Experiment Set Up
A THz image system with very high spatial resolution will
provide an important tool for research of material local
property.
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.
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.
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.
0.5
0.0
0
(b)
(c)
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 detected[2]. Another
way is to detect transmitted THz signal and the difference
between incident and transmitted THz signal resulted from
the sum of absorption and scattering[3]. Sub micron
resolution has been reported.
4
8
12
Tip Z position (nm)
16
Fig. 3. THz signal and Tip current signal measured at the same
time as tip approaches the sample.
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.
0
1. Improve scanning speed.
2. Improve the position control of the tip.
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-10
0
40
Acknowledgment
30
10
20
20
30
X (
m)
10
40 0
Y
)
m
(
Fig. 4, THz 2-D image of a gold grating on p-InAs wafer.
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.
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.
THz signal (a.u.)
Tip Current
THz
1.0 Step Size ~0.6nm
THz signal (a.u.)
Fig.2, A schematic graph of the experimental setup
Signal (a.u.)
(a)
We have built a scanning THz emission Microscope and
demonstrated its nanoscale resolution and 2D image scanning
ability.
Future work
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 subwavelength aperture and aperture-less method. In apertureless 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].
Discussion and conclusion
1.0
metal
InAs
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0
5
10
15
20
Tip X position (nm)
Fig. 5. The peak value of THz signal as tip scanning across a
metal film edge on p-InAs wafer.
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.
Reference
[1] F. Zenhausern, Y. Martin, H.K. Wickramasinghe,
“Scanning interferometric apertureless microscopy:
Optical imaging at 10 angstrom resolution”, Science, 269,
1083 (1995). [email protected]
[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)
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