Challenges and Opportunities

1
CdTe as Emitter for Compact THz Imaging Systems
Xu Xie, Jingzhou Xu and X.-C. Zhang Center for
THz Research, Rensselaer Polytechnic Institute,
Troy, NY 12180 Contact zhangxc_at_rpi.edu

Results and Discussion
Challenges and Opportunities Currently most
pulsed THz systems are based on Ti-Sapphire laser
systems with ZnTe crystals as THz emitters and
detectors. The bulky size and high cost of
Ti-sapphire lasers limit wide use of compact THz
systems. Recent advances of ultrafast fiber laser
systems have yielded smaller laser sources. Since
optical rectification as well as EO sampling
method are mostly determined by phase matching
conditions between THz radiation and laser
pulses, these two processes are laser wavelength
sensitive. This can be described by coherent
length as where ?THz is the frequency of THz
radiation, nopt is the refractive index of
emitter/sensor crystal in THz frequency region
and nTHz is the group index of the laser pulse.
The longer the coherent length, the stronger the
THz radiation will be generated and more
sensitive detection is possible. Calculations
show ZnTe is not suitable for use with fiber
laser wavelengths at 1550 nm and 1050 nm.
Destination Terahertz radiation is gentle but
piercing. THz imaging technology has been used in
industrial non-destructive evaluation and is
promising in mail and luggage examination,
biological sensing and imaging. A low cost and
compact THz system is crucial for its future use.
Fig.5, (a) Emitted THz power from a CdTe crystal
and a ZnTe crystal as a function of laser
wavelength. (b) Emitted THz power and power
transmission of CdTe crystal as a function of
laser wavelength.
Fig.1, CW THz images of three kinds of drugs. The
small polyethylene bags contain from left to
right MDMA, aspirin, and methamphetamine. The
bags were placed inside the envelope during
imaging. (Credit Kodo Kawase, Optics Express 11
2549 )
ZnTe crystal was compared with CdTe at different
laser wavelength as a THz emitter. Experimental
results show consistency with the calculations.
As a result, in the wavelength range from 710 to
970 nm, the longest laser wavelength can give
the most powerful THz radiation from CdTe which
is stronger than ZnTe at 970 nm .
Fig.1 CW THz image of a space shuttle thermal
insulating form.
Conclusion We measured THz radiation from CdTe
crystal at different wavelengths via the process
of optical rectification. It is demonstrated that
in the range from 710 nm to 970 nm, phase
mismatching affects the emitted THz power. As a
result the longer laser wavelength can give more
powerful THz radiation in this wavelength range.
This has made it possible to use Yb doped fiber
lasers with wavelength of about 1050 nm as laser
source to build compact THz imaging
systems. Acknowledgement This work was
supported in part by CenSSIS, the Center for
Subsurface Sensing and Imaging Systems, under the
Engineering Research Centers Program of the
National Science Foundation (Award Number
EEC-9986821).
Fig.3, Calculated coherent lengths at 2 THz of
different materials as a function of laser
wavelengths. (Credit Masaya Nagai, Applied
Physics Letters 85 3974 )
Calculations show that CdTe has a maximum
coherent length when the laser wavelength is 1050
nm that is close to a Yb doped fiber laser
wavelength. This implies possible use of CdTe
crystals with fiber laser systems for THz
generation and detection. Research of CdTe
properties in THz generation and detection at
different laser wavelengths has become important.
Here we report a study of CdTe in wavelengths
ranging from 710 nm to 970 nm. Our result shows
that at 970 nm CdTe can generate stronger THz
power compared with ZnTe crystal.
Fig.2, A CW THz imaging system with a GUNN diode
as THz source and a Schottky diode as THz
detector.
In order to obtain abundant spectrum information
of the image target, an image system with
broadband THz generation and detection
capabilities must be realized. Our goal is to
make a low cost, compact pulsed broadband THz
imaging system by using optical rectification for
THz generation and EO sampling for THz detection.