Wavelength (nm)
Wavelength (nm)
1. 2
1
0.8
0.6
0.4
0.2
1050 1055 1060 1065 1070 1075
1050 1055 1060 1065 1070 1075
0
1. 2
1
0.8
0.6
0.4
0.2
0
a) Intensity (a.u.)
b) Intensity (a.u.)
camera was first demonstrated more than 20 years ago using
broadband terahertz pulses generated by femtosecond lasers. 4
Terahertz images were upconverted by mixing laser and
terahertz pulses in a nonlinear crystal to generate an orthogonally polarized signal at the laser wavelength by the elec-tro-optic effect. Since this signal is proportional to the terahertz field, an image encoded in the terahertz beam can be
transferred to the laser’s wavelength. Detecting this image requires filtering out the laser beam at its original polarization,
which was accomplished by polarization filters. The contrast
of these images was not great, but it was the first demonstration of the concept.
High-power narrowband sources of terahertz waves (up to
3 m W of average power at 1. 5 THz) developed by Microtech
have enabled significant improvements in the contrast of images produced in a similar process. 5, 6
Mixing of narrowband picosecond lasers and terahertz
pulses in a nonlinear crystal produces spectral sidebands on
both sides of the laser line (see Fig. 3). A notch filter is used
to attenuate the background at 1064 nm and sidebands corresponding to ωpump + ω THz (at 1058 nm) and ωpump– ω THz (at
1070 nm) are observed. Replacing the notch filter by a long-
pass filter almost completely removes the pump (and the
shorter-wavelength sideband), leaving the sideband at 1070
nm. Spectral sidebands were also orthogonally polarized in
this experiment.
This upconverted signal can be used for transferring images
from terahertz beams to the near-IR. This is accomplished by
placing objects in a collimated terahertz beam,
then focusing and mix-
ing it with a near-IR
laser beam in a nonlinear crystal. Filtered sidebands in the laser beam
carry the image, which
is detected with CCD
and CMOS cameras.
A combination of spectral and polarization filtering has resulted in very
high-contrast images and
videos. Examples of terahertz movies made using
this method are available
at http://mtinstruments.
com/movies/leafppt-
video.mp4; http://mtin-struments.com/movies/
envelopepptvideo.mp4;
and http://mtinstru-ments.com/movies/tef-
lonvids.mp4.
Spectral separation of the sidebands is defined by the terahertz frequency, which was 1. 5 THz in the 1070 nm sideband experiment. This imaging method can be extended to
lower terahertz frequencies—potentially even to 100–300
GHz—by using narrower-spectrum lasers and filters. Higher-
power terahertz sources available at these lower frequencies
will lead to further improvements in the image contrast, and
increase demand for terahertz imaging systems in a variety
of emerging applications.
REFERENCES
1. I. V. Andreev et al., Appl. Phys. Lett., 105, 202106 (2014).
2. W. Knap et al., J. Infrared Millim. Terahertz Waves, 30, 1319 (2009).
3. V. M. Muravev et al., Phys. Rev. Lett., 114, 106805 (2015).
4. Q. Wu et al., Appl. Phys. Lett., 69, 1026–1028 (1996).
5. P. Tekavec et al., “Terahertz generation from quasi-phase matched gallium
arsenide using a type II ring cavity optical parametric oscillator,” Proc. SPIE,
8261, 82610V (Feb. 9, 2012).
6. P. Tekavec et al., “Video rate THz imaging based on frequency upconversion
using a near-IR CMOS camera,” Proc. CLEO 2014, STh4F. 7 (2014).
Viacheslav M. Muravev is vice president, Gombo E. Tsydynzhapov
is senior engineer, and Igor V. Kukushkin is president, all at TeraSense
Group, San Jose, CA, http://terasense.com, while Ian Mcnee is an applications engineer and Vladimir G. Kozlov is founder and CEO, both
at Microtech Instruments, Eugene, OR; e-mail: info@mtinstruments.com;
www.mtinstruments.com.
FIGURE 3. For the spectra of an
upconverted signal (a), a notch filter is
used to attenuate the pump at 1064
nm; next, a long-pass filter is used
to remove the pump at 1064 nm (b).
(Courtesy of Microtech)
