Terahertz imaging through self-mixing
in a quantum cascade laser
Paul Dean,1,* Yah Leng Lim,2Alex Valavanis,1Russell Kliese,2Milan Nikolić,2Suraj P. Khanna,1
Mohammad Lachab,1Dragan Indjin,1Zoran Ikonić,1Paul Harrison,1Aleksandar D. Rakić,2
Edmund H. Linfield,1and A. Giles Davies1
1School of Electronic and Electrical Engineering, University of Leeds, Leeds, LS2 9JT, UK
2The University of Queensland, School of Information Technology and Electrical Engineering, QLD, 4072, Australia
*Corresponding author: firstname.lastname@example.org
Received April 13, 2011; revised May 25, 2011; accepted May 31, 2011;
posted May 31, 2011 (Doc. ID 145642); published July 1, 2011
We demonstrate terahertz (THz) frequency imaging using a single quantum cascade laser (QCL) device for both
generation and sensing of THz radiation. Detection is achieved by utilizing the effect of self-mixing in the THz
QCL, and, specifically, by monitoring perturbations to the voltage across the QCL, induced by light reflected from
an external object back into the laser cavity. Self-mixing imaging offers high sensitivity, a potentially fast response,
and a simple, compact optical design, and we show that it can be used to obtain high-resolution reflection images of
exemplar structures.© 2011 Optical Society of America
OCIS codes:110.6795, 250.5960, 140.5960, 040.2235.
The quantum cascade laser (QCL) [1,2] is a compact
semiconductor source of terahertz (THz) frequency ra-
diation that is potentially well suited for imaging across
a broad range of applications. These inter-subband, het-
erostructure devices are capable of cw emission with
powers exceeding 100mW . Furthermore, narrowband
emission has been demonstrated at frequencies from 1.2
to 5:0THz [4,5], with operating temperatures as high as
186K being obtained in pulsed mode . The high powers
of THz QCLs, coupled with the possibility of tailoring the
emission to frequencies that exhibit low atmospheric at-
tenuation, has enabled real-time imaging over stand-off
distances exceeding 25m . Real-time imaging in both
transmission and reflection geometries has also been de-
monstrated at near-video-rates (20 frames per second)
through use of a microbolometer focal-plane array .
Other detector systems employed for imaging with
THz QCLs include room-temperature Schottky diodes
, Golay cells , pyroelectric detectors  and
temperature THz detectors avoid the reliance on cryo-
genic liquids, their response is typically slow and, with
the exception of Schottky diodes, sensitivities are signif-
icantly poorer than those achieved with cryogenically
cooled bolometric detection.
In this Letter, we demonstrate a THz frequency ima-
ging system that uses a single QCL to both generate
and sense the THz radiation through self-mixing, an ef-
fect that occurs when the radiation from a laser is re-
flected from an external target back into the laser
cavity [12,13]. The reflected light interferes (mixes) with
the intracavity field, producing variations in the thresh-
old gain, emitted power, lasing spectrum, and junction
voltage. To date, self-mixing in THz QCLs has been used
only to determine the linewidth enhancement factor of
the laser . In our scheme, imaging is performed by
monitoring perturbations to the voltage dropped across
the QCL as a reflective object is scanned through the
emitted THz beam. The QCL itself behaves as an inter-
ferometric sensor, thereby removing the need for an
external detector. We demonstrate high-resolution re-
flection imaging of exemplar metallic structures, includ-
ing imaging through visibly opaque screens.
The homodyne (coherent) nature of a self-mixing
scheme inherently provides very high-sensitivity detec-
tion, potentially at the quantum noise limit, and therefore
a high signal-to-noise ratio can be expected in imaging
data [15,16]. Furthermore, the maximum speed of
response to optical feedback is determined by the fre-
quency of relaxation oscillations in the laser. In the case
of THz QCLs, the lifetime of the upper state of the lasing
transition is limited by elastic and inelastic scattering
mechanisms to a few picoseconds [17,18], enabling re-
sponse frequencies of the order of 100GHz. Self-mixing
systems also have a simple optical design and offer the
advantage of potential implementation into arrays .
A schematic diagram of our imaging system is shown
in Fig. 1. The THz QCL consisted of a 10-μm-thick GaAs—
AlGaAs bound-to-continuum active region  that was
processed into a semi-insulating surface-plasmon ridge
waveguide with dimensions 3mm × 140μm. The QCL
was mounted on the cold finger of a continuous-flow
cryostat fitted with a polythene window and operated
in cw mode at a heat sink temperature of 25K. Measure-
ments of the source emission spectrum for a drive
THz imaging with a QCL. S, current source; OSC, oscilloscope;
LA, lock-in amplifier; C, mechanical chopper; O, object. Inset:
emission spectrum of the THz QCL, as measured using a
Schematic diagram of the experimental system used for
July 1, 2011 / Vol. 36, No. 13 / OPTICS LETTERS2587
0146-9592/11/132587-03$15.00/0© 2011 Optical Society of America
current of 900mA, obtained using a Fourier-transform in-
frared spectrometer with a spectral resolution of 7:5GHz,
indicate emission in a single longitudinal mode at
2:60THz (inset, Fig. 1). Radiation from the QCL was col-
limated using a 2 in.f=2 off-axis parabolic reflector and
focused at normal incidence onto the object, using a sec-
ond identical reflector. The total optical path between
source and object was 65cm, with ∼240μW of power
being incident on the object, as measured using a cali-
brated THz frequency powermeter. The laser beam
was mechanically modulated at a frequency of 215Hz
using an optical chopper and coupled back into the laser
cavity along the same optical path as the incident radia-
tion. The self-mixing-induced perturbations to the vol-
tage across the QCL terminals were amplified by an
ac-coupled differential amplifier, with a gain of 100. This
signal was then measured by a lock-in amplifier and syn-
chronized with the chopper frequency, as well as by
being observed directly on an oscilloscope. For image ac-
quisition, the object was raster scanned in two dimen-
sions using a two-axis, computer-controlled translation
stage, with the lock-in amplifier output being recorded
at each position. No atmospheric purging was employed.
Figure 2(a) shows a typical waveform obtained, after
amplification of the ac-coupled voltage across the QCL
terminals in response to a modulated feedback signal,
for the case in which the object was a reflective metallic
plate. The mean junction voltage was 3:1V, with a driving
current of 900mA. Figure 2(b) shows the rms self-mixing
signal measured as a function of the QCL drive current,
obtained by positioning a corner cube retroreflector in
the collimated portion of the beam. Also shown is the
power-current characteristic of the QCL. This demon-
strates that the QCL is most sensitive to optical feedback
at operating currents near threshold. Similar behavior
has been observed previously in junction semiconductor
lasers . We estimated the detection limit of our sys-
tem by inserting attenuators between the QCL and beam
focus. Instead of modulating the amplitude of the feed-
back signal using an optical chopper, for this measure-
ment the reflective plate was attached to a subwoofer
speaker that was vibrated at ∼20Hz to generate a time-
varying optical path length. This ensured that weak re-
flections from the attenuators remained constant and
made no contribution to the heterodyne signal. We found
that our system could tolerate ∼48dB of attenuation, in-
dicating a minimum detectable reflected power equal
Figure 3(a) shows an image of a scalpel blade ob-
scured by a high-density polyethylene FedEx envelope.
The step-size for this image was 250μm and the lock-
in time constant was 5ms. The magnitude of the self-
mixing signal depends on the phase of the field coupled
back into the laser cavity, or equivalently the length of
the extended cavity formed between the QCL and the
sample being imaged. This explains the fringes observed
in this image, which represent the surface morphology of
the object, with adjacent fringes corresponding to a long-
itudinal displacement of half a wavelength, or ∼58μm in
this case. This demonstrates the potential applicability of
this sensing technique to three-dimensional imaging. The
modulation transfer function for the system was deter-
mined by imaging a set of gold-on-quartz bar resolution
targets . By defining the resolution limit at the 20%
modulation threshold, Fig. 3(b) shows that our system
is capable of resolving features down to widths of 250μm
or lower. This allows high-resolution imaging to be
performed, as demonstrated in the exemplar image of
a British two-pence coin in Fig. 3(c). For this 25:9mm×
25:9mm image (corresponding to 259pixel × 259pixel),
the shortest acquisition time realizable using our system
was 19 minutes.
It should be noted that, in our scheme, the QCL is op-
erated in cw mode in order to achieve good spectral pur-
ity. Since the reported scheme is sensitive to the phase of
the reflected field, poorer spectral purity would result in
reduced detection sensitivity, and would also adversely
amplification of the ac-coupled voltage dropped across the QCL
in response to a square-modulated feedback signal. The TTL
feedback control signal (top trace) has been scaled and offset.
(b) Root-mean-square self-mixing signal (left axis) and QCL
power (right axis) as a function of the QCL driving current.
(a) Exemplar waveform (bottom trace) obtained afterFig. 3.
scalpel blade obscured behind a high-density polyethylene Fe-
dEx envelope. (b) Modulation transfer function of the system,
obtained from images of gold-on-quartz bar resolution targets.
(c) A high-resolution image of a British two-pence coin
(diameter ¼ 25:9mm).
(a) Exemplar image, obtained using our system, of a
2588OPTICS LETTERS / Vol. 36, No. 13 / July 1, 2011
affect the ability of the system to resolve the morphology Download full-text
of the object accurately . Nevertheless, the QCL
could in principle be operated in pulsed mode, thereby
removing the need for the mechanical chopper. This
would also facilitate faster modulation of the self-mixing
signal, which is currently limited by the maximum usable
In summary, we have demonstrated THz imaging using
a single QCL as both the source and detector, by moni-
toring the self-mixing voltage induced across the QCL
terminals by optical feedback from an object. This
technique is well-suited to fast, high-resolution, high-
sensitivity imaging at THz frequencies without the need
for an external THz detector.
This project is funded under the European Research
Council (ERC) Advanced Grants New Opportunities in
Terahertz Engineering and Science (NOTES) and Tera-
hertz Optoelectronics from the Science of Cascades to
Applications (TOSCA), and the Innovative Research Call
in Explosives and Weapons Detection (2007), a cross-
government program, sponsored by a number of
counter-terrorism strategy (CONTEST). This research
was supported under Australian Research Council’s Dis-
covery Projects funding scheme. DP0988072.
1. R. Köhler, A. Tredicucci, F. Beltram, H. E. Beere,
E. H. Linfield, A. G. Davies, D. A. Ritchie, R. C. Iotti, and
F. Rossi, Nature 417, 156 (2002).
2. B. Williams, Nat. Photon. 1, 517 (2007).
3. B. Williams, S. Kumar, Q. Hu, and J. L. Reno, Electron. Lett.
42, 89 (2006).
4. C. Walther, M. Fischer, G. Scalari, R. Terazzi, N. Hoyler, and
J. Faist, Appl. Phys. Lett. 91, 131122 (2007).
5. A. W. M. Lee, Q. Qin, S. Kumar, B. S. Williams, Q. Hu, and
J. L. Reno, Appl. Phys. Lett. 89, 141125 (2006).
6. S. Kumar, Q. Hu, and J. L. Reno, Appl. Phys. Lett. 94,
7. A. W. M. Lee, B. S. Williams, S. Kumar, Q. Hu,and J. L.Reno,
IEEE Photon. Technol. Lett. 18, 1415 (2006).
8. S. Barbieri, J. Alton, C. Baker, T. Lo, H. E. Beere, and
D. Ritchie, Opt. Express 13, 6497 (2005).
9. K. L. Nguyen, M. L. Johns, L. F. Gladden, C. H. Worrall,
P. Alexander, H. E. Beere, M. Pepper, D. A. Ritchie, J. Alton,
S. Barbieri, and E. H. Linfield, Opt. Express 14, 2123 (2006).
10. P. Dean,M. U. Shaukat,S.P.Khanna, M. Lachab,A.Burnett,
A. G. Davies, E. H. Linfield, and S. Chakraborty, Opt.
Express 16, 5997 (2008).
11. P. Dean, N. K. Saat, S. P. Khanna, M. Salih, A. Burnett, J.
Cunningham, E. H. Linfield, and A. G. Davies, Opt. Express
17, 20631 (2009).
12. T. Bosch, C. Bes, L. Scalise, and G. Plantier, in Encyclope-
dia of Sensors, C. A. Grimes, E. C. Dickey, and M. V. Pishko,
eds. (American Scientific, 2006), Vol. X, pp. 1–20.
13. S. Donati, Electro-Optical Instrumentation: Sensing and
Measuring with Lasers (Prentice Hall, 2004).
14. R. P. Green, J. H. Xu, L. Mahler, A. Tredicucci, F. Beltram,
G. Giuliani, H. E. Beere, and D. A. Ritchie, Appl. Phys. Lett.
92, 071106 (2008).
15. S. Donati, Photodetectors (Prentice Hall, 1999).
16. S. Donati and M. Sorel, IEEE Photon. Technol. Lett. 8,
17. G. Scalari, L. Ajili, J. Faist, H. Beere, E. H. Linfield,
D. Ritchie, and G. Davies, Appl. Phys. Lett. 82, 3165 (2003).
18. D. Indjin, P. Harrison, R. W. Kelsall, and Z. Ikonic, Appl.
Phys. Lett. 82, 1347 (2003).
19. Y. L. Lim, R. Kliese, K. Bertling, K. Tanimizu, P. A. Jacobs,
and A. D. Rakić, Opt. Express 18, 11720 (2010).
20. S. Barbieri, J. Alton, H. E. Beere, J. Fowler, E. H. Linfield,
and D. A. Ritchie, Appl. Phys. Lett. 85, 1674 (2004).
21. K. Rochford and A. Rose, Opt. Lett. 20, 2105 (1995).
22. J. R. Tucker, A. D. Rakic, C.J. O’Brien, and A. V. Zvyagin,
Appl. Opt. 46, 611 (2007).
July 1, 2011 / Vol. 36, No. 13 / OPTICS LETTERS2589