COL 12(12), 120401(2014) CHINESE OPTICS LETTERS December 10, 2014
1671-7694/2014/120401(4) 120401-1 © 2014 Chinese Optics Letters
Detection of a directly modulated terahertz light
with quantum-well photodetector
Qingzhao Wu (武庆钊), Li Gu (顾 立), Zhiyong Tan (谭智勇),
Chang Wang (王 长), and Juncheng Cao (曹俊诚)
*
Key Laboratory of Terahertz Solid-State Technology, Shanghai Institute of
Microsystem and Information Technology, Chinese Academy of Sciences,
Shanghai 200050, China
*
Corresponding author: jccao@mail.sim.ac.cn
Received July 28, 2014; accepted October 11, 2014; posted online December 3, 2014
We demonstrate a wireless transmission link at 3.9 THz over a distance of 0.5 m by employing a terahertz (Hz)
quantum-cascade laser (QCL) and a THz quantum-well photodetector (QWP). We make direct voltage
modulation of the THz QCL and use a spectral-matched THz QWP to detect the modulated THz light
from the laser. The small signal model and a direct voltage modulation scheme of the laser are presented. A
square wave up to 30 MHz is added to the laser and detected by the THz detector. The bandwidth limit of
the wireless link is also discussed.
OCIS codes: 140.5965, 040.2235, 060.2605.
doi: 10.3788/COL201412.120401.
Terahertz (THz) technology has attracted more and
more attention in the past two decades because of its
various kinds of potential applications, such as real-
time imaging, information, and short range wireless
communication among others
[1–4]
. For these applications,
THz emitter and receiver are two key components.
There are several dierent THz emitters, including
uni-traveling carrier photodiode
[5]
, resonant tunneling
diode
[6]
, and microwave multiplication
[7]
, which mostly
operate in the sub-THz region from 100 to 300 GHz.
However, there are not any devices mature enough to
build a high-speed THz communication system in the
range of 2.0–5.0 THz yet. Owing to the obvious advan-
tages such as high emitting power, fast response, THz
quantum-cascade laser (QCL) is a very promising emit-
ter of THz wave especially above 1 THz frequency
[8,9]
.
The study of high-speed modulation of the THz QCL
is important for the application of the device. Since the
lifetime of the internal transport carrier in the THz
QCL is very short, the direct modulation of device
can reach the frequency of tens of gigahertzs
[10,11]
. On
the other hand, the THz quantum-well photodetec-
tor (QWP)
[12,13]
is a good choice as a receiver. THz
QWP has some specic characteristics, including high
response speed, high sensitivity, and narrow band
response, which is potentially suitable for detecting
THz light. A wireless communication demonstration
with a THz QCL and THz QWP was rst reported
by Grant et al. at 3.8 THz in 2009
[14]
, which shows the
basic components needed for a wireless communication
can be performed by using a THz QCL as a source and
a THz QWP as a receiver. Till date, there are a few
reports about THz wireless communication based on
THz QCL and THz QWP
[15,16]
.
In this letter, rstly, we present a direct voltage mod-
ulation scheme of the THz QCL by employing a square
wave signal. A spectral-matched THz QWP is used to
detect the output THz light from the source. Secondly,
the direct modulation and the small signal model of the
THz QCL as well as the bandwidth limit of the wireless
link are analyzed.
The THz QCL is based on a GaAs/AlGaAs material
system with a four-well resonant phonon structure of
the active region, as well as a metal-metal waveguide,
emitting at 3.9 THz. The active region of the device
has a size of 15 mm×25 µm and a height of 10 µm.
The output power of the laser is around 2 mW in the
continuous-wave mode. In order to achieve the best
performance of the devices, the THz QCL is placed
on a copper heat sink and mounted on the cold nger
of a closed cycle helium cryostat, operating at 10 K,
and the THz QWP is cooled down to 4 K by using a
continuous-ow liquid-helium cryostat.
A schematic experimental setup is shown in Fig. 1.
The THz QCL is driven by a modulator. The modulator
module mainly includes a high-speed amplier, which
has a magnication of A = V
o
/V
IN
= (R
G
+ R
F
)/R
G
.
By using a bias-T, the direct current (DC) bias is
added to the alternate current (AC) component. In this
experiment, the DC bias added to the laser is 12.55 V
and the AC bias is 0.45 V. The high voltage is in the
linear operation range of the laser and at the same
time, the low voltage is below the threshold voltage of
the laser. The modulation depth of the signal is 100%,
and the modulation is on-o keying.
Two o-axis parabolic mirrors are used between the
laser and the detector to establish a 0.5 m optical path
in the room air. The emitting THz light is collected by
the two o-axis parabolic mirrors with a focal length of
101.6 mm, and then focused onto the detector through
the polyethylene window of the closed cycle cryostat
where the detector is placed. The transmission of the