Beat note analysis and spectral modulation of terahertz
quantum cascade lasers with radio frequency injection
Yonghao Zhu (朱永浩)
1
, Hua Li (黎 华)
1,
*, Wenjian Wan (万文坚)
1
,LiGu(顾 立)
1
,
Tao Zhou (周 涛)
1
, Stefano Barbieri
2
, and Juncheng Cao (曹俊诚)
1,3
1
Key Laboratory of Terahertz Solid State Technology, Shanghai Institute of Microsystem and Information
Technology, Chinese Academy of Sciences, Shanghai 200050, China
2
Laboratoire Matériaux et Phénomènes Quantiques, Université Paris Diderot and CNRS, UMR 7162,
10 rue A. Domont et L. Duquet, Paris 75205, France
3
e-mail: jccao@mail.sim.ac.cn
*Corresponding author: hua.li@mail.sim.ac.cn
Received March 30, 2016; accepted November 18, 2016; posted online December 21, 2016
We demonstrate the electrical beat note analysis and radio frequency (RF) injection locking of a continuous
wave (cw) terahertz quantum cascade laser (QCL) emitting around 3 THz (∼100 μm). In free running the beat
note frequency of the QCL shows a shift of ∼180 MHz with increasing drive current. The beat note, modulation
response, injection pulling, and terahertz emission spectral characteristics in the different current regimes I, II,
and III are investigated. The results show that in the current regime I close to the laser threshold we obtain a
narrower beat note and flat response to the RF modulation at the cavity round trip frequency. The pulling effect
and spectral modulation measurements verify that in the current regime I the RF injection locking is more ef-
ficient and a robust tool to modulate the mode number and mode frequency of terahertz QCLs.
OCIS codes: 140.0140, 040.0040, 350.0350, 060.0060.
doi: 10.3788/COL201715.011404.
Frequency mode locking has been extensively observed in
lasers
[1]
, spin-transfer torque oscillators
[2]
, matter waves of a
Bose gas
[3]
, newly reported central nervous systems
[4]
, and
so on. For optical lasers, mode locking is an useful technique
to produce a narrow optical pulse and high frequency sta-
bility for spectroscopic applications. The pioneering work
of active injection locking was reported in 1960s by inject-
ing the beam of the first laser directly into the cavity of the
second one
[1]
. Due to injection locking, we are allowed to
investigate the coherence of lasers
[5,6]
. Different from the in-
terband diode lasers, quantum cascade lasers (QCLs)
[7–9]
are unipolar devices based on electron intersubband tran-
sitions in the conduction band of multiple quantum well
structures. The fast radiative relaxation in QCLs results
in a flat modulation frequency response that makes QCLs
more suitable for fast modulation
[10,11]
. For multimode
QCLs with a Fabry–Perot cavity geometry, the emission
spectrum is inherently modulated due to the resonant
cavity. In the multimode operation regime, the beat note
frequency from the beating between two adjacent longi-
tudinal mod es is equal to the cavity round trip frequency
determined by the cavity length. Because of this, one can
easily do injection locking by injecting the radio frequency
(RF) signal at the cavity round trip frequency. It has al-
ready been verified that QCLs emitting in mid-infrared
and terahertz wavele ngth ranges can be modulated and in-
jection locked up to dozens of gigahertz
[12–15]
. The injection
locking technique plays an important role in the phase
locking
[16,17]
and broadband frequency comb operation
[18,19]
of terahertz QCLs.
In this Letter, we demonstrate the beat note analysis
and spectral modulation of a 2.5 mm long and 180 μm wide
terahertz QCL with a bound-to-continuum (BTC) active
region structure. The beat note, modulation response, in-
jection pulling, and terahertz emission spectral modula-
tion in different current regimes are investigated. In
the current regime that is close to the laser threshold,
we clearly observe the pulling effect and the strongly
modulated terahertz emission lines under RF injection
locking.
The BTC active region of the terahertz QCL is similar
to that in Ref. [
20]. The Al
0.15
Ga
0.85
As/GaAs active re-
gion was grown by using a gas source molecular beam epi-
taxy (MBE) system on a lattice-matched semi-insulating
GaAs (100) substrate. After the MBE growth, the wafer
was processed into single plasmon waveguide devices. Fi-
nally, the processed sample was cleaved and mounted on a
cooper cold finger of a continuous flow liquid helium cryo-
stat for electrical and optical measurements.
To facilitate the beat note and RF injection measure-
ments, the cryostat was wired using semi-rigid coax cable
and flexible microwave cable with a bandwidth up to
18 GHz. A transmission line was used to accomplish the
impedance match between the RF cable and the terahertz
QCL chip
[12]
. The electrical beat note was measured using a
Bias-Tee. Before we sent the beat note signal into the spec-
trum analyzer, a RF amplifier with a gain of 30 dB was in-
stalled to improve the signal-to-noise ratio. A RF generator
with a frequency up to 26 GHz with a maximum power of
25 dBm was used to inject an RF signal into the THz QCL.
COL 15(1), 011404(2017) CHINESE OPTICS LETTERS January 10, 2017
1671-7694/2016/011404(5) 011404-1 © 2016 Chinese Optics Letters
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