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Beat note analysis and spectral modulation of terahertz

quantum cascade lasers with radio frequency injection

Yonghao Zhu (朱永浩)

, Hua Li (黎华)

*, Wenjian Wan (万文坚)

,LiGu(顾立)

Tao Zhou (周涛)

, Stefano Barbieri

, and Juncheng Cao (曹俊诚)

1,3

Key Laboratory of Terahertz Solid State Technology, Shanghai Institute of Microsystem and Information

Technology, Chinese Academy of Sciences, Shanghai 200050, China

Laboratoire Matériaux et Phénomènes Quantiques, Université Paris Diderot and CNRS, UMR 7162,

10 rue A. Domont et L. Duquet, Paris 75205, France

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

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

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