Widely tunable terahertz source based on intra-cavity frequency mixing in quantumcascade laser arraysAiting Jiang, Seungyong Jung, Yifan Jiang, Karun Vijayraghavan, Jae Hyun Kim, and Mikhail A. Belkin Citation: Applied Physics Letters 106, 261107 (2015); doi: 10.1063/1.4923374 View online: http://dx.doi.org/10.1063/1.4923374 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/106/26?ver=pdfcov Published by the AIP Publishing Articles you may be interested in InAs/AlSb widely tunable external cavity quantum cascade laser around 3.2 μm Appl. Phys. Lett. 102, 011124 (2013); 10.1063/1.4774088 Room temperature single-mode terahertz sources based on intracavity difference-frequency generation inquantum cascade lasers Appl. Phys. Lett. 99, 131106 (2011); 10.1063/1.3645016 Terahertz sources based on intracavity frequency mixing in mid-infrared quantum cascade lasers with passivenonlinear sections Appl. Phys. Lett. 98, 151114 (2011); 10.1063/1.3579260 Surface-emitting terahertz quantum cascade laser source based on intracavity difference-frequency generation Appl. Phys. Lett. 93, 161110 (2008); 10.1063/1.3009198 Single-frequency coherent terahertz-wave generation using two Cr:forsterite lasers pumped using one Nd:YAGlaser Rev. Sci. Instrum. 79, 036101 (2008); 10.1063/1.2884926

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Widely tunable terahertz source based on intra-cavity frequency mixingin quantum cascade laser arrays

Aiting Jiang,1 Seungyong Jung,1 Yifan Jiang,1 Karun Vijayraghavan,1,2 Jae Hyun Kim,1

and Mikhail A. Belkin1,a)

1Department of Electrical and Computer Engineering, The University of Texas at Austin, Austin, Texas 78712,USA2ATX Photonics, 10100 Burnet Rd., Austin, Texas 78758, USA

(Received 6 May 2015; accepted 19 June 2015; published online 29 June 2015)

We demonstrate a compact monolithic terahertz source continuously tunable from 1.9 THz to 3.9

THz with the maximum peak power output of 106 lW at 3.46 THz at room temperature. The

source consists of an array of 10 electrically tunable quantum cascade lasers with intra-cavity tera-

hertz difference-frequency generation. To increase fabrication yield and achieve high THz peak

power output in our devices, a dual-section current pumping scheme is implemented using two

electrically isolated grating sections to independently control gain for the two mid-IR pumps.VC 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4923374]

Compact, widely tunable and mass-producible room tem-

perature terahertz (THz) sources are highly desired for imag-

ing and spectroscopy applications in 0.3–6 THz spectral

range. Electrically pumped monolithic semiconductor sources,

similar to diode lasers in ultraviolet, visible, and infrared or

quantum cascade lasers (QCLs) in mid-infrared, would be the

most desired solution owing to their compactness and

low-cost mass-production potential. However, THz power

generated by semiconductor electronic devices rolls off with

frequency and is well below the milliwatt level above 1 THz;

moreover, these sources are not widely tunable.1–4 THz QCLs

are still constrained to cryogenic operation temperatures and

have demonstrated a maximum tuning range of 380 GHz.5–7

THz sources based on intra-cavity difference-frequency

generation (DFG) in mid-infrared (mid-IR) QCLs, first dem-

onstrated in 2007 and known as THz DFG-QCLs,8,9 are cur-

rently the only mass-producible monolithic electrically

pumped semiconductor sources of 1–6 THz radiation that can

operate at room temperature. Upon application of bias current,

these devices generate two mid-IR pump frequencies x1 and

x2 that are converted into THz radiation at xTHz¼x1�x2

by DFG inside of the laser cavity. The active region in THz

DFG-QCLs is quantum-engineered to provide broadband

mid-IR gain and giant optical nonlinearity for THz DFG.

With the introduction of laser waveguides designed for

Cherenkov DFG phase matching,10,11 these devices are now

capable of room temperature operation with nearly 2 mW of

peak power output12 and show single-mode tuning over nearly

the entire 1–6 THz range using an external cavity setup.13

Monolithic electronically tunable semiconductor tera-

hertz sources are most desirable for low-cost mass-produc-

tion. Monolithic THz DFG-QCL single-mode tuners have

recently been demonstrated by our group using two electri-

cally isolated grating sections to independently control and

electrically tune the frequencies of the two mid-IR pumps

(x1 and x2) which result in tunable THz DFG output

xTHz¼x1�x2.14 A tuning of approximately 600 GHz in

the 3.4–4 THz range was achieved. THz DFG-QCLs with

even larger tuning range of 2.6–4.2 THz were demonstrated

recently by Razeghi group using a three-section device con-

figuration with one section controlling the wavelength of one

mid-IR pump via a distributed Bragg reflector grating and

the other two sections providing tuning of the second mid-IR

pump based on Vernier tuning mechanism in a sampled gra-

ting.15 We note that single-mode operation of tunable QCLs

based on sampled gratings is very challenging and the side

mode suppression ratio in these devices is typically worse

than 10–20 dB.16,17

In this report, we follow the QCL array approach, first

demonstrated for monolithic mid-IR tuners,18 to produce

monolithic THz source with continuous spectral tuning from

1.9 to 3.9 THz. We integrate 10 monolithic THz DFG-QCL

tuners described in Ref. 14, each designed for a slightly dif-

ferent center THz emission frequency, in a single laser array.

Figures 1(a) and 1(b) show the schematic and scanning

electron microscope image of a THz DFG-QCL array. The

array consists of ten 22-lm-wide ridge-waveguide THz

DFG-QCLs, each designed to emit at a specific THz fre-

quency spanning 1.9–3.9 THz range. The structure of an indi-

vidual device in the array is shown in Fig. 1(c). The devices

contain two first order distributed feedback (DFB) grating

sections fabricated to select two mid-IR pump frequencies.

The surface grating scheme11,19–21 is used for DFB gratings.

Grating periods are in the range from 1.4 lm to 1.6 lm; gra-

ting duty cycle is set to 50%. The gratings are defined in the

top cladding layer and are approximately 145 nm deep. Such

etching depth was computed to provide nearly the same cou-

pling constant of �11 cm�1 for both mid-IR pumps, based on

the analysis in Ref. 21. The grating in the front section of the

device is designed for short wavelength (x1) mid-IR pump

selection, and the grating at the back section is designed for

long wavelength (x2) mid-IR pump selection. Each grating

section is 1.2-mm-long, and the two sections are separated by

a 300 lm gap for electrical isolation.14

The device wafer is grown by a commercial foundry on

a semi-insulating InP substrate. The layer sequence is similara)mbelkin@ece.utexas.edu

0003-6951/2015/106(26)/261107/4/$30.00 VC 2015 AIP Publishing LLC106, 261107-1

APPLIED PHYSICS LETTERS 106, 261107 (2015)

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to that of the devices reported in Ref. 11. Lateral current-

extraction scheme is used, similarly to devices in Refs. 10,

11, and 14. THz radiation is extracted at a Cherenkov angle

into the semi-insulating InP substrate.10 The front substrate

facet is polished at 30 degree angle, similar to devices in

Refs. 10, 11, and 14 to facilitate Cherenkov THz DFG radia-

tion outcoupling in forward direction. Devices are tested at

room-temperature with 70 ns current pulses at 40 kHz repeti-

tion frequency. Mid-IR and THz power are measured using

thermopile and calibrated bolometer, respectively, from the

front facet using a nitrogen-purged setup with two 2-in.-di-

ameter off-axis parabolic-mirrors: one with focal length of

50 mm to collimate the light from the device and the second

one with 100 mm focal length to refocus the light onto the

detector. Recorded mid-IR power is corrected for an esti-

mated 70% collection efficiency of our setup; THz power is

not corrected for any collection efficiency. Fourier-transform

infrared spectrometer (FTIR) is used for spectral

measurements.

Dual-wavelength mid-IR lasing is required to produce

THz DFG output. Due to gain competition, dual color DFB

lasing requires very similar threshold gain for both of fre-

quencies, which is difficult to achieve in practice.13 To fabri-

cate a 10-element DFB THz DFG-QCL array fabrication

with any reasonable yield, we need to be able to fabricate

individual dual-color devices with nearly 100% yield. To

that end, we implement independent control of the net gain

for the two mid-IR pumps in our devices using the configura-

tion shown in Fig. 2. The two grating sections shown in

Fig. 2 are connected to 50-X coaxial power lines in series

with 50 X resistors that provide impedance matching (differ-

ential resistance of QCLs at operating bias is only a few

Ohms). The power lines are connected to the power supply

through 50-X-impedance-matched variable attenuators.22

The power supply is made of a DEI HV1000 pulse generator

(IXYS Colorado Corp.) connected to a 1000 V DC voltage

source (Kepco Corp.). Current is measured using coil current

sensors. In this biasing configuration, we can control the por-

tion of current from the power supply that goes to the front

and back device section by adjusting attenuation values on

the two attenuators.

Independent control of the current density through the

two grating sections is crucially important to enable dual-

color lasing with high yield. Figures 3(a)–3(d) plot the power

of mid-IR pumps, collected from the front facet of the de-

vice, and THz output for device #6 in the array designed for

emission at 3 THz. This device performance is typical for

the other elements in our array. Panels 3(a) and 3(b) show

the power of the short wavelength (x1) and long wavelength

(x2) mid-IR pumps, respectively, as a function of current

density through the front and back sections of the device.

Mid-IR inference filters (Spectrogon, Inc.) were used to sep-

arate the pump powers. Figure 3(a) indicates that the device

predominantly lases at the short wavelength x1, which

implies that the threshold gain at x1 is lower than that at x2.

White dashed line in all the panels in Fig. 3 represents the

uniform current pumping condition (same current density for

both sections), and only one mid-IR pump is lasing in this

case. To enable dual-color mid-IR emission, we need to

apply different current bias to the front and the back section

as shown in Fig. 3(b). To provide further detail of the device

mid-IR performance, insets in Figs. 3(a) and 3(b) show the

emission spectra of the laser for the case of uniform pumping

density at 16 kA/cm2 and non-uniform pumping density with

12 kA/cm2 pump current through the x1 DFB section and

16 kA/cm2 pump current through the x2 DFB section,

respectively. Figures 3(c) and 3(d) show the product of the

two mid-IR output powers and the power of THz output of

the device #6 as a function of the current density through the

two DFB sections. As expected, THz radiation is generated

only when both mid-IR pumps are lasing. Maximum THz

power is measured to be 46 lW for this device.

To understand the results in Fig. 3, we note that the mid-

IR modes x1 and x2 only interact strongly with front and

back gratings, respectively. Neglecting feedback from

cleaved mirrors x1-mode is expected to be localized pre-

dominantly in the front DFB section, while the mode at x2 is

expected to be localized predominantly in the back DFB sec-

tion. Thus, stronger electrical pumping of the front (back)

FIG. 1. (a) Schematic of a 10-ridge THz DFG-QCL array (only two ridges are shown). (b) Scanning electron microscope image of the fabricated THz DFG-

QCL array. Inset: image of an individual device. (c) Schematic of an individual device structure with the two DFB sections and the direction of THz output

shown.

FIG. 2. Device biasing scheme. Shown are the front DFB section for the

short wavelength mode selection (right), the back DFB section for the long

wavelength mode selection (left), and the electrical biasing scheme with

variable attenuators. Power and spectrum measurements are all taken from

the right (front) facet.

261107-2 Jiang et al. Appl. Phys. Lett. 106, 261107 (2015)

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section is expected to predominantly increase the round-trip

gain of the x1 (x2) pump. The feedback from the cleaved

facets, which are uncoated in case of our devices, may distort

the mode distribution.23,24 Detailed theoretical analysis of

our device performance in this case is beyond the scope

of this paper. We note, however, that even in the presence of

facet reflections, the x1- and x2-mode distribution along the

laser waveguide is expected to remain dissimilar and the two

modes are expected to benefit differently from the front- and

back-section pumping.

Similar search of optimal combination of pump current

densities through the two DFB sections for maximum THz

output is performed for other devices in the array. As a

result, dual-color lasing and THz output are achieved for all

devices in the array. We emphasize that fewer than 50% of

our devices showed dual-color mid-IR emission under uni-

form pump density and thus dual-section pumping was cru-

cial for high yield THz DFG-QCL array fabrication.

Figure 4(a) plots the mid-IR lasing spectra of the entire

10-ridge array. The mid-IR pump frequencies are selected so

as to produce THz emission lines spaced by approximately

210 GHz. Spectral tuning of mid-IR frequencies x1 and x2

and hence THz frequency xTHz¼x1�x2 are achieved by

applying DC bias current to either the front or the back DFB

section of the device through a bias tee as described in Ref.

14. Figure 4(b) shows the THz emission of all devices in the

array with zero DC bias (solid lines) and with DC bias

applied to front section only (dashed lines) so as to produce

THz emission precisely in-between the emission lines of the

array at zero DC bias. Maximum THz output for each line is

also shown. Maximum THz peak power from the array was

recorded to be 106 lW for a device #8 at zero DC tuning

bias with a mid-IR-to-THz conversion efficiency of

0.4 mW/W2, similar to that observed in external-cavity THz

DFG-QCL systems with QCL chips based on a similar active

FIG. 3. (a) Peak power output of the

short-wavelength mid-IR pump, (b)

peak power output of the long-

wavelength mid-IR pump, (c) the prod-

uct of the peak powers of the two mid-

IR pumps, and (d) peak THz power

output as a function of current density

through the front and back sections of

the device #6 in the array. White

dashed line in (a)–(d) corresponds to

the uniform current pumping situation.

Insets in panels (a) and (b) display

mid-IR emission spectra of the device

for the injection current densities

through the front and back section

indicated by yellow circles in the

panels.

FIG. 4. (a) Normalized mid-IR emission spectra of all 10 devices in the

array recorded at optimal currents densities through the two DFB sections in

each device. (b) THz emission spectra (bottom panel) and the maximum

THz peak power (top panel) for all devices in the array without applying DC

bias for tuning (solid lines and stars) and with DC tuning bias applied to the

front grating section so as to produce THz emission in-between the emission

lines of the array at zero DC bias (dashed lines and triangles).

261107-3 Jiang et al. Appl. Phys. Lett. 106, 261107 (2015)

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region design.13 Figures 5 shows continuous tuning capabil-

ity of our array source. Figure 5(a) shows emission spectra

of the device #8 for different DC tuning bias currents applied

to the front DFB section. Tuning range is wide enough to

cover the gap between the emission frequencies of the two

adjacent devices in the array. Figure 5(b) shows further

details of the DC current tuning around the circled region in

Fig. 5(a). As discussed in Ref. 14, due to cleaved facet reflec-

tivity, mid-IR pumps experience mode-hoping which leads

to a gap in THz frequency tuning. As further described in

Refs. 14 and 25, continuous THz tuning can still be obtained

by applying a second DC bias to the back DFB section. The

experimental demonstration is presented in Fig. 5(c).

To summarize, we demonstrated continuously tunable

monolithic room-temperature THz DFG-QCL source made

of an array of 10 DFG-QCL devices. Independent current

pumping scheme for two grating sections was implemented

to enable dual-color mid-IR pump emission in all devices in

the array with nearly 100% yield. The array achieves the

maximum THz peak power output of 106 lW at 3.46 THz

and provides continuous spectral coverage between 1.9 and

3.9 THz. Further improvement of tuning range is expected

by increasing the number of lasers in the array and/or by fur-

ther increasing the frequency spacing between the emitters

in the array. We expect that, similar to tunable mid-IR sour-

ces based on QCL arrays,26–28 tunable THz sources based on

this technology will provide a compact low-cost THz source

option for spectroscopic applications.

The University of Texas at Austin group acknowledges

support from the National Science Foundation Grant Nos.

ECCS-1150449 (CAREER) and ECCS-1408511. Sample

fabrication was carried out in the Microelectronics Research

Center at the University of Texas at Austin, which is a

member of the National Nanotechnology Infrastructure

Network.

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FIG. 5. Mode-hope free THz tuning using device #8 in the array as an example. (a) Emission spectra of device #8 in the array for different values of DC tuning

current though the front section. Also shown is the THz emission spectrum of the adjacent device #7 at zero DC tuning current. (b) THz tuning behavior of de-

vice #8 around the region circled in panel (a). Mode hoping is observed around 3.43 THz. (c) Mode-hope-free tuning of device #8 within the circled region

shown in (b) by applying DC bias to both the front and the back DFB sections of the device #8. From left to right, the spectra shown in (c) correspond to the

front/back DC tuning current of 110 mA/0 mA, 100 mA/0 mA, 100 mA/50 mA, 140 mA/100 mA, and 90 mA/0 mA.

261107-4 Jiang et al. Appl. Phys. Lett. 106, 261107 (2015)

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