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Case Study CS-LDTC07 Rev. A

Room Temperature Terahertz

Frequency Comb Using Quantum

Cascade Lasers

June, 2020

Page 1

ABSTRACT

Researchers at Northwestern University, Illinois have designed a terahertz (THz) frequency comb using

a Quantum Cascade Laser (QCL) with a Distributed Feedback (DFB) grating inside the cavity. The DFB

grating addition allows for dual wavelength emission from the QCL. A single-mode state and a harmonic

comb state combine inside the cavity for a THz frequency comb. The device generates THz frequencies

at room temperature without any external optical setup. This allows for a harmonic frequency comb at

3.0 THz with a range of 2.2 to 3.3 THz. With the single QCL dual wavelength emission, the THz comb is

more stable than multiple sources. This devices enables a greater current dynamic range for harmonic

comb operation, especially useful in spectroscopy applications. Two uses of QCL frequency combs

demonstrated by researchers at Harvard University are also discussed.

QCL FREQUENCY COMBS

Quantum Cascade Lasers (QCLs) can be useful for creating

optical frequency combs (OFCs) due to the engineered

properties of intersubband transition, fast phonon scattering,

high power efficiency, and dispersion management.1 QCLs

also exhibit four distinct laser states. Injection currents in

the QCL determine the state from lowest current to highest:

single mode, harmonic frequency comb, fundamental

frequency comb, and high-phase noise state.2 Single mode

state will only emit one mode or frequency peak. Harmonic

frequency combs emit multiple modes equally spaced by

higher harmonics of the cavity free spectral range (FSR).

Fundamental frequency combs emit multiple modes, but

the adjacent cavity modes are populated. High-phase noise

state is when the comb is destabilized and exhibits a high-

phase-noise pedestal.2

Terahertz (THz) QCL combs show potential for

spectroscopy and imaging, but the cryogenic aspect of

operation is a barrier to commercial production. Cryogenic

operation increase setup and application complexity and

cost. Room temperature QCLs for THz frequency combs

are difficult to design compared to the mid-infrared (mid-IR)

QCL.

METHODS & PROBLEMS

When operating in the mid-IR to the THz range, techniques

used to generate OFCs are not easily executed or possible.

Techniques such as difference-frequency generation,

optical parametric oscillation, and photoconductive antenna

have been employed to create OFCs up to the THz range.

However, these techniques require intricate optical layouts.1

Down-converting a mid-IR comb to THz range can prove

difficult. Northwestern University has a novel approach to

eliminate external optics while downconverting.

What is a Frequency comb?

A frequency comb is a laser with a spectrum of

equidistant-spaced frequency peaks (

Figure 1).

This can be used as an optical ruler for frequency

measurements. Performing measurements on a wide

range of frequencies requires a large overall bandwidth

of the frequency comb.3

Figure 1. Frequency Comb Illustration

Optical frequency combs (OFCs) can be generated

with a variety of techniques: mode-locked lasers

(amplitude-modulated),

microresonators,2

optical

parametric oscillation, difference-frequency generation,

and four-wave mixing (FWM).1 Applications include:

metrology, chemical sensing, atomic clocks, wireless

communication, data transmission and spectroscopy.

Optical Frequency

Equidistant

Spacing