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Expanding the spectral range of QCLs

Refinements to waveguides and active regions enable QCL room-temperature operation at 19 µm.

The QCL that lases via a diagonal intersubband transition (D605) has a higher threshold current density than the device that operates using a vertical transition (D686)

A team from Franceis claiming to have set a new benchmark for the performance of quantum cascade lasers (QCLs) operating in the long infrared.

One device based on the InAs/AlSb material system delivered emission at 19 µm at temperatures up to 291 K, while a cousin at 21 µm produced lasing up to 250 K. According to the partnership between researchers at University Paris Sud and the University of Montpellier, these values represent the best performance to date for QCLs operating above 16 µm.

The team’s QCLs operate in a spectral range that corresponds to an atmospheric transparency window. This spectral range is of interest to astronomers, because it allows signals from space to reach the earth without undergoing excessive absorption.

“Since these signals are very weak, lasers can be used as local oscillators to perform heterodyne detection,” explains Raffaele Colombelli from University Paris Sud.

It is possible to construct QCLs emitting in the 19 - 24 µm range with InGaAs-based and GaAs-based devices, but lasers built with these material systems more than a decade ago did not produce encouraging results.

The pairing of InAs and AlSb is far more promising, because very low effective electron mass in the quantum wells leads to elevated optical gain. In 2013, the team at Montpellier reported QCLs emitting at around 20 µm that were based on InAs and AlSb, and device improvements are detailed in its latest paper that is produced in collaboration with the University Paris Sud.

The latest lasers feature metal-metal waveguides. These structures produce very divergent far fields for terahertz QCLs, which are defined as emitting at 65 µm or more. However, for the French team’s lasers, undesirable diffraction effects appear to be absent. Laser structures were created in a Riber Compact 21 MBE reactor, using growth runs that could take 10 hours.

Roland Teissier from the University of Montpellier explains that one of the main difficulties associated with MBE growth is the control of the very thin AlSb layer – which has a thickness of the order of one atomic monolayer – with high interface quality. “[The second challenge is] the stability of the growth rate, in order to keepuniform layer thickness throughout the growth of the 7 µm-thick active region.”

To create the metal-metal waveguide, the researchers used wafer bonding and active region transfer. “This required the development of a specific etch stop layer and substrate removal procedure,” explains Teisser.

He and his co-workers produced two types of QCL: one design was very similar to the laser made in 2013, but employed a modified injector, plus higher doping of the active region to increase carrier dynamics; while the other had a modified active region, which replaces a diagonal intersubband transition with a vertical transition that maximises oscillator strength.

These lasers have a beam divergence that is only a little larger than that of commercial QCLs operating at shorter wavelengths, such as 8 µm. “However, a larger beam divergence can be corrected by a judicious optical system,” says Colombelli, who added that the team is also designing new laser geometries that should reduce divergence.

Compared to the QCL of 2013, the laser with higher doping had a similar threshold current, but a larger current dynamic – and the latter permitted a significantly higher operating temperature. An even higher operating temperature of 291K was possible with the QCL that featured vertical transitions in its active region. To obtain controllable side-mode emission, the team plans to use this laser design to make distributed feedback lasers capable of higher temperature operation.

" We are also planning to exploit the high optical gain of InAs in order to extend the wavelength, ideally up to 30 µm to 32 µm, where QCLs based on GaAs or InGaAs cannot operate, given the presence of optical phonons. InAs phonons have lower energy, hence it is possible to reach these wavelengths,” says Colombelli

D. Chastanet et. al.
Appl. Phys. Lett. 104 021106  (2014)

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