The quantum cascade laser (QCL) is based on inter-subband electron transitions inside a quantum well structure, which can be tailored to emit different wavelengths simply by changing the thickness of the constituent layers. In other words, we are no longer limited by inherent band gaps, and can demonstrate a very versatile source using one material system.

Conduction band schematic of GaInAs/AlInAs quantum cascade laser lattice matched to InP

Tunable Emission Over a Wide Spectral Range

For InP-based heterostructures, we have already utilized this principle to show pulsed laser emission from 3.7-16 μm at room temperature

Demonstrated pulsed laser wavelengths at room temperature

Trace and Remote Chemical Sensing

One of the primary applications for this laser is as a sensor for toxic chemicals, warfare agents, and explosives. Most chemicals have distinctive absorption lines in the mid- to far-infrared (3-16 μm wavelength), which makes quantitative analysis possible through laser absorption spectroscopy.

Example absorption regions in the mid- and far-infrared

Lasers can be used to respond to extremely low concentrations of these chemicals, before a danger is present. Multiple wavelengths can be used to filter out interferents and minimize the possibility of false positives. To help preserve the safety of the most people, such devices should be inexpensive and readily available. A self-contained prototype quantum cascade laser pointer realized at CQD is shown.

Photograph of a self-contained prototype quantum cascade laser pointer realized at CQD

Mid-Infrared Free-Space Communications

One application for quantum cascade lasers is for use in free-space wireless optical communication networks.

Lasers operating in the 3-5 and 8-12 μm wavelength ranges can make use of high atmospheric transmission for long distance applications such as remote chemical sensing, free space optical communication, and laser radar. Because of the long wavelengths these lasers are much less sensitive to bad weather and smoky conditions. Further, room temperature operation allows a more reliable transmitter in a smaller package. By combining high power, long wavelength emission, and room temperature operation into one package, the QCL becomes a clear candidate for realizing a compact, reliable, wireless free-space communications.

Distributed Feedback (DFB) Quantum Cascade Lasers

In order to show single mode output, CQD researchers have successfully fabricated preliminary distributed feedback quantum cascade lasers (DFBQCLs).

This structure uses a diffraction grating either buried within or on top of the ridge waveguide to isolate a single output wavelength. In-house facilities include an ele ctron- beam lithography system and a custom holographic lithography system, which can rapidly fabricate arbitrary grating periods and duty cycles. Isolated gratings and laser ridge examples are shown in the SEM micrographs.

At present, DFB-QCLs emitting at 4.8, 9 and 11 μm have been fabricated and tested. Several hundred milliwatts of continuous (CW) power have been observed at 4.8 μm with single-mode CW operation reported up to 60 °C. An plot is shown below illustrating the power and voltage behavior of this laser for different heatsink temperatures. A spectral plot of the single-mode output is also illustrated and shows the tunability of the single-mode output wavelength as a function of temperature. These devices also demonstrated low threshold current densities in pulsed (0.9 kA/cm²) and continuous mode (1.1 kA/cm²) and a >30 dB SMSR from 15-60 °C.

High Reliability QCLs
Operating at Room Temperature

Initial reliability testing has been performed on two randomly selected strain-balanced QCL samples without any pre-screening or pre-selection. Testing was begun at a typical application condition of 100 mW output power and a heatsink temperature of 300 K. Over 4000 hours of operation was reported in [A. Evans and M. Razeghi, Appl. Phys. Lett. 88, 261106 (2006)] and the lasers continue to operate with over 13,000 hours (1 year and 5 months) of operation recorded to date without any decrease in output power, despite numerous power fluctuations and testing system shutdowns. At 13,000 hours, the output power was increased to 150 mW to increase the stress on the device and record the aging process under increasingly harsh conditions. A plot of output power as a function of ageing time is included below. Demonstration of QCLs under these harshest of conditions are designed to be robust templates with the most versatility for any application.

Reliability testing of two randomly selected QCls: CW Output power as a function of ageing time

The P-I-V (power, current, voltage) characteristics are illustrated below before ageing (Time = 0) and after 12,000 hours of constant ageing. Comparison of the curves show nearly identical behavior of the laser diode before and after significant ageing time.

Reliability testing of two randomly selected QCls: P-I-V characteristics before and after ageing for sample B

High Performance QCLs
Operating at Room Temperature

At the Center for Quantum Devices, all lasers structures are grown in single growth step using gas-source molecular beam epitaxy (GSMBE). We have generated the highest power single -stripe (25 μm by 3 mm) emitters with 7 W peak power at room temperature at a wavelength of 9 μm. Power usage is also important, and, to help reduce battery drain, we have also demonstrated the lowest threshold current density for QCLs at room temperature (1.1 kA/cm²), which allows our devices to turn on with less power.

One of the main limitations for the quantum cascade laser at higher duty cycles is removal of waste heat. Similar to near-infrared lasers, the performance can theoretically be enhanced through more sophisticated fabrication and packaging techniques. Further, through use of highly-reflective (HR) coatings, the light can be re-directed to come out from a single mirror facet, which is more convenient from a system perspective.

Electrical and optical characteristics of a typical 9 μm quantum cascade laser operating in pulsed mode at room temperature. Peak output power of 2.5 W is the highest power for a quantum cascade laser in these conditions

Using advanced fabrication techniques, such a buried heterostructure QCLs and electoplated Au heat spreaders, we have further improved the power and efficiency of CW QCLs dramatically in recent years. At room temperature, we have now demonstrated over 600 mW of power for a single device and have achieved power efficiencies >5%. In pulsed operation, average power has been demonstrated up to 0.85 W at the same temperature

Cross section image of a buried-ridge QCLlaser, left. Cross section image of a Au electroplated QCL, right

Highest average power QCL

Highest power continuous wave room temperature quantum cascade laser

If room temperature is not a requirement, cryogenic cooling is also an option for these lasers. With cooling, the laser performance can increase dramatically, with up to 1.4 W of continuous power available at >100 K.

Cryogenic testing of CQD QCLs in continuous wave. Measurements courtesy of Naval Research Laboratory.

As mentioned previously, wavelength diversity is one of the key advantages of QCL technology. As such, significant effort was dedicated to achieve strong CW performance at both shorter and longer wavelengths than other groups have demonstrated. The CQD is currently the only research group, university or otherwise, to have demonstrated continuous wave (CW) operation of QCLs at room temperature and above with the highest output powers over a broad range of wavelengths from 3.8 μm to 11.5 μm.

Maximum continuous wave output power for QCLs at room temperature as a function of wavelength

Last Updated 03/24/2007

Band Structure Engineering