Band Structure Engineering

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, 7.8, 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 2,000 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 21,000 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 A

High Peak Power QCLs at Room Temperature

At the Center for Quantum Devices, we have demonstrated high peak power single emitters at room temperature by investigating power scaling.    Some of this work is shown in the figure below for shorter wavelength lasers.  For pulsed mode operation, the output power is nearly proportional to the ridge width.  For devices with a ridge width smaller than 50 μm, the WPE increases with the ridge width, mainly due to the decrease of waveguide loss. Above 50 μm we start to see a decreasing of the WPE. The internal heating in pulsed mode and high order transverse mode oscillation for extremely wide ridge widths are possible reasons for this behavior. In fact, the far field for the 100-μm- and 200-μm-ridge-width devices are distinctively double lobed, which is a signature of a high order transverse mode. Despite the slight reduction in WPE, the maximum total peak power for the 200-μm-ridge-width device still reaches 34 W at room temperature, which can be very attractive for applications that need a high power pulsed source. 

Room temperature pulsed mode operation of a 4.86 μm QCL with a 200 μm ridge width. The inset shows the maximum WPE and peak power for a number of devices with different ridge widths.

Similar work has also been done at longer wavelengths.  At a 10 μm wavelength, the power was scaled by increasing the number of emitting stages and the core doping level.  For a 75-stage emitter and a 95 μm ridge width, up to 25 W of peak power was observed from a single emitter, as shown below.  While the output far-field wasn't diffraction-limited, most of the power was in a single spatial lobe.

Photonic Crystal Distributed Feedback QCLs 

 One of our newer efforts is to combine simple power scaling with photonic crystal diffractive feedback.  In the section above above, the peak output power was very high, but this came at the expense of poor beam quality.  In addition, because the laser cavities used were simple Fabry-Perot (FP) cavities, they were also intrinsically multimode spectrally.  By combining a wide laser cavity with a two-dimensonal photonic crystal, the peak output power can still be scaled, but the laser far-field and emitting spectrum can remain single mode.

The photonic crystal distributed feedback (PCDFB) QCLs we have demonstrated can start with the same laser core and grating host layer used for standard DFB lasers described above.  After the grating layer is patterned with the 2-D photonic crystal structure, the waveguide top cladding are regrown directly on top, burying the diffractive structure within the waveguide for maximum coupling and minimal excess loss.  A schematic of the structure is shown below. 

 

Top: Schematic layer structure of PCDFB QCL.  Bottom: Top, oblique, and side view of the PCDFB pattern in PMMA, grating host layer, and finished heterostructure.

Though still very early in the development cycle, and despite the complexity of the laser design and fabrication, we have had some success with these devices.  The figures below show far-field and spectral comparisons between standard wide-ridge FP lasers and PCDFB lasers.  As predicted, the beam quality and spectral purity can be significantly enhanced.  Though inital lasers had only a few Watts of power, recent testing has demostrated, for wavelength near 4.8 μm, peak output powers near 12 W for 3 mm-long cavities with a width around 100 μm.

 

Comparison of FP and PCDFB (a) far-field and (b) emitting spectra.

High Power and High Efficiency Continuous Wave QCLs

One of the main limitations for the quantum cascade laser at higher duty cycles is removal of waste heat. This makes wide cavity designs impractical for high average or continuous output power operation.  In the following sections, we detail other mechanisms for improving device output power using narrower ridges that have intrinsically better beam quality.

Similar to near-infrared lasers, the CW laser performance can theoretically be enhanced through more sophisticated fabrication and packaging techniques. Using advanced fabrication techniques, such a buried heterostructure QCLs and electoplated Au heat spreaders and epilayer-down bonding of laser dies onto diamond submounts, we have further improved the power and efficiency of CW QCLs dramatically in recent years.

 

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

Besides basic packaging, significant effort has also been placed on improving the intrinsic wallplug (power conversion) efficiency of the QCL.  After analyzing various sources of inefficiency in our 4.6 μm lasers, several key problems were identified that could be addressed.  We are currently tackling these problems and have already shown dramatic (2-4 times) improvement in our wall plug efficiency over the last 18 months.

As mentioned above, 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.  The figure below represents a summary of our maximum cw output power at room temperature at various wavelengths.

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

As a highlight of this work, shown below are the power and efficiency characteristics of a buried ridge 4.6 μm laser bonded epilayer-down on a diamond submount.  2.8 W of power is measured with an power conversion efficiency of 15%.

Power and efficiency curves for buried ridge 4.6 μm QCL on a diamond submount.

Low Temperature Testing Results

Besides demonstrating very good performance at room temperature, our lasers show even higher power and efficiency when cooled to 80 K.  Shown below is the output characteristics of a wide ridge 4.6 μm laser in continous wave at 80 K.  10 W of power is output and a wallplug efficiency of 33% is demonstrated.

Power and efficiency curves for a mid-infrared QCL at 80 K.

Last Updated 03/20/2009