Quantum cascade lasers shape up for trace-gas sensing

CLAIRE GMACHL, FEDERICO CAPASSO, RAFFAELE COLOMBELLI, ROBERTO PAIELLA, DEBORAH L. SIVCO, and ALFRED Y. CHO

Quantum cascade (QC) lasers are semiconductor injection lasers based on intersubband transitions in a multiple quantum-well heterostructure. They are designed by means of band-structure engineering and grown by molecular beam epitaxy.¹ Since their invention only seven years ago at Bell Labs, they have reached an impressive level of maturity, demonstrated both by their recent transition into the marketplace and by the large number of research groups conducting related research worldwide (see "Quantum cascade lasers find use in trace-gas sensing applications").

Quantum cascade lasers grown in the indium gallium arsenide/aluminum indium arsenide material system have, until now, been fabricated for emission wavelengths as short as 3.5 µm and, recently, as long as 24 µm.² The wavelength coverage of these lasers extends nearly continuously through the mid-infrared and reaches into the far-infrared (see Fig. 1). This large range means that the primary application of these lasers is trace-gas sensing. Many trace gases have telltale absorption features in this wavelength range that result from vibrational and mixed rotational-vibrational transitions. To qualify as a superior source for such sensing applications, a laser needs to fulfill several requirements. It must be single mode and continuously tunable, must display a narrow linewidth (relative to the width of the trace gas absorption exotic lasers in question), and—for reasons of economics—must operate at high temperature, with good long-term stability and reliability. Quantum cascade lasers excel under all those requirements.

FIGURE 1. Transmission trace of the open atmosphere reveals two windows of atmospheric transmission, one between 3 and 5 µm and the other between 8 and 13 µm, along with the wavelength coverage of quantum cascade (QC) lasers to date (top). Emission wavelength as a function of the continuous-wave current at various constant heat-sink temperatures was measured for a 7.9-µm-emitting QC-DFB laser (bottom). The symbols represent measured data; the lines are quadratic functions fitted to the data.

Continuously tunable single-mode operation

Single-mode operation is achieved through integration of a Bragg grating into the laser waveguide, resulting in a distributed feedback (DFB) laser. The latest generation of QC-DFB lasers is based on a so-called "top-grating" approach that takes particular advantage of the characteristics of the mid-infrared waveguide (see Fig. 2).³ For wavelengths between 3.5 and approximately 15 µm, the latter is commonly a dielectric waveguide built from low-doped semiconductor layers that have the appropriate refractive-index contrasts.

FIGURE 2. Mode-intensity profiles are shown of a QC-DFB laser based on the top-grating approach; blue and red indicate the grating ridges and grooves, respectively (top left). The design wavelength is approximately 8.0 µm. Yellow bars indicate the metallization, while pink shading points to the surface-plasmon-enhanced Bragg modulation of the waveguide. Also shown is a mode-intensity profile of a surface-plasmon waveguide designed for a 17-µm wavelength (top right). The stacks of active regions and injectors are indicated through a shaded bar and the dashed lines show the respective substrate-epilayer interfaces. Calculated conduction band profile of two chirped active regions and interleaved injector regions are for a laser designed for operation at 24-µm wavelength (bottom). The green shaded areas indicate the extent of electron minibands. The laser transition occurs between levels 2 and 1 (red wavy arrow).

If such a waveguide—with tight confinement to condense long-wavelength light in a reasonably thin layer stack—is directly overlaid with metal (for example, top-contact metallization), then light is guided not only by the dielectric contrast of the semiconductor layers but also by the surface plasmon mode. The latter propagates at the metal-semiconductor interface. A pure surface plasmon waveguide is inherently more lossy than a dielectric waveguide; however, it can be designed with additional refractive-index contrast to display a much tighter optical confinement than the latter.

Combined with reduced penetration depth into metal at longer wavelengths, a surface plasmon waveguide is in fact favorable for longer wavelengths—those longer than approximately 15 µm. At the shorter wavelengths, however, the coupling to the surface plasmon waveguide can be suppressed by a topmost semiconductor waveguide layer so highly doped that it displays the anomalous dispersion of the plasma edge and a concomitant strongly reduced refractive index of less than or equal to 1.

The QC-DFB top grating is etched into this topmost layer such that the grating grooves fully penetrate through the layer while the grating ridges remain unetched. If this grating structure is overlaid with metal, a periodic coupling to the surface plasmon mode results for the guided mode, with strong periodic modulations. Single-mode operation results then from Bragg scattering off this grating, and continuous tunability is achieved through the temperature dependence of the above mentioned waveguide parameters. The temperature can either be varied by a temperature change of the heat sink and mounted device or—more rapidly—by an added direct current through the device and dissipative heating. Characteristic total tuning ranges per current sweep are around 0.3% to 0.5% of the emission wavelength, which permits the scanning of several (often up to 10) isolated absorption features of a trace gas.

In addition to continuous tunability, the narrowness or monochromaticity of the laser emission is a crucial factor. Coworkers at Pacific Northwest National Labs (Richland, WA) and JILA/NIST (Boulder, CO) have used known gas absorption features or high-finesse Fabry-Perot cavities to determine the linewidth of several QC-DFB lasers as being a few 100 kHz when free-running and a few kHz when frequency-stabilized.⁴ Such narrow lines are clearly atypical of semiconductor lasers and illustrate the uniqueness of the intersubband nature of light generation.

The remaining requirements of device reliability and long-term stability are fulfilled through the choice of well-tested and reliable materials, such as indium phosphide and gallium arsenide-based heterostructures. Finally, the operating temperature of QC lasers has long reached above room temperature for pulsed operation and encouraging progress has been made recently for continuous-wave operation as well.⁵

Far-infrared QC lasers

It is a key strength of the QC concept that the emission or peak gain wavelength can be chosen freely within a very wide range by design of layer thicknesses. A firm boundary only exists at the short wavelength side given by the band offset between the quantum wells and barriers of the respective materials. For ever-longer wavelengths, however, the question remains whether enough gain can be generated to overcome the increasingly larger waveguide loss. In general, free carrier losses even in the very low-doped semiconductor layers are noticeable. As a solution, a surface-plasmon waveguide maximizes overlap of the guided mode with active material and minimizes overlap with passive and lossy waveguide layers. This approach was taken in the most recent realization of the first far-infrared QC lasers at 21.5- and 24-µm wavelengths.² The active material was composed of so-called "chirped" superlattice active regions interleaved with injector regions, making optimal use of the cascading scheme by stacking up to 65 active regions and thus multiplying the modal net gain accordingly. The lasers operated in pulsed mode up to approximately 150 K heat-sink temperature and with peak output power levels of a few milliwatts. Threshold current density values of greater than 4 kA/cm² were obtained.

Similar to their mid-infrared counterparts, the long-wavelength QC lasers are expected to be used in trace-gas sensing—in particular of heavy molecules—and potentially as local oscillators for spectroscopy applictions in astrophysics.

Short-pulse operation of QC lasers

Most physical systems can be described on two time scales corresponding to their steady state and dynamic behaviors. The invention of an ultrafast (picosecond to femtosecond) light source in a certain spectral range typically opens up an entire new range of phenomena related to that time scale and frequency range. One prime example has been the Ti:sapphire laser operating at around 1 µm. It is therefore highly desirable to develop comparable short-pulse and high-power sources that operate in the mid-infrared wavelength range and beyond.

The electron dynamics at the heart of QC lasers occur on a picosecond time scale, sped up by optical phonon scattering between electron subbands. The QC laser therefore lends itself to ultrafast modulation and short-pulse emission. Recent work has demonstrated gain switching with pulse widths of approximately 50 ps, and active and passive modelocking leading to 3- to 5-ps short pulses with approximately 12-GHz repetition rate and an average power output of a few tens of milliwatts at both investigated wavelengths of 5 and 8 µm.⁷ Such ultrafast QC lasers should in the near future become sources for time-resolved mid-infrared spectroscopy.

ACKNOWLEDGMENT
The authors would like to acknowledge the help of M. I. Blakey, A. L. Hutchinson, A. M. Sergent, and S. N. G. Chu with device processing and characterization, and they thank H. C. Liu of NRC, Ottawa, Canada, for the loan of high-speed QWIP detectors. This work was partly supported by DARPA/US ARO under contract number DAAD19-00-C-0096.

REFERENCES

F. Capasso et al. IEEE J. Select. Topics Quant. Elect. 6, 931 (November/December 2001) and references therein.
R. Colombelli et al., Appl. Phys. Lett. 78, 2620 (2001).
R. Köhler et al. Appl. Phys. Lett. 76, 1092 (2000).
R. M. Williams et al., Opt. Lett. 24, 1844 (1999) and private communications.
D. Hofstetter et al., Appl. Phys. Lett. 78, 396 (2001) and Appl. Phys. Lett. 78, 1964 (2001).
C. Gmachl et al., Reports on Progress in Physics, in preparation.
R. Paiella et al., Science 290, 1739 (2000) and references therein.

Claire Gmachl, Federico Capasso, Raffaele Colombelli, Deborah L. Sivco, and Alfred Y. Cho are researchers at Bell Laboratories, Lucent Technologies, 600 Mountain Ave., Murray Hill, NJ 07974; e-mail: [email protected]. Roberto Paiella was with Bell Laboratories and is now a researcher at Agere Systems, Murray Hill, NJ.

Quantum cascade lasers find use in trace-gas sensing applications

Soon after single-mode tunable QC-DFB lasers were first fabricated, they were put to work in trace-gas sensing applications. Currently, more than ten research groups have reported successful demonstrations of the latter using a large variety of spectroscopic methods. The lasers are typically used under two quite distinct operating conditions: in continuous-wave mode at liquid nitrogen temperatures, or pulsed close to room temperature. The former has the advantage of a very narrow linewidth essential for high-resolution spectroscopy of trace gases at low pressure. Tunable QC-DFB lasers operated in pulsed mode have a broadened emission line because of the thermal chirp from the current pulse; they are, nevertheless, useful for the spectroscopy of pressure-broadened gas samples. The following represents a partial sampling of recent and very successful trace-gas sensing experiments conducted by various groups, all using QC-DFB lasers.

A. A. Kosterev, F. Tittel, and colleagues at Rice University (Houston, TX) developed a variable duty-cycle, quasi-continuous-wave frequency scanning technique for QC-DFB lasers that mitigates many of the thermally induced effects of pulsed operation.¹ Combined with a 100-m multipass gas cell and zero-air background subtraction, a detection sensitivity close to the parts-per-billion concentration level was achieved (see figure at top of this page). Around the 7.9-µm wavelength, various natural constituents of air were measured as well as ethanol vapor.

Another group at NASA, JPL, and Caltech (Pasadena, CA) reported the first atmospheric measurements using QC-DFB lasers.² The devices were flown on NASA's ER-2 high-altitude aircraft to monitor stratospheric methane and nitrous oxide over North America, Scandinavia, and Russia. The sensitivity limit of the wavelength modulation based technique was about 2 parts per billion for methane.

A group at Ford Research Laboratories (Dearborn, MI) used a continuous-wave QC-DFB laser emitting at 5.2 µm to obtain sub-Doppler-resolution-limited saturation features in a Lamb-dip experiment on nitrous oxide.³ The Lamb dips appear as transmission spikes with full-width half-maximum values around 4.3 MHz. This makes the 73-MHz wide L-doubling of the R(13.5)3/2 line easily visible beneath the 130-MHz Doppler broadening of the line.

Researchers at Physical Sciences Inc. (Andover, MA) used a multimode Fabry-Perot type QC laser around the 8.0-µm wavelength for quantitative backscatter absorption measurements on isopropanol vapor.⁴ They developed and employed a pseudo-random code modulation of the QC laser and explored its use for differential absorption lidar. Even a rather simple measurement configuration resulted in a detection limit of 12 ppmv or an absorption of about 3 x 10-3. Another group at Physical Sciences Inc. used QC-DFB lasers around 5.4 µm in quasi-continuous-wave mode close to room temperature in conjunction with a balanced ratiometric detection technique to achieve high sensitivity.⁵ Researchers have also demonstrated the first use of a GaAs-based QC laser for gas-sensing applications.⁶ Last, QC-DFB lasers are used in cavity ring-down and photo-acoustic spectroscopy.

REFERENCES

A. A. Kosterev et al., Appl. Opt. 39, 4425 and 6866 (2000).

C. R. Webster at al., Appl. Opt. 40, 321 (2001).

J. T. Remillard et al., Optics Express 7, 243 (2000).

C. M. Gittins et al., Opt. Lett. 25, 1162 (2000).

D. M. Sonnenfroh and coworkers, Appl. Opt. 40, 812 (2001).

L. Hvozdara et al., Appl. Opt. 39, 6926 (2000).