Quantum Cascade Lasers

Semiconductor lasers are an important light source for fiber-optic communications and are key components of such common appliances as compact disc players, supermarket scanners, laser printers, fax machines, and laser pointers. The lasers in these applications are so-called double heterostructure lasers, essentially diodes consisting of an active semiconductor region sandwiched between doped semiconductor cladding layers, one n-type, the other p-type. The cladding regions supply electrons and holes to the active region when an appropriate bias voltage is applied. They also have a higher bandgap and a lower refractive index than the active layer, so that the injected electrons and holes as well as the photons generated by their annihilation are confined to the active region. The share of the 2000 Nobel Prize in Physics awarded to Zhores Alferov and Herbert Kroemer for their role in developing the double heterostructure laser recognized the major impact of this device (see Physics Today, December 2000, page 17 ).

Double heterostructure lasers have demonstrated high performance and have been successfully commercialized for wavelengths ranging from blue to the near infrared (IR), up to about 1.6 μm. Unfortunately, few semiconductor materials emitting in the mid-IR (2–20 μm) are reliable, easily processed, and insensitive to temperature cycling—the repeated heating and cooling associated with laser operation.

The scientifically and technologically interesting mid-IR, often called the molecular-fingerprint region, is the part of the spectrum where gases and vapors have telltale absorption features associated with vibrorotational transitions—that is, those in which the vibrational and, generally, rotational quantum numbers change. These features can be mapped by a number of spectroscopic techniques in a wide range of industrial, military, and scientific applications. We discuss some applications later in this article.

Semiconductor diode lasers made of lead salts and emitting in the mid-IR have been commercially available for twenty years or more. Lead-salt lasers, however, have limited power (at most a few milliwatts of peak and continuous wave power), have a small continuous single-mode tuning range, and have yet to operate at room temperature. ¹ They also suffer from spectral degradation and reliability problems associated with thermal cycling.

In 1971, Rudolf Kazarinov and Robert Suris of the Ioffe Physico-Technical Institute in St. Petersburg, Russia, proposed that optical amplification could occur between the quantized electronic states of a multiple quantum-well structure under a high electric field. ² In the late 1970s, scientists at Bell Labs began a 15-year research effort that concentrated on designing and growing artificially structured semiconductor materials and devices of ever increasing complexity and functionality. ³ (See the article “Quantum Electron Devices,” by Federico Capasso and Supriyo Datta, Physics Today, February 1990, page 74 .) This effort culminated in 1994 with the invention of the quantum cascade (QC) laser. ⁴ At present, more than 20 groups worldwide are working on QC lasers.

The QC laser is a light source that fills the increasing need for compact and high-performance mid-IR semiconductor lasers. ^5,6 It is a high-power laser, radically different from double heterostructure lasers, and can be designed to emit essentially any wavelength in the mid-IR spectrum, extending into the far IR. ^5,6 QC lasers in pulsed operation at room temperature generate peak optical powers of up to hundreds of milliwatts concentrated in a single laser mode, and they can be continuously tuned over a significant wavelength range. Recently, continuous wave (cw) operation at room temperature has been reported. ⁷ Less than a decade after their invention, QC lasers greatly outperform mid-IR diode lasers and are commercially available. ⁸ They have been successfully used in trace gas analysis with a sensitivity of parts per billion in volume. ^5,6,8,9 Portable QC-laser-based sensors are opening up new market opportunities. ⁸

Band-structure engineering

In lasers, the chemical composition of the active material determines the energy levels between which laser action occurs. This simple but important point is reflected in conventional semiconductor lasers in that substantially changing the emitted wavelength requires selecting other active-region materials with different bandgaps. QC lasers are fundamentally different from conventional lasers in that their wavelengths can be programmed over an unprecedented range by choosing the thickness of the materials in the active region: The composition of those materials needn’t change at all. For example, QC lasers using active-region layers of the semiconductor alloys aluminum indium arsenide and gallium indium arsenide, lattice matched to an indium phosphide substrate and having suitably designed thicknesses, have demonstrated emission wavelengths ranging from 4 to 24 μm. ^4–6

To understand how thickness can determine wavelength, note that in QC lasers the optical transition does not occur across the bandgap as it does for double heterostructure lasers but, rather, occurs between discrete electronic states within the conduction band. These states arise from the quantization of electron motion in the active region’s nanometer-thick layers. To good approximation, electrons move freely in the direction parallel to these layers: Discrete energy levels arise from quantizing electron motion in the normal direction. The layers are called quantum wells by analogy to the potential wells posited in the well-known particle-in-a-box problem of introductory quantum mechanics. In particular, in both cases, the energy levels depend on the width of the well. The energy levels for a quantum well structure can be obtained by numerically solving the Schrödinger equation. By adjusting the width and shape of the quantum wells, one can design an energy-level difference that leads to a desired emitted wavelength. Likewise, one can maximize the matrix element of the optical transition and optimize the lifetimes of the quantum-well states to achieve the population inversion required for laser action.

Designing QC lasers is therefore a sophisticated exercise in band-structure engineering. Fabricating QC lasers requires molecular beam epitaxy, a crystal growth technique capable of depositing thin films down to a thickness of one molecular layer. Band-structure engineering combined with molecular beam epitaxy has played a key role in the invention and development of many modern semiconductor devices and materials with tunable electronic and optical properties. ³

In QC lasers, an electron remains in the conduction band after emitting a laser photon. The electron can therefore easily be recycled by being injected into an adjacent identical active region, where it emits another photon, and so forth. To achieve this cascading emission of photons, active regions are alternated with doped electron injectors and an appropriate bias voltage is applied. The active-region-injector stages of the QC laser give rise to an energy staircase in which photons are emitted at each of the steps. The number of stages typically ranges from 20 to 35 for lasers designed to emit in the 4–8 μm range, but working lasers can have as few as one or as many as 100 stages. ⁶ The cascade effect is responsible for the very high power that QC lasers can attain. Lasers emitting at wavelengths from 5 to 12 μm have generated up to 1 W in pulsed mode at room temperature. Similar power levels are obtained at the same wavelengths for cw operation at liquid-nitrogen temperature.

Figure 1(a) shows a micrograph of a cross section of a QC laser, displaying several stages of active and injector layers. Figure 1(b) is an electron microscope image of a portion of the same QC laser; note the diffraction grating etched on its upper face. Absent such a grating, a QC laser would generally emit many closely spaced wavelengths— called longitudinal modes—separated in frequency by the reciprocal of the roundtrip time of light in the laser cavity. The period of the grating, d, selects the wavelength that satisfies the Bragg condition λ = 2n _eff d, where n _eff is the effective refractive index of the waveguide. Light whose wavelength satisfies the Bragg condition is strongly reflected off the grating and so is selected for laser action. Lasers that exploit this Bragg reflection are called distributed feedback (DFB) lasers. They are widely used in telecommunications and have enabled QC lasers to achieve single-mode operation with extremely high rejection ratio (greater than 1000:1) of the other modes. ^5,6,9 Wavelength tuning in QC lasers is achieved by increasing the temperature of the active region either with a temperature controller or by using a sawtooth current waveform to drive the laser. ⁹ Raising the temperature increases the effective refractive index and thus also the emission wavelength.

Optical-transition design

Box 1 discusses the energy band diagram of a QC laser designed to emit at a wavelength of about 4.65 μm. The laser, which has 25 stages, operates in pulsed mode at room temperature. Its peak power exceeds 100 mW in a single mode near the design wavelength when operating in a DFB configuration. Such a power level is typical of room-temperature pulsed QC lasers of similar design operating at wavelengths up to about 10 μm. Even higher power levels (200–300 mW) are achieved when the lasers are operating without a grating. The impressive power outputs of QC lasers are due not only to the cascade effect, but also to their ability to carry large currents: The materials used in QC lasers have wider energy gaps than those used in mid-IR diode lasers. The energy gaps allow QC laser materials to transport currents that would damage devices made of narrow-gap semiconductors such as lead salts.

Jerome Faist’s group at the University of Neuchâtel in Switzerland has reported a new QC laser design that operates at record-high temperatures. ¹⁰ The active region of their laser has four quantum wells and a double phonon resonance—that is, three energy levels equally separated by the energy of an optical phonon instead of the two levels discussed in box 1. This active-region design results in a larger population inversion because electrons are more efficiently removed from the lower state of the laser transition. The Faist group’s pulsed laser had a peak power of 100 mW in a single mode at 5.3 μm when operating in the DFB configuration at 390 K. The same group used its new active-region design to achieve the first room-temperature cw operation of a QC laser. Their device ⁷ emitted 17 mW of power at 9.1 μm. This breakthrough will extend even further the range of applications of QC lasers.

Carlo Sirtori and his group at Thales (formerly Thompson–CSF) have spearheaded the development of aluminum gallium arsenide/gallium arsenide (AlGaAs/GaAs) heterostructure QC lasers. Last year they reported room-temperature operation of a laser emitting at 9.4 μm in pulsed mode. ¹¹

For lasers emitting wavelengths longer than about 10 μm, active regions designed differently than described in box 1 yield superior performance. These active regions contain many quantum wells coupled by very thin barriers. ^5,6 That is to say, the active regions are superlattices similar to the ones introduced in box 1. As explained there, electronic states form minibands separated by minigaps. In superlattice QC lasers, the optical transition is between the bottom and top of two adjacent minibands so that the laser frequency is determined by the width of the separating minigap. That width depends on the thickness of the layers making up the superlattice. Box 2 shows the design of a superlattice QC laser emitting at a wavelength of about 24 μm and discusses several advantages of the superlattice design. Lasers fabricated using this design with 65 stages emitted up to 2 mW of optical power inpulsed mode at 100 K.

Several groups have been working on superlattice QC lasers. Erich Gornik and his group at the Technical University of Vienna have successfully built AlGaAs/GaAs superlattice QC lasers. Manijeh Razeghi and her collaborators at Northwestern University demonstrated high output power for AlInAs/GalnAs superlattice QC lasers emitting at a wavelength of 11 μm: They obtained more than 1 W peak power at room temperature. Alessandro Tredicucci and his collaborators at the Scuola Normale Superiore in Pisa, Italy, ran a superlattice QC laser at a wavelength of about 70 μm. This wavelength is greater than the 30-μm maximum wavelength obtained to date with lead-salt diode lasers. ¹

Band-structure engineering offers the opportunity to design laser materials with properties not easily found in bulk semiconductors. We discuss two applications in box 3: a QC laser engineered so that laser action can occur simultaneously at two substantially different wavelengths ⁵ and a QC laser capable of emitting different wavelengths under opposite bias voltage. ¹²

Earlier this year, our group at Lucent Technologies reported a new QC laser that emits a continuum of wavelengths from 6 to 8 μm, with comparable power at all wavelengths. ¹³ The design innovation that leads to the continuum generation is to make active regions in different stages have different quantum-well thicknesses. In spectroscopic applications, one can select individual wavelengths from the continuum with an external grating.

Narrow linewidth and high-speed operation

The linewidth of QC lasers is important from both a fundamental and an applied point of view. For structures of the type discussed in box 1, the predicted linewidth is intrinsically much narrower than that of conventional semiconductor lasers. ⁴ This phenomenon is related to the optical transition being between discrete localized electronic states in the conduction band (similar to an atomic transition) rather than between the conduction and valence bands as in a conventional semiconductor laser. QC lasers operating continuously at a wavelength of 8.5 μm in the DFB configuration have achieved linewidths of around 10 kHz. ¹⁴

QC lasers can potentially be directly modulated at much higher frequencies than can diode lasers, whose bandwidth is typically limited to about 20 GHz. ⁵ The reason is that the intrinsic response of the active regions to variations in the photon density in the cavity is limited by the electron lifetimes in the upper state. These electron lifetimes are comparable to the photon lifetime (the roundtrip time of light in the laser divided by the transmission coefficient of the laser facets), so intrinsic bandwidths ranging from a few hundred gigahertz to 1 THz should be possible. ⁵ The short intrinsic response time of the QC laser material makes these light sources particularly well suited for generating ultrashort mid-IR pulses. In recent experiments, our group has generated picosecond pulses in QC lasers emitting at 5 and 8 μm, with repetition rates on the order of 10 GHz. ^5,6

Quantum cascade laser applications

The two wavelength regions (3-5 μm and 8-13 μm) in which the atmosphere is relatively transparent due to lack of water-vapor absorption are particularly important for chemical-sensing applications. These windows allow the measurement of traces of environmental and toxic gases or vapors that would otherwise be masked by a large atmospheric water-vapor background. Laser-based optical methods in trace-gas analysis and chemical sensing have a number of advantages, including their noninvasive nature, high sensitivity and selectivity, and real-time detection.

A variety of spectroscopic methods can determine the local concentration of a species with parts per billion in volume (ppbv) sensitivity. For example, in point sensing, a sample of air is introduced into a chamber in which laser light undergoes many reflections to increase its optical path. The large path length combined with direct-absorption spectroscopy or wavelength-modulation spectroscopy (in which the first or second derivatives of the spectrum are measured) allows one to determine the local concentration of a species with high accuracy and sensitivity.

Industrial uses of chemical sensing include combustion diagnostics in the power and automobile industries; medical diagnostics such as breath analysis for the early detection of ulcer, diabetes, colon cancer, and other diseases; and process control. Military and law enforcement applications include the detection of explosives, emissions from illicit drug production sites, and chemical and biological weapons of mass destruction. (QC lasers can also be used for such military countermeasures as blinding the IR sensor of a heat-seeking missile.) Atmospheric science uses spectral data in the mid-IR to determine chemical concentration profiles, which are important for the development of reliable global climate models.

Most of these applications require tunable singlemode mid-IR lasers with power levels sufficient to obtain a good signal-to-noise ratio. Commercial devices such as field point sensors also need to be portable, so they require compact, room-temperature battery-operated light sources. Existing mid-IR sources barely meet these requirements, but QC lasers readily satisfy them and have already been used in many trace-gas analyses. ^{6,8,9,15–17} In a typical spectroscopic measurement used in these analyses, the temperature of the laser is adjusted so that the emitted wavelength is initially positioned on the short-wavelength side of a particular absorption peak. Then the wavelength is repetitively scanned across the peak using a low frequency (100–10 000 Hz) current ramp.

Chris Webster and coworkers at the Jet Propulsion Laboratory flew one of our QC lasers aboard a NASA ER-2 high-altitude aircraft to measure trace concentrations of atmospheric gases including methane and nitrous oxide (N₂O). ¹⁵ (See figure 2(a) for a photograph of the aircraft.) The cryogenically cooled laser operated near 8 μm in the DFB configuration. The air surrounding the NASA craft was sucked into a multipass gas cell aboard the plane over the course of eight-hour long flights. Then it was analyzed by derivative spectroscopy so as to map the vertical profile of trace gases. From September 1999 to March 2000, Webster and company took measurements of air over North America, Scandinavia, and Russia during a series of 20 flights achieving stratospheric altitudes as high as about 20 km. Methane data for one flight are graphed in figure 2(b): The detection limit, or sensitivity, of the measurements is about 2 ppbv.

In 2001, Anatoliy Kosterev, Frank Tittel, and Robert Curl (Rice University) along with collaborators from Physical Sciences Inc (PSI) demonstrated the first application of a QC laser for continuous monitoring of carbon monoxide in ambient air. A typical day’s data are shown in figure 3. The Rice group measured CO concentrations using direct absorption spectroscopy over a path length of 1 m. The sensitivity they achieved, 12 ppbv, is much greater than that obtained with standard nonoptical methods and holds out promise for the commercial development of QC-laser–based CO sensors.

The Rice group has pioneered trace-gas detection in open air with near–room temperature QC lasers, measuring concentrations near one part per million in volume (ppmv) for CH₄ and HDO, and less than one ppbv for N₂O. ¹⁶ PSI is developing QC-laser–based compact sensors for nitric oxide (NO) as well as for sulfur dioxide and sulfur trioxide, both important pollutant species emitted by aircraft engines. ⁸ They have achieved NO detection sensitivities in the lab of 55 ppbv. ¹⁷

Bill Weber’s group at the Ford Research Laboratory in Dearborn, Michigan, has used wavelength-modulated QC lasers from our group, combined with a multipass absorption cell, to measure concentrations of nitrogen oxides (NO _x ), which are pollutants in vehicle exhaust. Certification of future ultralow-emission vehicles will require measurements of NO _x concentrations at levels below 1 ppmv. Weber’s group measured concentrations of a few ppbv in diluted exhaust-gas bag samples collected during vehicle certification.

QC lasers may potentially be applied for optical wireless communications in the eye-safe atmospheric transmission windows at wavelengths of 3–5 μm and 8–13 μm. Losses associated with atmospheric light scattering at these wavelengths are many orders of magnitude lower than they are for visible light, particularly in conditions of poor visibility such as fog. Recently we used a QC laser emitting at 8.1 μm to transmit complex data (multimedia satellite channels comprising 800 television channels and 100 radio channels) over a distance of 200 m.

The authors wish to thank Mark Allen, James Baillargeon, Raffaele Colombelli, Robert Curl, Jerome Faist, John Hall, John Hartman, Albert Hutchinson, James Kelly, Anatoliy Kosterev, Rainer Martini, Tanya Myers, Roberto Paiella, Gaetano Scamarcio, Carlo Sirtori, David Sonnenfroh, Matthew Taubman, Frank Tittel, Alessandro Tredicucci, Michael Wanke, Christopher Webster, Edward Whittaker, Richard Williams, and Richard Zare for collaborations and discussions.