Thomas Boggess Research

Mid-Infrared Semiconductor Diode Laser Development:

While most of the measurements we perform on narrow band gap semiconductor superlattices and quantum wells are aimed at improving our fundamental understanding of these materials, a practical goal is to use this information to develop the next generation of electronic and optoelectronic devices. Over the past seven years, much of our effort has been directed toward the development and improvement of mid-IR diode lasers. Potential applications for these lasers include remote sensing of pollutants and trace gases, medical diagnostics, IR spectroscopy, and IR countermeasures. These lasers are plagued, however, by Auger recombination, a nonradiative carrier recombination process that tends to increase with increasing temperature and decreasing band gap energy. Hence, this process is a severe impediment to room temperature operation of mid-IR diode lasers. Another process of concern is intersubband absorption at photon energies near that of the band gap. This process is associated with, e.g., optical excitation of an electron in a low-lying valence band to the uppermost valence band. Intersubband absorption in laser materials is only relevant under nonequilibrium conditions and is of concern only when the absorption occurs near the band edge, i.e., when it can provide loss at the lasing wavelength. Finally, since many of the current designs for mid-IR lasers rely on many layers of semiconductors with widely differing band gaps and large band offsets at the interfaces, carrier injection into these structures can be a problem. We are currently using ultrafast optical techniques, such as those discussed above, to examine all of these problems in an attempt to understand them and ultimately suppress them.

Our measurements are conducted in close collaboration with Professors Hasenberg and Flatté of the Department of Physics & Astronomy and Professor Winston Chan of the Department of Electrical & Computer Engineering. Professor Flatté designs laser active regions optimized for suppression of Auger recombination and intersubband absorption, Professor Hasenberg grows test structures based on these designs for optical characterization in our laboratories. This process may be iterated several times to refine the laser design, growth, and optical measurements. Once a satisfactory active region is arrived at, device issues are addressed, including issues of carriers injection into the active material and carrier transport across the structure. This may lead to further refinements of the design and/or compromises between active layer optimization and electrical injection. Professor Hasenberg's group then grows and actual laser structure which is subsequently processed and packaged in Professor Chan's lab and tested in our labs. This closed-loop approach to laser design is very dynamic and productive - new laser designs are constantly being discussed, grown, and tested. This effort has led to a 3.2 micron diode laser operating at temperatures as high as 255K, a 3.5 micron optically-pumped laser operating at room temperature, and a 5.2 micron laser operating up to 185K.

In addition to our "in-house" program, we have collaborations with numerous government and industrial labs. A particularly significant collaboration is with Dr. George Turner and his group at MIT Lincoln Laboratory. Recently, this group has been interested in optically-pumped mid-IR lasers that are designed to optimally utilize the pump radiation by incorporating integrated separate absorbing layers in the laser heterostructure (the sample structure is shown below). A concern in these designs is the speed and efficiency with which carriers in these separated absorbing regions are transferred into the wells. We have used time-resolved mid-IR optical spectroscopy to examine this process. . We observe a rapid onset of emission from the absorber regions, but the emission from the wells develops on a time scale of more than 10 ps. This is associated with carrier transport to and capture by the wells. While on an absolute scale, this is quite fast, the time scale is somewhat slower than in simpler laser structures (e.g., those based on GaAs/AlGaAs quantum wells), a characteristic that is attributed to the presence of large InAs hole barriers that restrict the flow of holes into the lasing states.

Time-resolved emission spectra for such a structure are shown below. The peak near 600 meV arises from radiative recombination in the barrier regions, while the peak near 300 meV is associated with optical transitions in the quantum well. The latter signal grows as the former decays, reflecting the capture by the wells of carriers in the barrier states. The elongated high-energy tail of the quantum well emission indicates a hot carrier distribution, consistent with the large excess energy associated with the captured carriers.