Recent Progress of Mid-IR Type-II Interband Cascade Lasers
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35 Mat. Res. Soc. Symp. Proc. Vol. 607 © 2000 Materials Research Society
EXPERIMENT and RESULTS The type-II cascade structure utilizes the interband transitions instead of intersubband transitions, which eliminates the phonon relaxation path while retaining the advantages of electron recycling, therefore, the threshold current density could be much lower. Furthermore, the Auger recombination, which usually dominates the laser performance at higher temperatures and longer wavelengths for interband transitions, can be suppressed in type-II quantum wells with band structure engineering. Similar to QC lasers, each injected electron is reused in IC laser for sequential photon emission, and can in principle generate as many photons as the number of active regions to achieve high quantum efficiency. Figure 1 shows the schematic band diagram of a type-IT IC laser with a W active region [12].
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Figure 1. Schematic band diagram of one stage of active region and injection region of a type-II IC laser with a W active region [12]. Dual-Wavelength Type-IT IC Lasers For many applications, like differential absorption lidar (DIAL), the light scattering has to be evaluated and compared at two different frequencies [1]. A compact, mid-IR (MIR) dualwavelength or multi-wavelength laser would be extremely useful. Here, we report the first dualwavelength laser based on type-II IC structure. The sample was grown in a Riber 32 MBE system on a p-type GaSb substrate using methods similar to those described in Ref. 17. Here, we have used two InAs layers in the active region, so called "W" active region. The purpose is to reduce the leakage current, proposed and demonstrated by Jerry Meyer et. al [12]. The active region is composed of two different 10-stage structures with a total thickness of 1.54 pm. For the short wavelength section, the active region is composed of 24.6-A InAs/35.9-A InGaSb/21.5-A InAs/15.3-A AlSb/38.9-A InGaSb/15.3-A AlSb/45-A GaSb/12.2-A AlSb. For the long wavelength section, the active region is composed of 25.2-A InAs/36.7-A InGaSb/22-A
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InAs/15.3-A AlSb/38.9-A InGaSb/15.3-A AlSb/45-A GaSb/12.2-A AlSb. The top and bottom cladding layers are composed of n-type InAs/AlSb SLs, each having a thickness of 1.85 pm. After x-ray measurements, the sample was processed into laser bars with different cavity lengths without facet coating. To suppress the current spreading, we have etched the mesa down to the bottom cladding layer to isolate devices. Figure 2a shows the lasing spectra of a short device at 100 K with an injection current of 185 and 260 mA. With a current of 185 mA, the lasing is occurred at 4.485 and 4.565 ptm. To verify the two-color lasers, Figure 2b shows the lasing spectrum of a long device at 120 K. To the best of our knowledge, this is the first twocolor laser based on type-II IC configuration. The physics of this two color type-IT IC laser is different from the dual wavelength type-I quantum cascade
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