While conventional semiconductor lasers employ electrical injection for carrier excitation, optically pumped
semiconductor lasers (OPSLs) have demonstrated high output powers and high brightness in the mid-infrared. An
important consideration for optically pumped lasers is efficient absorption of the pump beam, which can be achieved
through increasing the number of periods in the active region, by placing the active region in a cavity with an optical
thickness of twice the pump wavelength between distributed Bragg reflectors (Optical Pumping Injection Cavity), or by
periodically inserting the active quantum wells into an InGaAsSb waveguide designed to absorb the pump radiation
(Integrated Absorber). A tunable optical pumping technique is utilized by which threshold intensities are minimized and
efficiencies are maximized. The near-IR idler output of a Nd:YAG-pumped optical parametric oscillator (10 Hz, ~4 ns)
is the tunable optical pumping source in this work. Results are presented for an OPSL with a type-II W active region
embedded in an integrated absorber to enhance the absorption of the optical pump beam. Emission wavelengths range
from 4.64 μm at 78 K to 4.82 μm at 190 K for optical pump wavelengths ranging from 1930-1950 nm. The effect of
wavelength tuning is demonstrated and compared to single wavelength pumping (1940 nm) at a higher duty cycle (20-
30%). Comparisons are also made to other OPSLs, including a discussion of the characteristic temperature and high
temperature performance of these devices.
The optical pumping injection cavity (OPIC) laser concept was developed to enhance the efficiency of optical pump
beam absorption, and this work focuses on epitaxial configurations designed for broadband absorption around 1850 nm,
an optimal pump wavelength for transmission through GaSb substrates that allows for epi-down mounting for improved
heat management, while minimizing the photon decrement. The OPIC devices presented in this work have
InAs/GaSb/InAs/AlSb type-II W active regions with a thicker GaSb/AlAsSb distributed Bragg reflector on top in order
to enhance reflection back into the active region for epi-down mounting. Results are presented for optical pumping at
1850 nm as well as for resonant optical pumping, as the cavity resonance varies with temperature due to shifts in lattice
constant, refractive index, and gain. Pumping at 1850 nm resulted in lasing from 78 K up through 310 K. At 78 K, the
actual pump cavity resonance is ~1840 nm, and with increasing temperature the resonance shifts to longer wavelengths
beyond 1850 nm. Emission wavelengths range from 3.59 μm at 78 K to 4.01 μm at 310 K for 1850 nm optical pumping.
The broadened OPIC configuration presents a distinct advantage over earlier reported OPIC devices as the broader
resonance allows for efficient emission across a wide temperature range for a single pump wavelength (e.g., 1850 nm),
providing over 400 nm of wavelength tuning. Results will be compared with a second broadened OPIC with emission
wavelengths beyond 4 μm that temperature tunes across the carbon dioxide spectral line at 4.2 μm.
Tight control of blood glucose levels has been shown to dramatically reduce the long-term complications of diabetes. Current invasive technology for monitoring glucose levels is effective but underutilized by people with diabetes because of the pain of repeated finger-sticks, the inconvenience of handling samples of blood, and the cost of reagent strips. A continuous glucose sensor coupled with an insulin delivery system could provide closed-loop glucose control without the need for discrete sampling or user intervention. We describe an optical glucose microsensor based on absorption spectroscopy in interstitial fluid that can potentially be implanted to provide continuous glucose readings. Light from a GaInAsSb LED in the 2.2-2.4 μm wavelength range is passed through a sample of interstitial fluid and a linear variable filter before being detected by an uncooled, 32-element GaInAsSb detector array. Spectral resolution is provided by the linear variable filter, which has a 10 nm band pass and a center wavelength that varies from 2.18-2.38 μm (4600-4200 cm<sup>-1</sup>) over the length of the detector array. The sensor assembly is a monolithic design requiring no coupling optics. In the present system, the LED running with 100 mA of drive current delivers 20 nW of power to each of the detector pixels, which have a noise-equivalent-power of 3 pW/Hz<sup>1/2</sup>. This is sufficient to provide a signal-to-noise ratio of 4500 Hz<sup>1/2</sup> under detector-noise limited conditions. This signal-to-noise ratio corresponds to a spectral noise level less than 10 μAU for a five minute integration, which should be sufficient for sub-millimolar glucose detection.
The performances of a pin versus a pn structure from GaInAsSb materials operating at room temperature are compared both from a theoretical point of view and experimentally. Theoretically, it is found in materials limited by generation-recombination currents, pn junctions have a higher D* than pin junctions. The thinner depletion region of pn junctions results in a lower responsivity but a higher dynamic resistance, giving an overall higher D* compared to a pin structure. A series of five p+pn+ Ga<sub>0.80</sub>In<sub>0.20</sub>As<sub>0.18</sub>Sb<sub>0.82</sub> detector structures latticed matched to GaSb substrates and with 2.37 μm cut off wavelength were grown by molecular beam epitaxy and processed into variable size mesa photodiodes. Only the doping of the absorbing (p) region was varied from sample to sample, starting with nominally undoped (~1x10<sup>16</sup> cm<sup>-3</sup> pbackground doping due to native defects) and increasing the doping until a p+n+ structure was attained. Room temperature dynamic resistance-area product R0A was measured for each sample. A simple method is presented and used to disentangle perimeter from areal leakage currents. All five samples had comparable R0A's. Maximum measured R0A was 30 Ω-cm<sup>2</sup> in the largest mesas. Extracted R0A's in the zero perimeter/area limit were about ~50 Ω-cm<sup>2</sup> (20-100 Ω-cm<sup>2</sup>) for all samples. Within uncertainty, no clear trend was seen. Tentative explanations are proposed.
A focal plane array detector sensitive from 2.0-2.5 μm and consisting of 32, 1.0 mm x 50 μm pixels, all functional, is demonstrated. Mean room-temperature R<sub>0</sub>A is found to be 1.0 Ωcm<sup>2</sup>, limited by sidewall leakage. The focal plane array is fabricated from an MBE-grown homojunction <i>p-i-n </i>GaInAsSb grown on an <i>n</i>-type GaSb substrate. Back-illumination geometry is compared to front-illumination geometry and is found to be favorable, particularly the improved responsivity (1.3 A/W at 2.35 μm corresponding to 68% quantum efficiency) due to reflection of light off the metal contact. Further, back-illumination is the most convenient geometry for mounting the array onto a compact blood glucose sensor because both contacts can be mounted on one side, while detector illumination occurs on the other.
Recent progress towards the realization of high-power, non- cryogenic (quasi-)cw mid-IR lasers based on the `W' configuration of the active region is reported. Type-II diodes with AlGaAsSb broadened-waveguide separate confinement regions are the first III-V interband lasers to achieve room-temperature pulsed operation at a wavelength longer than 3 micrometers . For cw operation, T<SUB>max</SUB> was 195 K and P<SUB>out</SUB> equals 140 mW was measured at 77 K. Optically- pumped W lasers recently attained the highest cw operating temperatures (290 K) of any semiconductor laser emitting in the 3 - 6 micrometers range. For a (lambda) equals 3.2 micrometers device at 77 K, the maximum cw output power was 0.54 W per uncoated facet. In order to maximize the absorption of the pump in the active region, an optical pumping injection cavity structure was used to create an etalon cavity for the 2.1 micrometers pump beam. The pulsed incident pump intensity at threshold was only 8 kW/cm<SUP>2</SUP> at 300 K for this edge- emitting mid-IR laser. The differential power conversion efficiency was 9% at 77 K and 4% at 275 K, which indicates promising prospects for achieving high cw output powers at TE-cooler temperatures following further optimization.
Optically-pumped type-II W lasers have exhibited improved high-temperature performance throughout the wavelength range of 2.7 micrometer to 7.3 micrometer. Low duty cycle pumping at 2.1 micrometer yielded maximum operating temperatures as high as 360 K at (lambda) less than or equal to 4 micrometer for 3 devices, with peak output powers exceeding 1.5 W at ambient temperature. Internal losses of 90 cm<SUP>-1</SUP> at 300 K were seen for one device and suppressed Auger recombination coefficients were observed for all three. Pulsed operation at wavelengths as long as 7.3 micrometer was seen in another device which had a maximum operating temperature of 220 K. For 1.064 micrometer optical pumping, the same laser was able to operate in continuous-wave (cw) mode to 130 K. Cw operation was also observed at temperatures as high as 290 K for lambda approximately equals 3.0 micrometer. Maximum cw output powers (per uncoated facet) of 260 mW at (lambda) equals 3.1 micrometer and 50 mW at (lambda) equals 5.4 micrometer were observed at T equals 77 K. With further improvements in the design and growth quality of these W laser structures, it is projected the cw output powers of 0.5 W or more should be achievable at thermoelectric cooler temperatures.
Recently, we demonstrated a new type of quantum cascade lasers based on interband transitions in type-II heterostructures. It takes advantage of the broken-gap band alignment in the InAs/Ga(In)Sb heterostructure to recycle electrons from the valence band back to the conduction band, thus enabling sequential photon emission from active regions stacked in series. A peak optical output power of approximately 0.5 W/facet from a broad area gain-guided interband cascade laser with a threshold current density of 290 A/cm<SUP>2</SUP>, and a slope of 211 mW/A per facet, corresponding to a differential external quantum efficiency of 131%, were obtained at 80 K and at a wavelength of approximately 3.9 micrometer. Differential quantum efficiencies exceeding 200% were also observed from mesa structure lasers. Comparable device performance was also achieved based on a 'W' configuration cascade laser at approximately 2.9 micrometer, which has been operated at temperatures up to 225 K. Another W interband cascade laser has displayed lasing at 3.6 micrometer and nearly to room temperature (286 K).