Using a Green’s function solution to the photoacoustic wave equation, we compare intensity-modulated continuous-wave (CW) lasers with a chirped modulation frequency to pulsed lasers for photoacoustic imaging applications. Assuming the same transducer is used in both cases, we show that the axial resolution is identical and is determined by the transducer and material properties of the object. We derive a simple formula relating the signal-to-noise ratios (SNRs) of the two imaging systems that only depends on the fluence of each pulse and the time-bandwidth product of the chirp pulse. We also compare the SNR of the two systems assuming the fluence is limited by the American National Standards Institute (ANSI) laser safety guidelines for skin. We find that the SNR is about 20 dB to 30 dB larger for pulsed laser systems for reasonable values of the parameters. However, CW diode lasers have the advantage of being compact and relatively inexpensive, which may outweigh the lower SNR in many applications.

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In As ∕ Ga Sb type-II superlattices (SLs) were grown on (001) GaSb substrates by metal organic chemical vapor deposition. Besides the expected tensile stress introduced by the InAs layers in the SLs, additional tensile stress is found in the InAs∕GaSb SLs from the simulation of x-ray diffraction (XRD) curves of the SLs. High-resolution transmission electron microscopy and XRD of the SLs grown with different interface gas switching procedures suggest that the additional tensile stress is mainly located at the GaSb→InAs interface. To compensate for the tensile stress in the SL structures, we show that introducing ∼2-ML-thick InAs0.8Sb0.2 layer at the interfaces of the SL improves the morphology and the structural properties of the SLs significantly.

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We report on the characterization and performance of epitaxial structures and photodiodes based on InAs/GaSb type-II superlattices grown by metalorganic chemical vapor deposition. Interfacial layers were introduced at the superlattice interfaces to compensate the tensile strain and hence to improve the overall material quality of the superlattice structures. The optimal morphology and low strain was achieved via a combined interfacial layer scheme with InAsSb+InGaSb layers. Using this scheme, a p-i-n photodiode structure with a 360-period InAs/GaSb superlattice was grown on a GaSb substrate, which operates at 78 K with a cut-off wavelength of ∼8 μm and a peak responsivity of 0.6 A/W at ∼6 μm.