Semiconductor Lasers: Physics and Applications

Contains an introduction and reports on eight research projects.MIT Lincoln Laboratory Contract BX-6558MIT Lincoln LaboratoryU.S. Navy - Office of Naval Research/MUR

Use of Standard Reference Material 2242 (Relative Intensity Correction Standard for Raman Spectroscopy) for microarray scanner qualification

As a critical component of any microarray experiment, scanner performance has the potential to contribute variability and bias, the magnitude of which is usually not quantified. Using Standard Reference Material (SRM) 2242, which is certified for Raman spectral correction, for monitoring the microarray fluorescence at the two most commonly used wavelengths, our team at the National Institute of Standards and Technology (NIST) has developed a method to establish scanner performance, qualifying signal measurement in microarray experiments. SRM 2242 exhibits the necessary photostability at the excitation wavelengths of 635 nm and 532 nm, which allows scanner signal stability monitoring, although it is not certified for use in this capacity. In the current study, instrument response was tracked day to day, confirming that changes observed in experimental arrays scanned are not due to changes in the scanner response. Signal intensity and signal-to-noise ratio (S/N) were tracked over time on three different scanners, indicating the utility of the SRM for scanner qualification

Distinguishing Thermal and Electronic Effects in Ultrafast Optical Spectroscopy Using Oxide Heterostructures

Measuring time-resolved photoexcited properties in semiconductors is critical to the design and improvement of light-harvesting devices. Although ultrafast pump–probe spectroscopy offers a promising route to understand carrier recombination mechanisms and quantify lifetimes, thermal contributions to the transient optical response can be significant and need to be properly accounted for to isolate carrier-induced contributions. We demonstrate the use of broadband ultrafast optical spectroscopy on type I heterostructures as a means to isolate transient effects that are solely thermal in nature. Specifically, we use transient absorption and reflectance spectroscopy to measure the time-resolved optoelectronic changes in photoexcited epitaxial bilayers of LaFeO₃/LaMnO₃ and monolithic thin films of these materials. Experiments and complementary numerical modeling reveal that thermal effects dominate the transient absorption and reflectance spectra above the band gap. Fitting the dynamics with a thermal diffusion model yields thermal conductivities of 6.4 W m^–1 K^–1 for LaFeO₃ and 2.2 W m^–1 K^–1 for LaMnO₃. In LaFeO₃, an additional photoinduced absorption feature below the band gap at ∼1.9 eV is assigned primarily to photoexcited carriers and persists for over 3 ns. This work provides a direct demonstration of how thermal and electronic contributions can be separated in transient optical spectroscopies, enabling new insights into dynamical optical properties of semiconductors

Static and Dynamic Optical Properties of La_1–<i>x</i>Sr_<i>x</i>FeO_3−δ: The Effects of A‑Site and Oxygen Stoichiometry

Perovskite oxides are a promising material class for photovoltaic and photocatalytic applications due to their visible band gaps, nanosecond recombination lifetimes, and great chemical diversity. However, there is limited understanding of the link between composition and static and dynamic optical properties, despite the critical role these properties play in the design of light-harvesting devices. To clarify these relationships, we systemically studied the optoelectronic properties in La_1–<i>x</i>Sr_<i>x</i>FeO_3−δ epitaxial films, uncovering the effects of A-site cation substitution and oxygen stoichiometry. Variable-angle spectroscopic ellipsometry was used to measure static optical properties, revealing a linear increase in absorption coefficient at 1.25 eV and a red-shifting of the optical absorption edge with increasing Sr fraction. The absorption spectra can be similarly tuned through the introduction of oxygen vacancies, indicating the critical role that nominal Fe valence plays in optical absorption. Dynamic optoelectronic properties were studied with ultrafast transient reflectance spectroscopy, revealing similar nanosecond photoexcited carrier lifetimes for oxygen deficient and stoichiometric films with the same nominal Fe valence. These results demonstrate that while the static optical absorption is strongly dependent on nominal Fe valence tuned through cation or anion stoichiometry, oxygen vacancies do not appear to play a significantly detrimental role in the recombination kinetics