15 research outputs found
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
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 LaFeO<sub>3</sub>/LaMnO<sub>3</sub> 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<sup>–1</sup> K<sup>–1</sup> for LaFeO<sub>3</sub> and 2.2
W m<sup>–1</sup> K<sup>–1</sup> for LaMnO<sub>3</sub>. In LaFeO<sub>3</sub>, 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<sub>1–<i>x</i></sub>Sr<sub><i>x</i></sub>FeO<sub>3−δ</sub>: 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<sub>1–<i>x</i></sub>Sr<sub><i>x</i></sub>FeO<sub>3−δ</sub> 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
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Continuous Wave Operation of 1.3{sub micro}m Vertical Cavity InGaAsN Quantum Well Lasers
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Room Temperature Continuous Wave InGaSsN Quantum Well Vertical Cavity Lasers Emitting at 1.3 um
Selectively oxidized vertical cavity lasers emitting at 1294 nm using InGaAsN quantum wells are reported for the first time which operate continuous wave at and above room temperature. The lasers employ two n-type Al{sub 0.94}Ga{sub 0.06}As/GaAs distributed Bragg reflectors each with a selectively oxidized current aperture adjacent to the optical cavity, and the top output mirror contains a tunnel junction to inject holes into the active region. Continuous wave single mode lasing is observed up to 55 C. These lasers exhibit the longest wavelength reported to date for vertical cavity surface emitting lasers grown on GaAs substrates