We report on a novel quantum cascade laser (QCL) capable of operating in pure amplitude or wavelength modulation configuration thereby allowing the acquisition of background-free gas absorption-line profiles using quartz-enhanced photoacoustic spectroscopy (QEPAS). The QCL is composed of three electrically independent sections: Gain, Phase (PS) and Master Oscillator (MO). The non-uniform pumping of these three QCL sections allows laser wavelength tuning with constant optical power and vice-versa. Pure QEPAS amplitude modulation operating conditions were obtained by modulating the PS current, while pure wavelength modulation was obtained by modulating the MO section and slowly scanning the PS current.
The equations for the threshold-current density J<sub>th</sub>, differential quantum efficiency η<sub>d</sub> and maximum wallplug efficiency
η<sub>wp,max</sub> for quantum-cascade lasers (QCLs) have been modified for electron leakage and backfilling. We used a thermalexcitation
model of "hot" injected electrons from the upper laser state to upper active-region energy states to calculate
leakage currents. Then the calculated characteristic temperature T<sub>0</sub> for J<sub>th</sub> was found to agree well with experiment for
both conventional and deep-well QCLs. The characteristic temperature T<sub>1</sub> for η<sub>d</sub> was deduced to be due to both electron
leakage and an increase in the waveguide-loss coefficient. For conventional mid-infrared QCLs η<sub>wp,max</sub> is found to be
strongly temperature dependent which explains experimental data. By using a new concept: tapered active-region (TA),
deep-well QCLs have been optimized for virtual suppression of the electron-leakage currents. In turn, at room
temperature, for continuous-wave (CW)-operating, 4.5-5.0 μm-emitting TA QCLs we estimate the threshold current to
decrease by ~ 25 %, the active-region temperature rise at the η<sub>wp,max</sub> point to decrease by ~ 30 %, and the single-ended,
η<sub>wp,max</sub> value to become at least 22 %. Preliminary results from TA QCLs include T1 values as high as 454 K, over the
20-60 oC heatsink-temperature range.
It is well known by far that biological organisms could exhibit sophisticated optical system, which compete or overcame the top technology products available. The diatoms are microscopic algae enclosed in intricate amorphous silica cells, called frustules. In this work the optical reflectivity data, infrared spectroscopy, scanning electron microscopy and photoluminescence (PL) characterizations are presented for silica shells of <i>Coscinodiscus wailesii</i>, which is a centric diatom characterized from a diameter that varies in the range between 100 and 500 μm. Preliminary results suggest that the <i>Coscinodiscus wailesii</i> can be used as photonic material and sensor transducer.