Remote gas sensing for atmospheric and environmental studies using single mode emitting semiconductor lasers, e.g. in LIDAR applications has gained wide interest in the last few years. This technique has been brought to sophisticated sensitivity levels and nowadays detection limits are in the range of a few ppb. However, up until recently only semiconductor laser diode sources with wavelengths below 2.3 μm have been available, which inherently limits the detection sensitivity due to the fact that the fundamental absorption band of many gases lies in the spectral range beyond 2.3 μm. With novel distributed feedback laser diodes at wavelengths up to 2.9 μm higher detection sensitivities as compared to currently available laser based sensors are possible.
We present an overview of the current status of laser diodes used in remote sensing application including novel
laser types such as single mode emitting DFB lasers operating at wavelengths up to 3 μm and quantum cascade
lasers for mid infrared absorption spectroscopy. In particular we will focus on applications of these devices in the
frame of safeguard measures and home security.
Monomode laser diodes in the wavelength range around 1.15μm are of particular interest for various kinds of
applications, including frequency doubling where a large part of the spectral range from yellow to green currently
remains inaccessible. For efficient frequency conversion, single-frequency laser light, as e.g. obtained from Distributed
Feedback Laser (DFB) laser diodes, is an essential prerequisite. One particular challenge at this wavelength range around
1.15μm is to find a gain medium with high internal efficiency. For broad area (BA) lasers, good results have recently
been achieved using quantum dots (QDs) or highly strained InGaAs quantum wells (QWs) .
In the following, first results for high performance monomode QD DFB laser diodes in the wavelength range of interest
are discussed. The spectral gain properties of the underlying QD active region allow to realize DFB lasers with emission
spanning an extremely broad wavelength range of 65nm ranging from around 1095nm to 1160nm based on the identical
We report on novel distributed feedback laser diodes with emission wavelengths as long as 2.9 µm. Single mode laser emission is realized by making use of metal Bragg gratings patterned laterally to the laser ridge. The laser diodes work at room temperature in cw mode and deliver output powers of a few mW. The high side mode suppression ratio of < 35 dB ensures high spectral selectivity within a broad wavelength tuning range of up to 10 nm. These devices promise to be key enablers for a new generation of laser based gas sensing systems featuring previously unreached detection limits.
We report on time-resolved optical investigations on the polarization and the spin dynamics in non-magnetic and magnetic self-assembled II VI semiconductor quantum dots. In case of CdSe/ZnSe quantum dots, no transient loss of polarization is found within the time scale of exciton recombination, if one excites the excitons strictly resonant in the quantum dot ground state with a laser pulse linearly polarized along the  or [1-10] crystal axes. This indicates a high temporal stability of the exciton state, which is a coherent superposition of spin-up and spin-down exciton states. Even after replacing some Cd atoms in the crystal matrix by magnetic ions Mn2+, the polarization is being conserved as long as the average Mn2+ concentration is about (formula available in paper)or less, despite the pronounced exchange interaction between the manganese ion spins and the carrier spins. In case of magnetic semiconductor quantum dots with a large concentration of (formula available in paper)ions, the spin spin interaction between charge carriers and manganese ions results in the formation of a quasi-zero dimensional ferromagnetically aligned spin complex, the exciton magnetic polaron. For (formula available in paper)Se quantum dots this transient spin alignment is directly evidence by a transient shift of the emission energy. We deduce a typical time constant of 125 ps at T = 2 K for the dynamical response of the magnetic ion spins.
Single epitaxially grown CdSe/ZnSe quantum dots have been studied by using photoluminescence spectroscopy with a high spatial resolution. The lifting of the spin degeneracy due to exchange interaction results in a splitting of the exciton ground state, strongly dependent on the symmetry of the quantum dot. By applying a magnetic field in Faraday geometry, the energy splitting as well as the polarization properties of the exciton transition can be varied and remarkably, even at high magnetic field a spin coherence time of about 3 ns is found, which exceeds the recombination lifetime of single excitons significantly. As the biexciton state is a spin singlet, both its fine structure splitting as well as its degree of polarization are shown to be controlled by the final state of recombination, the single exciton state. Besides the discrete energy splitting of optical transitons in single quantum dots, we observe a rather statistical, but strongly correlated energy shift was well as a correlated on-off switching behavior of the exciton and the biexciton emission on a typical time constant of seconds. These effects are related to the influence of charge carriers in the nanoenvironment of the dot and to thermal or Auger-driven carrier escape into trap states in the vicinity of the dot, respectively.