Non-uniform lateral current distribution in quantum cascade lasers.

Non-uniform lateral current distribution in quantum cascade (QC) lasers is investigated, as a result of stimulated-optical-emission-assisted electron transport, the rate of which depends on the local photon density and is comparable to the longitudinal optical (LO) phonon scattering rate. A microscopic model based on rate equations is built to study the self-consistent process of interaction between local electrons and photons in QC lasers. The non-uniform distribution of lateral current and the corresponding spatial hole burning are simulated from this model.

Ultrafast laser-based spectroscopy and sensing: applications in LIBS, CARS, and THz spectroscopy.

Ultrafast pulsed lasers find application in a range of spectroscopy and sensing techniques including laser induced breakdown spectroscopy (LIBS), coherent Raman spectroscopy, and terahertz (THz) spectroscopy. Whether based on absorption or emission processes, the characteristics of these techniques are heavily influenced by the use of ultrafast pulses in the signal generation process. Depending on the energy of the pulses used, the essential laser interaction process can primarily involve lattice vibrations, molecular rotations, or a combination of excited states produced by laser heating.

Femtosecond and nanosecond laser-induced breakdown spectroscopy of trinitrotoluene.

Femtosecond and nanosecond laser-induced breakdown spectroscopy (LIBS) were used to study trinitrotoluene (TNT) deposited on aluminum substrates. Over the detection wavelength range of 200-785 nm, we have observed emission from CN and C(2) molecules as the marker for the explosive with femtosecond LIBS. In contrast, the signal for nanosecond LIBS of TNT is dominated by emission from the elemental constituents of the explosive.

Effects of surface roughness on reflection spectra obtained by terahertz time-domain spectroscopy.

We present an analytical model that shows that reflection from a rough surface causes a Gaussian frequency roll-off for the spectral magnitude of a terahertz wave and reduces the signal-to-noise ratio of terahertz time-domain spectroscopy. The parameter that determines the width of the frequency roll-off is the standard deviation of the surface height distribution. Measurements of terahertz waves reflected from copper powder samples provide experimental evidence for this effect.

Fiber-coupling efficiency for free-space optical communication through atmospheric turbulence.

High-speed free-space optical communication systems have recently used fiber-optic components. The received laser beam in such a system must be coupled into a single-mode fiber at the input of the receiver module. However, propagation through atmospheric turbulence degrades the spatial coherence of a laser beam and limits the fiber-coupling efficiency.