We discuss several coherent nonlinear optical phenomena that occur in active quantum-cascade (QC) structures that support both laser action and, at the same time, nonlinear self-conversion of laser light into coherent radiation at other frequencies. In other words, the laser field serves as an intracavity optical pump for a desired nonlinear optical process. In such systems, resonant absorption of the pump field is overcome by laser gain, and giant resonant nonlinearities of the intersubband transitions can be fully exploited. The proposed approach holds promise to extend the operating wavelength of QC lasers to the spectral regions where they become less efficient, constrained by material limitations, or operate only at cryogenic temperatures. It may also lead to the development of new broadly tunable injection-pumped sources. We review recent development in this research direction including the demonstration of the Raman injection laser.

The interaction between electrically neutral surfaces at sub-micron separation is dominated by the force arising from quantum fluctuations of the electromagnetic field, known as the Casimir force. This effect has been witnessing a renewed interest because of its potential impact in micro- and nanotechnology. Most recent literature has focused on the study of the attraction between bulk-like metallic surfaces in vacuum. Because electromagnetic fluctuations depend on the dielectric function of the surfaces, the use of different materials might reveal new aspects of the Casimir force and suggest novel solutions for the design of micro- and nanofabricated devices. Following this approach, we have measured the Casimir force using Hydrogen Switchable Mirrors-a metallic mirror that switches from highly reflective to transparent when exposed to hydrogen. The comparison of the results obtained in air and in hydrogen sheds light on the relative contribution of visible and infrared wavelengths to the Casimir interaction. We have also studied the dependence of the Casimir force on the metallic film thickness and have shown the effect of the skin-depth. The final section of the paper discusses the torque induced by quantum fluctuations on two birefringent plates and describes an experiment that should allow us to observe this phenomenon. (C) 2005 Elsevier Ltd. All rights reserved.

Forces arising from overlap between the guided waves of parallel, microphotonic waveguides are calculated. Both attractive and repulsive forces, determined by the choice of relative input phase, are found. Using realistic parameters for a silicon-on-insulator material system, we estimate that the forces are large enough to cause observable displacements. Our results illustrate the potential for a broader class of optically tunable microphotonic devices and microstructured artificial materials. (c) 2005 Optical Society of America.

We report the development of a novel laser spectrometer for high-sensitivity detection of methane and nitrous oxide. The system relies on a quantum-cascade laser source emitting wavelength of around 8.06 mu m, where strong fundamental absorption bands occur for the considered species and their isotopomers. The detection technique is based on audio-frequency and radio-frequency modulation of laser radiation. First experimental tests have been performed to estimate the achievable detection limits and the signal reproducibility levels in view of possible measurements of C-13/C-12, O-18/O-16, O-17/O-16 and N-15/N-14 isotope ratios.

We have calculated the optically-induced force between coupled high-Q whispering gallery modes of microsphere resonators. Attractive and repulsive forces are found, depending whether the bi-sphere mode is symmetric or antisymmetric. The magnitude of the force is linearly proportional to the total power in the spheres and consequently linearly enhanced by Q. Forces on the order of 100 nN are found for Q= 10(8), large enough to cause displacements in the range of 1 mu m when the sphere is attached to a fiber stem with spring constant 0.004 N/m. (c) 2005 Optical Society of America.

We have performed measurements of the Casimir force between a metallic plate and a transparent sphere coated with metallic films of different thicknesses. We have observed that, if the thickness of the coating is less than the skin-depth of the electromagnetic modes that mostly contribute to the interaction, the force is significantly smaller than that measured with a thick bulk-like film. Our results provide direct evidence of the skin-depth effect on the Casimir force between metallic surfaces.

Stimulated Raman scattering is a nonlinear optical process that, in a broad variety of materials, enables the generation of optical gain at a frequency that is shifted from that of the incident radiation by an amount corresponding to the frequency of an internal oscillation of the material(1,2). This effect is the basis for a broad class of tunable sources known as Raman lasers(2,3). In general, these sources have only small gain (similar to10(-9) cm W(-1)) and therefore require external pumping with powerful lasers, which limits their applications. Here we report the realization of a semiconductor injection Raman laser designed to circumvent these limitations. The physics underlying our device differs in a fundamental way from existing Raman lasers(3-8): it is based on triply resonant stimulated Raman scattering between quantum-confined states within the active region of a quantum cascade laser that serves as an internal optical pump-the device is driven electrically and no external laser pump is required. This leads to an enhancement of orders of magnitude in the Raman gain, high conversion efficiency and low threshold. Our lasers combine the advantages of nonlinear optical devices and of semiconductor injection lasers, and could lead to a new class of compact and wavelength-agile mid- and far-infrared light sources.

We present detailed numerical calculations of the mechanical torque induced by quantum fluctuations on two parallel birefringent plates with in-plane optical anisotropy, separated by either vacuum or a liquid (ethanol). The torque is found to vary as sin(2 theta), where theta represents the angle between the two optical axes, and its magnitude rapidly increases with decreasing plate separation d. For a 40 mu m diameter disk, made out of either quartz or calcite, kept parallel to a barium titanate plate at d similar or equal to 100 nm, the maximum torque (at theta=pi/4) is of the order of similar or equal to 10(-19) N m. We propose an experiment to observe this torque when the barium titanate plate is immersed in ethanol and the other birefringent disk is placed on top of it. In this case the retarded van der Waals (or Casimir-Lifshitz) force between the two birefringent slabs is repulsive. The disk would float parallel to the plate at a distance where its net weight is counterbalanced by the retarded van der Waals repulsion, free to rotate in response to very small driving torques.