Nonlinear effects in electricity and magnetism have been recognized since Maxwell’s time. However, much progress has been made in the field of nonlinear optics since the discovery of the laser,¹ which made high-intensity optical fields easily feasible. The field started to grow with the first experimental work of Franken et al.² on optical second-harmonic generation (SHG) in 1961 and the theoretical work of Bloembergen et al.^3,4 on optical wave mixing in 1962. Over the following decades, the field of nonlinear optics witnessed enormous growth, leading to the observation of new physical phenomena and giving rise to novel concepts and applications including high-harmonics generation and frequency mixing that can act as new light sources or as amplification schemes, light modulators for controlling the phase or amplitude of a light beam, optical switches, optical logic, optical limiters, and numerous ways of processing the information content of data images, which created revolutionary change in photonics technology in the 20th century.^5,6 Almost all those achievements were made on conventional bulk crystals where cumbersome phase-matching conditions limit the efficiency of the nonlinear processes.

The formation of pure single crystalline silicon nanoparticles by microwave induced decomposition of silane in a low pressure flow reactor is reported. The morphology and crystal structure of the particles are characterized in situ by particle mass spectrometry (PMS) and ex situ by means of X-ray diffraction, high resolution transmission electron microscopy, electron energy loss spectroscopy, and infrared spectroscopy. The preparation method allows for the adjustment of the mean particle diameter in the range 6 nm ≤ d_PM ≤ 11 nm by controlling the precursor concentration, gas pressure, and microwave power. Spectroscopic investigations reveal that the particles are single crystal silicon. The potential on n- or p-type doping is in progress.

Silicon nanoparticles synthesized in the gas phase are studied. From time-resolved photoluminescence measurements we determine, quantitatively, the size-dependence of the oscillator strength of the nanoparticles. We investigate experimentally the absorption and photoluminescence emission of nanoparticle ensembles with a broad size distribution. Using a model which accounts for size-effects in both oscillator strength and quantum-confinement, we are able to calculate absorption and emission spectra of ensemble samples. From these results we have determined, whether silicon nanoparticles should be regarded as indirect or direct semiconductors. Moreover, we systematically study the influence of the particle size-distribution on the optical spectra.

We observe experimentally by photoluminescence the band structure and specific emission properties of an in-plane, light-diffracting photonic crystal formed onto a multimode gallium nitride waveguide. Clear-cut two-dimensional photonic crystal effects are reported. Comparison with modeling results in identification of the band structure, provides insight into the light diffraction mechanism and points out design issues for enhancement of the extraction efficiency.