A multi-mode super-fano mechanism for enhanced third harmonic generation in silicon metasurfaces.

We present strong enhancement of third harmonic generation in an amorphous silicon metasurface consisting of elliptical nano resonators. We show that this enhancement originates from a new type of multi-mode Fano mechanism. These 'Super-Fano' resonances are investigated numerically in great detail using full-wave simulations. The theoretically predicted behavior of the metasurface is experimentally verified by linear and nonlinear transmission spectroscopy. Moreover, quantitative nonlinear measurements are performed, in which an absolute conversion efficiency as high as ηmax ≈ 2.8 × 10-7 a peak power intensity of 1.2 GW cm-2 is found. Compared to an unpatterned silicon film of the same thickness amplification factors of up to ~900 are demonstrated. Our results pave the way to exploiting a strong Fano-type multi-mode coupling in metasurfaces for high THG in potential applications.

Electric-field-induced second harmonic generation in silicon dioxide.

Electric-field-induced second harmonic generation (EFISH) as a third order nonlinear process is of high practical interest for the realization of functional nonlinear structures. EFISH in materials with vanishing χ(2) and non-zero χ(3) offers huge potential, e.g., for background-free nonlinear electro-optical sampling. In this work, we have investigated SiO2 as a potential EFISH material for such applications using DC-electric fields. We were able to observe significant second harmonic generation (SHG) in comparison to the background SHG signal. The fundamental excitation at 800 nm results in a SHG signal at 400 nm for high applied DC electric fields, which is a clear indication for EFISH. Additionally, we were are able to precisely model the EFISH signal using time-domain simulations. This numerical approach will be of great importance for efficiency enhancement and prove as a valuable tool for future device design.

Ultrathin Nonlinear Metasurface for Optical Image Encoding.

Security of optical information is of great importance in modern society. Many cryptography techniques based on classical and quantum optics have been widely explored in the linear optical regime. Nonlinear optical encryption in which encoding and decoding involve nonlinear frequency conversions represents a new strategy for securing optical information. Here, we demonstrate that an ultrathin nonlinear photonic metasurface, consisting of meta-atoms with 3-fold rotational symmetry, can be used to hide optical images under illumination with a fundamental wave. However, the hidden image can be read out from second harmonic generation (SHG) waves. This is achieved by controlling the destructive and constructive interferences of SHG waves from two neighboring meta-atoms. In addition, we apply this concept to obtain gray scale SHG imaging. Nonlinear metasurfaces based on space variant optical interference open new avenues for multilevel image encryption, anticounterfeiting, and background free image reconstruction.

Optical spectrum, perceived color, refractive index, and non-adiabatic dynamics of the photochromic diarylethene CMTE.

Photochromism allows for reversible light-induced conversion of a molecular species into a different form with significantly altered optical properties. One promising compound that excels with high fatigue resistance and shows its photochromic functionality both in solution and in molecular solid films is the diarylethene derivative CMTE. Here we present a comprehensive study of its photophysical properties with density-functional theory based methods and benchmark the results against higher-level quantum-chemical approaches and experiments. In addition to static properties such as optical absorption, perceived color, and refractive index, we also investigate reaction dynamics based on non-adiabatic ab initio molecular dynamics. This gives detailed insight into the molecules' ultrafast reaction dynamics and enables us to extract reaction time scales and quantum yields for the observed electrocyclic reaction following photoexcitation.

Blue-green emitting microdisks using low-temperature-grown ZnO on patterned silicon substrates.

Zinc oxide (ZnO) as an extremely bright emitter is an attractive material for photonic devices. However, devices made of epitaxially grown ZnO are difficult to fabricate due to the lack of selective etching processes. Here, we demonstrate that by a low-temperature growth process on pre-patterned silicon dioxide (SiO2) microdisks (MDs) high quality ZnO resonators are created. The devices exhibit whispering gallery modes (WGMs) over the blue-green part of the visible spectrum with quality factors exceeding Q = 3500, which are among the highest values reported in this material system so far. By deposition of SiO2 capping layers we find an enhanced coupling of the spontaneous emission from the active medium into the MDs, observed by sharp WGMs up to a radial quantum number of N = 3.

Electroluminescence from silicon nanoparticles fabricated from the gas phase.

Electroluminescence from as-prepared silicon nanoparticles, fabricated by gas phase synthesis, is demonstrated. The particles are embedded between an n-doped GaAs substrate and a semitransparent indium tin oxide top electrode. The total electroluminescence intensity of the Si nanoparticles is more than a factor of three higher than the corresponding signal from the epitaxial III-V semiconductor. This, together with the low threshold voltage for electroluminescence, shows the good optical properties of these untreated particles and the efficient electrical injection into the device. Impact ionization by electrons emitted from the top electrode is identified as the origin of the electrically driven light emission.

Tuning quantum-dot based photonic devices with liquid crystals.

Microdisks made from GaAs with embedded InAs quantum dots are immersed in the liquid crystal 4-cyano-4'-pentylbiphenyl (5CB). The quantum dots serve as emitters feeding the optical modes of the photonic cavity. By changing temperature, the liquid crystal undergoes a phase transition from the isotropic to the nematic state, which can be used as an effective tuning mechanism of the photonic modes of the cavity. In the nematic state, the uniaxial electrical anisotropy of the liquid crystal molecules can be exploited for orienting the material in an electric field, thus externally controlling the birefringence of the material. Using this effect, an electric field induced tuning of the modes is achieved. Numerical simulations using the finite-differences time-domain (FDTD) technique employing an anisotropic dielectric medium allow to understand the alignment of the liquid crystal molecules on the surface of the microdisk resonator.

Synthesis of high purity silicon nanoparticles in a low pressure microwave reactor.

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 < or = dPM < or = 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.