G. von Freymann

Three-dimensional face-centered-cubic photonic crystal templates by laser holography: fabrication, optical characterization, and band-structure calculations

We fabricate three-dimensional photoresist templates by means of laser holography. In particular, fcc structures are achieved by placing a specially designed “prism” onto the photoresist surface. This solves the problem of previous work, in which the refraction at the air–photoresist interface made it impossible to obtain the required angles of the light wave vectors inside the photoresist. The photoresist templates are characterized by scanning electron microscopy as well as by optical transmission spectroscopy, which agree well with numerical band-structure calculations.

Direct laser writing of three-dimensional submicron structures using a continuous-wave laser at 532 nm

Three-dimensional direct laser writing is commonly associated with tightly focused femtosecond laser pulses. Although few reports have used continuous-wave lasers instead, it is unclear whether state-of-the-art three-dimensional submicron structures for photonics can be fabricated along these lines. Here, we systematically investigate the underlying mechanisms using a 532 nm continuous-wave laser operating at power levels of only some 10 mW and three different commercially available photoresists. Body-centered cubic woodpile photonic crystals composed of 24 layers with rod spacings as small as 450 nm serve as a demanding benchmark example for illustrating “state-of-the-art.”

Fabrication and characterization of silicon woodpile photonic crystals with a complete bandgap at telecom wavelengths

By using direct laser writing into a novel commercially available photoresist and a silicon-double-inversion procedure followed by tempering of the silicon structures, we realize high-quality centered-tetragonal woodpile photonic crystals with complete photonic bandgaps near 1.55 microm wavelength. The 6.9% gap-to-midgap ratio bandgap is evidenced by the comparison of measured transmittance and reflectance spectra with band-structure and scattering-matrix calculations.

Layer-by-layer three-dimensional chiral photonic crystals

We fabricate and characterize polymeric three-dimensional layer-by-layer chiral photonic crystals. The obtained circular dichroism from polarization stop bands is comparable with that of recently demonstrated circular-spiral photonic crystals. Moreover, telecommunication wavelengths are easily accessible with the layer-by-layer approach; even visible wavelengths are in reach.

3D-2D-3D photonic crystal heterostructures fabricated by direct laser writing

Using direct laser writing, we fabricate photoresist templates for 3D-2D-3D photonic crystal heterostructures for what we believe to be the first time. The optical properties of these structures are directly compared with the theoretical ideal, revealing good agreement and hence good sample quality. This provides an experimental starting point for the microfabrication and testing of broadband, 3D air-waveguide microcircuitry in photonic bandgap materials.

Three-dimensional chiral photonic superlattices.

We study the chiral optical properties of three-dimensional photonic superlattices composed of polymeric helices in various spatial checkerboard-like configurations. We fabricate and characterize four arrangements of circular-spiral photonic crystals with different relative phase and handedness.

Waveguides in three-dimensional photonic-bandgap materials by direct laser writing and silicon double inversion

Three-dimensional complete photonic-bandgap materials offer unique opportunities regarding the integration of optical waveguide architectures in three dimensions. However, corresponding experimental realizations are truly sparse. Here, we fabricate such waveguides using direct laser writing and a silicon double-inversion procedure. The optical characterization is in good agreement with theoretical calculations, raising hopes that even more complex architectures may soon come into reach.

Diffraction properties of two-dimensional photonic crystals

We show that the envelope of the diffraction efficiency of a two-dimensional photonic crystal can exhibit spectral regions of very small diffraction efficiency (<5×10−3), while in other regions, the diffraction efficiency is near unity. The experimental results on higher bands of hexagonal, silicon-based photonic crystals agree well with corresponding numerical calculations and highlight the prominent role of the surface termination, an aspect which cannot be described by the photonic band structure alone. We speculate about possible applications of such additional spectral filters in Raman and photoluminescence spectroscopy.

Statistical analysis of near-field photoluminescence spectra of single ultrathin layers of CdSe/ZnSe

The statistical analysis of thousands of near-field photoluminescence spectra of single ultrathin CdSe layers at 20 K exhibits a strong positive correlation peak around 20 meV energy with a width of 5 meV. Our data are consistent with individual spectra which consist of sets of many pairs of lines. In each pair, the two lines must have comparable strength. We speculate about the origin of these pairs.

COMPUTER SIMULATIONS ON NEAR-FIELD SCANNING OPTICAL MICROSCOPY : CAN SUBWAVELENGTH RESOLUTION BE OBTAINED USING UNCOATED OPTICAL FIBER PROBES?

Recent experiments claim that subwavelength resolution can be obtained with an optical scanning microscope using uncoated optical fiber probes. In these experiments, linearly polarized light is sent down the fiber which is reflected and depolarized in the tip-sample region. The internally reflected signal in the orthogonal polarization is detected. Here, numerical solutions of the vector Maxwell equations for a model are discussed. In this model, subwavelength resolution can indeed be obtained in the above mode, while this is not possible without polarization sensitivity. The influence of parameters such as polarization, different scanning modes and tip-sample distance is discussed.