Light Emission of Single Emitters in 3-dimensional Photonic Crystals
Final Report Abstract
This project was devoted to the development and application of experimental methods for the study of the local optical properties of 3-dimensional photonic crystals and their correlation with its structure. Two fluorescence microscopy setups have been developed, which record the angular and spectral variation of the emission intensity propagating from an excited spot inside the photonic crystal. These techniques are based on Fourier- or so called back focal plane imaging techniques and use the autofluorescence or the fluorescence of fluorescent beads inside the photonic crystal. This technique has turned out to be much easier to apply and the results are much easier to analyze as compared to the originally suggest defocused imaging. Both techniques, however, deliver the same kind of information on the light propagation in photonic crystals. The first technique, which is back focal plane imaging (BFPI) records the back focal plane image directly with a CCD camera and a band pass filter giving spectral resolution to this method. The second method of back focal plane spectroscopy (BFPS) is imaging the back focal plane image onto the entrance slit of an imaging spectrograph. By sliding the back focal plane image across the slit one obtains a complete angular emission spectrum with much higher spectral resolution as compared to BFPI. BFPI on the other side is very fast as compared to other techniques used in the literature and allows for a quick characterization of the local optical properties of photonic crystals. Both methods allowed a local study of light propagation inside 3-dimensional photonic-crystals up to emission angle of 80° without moving the sample or even by a single camera image. Both techniques therefore deliver a wealth of new information on the local structure and optical properties of colloidal photonic crystals of colloidal crystal formation. The methods have been characterized in its spectral and spatial resolution within this project by comparing the experimental results from colloidal photonic crystals with numerical simulations of band structure and light propagation within photonic crystals. All results reveal a very good agreement of the simulated and experimentally determined stop band positions of these crystals in the case when a slight compression of the photonic lattice structure is assumed. What is currently not understood, is the contrast reversal in the back focal plane images for photonic crystal with large bead diameters. The studies were further extended to measurements on structural defects in the colloidal crystals. Strong structural variations on length scales below 100 µm are found even though the surface of the colloidal crystals were well ordered. The main defects found are stacking faults and structural cracks which form as the result of stresses in the system or the simultaneous growth of multiple domains. A set of parameters characterizing the back focal plane images has been developed to allow a mapping of the spatial variation of the optical properties in the prepared photonic crystals. The study highlights that local structural changes in crystal symmetry as well as in stacking faults may occur without crack formation in the crystal structure. On the other side, the existence of cracks does not necessarily lead to a change in crystal structure or orientation. The results demonstrate that colloidal photonic crystals, which are commonly studied at macroscopic length scales reveal many structural details when studied by microscopic techniques. Back focal plane imaging has also been applied to the study of colloidal crystal growth itself. The experiments revealed 3 stages of crystal formation. The system starts with a disordered suspension in which in a second stage a low density crystalline structure is formed presumable due to the Debye layer repulsion in the colloidal system. This crystalline structure is continuously decreasing its lattice constant until water is released from interstitials in the crystal. During the release of water mechanical stresses appear, which lead to a partial delimitation of the photonic crystal film and the formation of cracks perpendicular to the growth direction. Overall the set of developed methods and analysis techniques will allow future studies of microscopic properties of three dimensional photonic crystals.
Publications
- Imaging single quantum dots in three-dimensional photonic crystals. Physical Review Letters 07/2006; 96(24):243902
Michael Barth, Roman Schuster, Achim Gruber, Frank Cichos
- Back focal plane imaging spectroscopy of photonic crystals. Appl. Phys. Lett. 08/2012; 101(8)
Rebecca Wagner, Lars Heerklotz, Nikolai Kortenbruck, Frank Cichos
(See online at https://doi.org/10.1063/1.4746251) - Fast measurement of photonic stop bands by back focal plane imaging. Physical Review. B, 04/2013; 87(16):165438
Rebecca Wagner, Frank Cichos
(See online at https://doi.org/10.1103/PhysRevB.87.165438)
