Photonic Crystals

tl_files/mona/images/Photonic Crystals/WidefieldSetup.pngPhotonic Crystals (PCs) are materials where the dielectric constant is varying periodically. These dielectric structures are able to superimpose the scattering waves in such a way that photonic band structures form in a similar way as electronic band structures in common crystals. Thus many features of electronic band structures such as band gaps can be found. Unlike real crystals, photonic crystal can be designed and structured without any limits. Thus the dispersion of photons inside the photonic crystal can be shaped in well defined ways.

Our group is interested in how the propagation of light is modified by a 3-dimensional photonic crystal. For this purpose we employ internal emitters and single chromophores which are studied by fluorescence microscopy techniques developed in the group.

Angle Resolved Emission Microscopy

tl_files/mona/images/Photonic Crystals/animated_Bandstructure.gifThe dispersion relation in photonic crystal, i.e. the photonic band structure can be studied by angle resolved transmission and reflection spectroscopy. This is in general done by varying the incident and detection angle step by step recording at each position a full spectrum. This method is rather time consuming and requires to couple the incident light into the photonic structure. 

We have developed a promising method relying on an optical microscopy setup. Our approach collects the emission of a photonic structure. To obtain angular resolved spectra, the back focal plane of the microscopy lens is imaged to a spectrograph. This allows a fast recording of the photonic band structure symmetry and a detection of defects in the crystal. 

Defocused Imaging of Single Emitters in Photonic Crystals

In a Photonic Crystal the dielectric constant in the material is varying locally, so the density of states and all the optical properties of PCs are local properties. For this reason there is a need for local measurements, which has not been possible so far.
Our idea is to use single emitters, incorporated into the PC, as local probes and detect the influence of the PC on their emission. Their emission wavelength is inside the stop band of the PC.
To take into account that our PCs possess a band gap for certain directions only, a new value, the FLDoS (Fractional Local Density of States), is introduced [1]. It describes the angular dependence of the optical density of states at a certain wavelength.
The fluorescence of single Quantum Dots (QDs) is detected with a home build wide field microscope. However, when focusing on a single emitter we only see the point spread function from which one can not obtain any information about the angular dependence of the QDs emission. To gain this, we use a technique called defocused imaging. It was developed to determine the orientation of molecules or QDs on a substrate [2], because this technique makes it possible to observe the anisotropy of emission which is inherent to those emitters. Our idea is that defocused imaging will enable us to also observe an additional anisotropy which is induced by a photonic crystal [3].

 

[1] M. Barth, A. Gruber und F. Cichos: Spectral and angular redistribution of photoluminescence near a photonic stop band, Phys. Rev. B. 72, 085129 (2005)
[2] Böhmer und J. Enderlein: Orientational imaging of single molecules by wide-field epifluorescence microscopy, J. Opt. Soc. Am. B 20, 554-559 (2003)
[3] M. Barth, R. Schuster, A. Gruber und F. Cichos: Imaging single quantum dots in three-dimensional photonic crystals, Phys. Rev. Lett. 96, 243902 (2006)