Mid-infrared characterization of two-dimensional photonic crystal slabs fabricated in silicon with laser interference lithography

The goal of the present work was to perform mid-infrared characterization of two dimensional photonic crystal slabs fabricated in silicon with laser interference lithography. A two-dimensional (2D) silicon photonic crystal (PhC), which is designed to provide a modified dispersion for photon energies of less than half of the electronic band gap of silicon, and which has been fabricated by a novel modification of laser interference lithography (LIL), is studied by angular dependent infrared reflectivity and transmission measurements. The existence of resonance features is experimentally demonstrated as observed in the polarized reflectivity and transmission spectra, and which arises from resonant coupling of the incident infrared radiation to photonic modes. The measured photonic crystal resonances are used to derive the quality factors of the probed photonic modes via fits to the Fano-type line shapes found. The quality factor of the corresponding photonic modes is also theoretically calculated. The obtained theoretical values, and comparison with the experimentally obtained quality factors, provide the first information on the LIL fabrication-inherent quality of the crystals. In the named experiments, the LIL fabricated crystals are investigated with low intensity probe beams in the near to mid-infrared range, such that an optically nonlinear response cannot be detected. To provide also information on the nonlinear response, we present initial results also at high intensities. For this, we have studied the light induced change in the optical phase upon reflection from the photonic crystal using a Mach-Zehnder setup. The technique involves measuring the time dependent reflection of a pulsed probe beam on the 2D PhC sample due to the excitation from an additional, pulsed drive beam. With this technique we have realized what we believe to be the first experimental observation of optical switching of the reflection phase from a guided resonance of a 2D PhC slab. In conclusion, the goals that have been achieved are the development of a novel fabrication process for high-index 2D photonic crystals, the optically linear characterization of the properties of the fabricated crystal using specular reflectivity and normal incidence transmission experiments, and the first interferometric characterization of the optically nonlinear response of photonic crystal in terms of an optical switching of the reflection phase from a photonic resonance