Bartl Group Research
Research in the Bartl group focuses on the fabrication and characterization of nanophotonics – optically active 3-dimensional nano- and microstructured materials. Nanophotonics – nanotechnology utilizing photons rather than electrons as the main information carrier – have emerged over the last years as one of the most promising concepts of next generation optical device configurations such as photonic chips and all-optical integrated circuits. However, in order to become the 21st century’s driving force in the development of novel information technology concepts a thorough understanding of the photophysics of nanophotonics – both theoretically and experimentally – is needed. The primary research goals in the Bartl group are directed exactly towards this challenge: The development of novel photonic structures and a thorough study (theoretically and experimentally) of their structural and optical properties. To meet this end we employ state-of-the-art micro-spectroscopy techniques as well as optical and electron microscopy imaging to systematically study non-classical optical phenomena and manipulation of photons in specifically engineered electromagnetic environments such as photonic band gap crystals, colloidal nanocrystals (quantum dots), and magneto-optically active nanostructures.
Photonic Band Structure Crystals
Photonic band gap structures (photonic crystals) are artificial electromagnetic crystals for which the band structure concepts of solid-state physics are applied to electromagnetism. A central theme of our research is the design and characterization of photonic crystals with specifically tailored optical properties. Photonic crystals are the electromagnetic analog to solid state crystals with arrangement of building blocks, or features, with spacings comparable to the wavelength of light. Due to the diffraction of light from the different crystal planes of these artificial electromagnetic crystals, certain frequency ranges of light become forbidden to propagate through the structure, resulting in distinct photonic band structures (dispersion relations).
Figure1. Titania inverse opal photonic crystals. Views along the 111 (left) and 100 (right) face-centered cubic crystal axes. Scale bars are 300 nm.
We are interested in studying the structure – property relationship in these materials with a specific focus on structures with photonic properties in the visible region of the electromagnetic spectrum. Our central focus is the titania inverse opal (fcc air-spheres surrounded by a titania matrix), which we fabricate by combining the simplicity of self-assembly with sol-gel chemistry. Using this easily fabricated template, we are currently exploring novel routes to engineer systems with unique and enhanced photonic properties. Here we take inspiration from the wealth of photonic structures found in Nature. Current research efforts focus on the 3-dimensional photonic structures found in the exoskeleton of various weevils, producing the beautifully iridescent green color. Of particular interest to our group is to gain fundamental insights into how and why these bio-photonic systems have developed and to use this information to develop better fabrication strategies that build upon and surpass conventional techniques for the visible.
Figure 2. Left: Photograph of the weevil Lamprocyphus augustus. Right: Optical microscope image of individual green iridescent scales attached to the exoskeleton of the beetle.
A further nanophotonic research area in the Bartl group is directed towards combining magnetism and photonic band structure concepts. The goal is to gain insights into how magneto-optical phenomena are affected in photonic crystals. Our research focuses on fabricating nano- and micro-structured magnetic composites exhibiting distinct photonic band structures and studying the effect of photonic band structure phenomena such as localization of light and enhancement of density of optical states at a photonic band edge on basic magnetic and magneto-optical properties (Faraday rotation, Kerr effect, magnetization-induced 2nd harmonic generation, etc.). Photonic crystal structures of main interest for these studies are dilute magnetic semiconductor multilayer films (1-dimensional photonic crystals) and inverse opals (3-dimensional photonic crystals).
Colloidal Nanocrystal Quantum Dots
Figure 3. SEM micrographs showing cross-cut views of the photonic crystal structure found inside individual scales of the weevil Lamprocyphus augustus.
Colloidal nanocrystal quantum dots are nanometer-sized inorganic semiconductors. Utilizing colloidal organometallic synthesis routes, quantum dots are fabricated in sizes smaller or approaching their exciton Bohr radius (1-10 nm in diameter), resulting in size-dependent electronic and optical properties due to the quantum confinement effect. These size-dependent tunable properties make quantum dots interesting for an extensive range of applications, including imaging, sensing, biological labeling, and as photon sources in novel nanophotonic devices.
Figure 4. Absorption spectra of a nanocrystal QD sample before (red spectrum) and after (green spectrum) the size reduction treatment. The inset shows the corresponding emission properties. Both samples were excited by a UV lamp at 365 nm.
A central focus in the Bartl group is to develop strategies to post-synthetically manipulate and fine-tune the size and size-dependent properties of colloidal quantum dots. Such studies not only provide us with valuable insights into the growth evolution, stability, and surface phenomena of colloidal nanocrystals, but also enable us to precisely tune their electrical and optical properties. In addition, we are also interested in using these colloidal nanocrystal quantum dots as nanoscale photon source in combination with various nanophotonic environments. We are particular interested in studying and understanding how spontaneous emission from can be manipulated in the presence of specifically engineered photonic environments such as microcavities and/or photonic band gap crystals for applications in light localization, excitation/energy transfer, and photon amplification.