Organic microcavities and photonics
The interaction between light and matter is of fundamental importance in a range of optoelectronic technologies. Modifying the environment around an excited state can profoundly change its emission properties; for example, by placing a fluorescent dye molecule between two highly reflective mirrors, the basic emission processes goes from one where spontaneous emission dominates (characterized by incoherent, broadband emission), to one where lasing dominates (characterized by a spectrally-narrow and coherent emission). Here, we are interested in developing new types of photonic structures that can modify emission processes. One example of this is an optical nanocavity. Nanocavities can be fabricated into a 'photonic-crystal'. This is a 2D structure based on a periodic array of 'holes' in a dielectric slab. By placing a physical defect into a photonic-crystal (for example by missing out some holes), a cavity is created in which light can be very efficiently trapped (see Figure 1). When an electromagnetic dipole is placed within a nano-cavity, many fascinating physical processes can occur, including significant enhancements of radiative-emission rates. We are interested in placing molecular materials into nanocavites to explore fundamental physics and also create new types of bio-sensors and chemical sensors.
Fig 1: Noda optical cavities overcome problems with leakage that reduces the efficiency of very tightly confined nanocavities. Above is a scanning electron micrograph of an asymmetric Noda cavity, move your mouse over the image to see the effect on field distribution.
We also have a long-standing interest in the physics of organic materials placed in high finesse 1-dimensional optical cavities (see figure 2). Here, two high reflectivity mirrors are placed in close proximity - usually a few hundred nanometers. The mirrors quantize the optical field within the cavity, meaning that only photons of certain energy can be confined within the structure. Within the so-called 'strong-coupling' regime, the trapped cavity photons and the electronic states of a material placed in the cavity can undergo a mixing process, where the new states formed (termed cavity polaritons) are a superposition of optical and electronic states. This can be seen by measuring either photoluminescence emission or white-light reflectivity from the cavity (see Figure 3). Here, we plot a 'dispersion' curve for an organic microcavity (photon-energy against in-plane momentum) with 'anticrossing' between photon and exciton states detected. We are currently studying non-linear optical processes in organic microcavities, and making electrically driven strong-coupled devices.
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| Fig 2: Cavity schematic. | Fig 3: White-light reflectivity from the cavity. |
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LEFT: NEW Cover Article: Vibrationally Assisted Polariton-Relaxation Processes in Strongly Coupled Organic-Semiconductor Microcavities Coles et al, Advanced Functional Materials 21 (2011) 3691-3696" |
