Circularly polarized light emission from microcavity light emitting devices based on sculptured chiral reflectors

In this presentation, a novel microcavity light emitting device is demonstrated to produce pure circularly polarized light with simultaneous control over emission bandwidth and center wavelength. This is achieved by sandwiching fluorescent molecules (Alq3) between two sculptured thin film (STF) chiral reflectors to form a resonant microcavity device of circular polarization selectivity, and optically pumping the device with unpolarized violet light. The microstructure of the STF chiral reflector consists of a large number of bent and curved columns that are parallel and identical to each other. The film possess a distinguished axis about which the constitutive property tensors rotate, much like the way a screw turns, and are therefore structurally chiral (Figure 1(a))[1]. On axial excitation, i.e., when all fields vary spatially along only the axis of rotation, chiral reflectors display optical activities as circular Bragg phenomenon in a narrow wavelength range (Bragg regime) [2], as shown in Figure 1(b). When a pair of STF chiral reflectors is utilized to create a resonant microcavity structure, in the Bragg reflection regime, the polarization dependent reflection of the structurely left-handed (right-handed) STF reflectors inhibits the presence of optical modes for the right (left) circular-polarization handness in the microcavity, which induces the resonance of left-handed (right-handed) circular polarization. Figure 2 shows the schematic structure of an STF reflector-based microcavity light emitting device. Glass is used as the substrate material due to its transparency in visible and near infrared parts of the spectrum. The bottom TiO ₂ STF reflector was deposited on the glass with the serial bideposition (SBD) technique [3]. The reflector was coated with a thin film of Alq3 molecules in a subsequent thermal evaporation process. Finally, the binding of a top STF reflector to the bottom layers completed the device fabrication process. The thickness of the Alq3 active region was tailored to match the resonance condition in the microcavity for the intended emission wavelength, while the period of the STF chiral reflectors was designed to achieve the intended emission bandwidth. We have observed spectrally narrowed fluorescent emission of pure circular polarization from the fabricated microcavity light emitting device, as shown in Fig. 3. The Full Width at Half Maximum (FWHM) of the left-circularly polarized fluorescent emission is 12 nm, which is much narrower than that of the non-cavity Alq3 fluorescence. The measure emission peak wavelength matches well with the resonance wavelength of the microcavity. The ongoing effort involves the development of a physical model to describe how the confinement applied by the circular-polarization-selective STF chiral reflectors alters the optical mode density within the microcavity, and spatially and spectrally redistributes the emission output. Using this model, the coupling of excitons to the confined circularly polarized electromagnetic field in the cavity will be investigated within the theoretical frame.