| Summary: | Over the past decade, light-emitting diodes (LEDs) based on III-V and organic semiconductor have transformed the landscape of displays and ambient lighting, supplanting traditional technologies such as liquid-crystal and plasma displays, incandescent bulbs, and fluorescent lights. While LEDs boast high luminance and energy efficiency, they fall short of fulfilling the needs of emerging immersive visual technologies. These technologies require light sources with novel optical functionalities, that is not only with high brightness, but also with dynamically tunable spectrum, wavefront, polarization and directionality. Consequently, advancing light-emitting devices to the next level hinges on two key factors: shrinking electronic components through miniaturization and advancing integrated subwavelength optics to achieve functional emission properties. However, the efficient integration of light-manipulating nanophotonic structures with traditional LED devices remains a challenge.
In this thesis, we introduce a novel light emitting device concept, the light emitting transistor with monolithically integrated dielectric metasurfaces, based on the metal halide perovskite semiconductor with excellent luminescence properties and high refractive index. Employing arrays of flat dielectric optical resonators with spatially varying geometric parameters at subwavelength scales, these metasurfaces enable fine control of light-matter interactions and light emission characteristics. In order to achieve intensity enhancement for high brightness luminescence, high finesse photonic resonances are employed through careful design and engineering of the spatially varying optical response. Interestingly, the monolithic metasurface with high quality factor based on excitonic material allows the device operating in the strong coupling regime, enabling electrical generation of exciton-polaritons, bosonic quasiparticles arising form the strong interaction between material excitons and cavity photons. The light-emitting metatransistor device concept, combined with high quality factor metasurface design, offers great possibilities for the realization of novel quantum phenomena in practical polaritonic devices.
The investigation begins with the development of a monolithic light-emitting transistor integrated with perovskite metasurfaces to achieve actively tunable emission polarization. The metasurface is directly patterned into its emission channel using focused ion beam lithography. The lateral configuration of the device, inspired by conventional field-effect transistors, emits light from the exposed surface between its top electrodes. Metasurfaces composed of arrays of nanogratings, exhibiting a resonant response that can be finely tuned across the electroluminescence spectrum of the device, result in nearly a tenfold increase in spectral enhancement of the electroluminescence. Additionally, by leveraging the polarization properties of the nanograting and precise spatial control of the recombination zone via electrical bias, we demonstrate dynamic tunability of the emission polarization.
Further delving into the strong coupling regime, we incorporate a metasurface supporting photonic bound states in the continuum, exotic photonic modes with theoretically infinite Q-factor, into the light emitting transistor. The high-finesse photonic Bloch mode of the metasurface exhibits efficient energy exchange with the electrically injected excitons in the perovskite material, forming electrically driven exciton-polaritons. As a result of this strong coupling, a significant Rabi splitting energy of around 200 meV is achieved, accompanied by a more than 50-fold enhancement of electroluminescence from the polaritons compared to the intrinsic excitonic emission. Furthermore, the unique transistor configuration enables unbalanced charge carrier distribution within the active metasurface, which can be adjusted by the source-drain bias. This functionality can be exploited to electrically control the directionality of polaritonic emission.
As bosonic quasiparticles, exciton polaritons exhibit two spin projections of σ=±1. Exploiting the additional spin degree of freedom in polaritonic devices can open new avenues for spintronic applications such as functional opto-spintronic devices, quantum information processors and neuromorphic computers. However, this is difficult to achieve in electrically driven polaritonic devices. In the final part of the thesis, we report a dielectric metasurface with broken in-plane and out-of-plane inversion symmetry incorporated into the light emitting transistor and demonstrate exciton polaritons with high spin purity (S3~0.8) under electrical injection. The spin-polarized polaritons in the system exhibit a pronounced polaritonic Rashba effect, where the polariton bands split in opposite halves of the momentum space due to their spin properties. Furthermore, we demonstrate electrical selection of individual polaritonic spin states by unbalanced charge injection, thereby allowing to control directionality and helicity of the circularly polarized electroluminescence.
Overall, this thesis introduces a novel perovskite light emitting metatransistor concept, where light-matter interactions can be tuned by metasurface design. This results in excitonic and polaritonic electroluminescence with actively tunable emission characteristics, including brightness, polarization and directionality. Our study based on the electrically driven perovskite light emitting metatransistor in the weak or strong coupling regime represents a significant step toward functional light sources that may lead to the integration of spintronic technologies with electronics and even the realization of solution-processed electrically driven lasers.
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