Manipulation of emitted light by structured lead halide perovskite nanocrystals for photonics applications

Resumo

Historically, active materials in photonic integrated circuits (PICs) have been implemented using III-V semiconductors, glasses, and ferroelectrics doped with rare-earth ions. However, there is a low-cost alternative based on (nano)materials synthesized using colloidal chemistry techniques. Their use in colloidal suspensions allows for the easy integration into any optical architecture via coating or printing techniques. The structure of perovskite nanocrystals (PNCs) are compounds with general formula ABX3, being A an inorganic or organic bulky cation, B a metal cation such as Pb2+ or Sn2+ and X the halide anion. In this context, all inorganic CsPbX3 (with X = Cl, Br, I) PNCs have recently emerged as an outstanding material with fascinating optical properties, such as a high absorption efficiency, a quantum yield of emission exceeding 90% at room temperature, a tunable band gap depending on chemical composition or size and shape tuning, and high nonlinear optical coefficients. A remarkable point of the CsPbX3 PNCs is the tuning of their band gap, and consequently of their light emission spectrum, by modifying the composition of the halide anion (X): their peak wavelength is observed at around 400 nm (near UV), 510 nm (green) and 680 nm (deep red) for X = Cl, Br and I, respectively, in addition to the ClxBryI1-x-y combinations with 0≤x, y≤1. Moreover, light emission of these PNCs is characterized by high color purity, with PL Full Width at Half Maximum (FWHM) as low as 20 nm for CsPbBr3(green emission) and less than 15 nm for those of CsPbCl3 (emission in blue-violet). In light of these considerations, the goal of this Ph.D. thesis is to fully reveal the significance of PNCs as an active material for photonics and quantum technologies, from both a fundamental and an application standpoint. For the first research objective within that goal, it is mandatory to examine all physical mechanisms responsible for spontaneous emission in PNCs. In the first step, single PNC samples are analyzed as basic building blocks from which more sophisticated architectures can be built. Controlling emitted light in single PNCs and fully characterizing its dependence on excitation fluence, temperature, and ambient conditions, as well as its dynamics, is a major step forward. Once the optimal conditions for PNCs are established, the PNCs can be used to grow super-crystals (SCs) to study super-fluorescence (SF) coherent light and cavity modes within SCs in the second step. The thermal decoherence of this SF will be the next step to be studied, because it is necessary to get coherent light from low to room temperature (RT) for use in photonics and quantum devices. All these results provide novel knowledge on the possible use of cesium lead halide PNCs as an active material and can pave the road for new quantum photonic devices based on PNCs. A second objective is focused on the applications of these PNCs in photonics. Particularly, Hyperbolic metamaterials (HMMs) have recently grasped much attention because they possess the ability for broadband manipulation of the photon density of states and sub-wavelength light confinement. These exceptional properties arise due to the excitation of electromagnetic states with high momentum (high-k modes). Accordingly, HMMs are properly designed, simulated, and fabricated as a fantastic photonic structure able to control the spontaneous emission rate (to achieve Purcell enhancement) of lead halide PNCs deposited on the top. Finally, in order to overcome the PL intensity reduction of the emitters deposited on top of the HMM structures caused by the coupling of perovskite emitters to the HMM modes, due to preferential emission of light into the high-k HMM modes, these HMM structures were modified by light scattering centers. The modifying strategy is easily implemented by dispersing spherical dielectric Mie scatterers onto the HMM/PMMA substrate at the same time. In light of the above-written results, this Ph.D. thesis suggests that colloidal PNCs are promising candidates for opening the way for a new generation of quantum and photonic applications and devices.