Nanoopticas and photonics

Optical amplification and lasing from compact films of perovskite nanocrystals
Sergii Yakunin1,2, Loredana Protesescu1,2, Franziska Krieg1,2, Maryna I Bodnarchuk1,2, Maksym V Kovalenko 1,2
1 Department of Chemistry and Applied Biosciences, Laboratory of Inorganic Chemistry, ETH Zürich, Vladimir-Prelog-Weg 1, CH-8093 Zürich, Switzerland
2 Laboratory for Thin Films and Photovoltaics, Empa—Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland 
E-mail: yakunin@inorg.chem.ethz.ch
We recently introduced fully-inorganic perovskite nanocrystals (NCs) into research as very promising material for optoelectronics [1]. During the comprehensive optical characterization of these materials we found that this class of NCs – due to their high photoluminescence quantum yield (PLQY, 50-90%), long exciton lifetime and large exciton delocalization – has great potential as an active material for devices exploiting optical gain such as lasers.  We found that the effects of amplified spontaneous emission (ASE) appear in CsPbBr3 NC films with a very low threshold in optical pumping fluence of about 5 μJ cm−2 [2]. The optical gain in perovskite NCs is tunable through the whole visible spectrum range by variation of the anion composition. By applying conformal deposition of thin layer of CsPbBr3 NCs over an optical resonator (silica microsphere) it was relatively easy to demonstrate whispery gallery modes of lasing. Additionally, stochastic modes of random lasing appear in the highly scattering spectral region of blue emission of CsPb(Br/Cl)3 NC film - an interesting property for using in cryptography and security applications. We also tested perovskite NCs with organic and mixed (organic-inorganic) cations. There found that aggregation of nanoplatelets and NCs or thermally inducing merging of NCs also decrease threshold for ASE [3-5]. 
1.	Protesescu, L. et. al. Nanocrystals of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut. Nano Letters 2015, 15 (6), 3692-3696.
2.	Yakunin, S. et.al Low-threshold amplified spontaneous emission and lasing from colloidal nanocrystals of caesium lead halide perovskites. Nature Communications 2015, 6, 8056.
3.	Vybornyi, O. et. al. Polar-solvent-free colloidal synthesis of highly luminescent alkylammonium lead halide perovskite nanocrystals. Nanoscale 2016, 8 (12), 6278-6283.
4.	Protesescu, L. et. al. Dismantling the “Red Wall” of Colloidal Perovskites: Highly Luminescent Formamidinium and Formamidinium–Cesium Lead Iodide Nanocrystals. ACS Nano 2017, 11 (3), 3119-3134.
5.	Krieg, F. et. al. Colloidal CsPbX3 (X = Cl, Br, I) Nanocrystals 2.0: Zwitterionic Capping Ligands for Improved Durability and Stability. ACS Energy Letters 2018, 3 (3), 641-646.