Organic-inorganic lead-halide perovskites have been the subject of recent intense interest due to their unusually strong photovoltaic performance. A new addition to the perovskite family is all-inorganic Cs-Pb-halide perovskite nanocrystals, or quantum dots, fabricated via a moderate-temperature colloidal synthesis. While being only recently introduced to the research community, these nanomaterials have already shown promise for a range of applications from color-converting phosphors and light-emitting diodes to lasers, and even room-temperature single-photon sources. Knowledge of the optical properties of perovskite quantum dots still remains vastly incomplete. Here we apply various time-resolved spectroscopic techniques to conduct a comprehensive study of spectral and dynamical characteristics of single- and multiexciton states in CsPbX3 nanocrystals with X being either Br, I, or their mixture. Specifically, we measure exciton radiative lifetimes, absorption cross-sections, and derive the degeneracies of the band-edge electron and hole states. We also characterize the rates of intraband cooling and nonradiative Auger recombination and evaluate the strength of exciton-exciton coupling. The overall conclusion of this work is that spectroscopic properties of Cs-Pb-halide quantum dots are largely similar to those of quantum dots of more traditional semiconductors such as CdSe and PbSe. At the same time, we observe some distinctions including, for example, an appreciable effect of the halide identity on radiative lifetimes, considerably shorter biexciton Auger lifetimes, and apparent deviation of their size dependence from the "universal volume scaling" previously observed for many traditional nanocrystal systems. The high efficiency of Auger decay in perovskite quantum dots is detrimental to their prospective applications in light-emitting devices and lasers. This points toward the need for the development of approaches for effective suppression of Auger recombination in these nanomaterials, using perhaps insights gained from previous studies of II-VI nanocrystals.

We present absolute two-photon absorption (2PA) spectra of 15 commercial organic dyes covering an extended range of excitation wavelengths, 550-1600 nm. The 2PA is measured with an estimated accuracy +/-10% using a femtosecond fluorescence excitation method. The data are corrected for the variations of the pulse duration and the beam profile with the excitation wavelength, and are applicable as reference standards for 2PA measurements.

Two-photon excitation of fluorescent proteins is an attractive approach for imaging living systems. Today researchers are eager to know which proteins are the brightest, and what the best excitation wavelengths are. Here we review the two-photon absorption properties of a wide variety of fluorescent proteins, including new far-red variants, to produce a comprehensive guide to choosing the right FP and excitation wavelength for two-photon applications.Two-photon laser scanning microscopy (2PLSM) 1,2 of cells and tissues expressing fluorescent proteins is becoming a powerful tool for biological studies at different levels of organization [2][3][4] . The advantages of two-photon excitation (2PE) include reduced out-offocus photobleaching, less autofluorescence, deeper tissue penetration, and intrinsically high three-dimensional resolution 1,2 . 2PLSM should make it possible to obtain even better optical recordings of ion concentration and cell signaling with genetically targeted sensors 5,6 . Twophoton excitation of fluorescent proteins can also be considered as potentially advantageous in the contexts of genetically targeted deep photodynamic therapy or chromophore-assisted light inactivation 6 , three-dimensional optical memory 7 , as well as superresolution (subdiffraction limited) imaging techniques, such as stimulated emission depletion 8 , photoactivated localization microscopy, and stochastic optical reconstruction microscopy 9 .To fully realize the potential of 2PE of the fluorescent proteins, it is important to know their two-photon absorption (2PA) spectra, cross sections, σ 2 , and 2PE action cross sections, or brightness, σ 2 ', (σ 2 ' = σ 2 × φ, where φ is the fluorescence quantum yield). The linear, onephoton absorption (1PA) spectra and extinction coefficients of many fluorescent proteins have been described and reviewed 5,6,10 (Supplementary Table 1 online), but the 1PA properties are not sufficient to predict the key 2PA properties such as optimum excitation wavelength and maximum brightness (Box 2).Correspondence should be addressed to M.D. (drobizhev@physics.montana.edu).. 4 Present address: School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia. 5 Present address: Vollum Institute, Oregon Health and Science University, Portland, Oregon. HHS Public Access Author Manuscript Author ManuscriptAuthor Manuscript Author ManuscriptHere we present a systematic characterization of the 2PA properties of 48 different fluorescent proteins using the same experimental setup, common 2PA reference standards, and an all-optical method for measuring mature chromophore concentration 11 (Supplementary Methods online). Briefly, we use a relative fluorescence method with femtosecond excitation and coumarin 485 (Exciton), coumarin 540A (Exciton), rhodamine 610 (Exciton), fluorescein (Aldrich), and styryl 9M (Aldrich) as 2PA standards 12 , which eliminates the necessity to calibrate the laser parameters. The power dependence of the fluorescence signal was quadratic for all data ...