Two-Photon-Pumped Lasing From Perovskite Semiconductor Nanocrystals

Ultrasmall nanocrystals have always been the research focus among researchers in the field of biophotonics since its discovery.Among them,photodynamic therapy?PDT?is an emerging treatment modality for cancer based on excitation of the localized photosensitizers with appropriate wavelength of light to generate singlet oxygen which will finally help kill cancer cells.However,the restricted choice of the photosensitizers and the corresponding excitation wavelength makes photodynamic therapy limited in vitro.Utilizing the biological window of maximum optical transparency of near-infrared light,we may expand PDT from in vitro to in vivo if we demonstrate two-photon-pumped nanolasers.Two-photon pumped lasers utilize the infrared light to enter into tissue cells.In contrast to single-photon excitation,the two-photon excitation features several merits,including a large penetration depth,high spatial resolution and little damage to the targeted issues.Therefore,the key here is to demonstrate efficient and stable two-photon-pumped lasing which may be of great significance in the filed of biophotonics.However,those kinds of lasers have strict demands on the optical properties of the gain media,i.e.both efficient two-photon absorption and ease of achieving population inversion,which have hindered their practical applications.Although two-photon-pumped lasers have been accomplished in some kinds of materials,they mostly cannot provide the stable lasing under high excitation density for a long time.The high threshold and limited photostability have been regarded as a critical issue facing the two-photon-pumped lasers.Halide perovskite materials have attracted intense research interest due to striking performance in photo-harvesting photovoltaics as well as light-emitting applications.Recently,the emerging perovskite semiconductor nanocrystals have not only strong light-matter interaction but also photoluminescence quantum yield as high as?90%.In our experiments,we observe highly efficient two-photon absorption?with a cross section of 2.7 x 106 GM?in toluene solutions of CsPbBr₃ nanocrystals and successfully demonstrate stable two-photon-pumped lasing at a remarkable low threshold.Our findings suggest perovskite nanocrystals can be used as excellent gain medium for two-photon-pumped lasers toward practical applications.The two main results are summarized below:We first employ ultrafast transient absorption spectroscopy to study the optical gain in perovskite semiconductor nanocrystals.From the transient absorption spectra,we observe the emergence of the optical gain in films of perovskite nanocrystals and further analyze the fundamental physical mechanism of the optical gain,which is the first step to demonstrate two-photon-pumped lasers.During the development of optical gain,the presence of biexciton indicates that optical gain here may originate from the multi-exciton effect.Thus,Auger recombination is further analyzed by power-dependent ultrafast spectroscopic experiment,from which we obtain the dynamics of biexction and indicate its contribution to the optical gain.We then focus on lasing from the perovskite nanocrystals using the optical gain observed in the transient absorption experiment.Under a low excitation fluence,we demonstrate the amplified spontaneous emission from thick CsPbBr₃ nanocrystal films.We obtain large optical gain(>500cm^-1)in thin films by the well-established varied stripe length experiment.Finally,we have succeeded in demonstrating stable two-photon-pumped lasing at a remarkable low threshold by coupling CsPbBr₃ nanocrystals with microtubule resonators.