Low Threshold Two-Photon-Pumped Amplified Spontaneous Emission in CH3NH3PbBr3 Microdisks

Ultraviolet-to-infrared broadband photodetector and imaging application based on a perovskite single crystal

The organic-inorganic hybrid perovskite CH₃NH₃PbBr₃(MAPbBr₃) has been well developed in the X-ray to visible light band due to its superior optoelectronic properties, but this material is rarely studied in the infrared band. In this paper, a UV-NIR broadband optical detector based on MAPbBr₃ single crystal is studied, and the response range can reach the near-infrared region. In the visible light band, the optical response of the device is mainly caused by the photoelectric effect; in the near-infrared band, the optical response of the device is mainly caused by the thermal effect. The carrier response of MAPbBr₃ material under different wavelengths of light was investigated using a non-contact measurement method (optical pump terahertz (THz) probe spectroscopy). This paper also builds a set of photoelectric sensor array components, and successfully realizes the conversion of optical image signals to electrical image signals in the visible light band and infrared band. The experimental results show that MAPbBr₃ crystals provide a new possibility for UV-NIR broadband photodetectors.

Two-photon-pumped amplified spontaneous emission from Ruddlesden-Popper perovskite flakes

Herein, we report the two-photon pumped amplified spontaneous emission (ASE) in the 2D RPPs flakes at room temperature. We prepared high-quality (BA)₂(MA)_n-1Pb_nI_3n+1 (n = 1, 2, 3, 4, 5) flakes by mechanical exfoliating from the fabricated crystals. We show that the (BA)₂(MA)_n-1Pb_nI_3n+1 flakes display a tunable two-photon pumped emission from 527 nm to 680 nm, as n increases from 1 to 5. Furthermore, we demonstrated two-photon pumped ASE from the (BA)₂(MA)_n-1Pb_nI_3n+1 (n = 3, 4, 5) flakes. The two-photon pumped ASE thresholds of the RPPs are lower than lots of the other semiconductor nanostructures, indicating an excellent performance of the RPPs for two-photon pumped emission. In addition, we investigated the pump-wavelength-dependent two-photon pumped ASE behaviors of the RPPs flakes, which suggest that the near-infrared laser in a wide wavelength range can be converted into visible light by the frequency upconversion process in RPPs. This work has opened new avenues for realizing nonlinearly pumped ASE based on the RPPs, which shows great potential for the applications in wavelength-tunable frequency upconversion.

One-Step Prepared Water-Resistant Organic-Inorganic-Hybrid Perovskite Quantum Dots with Zn-Oxygen Vacancies for Attempts at Nitrogen Fixation

Applying organic-inorganic hybrid perovskite quantum dots (PQDs) to photocatalytic nitrogen fixation is hindered long-term by the inherent instability in water and tedious preparations. Here, to realize PQD-catalyzed photocatalytic N₂ reduction reaction (NRR), water-resistant PQDs are simply prepared through one-step electrospray synthesis in microseconds. During the fast electrospray, PQDs of Zn/PbO-doped methylammonium lead bromide (Zn/PbO/PC-Zn/MAPbBr₃ , MA: CH₃ NH₃ ) are prepared and part-encapsulated by polycarbonate. The synthesis maintains good water resistance, whose restriction on charge transport is overcome skillfully. Simultaneously, substitution of Zn with Pb on water-resistant surface is also achieved, which fabricates new Zn-oxygen vacancies (Zn-OVs) with Zn/PbO-Zn/MAPbBr₃ type I heterojunction. This facilitates efficient electron transfer from internal heterojunction interface of Zn/MAPbBr₃ PQDs to the surface of Zn/PbO. Demonstrated by theoretical calculations, Zn-OVs promote chemisorption and polarization of N₂ . In addition, s-electrons in exposed Zn become active due to changes of electron filling of Zn orbitals under OVs' co-doping. Thus, photocatalytic N₂ reduction reaction catalyzed by organic-inorganic hybrid PQDs is first achieved in aqueous phase without sacrificial agents being added. This initiates possibilities for photocatalytic applications of organic-inorganic hybrid PQDs in aqueous phase.

Nonlinear Photonics Using Low-Dimensional Metal-Halide Perovskites: Recent Advances and Future Challenges

Low-dimensional metal-halide perovskites have exhibited significantly superior nonlinear optical properties compared to traditional semiconductor counterparts, thanks to their peculiar physical and electronic structures. Their exceptional nonlinear optical characteristics make them excellent candidates for revolutionizing widespread applications. However, the research of nonlinear photonics based on low-dimensional metal-halide perovskites is in its infancy. There is a lack of comprehensive and in-depth summary of this research realm. Here, the state-of-the-art research progress related to third-and higher-order nonlinear optical properties of low-dimensional metal-halide perovskites with diverse crystal structures from 3D down to 0D, together with their practical applications, is summarized comprehensively. Critical discussions are offered on the fundamental mechanisms beneath their exceptional nonlinear optical performance from the physics viewpoint, attempting to disclose the role of intrinsic attributes (e.g., composition, bandgap, size, shape, and structure) and external modulation strategies (e.g., developing core-shell structures, transition metal ion doping, and hybridization with dielectric microspheres) in tuning the response. Additionally, their potential applications in nonlinear photonics, nonlinear optoelectronics, and biophotonics are systematically and thoroughly summed up and categorized. Lastly, insights into the current technical challenges and future research opportunities of nonlinear photonics based on low-dimensional metal-halide perovskites are provided.

Photostable and Uniform CH3NH3PbI3 Perovskite Film Prepared via Stoichiometric Modification and Solvent Engineering

Solution-processed organometal halide perovskites (OMHPs) have been widely used in optoelectronic devices, and have exhibited brilliant performance. One of their generally recognized advantages is their easy fabrication procedure. However, such a procedure also brings uncertainty about the opto-electric properties of the final samples and devices, including morphology, stability, coverage ratio, and defect concentration. Normally, one needs to find a balanced condition, because there is a competitive relation between these parameters. In this work, we fabricated CH₃NH₃PbI₃ films by carefully changing the ratio of the PbI₂ to CH₃NH₃I, and found that the stoichiometric and solvent engineering not only determined the photoluminescence efficiency and defects in the materials, but also affected the photostability, morphology, and coverage ratio. Combining solvent engineering and the substitution of PbI₂ by Pb(Ac)₂, we obtained an optimized fabrication condition, providing uniform CH₃NH₃PbI₃ films with both high photoluminescence efficiency and high photostability under either I-rich or Pb-rich conditions. These results provide an optimized fabrication procedure for CH₃NH₃PbI₃ and other OMHP films, which is crucial for the performance of perovskite-based solar cells and light emitting devices.

Noise-like pulse generation around 1.3-µm based on cascaded Raman scattering

Based on cascaded Raman scattering, near-infrared (NIR) noise-like pulses (NLPs) were successfully demonstrated using a Yb-doped fiber amplifier system. Through a nonlinear fiber amplifier using a germanium-zirconia-silica Yb³⁺-doped single mode fiber as a gain fiber, the fourth-order Stokes wave (4^th-SW) can be excited to extend the emission peak of approximately 1.2-µm and a 3-dB bandwidth of approximately 130 nm. To further shift the wavelength more efficiently toward 1.3 µm, filtered NLPs with an emission peak at 1075 nm were adopted as seeded pulses to excite the fifth-order Stokes wave (5^th-SW) because of the better conversion efficiency of stimulated Raman scattering without gain competition with Yb-doped fiber. The generated NIR NLPs were shown to be an excellent light source for the photoluminescence emission from three photon absorption of perovskite to illustrate the red shift of the emission peak owing to the reabsorption effect.

Photo-induced dual passivation via Usanovich acid-base on surface defects of methylammonium lead triiodide perovskite

Post-fabrication defect passivation of organometal halide perovskites has become an efficient way to improve their photophysical properties, but the underlying mechanisms are still in debate. In this work, we used p-benzoquinone (p-BQ) to generate surface defects on methylammonium lead triiodide perovskite (MAPbI3), and found that a Usanovich acid-base (O2, acetone or acetonitrile) treatment can effectively passivate those defects and lead to photoluminescence (PL) enhancement. The passivation effect arose from partial neutralization of defect charges via electron transfer between passivation reagents and relevant defects. O2 accepted photo-generated electrons, formed negatively charged oxygen species and attached to the I vacancy site to reduce its PL quenching efficiency by neutralising the defects positive charge. Likewise, acetone accepted photo-generated holes, formed positively charged species and partially neutralised the defects negative charge. The reduced trapping ability of defects caused PL enhancement. In addition, the observed photo-catalysed oxidation of acetone by O2 on the crystal surface supported the single electron transfer mechanism, and showed the potential of MAPbI3 as a photo-catalyst.

Toward Metal Halide Perovskite Nonlinear Photonics

The possibility of controlling light using the nonlinear optical properties of photonic devices opens new points of view in information and communications technology applications. In this Perspective, we review and analyze the potential role of metal halide perovskites in a framework different from their usual one in photovoltaic and light-emitting devices, namely, the one where they can play as nonlinear photonic materials. We contextualize this new role by comparing the few extant results on their nonlinear optical properties to those of other known nonlinear materials. As a result of this analysis, we provide a vision of future developments in photonics that can be expected from this new perspective on metal halide perovskites.

High Resolution Mapping of Two-Photon Excited Photocurrent in Perovskite Microplate Photodetector

We fabricate photodetectors based on solution-processed single CH₃NH₃PbBr₃ microcrystals (MCs) and map the two-photon absorption (TPA) excited photocurrent (PC) with spatial resolution of 1 μm. We find that the charge carrier transport length in the MCs depends on the applied electric field, and increases from 5.7 μm for 0.02 V bias (dominated by carrier diffusion) to 23.2 μm for 2 V bias (dominated by carrier drift). Furthermore, PC shows strong spatial variations. Combining the PC mapping results with time-resolved photoluminescence microscopy, we demonstrate that the spatial distribution of PC mainly originates from the inhomogeneous distribution of trap-states across perovskite MCs. This suggests that there is still large margin for improvement of perovskite single crystal devices by better controlling of the traps.

Inorganic Ions Assisted the Anisotropic Growth of CsPbCl3 Nanowires with Surface Passivation Effect

All-inorganic halide perovskite nanowires (NWs) exhibit improved thermal and hydrolysis stability and could thus play a vital role in nanoscale optoelectronics. Among them, blue-light-based devices are extremely limited because of the lack of a facile method to obtain high-purity CsPbCl₃ NWs. Herein, we report a direct and facile method for the synthesis of CsPbCl₃ NWs assisted by inorganic ions that served both as a morphology controlling agent for the anisotropic growth of nanomaterials and a surface passivation species modulating the surface of nanomaterials. This new approach allows us to obtain high-purity and size-uniform NWs as long as 500 nm in length and 20 nm in diameter with high reproducibility. X-ray photoelectron spectroscopy and ultrafast spectroscopic measurements confirmed that a reduced band gap caused by the surface species of NWs relative to nanocubes (NCs) was achieved at the photon energy of 160 eV because of the hybrid surface passivation contributed by adsorbed inorganic ions. The resulting NWs demonstrate significantly enhanced photoelectrochemical performances, 3.5-fold increase in the photocurrent generation, and notably improved stability compared to their NC counterparts. Our results suggest that the newly designed NWs could be a promising material for the development of nanoscale optoelectronic devices.

Robust Subwavelength Single-Mode Perovskite Nanocuboid Laser

On-chip photonic information processing systems require great research efforts toward miniaturization of the optical components. However, when approaching the classical diffraction limit, conventional dielectric lasers with all dimensions in nanoscale are difficult to realize due to the ultimate miniaturization limit of the cavity length and the extremely high requirement of optical gain to overcome the cavity loss. Herein, we have succeeded in reducing the laser size to subwavelength scale in three dimensions using an individual CsPbBr₃ perovskite nanocuboid. Even though the side length of the nanocuboid laser is only ∼400 nm, single-mode Fabry-Pérot lasing at room temperature with laser thresholds of 40.2 and 374 μJ/cm² for one- and two-photon excitation has been achieved, respectively, with the corresponding quality factors of 2075 and 1859. In addition, temperature-insensitive properties from 180 to 380 K have been demonstrated. The physical volume of a CsPbBr₃ nanocuboid laser is only ∼0.49λ³ (where λ is the lasing wavelength in air). Its three-dimensional subwavelength size, excellent stable lasing performance at room temperature, frequency up-conversion ability, and temperature-insensitive properties may lead to a miniaturized platform for nanolasers and integrated on-chip photonic devices in nanoscale.

Constructing Sensitive and Fast Lead-Free Single-Crystalline Perovskite Photodetectors

We developed a high-performance photodetector based on (CH₃NH₃)₃Sb₂I₉ (MA₃Sb₂I₉) microsingle crystals (MSCs). The MA₃Sb₂I₉ single crystals exhibit a low-trap state density of ∼10¹⁰ cm^-3 and a long carrier diffusion length reaching 3.0 μm, suggesting its great potential for optoelectronic applications. However, the centimeter single crystal (CSC)-based photodetector exhibits low responsivity (10^-6 A/W under 1 sun illumination) due to low charge-carrier collection efficiency. By constructing the MSC photodetector with efficient charge-carrier collection, the responsivity can be improved by three orders of magnitude (under 1 sun illumination) and reach 40 A/W with monochromatic light (460 nm). Furthermore, the MSC photodetectors exhibit fast response speed of <1 ms, resulting in a high gain of 108 and a gain-bandwidth product of 10⁵ Hz. These numbers are comparable to the lead-perovskite single-crystal-based photodetectors.

Frequency upconverted amplified spontaneous emission and lasing from inorganic perovskite under simultaneous six-photon absorption

Multiphoton pumped stimulated emission requires simultaneous absorption of photons for the creation of population inversion to sustain optical amplification. Recently, stimulated emission by simultaneous absorption of up to five photons has been realized. To achieve more diverse nonlinear optical applications, it is desired to have more photons involved in the upconversion process. Here, we demonstrate unambiguously frequency upconverted amplified spontaneous emission and lasing via simultaneous six-photon absorption from inorganic perovskite. Our finding allows the utilization of inorganic perovskite as the novel alternative for higher-order multiphoton fluorescent applications.

Formamidinium Lead Bromide (FAPbBr3) Perovskite Microcrystals for Sensitive and Fast Photodetectors

Because of the good thermal stability and superior carrier transport characteristics of formamidinium lead trihalide perovskite HC(NH₂)₂PbX₃ (FAPbX₃), it has been considered to be a better optoelectronic material than conventional CH₃NH₃PbX₃ (MAPbX₃). Herein, we fabricated a FAPbBr₃ microcrystal-based photodetector that exhibited a good responsivity of 4000 A W^-1 and external quantum efficiency up to 10⁶% under one-photon excitation, corresponding to the detectivity greater than 10¹⁴ Jones. The responsivity is two orders of magnitude higher than that of previously reported formamidinium perovskite photodetectors. Furthermore, the FAPbBr₃ photodetector's responsivity to two-photon absorption with an 800-nm excitation source can reach 0.07 A W^-1, which is four orders of magnitude higher than that of its MAPbBr₃ counterparts. The response time of this photodetector is less than 1 ms. This study provides solid evidence that FAPbBr₃ can be an excellent candidate for highly sensitive and fast photodetectors.

Benzyl Alcohol-Treated CH3NH3PbBr3 Nanocrystals Exhibiting High Luminescence, Stability, and Ultralow Amplified Spontaneous Emission Thresholds

We report the high yield synthesis of about 11 nm sized CH₃NH₃PbBr₃ nanocrystals with near-unity photoluminescence quantum yield. The nanocrystals are formed in the presence of surface-binding ligands through their direct precipitation in a benzyl alcohol/toluene phase. The benzyl alcohol plays a pivotal role in steering the surface ligands binding motifs on the NC surface, resulting in enhanced surface-trap passivation and near-unity PLQY values. We further demonstrate that thin films from purified CH₃NH₃PbBr₃ nanocrystals are stable >4 months in air, exhibit high optical gain (about 520 cm^-1), and display stable, ultralow amplified spontaneous emission thresholds of 13.9 ± 1.3 and 569.7 ± 6 μJ cm^-2 at one-photon (400 nm) and two-photon (800 nm) absorption, respectively. To the best of our knowledge, the latter signifies a 5-fold reduction of the lowest reported threshold value for halide perovskite nanocrystals to date, which makes them ideal candidates for light-emitting and low-threshold lasing applications.

Multiphoton Absorption Order of CsPbBr3 As Determined by Wavelength-Dependent Nonlinear Optical Spectroscopy

CsPbBr₃ is a direct-gap semiconductor where optical absorption takes place across the fundamental bandgap, but this all-inorganic halide perovskite typically exhibits above-bandgap emission when excited over an energy level, lying above the conduction-band minimum. We probe this bandgap anomaly using wavelength-dependent multiphoton absorption spectroscopy and find that the fundamental gap is strictly two-photon forbidden, rendering it three-photon absorption (3PA) active. Instead, two-photon absorption (2PA) commences when the two-photon energy is resonant with the optical gap, associated with the level causing the anomaly. We determine absolute nonlinear optical dispersion over this 3PA-2PA region, which can be explained by two-band models in terms of the optical gap. The polarization dependence of 3PA and 2PA is also measured and explained by the relevant selection rules. CsPbBr₃ is highly luminescent under multiphoton absorption at room temperature with marked polarization and wavelength dependence at the 3PA-2PA crossover and therefore has potential for nonlinear optical applications.

Ultrasensitive and Fast All-Inorganic Perovskite-Based Photodetector via Fast Carrier Diffusion

Low trap-state density, high carrier mobility, and efficient charge carrier collection are key parameters for photodetectors with high sensitivity and fast response time. This study demonstrates a simple solution growth method to prepare CsPbBr₃ microcrystals (MCs) with low trap-state density. Time-dependent photoluminescence study with one-photon excitation (OPE) and two-photon excitation (TPE) indicates that CsPbBr₃ MCs exhibit fast carrier diffusion with carrier mobility over 100 cm² V^-1 S^-1 . Furthermore, CsPbBr₃ MC-based photodetectors with high charge carriers' collection efficiency are fabricated. Such photodetectors show ultrahigh responsivity (R) up to 6 × 10⁴ A W^-1 with OPE and high R up to 6 A W^-1 with TPE. The R for OPE is over one order of magnitude higher (the R for TPE is three orders of magnitude higher) than that of previously reported all-inorganic perovskite-based photodetectors. Moreover, the photodetectors exhibit fast response time of ≈1 ms, which corresponds to a gain ≈10⁵ and a gain- bandwidth product of 10⁸ Hz for OPE (a gain ≈10³ and a gain-bandwidth product of 10⁶ Hz for TPE).

Facet-controlled preparation of hybrid perovskite microcrystals in the gas phase and the remarkable effect on optoelectronic properties

Particle shape of hybrid perovskite microcrystals influences PL properties via differences in the abundant facets and associated surface trap states.

Enhancing perovskite film fluorescence by simultaneous near- and far-field effects of gold nanoparticles

Gold nanoparticles are incorporated into PEDOT:PSS for enhanced perovskite fluorescence, which originates from simultaneous near- and far-field effects.

Unravelling the low-temperature metastable state in perovskite solar cells by noise spectroscopy

The hybrid perovskite methylammonium lead iodide CH₃NH₃PbI₃ recently revealed its potential for the manufacturing of low-cost and efficient photovoltaic cells. However, many questions remain unanswered regarding the physics of the charge carrier conduction. In this respect, it is known that two structural phase transitions, occurring at temperatures near 160 and 310 K, could profoundly change the electronic properties of the photovoltaic material, but, up to now, a clear experimental evidence has not been reported. In order to shed light on this topic, the low-temperature phase transition of perovskite solar cells has been thoroughly investigated by using electric noise spectroscopy. Here it is shown that the dynamics of fluctuations detect the existence of a metastable state in a crossover region between the room-temperature tetragonal and the low-temperature orthorhombic phases of the perovskite compound. Besides the presence of a noise peak at this transition, a saturation of the fluctuation amplitudes is observed induced by the external DC current or, equivalently, by light exposure. This noise saturation effect is independent on temperature, and may represent an important aspect to consider for a detailed explanation of the mechanisms of operation in perovskite solar cells.

Excitation Wavelength Independence: Toward Low-Threshold Amplified Spontaneous Emission from Carbon Nanodots

Carbon nanodots (CDs) are known to be a superior type of lasing material due to their low cost, low toxicity, high photostability, and photobleaching resistance. Significant attention has been paid to synthesizing CDs with high fluorescence quantum yields (FLQYs) to achieve higher optical gains. In this report, we reveal that excitation wavelength-independent (λex-independent) photoluminescence (PL) characteristics, rather than high FLQYs, should be given priority to realize CD-based light amplification. CDs with excitation wavelength-dependent (λex-dependent) PL characteristics and FLQYs as high as 99% and 96% were found not to exhibit amplified spontaneous emission (ASE), while those with λex-independent PL characteristics and FLQYs of only 38% and 82% realized ASE with low thresholds. The difficulty of achieving ASE using CDs with λex-dependent PL characteristics is likely attributable to their high contents of C-O-H or C-O-C groups. These groups can induce numerous localized electronic states within the n-π* gap, which could decentralize the excited electrons, thus increasing the difficulty of population inversion. In addition, the radiative transition rates and stimulated emission cross sections of CDs with λex-independent PL characteristics were found to be significantly higher than those of CDs with λex-dependent PL characteristics. ASE in a planar waveguide structure, which is a practical structure for solid-state lasing devices, was also demonstrated for the first time using CDs with λex-independent PL characteristics. These results provide simple and effective guidelines for synthesizing and selecting CDs for low-threshold lasing devices.