Runze Chen

Temperature-dependent excitonic photoluminescence excited by two-photon absorption in perovskite CsPbBr 3 quantum dots.

Recently, lead halide perovskite quantum dots have been reported with potential for photovoltaic and optoelectronic applications due to their excellent luminescent properties. Herein excitonic photoluminescence (PL) excited by two-photon absorption in perovskite CsPbBr3 quantum dots (QDs) has been studied at a broad temperature range, from 80 to 380 K. Two-photon absorption has been investigated and the absorption coefficient is up to 0.085 cm/GW at room temperature. Moreover, the PL spectrum excited by two-photon absorption shows a linear blue-shift (0.32 meV/K) below the temperature of 220 K. However, for higher temperatures, the PL peak approaches a roughly constant value and shows temperature-independent chromaticity up to 380 K. This behavior is distinct from the general red-shift for semiconductors and can be attributed to the result of thermal expansion, electron-phonon interaction and structural phase transition around 360 K. The strong nonlinear absorption and temperature-independent chromaticity of CsPbBr3 QDs observed in temperature range from 220 to 380 K will offer new opportunities in nonlinear photonics, light-harvesting, and light-emitting devices.

Characterization of nonlinear properties of black phosphorus nanoplatelets with femtosecond pulsed Z-scan measurements.

The nonlinear properties of black phosphorus (BP) nanoplatelets (NPs) have been characterized with Z-scan measurements under 800-nm femtosecond pulsed laser excitation. A transition from saturable absorption (SA) to reverse saturable absorption (RSA) with the increase of laser intensity was observed in the open-aperture (OA) measurements. Simultaneously, closed-aperture (CA) measurements were carried out to investigate the nonlinear refractive index of BP NPs together, and a value of n(2) ≃(6.8±0.2)×10(-13) m2/W was obtained. The nonlinear absorption properties were analyzed according to the band structure of BP. A theoretical analysis based on SA and two-photon absorption (TPA) was used to determine the nonlinear absorption coefficients from the experimental results, and the TPA coefficient at 800 nm was estimated about (4.5±0.2)×10(-10) m/W.

Giant nonlinear absorption and excited carrier dynamics of black phosphorus few-layer nanosheets in broadband spectra

Layered black phosphorus nanosheets (BPNs) offer potential uses in nanoelectronic and photonics applications. This work demonstrates giant saturable absorption (SA) and two-photon absorption (TPA) of BPNs under femtosecond visible-to-mid-infrared laser-pulse excitation. In addition, the excited carrier dynamics of BPNs were also studied with ultrafast pump-probe technology. Measurements demonstrated that the nonlinear absorption properties of BPNs strongly depend on excitation wavelength and intensity. BPNs moved from SA to TPA under increasing laser intensities at 800, 1160, 1300, and 1550 nm but showed a purely TPA response at 2000 nm, as gauged by measuring BPN nonlinear absorption coefficients. The BPNs showed broadband photon-induced absorption (PIA) signals from 470 to 700 nm, and the excited carrier recombination time was determined to be 150±10  ps. The excitation intensity dependence of excited carrier dynamics were also studied.

Broadband ultrafast nonlinear absorption and ultra-long exciton relaxation time of black phosphorus quantum dots

Black phosphorus (BP) has recently attracted significant attention for its brilliant physical and chemical features. The remarkable strong light-matter interaction and tunable direct wide range band-gap make it an ideal candidate in various application regions, especially saturable absorbers. In this paper, ultrasmall black phosphorus quantum dots (BPQDs), a unique form of phosphorus nanostructures, with average size of 5.7 ± 0.8 nm are synthesized. Compared with BP nanosheets (BPNs) with similar thickness, the ultrafast nonlinear optical (NLO) absorption properties and excited carrier dynamics are investigated in wide spectra. Beyond the saturation absorption (SA), giant two photon absorption (TPA) is observed in BPQDs. BPQDs exhibit quite different excitation intensity and wavelength dependent nonlinear optical (NLO) response from BPNs, which is attributed to the quantum confinement and edge effects. The BPQDs show broadband photon-induced absorption (PIA) under the probe wavelength from 470 nm to 850 nm and a fast and a slow decay time are obtained as long as 92 ± 10 ps and 1100 ± 100 ps, respectively. The substantial independence for ultra-long time scales of pump intensity and temperature reveals that the carrier recombination mechanism may be attributed to a defect-assisted Auger capture process. These findings will help to develop optoelectronic and photonic devices operating in the infrared and visible wavelength region.

Photo-induced excitonic structure renormalization and broadband absorption in monolayer tungsten disulphide

Atomically thin transition metal dichalcogenides (TMDCs) have emerged as a new class of two-dimensional (2D) material for novel optoelectronic applications. In particular, 2D TMDCs are viewed as intriguing and appealing materials to construct Q-switching and mode-locked modulators, due to their broadband saturable absorption even of photon energy below their excitonic energies. However, the dynamics and mechanism of saturable absorption inside TMDCs has yet to be investigated. In this paper, the relaxation dynamics of monolayer tungsten disulphide (WS2) was investigated considering different excitonic transitions. WS2 illustrates dramatic changes in optical responses when excited by intense laser pulses, which are characterized by the broadband photo-induced nonresonance absorption and the giant excitonic bands renormalization process. The experimental results show that strong photo-induced restructuring of excitonic bands has picosecond lifetime and full recovery of optical responses takes hundreds of picosecond. Additionally, our observations reveal that heavy renormalization and overlap of excitonic bands are induced by strong many-body Coulomb interactions. Moreover, the broadband absorption feature of WS2 opens up new applications in broadband saturable absorbers and ultrafast photonic devices.