Feifei Qin

SERS-active ZnO/Ag hybrid WGM microcavity for ultrasensitive dopamine detection

Dopamine (DA) is a potential neuro modulator in the brain which influences a variety of motivated behaviors and plays a key role in life science. A hybrid ZnO/Ag microcavity based on Whispering Gallery Mode (WGM) effect has been developed for ultrasensitive detection of dopamine. Utilizing this effect of structural cavity mode, a Raman signal of R6G (5 × 10−3 M) detected by this designed surface-enhanced Raman spectroscopy (SERS)-active substrate was enhanced more than 10-fold compared with that of ZnO film/Ag substrate. Also, this hybrid microcavity substrate manifests high SERS sensitivity to rhodamine 6 G and detection limit as low as 10−12 M to DA. The Localized Surface Plasmons of Ag nanoparticles and WGM-enhanced light-matter interaction mainly contribute to the high SERS sensitivity and help to achieve a lower detection limit. This designed SERS-active substrate based on the WGM effect has the potential for detecting neurotransmitters in life science.

Crystal structure and electron transition underlying photoluminescence of methylammonium lead bromide perovskites

Bromine-based methylammonium lead hybrid perovskites (CH3NH3PbBr3 or MAPbBr3) have exhibited remarkable charge transport and optical properties. Nonetheless, the photoluminescence (PL) behavior and electronic transition state are still obscure. In this paper, the intrinsic emission mechanisms of two peaked CH3NH3PbBr3 microcuboid crystals have been investigated. A systematic analysis of the stable-state, transient-state and temperature-dependent spectra demonstrated the structure–activity relationship between optical properties and crystal phase. The lattice symmetry was also confirmed by the two-photon absorption induced PL. The findings can be assigned to the fact that the two emission states with band-energy ∼2.22 eV and ∼2.31 eV are originated from free exciton and free carrier recombination which are attributed to the coexistence of a non-centrosymmetric tetragonal phase and a centrosymmetric cubic phase for CH3NH3PbBr3 microcrystals at higher temperature (>160 K).

Plasmon enhancement for Vernier coupled single-mode lasing from ZnO/Pt hybrid microcavities

It is essential to develop a single mode operation and improve the performance of lasing in order to ensure practical applicability of microlasers and nanolasers. In this paper, two hexagonal microteeth with varied nanoscaled air-gaps of a ZnO microcomb are used to construct coupled whispering-gallery cavities. This is done to achieve a stable single mode lasing based on Vernier effect without requiring any complicated or sophisticated manipulation to achieve positioning with nanoscale precision. Optical gain and the corresponding ultraviolet lasing performance were improved greatly through coupling with localized surface plasmons of Pt nanoparticles. The ZnO/Pt hybrid microcavities achieved a seven-fold enhancement of intensity of single mode lasing with higher side-mode suppression ratio and lower threshold. The mechanism that led to this enhancement has been described in detail.

Synergistic graphene/aluminum surface plasmon coupling for zinc oxide lasing improvement

Collective oscillations of free electrons generate plasmons on the surface of a material. A whispering-gallery microcavity effectively confines the light field on its surface based on the total reflection from its internal wall. When these two kinds of electromagnetic waves meet each other, the stimulated emissions from an individual ZnO microrod were enhanced more than 50-fold and the threshold was reduced after the whispering-gallery microcavity was coated with a monolayer of graphene and Al nanoparticles. The improvement of the lasing performance was attributed to the synergistic energy coupling of the graphene/Al surface plasmons with ZnO excitons. The lasing characteristics and the coupling mechanism were investigated systematically.

Burstein-Moss Effect Behind Au Surface Plasmon Enhanced Intrinsic Emission of ZnO Microdisks

In this paper, ZnO microdisks with sputtering of Au nanoparticles were prepared to explore their plasmon/exciton coupling effect. An obvious blue shift and enhanced excitonic emission intensity were observed in the PL spectra of as-grown and Au-sputtered ZnO samples at room temperature. The investigation on the absorption spectra and temperature-dependent PL spectra has been demonstrated the Burstein-Moss effect behind the optical phenomena. These results revealed the coupling dynamics between the metal localized surface plasmon and semiconductor exciton.

Underlying mechanism of blue emission enhancement in Au decorated p-GaN film

Localized surface plasmons (LSPs) excited on metallic structures often play a significant role in mediating the photoluminescence (PL) of semiconductors. For p-GaN film, due to the LSP coupling, blue emission was enhanced while defect-related green emission was quenched to noise level after the decoration with Au nanoparticles (NPs). Why could the Au SP in the green light region enhance the blue and even ultraviolet emission? In this paper, a series of near/far-field spectral analyses and simulations were conducted to understand this process. A clear physical model of LSP-induced electron transfer was proposed to explain the defect-related LSP generation, coupling, electron transfer, and further blue emission increase with green emission reduction. Based on the PL measurement, an insulating SiO2 layer was introduced to confirm the LSP-induced electron transfer between Au and GaN. Additional green light was introduced to observe the LSP-induced PL enhancement, in the same way as for samples with defects. Our study provides a full understanding of the mechanism of PL enhancement in Au decorated GaN and this model should be universal for similar metal/semiconductor systems.

Plasmon-enhanced ZnO whispering-gallery mode lasing

Collaborative enhancements from surface plasmons (SPs) and whispering-gallery modes (WGMs) can induce intense near-field effects with high spatial localization around the surface of a semiconducting material. One can construct a highly efficient hybrid microcavity using semiconducting materials through resonant coupling between SPs and WGMs. Hexagonal ZnO micro-/nanostructures, which have been employed as natural WGM microcavities for ultraviolet (UV) lasing, can be used as ideal platforms to construct such hybrid microcavities. Here, we comprehensively review the recent efforts for improving lasing performance by resonant coupling between SPs and WGMs. Traditional SPs originating from various metals as well as novel SPs originating from atomic layers such as graphene are considered. Moreover, we discuss the mechanism of light-matter interactions beyond the improvements in lasing performance.

Low threshold lasing from novel thulium incorporated C(NH2)3PbI3 perovskite thin films in Fabry-Pérot resonator

Organic–inorganic hybrid halide perovskites are desirable candidates for application in photovoltaics and lasing. However, enhancing the lasing performances of these perovskites with a lower threshold and higher quality factor is still a challenging issue. Thus, herein, we fabricated a Fabry–Perot resonator using an active layer of novel C(NH2)3PbI3:Tm thin films with a reflecting layer of cholesteric liquid crystal (CLC), spacer layers of Al2O3 and poly(3-hexylthiophene) (P3HT) and back reflecting layer of Au to improve the lasing performances of hybrid halide perovskites. The incorporation of thulium further improved the optical properties of the C(NH2)3PbI3 perovskite thin films by enhancing the insertion of organic and inorganic molecules. Furthermore, the introduction of a cholesteric liquid crystal layer greatly enhanced the single-mode lasing performances of GAPbI3:Tm with a low threshold of 0.70 μJ cm−2. Interestingly, the Fabry–Perot resonator of CLC/Al2O3/C(NH2)3PbI3:Tm/P3HT/Au achieved a high quality factor of 2500. This work proves the potential of C(NH2)3PbI3:Tm perovskites in lasing. Therefore, rare earth element-incorporated guanidinium lead iodide can result in a dramatic change in future lasing applications owing to its desirable optical properties.

Optical performance improvement in hydrothermal ZnO/graphene structures for ultraviolet lasing

Ever since the exploration of ZnO for ultraviolet (UV) lasers, realization of high performance lasing in hydrothermal samples is still a challenge due to the poor crystal quality. In this paper, high quality ZnO microcavities were fabricated by a hydrothermal method with graphene as a growth medium. The structural, optical as well as the exciton dynamic properties were studied systematically. Compared with the one grown without graphene, structured cavity morphology, higher light emission intensity and faster exciton recombination dynamics process were observed for ZnO grown on graphene, which make the ZnO samples suitable for lasing. When it was excited by a 325 nm femtosecond laser, single mode lasing with a central wavelength of about 393.2 nm and a Q value of about 936 was realized in a vertical cavity. By further increasing the exciton recombination rate through Al decoration, intensity improved multimode lasing was realized. Our results propose a strategy for yielding single mode lasers in several micrometers with improved performances.

Self-Assembled Growth of Ultrastable CH3NH3PbBr3 Perovskite Milliwires for Photodetectors

The unstability of organolead halide perovskite under continuous illumination, moisture, and high temperature has seriously impeded its commercial development for long-period applications. Here, a facile method was developed to grow ultrastable CH3NH3PbBr3 milliwires through the reaction of self-assembled PbBr2 milliwire with CH3NH3Br at room temperature. The initial self-assembled PbBr2 milliwire is that PbBr2 complexed with dimethylformamide (DMF) molecular self-assemble into perovskite-type PbBr2. Crystal conversion from PbBr2 to CH3NH3PbBr3 milliwire occurred in the molecular exchange between CH3NH3Br and DMF. The synthesized CH3NH3PbBr3 milliwires present high stability under high humidity ∼75%, continuous illumination, heating, and sustain ultrastability in air for more than 255 days. In addition, the CH3NH3PbBr3 milliwire can be dynamically degraded and reconstructed in the presence of water molecules. The milliwires have strong band-edge photoluminescence (PL) with PL lifetime of ∼110 ns. On the basis of the mono-milliwire-constructed photodetector, it exhibits high photoresponse and fast response time of 0.407 s.