Qiangbing Guo

Topological engineering of glass for modulating chemical state of dopants.

A novel approach to modulating the chemical state of dopants by engineering the topological features of a glass matrix is presented. The method allows selective stabilization of dopants on a wide range of length scales, from dispersed ions to aggregated clusters to nanoparticles, leading to various intriguing optical phenomena, such as great emission enhancement and ultra-broadband optical amplification.

Emerging Low-Dimensional Materials for Nonlinear Optics and Ultrafast Photonics

Low-dimensional (LD) materials demonstrate intriguing optical properties, which lead to applications in diverse fields, such as photonics, biomedicine and energy. Due to modulation of electronic structure by the reduced structural dimensionality, LD versions of metal, semiconductor and topological insulators (TIs) at the same time bear distinct nonlinear optical (NLO) properties as compared with their bulk counterparts. Their interaction with short pulse laser excitation exhibits a strong nonlinear character manifested by NLO absorption, giving rise to optical limiting or saturated absorption associated with excited state absorption and Pauli blocking in different materials. In particular, the saturable absorption of these emerging LD materials including two-dimensional semiconductors as well as colloidal TI nanoparticles has recently been utilized for Q-switching and mode-locking ultra-short pulse generation across the visible, near infrared and middle infrared wavelength regions. Beside the large operation bandwidth, these ultrafast photonics applications are especially benefit from the high recovery rate as well as the facile processibility of these LD materials. The prominent NLO response of these LD materials have also provided new avenues for the development of novel NLO and photonics devices for all-optical control as well as optical circuits beyond ultrafast lasers.

Universal Near-Infrared and Mid-Infrared Optical Modulation for Ultrafast Pulse Generation Enabled by Colloidal Plasmonic Semiconductor Nanocrystals

Field effect relies on the nonlinear current-voltage relation in semiconductors; analogously, materials that respond nonlinearly to an optical field can be utilized for optical modulation. For instance, nonlinear optical (NLO) materials bearing a saturable absorption (SA) feature an on-off switching behavior at the critical pumping power, thus enabling ultrafast laser pulse generation with high peak power. SA has been observed in diverse materials preferably in its nanoscale form, including both gaped semiconductor nanostructures and gapless materials like graphene; while the presence of optical bandgap and small carrier density have limited the active spectral range and intensity. We show here that solution-processed plasmonic semiconductor nanocrystals exhibit superbroadband (over 400 THz) SA, meanwhile with large modulation depth (∼7 dB) and ultrafast recovery (∼315 fs). Optical modulators fabricated using these plasmonic nanocrystals enable mode-locking and Q-switching operation across the near-infrared and mid-infrared spectral region, as exemplified here by the pulsed lasers realized at 1.0, 1.5, and 2.8 μm bands with minimal pulse duration down to a few hundreds of femtoseconds. The facile accessibility and superbroadband optical nonlinearity offered by these nonconventional plasmonic nanocrystals may stimulate a growing interest in the exploiting of relevant NLO and photonic applications.

Enhanced broadband excited upconversion luminescence in Ho-doped glasses by codoping with bismuth.

We report enhanced green and red upconversion (UC) luminescence in Ho3+-doped oxyfluoride germanate glass by introducing bismuth near-infrared active centers as sensitizers. The UC excitation bands at 750 and 970 nm show a full width at half-maximum of 20 and 45 nm, respectively. Energy transfer from sensitizers, the excited-state absorption, and phonon-coupled absorption of Ho3+ jointly contribute to the enhanced UC luminescence. Our approach provides an efficient methodology to broaden the excitation bandwidth of UC luminescent materials, which may have the potential for promising application in solar cells.

Ultrabroadband near-infrared luminescence and efficient energy transfer in Bi and Bi/Ho co-doped thin films

Ultrabroadband near-infrared luminescence in the 1.0–2.4 μm range has been observed in bismuth (Bi)-doped oxyfluoride germanate thin films prepared by pulsed laser deposition (PLD). The emission peak position shows a red-shift with decreasing oxygen pressure during PLD growth. Systematic investigation reveals that the origin of the luminescence could be ascribed to Bi clusters. With the sensitization of Bi near-infrared active centers, enhanced broadband ∼2 μm luminescence of Ho3+ is realized in Bi/Ho co-doped films, and a high energy transfer efficiency is obtained. These results may provide promise to realize planar waveguide lasers in the near-infrared region for integrated optics.

Regulation of structure rigidity for improvement of the thermal stability of near-infrared luminescence in Bi-doped borate glasses

The effect of heat-treatment on the near-infrared (NIR) luminescence properties was studied in Bi-doped borate glasses. The luminescence intensity generally decreases with the increase of temperature, and the thermal stability can be improved by nearly 4.5 times with addition of 5 mol% La2O3. Collaborative studies by using steady photoluminescence (PL) and photoluminescence excitation (PLE) spectra, luminescence decay curve, differential thermal analysis (DTA), Raman spectra and X-ray diffraction (XRD) indicate that the luminescence decrement is associated with the agglomeration of Bi active centers during heat-treatment. The improvement of the thermal stability of NIR luminescence with the addition of La2O3 is benefited from the enhancement of structure rigidity due to the strong cationic field strength of La3+. The results not only provide valuable guidance for suppressing performance degradation of Bi-doped glass during fiber drawing process, but also present an effective way to control the luminescence properties of main group elements in glasses from the perspective of glass structure.

A Solution‐Processed Ultrafast Optical Switch Based on a Nanostructured Epsilon‐Near‐Zero Medium

Dynamical materials that capable of responding to optical stimuli have always been pursued for designing novel photonic devices and functionalities, of which the response speed and amplitude as well as integration adaptability and energy effectiveness are especially critical. Here we show ultrafast pulse generation by exploiting the ultrafast and sensitive nonlinear dynamical processes in tunably solution-processed colloidal epsilon-near-zero (ENZ) transparent conducting oxide (TCO) nanocrystals (NCs), of which the potential respond response speed is >2 THz and modulation depth is ~23% pumped at ~0.7 mJ/cm2, benefiting from the highly confined geometry in addition to the ENZ enhancement effect. These ENZ NCs may offer a scalable and printable material solution for dynamic photonic and optoelectronic devices.All the optical properties of materials are derived from dielectric function. In spectral region where the dielectric permittivity approaches zero, known as epsilon-near-zero (ENZ) region, the propagating light within the material attains a very high phase velocity, and meanwhile the material exhibits strong optical nonlinearity. The interplay between the linear and nonlinear optical response in these materials thus offers unprecedented pathways for all-optical control and device design. Here the authors demonstrate ultrafast all-optical modulation based on a typical ENZ material of indium tin oxide (ITO) nanocrystals (NCs), accessed by a wet-chemistry route. In the ENZ region, the authors find that the optical response in these ITO NCs is associated with a strong nonlinear character, exhibiting sub-picosecond response time (corresponding to frequencies over 2 THz) and modulation depth up to ≈160%. This large optical nonlinearity benefits from the highly confined geometry in addition to the ENZ enhancement effect of the ITO NCs. Based on these ENZ NCs, the authors successfully demonstrate a fiber optical switch that allows switching of continuous laser wave into femtosecond laser pulses. Combined with facile processibility and tunable optical properties, these solution-processed ENZ NCs may offer a scalable and printable material solution for dynamic photonic and optoelectronic devices.

Magnetic field modulated upconversion luminescence in NaYF4:Yb,Er nanoparticles

Upconversion, an anti-Stokes process that converts two or more lower energy photons into a higher energy photon, has been paid growing attention due to its wide range of applications ranging from photonics to bioscience. This process, however, suffers from poor efficiency which strongly hampers its application and commercialization. We show here that the upconversion luminescence of NaYF4:Yb,Er nanoparticles can be modulated by the magnetic field and an enhancement of upconversion intensity by a factor of 2.5 is obtained at 20 T. The increased upconversion luminescence is interpreted in terms of enhanced energy transfer from Yb3+ to Er3+ and the enhanced non-radiative transition from 4S3/2 to 4F9/2 and 4I11/2 to 4I13/2 of Er3+ ions. In addition, continuous spectral broadening and shift of f–f transitions with increasing magnetic field intensity are observed, which are ascribed to the Zeeman effect and the difference in the g factor of Zeeman levels. The results demonstrated here may open a new gate towards the modulation of the excited state process by the magnetic field.

Influence of high magnetic field on the luminescence of Eu3+-doped glass ceramics

Rare earth (RE) doped materials have been widely exploited as the intriguing electronic configuration of RE ions offers diverse functionalities from optics to magnetism. However, the coupling of magnetism with photoluminescence (PL) in such materials has been rarely reported in spite of its fundamental significance. In the present paper, the effect of high pulsed magnetic field on the photoluminescence intensity of Eu3+-doped nano-glass-ceramics has been investigated. In our experiment, Eu-doped oxyfluoride glass and glass ceramic were prepared by the conventional melt-quenching process and controlled heat treatment. The results demonstrate that the integrated PL intensity of Eu3+ decreases with the enhancement of magnetic field, which can be interpreted in terms of cooperation effect of Zeeman splitting and magnetic field induced change in site symmetry. Furthermore, as a result of Zeeman splitting, both blue and red shift in the emission peaks of Eu3+ can be observed, and this effect becomes more promin...

Cu-Sn-S plasmonic semiconductor nanocrystals for ultrafast photonics

Here, we show that solution-processed Cu-Sn-S semiconductor nanocrystals (NCs) demonstrate a tunable localized surface plasmon resonance band in the near infrared region, where strong saturable absorption occurs. A saturable absorber based on these plasmonic NCs enables the construction of a stable mode-locked femtosecond fiber laser operating at the telecommunication band.