Zhaona Wang

Optimizing performance of silicon-based p-n junction photodetectors by the piezo-phototronic effect.

Silicon-based p-n junction photodetectors (PDs) play an essential role in optoelectronic applications for photosensing due to their outstanding compatibility with well-developed integrated circuit technology. The piezo-phototronic effect, a three-way coupling effect among semiconductor properties, piezoelectric polarizations, and photon excitation, has been demonstrated as an effective approach to tune/modulate the generation, separation, and recombination of photogenerated electron-hole pairs during optoelectronic processes in piezoelectric-semiconductor materials. Here, we utilize the strain-induced piezo-polarization charges in a piezoelectric n-ZnO layer to modulate the optoelectronic process initiated in a p-Si layer and thus optimize the performances of p-Si/ZnO NWs hybridized photodetectors for visible sensing via tuning the transport property of charge carriers across the Si/ZnO heterojunction interface. The maximum photoresponsivity R of 7.1 A/W and fastest rising time of 101 ms were obtained from these PDs when applying an external compressive strain of -0.10‰ on the ZnO NWs, corresponding to relative enhancement of 177% in R and shortening to 87% in response time, respectively. These results indicate a promising method to enhance/optimize the performances of non-piezoelectric semiconductor material (e.g., Si) based optoelectronic devices by the piezo-phototronic effect.

Ultrafast Response p-Si/n-ZnO Heterojunction Ultraviolet Detector Based on Pyro-Phototronic Effect.

A light-self-induced pyro-phototronic effect in wurtzite ZnO nanowires is proposed as an effective approach to achieve ultrafast response ultraviolet sensing in p-Si/n-ZnO heterostructures. The relatively long response/recovery time of zinc-oxide-based ultraviolet sensors in air/vacuum has long been an obstacle to developing such detectors for practical applications. The response/recovery time and photoresponsivity are greatly improved by the pyro-phototronic effect.

High performance plasmonic random laser based on nanogaps in bimetallic porous nanowires

A plasmonic random laser is fabricated using gold-silver bimetallic porous nanowires with abundant nanogaps that provide strong feedback or gain channels for coherent lasing from dye molecules. The strong confinement of the nanogaps allows the bimetallic porous nanowire-based random laser, which is pumped by ns pulses, to operate with a very low threshold and extremely low concentrations of Rhodamine 6 G (as low as 0.067 mM). This random laser can be used as a pump source for another coherent random laser based on oxazine. These results provide a basis for studies of coherent random lasing pumped by another random laser.

Cavity coupling in a random laser formed by ZnO nanoparticles with gain materials

Cavity coupling in a random laser with a weakly scattering disordered structure formed by ZnO nanoparticles is observed experimentally. The lasing characteristics are quite different from those of a traditional random laser. It is found that the threshold of coherent radiation with gain materials in such a structure is considerably low, and the emission spectrum and the threshold of each peak are orientationally uniform; the possible positions of the coherent peaks are fixed. These characteristics will be very useful in its applications. A new physical mechanism, cavity coupling, is suggested to discuss the lasing system. Nano-scale scatterers play an important role in providing randomly distributed feedback.

Investigation of a peculiar bifurcation phenomenon in diffraction spectra of volume holograms

A peculiar bifurcation phenomenon in the diffraction spectra of volume holograms is investigated. A linear expansion model modulated by a random number was built to explain the phenomenon. In our model, a random number was first introduced to describe the random property of swelling or shrinking of the emulsion during the process after exposure. The calculated results show good consistency with the experimental observations.

Cascade-pumped random lasers with coherent emission formed by Ag–Au porous nanowires

A series of sequentially cascade-pumped random lasers is reported. It consists of three random lasers in which the Ag-Au bimetallic porous nanowires play the role of scatterers, and the gain materials are coumarin 440 (C440), coumarin 153 (C153), and rhodamine 6G (R6G), respectively. The random laser with C440 is first pumped by a 355 nm pulsed laser. The emission of C440 pumps the C153, and the emission of C153 pumps the R6G sequentially. Low-threshold coherent emissions from the three random lasers are observed. The cascade-pumped random lasers can be achieved easily with low cost and can be used in applications conveniently.

Coherent plasmonic random laser pumped by nanosecond pulses far from the resonance peak of silver nanowires

Silver nanowires were used to enhance stimulated emission of Rhodamine 6G in a liquid random laser. Low-threshold coherent emission from the nanosecond-pulse-pumped random laser was achieved. Surface plasmonic resonance plays a key role in low-threshold operation of this random laser. The results demonstrate the ability of silver nanowires to enhance the stimulated emission, especially when the emission light from the dye molecules is far from the plasmonic resonance peak. Although the random laser shows different emission spectra in different directions, universal properties of strong interaction among multiple modes under different pump power densities were also demonstrated.

Two-threshold silver nanowire-based random laser with different dye concentrations

The feedback mechanisms of silver nanowire-based random lasers with different concentrations of laser dye rhodamine 6 G pumped by a nanosecond pulsed laser were demonstrated. It was shown that dye concentration greatly impacts on the optical amplification mechanism. At lower or higher dye concentrations, random lasers have a single threshold. At a proper concentration, the system shows transition from incoherent emission to coherent lasing and has two thresholds corresponding to incoherent feedback and coherent feedback, respectively. The corresponding physical mechanism was displayed. Also, the processes of fluorescence, incoherent feedback and coherent feedback were distinguished by the emission spectra in the time domain. The results will supply some guidance to clear the working mechanism of random lasers.

Electromagnetic localization based on transformation optics

Localization of an electromagnetic field can be achieved by transformation optics using metamaterials. A coordinate transformation structure different from traditional resonator is proposed. Wherein, arbitrary frequency of the whole band of electromagnetic wave can be localized without energy loss, i.e., the modes in this structure are continuous. Theoretical analysis and numerical simulation show that the material parameter variations at the outer boundary of the structure have little influence on the localization property. When realizable physical structure is considered, multi-layer approximation should be applied. The calculated results show that the estimated localization time is about 100 ns for an 8-layer inhomogeneous approximation, and it could reach several seconds for a 30-layer homogeneous approximation. The present work may present a new application of transformation optics.

A Dnv point group structure possessing complete band gap based on gradual heterostructure and self-simulating sphere

A structure with Dnv point group was fabricated by compounding a gradual heterostructure and a self-simulating sphere. The gradual heterostructure has two-dimensional omnidirectional band gap and the self-simulating sphere makes it possess a complete band gap. The experimental results were explained by theoretical analysis. This kind of structure can be implemented using low refractive index materials.