Chih-Ming Wei

Surface plasmon enhanced energy transfer between type I CdSe/ZnS and type II CdSe/ZnTe quantum dots

Fluorescence resonant energy transfer (FRET) has been investigated between donor-acceptor pairs of type I CdSe/ZnS and type II CdSe/ZnTe quantum dots (QDs). An Au nanoparticles assisted FRET enhancement was clearly demonstrated. It is found that the efficiency of the energy transfer depends on the excitation wavelength and is largest when in resonance with the Au surface plasmon mode. With the large tunability of the emission intensity in near infrared region, our finding paves an excellent route for creating highly efficient optoelectronic devices and bioimaging labels derived from type II QDs.

Tunable emission based on the composite of Au nanoparticles and CdSe quantum dots deposited on elastomeric film

A simple approach to investigate the dependence of emission on the separation distance between metal nanoparticles and semiconductor quantum dots is demonstrated. Without varying the mixed concentrations, a tunable emission is achieved based on the deposition of the composite of Au nanoparticles and CdSe quantum dots on elastomeric film. By utilizing the inherent nature of the elasticity of the elastomeric film, it is found that depending on the separation distance, the emission intensity can be quenched or enhanced. The underlying mechanism can be explained quite well by the interplay between the local field excitation due to surface plasmons and electrons transfer to metal nanoparticles.

Highly sensitive MOS photodetector with wide band responsivity assisted by nanoporous anodic aluminum oxide membrane

A new approach for developing highly sensitive MOS photodetector based on the assistance of anodic aluminum oxide (AAO) membrane is proposed, fabricated, and characterized. It enables the photodetector with the tunability of not only the intensity but also the range of the response. Under a forward bias, the response of the MOS photodetector with AAO membrane covers the visible as well as infrared spectrum; however, under a reverse bias, the near-infrared light around Si band edge dominates the photoresponse. Unlike general MOS photodetectors which only work under a reverse bias, our MOS photodetectors can work even under a forward bias, and the responsivity at the optical communication wavelength of 850nm can reach up to 0.24 A/W with an external quantum efficiency (EQE) of 35%. Moreover, the response shows a large enhancement factor of 10 times at 1050 nm under a reverse bias of 0.5V comparing with the device without AAO membrane. The underlying mechanism for the novel properties of the newly designed device has been proposed.

Magnetically tunable surface plasmon resonance based on a composite consisting of noble metal nanoparticles and a ferromagnetic thin film

We demonstrate magnetically tunable surface plasmon resonance based on a composite consisting of noble metal nanoparticles and ferromagnetic thin film. We found that both the frequency and linewidth of the localized surface plasmon resonance can be manipulated by applying an external magnetic field. The underlying mechanism is attributed to the variation of the dielectric constant in the ferromagnetic thin film resulting from the change of magnetization. Our result shown here paves an alternative route for manipulation of the characteristics of the surface plasmon resonance, which may serve as a new design concept for the development of magneto-optical devices.

Optical detection of magnetoelectric effect in the composite consisting of InGaN/GaN multiple quantum wells and FeCo thin film

The magnetoelectric effect has been demonstrated based on the composite of InGaN/GaN multiple quantum wells (MQWs) and FeCo thin film. By applying an external magnetic field, the ferromagnetic layer will be deformed due to magnetostriction. This deformation is transmitted to the piezoelectric layers and results in piezoelectric effect, which induces electric polarization in the piezoelectric layers. The induced electric polarization changes the strain and the built-in internal electric field in the InGaN/GaN MQWs and therefore, the optical properties of the InGaN/GaN MQWs change. The results shown here open up a possibility for the application of nitride semiconductors in magneto-optical and magnetoelectric engineering.

Photogalvanic effects for interband transition in p-Si0.5Ge0.5∕Si multiple quantum wells

Circular photogalvanic effect (CPGE) and linear photogalvanic effect for interband transition have been observed simultaneously in Si0.5Ge0.5∕Si multiple quantum wells. The signature of the CPGE is evidenced by the change of its sign upon reversing the radiation helicity. It is found that the observed CPGE photocurrent is an order of magnitude greater than that obtained for intersubband transition. The dependences of the CPGE on the angle of incidence and the excitation intensities can be well interpreted based on its characteristics. The large signal of spin generation observed here at room temperature should be very useful for the realization of practical application of spintronics.

Tunable photoluminescence and photoconductivity in ZnO one-dimensional nanostructures with a second below-gap beam

Tunable photoluminescence (PL) and photoconductivity (PC) with a second below-gap beam were demonstrated on ZnO nanorods and nanoribbons. We found that both PL and PC could be quenched as the second beam was applied to the nanostructures, and this behavior was excluded from thermal effect by comparing the phonon replica spectra with that from heating the sample directly. The most quenching effect occurred near the defect transition located at 520 nm. The underlying mechanism of the quenching behavior was attributed to the defect transition between different states of oxygen vacancies. Size-dependence measurement lets us know that the effect occurs near the surface of nanostructures, and the power-dependent measurement further confirms the underlying mechanism we proposed.

Magnetic field modulation of photonic bandgap on FeCo/NiO half-shell array

FeCo/NiO half-shell arrays were fabricated based on the periodic monolayer polystyrene spheres. The two-dimensional magnetic periodic arrays form well-defined photonic crystals with pronounced stop bands. Quite interestingly, it is found that the stop bands can be tuned by an external magnetic field. The underlying mechanism is attributed to the controllable dielectric constant of the magnetic FeCo film under an applied magnetic field. The results shown here may open up an avenue for magnetically tunable photonic crystal stop bands, which may be useful for the creation of new magneto-optical devices.

Selectively enhanced emission and suppression in Si0.5Ge0.5/Si multiple quantum wells by photonic crystals

Selective enhancement and suppression of the photoluminescence arising from Si0.5Ge0.5/Si multiple quantum wells by photonic crystals (PCs) have been demonstrated. The formation of the stop band in PCs is designed to be a filter as well as a reflector. It is found that the self-assembled PCs are able to selectively enhance the luminescence of the type-II transitions at the interface between Si and Si0.5Ge0.5/Si layers and suppress the emission from Si. Our working principle shown here can be extended to many other material systems and should be very useful for creating high power solid-state emitters.

Optically tunable and detectable magnetoelectric effects in the composite consisting of magnetic thin films and InGaN/GaN multiple quantum wells

An optically tunable and detectable magnetoeletric (ME) effect has been discovered in the composite consisting of InGaN/GaN multiple quantum wells and magnetostrictive ferromagnetic Ni or FeCo thin films at room temperature. Due to the interactively optical and piezoelectric properties of nitride semiconductors, this composite provides an intriguing optically accessible system, in which the magnetoelectric effect can be both easily tuned and detected. The underlying mechanism can be well accounted for by the interplay among magnetostrictive, piezoelectric and optical transition. It thus offers a new paradigm to generate artificial material systems with magnetic/electric/optical inter-related/controllable properties.