Zhichao Zhu

Large-scale uniform Au nanodisk arrays fabricated via x-ray interference lithography for reproducible and sensitive SERS substrate

Large-scale Au nanodisk arrays on Si substrate are successfully fabricated via x-ray interference lithography and followed by electron-beam vapor deposition. The Au nanodisk arrays exhibit a significant, uniform, and reproducible surface enhancement on Raman scattering signal, which enables the detection of R6G as low as 10(-8) M with an enhancement factor of 10(6). Importantly, the Au nanodisk arrays SERS-active substrates with uniformly high sensitivity also have high reproducibility and stability. The diameters of the nanodisks and the inter-disk distance can be simply optimized to obtain high enhancement in Raman signal by varying exposure time and development time in XIL process. The electric fields of the Au nanodisks with various diameters and inter-disk distance simulated by the finite difference time domain (FDTD) techniques further confirm that the Raman signal enhancement of Au nanodisks is determined by the diameters of nanodisks and the inter-disk distance of nanodisks. The Au/Ag double-layer bimetal nanodisk arrays are also fabricated which show a significant increase in the Raman signal enhancement than that of the Au nanodisk arrays. XIL nanofabrication appears to be a feasible approach to prepare uniform and reproducible SERS-active substrates with high sensitivity for practical SERS applications.

Enhanced light extraction of scintillator using large-area photonic crystal structures fabricated by soft-X-ray interference lithography

Soft-X-ray interference lithography is utilized in combination with atomic layer deposition to prepare photonic crystal structures on the surface of Bi4Ge3O12 (BGO) scintillator in order to extract the light otherwise trapped in the internal of scintillator due to total internal reflection. An enhancement with wavelength- and emergence angle-integration by 95.1% has been achieved. This method is advantageous to fabricate photonic crystal structures with large-area and high-index-contrast which enable a high-efficient coupling of evanescent field and the photonic crystal structures. Generally, the method demonstrated in this work is also suitable for many other light emitting devices where a large-area is required in the practical applications.

Improvement of light extraction of LYSO scintillator by using a combination of self-assembly of nanospheres and atomic layer deposition

The self-assembled monolayer periodic array of polystyrene spheres conformally coated with TiO₂ layer using atomic layer deposition is designed to obtain a further enhancement of light extraction for LYSO scintillator. The maximum enhancement is 149% for the sample with polystyrene spheres conformally coated with TiO₂ layer, while the enhancement is only 76% for the sample with only polystyrene spheres. Such further enhancement could be contributed from the additional modes forming by TiO₂ layer due to its high refractive index, which can be approved by the simulation of electric field distribution. The experimental results are agreement with the simulated results. Furthermore, the prepared structured layer exhibits an excellent combination with the surface of scintillator, which is in favor of the practical application. Therefore, it is safely concluded that the combination of self-assembly method and atomic layer deposition is a promising approach to obtain a significant enhancement of light extraction for a large area. This method can be extended to many other luminescent materials and devices.

Guided-mode resonance assisted directional emission of a wavelength-shifting film for application in scintillation detection

Thin-film luminescent layers as wavelength shifters using in the scintillation detection system suffer with low efficiency due to the total internal reflection and the non-directional emission. In the present work, we design and fabricate a photonic crystal on the surface of LuTaO(4):Eu(3+) thin-film which is a newly developed luminescent material using in radiation detection systems. The entire structure shows guided-mode resonances with only one TE and TM mode. As a result, the emitting light is effectively extracted. Furthermore, due to only two modes existing in the layer, the directionality of emission is strongly controlled. This result enables the structured LuTaO(4):Eu(3+) thin-film to be a potential wavelength shifter with high-efficiency.

Enhanced light extraction of Bi3Ge4O12 scintillator by graded-refractive-index antireflection coatings

A three-layer graded-refractive-index antireflection coating is designed and prepared on the one surface of the Bi3Ge4O12 scintillator by sol-gel technology. The emission intensity of the Bi3Ge4O12 with a graded-refractive-index antireflection coating exhibits a broadband and omnidirectional enhancement of 15.9% compared with the reference sample without coating. This significant enhancement is attributed to the decrease of Fresnel reflection, which is consistent with the measurement of transmission spectra. Additionally, it is evident that the graded-refractive-index coating is superior to the conventional quarter-wave coating due to the omnidirectionality advantage.

An approach to achieve significantly faster luminescence decay of thin-film scintillator by surface plasmons

A fast component of 2.2 ns from the LSO thin-film scintillator was achieved through coupling of scintillator with surface plasmons of silver nanoparticles. From the emission spectra, the observed fast component is from the transition of 5d to 4f level of Ce3+ in LSO. The fast component is attributed to the enhanced spontaneous recombination rate due to the surface plasmons. The present demonstration provides an interesting approach to improve the timing resolution of scintillator, which is distinguished from these conventional methods.

Enhancement of directional broadband luminescence from a scintillation film via guided-mode resonance in a photonic crystal structure

Scintillation films play an important role in radiation detection. Improved light output and control of emission directionality are critical for practical applications. To obtain enhancement of broadband directional luminescence from a Lu2SiO5:Ce3+ scintillation film, a special photonic crystal structure is deposited on the film surface to provide multiple guided-mode resonances. The structure can be designed according to the application requirements. Numerical simulations are performed to analyze the enhancement. Overall, this method could be used when directional emission is required for radiation detection.

Modified timing characteristic of a scintillation detection system with photonic crystal structures

It is intuitively expected that an enhanced light extraction of a scintillator can be easily achieved by photonic crystal structures. Here, we demonstrate a modified timing characteristic for a detection system induced by enhanced light extraction with photonic crystal structures. Such improvement is due to the enhanced light extraction which can be clearly proven by the independent measurements of the light output and the timing resolution. The present investigation is advantageous to promote the development of a scintillation detection system performance based on the time-of-flight measurement.

Plasmonic lattice resonance-enhanced light emission from plastic scintillators by periodical Ag nanoparticle arrays

We have demonstrated that periodical arrays of silver nanoparticles can enhance the light emission from a plastic scintillator layer on the surface of a silicon substrate. The enhancement is attributed to surface lattice resonances with a photonic-plasmonic nature. Although the enhancement exhibits directional characteristics for individual wavelengths, the wavelength-integrated enhancement shows a monotonous increase with increasing emission angle. As a result, an overall 1.81-fold wavelength- and angle-integrated enhancement has been obtained. This observation is promising for fundamental and applied research into enhanced luminescent material layers on opaque substrates.

Directional emission of quantum dot scintillators controlled by photonic crystals

We have demonstrated enhanced light emission from a CdSe/ZnS quantum dot scintillator film in the normal direction by photonic crystal structures. With the control of the photonic crystal structures, a two-fold enhancement was achieved for the wavelength-integrated emission spectra under the excitation of both ultra-violet and X-rays, which is beneficial to radiation detection applications. Furthermore, it is found that the optical properties such as the bandwidth and peak wavelength can be controlled by adjusting the thickness of a TiO2 conformal layer which was deposited on the surface of the photonic crystal slab formed from solidified resist.