Bing Cao

Design and fabrication of silicon-based linear polarizer with multilayer nanogratings operating in infrared region

Abstract. We have proposed and experimentally demonstrated a silicon-based linear polarizer with multilayer nanogratings working in 3 to 5u2009u2009μm of an infrared region. A dielectric grating is first fabricated in a low-refractive index thin layer on a Si-substrate and then double-layer metallic gratings are formed by evaporating a metallic film onto the dielectric grating. With the designed structure of multilayer nanogratings coupled with a low-refractive-index dielectric layer on the high-refractive index silicon substrate, both high transverse magnetic transmission (TMT) and high extinction ratio (ER) can be effectively achieved across 3- to 5-μm range in the infrared band without the complicated metallic ion etching process that is required in conventional nanowire grids. An ER of 40 dB and TMT of averagely higher than 80% were obtained experimentally from a linear polarizer with a multilayer grating of 280-nm period. The Si-based multilayer grating structure shows possibilities of implementing polarization in a fashion of relatively easy-fabrication, semiconductor process compatible, and high performance.

Graphene-based optical absorbers in middle-infrared wavelengths

In the visible and near infrared regions, graphene is essentially transparent with a constant absorptivity of 2.3%. On contrast, in longer wavelengths, the absorptivity can be enhanced by graphene plasmons motivated by simple nanostructures. Besides, the graphene plasmons can be further enhanced via electrostatic doping when voltage is applied. This work numerically demonstrates that in optimized configuration the absorptance in monolayer graphene can be greatly enhanced and reach to 98.6% of the impinging light for transverse magnetic (TM) polarizations. Graphene can interact with light via plasmonic resonance. Towards this, we utilize a subwavelength-thick optic cavity, which composed of graphene grating, a dielectric spacing layer and a metal film to further enhance the interaction. When we use the TM mode source, the incident light matched the graphene plasmons, a strong drastic cut in the energy of the reflected light, which means obvious resonance absorption occurred. Meanwhile, the reflection can approach 0 when voltage applied. Finally, great absorption in 6.94 μm has been achieved by the graphene grating with the addition of a subwavelength-thick optic cavity via different voltage.

High linearly polarized light emission from GaN-based LED with patterned dielectric/metal structures

We proposed and demonstrated an integrated high linearly polarized InGaN/GaN green LED grown on (0001) oriented sapphire with a structure of combined dielectric/metal wire grids (CDMWG). Both theoretical and experimental results show that the CDMWG can effectively loosen the requirement on the dimension of the grating, and the introduction of a low-refractive dielectric layer can further enhance both TMT and ER significantly for the GaN-type LED. An InGaN/ GaN green LED with an integrated CDMWG of 220 nm period has been fabricated, and a measured extinction ratio(ER) of higher than 20 dB and TMT of 65% within an angle of ±40° is obtained directly from a InGaN/GaN LED.

Infrared stop-band filter based on a subwavelength structure

We propose a stop-band filter in infrared region using a periodically chirped subwavelength structure. The structure is made of a stack of metal and dielectric pattern made on a thick metal layer that is deposited a PMMA substrate. It is found that an appropriately designed microstructure of metal-insulator-metal patches can generate a wideband infrared absorption, resulting in an infrared stop-band filter. Different width of metal-insulator-metal resonator arranged in one unit cell generate trough in the reflection spectrum at different wavelengths. The full width at half magnitude (FWHM) of the stop-band filter can thus be adjusted by tuning the width of the resonators. The larger the range of the resonator width, the wider the bandwidth will be. Under the condition of subwavelength dimension of the structure compared with the working wavelength, it is found that a FWHM of 4μm at central wavelength of ~9μm and a high absorption efficiency of up to 80% can be achieved. The proposed structure provides a novel method in the design of wideband efficient plasmonic absorbers in infrared or THz spectral regions with simultaneously wide bandwidth and high efficiency of absorption.

Direct Growth of Graphene on SiO2 by Using Ga-Ni Flux

A new growth method of graphene directly on the bare dielectric substrates by CVD (Chemical Vapor Deposition) method with Ga-Ni flux as sacrificial layer is reported in this work. Ga-Ni flux acts as not only a container to store carbon atoms which are dissolved from amourphous carbon film when heated, but also as a catalyst to promote the formation of graphene when cooled. In the process of growth, the Ga-Ni flux dewets and evaporates during the rearrangement of carbon atoms, resulting in graphene synthesized directly on the bare dielectric substrates. Scanning Raman Mapping and Spectroscopy, Scanning Electron Microscopy, and Atomic Force Microscopy were adopted to characterize the graphene film.

Polarization Properties of a Nano-Particle Array Grating on GaN-Based LED

A nano-particle array grating is proposed to achieve fine polarized output and transmittance on GaN-based LED. This nanostructure enables simple fabrication, since the novel arrays can be directly coated on the surface of GaN. In this letter, Aluminum nano-particle array gratings are optimized at blue and green band respectively. Extinction ratio up to 26dB and a TM polarized transmittance of 76% are achieved. Theoretical results show the nano-particle grating can be a guidance in designing the integrated GaN based and polarized photonic devices. Numerical simulations are performed using the Finite-difference time-domain (FDTD) solutions.

Narrowband Notch Filters with Composite Nanostructure Layers on a GaN-Based Light Emitting Diode

In this paper, novel active narrowband notch filters with triple-layer composite nanostructures on a GaN-based LED are obtained by mainly adjusting the grating period and duty cycle. The three layers consist of two dielectric thin layers and one metallic / dielectric grating layer. The grating layer composes of subwavelength period and thickness rectangular stripes, which lies between a transition layer and a protecting layer. Line-width and attenuation peak properties of the resonance filters are calculated and investigated by using a full vector implementation of Rigorous Coupled Wave Analysis (RCWA) algorithm. It is shown that the grating period can significantly change the filter peak wavelength, and the grating duty cycle heavily changes the filter line-width. The filter attenuation peak has a red shift with 23.3nm as the grating period increases 18nm. The FWHM (Full Width at Half Maximum) of the filter reduces from 1.9nm to 0.28nm as the duty cycle changes from 0.55 to 0.3, which compressed more than six times. Moreover, thickness of each composite nanostructure layer can also affect the narrowband width and peak wavelength of the filter. The results provide guidance in designing, optimizing and fabricating such an active narrowband filter with highly integrated photonic devices.

Subwavelength patterning based on a surface plasmon resonant cavity

With the development of Super Large Scale Integration (SLSI) and integrated optics, high-resolution photolithography has become more and more important. Traditional photolithography is limited by the optical diffraction of the system. Recent discovery of extraordinary behaviors of the surface plasmon polaritons suggests a novel method of photolithography beyond the diffraction limit. In this paper, we report on a novel subwavelength nanolithography technique using a surface plasmonic resonant cavity formed by two metallic layers separated by a photoresist layer with two incident beams illuminating from two sides. Finite-difference time-domain (FDTD) simulations show that a two-dimensional (2D) dot array pattern with a period of 70 nm can be obtained using an exposure radiation of 436nm wavelength. It is also found that the period of the 2D dot array is tunable which can be implemented by varying the cavity length.

High Polaried Transmission Effects for Double-Layer Metallic Grating Films on GaN Substrate

Highly polarized light transmission from GaN based light emitting diode is proposed using a double-layer metallic grating film and a dielectric transition layer. TM mode transmission and the polarized extinction ratio (ER) are calculated using commercial software, based on a full vector implementation of Rigorous Coupled Wave Analysis (RCWA) algorithm. Such a thin-film double-layer grating with subwavelength metallic stripes are designed and simulated by perfect parameters of period, thickness and filling factor for achieving good polarization properties. It is found that TM transmission and ER are almost stable and flat under different slit arrays of the double-layer grating. The polarized structure shows larger width of incident wavelength with a transition layer of a low refractive index than that of a high refractive index, but higher TM transmission and ER can be obtained for low refractive index transition layer. Flat sensitivity and high transmission of the TM mode on the double-layer metal grating thickness have been achieved. Up to 100nm range of the grating height can be employed to achieve TM transmission more than 92% while ER> 20dB. The results provide guidance in designing, optimizing and fabricating the integrated GaN-based and polarized photonic devices.

Optical properties of subwavelength wiregrid polarizer designed for GaN-based LED

A theoretical simulation is presented on the optical transmission and polarized extinction ratio through subwavelength wiregrid polarizer for GaN-based LED. The micro-polarizers are specially designed based on rigorous coupled wave analysis method for operation wavelengths of blue light (470nm) and green light (520nm) with metal wiregrid on the GaN substrate. The TM transmittance and the extinction ratio of TM and TE transmittances are influenced by period, thickness and duty cycle of the metal wiregrid. Aluminum and silver metal film grating under subwavelength is separately used for optimization design to improve the optical properties at the visible wavelengths. Simulation results show that, as the thickness of the metal grating decreases, the coupled light shifts the TM transmittance peak to shorter wavelength. As the same time, the extinction ratio remains a high value. TM transmittance coefficients separately greater than 95% and 97% with extinction ratio greater than 33dB are achieved for GaN-based aluminum grating and silver grating at the wavelength 470nm. And TM coefficients are greater than 92% and 96% with extinction ratios greater than 33dB, respectively at the wavelength 520nm. The thicknesses and periods of the metal grating are with sizes about 300nm, which are under subwavelength structures. The numerical results are useful for designing and fabricating novel photonic nanostructure polarized optical devices.