Metallic nanowire gratings have been proposed for use as transmitted-type non-absorptive colorfilters and polarizers that take the place of the conventional absorptive ones in liquid crystal displays (LCDs), which can improve the light efficiency by recycling the reflected lights. To achieve a high recycling rate, the designed reflected light should be as high as possible, meaning absorption should be as low as possible. In this work, we find that higher reflection and lower loss can be obtained for the light incident to the grating side than to the substrate side in bi-layered aluminum nanowire gratings (BANGs), by decreasing light localization and waveguiding loss in the substrate. Taking full advantage of the reflection characteristics, we firstly demonstrate that when a BANG-based integrated polarizer and colorfilter is placed with its grating side facing the backlight in LCDs, more than a 30% light enhancement is obtained than the case with the substrate side facing the backlight. This work affords an essential guide for the design of eco-displays by using MNGs.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Metallic nano-structures with sub-wavelength pitches have been intensively investigated in biosensors [1–3], displays , photovoltic [5,6] and photothermal  devices during the past several decades. Among those researches, metallic nanowire gratings (MNGs) polarizers [8–10], colorfilters [11,12] and the integration of both  are of particular interest for the potential application in displays. Especially, Bi-layered MNGs with metallic nanowires [9,10] on top of dielectric grating lines and in the grooves are suitable for large size fabrication by utilizing simple lithography and imprinting process. In addition, bi-layered MNGs has the merits of giant extinction ratio of polarization.
Polarizers and colorfilters are both key components to fulfill the gray level and color modulation in liquid crystal displays (LCDs). Conventional iodine doped polyvinyl alcohol plastic polarizers  and dyes colorfilters  restrain the light efficiency of LCDs by absorbing 50% and 66.6% of incident light, respectively, as shown in the left part of Fig. 1. Based on the configuration, by replacing absorptive-type bottom polarizers or colorfilters with reflective types, the aboriginal absorptive lights can be recycled to improve the light efficiency of the LCDs. Kim et al.  utilized a kind of single layer MNGs as the bottom polarizer in LCDs, where the light efficiency of the new device is increased by about 30%.
Besides the efficiency, the contrast ratio and color gamut are the other two key parameters to evaluate the performance of the devices. Ge et al. concluded that by using MNG polarizers, contrast ratio of the LCDs could be substantially improved . Since plasmonic colorfilters have narrower transmission bands than that of pigments, LCDs based on MNGs also have wider color gamut as shown in Refs. [18,19].
In this work, bi-layered aluminium nanowire gratings (BANGs), integrated with polarization and colorfilter functions, are proposed to insert between the backlight and LCD module. It should be noted that, in order to avoid the reflection of ambient lights, which may interfere the reading of users, the bottom absorptive polarizers could still be used by inserting between BANG and liquid crystal layer. The detailed process are shown in the right part of Fig. 1. Where, when the un-polarized white lights emitted from the light guiding plate are incident on the BANG typed colorfilters and polarizers, the TE polarized white lights and the other two colors of TM lights not permitted to transmit by a specific pixel of BANG, are reflected back to the backlights. Via the reflection of metal reflector, the reflected lights by BANGs are sent back to the BANGs, where they are transmitted through corresponding right color pixels of BANGs, thus the originally absorbed lights in the conventional LCDs are recycled in the BANGs based LCDs. Thus, the light efficiency is improved.
Different from the incident angle sensitive surface plasmon resonant (SPR) transmission,  slit plasmonic waveguiding (SPW) and grating diffraction utilized in this work can alleviate color shifts with incident angles. The reflection from the grating side is stronger than that from the substrate. Based on the reflection character, a fundamental principle of LCD backlight unit with nanowire grating side facing the light source for further improving the light efficiency is firstly proposed, to the best of our knowledge. The light efficiency in the demonstrated LCD is further increased by more than 30%.
2.1 Device structure
The schematic of the BANG based integrated polarizer and colorfilter is presented in Fig. 2(a1) and 2(a2), where the reflection and transmission characters of both TM- and TE- polarized light are illustrated. The transmission of TM-polarized light via SPW in the slits (vertical arrows in Fig. 2(a1)) has color-filtering effect induced by the lateral SPR (horizontal arrows in Fig. 2(a1)). Without the SPW effect, TE-polarized light cannot enter the grating slits and are reflected mostly. When the light is incident to the substrate side (Incident-to-Substrate), the diffracted and reflected light by the grating can be partly located and guided in the substrate (oblique arrows in Figs. 2(a1 and a2), which induce the lower reflection than the case of incidence to the grating side (Incident-to-Grating), for both TM- and TE-polarized light
The photoresist (PR) nanowire gratings with pitch of 300 nm, duty ratio of 0.5, and thickness of 80 nm were fabricated by two-beam laser interference lithography using a diode pumped solid state laser with wavelength λ0 = 457.8 nm (CVI-MG). The Al film with thickness of 70 nm was deposited on the PR gratings by electron beam thermal deposition (EI5z, ULVAC), which cloned the profile of photoresist and formed periodical bi-layer one-dimensional nanowire gratings with step of 80 nm between the two layers. The scanning electron microscopy (SEM) images of the top and side views and atomic force microscopy (AFM) image of the fabricated device are shown in Figs. 2(b1-b3).
2.2 Symmetric polarized transmission
The measured transmitted spectra for TM- and TE-polarized lights of Incident-to-Substrate cases are shown in Figs. 3(a1 and b1), respectively, which are same with that of the Incident-to-Grating case. Generally, the transmission of TM light (about 15%) is higher than of the TE light(less than 0.3%), showing the characteristic of a polarizer with extinction ratio larger than 50 at wavelength of 650 nm. Furthermore, the transmission increases gradually with the increase of wavelengths and decreases slowly with increase of incident angles, which presents as an incident angle insensitive colorfilter with red color. The simulated results by using the software of DiffractMOD in RSOFT  are shown in Figs. 3(a2 and b2) with extinction ratio larger than 600, where the refractive index of PR is set with 1.5, aluminum is set with the built-in material in the software, and other structure parameters are same with that in Fig. 2. These simulations agree well with the measurements, further confirming the symmetric character. The difference in quantity between the simulations and experiments comes from imperfect fabrication, e.g. the grating lines are not uniform and their profiles are not steep enough, which reduce the transmission of the TM polarized lights.
The difference between TM and TE transmission comes from the well-known SPW effect where the metal-slit-metal waveguides permit TM-polarized light to pass while prohibit TE-polarized light. The dispersion curves of the Al-slit-Al waveguide modes are shown in Fig. 4, which obey the equation . Where, R=ɛikmx/ (ɛmkx) for TM-polarized light and kmx/kx for TE-polarized light, kmx= (ɛmk02-kz2)1/2 and kx= (ɛik02-kz2)1/2, ɛm and ɛi are the permittivity of metallic and dielectric, respectively. The blue star and red dot lines in Fig. 4 represent the real (kxr) and imaginary (kxi) parts of the kx. The dashed black lines show the range of visible light with wavelengths from 400 to 800 nm. The solid black lines represent the dispersion of light in the dielectric, i.e. light cone. The curve below light cone corresponds to SPW modes of TM-polarized light with a negligible kxi. Thus, TM-polarized light can always enter the slits in the SP modes illustrated as vertical arrows in Fig. 2(a1). The curves above the light cone are the first order of TE-polarized light and the second order of TM-polarized light. The frequencies 3.57*1015 Hz (527.4 nm, in Fig. 4(a1 and a2) in PR slits and 5.39*1015 Hz (349.4 nm, in Figs. 4(b1 and b2) in the air slits, where kxi=kxr, are the cut-off frequencies ωc. It means that the light with wavelengths larger than 527.4 nm and 349.4 nm cannot enter the photoresist and air slits in normal waveguide modes, respectively. As a result, TE-polarized light cannot transmit the grating. Thus, the bi-layer grating functions as a TM-transmitted polarizer in the visible light.
2.3 Asymmetric polarized reflection
2.3.1 TM polarization
The measured and simulated reflection and diffraction of TM spectra are shown in Fig. 5. Different from the transmission, the reflection behaves obviously asymmetric character as the reflection of Incident-to-Grating case in Figs. 5(b1 and b2) is higher than that of Incident-to-Substrate case in Figs. 5(a1 and a2), especially for the longer wavelengths. Three distinct zones divided by grating diffraction, which can well address the colorfilter effect, are listed as followed:
- (1) DA-Zone with |k0sinθi-G|< k0, where the light is Diffracted to air. In this zone, the 0th transmission and reflection are both weak and the incident light is mainly reflected back with angle of θog obeying the −1st order diffraction equation k0sinθi − G = k0sinθog, as shown in Figs. 5(a3 and b3). Figures 5(a4 and b4) present the electric-magnetic field distribution of wavelength 450 nm at incident angle 60° in both Incident-to-Substrate and -Grating cases, which clearly show light diffraction in the air.
- (2) DSW-Zone with k0<|k0sinθi-G|<ns*k0, where the light is Diffracted to substrate waveguide. In Incident-to-Substrate case, the diffracted angle is bigger than the total reflected angle at substrate-air interface, thus a part of the light is coupled into the waveguide mode of substrate and the rest enters into the top layer of air-Al grating. While, in Incident-to-Grating case, the light is reflected back to air firstly before entering into the bottom layer Photoresist-Al grating. Thus, the reflection of Incidence-to-Grating is higher than that of incidence-to-Substrate case with smaller loss to the substrate. The simulated electric-magnetic field for wavelength of 600 nm in Figs. 5(a5 and b5) further confirms the above analysis.
- (3) RT-Zone with ns*k0<|k0sinθi-G|, where, diffraction does not exit and the incident light is transmitted as shown in Figs. 3(a1 and a2) or otherwise reflected as shown in Figs. 5(a1, a2, b1 and b2). In the Incident-to-Substrate case, the light can enter the PR slits in the SPW mode with little loss (small kxi) and 2nd order mode with large loss (large kxi), as shown in Fig. 3(a1). In the Incident-to-Grating case, the light can enter the air slits in SPW mode only, as shown in Fig. 3(b1), which leads to less loss. Thus, the reflection in Incident-to-Grating case is higher than that in Incident-to-substrate case.
2.3.2 TE Polarization
The reflection of TE light has similar asymmetric character to the TM light, except it is much higher than that of TM, as shown of the measured results in Figs. 6(a1-b1) and simulated results in Figs. 6(a2-b2). The higher reflection of TE light than that of TM light is because that: without SPW mode, the TE-polarized light with wavelength larger than the cut-off value is prohibited to enter into the slits, thus the diffraction and transmission effects are very weak and the light is mainly reflected as the simulated fields in Figs. 6(a6 and b6). For the asymmetric reflection, it is because that the SPW modes in the PR slits have smaller imaginary parts of wavenumbers than that of Air slits (as shown in Figs. 4(a2 and b2)), thus the light in Incident-to-Substrate case can enter the device deeper, which leads to stronger diffraction and larger loss than that in Incident-to-Grating case. The diffracted spectra in Figs. 7(a3 and b3) and field distributions in Figs. 6(a4, b4, a5 and b5) further confirms the above analysis.
2.4 LCD backlight applications
To prove that the asymmetric reflection is adoptable in LCD backlight systems for further improving the light efficiency, a fabricated BANG with integrated transmitted colorfilter and polarizer functions was inserted in a cell phone between the backlight unit and LCD module. Snapshots with grating and substrate facing the backlight unit are shown in Figs. 7(a1 and a2), respectively. The characters of ‘SJTU’ in LCD were set to be red to meet the red color transmission of the BANG. The spectra for grating and substrate sides facing the backlight unit in red and white color are shown in Figs. 7(b1 and b2) with solid and dashed lines, respectively, which shows an increase of transmittance by more than 30% for Incident-to-Grating than that of Incident-to-Grating.
In summary, asymmetric reflection has been revealed for BANGs. The reflection from the grating side has less diffraction loss into the substrate, which leads to stronger reflection and diffraction into air for light recycling in LCDs. A large-size BANG integrated with colorfilter and polarizer effects is fabricated. A LCD backlight using the BANG is demonstrated. More than 30% of the light is obtained for grating side facing the light source compared with the substrate side doing. Although only the red light colorfilter is presented, we believe there will be similar for the blue and green color filters. The results are a significant hint for using reflective plasmonic colorfilters and polarizers to make power-saving LCDs and even the organic light emitting displays (OLEDs).
National Natural Science Foundation of China (11721091, 61775136); Research Fund of Yantai Information Technology Research Institute of Shanghai Jiao Tong University (G19YTJC007).
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