Ultrathin nanogratings enable high-resolution spectral filtering and imaging

Current LCDs use several layers of optical films to color and polarize light from a backlight, which complicates manufacturing and increases cost. Additionally, color filters frequently suffer chemical defects, such as fading and embrittlement, and involve separate steps to deposit the red-green-blue sub-pixels, wasting considerable materials and chemicals during fabrication.

Thanks to the rapid development of nanofabrication and characterization techniques, surface plasmons (SPs) and related plasmonic nanostructures have generated considerable interest.¹ By exploiting plasmonic nanostructures, such as nanohole or nanoslit arrays, efficient conversion between photons and plasmons can be controlled at a subwavelength scale. This may provide new solutions to traditional optical processes, such as spectral filtering and imaging.

We demonstrated an ultrathin plasmonic nanograting that is capable of filtering white light into individual colors across the entire visible band. The key concept here is to use the nanograting to realize the photon–plasmon–photon conversion efficiently at specific resonance wavelengths. Compared to other novel spectral filtering methods,^2–4 our new design significantly improves absolute transmission, polarization, and pass bandwidth.

As a proof of principle and to enable easy fabrication, we designed the device as a subwavelength periodic metal-insulator-metal (MIM) stack array on a magnesium fluoride (MgF₂) transparent film. For each stack, a 100nm-thick zinc selenide (ZnSe) layer is sandwiched between two 40nm-thick aluminum (Al) layers. The ZnSe ensures the efficient coupling of SP modes at the top and bottom edges of the stack. The Al prohibits the direct transmission of incident light. The bottom Al grating is used to couple selectively the incident light into plasmon waveguide modes by diffraction. The top Al grating efficiently reconverts the confined plasmons to propagating waves by scattering, and then it transmits the light to the far field in the forward direction.

Figure 1(a) shows the optical microscopy images of the seven square-shaped plasmonic color filters illuminated by the white light. Fabricated using focus ion beam milling, the color filters have the stack period changing from 200–360nm, corresponding to colors from violet to red. All the filters have the same area dimension of approximately 10μm × 10μm. The measured transmission spectra of red-green-blue filters are given in Figure 1(b). For transverse magnetic illumination (the E-field is perpendicular to the Al grating direction), stack arrays show the expected filtering behavior with absolute transmission over 50% around the resonant wavelengths. This transmission is comparable with the prevailing colorant-based filter used in an LCD panel. But the plasmonic device is one to two orders of magnitude thinner than that of the colorant device. However, the transverse electric polarized light (the E -field is parallel to the Al wire direction) does not support the excitation of SP modes, and thus there is no obvious light-conversion process. As a result, the transverse electric polarized light is strongly suppressed at resonance wavelengths, and the transmissions are extremely low. This feature indicates that the structure itself can simultaneously function as a color filter and polarizer, which could greatly benefit LCDs by eliminating the need for a separate polarizer layer.

Figure 1. (a) Optical microscopy images of seven plasmonic color filters illuminated by white light. Scale bar is 10μm. (b) Measured transmission spectra of three fabricated color filters corresponding to the red-green-blue colors. Circle and triangle symbols correspond to transverse magnetic and transverse electric illuminations, respectively.

By gradually changing the period of the plasmonic nanograting array, we also designed and demonstrated a plasmonic spectroscope for spectral imaging. The device period varied from 200nm to 400nm, covering all colors in the visible range: see Figure 2(a). When illuminated with white light, the structure becomes a rainbow stripe with light emitting from the stack array: see Figure 2(b). Plasmonic spectroscopes can disperse the whole visible spectrum over a distance of just a few micrometers. This is orders of magnitude smaller than the dispersion of a conventional prism-based device. In fact, our thin-film stack structures can be directly integrated on top of focal plane arrays to implement high-resolution spectral imaging or to create chip-based ultracompact spectrometers.

Figure 2. (a) SEM image of fabricated 1D plasmonic spectroscope with gradually changing periods from 400nm to 200nm (from left to right). Scale bar is 2μm. (b) Optical microscopy image of plasmonic spectroscope illuminated with white light.

Considering that human eyes typically have a resolution limit of about 80μm at 35mm, these plasmonic filters can be used to build colored ‘super-pixels,’ which are much smaller than the resolution limit of human sight. Furthermore, these plasmonic devices are at least one order of magnitude thinner than current colorant-based filters. These artificial structures provide an opportunity for display and imaging devices with a higher spatial resolution, as well as much smaller device dimensions than those currently available.

Going forward, we aim to develop scalable nanofabrication techniques to produce large-area plasmonic color filters for display applications, employing methods such as roll-to-roll nanoimprinting process. We are also investigating reflective-type color filters.