Yeo-Taek Yoon

Color filter based on a subwavelength patterned metal grating

A color filter incorporating a subwavelength patterned grating in a metal film perforated with a square array of circular apertures on a quartz substrate was accomplished. Its performance was enhanced by applying a dielectric overlay to the grating layer so as to match the refractive indices of the media on either side of it. The device was designed by utilizing the finite-difference time-domain method and implemented by adopting the electron-beam direct-writing technique. Two different devices were fabricated with the structural parameters: the grating height of 50 nm and the pitch of 340 nm for the red color and 260 nm for the green color. For the red color filter the center wavelength was 680 nm and the peak transmission 57%, while for the green color one the center wavelength was 550 nm and the peak transmission 50%. It was confirmed the introduction of the index matching overlay led to an increase of ~15% in the transmission efficiency and helped combine double bands into a single dominant band as well, thereby improving the color selectivity of the filter.

Color filter incorporating a subwavelength patterned grating in poly silicon

A color filter based on a subwavelength patterned grating in poly silicon was proposed and realized on a quartz substrate. It was produced by utilizing the laser interference lithography technique to feature wide effective area compared to the costly e-beam lithography. An oxide layer was introduced on top of the silicon grating layer as a mask to facilitate the silicon-etching and to enhance the filtering selectivity as well. The structural parameters for the device include the grating pitch and height of 450 nm and 100 nm respectively, the silicon stripe width of 150 nm, and the oxide thickness of 200 nm. The fabricated device offered a spectral response suitable for a blue color filter, exhibiting the center wavelength of approximately 460 nm, the bandwidth approximately 90 nm and the peak transmission 40%. The positional dependence of its performance was examined to find the effective area of 3 x 3 mm(2), where the variation in the relative transmission efficiency and in the center wavelength was less than 10% and 2 nm respectively. Finally, the influence of the angle of the incident beam upon the transfer characteristics of the device was investigated to reveal that the rate of change in the relative transmission was equivalent to about 1.5%/degree.

Transmission type color filter incorporating a silver film based etalon.

Transmission type color filters based on a thin film Ag-SiO(2)-Ag etalon were built on a quartz substrate, enabling the infrared suppressed transmission and large effective area. They were designed by taking into account the influence of the dispersion characteristics and the thickness of the silver metal. Three different color filters were devised: The cavity length for the red, green and blue filter was 160 nm, 130 nm, and 100 nm respectively, while the metal layer was fixed at 25 nm. The observed spectral pass band was centered at 650 nm, 555 nm, and 480 nm for the red, green, and blue device; the corresponding bandwidth was about 120 nm, 100 nm, and 120 nm; and the peak transmission was all ~60%. For the oblique light incidence the angular dependence of the peak relative transmission was measured to be approximately 1%/degree. The spectral response of the device was also analyzed for two different polarizations as the tilt angle varied up to 12(o), and it was found to be hardly polarization dependent. Finally, as for the positional dependence the relative transmission and the center wavelength were found to vary within 10% and 5 nm respectively over an effective area of 4x4 cm(2).

Polarization-independent visible wavelength filter incorporating a symmetric metal-dielectric resonant structure

A nanophotonic polarization-independent visible wavelength filter is presented, incorporating a symmetric metal-dielectric resonant structure on quartz substrate, where a sub-wavelength grating, made up of a two-dimensional array of Al square sheets, is integrated with a Si(3)N(4) slab waveguide via an oxide layer. Incident light is orthogonally diffracted by the symmetric grating towards two directions of the grating groove, and then resonantly coupled to both transverse electric and transverse magnetic guided modes associated with the underlying waveguide, irrespective of light polarization. Polarization independent bandpass filtering was thus achieved around specific wavelengths, determined by the grating pitch and the effective index of the waveguide. Three devices, operating in the blue, green and red spectral bands, were built through design and analysis drawing upon the finite-difference time-domain method. The devices, DEV I, II, and III, were constructed with grating pitches of 285, 355 and 395 nm, respectively, while the core was 100 nm thick. They were inspected to function as an efficient bandpass filter, centered at 460, 560 and 610 nm, with bandwidths of about 13, 14 and 17 nm, respectively; the peak transmission efficiencies were consistently over 85%. Furthermore, the transfer characteristics, insensitive to light polarization, were satisfactorily confirmed for normal incidence.

Highly Efficient Color Filter Incorporating a Thin Metal–Dielectric Resonant Structure

A highly efficient color filter that takes advantage of an ultrathin metal (Al)-dielectric (TiO2) resonant structure, where a subwavelength metallic grating is deposited as cladding in a planar dielectric waveguide, is demonstrated. A selective spectral response was obtained by virtue of the guided mode resonance between the diffracted mode and waveguide mode. The center wavelengths for the blue, green, and red filters were found to be 430, 520, and 630 nm, and the corresponding 3 dB bandwidths were 67, 84, and 80 nm, respectively. All three filters provided an overall transmission exceeding 70% and a satisfactory color image.

Electrically tunable color filter based on a polarization-tailored nano-photonic dichroic resonator featuring an asymmetric subwavelength grating

We have demonstrated a highly efficient electrically tunable color filter, which provides precise control of color output, taking advantage of a nano-photonic polarization-tailored dichroic resonator combined with a liquid-crystal based polarization rotator. The visible dichroic resonator based on the guided mode resonance, which incorporates a planar dielectric waveguide in Si3N4 integrated with an asymmetric two-dimensional subwavelength Al grating with unequal pitches along its principal axes, exhibited polarization specific transmission featuring high efficiency up to 75%. The proposed tunable color filters were constructed by combining three types of dichroic resonators, each of which deals with a mixture of two primary colors (i.e. blue/green, blue/red, and green/red) with a polarization rotator exploiting a twisted nematic liquid crystal cell. The output colors could be dynamically and seamlessly customized across the blend of the two corresponding primary colors, by altering the polarization via the voltage applied to the polarization rotator. For the blue/red filter, the center wavelength was particularly adjusted from 460 to 610 nm with an applied voltage variation of 2 V, leading to a tuning range of up to 150 nm. And the spectral tuning was readily confirmed via color mapping. The proposed devices may permit the tuning span to be readily extended by tailoring the grating pitches.

Highly Angle-tolerant Spectral Filter Based on an Etalon Resonator Incorporating a High Index Cavity

A high angular tolerance spectral filter was realized incorporating an etalon, which consists of a TiO2 cavity sandwiched between a pair of Ag/Ge mirrors. The effective angle was substantially extended thanks to the cavitys high refractive index. The device was created by embedding a 313-nm thick TiO2 film in 16-nm thick Ag/Ge films through sputtering, with the Ge layer alleviating the roughness and adhesion of the Ag layer. For normal incidence, the observed center wavelength and transmission were ~900 nm and ~60%, respectively; throughout the range of 50°, the relative wavelength shift and transmission variation amounted to only ~0.06 and ~4%, respectively.

Polarization-independent visible wavelength filter incorporating a 2D metal-dielectric resonant structure

A visible wavelength filter has been built based on a two-dimensional (2D) metal-dielectric resonant structure, featuring a polarization-independent operation and a high transmission over 90%.

Visible filter integrated with an image sensor fabricated by a 90-nm standard CMOS process

A visible filter, enabling a homogeneous integration with an image sensor, was demonstrated through an advanced standard 90-nm CMOS process. The overall transmission was as high as about 85% in the visible band.

Highly angle tolerant filter incorporating serially cascaded a-Si based etalons and its application to a compact receiver

A highly angle tolerant spectral filter has been implemented taking advantage of three-stage serially concatenated resonators in dielectric films, each of which involves a high-index cavity in a-Si, sandwiched with a pair of SiO2 films. For the constituent etalons, the free spectral range is individually adjusted by differentiating the thickness of the cavity, so that a primary infrared pass-band could be attained to present enhanced roll-off characteristics in conjunction with an appropriate bandwidth. The a-Si cavities relating to the three etalons are selected to be 117, 234, and 468-nm thick, while the SiO2 layer is uniformly 150-nm thick. The filter is actually created on a silica glass substrate, by alternately depositing SiO2 and a-Si films. The observed center wavelength, bandwidth, and peak transmission efficiency are about 900 nm, 120 nm, and over 90%, respectively, for normal incidence. In response to an angle change amounting to 60°, the relative wavelength shift and the variation in peak transmission become barely 0.03 and 8%, respectively. Finally, a detecting cell is constructed by integrating the prepared filter with a photodiode, thus rendering a 3-dB angular bandwidth of 90°. By adequately arranging three detecting cells in a fixture, a compact, portable optical receiver could then be constructed. For incoming collimated light at λ = 905 nm, the infrared receiver exhibits an extended 3-dB angular acceptance as large as 160°.