Duty cycle dependency of the optical transmission spectrum in an ultrathin nanostructured Ag film
Introduction
Plasmonic color filters based on periodically nanostructured metal films have been widely studied recent years [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. They represent an attractive approach for on chip color filters, which are vital components for future displays, image sensors, digital photography, projectors and other optical measurement instrumentation due to their stability, ease of fabrication, high space resolution, and high color tunability. While traditional colorant filters are vulnerable to processing chemicals, and suffer from performance degradation under long-duration ultraviolet irradiation or at high temperatures. Additive color filters (ACFs) based on the well-known extraordinary optical transmission (EOT) phenomenon [12], [13], [14] could accomplish color filtering by reject the entire visible spectrum except for the selective transmission band that is associated with the excitation of Surface Plasmon Polaritons (SPPs) on the surface of a single optical thick nanostructured metal film. The transmission band could be easily tuned through the entire visible spectrum by changing the geometric parameters, leading to high color tunability. However, the transmission peak of this color filter is relatively low (40–50%) [5] for the rejection of the major part of the visible spectrum. A plasmonic subtractive color filter (SCF) based on extraordinary low transmission (ELT) was proposed by Zeng et al. [8] to improve the transmission (up to 60–70%). They used an ultrathin nanostructured metals to reject one particular wavelength band and transmit the remaining major part of the visible spectrum, thus improved the total transmission and achieved the color filtering of the complementary color corresponding to the transmission minimum wavelength. Seemingly, the transmission minimum can be spectrally tuned by simply adjusting the geometric parameters, namely periodicity, shape and size of the nanostructure.
To our knowledge, most previous studies to tune the spectrum of the color filter are focused on the material selection [15], [16], [17], periodicity [8], [18], array shape [5], and thickness [18] of the nanostructure. West et al. [15] studied pure metals, alloys, heavily doped wide-band semiconductors and graphene to find the best low-loss plasmonic material for particular frequency and application. They found that silver is the better material in local surface plasmon polaritons (LSPPs) and SPP applications in visible and near infrared ranges in terms of its quality factor. The authors also noted that there is not one single clear choice for the best low-loss plasmonic material for all applications and many other aspects, such as fabrication practicality and cost, should be considered to make the final choice. Yang et al. [16] considered aluminum silver alloy as plasmonic material, and provided a method to change the interband transition energy. This alloy significantly decreases the dissipative loss by varying the annealing temperature. They further compared aluminum silver alloy with typical plasmonic material Cu and Au and came to a conclusion that the alloy is a better choice only within wavelengths below 650 nm. An aluminum platinum alloy for a plasmonic material in a particular frequency was also reported [17]. Yokogawa et al. [5] reported on the optical performance of a hexagonally packed subwavelength sized hole array on Al film, and found that the transmission properties of the hexagonal hole array filters are extremely robust with respect to array size, random defects, and spatial crosstalk from neighboring filters of different color compared with square hole array. In our previous work [18], effects of film thickness and grating period on color filter with one-dimensional Ag grating film were systematically studied. It is found that the wavelength of transmission minimum undergoes a redshift with increasing grating period and decreasing film thickness. However, so far no systematical investigation has been reported on the effect of duty cycle (the ratio of Ag width to the period) on the periodic nanostructure to color filter. In this work, we carried out a systematical study on the effect of duty cycle on the spectral performance in one-dimensional Ag grating nanostructure deposited on a glass substrate with different film thicknesses and periods. We reveal that the duty cycle can indeed largely influence the color filter behavior. Possible mechanisms for obtained results are discussed.
Section snippets
Simulation details
To systematically investigate the influence of duty cycle to the performance of plasmonic color filter, we simulated a series of periods and film thicknesses of nanostructured Ag films as depicted in Table 1.
Fig. 1 shows the schematic illustration of the structure studied here. w is the width of the Ag line, s is the width of the slit, d is the thickness of the Ag film, p is the period and the duty cycle c = w/p.
All of the simulations were carried out by three dimensional FDTD methods using the
Results
In the present work, we studied the effect of duty cycle on spectral performance of a one-dimensional Ag nanograting film deposited on a glass substrate. Firstly, gratings with fixed period p = 100 nm and film thickness d = 15 nm, but various duty cycles c = 0.4, 0.55 and 0.7 were simulated, respectively. Fig. 2 shows the simulation transmission spectra for the three different duty cycles. The blue, green, and red solid lines corresponding to c = 0.4, 0.55 and 0.7 clearly show a transmission minimum
Discussion
From Fig. 3, Fig. 4, Fig. 5, Fig. 6, it is found that for larger periods when the duty cycle increases, additional transmission minimum appears, degrading the transmission spectrum. Actually, this feature is a universal phenomenon when the duty cycle increases, for which the origin is still in debate. Some suggested SRSPP [20], [21] (short-range SPPs) as a key factor while LSPP [22] (local SPPs) was also proposed. Recently, Zeng et al. [8]. proposed that the ELT phenomenon attributes to both
Conclusions
In conclusion, we have theoretically and experimentally studied the influence of duty cycle to the transmission spectrum of ultrathin Ag film patterned with one dimension periodic nanogratings. Some selected one dimensional Ag film nanogratings (film thickness of 25 nm and period of 350 nm) with duty cycle of 0.2, 0.28, and 0.39 were experimentally fabricated. They illustrate yellow, purple red, and purple blue color by changing the duty cycle, which are consistent with simulated results,
Acknowledgements
Financial supports from National Natural Science Foundation of China (Grant Nos. 11179026 and U1432110), National Key Basic Research Program of China (2012CB825700), Natural Science Foundation of Zhejiang Province (Grants Z1110196 and Y4110192), and the Fundamental Research Funds for the Central Universities, the support of Soft-X Ray Interference Lithography Beamline (BL08U1B) in SSRF for sample preparation, and the support from Open Research Project of Large Scientific Facility from Chinese
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