Dichroic Optical Diode Transmission in Two Dislocated Parallel Metallic Gratings
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An optical diode structure with two dislocated parallel metallic gratings is proposed and investigated numerically. Dichroic optical diode transmission is realized in this structure, i.e., optical diode effect is observed in two wavebands corresponding to inverse transmission directions. In the structure, two parallel metallic gratings with different grating constants are separated by a dielectric slab in between. The first illuminated grating acts as a selector for exciting surface plasmons at a proper wavelength. The other grating acts as an emitter to realize optical transmission. When the incident direction is reversed, the roles of two gratings exchange and surface plasmons are excited at another wavelength. In dichroic transmission wavebands, the optical diode structure exhibits extraordinary transmission and possesses high optical isolation up to 1. Furthermore, the operating wavebands can be modulated by changing structure parameters.
KeywordsDichroic optical diode Metallic gratings Diffraction Surface plasmons
Induced surface plasmons.
Structured surface plasmons
Optical diode, which transmits photons toward one direction and forbids the transmission in the reverse direction, has attracted considerable attention by virtue of the unidirectional transmission property . Optical diode phenomena can be observed when time-reversal symmetry of light-matter interaction is broken. External magnetic field , bias voltage , acoustic wave , or time-dependent modulation [5, 6] can be applied to achieve the optical diode effect. In addition, the structure of spatial inversion symmetry breaking is an alternative choice, such as asymmetric multilayer structures , asymmetric photonic crystals , and asymmetric gratings . In recent decades, metallic micro-nano structures gained great interest due to the promising properties of surface plasmons (SPs). Plasmonic devices are proposed in many research fields such as metasurface holography [10, 11, 12, 13, 14], refractive index sensor [15, 16], and filter [17, 18]. Plasmonic devices can strongly modify the interaction of electromagnetic fields in nanoscale . The modulation on SPs can be realized through changing the surrounding dielectric environment and geometric parameters of metallic structures [20, 21]. Optical diodes composed of nanoscale metallic structures, for example, plasmonic layer sandwiched gratings [22, 23], cascaded plasmonic gratings [24, 25], plasmonic nanoholes , plasmonic slot waveguide , and plasmonic nanoparticle aggregates , are widely investigated for the purpose of optical information processing.
In this paper, dichroic optical diode transmission is obtained in two dislocated parallel metallic gratings sandwiching a dielectric slab. Both transmission enhancement and high isolation contrast ratio are achieved in the two operating wavebands with reverse transmission directions, because metallic gratings consisting of narrow slits exhibit extraordinary light transmission [29, 30] and asymmetric structures realize unidirectional transmission [27, 28, 29, 30, 31]. According to the illuminated order, two metallic gratings with different grating constants act as a selector and an emitter respectively. The selector selects the resonance wavelength by exciting SPs and, with the contribution of SPs, the emitter realizes light transmission. When the incident direction is reversed, the roles of two gratings exchange and SPs are excited at another wavelength. Therefore, the dichroic optical diode transmission is obtained. The thickness of the optical diode structure proposed in this paper is as small as 160 nm. With the development of nanofabrication technologies, many methods can be applied to the fabrication of metallic gratings structures, such as ultraviolet nanoimprint lithography , laser-direct-writing lithography , and electron-beam lithography . The optical diode character is independent of the incident intensity. These properties imply that our structure has extensive potentials in optical integration.
Hence, η = 1 means the best optical diode performance.
Results and Theoretical Analyses
And it decides the critical wavelength λC (λC = 2π/|κ|) for TD ≠ TU. According to Eq. (3), λC is 1800 nm for our structure mentioned above, which is in good agreement with the simulation results λC = 1806 nm shown in Fig. 2. Optical diode effects appear in the range of λ ≤ λC. According to the simulation results, the period of the integrated gratings (1800 nm) is larger than the diode operating wavelengths (1315 nm and 921 nm). Multi-order diffraction components can be obtained with light scattering from the integrated gratings. Thus, the transmission field is not uniform along the direction parallel to gratings, even when the light is transmitted to the far field.
In the optical diode structure, G1 is the selector to excite SSPs for downward incidence and G2 is the selector for upward incidence. G1 and G2 have different grating constants, so SSPs are excited at different wavelengths for reverse incident directions. In Fig. 5, the photon energy at the red point is 0.91 eV and the wavelength is 1365 nm, which is corresponding to λD (1315 nm) shown in Fig. 2. Similarly, the photon energy indicated by the black point is 1.04 eV and its wavelength is 924 nm, corresponding to λU (921 nm) in Fig. 2. As the approximation of grating to plate, the SSPs resonance wavelengths calculated by using Eq. (4) and Eq. (6) are not exactly equal to the ones simulated by using FDTD methods shown in Fig. 2.
Investigation and Discussion
In this section, we investigate the influence of structure parameters on transmission spectra and isolation contrast ratio.
As shown in Fig. 1, the optical diode structure is periodic and it has the same unit cell when Δ = a ± MΛ2/2 (0 nm < a < Λ2/2 and M = 0, 1, 2…). Besides, the unit cell of Δ = a is left-right flip symmetric with that of Δ = − a ± MΛ2/2 and they can realize the same transmission effect. So, the transmittance of the optical diode structure is affected by Δ as: T(Δ) = T(Δ + Λ2/2) = T(− Δ + Λ2/2). As shown in Fig. 8, optical diode effect at λ~921 nm turns on and off within a period of Λ2/2 as Δ increases. However, transmission peak of TD exhibits a slight blueshift and the optical diode effect at λ~1315 nm is always on when Δ increases. Seen in Fig. 8a, a new transmission peak at λN emerges in TU curve near λU. When Δ increases from Λ2/12 to Λ2/6, the peak at λN exhibits a blueshift while the peak at λU exhibits a redshift (Fig. 8a, b). Ey distributions for transmission resonances at λU and λN are inserted in Fig. 8b. According to the simulation results, the resonance at λN generates because of the energy splitting. When Δ increases to Λ2/4, shown in Fig. 8c, TU is suppressed and two transmission resonances disappear, which makes the optical diode effect turn off at λ~921 nm.
The dichroic optical diode transmission based on SPs is realized in our structure, which consists of two dislocated parallel silver gratings and a silica interlayer. The first illuminated metallic grating selects the transmission waveband by exciting SSPs, and the other metallic grating emits electromagnetic energy forward through the surficial electrons oscillations. When the incident direction of light is reversed, the roles of two gratings exchange and another optical diode transmission waveband appears. The optical isolation ratio can almost reach up to 1. Optical diode transmission wavebands can be adjusted to be in different regions by changing the structure parameters. The optical diode operating wavebands and transmittance are independent of the incident intensity. The thickness of the structure is only a few hundred nanometers. These properties of our structure provide a wide range of applications in integrated circuits.
National Natural Science Foundation of China (11504185, 61178004, 11874229); Fundamental Research Funds for the Central Universities; Natural Science Foundation of Tianjin City (06TXTJJC13500); Science and Technology Commission of Tianjin Binhai New Area (BHXQKJXM-PT-ZJSHJ-2017003).
Availability of Data and Materials
The datasets supporting the conclusions of this article are included within the article.
PX, QS, and JC initiated the idea. JQ provided FDTD Solutions for simulation and provided technical help. PX built the basic model and performed the simulations. PX, XL, JC, and QS participated in the analyses and discussion. PX and JC prepared the manuscript. All authors contributed to the revision of the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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