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Abstract
Coupling light from in-plane guided light into free space or optical fibers is crucial for many photonic integrated circuits and vice versa. However, traditional grating couplers or waveguide grating antennas suffer from low upward coupling efficiency due to the light radiating in both upward and downward directions simultaneously. In this paper, a compact aperture-coupling nanoslot antenna array is proposed for high-efficiency unidirectional radiation, where a two-dimensional high-contrast grating (HCG) is employed as a mirror to reflect the undesired downward radiation. Upon the HCG separated by a low-index spacing layer, a thin silver layer is deposited. Finally, a series of H-shaped slots are patterned on the silver thin film to arrange the aperture fields and radiate the in-plane guided light into free space. The proposed nanoslot antenna array features a front-to-back ratio (F/B) over 10 dB within the wavelength range of 1500 ∼ 1600 nm. At the same time, a high radiation efficiency of over 75% and a maximum radiation efficiency of 87.6% are achieved within the 100 nm bandwidth. The high-efficiency unidirectional antenna array is promising for the integrated photonic applications including wireless optical communications, light detection and ranging, and fiber input/output couplers.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Photonic integrated circuits (PICs) provide alternative low-cost and high-performance solutions for achieving complex optical systems. Such systems are highly promising for the applications such as free-space optical communications, light detection and ranging (LiDAR), holographic displays, energy-harvesting and quantum light source [1–7]. In these applications, efficiently converting in-plane guided light into free-space radiation is essential and vice versa. Besides, the vertical input/output coupling to or from an optical fiber has also been extensively investigated in the telecom C-band [8–12]. Waveguide grating antennas are widely adopted to achieve high-efficiency couplings between the guided mode and the radiating mode [13–15]. However, a major issue associated with the waveguide grating antennas without special engineering is the nearly-equal radiated energy in both upward and downward directions, because of insufficient asymmetry in vertical direction. As examples, the optical power emitted upwards are typical below 50% for traditional waveguide grating antennas [16–18]. Although a partial etching operation can be introduced to break up-down symmetry of a grating antenna, achieved 51% upward-radiation energy is still far from the ideally near-unity efficiency [19].
To deal with this problem, several methods have been proposed so far. Subwavelength L-shaped radiating elements are designed to break up-down symmetry of radiating antennas. As a result, high radiation efficiencies (ηrad) of 89% [20] and 72% [21] are achieved, respectively. Another solution to tackle the bidirectional emission issue is to totally reflect the downward-radiation energy. A metallic reflector can be placed underneath antenna structures to achieve an ideal unidirectional radiation, which has been widely adopted in microwave antennas. As an example, limited by the absorption loss of the metallic radiation aperture, a unidirectional radiation antenna with maximum ηrad of only 65% is achieved by employing an aluminum ground plane underneath the antenna [22]. Although the efficiency can be further enhanced by replacing the metallic antenna with dielectric grating antenna, the main issue is that the introduction of bottom metallic reflectors is usually not compatible with standard CMOS processes, limiting many applications of photonic integrated circuits. Furthermore, instead of the bottom metallic reflector, two pairs of distributed Bragg reflector (DBR) are utilized to reflect the undesired downward radiation [23]. Consequently, a maximum upward ηrad of 95% is obtained while only 46% can be achieved without the DBR. In addition, destructive interference in the downward emission is designed by stacking up two waveguide gratings vertically, and an upward ηrad of 93% is achieved [24]. However, the multilayer structures in [23,24] are very complicated and challenging for fabrication.
Surface plasmonic waveguides support highly confined localized optical fields at the interface of metal-dielectric layer. Plasmonic nanoantennas (PNAs) are able to convert the confined localized fields to free-space radiation. By properly designing the radiating aperture, desired far-field radiation can be obtained, accordingly. Due to the strong light-matter interaction and highly oriented radiation pattern, these devices based on PNAs have enabled functionalities including nonlinear plasmonic [25], high-speed optical wireless links [26], and optical beam steering [27]. However, high propagation loss of surface plasmons severely restricts many practical applications [28,29]. Compared to the surface plasmonic waveguides, hybrid plasmonic waveguides (HPWs), which is based on a metal layer separated from a high -index slab by a low-index spacer, can confine energy in the low-index material with smaller propagating loss [30]. Previously, our group developed an aperture-coupled HPW-based nanopatch antenna array to achieve broadband unidirectional radiation [31]. However, limited by the intrinsic loss of metal, the maximum overall efficiency is only 71.33% after many optimizations. Besides, the dual-layer metallic structure in that design is also challenging for fabrication.
Recently, high-contrast gratings (HCGs), which are made up of periodic high-index material surrounded by low-index material, have captured many interests due to their broadband high reflectivity [32,33]. Particularly, the HCGs have been used as planar focusing reflectors [34], high-Q resonators [35], hollow-core low-loss waveguides [36] and so on. Furthermore, the HCG-based broadband mirror is implemented in vertical cavity surface emitting lasers (VCSELs) to replace conventional DBRs [37].
In this paper, a nanoslot antenna array for high-efficiency unidirectional radiation is introduced by combining the low-loss characteristic of HPWs and the high reflectivity of HCGs. The presented designs are based on a silicon-on-insulator (SOI) platform and are compatible with CMOS technology. Simulated results show that our proposed array achieves a front-to-back ratio (F/B) over 10 dB within the 1500 ∼ 1600 nm bandwidth, which is defined as the ratio between upward- and downward-emission optical power. Besides, compared to the dual-layer metallic structure in our previous work [31], higher ηrad of over 75% and a maximum ηrad of 87.6% are achieved within the 100 nm bandwidth. Therefore, the proposed high-efficiency unidirectional-radiation nanoslot antenna array is promising for many integrated photonics applications, including wireless optical communications, light detection and ranging, fiber input/output coupler and energy harvesting.
2. Design and results
The schematic overviews of the proposed structure are shown in Figs. 1(a) – 1(c). This design is based on a SOI platform with a silicon layer thickness of H = 220 nm and a buried oxide thickness of 2 μm, but an additional patterned metallic layer on top. Firstly, a two-dimensional (2-D) HCG is formed on the silicon layer acting as a broadband mirror to reflect the undesired downward radiation, which is the key to achieve unidirectional radiation.
Fig. 1. Structure of the proposed HCG reflection-mirror-based slot antenna array. (a) Illustration of the structure layer by layer. (b) Side view of the structure along yoz plane. (c) Cross-section view of the structure. The inset shows the electric field distributions of hybrid plasmonic mode at the wavelength of 1550 nm.
The period of the 2-D HCG in longitudinal direction (propagating direction of light) is Dy with a duty cycle of η. In transverse direction, the waveguide width is W with an element pitch of Dx. Then, separated by a thin SiO2 spacing layer with a thickness of H1, a silver thin film with a thickness of T is deposited. Therefore, a hybrid plasmonic mode is supported in this HPW formed by the metallic layer, the SiO2 spacer and the silver thin film. The electric field distributions of the eigenmode are depicted in the inset of Fig. 1(c). The refractive indices of silicon and silica are set to 3.48 and 1.444, respectively, and the permittivity of silver is fitted using Drude model with εinf = 5, ωp = 13.4 × 1015 rad/s and Г = 1.12 × 1014 1/s [31]. Because light is concentrated in the spacing layer with low refractive index (SiO2 in our case), the propagation loss can be substantially reduced compared to the surface plasmonic mode, where optical fields are highly confined at the interface of metal-dielectric layer in the latter. In addition, other materials with low refractive index, e.g., BCB, polymer and SU-8 photoresist, can also be used in the proposed structure for separation and planarization. Finally, a series of H-shaped slots are patterned on the silver thin film to radiate the energy into free space. Plasmonic slot antennas have been widely investigated for a variety of applications, e.g., polarization [38], polarization splitter [39] and biological sensing [40], because of their unique abilities to tailor the aperture fields. In our proposed structure, by carefully tuning physical parameters of the H-shaped slots, the radiating aperture fields can be arranged, accordingly. Combining with the designed HCG reflection mirror, the proposed nanoslot antenna array for high-efficiency and unidirectional radiation can be achieved.
2.1 2-D HCG design and analysis
It has been demonstrated that multipolar interference between a series of resonant modes leads to fascinating phenomena [41]. For example, the HCGs harness a special reflection mechanism based on the constructive and destructive interference of two modes supported by the grating [42]. Therefore, firstly, the transmission characteristics of 2-D HCGs are investigated. In the numerical model, Floquet-bloch boundaries are employed in a unit cell to calculate the silicon block in a periodic environment, as shown in Fig. 2(a). The incident wave with y-axis polarized electric-field component is launched from the incident port 1. The transmissivity is calculated by adding an additional port (named transmission port 2) to receive the transmitted power. Figure 2(b) gives the transmission coefficient from the input port 1 to the transmission port 2. It can be seen that two resonances are built at the wavelengths of around 1477 nm and 1625 nm, respectively. Furthermore, to investigate the multipole contributions to the two resonant modes, a multipole expansion operation is performed. This operation is based on the developed exact multipole expansion formulas [41], in which total scattering cross sections are obtained from the full-wave simulations as shown in Fig. 1(a) and then decomposed into the electric dipolar term Cpsca, magnetic dipolar term Cmsca, electric quadrupole CQesca and magnetic quadrupole CQmsca, respectively. The results in Fig. 2(c) reveal that the resonance at lower frequency corresponds to the electric resonance while the magnetic resonance and electric quadrupole resonance concertedly contribute to the resonance at higher frequency, which are in good agreement with the transmission spectrum in Fig. 2(b). Besides, the corresponding electric-field (E-field) y-component and magnetic-field (H-field) x-component distributions at the resonant wavelengths of 1477 nm and 1625 nm are depicted in Fig. 2(d), respectively. In our preliminary design, two resonances are first built up around the target frequency band. Although the transmissivity between the two resonances is still relatively high for the single 2-D HCG, when combining the 2-D HCG with optimized antenna radiating aperture, broadband reflection can be achieved.
Fig. 2. (a) Numerical model of the unit cell of the 2-D HCG. (b) Transmission spectrum from the incident port 1 to the transmission port 2. (c) Scattering spectrum of the designed 2-D HCG based on multipole expansion technology. (d) The corresponding E-field y-component and H-field x-component distributions of the 2-D HCG at the resonant wavelengths of 1477 nm and 1625 nm, respectively. The parameters are: W = 500, Dx = 1000, Dy = 880, ηDy = 520 and H = 220, all in nm.
2.2 Performances of the proposed nanoslot antenna array
Based on the above 2-D HCG, a high-efficiency unidirectional-radiation nanoslot antenna array is designed and numerically demonstrated. Due to the periodic nature of the antenna array in the x-direction, the calculated domains can be reduced by applying periodic boundary conditions on two sidewalls of a waveguide antenna element, as shown in Fig. 3(a). The parameters are given as follows: W = 500, Dx = 1000, Dy = 880, ηDy = 520, H = 220, H1 = 180, T = 80, ls = 360, ws = 200, lss = 180 and wss = 340, all in nm. The structure is 18.5 μm in length with an effective radiating region of 17.6 μm, where 20 repeated H-shaped slots are employed. The calculations are carried out using finite-element method (FEM). In this model, transverse-magnetic (TM) polarized light is launched into the waveguide and a wave port is added to excite the hybrid plasmonic mode. As explained in [43], for the HPW mode, the induced currents on the silver film are dominated by their y-component and are in parallel to the wave propagation direction. When introducing the transverse nanoslots, the induced surface currents are broken. To fulfill the current continuity formula, displacement currents polarized along y-direction within the nanoslots will be inspired, which equivalently acts as sources to radiate the energy into free space.
Fig. 3. (a) 3D schematic view of the simulated antenna element. (b) Electric-field y-component distributions of the proposed slot antenna array on the yoz plane at 1550 nm. (c) Magnetic-filed x-component distributions of the proposed slot antenna array on the yoz plane at 1550 nm.
Figure 3(b) gives the y-component distributions of E-fields of the designed nanoslot antenna array on the yoz plane at 1550 nm. As seen in Fig. 3(b), strong E-fields are excited in the nanoslots, and the in-plane guided light can be coupled into free space via the aperture. Therefore, a desired far-field radiation can be obtained by carefully designing the shape of nanoslots. Besides, the E-field y-component and the H-field x-component of the proposed design depicted in Fig. 3(b) and (c) show similar field distributions with the corresponding one of an isolated 2-D HCG depicted in the Fig. 2(d), indicating weak electric quadrupole resonance and magnetic resonance of the designed 2-D HCG. The multipolar interferences among these resonant modes contribute to its high reflectivity. Therefore, the proposed array with unidirectional radiation can be achieved.
The near-field distributions of the proposed nanoslot antenna array are depicted in Fig. 4(a) and compared with those of a traditional one as a reference in Fig. 4(b). Their physical parameters are identical but the periodic silicon blocks along y-direction are replaced by continuous silicon strip in the latter. As seen in Fig. 4(b), without special engineering, the referenced array is allowed to radiate in both upward and downward directions. The F/B, which is defined as the ratio of total power in the upper-half space to that in the lower-half space, is calculated to -2.6 dBc, indicating more downward radiation. However, in the proposed design as shown in Fig. 4(a), clear wave front in the upper-half space is formed due to the interference of the radiation fields from the nanoslots, while the behaviors are not observed in the lower-half space, corresponding to an F/B up to 13 dBc. More intuitively, the corresponding far-field radiation patterns of the proposed and the referenced array are shown in Fig. 4(c), indicating a clear unidirectional radiation of the proposed array. Meanwhile, the directivity of the referenced nanoslot array suffers from a drop of 3.59 dB (from 18.22 dBi to 14.63 dBi) compared to that of the proposed array, due to the bidirectional light radiation. The antenna directivity D is defined as: D = 4π*P(θ,φ)/Pe, where P(θ,φ) and Pe represent the radiation power density at the specified angle in free space and total radiated power, respectively [31]. On the other hand, because the radiation fields have exponentially decaying power P = P0 exp(-2αy) along the propagation direction for a uniform slot aperture, where α is the leakage factor of the antenna [21], it can be seen from Fig. 4(a) and (b) that the field distributions show a strong radiation at the beginning of the coupler. However, it can be prevented by employing an apodized H-shaped slot design, so that a uniform radiation can be achieved. Besides, reducing the sizes of H-shaped slots can also tackle the issue.
Fig. 4. (a) The near-field distributions of the proposed HCG-reflector-based nanoslot antenna array. (b) The near-field distributions of the referenced array (c) The far-field radiation patterns of the HCG-reflector-based nanoslot antenna array and the traditional nanoslot antenna array.
The directivity, F/B and ηrad of the proposed array versus wavelength are shown in Fig. 5(a). Stable antenna directionalities around 18 dB indicate good radiation characteristics of the proposed array. Besides, F/B of over 10 dB is achieved within the wavelength ranges of 1500 ∼ 1600 nm and the maximum F/B is 13.24 dB at 1554 nm. Furthermore, a high ηrad of over 75% and a maximum ηrad of 87.6% are achieved within the 100 nm bandwidth after taking the metallic intrinsic absorption loss into account. In addition, the port reflection coefficient below -15 dB is maintained within the operation frequency band, as shown in Fig. 5(b), suggesting that the introductions of the 2-D HCG on the silicon layer and H-shaped slots on the silver thin film will not degrade the port matching. Figure 5(c) illustrates the normalized radiation patterns of the proposed array at various wavelengths and the inset in Fig. 5(c) shows the partial enlarged details. The main beam can be steered from -5.8° to -15.5° as the wavelength increases from 1500 to 1596 nm. Meanwhile, within the scanning ranges, the main beam directivity in free space is at least 16 dB higher than the maximum of that on the substrate side at the scanning yoz plane, as indicated by Fig. 5(c). Frequency-controlled beam-scanning antennas or arrays have been widely researched in microwave frequency bands for their applications in automotive radar, multi-spot heating/sensing, spatial filtering and wireless communication systems (such as point-to-point satellite communication systems) [44]. In optical frequency range, the frequency-scanning characteristics of the proposed array is also useful in various applications, such as wireless optical links, holographic displays, light detection and ranging (LiDAR) and biological sensing [2,3,45,46].
Fig. 5. (a) Red solid curve: antenna directivity versus wavelength; Black solid curve: F/B versus wavelength; Blue dash line: ηrad versus wavelength. (b) The reflection s-parameter of input port within the operation bandwidth. (c) Normalized far-field radiation patterns at various wavelengths. The inset in (c) shows the partial enlarged details.
2.3 Parameter analysis
In this section, the antenna ηrad and F/B varied with the physical parameters of the proposed array are investigated. Because the hybird TM mode of the HPW is confined in the low-index layer, the thickness H1 is associated with mode characteristics, e.g., transmission loss, mode confinement factor, and propagation constant, which will have a significant influence on ηrad and F/B. Figures 6(a) and (b) give the ηrad and F/B versus wavelength under different H1, respectively. It can be seen that with the increasing of H1, the resonance (located at the wavelength above 1600 nm) shifts toward higher frequency and is closed to higher end of the operating wavelength band, resulting in a drop of ηrad. It can be seen that as H1 = 240 nm, ηrad drops to below 60% at 1600 nm. On the other hand, the downward radiation reflected by the 2-D HCG interferes with the upward radiation waves on the antenna aperture. The constructive interference depends on the travling paths of the reflected waves, i.e., H1. Therefore, H1 = 180 nm is properly chosen for higher F/B and a moderate ηrad. Similar tendencies can be observed from Figs. 6(c) and (d), where ηrad and F/B versus wavelength at different T are illustrated.
Fig. 6. At different spacing layer thickness, (a) ηrad versus wavelength, and (b) F/B versus wavelength. At different silver thin film thickness, (a) ηrad versus wavelength, and (b) F/B versus wavelength.
Besides, the H-shaped slots are also the key to dominate ηrad and F/B. As mentioned above, for the hybrid TM mode, the induced currents are cut off by the transverse slots on the silver thin film, resulting in the radiation of electromagnetic waves. Therefore, larger sizes of the H-shaped slots indicate a larger radiation aperture and stronger radiation can be inspired, accordingly. In addition, larger slot aperture leads to smaller light-metal interaction area. Consequently, the metallic absorption loss can be reduced. As seen in Fig. 7(a), ηrad decreases as wss become smaller, and the resonance located at around 1600 nm also shifts to higher frequency. Especially, when wss is below 200 nm, ηrad will experience a significant drop at low frequency due to the resonance. A similar tendency of F/B can also be observed in Fig. 7(b). As wss decreases, the resonance is getting closer and closer to the higher end of the operating wavelength band, and F/B gradually deteriorates. It is further confirmed that the achievement of high antenna F/B is associated with the resonant modes supported by the 2-D HCG.
Fig. 7. At different wss, (a) ηrad versus wavelength, and (b) F/B versus wavelength. At different ls, (c) ηrad versus wavelength, and (d) F/B versus wavelength. At different ws, (e) ηrad versus wavelength, and (f) F/B versus wavelength.
The antenna ηrad and F/B versus the longitudinal slot parameters ls and ws are also shown in Figs. 7(e)-(f), respectively. The longitudinal slots here are employed to enlarge the antenna radiation aperture and adjust the resonance of 2-D HCG. It can be seen that the resoance at about 1600 nm, corresponding to the electric dipolar resonance according to the multipolar expansion results shown in Fig. 2(c), can be adjusted by arranging ls and ws. Decreasing the sizes of the H-shaped slots, the resonance shifts to higher frequency, resulting in a drop of ηrad and F/B. However, for larger slot sizes, taking ls = 420 nm as an example, a ηrad of up to 85% can be obtained in the 1500 ∼ 1600 bandwidth, but F/B shows a little deterioration compared to that with ls = 360 nm. Therefore, in our case, the sizes of H-shaped slots are chosen for high F/B and a moderate ηrad in 1500 ∼ 1600 nm.
In this work, the proposed design can be fabricated by standard electron beam lithography (EBL) and lift-off processes. The EBL technique provides the capability of high-precision fabrication with the fabricated deviations on the order from a few nanometers to a few dozens of nanometers. The fabricated processes are described in detail as follows: Firstly, the 2-D HCG can be obtained by a single reactive-ion etching (RIE) process. Then, the SiO2 can be deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD). After a polishing operation, the silver thin film can be deposited onto SiO2 spacing layer by using the thermal evaporator. Finally, the H-shaped slots on the silver thin film can be fabricated by EBL and lift-off procedures. According to the above parameter analysis, these results strongly indicate that the structure is robust under a reasonable deviation of H-shaped slot sizes and other structural parameters such as the thickness of the SiO2 spacing layer and the silver thin film. Therefore, the proposed design holds considerable merits in favorable fabrication tolerances.
3. Discussion
Traditional grating couplers without special engineering suffer from low coupling efficiency due to the light leakage into the substrate. Taking full-etched gratings as an example, although the buffer layer thickness is optimized, the low coupling efficiency of only -2.4 dB is achieved [47]. To improve the coupling efficiency, shallow-etched grating couplers are proposed to suppress the downward leakage [10,48,49]. In this section, the feasibility of the proposed structure used as fiber input/output coupler is discussed.
At the wavelength of 1550 nm, the port transmission coefficient, antenna F/B and ηrad versus the period number Ns of the H-shaped slots in y-direction are illustrated in Fig. 8, respectively. Specially, the port transmission is obtained by adding an additional port at the end of the waveguide to receive the remaining power and the efficient radiation length can be roughly calculated as L = Dy*(Ns-1). As seen in Fig. 8, stable F/B of over 12 dB are maintained at different Ns. Besides, a slight drop of ηrad can be observed as antenna length L increases, which can be attributed to the metallic absorption. Because the longer the antenna length is, the more the metallic absorption. Identical with general traveling wave antennas, as L increases, more energy can be radiated into free space, resulting in less energy transmitted through the receiving port. As an example, when Ns = 24, corresponding to an efficient radiation length about 20.24 μm, only 5.5% of the energy is not radiated. Moreover, faster radiation rate can be achieved by further enlarging the radiation aperture. Considering the mode field diameter of standard single-mode optical fiber is about 10.4 μm, a typical grating length in longitudinal direction above 20 μm and grating width in transverse direction between 10-15 μm is required to match the size of the fiber core [10,50]. It suggests the short radiation length of the proposed array is suitable for bridging the coupling between in-plane guided light and optical fiber mode.
Fig. 8. (a) At the wavelength of 1550 nm, red solid curve: F/B versus the number of H-shaped slots; black solid curve: port transmission coefficient versus the number of H-shaped slots; blue dash curve: ηrad versus the number of H-shaped slots. (b) The schematic view of the proposed structure and the single-mode optical fiber at yoz plane. (c) The normalized power intensity of the radiation fields and the optical fiber mode along the red reference line at 1550 nm.
The overlap integral of the proposed design and a conventional single-mode optical fiber is roughly evaluated in 2-D calculations, as shown in Fig. 8(b). The optical fiber at a title angle θ = -11° has a core diameter and a cladding diameter of 9 μm and 125 μm, respectively. Figure 8(c) shows the normalized power intensity of the radiation fields and the optical fiber mode along the reference line. It can be seen that the optical fiber mode is approximated with a Gaussian beam, while the radiation fields have exponentially decaying power along the propagation direction due to the uniform slot aperture. It has been demonstrated that for such a uniform slot aperture, a maximum overlap integral of approximately 80% can be obtained [51]. In our case, the overlap integral of the radiated near fields and the fiber mode is calculated to be about 58%. However, by properly designing the physical sizes of each H-shaped slot, the leakage factor can be arranged and a high overlap integral between the radiated fields of the proposed structure and the near-Gaussian optical fiber mode can be achieved, accordingly [52]. Therefore, our proposed design can be very promising for high-efficiency unidirectional coupling in-plane guided light to optical fiber or vice versa. The optimization of the apodized H-shaped slots for high-efficiency coupling between the proposed structure and the single-mode optical fiber will be carried out in our future work.
4. Conclusion
In this paper, an on-chip plasmonic HCG-reflector-based nanoslot antenna array is proposed for high-efficiency unidirectional radiation. Firstly, the 2-D HCG is designed by transmission spectrum analysis and multipole expansion technology is presented to further verify the resonant characteristics. Then, based on the designed 2-D HCG, the nanoslot antenna array with unidirectional radiation is designed and numerically demonstrated. Simulated results show that our proposed nanoslot antenna array achieves an F/B of over 10 dB within the 1500 ∼ 1600 nm bandwidth. Compared with the referenced nanoslot antenna array without the HCG reflector, a directivity enhancement of 3.59 dB is achieved. Besides, a high ηrad of over 75% and a maximum ηrad of 87.6% are achieved within the 100 nm bandwidth. Finally, parametric analysis of the proposed design is presented. These results also indicate that our design is robust to reasonable fabrication errors from structural parameter variations. In addition, the feasibility of the proposed structure used as fiber input/output coupler is discussed. It is demonstrated that the short controllable radiation length and steerable aperture fields make the proposed structure suitable for the high-efficiency couplings between in-plane guided light and optical fiber mode. Besides, our proposed high-efficiency unidirectional radiation nanoslot antenna array can also find applications in wireless optical communications, inter-chip photonic interconnects, light detection and ranging, and energy harvesting and so on.
Funding
National Natural Science Foundation of China (U20A20165); Fundamental Research Funds for the Central Universities (ZYGX2019Z005).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. W. M. Neubert, K. H. Kudielka, W. R. Leeb, and A. L. Scholtz, “Experimental demonstration of an optical phased array antenna for laser space communications,” Appl. Opt. 33(18), 3820–3830 (1994). [CrossRef]
2. G. Yang, W. Han, T. Xie, and H. Xie, “Electronic holographic three-dimensional display with enlarged viewing angle using non-mechanical scanning technology,” OSA Continuum 2(6), 1917–1924 (2019). [CrossRef]
3. C. V. Poulton, A. Yaacobi, D. B. Cole, M. J. Byrd, M. Raval, D. Vermeulen, and M. R. Watts, “Coherent solid-state LIDAR with silicon photonic optical phased arrays,” Opt. Lett. 42(20), 4091–4094 (2017). [CrossRef]
4. K. Wang, A. Nirmalathas, C. Lim, E. Wong, K. Alameh, H. Li, and E. Skafidas, “High-speed indoor optical wireless communication system employing a silicon integrated photonic circuit,” Opt. Lett. 43(13), 3132–3135 (2018). [CrossRef]
5. C. P. Dietrich, A. Fiore, M. G. Thompson, M. Kamp, and S. Höfling, “GaAs integrated quantum photonics: Towards compact and multi-functional quantum photonic integrated circuits,” Laser Photonics Rev. 10(6), 870–894 (2016). [CrossRef]
6. A. Nourmohammadi and M. Nikoufard, “Ultra-wideband photonic hybrid plasmonic horn nanoantenna with SOI configuration,” Silicon 12(5), 193–198 (2020). [CrossRef]
7. J. K. Doylend, M. J. R. Heck, J. T. Bovington, J. D. Peters, L. A. Coldren, and J. E. Bowers, “Two-dimensional free-space beam steering with an optical phased array on silicon-on-insulator,” Opt. Express 19(22), 21595–21604 (2011). [CrossRef]
8. X. Chen and H. K. Tsang, “Polarization-independent grating couplers for silicon-on-insulator nanophotonic waveguides,” Opt. Lett. 36(6), 796–798 (2011). [CrossRef]
9. X. Chen, K. Xu, Z. Cheng, C. K. Fung, and H. K. tsang, “Wideband subwavelength gratings for coupling between silicon-on-insulator waveguides and optical fibers,” Opt. Lett. 37(17), 3483–3485 (2012). [CrossRef]
10. D. Benedikovic, C. Alonso-Ramos, S. Guerber, X. L. Roux, P. Cheben, C. Dupre, B. Szelag, D. Fowler, E. Cassan, D. Marris-Morini, C. Baudot, F. Boeuf, and L. Vivien, “Sub-decibel silicon grating couplers based on L-shaped waveguides and engineered subwavelength metamaterials,” Opt. Express 27(18), 26239–26250 (2019). [CrossRef]
11. D. Benedikovic, C. Alonso-Ramos, P. Cheben, J. H. Schmid, S. Wang, R. Halir, A. Ortega-Moñux, D.-X. Xu, L. Vivien, J. Lapointe, S. Janz, and M. Dado, “Single-etch subwavelength engineered fiber-chip grating couplers for 1.3 µm datacom wavelength band,” Opt. Express 24(12), 12893–12904 (2016). [CrossRef]
12. A. Michaels and E. Yablonovitch, “Inverse design of near unity efficiency perfectly vertical grating couplers,” Opt. Express 26(4), 4766–4779 (2018). [CrossRef]
13. K. Shang, C. Qin, Y. Zhang, G. Liu, X. Xiao, S. Feng, and S. J. B. Yoo, “Uniform emission, constant wavevector silicon grating surface emitter for beam steering with ultra-sharp instantaneous field-of-view,” Opt. Express 25(17), 19655–19661 (2017). [CrossRef]
14. S. W. Chung, H. Abediasl, and H. Hashemi, “A monolithically integrated large-scale optical phased array in silicon-on-insulator CMOS,” IEEE J. Solid-State Circuits 53(1), 275–296 (2018). [CrossRef]
15. R. Fatemi, A. Khachaturian, and A. Hajimiri, “A nonuniform sparse 2-D large-FOV optical phased array with a low-power PWM drive,” IEEE J. Solid-State Circuits 54(5), 1200–1215 (2019). [CrossRef]
16. J. Sun, E. Timurdogan, A. Yaacobi, Z. Su, E. S. Hosseini, D. B. Cole, and M. R. Watts, “Large-scale silicon photonic circuits for optical phased arrays,” IEEE J. Sel. Top. Quantum Electron. 20(4), 264–278 (2014). [CrossRef]
17. K. V. Acoleyen, W. Bogaerts, J. Jágerská, N. L. Thomas, R. Houdré, and R. Baets, “Off-chip beam steering with a one-dimensional optical phased array on silicon-on-insulator,” Opt. Lett. 34(9), 1477–1479 (2009). [CrossRef]
18. C.-S. Im, B. Bhandari, K.-P. Lee, S.-M. Kim, M.-C. Oh, and S.-S. Lee, “Silicon nitride optical phased array based on a grating antenna enabling wavelength-tuned beam steering,” Opt. Express 28(3), 3270–3279 (2020). [CrossRef]
19. J. Sun, E. Timurdogan, A. Yaacobi, E. S. Hosseini, and M. R. Watts, “Large-scale nanophotonic phased array,” Nature 493(7431), 195–199 (2013). [CrossRef]
20. S. Khajavi, D. Melati, P. Cheben, J. H. Schmid, Q. Liu, D. X. Xu, and W. N. Ye, “Compact and highly-efficient broadband surface grating antenna on a silicon platform,” Opt. Express 29(5), 7003–7014 (2021). [CrossRef]
21. P. Ginel-Moreno, D. Pereira-Martín, A. Hadij-ElHouati, W. N. Ye, D. Melati, D. Xu, S. Janz, A. Ortega-Moñux, J. Wangüemert-Pérez, R. Halir, Í Molina-Fernández, J. Schmid, and P. Cheben, “Highly efficient optical antenna with small beam divergence in silicon waveguides,” Opt. Lett. 45(20), 5668–5671 (2020). [CrossRef]
22. A. Yaacobi, E. Timurdogan, and M. R. Watts, “Vertical emitting aperture nanoantennas,” Opt. Lett. 37(9), 1454–1456 (2012). [CrossRef]
23. Y. Zhang, Y. Ling, K. Zhang, C. Gentry, D. Sadighi, G. Whaley, J. Colosimo, P. Suni, and S. J. B. Yoo, “Sub-wavelength-pitch silicon-photonic optical phased array for large field-of-regard coherent optical beam steering,” Opt. Express 27(3), 1929–1940 (2019). [CrossRef]
24. M. Raval, C. V. Poulton, and M. R. Watts, “Unidirectional waveguide grating antennas with uniform emission for optical phased arrays,” Optica 42(13), 2563–2566 (2017). [CrossRef]
25. S. B. Hasan, C. Etrich, R. Filter, C. Rockstuhl, and F. Lederer, “Enhancing the nonlinear response of plasmonic nanowire antennas by engineering their terminations,” Phys. Rev. B 88(20), 205125 (2013). [CrossRef]
26. G. Bellanca, G. Calo, A. E. Kaplan, P. Bassi, and V. Petruzzelli, “Integrated Vivaldi plasmonic antenna for wireless on-chip optical communications,” Opt. Express 25(14), 16214–16227 (2017). [CrossRef]
27. C. T. Derose, R. D. Kekatpure, D. C. Trotter, A. Starbuck, J. R. Wendt, A. Yaacobi, M. R. Watts, U. Chettiar, N. Engheta, and P. S. Davids, “Electronically controlled optical beam-steering by an active phased array of metallic nanoantennas,” Opt. Express 21(4), 5198–5208 (2013). [CrossRef]
28. J. Park, S. Kim, J. Lee, S. G. Menabde, and M. S. Jang, “Ultimate light trapping in a free-form plasmonic waveguide,” Phys. Rev. Appl. 12(2), 024030 (2019). [CrossRef]
29. Z. Li, L. Liu, H. Sun, Y. Sun, C. Gu, X. Chen, Y. Liu, and Y. Luo, “Effective surface plasmon polaritons induced by modal dispersion in a waveguide,” Phys. Rev. Appl. 7(4), 044028 (2017). [CrossRef]
30. M. Khodadadi, N. Nozhat, and S. M. M. Moshiri, “Analytic approach to study a hybrid plasmonic waveguide-fed and numerically design a nano-antenna based on the new director,” Opt. Express 28(3), 3305–3330 (2020). [CrossRef]
31. Y. S. Zeng, S. W. Qu, C. Wang, B. J. Chen, and C. Chen, “Efficient unidirectional and broadband vertical-emitting optical coupler assisted by aperture-coupled nanopatch antenna array,” Opt. Express 27(7), 9941–9954 (2019). [CrossRef]
32. V. Karagodsky, F. G. Sedgwick, and C. J. Chang-Hasnain, “Theoretical analysis of subwavelength high contrast grating reflectors,” Opt. Express 18(16), 16973–16988 (2010). [CrossRef]
33. V. Karagodsky and C. J. Chang-Hasnain, “Physics of near-wavelength high contrast gratings,” Opt. Express 20(10), 10888–10895 (2012). [CrossRef]
34. F. Lu, F. G. Sedgwick, V. Karagodsky, C. Chase, and C. J. Chang-Hasnain, “Planar high-numerical-aperture low-loss focusing reflectors and lenses using subwavelength high contrast gratings,” Opt. Express 18(12), 12606–12614 (2010). [CrossRef]
35. Y. Zhou, M. Moewe, J. Kern, M. C. Y. Huang, and C. J. Chang-Hasnain, “Surface-normal emission of a high-Q resonator using a subwavelength high-contrast grating,” Opt. Express 16(22), 17282–17287 (2008). [CrossRef]
36. Y. Zhou, V. Karagodsky, B. Pesala, F. G. Sedgwick, and C. J. Chang-Hasnain, “A novel ultra-low loss hollow-core waveguide using subwavelength high-contrast gratings,” Opt. Express 17(3), 1508–1517 (2009). [CrossRef]
37. C. J. Chang-Hasnain, Y. Zhou, M. C. Y. Huang, and C. Chase, “High-contrast grating vcsel,” IEEE J. Sel. Top. Quantum Electron. 15(3), 869–878 (2009). [CrossRef]
38. J. Yang and J. Zhang, “Nano-polarization-converter based on magnetic plasmon resonance excitation in an L-shaped slot antenna,” Opt. Express 21(7), 7934–7942 (2013). [CrossRef]
39. B. Chen, J. Yang, C. Hu, S. Wang, Q. Wen, and J. Zhang, “Plasmonic polarization nano-splitter based on asymmetric optical slot antenna pairs,” Opt. Lett. 41(21), 4931–4934 (2016). [CrossRef]
40. K. J. Ahn, Y.-M. Bahk, D.-S. Kim, J. Kyoung, and F. Rotermund, “Ultrasensitive molecular absorption detection using metal slot antenna arrays,” Opt. Express 23(15), 19047–19055 (2015). [CrossRef]
41. T. Hinamoto and M. Fujii, “MENP: an open-source MATLAB implementation of multipole expansion for nanophotonics,” OSA Continuum 4(5), 1640–1648 (2021). [CrossRef]
42. D. Urbonas, R. F. Mahrt, and T. Stöferle, “Low-loss optical waveguides made with a high-loss material,” Light: Sci. Appl. 10(1), 15 (2021). [CrossRef]
43. Y. S. Zeng, S. W. Qu, and J. Y. Wu, “Polarization-division and spatial-division shared-aperture nanopatch antenna arrays for wide-angle optical beam scanning,” Opt. Express 28(9), 12805–12826 (2020). [CrossRef]
44. D. Liao, Y. Zhang, and H. Wang, “Wide-Angle Frequency-Controlled Beam-Scanning Antenna Fed by Standing Wave Based on the Cutoff Characteristics of Spoof Surface Plasmon Polaritons,” Antennas Wirel. Propag. Lett. 17(7), 1238–1241 (2018). [CrossRef]
45. G. Calo, G. Bellanca, B. Alam, A. E. Kaplan, P. Bassi, and V. Petruzzelli, “Array of plasmonic Vivaldi antennas coupled to silicon waveguides for wireless networks through on-chip optical technology – WiNOT,” Opt. Express 26(23), 30267–30277 (2018). [CrossRef]
46. J. Vörös, J. J. Ramsden, G. Csúcs, I. Szendrő, S. M. De Paul, M. Textor, and N. D. Spencer, “Optical grating coupler biosensors,” Biomaterials 23(17), 3699–3710 (2002). [CrossRef]
47. X. Xu, H. Subbaraman, J. Covey, D. Kwong, A. Hosseini, and R. T. Chen, “Complementary metal–oxide–semiconductor compatible high efficiency subwavelength grating couplers for silicon integrated photonics,” Appl. Phys. Lett. 101(3), 031109 (2012). [CrossRef]
48. D. Benedikovic, C. Alonso-Ramos, P. Cheben, J. H. Schmid, S. Wang, D. X. Xu, J. Lapointe, S. Janz, R. Halir, A. Ortega-Monux, J. G. Wanguemert-Perez, I. Molina-Fernandez, J. M. Fedeli, L. Vivien, and M. Dado, “High-directionality fiber-chip grating coupler with interleaved trenches and subwavelength index-matching structure,” Opt. Lett. 40(18), 4190–4193 (2015). [CrossRef]
49. Y. Chen, T. D. Bucio, A. Z. Khokhar, M. Banakar, K. Grabska, F. Y. Gardes, R. Halir, I. Molina-Fernandez, P. Cheben, and J. J. He, “Experimental demonstration of an apodized-imaging chip-fiber grating coupler for Si3N4 waveguides,” Opt. Lett. 42(18), 3566–3569 (2017). [CrossRef]
50. R. Halir, P. J. Bock, P. Cheben, A. Ortega-Monux, C. Alonso-Ramos, J. H. Schmid, J. Lapointe, D. X. Xu, J. G. Wanguemert-Perez, I. Molina-Fernandez, and S. Janz, “Waveguide sub-wavelength structures: a review of principles and applications,” Laser Photonics Rev. 9(1), 25–49 (2015). [CrossRef]
51. D. Taillaert, P. Bienstman, and R. Baets, “Compact efficient broadband grating coupler for silicon-on-insulator waveguides,” Opt. Lett. 29(23), 2749–2751 (2004). [CrossRef]
52. Z. Zhao and S. Fan, “Design Principles of Apodized Grating Couplers,” J. Lightwave Technol. 38(16), 4435–4446 (2020). [CrossRef]
