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Abstract
Plasmonic demultiplexers hold promise for the realization of the subwavelength and high-splitting ratio dichroic splitter and have a wide range of applications from optical communication, and manipulation to ultrafast data treatment. However, this vision has not been realized for a long time due to lacking the suitable splitting structure design, which limits its further development of integrated photonic circuits. Here, we demonstrate a plasmonic demultiplexer with subwavelength feature size (0.54 µm) and broadband spectral (620-870 nm) range, and high-splitting ratio (17 dB in experiments and 20 dB in calculations). It consists of two adjacent Fabry–Perot cavities (covered by PMMA polymer) and coupling gratings, which are integrated with the Au waveguide. The relatively simple double cavities design of our device has a simple theoretical analysis and fabrication process. Our work has relevance for various optical applications, such as multiple wavelength photodetectors and optical multichannel interconnects.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Plasmonic demultiplexer (PD), devices for multichannel interconnection of optical signals, have been utilized in various optical techniques and devices, ranging from optical communication [1,2] and detection [3–5] to manipulation [6,7]. The development of functionality, miniaturization, and integration of PD has played a positive role in integrated photonic circuits. In response to the above requirements, a variety of schemes have been shown, including the use of nanocavities [8,9], optical antennas [10,11], or spin-orbit interaction effects [12–14], in which the modulated optical parameters involve the wavelength [15–17], polarization [18,2], and circularly polarized of the incident light [19–23]. Among all kinds of PDs, the wavelength PD in waveguides has attracted much attention because of its integration and flexibility since it was proposed in 2011 [24]. However, the feature size of other types of PD has reached the subwavelength, and the wavelength PD utilized in waveguide still has a large feature size which will hinder its application in the compact integrated photonic circuit. It is natural and necessary to break through the subwavelength scale through appropriate structural design.
Fabry–Perot (FP) cavities are classical optical elements that possess a simple structure, high Q-factor, and spectral tunability [25–27], where various fabrication techniques have been utilized. It consists of two reflecting mirrors and one spacing layer, in which the optical properties are mainly dependent on the reflection efficiency of the mirror and the feature size of the cavity [28,29]. Efforts to bring the dimensions of the FP cavity to the sub-wavelength range require effective light-matter interaction at the nanoscale, and surface plasmon polaritons (SPPs) show this ability [30–32]. Subwavelength and high Q-factor FP cavities [33] have been demonstrated in the plasmonic structures and SPPs laser based on FP resonators have also been proved [34]. Recently, plasmonic FP cavities have been extended to photodetectors [35], sensing [36], and nano-sources [37,38] by efficiently improving light absorption. This crucial progress of reducing the feature size of the wavelength PD in the waveguide based on cavities, that is, subwavelength and suitable Q-factor cavity can gain opportunities from the subwavelength plasmonic FP cavities.
In the present work, we experimentally demonstrate a subwavelength optical demultiplexer consisting of two adjacent FP cavities coupling to the Au waveguide that includes two out-coupling gratings. Poly (methyl methacrylate) (PMMA) polymers cover FP cavities to modulate the beam splitting characters of the device. The beam splitting mechanism is based on the spectral tunability of FP cavities which enables a clear relationship between the SPPs excitation efficiency and wavelength. Standard electron-beam lithography (EBL) patterning and induction coupling plasma (ICP) etching techniques are used to manufacture the demultiplexer, in which the splitter region is covered by a layer of PMMA polymer through an EBL alignment process. The demultiplexer has a 0.54 µm feature size, a broadband spectral (620-870 nm) range, and high splitting ratio (-8 dB at 640 nm and 17 dB at 840 nm in experiments). The PD has good technical compatibility, which helps to improve the integration and functionality of optoelectronic devices.
2. Theory and calculations
Figure 1(a) shows the schematic of the design of the wavelength demultiplexer, where there are three parallel slits in Au waveguide acting as ‘reflecting mirrors’ in FP cavities [39]. The adjacent slits form one FP cavity where the excited SPPs efficiency of different wavelengths is mainly dependent on the distance between two adjacent slits. We utilize two asymmetric cavities to build the demultiplexer, where the excited SPPs mainly propagates to the side of the large-size FP cavity at the short wavelength, and the propagation direction of the long wavelength SPPs is vice versa. There is a layer of PMMA polymer (nPMMA = 1.49) covering the demultiplexer region to modulate the resonant properties of the cavity as shown in Fig. 1(b). The feature size of both cavities are subwavelength in the demultiplexer that is benefit to reduce the size of the total device. The feature size of the plasmonic dichroic splitter is 0.54 µm that is small than the excited wavelength due to utilizing two subwavelength plasmonic FP cavities.
Fig. 1. Subwavelength dichroic demultiplexer. (a) Schematic of the plasmonic beam splitter consisting of double Fabry–Perot cavities. Different wavelength lasers were focused through a 100× objective onto the splitter, the propagation direction of the excited SPPs is dependent on the incident wavelength and the cavity size. (b) Cross-section view of the demultiplexer. The dichroic splitter is fabricated on the silicon substrate, and the PMMA polymer covers two cavities. The thickness of Au film (tau) and PMMA (tPMMA) both is 100 nm. The length of the two cavities are L1 and L2, and the width of the slit is 80 nm. The total length of two cavities is smaller than the incident wavelength. The short wavelength mainly propagates to the side with the large cavity and the long-wavelength vice versa.
The optical demultiplexer is fabricated on a 0.5 mm thick silicon wafer. A layer of gold film (2 nm Ti/100 nm Au) on the silicon wafer was evaporated by the e-beam evaporation. The fabrication process consists of two standard EBL steps and ICP treatment processes. First, we fabricated parallel slits and out-coupling grating on the Au film by consisting the EBL patterning and Ar plasma etching for Au film, the fabrication accuracy of the method can reach 20 nm [40]. The wide of the slit is 80 nm and the spacing between the center of two adjacent slits is 240 nm and 300 nm respectively. The period of the out-coupling grating is 600 nm and the duty cycle is 1. Second, pattern structures were covered by a layer of PMMA, which is reducing to the suitable thickness by the O2 plasma by ICP. The second EBL alignment pattern is utilized to cover the PMMA polymer on the splitter region, the alignment accuracy between splitter and pattern PMMA is better than 100 nm. Fabrication details are shown in the method section of Supplement 1.
We now discuss the design of the wavelength dependent demultiplexer, which relies on the design of the resonant characters for plasmonic FP cavities consisting of two slits covered by PMMA polymer. The relationship between excited SPPs efficiency and wavelength is dependent on the resonant property of the cavity which are influenced by both the width of the cavity and the covered PMMA layer. The excited SPPs characters of the single FP cavity is studied at different wavelength, which will help get the FP cavity with a suitable resonant peak, wavelength separation between adjacent peaks, and Q-factor. Figure 2(a) shows the relationship between the excitation efficiency of SPPs of the single FP cavity and the width of the cavity at different wavelength. There is a layer of PMMA polymer covering the cavity region and the thickness is 100 nm and the width is 2 µm. The resonant peak of excitation SPPs (same order) shift red with the increase of cavity size. We choose two cavities used in the demultiplexer that are 240 and 300 nm respectively. The peak spacing of the excited SPPs in 240 and 300 nm cavities is ∼260 nm (from 640 to 900 nm). The subwavelength size of the cavity will make the separation between adjacent peaks hundred nanometers in the spectrum according to Eq. (1), have shown in the previous works, [33] the value will decide the operation bandwidth of the final splitter.
where, the $\Delta \mathrm{\lambda }$ is the wavelength separation between the adjacent resonance peaks, ${\lambda _0}$ is the central wavelength of the nearest peak, ng is effective refractive index, l is the length of the cavity. Too high Q-factor of the FP cavity will also hinder the improvement of the bandwidth of the final device.Fig. 2. Simulated behavior of the demultiplexer. (a) Excitation efficiency of SPPs of the single FP cavity with the width of the cavity at different wavelength. It is calculated using the 2D FDTD model where there is a monitor on the waveguide to collect the excitation SPPs intensity. The grey semi-transparent bands indicate the spectrum of excitation SPPs at the cavity size (240 and 300 nm) used in the demultiplexer. (b) The intensity of SPPs reaches the left side and the right side for different incident wavelengths from 620 to 870 nm. Power flow distributions at the wavelength of 640 nm (c) and 840 nm (d).
To study the beam splitting characters with the wavelength in our device, we use the finite-difference time-domain (FDTD) method to model the optical splitting characters of the demultiplexer (details in Supplement 1). The polarization of the incident light is perpendicular to the demultiplexer. There are two monitors that are put in the left and right side of the demultiplexer, where the distance between monitor and the center of splitter is same. We obtain the splitting ratio by comparing the collection intensity of SPPs power flow in the different monitors. It is observed that the excited SPPs with short wavelength mainly propagate to the right side (large FP cavity) and the SPPs with long wavelength is opposite situation, which is shown in Fig. 2(b). We clear see the beam splitting phenomenon in Fig. 2(c)-(d) that shows the power flow distribution at a wavelength of 640 and 840 nm.
3. Experiment
Next, we experimentally demonstrated the presented concepts. Figure 3(a) shows the scanning electron microscope (SEM) images of the demultiplexer where the distance between the splitter and the out-coupling grating is 10 µm. The zoom SEM image of the splitter region is shown in the inset. The average width of the slit of the splitter is ∼80 nm and the depth is equal to the thickness of Au films (100 nm). The distance between the center of two end slits is 540 nm, the width of the PMMA covering the splitter is 2 µm and the thickness is 100 nm. The optical measurement is performed at the room temperature and ambient condition (details setup in Supplement 1), where the polarization of the incident light is perpendicular to the long axis direction of the splitter. The relationship of the splitting ratio and the incident wavelength is shown in the Fig. 3(b), where the experimental results agree with the calculation. We get the splitting ratio by integrating the photon intensity at the two emitting grating regions. The ratio (dB) is obtained from 10×log (ILeft/IRight) where the ILeft is intensity of the left grating and IRight is intensity of the right grating. The spectral range in our study is from 620 nm to 870 nm, the maximum ratio is –8 dB at the short wavelength (620-740 nm) and is 17 dB at the long wavelength (740-870 nm) in experiments. The inset of Fig. 3(b) shows the optical images of the demultiplexer under 640 and 840 nm incident lasers. The excited SPPs mainly propagates to the right side and obvious scattering light is observed in the right grating at the wavelength of 640 nm. The phenomenon is opposite at the wavelength of 840 nm. All optical images of the splitter under the different wavelengths are shown in Fig. S4. The unidirectional character of the demultiplexer without PMMA polymer is shown in Fig. S1 (in Supplement 1).
Fig. 3. Experimental characterization of the plasmonic wavelength splitter. (a) Scanning electron microscope (SEM) images of the demultiplexer, where the distance between the center of the splitter and out-coupling grating is 10 µm, the period of gratings is 600 nm. Inset: Magnified view of the dashed box region, the middle deep color area is the PMMA layer and the width is 2 µm. The distance between the center of two end slit the is ∼540 nm, the deep of all slits is 100 nm and the width is ∼80 nm. The structure was fabricated through EBL patterning and ICP etching. (b) Relationship of the splitting ratio and the incident wavelength. The ratio is the intensity ration of emitted photons by the left grating to the right grating. The splitting ratio is -8 dB (640 nm) and 17 dB (840 nm). Inset shows the optical images of the splitter under 640 and 840 nm. The red triangle is the experiment data and the black line is the calculation results.
The size and edge roughness of the final device is limited by the fabrication accuracy which will cause the difference between experiments and calculations. Further performance optimization can be achieved by using the single-crystal Au microplate and etched by a helium-focused ion beam (h-FIB), where the plasmonic loss will significantly be reduced through using the single-crystal material, and fabrication accuracy of h-FIB could reach several nanometers [41,42]. The beam splitting characters of the demultiplexer with all or part of covering PMMA polymer are similar, and the part covering will efficiently reduce the SPPs propagation loss (Fig. S2 in Supplement 1). The device is fabricated on the silicon substrate which could be combined with on-chip photodetectors and will bring some exciting combinations for optoelectronic devices. We expect the subwavelength, broad bandwidth, and high splitting ratio beam splitters to help promote the development of the integrated multichannel photonic circuit.
4. Conclusion
In summary, we propose and realize the subwavelength dichroic demultiplexer (0.54 µm feature size) which consists of two adjacent FP cavities on the Au waveguide. Utilizing the PMMA layer covering the splitter to modulate the optical characters of the devices. It supports a broad spectral bandwidth from 620 nm to 870 nm. The ratio of scattering intensities of gratings is up to -8 dB at the wavelength of 640 nm and 17 dB at the wavelength of 840 nm. Such a concept is based on the suitable combination between two FP cavities, which can provide a simple and predictable scheme for on-chip wavelength demultiplexer. Our approach opens a new perspective on ultracompact multichannel optical interconnection and processing.
Funding
China Postdoctoral Science Foundation (2020M682223, 2020M682224); National Natural Science Foundation of China (12004222, 12004223, 62004110).
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.
Supplemental document
See Supplement 1 for supporting content.
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