1. Introduction

Photoelectric conversion at Near-infrared II (NIR-II) windows (1000-1700nm) has attracted much attention, due to their tremendous potential in optical communication, bioimaging, and machine vision [1,2]. Wavelength and polarization are the critical degrees of freedom in optical information technology. These advanced applications put forward higher requirements for (NIR) photodetectors on spectral discrimination and polarization-sensitive characters [3,4]. However, the current narrowband photodetection is commonly realized by integrating a filter on bulk spectrometer. Besides, polarization detection mainly relies on rare anisotropic materials [5]. Challengingly, novel photodetectors that respond to specific narrowband NIR-II photons with polarization-sensitive ability are highly demandable. Graphene emerges as a promising candidate for on-chip optoelectronics due to the ultrahigh mobility of the carrier as well as tunable optical properties via electrostatic doping [6,7]. The surface without dangling bonds makes it easy to integrate graphene with photonic structures such as cavities, waveguides, and plasmonic structures [8–13]. This endows graphene great potential in constructing high-performance multifunctional photoelectric devices. However, it is a great challenge for graphene to obtain spectral discrimination and polarization-sensitive response due to its broadband but weak absorption and isotropy properties [14].

Optical Tamm state (OTS), an analog of Tamm state in an electronic system [15], is discovered in one-dimensional multi-layer photonic structure [16], which confines the electromagnetic wave at the interface [17–22]. The interface usually constructs between a dielectric distributed Bragg reflector (DBR) and a metallic mirror [23]. If the metallic mirror is replaced by a metasurface, the narrowband absorption caused by Tamm plasmon in a certain band gap of the DBR can be tuned not only in wavelength [24] but also in polarization [25]. This wavelength-and-polarization-sensitive Tamm state provides a localized electromagnetic field to enhance light-matter coupling, which prepares for NIR photodetection.

Here, we report the fabrication and test of hybrid graphene NIR photodetector based on the tunable OTS to detect the wavelength and polarization simultaneously. Tamm plasmon manipulated by the metallic structure and the spacer on the top of the device provides different electromagnetic field distributions in polarizations and wavelengths, which effectively controls the carrier generation. The device shows a tunable narrowband response at wavelengths from 1300 nm to 1600 nm. The full width at half maximum (FWHM) of the response peak reaches ∼100 nm and can be further improved to a 10-nm level by increasing the number of periods in the DBR. The responsivity reaches 187 mA/W at 1550 nm, nearly 100 times higher than that based on bare graphene. Furthermore, the device keeps a high-speed photoresponse (rise time ∼530 µs, decay time ∼290 µs), which is faster than most of the photoconductive devices (>1 ms) [14]. Additionally, a polarization-sensitive narrowband photodetection is realized by setting the metallic structure as a gold grating. The anisotropic features, high dichroic ratios of ∼4.6 at 1300 nm and ∼2.5 at 1500 nm, are achieved. These results provide a new approach for realizing high-performance multi-parameter detection in a single device.

2. Experimental design of NIR photodetector based on Tamm plasmonic structure

The designed Tamm plasmonic structure contains a DBR, a dielectric spacer, and a metallic film. The DBR, shown in Fig. 1(a), has six periods, and each period is composed of three layers: 79 nm HfO₂, 292 nm SiO₂, and 79 nm HfO₂. Through continuously depositing HfO₂/SiO₂ on a silicon wafer, the DBR has a band gap of about 0.35 µm around the wavelength of 1.5 µm. Next, deposit a dielectric spacer of SiO₂ on the DBR with tunable thickness for the compensation in the reflection phase [25], and cover a gold film of 18 nm on the top. The elemental distribution of the Tamm plasmonic structure was characterized by using energy-dispersive x-ray spectrometry (EDS).

 

Fig. 1. The Tamm state in plasmonic structure and the corresponding simulations. (a) The upper cross-section figure shows the plasmonic structure composed of a gold film, a SiO₂ dielectric spacer, and a DBR (alternately placing SiO₂ and HfO₂) on the Si substrate. The lower figure provides the electromagnetic energy distribution along z for normal incidence at 1.5 µm through simulation calculated by commercial software, Lumerical FDTD Solutions. Inset in the bottom panel is the two-dimensional cross-section density picture that provides electromagnetic energy distribution of the Tamm state in plasmonic structure for normal incidence at 1.5 µm calculated by FDTD Solutions [26]. (b) The absorption of the Tamm plasmonic structure shows the maximum appears at 1.5 µm for normal incidence calculated by FDTD Solutions [26]. Inset is the improved narrowband absorption of the Tamm plasmonic structure with the DBR of 30 periods. (c) The elemental distribution of bare Tamm plasmonic structure is characterized by using EDS.

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Then, we propose a polarization-sensitive narrowband NIR photodetector with OTS engineering. The graphene detector is inserted into the area where the electromagnetic field is localized, and the corresponding fabrication process is provided as follows. Bi-layer graphene was mechanically exfoliated on the prepared DBR through scotch tape owing to the weak van der Waals interlayer interactions. The Raman spectrum of bi-layer graphene is shown in Fig. S1 (Supplement 1). Electrodes (gold 50 nm/nickel 5 nm) were then fabricated through a standard photolithography process of using electron beam lithography (EBL) and thermal evaporation. Afterwards, PMMA was spin-coated then on the surface of device, and a window was patterned at the center of graphene channel. SiO₂ dielectric layer and gold film/grating were deposited. Through changing the thickness of SiO₂ dielectric spacer, the response wavelength can be flexibly tuned. It is note that the inserted graphene has little effect on the electromagnetic response of Tamm structure. The OTS-coupled graphene photodetector is based on a photoconductor mechanism. The equivalent circuit model of the hybrid device is shown in Fig. S2 (Supplement 1). The photoelectric measurements are conducted for evaluating the performance of polarization-sensitive narrowband infrared response. The NIR response was measured using Keithley 2612 and a light source of Chameleon with Compact OPO (Coherent Inc.).

3. Results and discussion

3.1 Narrowband NIR photodetection based on OTS-coupled graphene devices

An OTS appears at the wavelength of the absorption peak inside the band gap of the DBR when a plane wave is normally incident. For an OTS, the energy of electromagnetic field is localized to the area under the gold film, and its magnitude decays rapidly along z-direction, as shown at the bottom panel in Fig. 1(a). When the thickness of the SiO₂ dielectric spacer is 105 nm, the absorption peak of the Tamm plasmonic structure locates at 1.5 µm. Inset in Fig. 1(a) displays the electromagnetic energy distribution of the Tamm state in plasmonic structure for normal incidence at 1.5 µm calculated by commercial software, Lumerical FDTD Solutions [26], which further proved that the energy of the electromagnetic field is highly localized in the area close to the gold film. As shown in Fig. 1(b), the FWHM and the maximum amplitude of narrow absorption peak reach 100 nm and 35.3%, respectively. It is noted that the FWHM can be further pushed to 20 nm by integrating the DBR of 30 periods under the gold film, with a significant absorption approach to 100% shown in the Inset in Fig. 1(b). The elemental distribution of the Tamm plasmonic structure was characterized by using energy-dispersive x-ray spectrometry (EDS), as illustrated in Fig. 1(c). Absorptivity of the plasmonic structures at oblique incidence situation has also been studied, shown in Fig. S3 (Supplement 1).

Next, we measured the photoelectric performance of an OTS-enabled narrowband NIR photodetector by integrating a metasurface of a gold film as shown in Fig. 2(a). Bi-layer graphene is used as the conductive channel in the devices. By changing the thicknesses of the SiO₂ dielectric layer, a narrowband absorption can be realized in Tamm plasmonic structure shown in Fig. 2(b). The center wavelengths locate at 1.35 µm, 1.45 µm, and 1.53 µm, when the thicknesses of the SiO₂ dielectric spacer are 452 nm, 66 nm, and 124 nm, respectively. Consistently, the OTS-coupled device shows a narrowband response at these bands, as shown in Fig. 2(c). The FWHM of the response peak reaches ∼100 nm. It is noted that the photocurrent is mainly generated at the area where the metasurface exists (Fig. 2(d)), which further proves that the energy of the electromagnetic field for the OTS is localized under the gold film and enhances the light absorption of graphene. As shown in the Inset of Fig. 2(d), the output characteristic of the integrated graphene device is a straight line, which is a typical feature of ohmic contact. This ensures the effective extraction of photogenerated carriers when operating under the excitation of an NIR laser. Responsivity and response time are the two crucial parameters for photodetectors. The OTS-sensitized graphene hybrid device with a SiO₂ dielectric spacer of 124 nm was employed. As shown in Figs. 2(e) and 2(f), the hybrid device demonstrates high responsivity that reaches 187 mA/W at 1550 nm. The responsivity here is nearly 100 times higher than that based on bare graphene [27]. Meanwhile, the device demonstrates a fast response with a rise time of ∼530 µs and a decay time of ∼290 µs at 1550 nm, extracted by curve fitting according to exponential function [28]. The response speed of this device is faster than most photoconductive devices (>1 ms) [14].

 

Fig. 2. Photoelectric performance of the OTS-coupled graphene narrowband photodetector. (a) shows schematic of the OTS-enabled narrowband NIR photodetector with the integration of gold metasurface. (b) Narrowband absorption spectra of Tamm plasmonic structure. The center wavelengths locate at 1.35 µm, 1.45 µm, and 1.53 µm, when the thicknesses of the SiO₂ dielectric layer are 452 nm, 66 nm, and 124 nm, respectively. (c) Normalized photocurrent of the device with the narrowband responses at wavelengths of 1.35 µm, 1.45 µm, and 1.53 µm. V_ds= 0 V. The FWHM of the corresponding exciting laser is about 20 nm. (d) The photocurrent line scan of the device, V_ds= 0 V. Top left Inset: The output characteristic of the OTS coupled graphene device. Top right Inset: Optical image of the OTS coupled graphene device. (e) Responsivity as a function of laser power at the wavelength of 1550 nm, the SiO₂ dielectric layer is 124 nm. Inset: Photoresponse of the device at the wavelength of 1550 nm. (f) Transient response of the device at 1550 nm. The lines are fitting curves according to an exponential function. The rise and decay times are 530 and 290 µs, respectively.

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3.2 Polarization-sensitive narrowband NIR photodetection based on OTS-coupled graphene devices

When the gold grating replaces the top gold film (Fig. 3(a)), the reflection phase changes according to the anisotropic permittivity of the gold strips, which shifts the wavelength of the Tamm state in different directions for two orthogonal polarizations (x-direction and y-direction) [29]. Here the thickness of the SiO₂ spacer is changed to 50 nm, and the gold grating fabricated on the spacer maintains a thickness of 18 nm, where the width of the gold grating is 300 nm with a period of 500 nm. For the electric field component of normally incident light perpendicular to the metallic strips, considering the band gap of the DBR from 1.292 µm to 1.640 µm, the maximum absorption inside this area shows the Tamm state at 1.295 µm in Fig. 3(b), and the corresponding field pattern of energy (use the insert case in Fig. 3(b)) is shown in Fig. 3(e), where the field is localized under the gold grating. For y-polarized normal incidence, the maximum absorption inside this area shows at 1.482 µm in Fig. 3(c), and the field pattern of energy (use the insert case in Fig. 3(c)) is shown in Fig. 3(f), where the field is also localized under the gold grating with a little difference in attenuation distance in z-direction. These polarization-sensitive Tamm states localize the electromagnetic field for different polarizations at different wavelengths, and the two absorption peaks caused by two orthogonal incident polarizations appear in Fig. 3(d). If the structural parameters are properly designed, the absorptions peaks for different polarizations can be tuned inside the band gap, which can be utilized to detect polarization-loaded signals, especially in NIR.

 

Fig. 3. The design of the polarization-sensitive narrowband NIR photodetector. (a) shows the schematic picture of the photodetector, where the gold film in the plasmonic structure in the previous case is replaced by a metasurface of a gold grating. (b, c) The absorption of the polarization-sensitive Tamm plasmonic structure (six periods in DBR), calculated by FDTD Solutions [26], shows the maximum (inside the band gap of the DBR from 1.292 µm to 1.640 µm) appears at (b) 1.295 µm for x-polarized normal incidence, and (c) 1.482 µm for y-polarized normal incidence. Insets in (a, b) are the improved narrowband absorption of Tamm plasmonic structure with 30 periods in DBR. (d) The absorption is calculated by FDTD Solutions [26] for x-polarized and y-polarized normal incidence simultaneously (equivalence to circular polarization). (e, f) The two-dimensional density picture provides electromagnetic energy distribution of the Tamm state in periodic plasmonic structure for normal incidence of (e) x-polarization at the corresponding absorption peak inside the band gap and (f) y-polarization at the corresponding absorption peak inside the band gap calculated by FDTD Solutions [26].

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Experimentally, we further replace the top gold film with gold grating for construct anisotropic electric field and polarization-sensitive response in the OTS-coupled graphene device, see Figs. 4(a) and 4(b). In polar coordinates, the photocurrent in grating-based devices exhibits prominent anisotropic features. The dichroic ratio (the ratio of maximum and minimum photocurrent, I_pmax/I_pmin) is utilized here to characterize the polarization performance of the device [30]. As illustrated in Figs. 4(c) and 4(d), the grating-based devices show high dichroic ratios of ∼4.6 at the wavelength of 1300 nm and ∼2.5 at the wavelength of 1500 nm, for two orthogonal polarizations. The maximum photocurrent arises at 1300 nm for x-polarization and 1500 nm for y-polarization. Polarization-sensitive narrowband infrared photodetection can thus be realized with flexible, adjustable OTS effects. The wavelength of narrowband response can be tuned by changing the thickness of SiO₂ spacer and structural parameters of the metasurface, and the polarization-sensitive feature can be flexibly tuned by designing the orientation of grating.

 

Fig. 4. Photoelectric performance of the OTS-coupled graphene polarization-sensitive narrowband photodetector. (a) Scanning electron microscope (SEM) image of the hybrid device. The dark-colored area under the gold grating indicates the graphene channel. The scale bar is 4 µm. (b) The SEM image of gold grating, where the scale bar is 1 µm. Experimental polarization-sensitive photocurrents are plotted with the linear-polarization laser of (c) 1300 nm and (d) 1500 nm in polar coordinates.

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It is noted that the FWHM of absorption spectra for Tamm plasmonic structure can be dramatically optimized by altering the number of periods in the DBR, see the Insets in Figs. 3(b) and 3(c). The FWHM of absorption spectra with gold grating reaches 10 nm at the wavelength of 1.333 µm for x-polarized normal incidence and 60 nm at the wavelength of 1.469 µm for y-polarized normal incidence. Significantly, the maximum absorptions for two polarizations have been promoted simultaneously, which reaches 80% at NIR wavelengths. The performance of such a narrowband absorption architecture is superior to most structures in literature [31–34], which exhibits great potential for application in on-chip adjustable polarization-sensitive narrowband devices.

4. Conclusion

In summary, we report a novel polarization-sensitive narrowband NIR photodetector based on OTS-coupled graphene hybrid structure. The devices show narrowband response at NIR wavelengths empowered by the tunable Tamm state, with an FWHM of ∼100 nm. Moreover, the FWHM of the response peak can be further improved to 10 nm by optimizing the periods of the DBR. Meanwhile, the device shows high sensitivity and fast response at a communication band of 1550 nm. After integrating the metasurface, the device delivers a prominent anisotropic feature with a high dichroic ratio of ∼4.6 at 1300 nm. It is noted that the FWHM of narrowband response and the polarization-sensitive character are highly-tunable, and can be flexibly controlled by changing the architecture of the SiO₂ dielectric layer, Bragg reflector, and the top grating. Our results pave a new way for tunable multi-parameter optical signal detection. The proposed architecture can also be coupled with current plasmonic-based nanodevices [35–37], and has great potential in on-chip integrated applications in optical communication, environmental monitoring, and intelligent recognition system.