Subwavelength perfect optical absorption structures based on monolayer-graphene are analyzed and demonstrated experimentally. The perfect absorption mechanism is a result of critical coupling relating to a guided mode resonance of a low index two-dimensional periodic structure. Peak absorption over 99% at wavelength of 1526.5 nm with full-width at half maximum (FWHM) about 18 nm is demonstrated from a fabricated structure with period of 1230 nm, and the measured results agree well with the simulation results. In addition, the influence of geometrical parameters of the structure and the angular response for oblique incidence are analyzed in detail in the simulation. The demonstrated absorption structure in the presented work has great potential in the design of advanced photo-detectors and modulators.
© 2017 Optical Society of America
The electrical and optical properties of graphene have been intensively studied since it was separated from graphite experimentally in 2004 [1,2]. The ultra-broad spectral response and the ultrahigh carrier mobility of graphene make it an ideal photonic and optoelectronic material. However, the absorption of suspended graphene toward normal incident light is weak, which seriously affects the applications of graphene in the optoelectronics area. So the absorption enhancement of graphene is critical, and the perfect absorption of graphene is highly desirable for graphene-based high-performance optoelectronic devices [3–6].
Up to now, many approaches have been investigated to enhance the optical absorption of graphene. In the mid- to far-infrared, the strong plasmonic resonances of graphene have been wildly utilized to enhance absorption [7–11]. In the visible and near-infrared regime, the graphene can hardly support plasmonic resonance since the doping level of graphene is limited, so the absorption enhancement of graphene was normally realized by coupling the graphene with some resonant structures [12–20]. Polarization-dependent graphene-based perfect absorption structures were theoretically demonstrated by coupling graphene with one-dimensional (1D) subwavelength dielectric gratings , and peak absorption over 99% at wavelength around 1500 nm for TE polarization was measured from graphene-based subwavelength structures . For some applications, polarization-independent absorption structures at normal incidence are more desirable. Piper and Fan numerically demonstrated that the polarization-independent perfect absorption of graphene can be achieved in the near-infrared by critical coupling the monolayer graphene with the guided resonance of a two-dimensional (2D) photonic crystal slab with high refractive index , and in the experiment, total absorption of 85% and graphene absorption of 77% were measured from monolayer graphene coupled with 2D photonic crystal slabs on top of back metal mirrors [19,20]. When a photonic crystal slab with high refractive index contrast is used to form an optical resonator in the absorption structure, the resonant mode will be well confined in the slab and sensitive to the slab parameters. In this case, it is difficult to realize perfect absorption in the experiment. In addition, the fabrication processes of the structure with high refractive index contrast are more complicated.
In this paper, we propose and experimentally demonstrate a polarization-independent graphene-based perfect absorption structure at normal incidence by coupling monolayer graphene with a 2D periodic dielectric structure with low refractive index contrast. Total absorption over 99% with FWHM about 18 nm is measured from a fabricated structure, and the peak absorption of the monolayer graphene in the demonstrated structure is close to 92%. Meanwhile, the demonstrated structure has relative large fabrication tolerances and simple fabrication processes, which are very important to realize perfect absorption in the experiment.
2. Structure and simulation
The schematic of the proposed graphene-based perfect absorption structure is shown in Fig. 1(a). A monolayer graphene is placed between a silica layer and a 2D polynethy1-methacrylate (PMMA) periodic structure with square lattice, and a gold layer is deposited at the back side of the silica layer to block the transmission of the incidence light.
The absorption structure shown in Fig. 1(a) supports several guided mode resonances which can couple to external fields [21–23]. A guided mode resonance has its electromagnetic field strongly confined within the structure, and in the vicinity of the frequency of a guided mode resonance, the field inside the structure can be significantly enhanced. So when the incident wave is coupled with a guided mode resonance, the absorption of the structure could be greatly enhanced due to the field enhancement. Furthermore, under the critical coupling condition, which means the leakage rate of a guided mode resonance out of the structure is equal to the absorption rate of that resonance in the absorptive materials of the structure, all the incident wave will be absorbed and perfect absorption will be achieved .
The proposed graphene-based perfect absorption structure is analyzed by using finite-element method-based software (Comsol Multiphysic). In the simulation, the monolayer graphene was regarded as a conductive surface with optical conductance of G0 ≈6.08 × 10−5Ω−1, which corresponds to the absorption about 2.3% for a free standing graphene. The refractive indices of PMMA and silica were taken to be 1.48 and 1.45, and the dielectric constant of gold was given by the Drude model as ε(ω) = ε∞ - ωp2/(ω2 + iγω) with ε∞ = 1.0, ωp = 1.37 × 1016 s−1 and γ = 8.17 × 1013 s−1 . Figure 1(b) shows the calculated absorption spectra of a designed graphene-based perfect absorption structure under normal incidence. The black line denotes the total absorption spectrum of the structure, and the red line and the blue line represent the absorption spectra of the graphene and the gold layer, respectively. Simulation results show that the peak absorptions of the graphene and the gold layer in the structure are 91.6% and 8.4%, respectively. In the simulation, the thicknesses of the PMMA layer, the silica layer and the gold layer were taken to be 280nm, 520nm, and 200nm, and the lattice period and the air hole radius of the PMMA pattern were chosen to be 1250nm and 300nm.
As mentioned above, the perfect absorption of the structure shown in Fig. 1(b) is realized by critical coupling to a guided mode resonance. The Q factor and the center wavelength of the resonance are 86 and 1541 nm, respectively. The normalized electric field amplitude distributions of the structure under normal incidence of a plane wave (Ey) at the horizontal plane (x-y plane, at the graphene layer) and the vertical planes (x-z and y-z planes) are plotted in Fig. 2, for on-resonant (1541 nm) and off-resonant (1600 nm) conditions. As shown in Fig. 2(a), a well confined resonant mode is excited by the outside incident wave, and there is no interference between the incident and the reflected wave since the incident wave is totally absorbed by the structure. Meanwhile, the field in the graphene is much higher than the field in the surface of the gold layer, so most of the incident wave is absorbed by the graphene at the on-resonant condition.
Next, we will discuss the dependence of the structure absorption on the geometric parameters of the structure. With a fixed lattice period, the air hole radius and the thicknesses of the PMMA layer and the silica layer will affect the Q factor of the guided mode resonance and the field intensity in the graphene. So the geometric parameters will affect the critical coupling condition and the peak absorption of the structure. Meanwhile, the wavelength of the guided mode resonance is related to the geometric parameters. If we increase the PMMA layer thickness or the silica layer thickness, and if we reduce the air hole radius, the effective index of the structure will increase, then the wavelength of the guided mode resonance and the absorption peak of the structure will exhibit a redshift. Figures 3(a)-3(c) show the calculated peak absorption and peak wavelength as functions of the air hole radius, the PMMA slab thickness, and the silica layer thickness, respectively. Simulation results show that over 99% peak absorption can be maintained when the air hole radius ranges from 290nm to 320nm, or the PMMA layer thickness ranges from 270nm to 300nm, or silica layer thickness ranges between 500nm to 560nm while other parameters are fixed. From Fig. 3 we can see that the designed structure has relative big fabrication tolerances.
The characteristics of the presented structure have been analyzed at normal incidence, and the absorption of the structure is polarization independent at normal incidence. However, since the TE and TM polarizations possess different symmetries with respect to the yz-mirror plane, different guided mode resonances will be excited by incident waves with different polarizations at tilted incidence . As a result, the absorptions of the structure for TE and TM polarizations are different at tilted incidence. The wavelengths of the guided mode resonances as a function of kx for the structure shown in Fig. 1 are plotted in Fig. 4. As shown in Fig. 4, there are two guided mode resonances which can be excited by external wave with TM polarization and there are three guided mode resonances which can be excited by external wave with TE polarization in the simulated wavelength range. And when kx = 0, only the double-degenerate resonances TE1 and TM1 can couple to the external wave, so we can see only one absorption peak at normal incidence as shown in Fig. 1(b).
Figures 5(a) and 5(b) show the absorption of the structure as functions of wavelength and incident angle for TM polarization and TE polarization, respectively. As shown in Fig. 5(a), the wavelength of the major absorption peak decreases very slowly along with the increase of incident angle for TM polarization. Since the major absorption peak for TM polarization caused by the guided mode resonance TM1 is not sensitive to the incident angle, it could be of valuable applications for integrated optoelectronic devices. As for the TE polarization, two major absorption peaks and a minor absorption peak appear in the absorption spectrum when the incident wave is tilted, and those absorption peaks originate from the three guided mode resonances excited by TE polarization in Fig. 4. As shown in Fig. 5(b), the two major absorption peaks are sensitive to the incident angle, so the absorption of the structure for TE polarization is not suitable for the applications which need large incident angular tolerance, but it could be of potential applications for spatial optical measurement .
3. Fabrication and measurement results
The proposed graphene-based absorption structures with different lattice periods were fabricated on a 500 μm silicon substrate. The fabrication processes are as follows. A 4 nm chromium (Cr) layer was deposited on the silicon substrate by electron-beam evaporation, and a 200 nm gold layer was deposited on the Cr layer by magnetron sputtering. Next, a 520 nm silica layer was deposited on the gold layer by plasma-enhanced chemical vapor deposition, and then a monolayer graphene grown by chemical vapor deposition was transferred on the silica layer by using wet transfer technique . Finally, a PMMA layer with thickness about 300 nm was spin-coated on the substrate and square lattice patterns with different periods were formed in the PMMA layer by standard e-beam lithography, and the lateral size of the square lattice patterns is 450 μm. The top view scanning electron microscope (SEM) image of a fabricated pattern is shown in Fig. 6(a), and the Raman spectrum of the transferred monolayer graphene after device fabrication is shown in Fig. 6(b). As shown in Fig. 6(b), the intensity ratio of 2D peak to G peak is over 2.0, which identifies the single layer of graphene.
The fabricated structures were measured by our homebuilt microscope setup , and a tunable laser (81600B, Agilent) with divergence angle less than 0.2 degree was used as the light source in the setup. An optical iris was used to confine the incident light beam inside the pattern area, resulting in about 300 μm incident beam spot in diameter. Figure 7(a) shows the reflection (R) and absorption (A) spectra of a fabricated structure with period of 1230 nm at normal incidence, where the reflection spectrum was normalized by the reflected light from the gold layer on the sample, and the absorption spectrum was derived based on the equation A = 1-R since the transmission was blocked by the gold layer. Measurement results (dotted line) show that the peak absorption of the fabricated structure is 99.4% and the absorption FWHM is 18 nm, which agrees well with the simulation results (dashed line), as shown in Fig. 7(a). The total absorption of the fabricated structure is the sum of the graphene absorption and the gold layer absorption, and the peak absorption of the monolayer graphene is 91.9% according to the simulation result. However, it is hard to distinguish the graphene absorption and the gold layer absorption directly in the experiment. So we fabricated a pattern with the same period in the sample outside the graphene area to check the gold layer absorption. Figure 7(b) shows the reflection and absorption spectra of the fabricated structure without graphene. Simulation results show that, without the graphene absorption, the peak absorption of the gold layer is increased from 8.1% to 27.1% due to the Q factor enhancement of the resonant mode. And the measured peak absorption of the structure without graphene is 32.2%, which is in agreement with the simulated value.
Finally, an IR camera (7292M, Electrophysics) was used to capture the reflection images of the fabricated graphene-based absorption structure under the irradiations of a tunable laser with different wavelengths. As shown in Figs. 8(a)-8(d), we can barely see the structure pattern when the incident wavelength is far from the on-resonant wavelength, and the color of the pattern becomes darker as the incident wavelength moves closer to the resonance, and there is nearly no light reflected from the pattern at the on-resonant wavelength.
In summary, perfect absorption structures based on monolayer-graphene coupled with low index 2D periodic structures on top of a metal mirror are proposed and demonstrated. Polarization-independent perfect absorption is realized for the proposed structure at normal incidence, and the fabrication tolerance and the angular characteristics of the designed structure are discussed by simulation. Peak absorption of 99.4% at wavelength of 1526.5 nm with absorption FWHM of 18 nm are measured from a fabricated structure and the measured data agree well with the simulation results. The demonstrated absorption structure with subwavelength thickness and ultra-high graphene absorption would be of valuable applications for the graphene-based optoelectronic devices.
National Natural Science Foundation of China (NSFC) (61404174 and 11674396).
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