Si NW array as surface enhanced IR sensor

Heavily doped Si has plasmonic property [205,206] with plasma frequency around 0.14 eV (8.86 μm, for heavily doped p-typed Si with carrier concentration 3–6 1019 _cm−3_{). [205] Figure 90(a)}

shows the real part and imaginary part of the permittivity of the heavily doped Si by using Drude model. For Si NW, it naturally forms an effective anisotropic material, which is a potential in the

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regime where Si has plasmonic property (real part of permittivity is negative). When treated as an effective medium, the effective permittivity of the Si NW forests can be expressed as

𝜖𝑥𝑥 = 𝜖𝑦𝑦 = 𝜖ℎ

(1 + 𝑓)𝜖𝑖+ (1 − 𝑓)𝜖ℎ

(1 − 𝑓)𝜖ℎ+ (1 + 𝑓)𝜖ℎ

(82)

𝜖𝑧𝑧 = 𝑓𝜖𝑖 + (1 − 𝑓)𝜖ℎ (83)

By using the effective medium approach, the fill ratio of the Si can be obtained by Ellipsometry by comparing the phase spectra with fitting data. Figure 90(b) illustrates the effective parallel and perpendicular permittivity of Si NW hyperbolic metamaterial (HMM) in air with a Si NW filling ratio of 0.3. It’s an effective type I HMM (𝜖_𝑥𝑥 > 0 𝜖_𝑧𝑧< 0) at the wavelength larger than 10 μm, which provides a potential application in surface-enhanced infrared absorption spectroscopy (SEIRAS). [207,208] Infrared (IR) spectroscopy deals with the infrared light matter interactions, which is mostly based on absorption. SEIRAS [208–210] is a variation of conventional IR spectroscopy, which exploits the enhanced surface field usually caused by metal particles to detect monolayer chemicals.

Figure 90 Optical property of Si NW.

(a) The real and imaginary part of the permittivity of heavily doped Si by using Drude model. The metallic property appears beyond 10 μm. (b) The effective parallel and perpendicular permittivity of Si NW HMM in air with Si NW filling ratio 0.3.

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The absorptivity of molecules is proportional to the light intensity. When shined by mid-IR light, the strong enhanced light intensity around the Si NW can be employed to improve the absorption. Figure 91 shows the corresponding simulated results. The 10 μm thick Si NW (with diameters varying from 40 nm to 400 nm) forests on Si substrate is exposed in air, as shown in Figure 91(a). 15 μm wavelength light incidents normally onto the NW forest and couple in to it, forming a Fabry Perot (FP) strong resonance along the vertical direction around each NW. Figure 91(b) illustrates a standing wave along the 𝑧 direction, where the thickness dependence of the FP resonance can be clearly seen. Figure 91(c) shows the top view of the dipole-like field distribution around a single NW, and Figure 91(d) is the coupled field of multiple NWs on the substrate plane.

Figure 91 The simulation of Si NW with thickness 10 μm on Si substrate.

(a) Schematic os NW arrays in COMSOL 4.3. (b) The electric field distribution in 𝑥𝑧 plane, and the bottom of every NW is a node due to the strong reflection. (c) Dipole-like electric field distribution of a single nanowire in xy plane. (d) Coupled field of multiple nanowires in 𝑥𝑦 plane. HMM have high local density of states, they can be applied in light emission devices such as light emission diodes (LEDs). [37,211] However, the high local density of states inside the HMM is difficult to be extracted out. Therefore, some groups demonstrated the HMM structure with grating on top of it for the applications using high density of states. [37,211] In this case, since Si NW array is effectively a type I HMM, the light can be coupled in and out with no extra moment required. Compared with multilayer structure HMM, NWs act as small scatters and create strong

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coupled light interactions. Furthermore, it is a three-dimensional (3D) metamaterial, which means the effective working area is larger other than that of other nanoparticles including Si particles or metal particles. The detected objects are not limited to monolayer in a 3-D metamaterial.

Figure 92 SEM images of Si nanowires with different conditions.

(a) top view (b) cross section (c) tiltited view of the Si NW. (a) top view (b) cross section (c) tiltited view of the Au covered Si NW. The white bar at the bottom indicates the scales.

The SEM images of the fabricated Si NW are shown in Figure 92 (a)- (c). The Si NW is vertical and straight, and the surface of the Si NW forest is flat. Figure 92 (d)- (f) shows the gold (Au) covered Si NW (physically deposited), which shows metallic properties. The Au layer was deposited by sputtering, and the set thickness of Au film is around 50 nm. The roughness on top of the Si NW shown in Figure 92 (d)-(f) corresponds to the Au grains. The deposited Au stuck on the sidewall of NW is around 10~30 nm, about less than half of the set thickness. Figure 93 shows the simulation result of the Si NW with a thickness of 1.5 μm coated with Au films with a thickness of 50 nm. Incident light with a wavelength of 440 nm travels from 𝑧+ direction, hits the core shell wire and reaches Si substrate. A standing wave along the wire on the surface of Au is formed with the bottom of each wire as a node, and the near field around the wire is dramatically enhanced. The Si NW array coated with Au film is also able to couple the light out by itself. Therefore, we

(a)

(b)

(c)

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propose to use it as an easily-fabricated candidate to enhance the light emission and fluoresce light of molecules, for example, to get enhanced Raman signals [204,212] of toluene on top of the Au covered Si NW samples.

Figure 93 Simulation of the effective modes in Si NW covered by Au layer.

The NW has a diameter of 20 nm coated with 20 nm Au. Total length of the Si NW on Si substrate is 1.5 μm. Incident light has a wavelength of 440 nm. (a) The electric field distribution of the core shell in 𝑥𝑧 plane. (b) The electric field distribution of a single nanowire in 𝑥𝑦 plane.