Research on Bulk-Silicon-Based Photonic Waveguides in the Mid-Infrared

Introduction

Importance of the mid-infrared

Currently, visible light and near-infrared (near-IR) light, with wavelengths between 200 and 1600 nm, are mainly used for optical communications and optical sensing areas. However, the mid-IR has not been actively used compared to the visible or near-IR. Recently, mid-IR photonic integrated circuits (PICs) have attracted significant attention due to their potential for use in free-space communications and chemical sensing.

The current bandwidth of optical communication in the near-IR can be expanded by using free-space communication in the mid-IR, and this leads to a significant increase in the data transmission rate [1, 2]. In addition, the mid-IR is well suited for chemical or biochemical sensors, as many molecules have unique absorption spectra in the mid-IR due to the vibrational properties of molecular bonds such as C-H, N-H, O-H, C-C, N-O, C=O [3, 4] . Therefore, molecules can be detected by analyzing their absorption spectra, allowing label-free detection [5, 6].

Previous Si-based waveguides in the mid-infrared

However, it has disadvantages that the price of the substrate is expensive and the permeability of the sapphire decreases rapidly when λ>5 µm. Because the holes in the waveguide must be far enough away not to affect the optical mode, it needs a wide membrane width. Recently, the air-clad silicon waveguide using the bulk silicon substrate was proposed, which can be manufactured at a low cost because the substrate price is low.

This waveguide has CMOS compatibility and thus possibility of mass production, and it can be realized with the simple process than previous silicon-based waveguides. Second, the bulk silicon-based photonic waveguides, operating in the mid-IR, are theoretically investigated, designed and analyzed. Finally, the realized bulk silicon-based photonic waveguides, which can be used in the mid-infrared, are experimentally investigated.

The propagation loss of the manufactured waveguides is measured at a wavelength of 1550 nm and is 4 dB/cm. This waveguide would be useful as a chemical sensor that can be realized at low cost.

Figure 1-2. Example of silicon-on-sapphire waveguides for the mid-infrared [12]

Waveguide analysis

The silicon structure of the inverted triangle plays a role as the core of the photonic waveguide. The electric field profile for a fundamental mode of the photonic waveguide is shown in Figure 2.3(b). Figure 2.7 shows that the fundamental mode is well confined to the silicon structure of the inverted triangle.

We measured the fiber-to-fiber insertion loss ILw of the combination of the input waveguide, the photonic waveguide, and the output waveguide. The intensity profile of the fundamental mode of the input waveguide or output waveguide is shown in Figure 4.5(a). The profile profile of the tapered region where the fundamental mode of the input waveguide is coupled to the fundamental mode of the photonic waveguide, as shown in Figure 4.5(b), is shown in Figure 4.5(c).

And the large coupling loss in the tapered region may be caused by the wet etching process. The propagation loss of the fabricated photonic waveguide is 4 dB/cm at a wavelength of 1550 nm.

Bulk-Silicon-Based Photonic Waveguide in the Mid-Infrared

Fabrication process and waveguide structure

To fabricate the photonic waveguide, a multi-step process is used that includes oxide deposition, photolithography, dry etching and wet etching. In step (c), the waveguide mask patterns are formed by etching the 250 nm thick oxide layer using oxide-reactive ion etching (RIE). In step (d), the (100)-oriented bulk silicon substrate is etched to the appropriate depth using deep reactive ion etching (DRIE).

The structural parameters of the waveguide are defined: core width is ws; core height is hs; pillar width is wp; pillar height is pk. These four parameters determine the performance of the waveguide and they can be modified by changing associated pattern design or by controlling the etch depth of dry etch and the time of wet etch. In other words, if the etching rate of wet etching according to the orientation of silicon crystal and the etching angle is known and the three independent design parameters of core.

The expected etch depth hDRIE and the wet etch time tetch can be obtained via Eq. Therefore, if the independent design parameters of core width ws, pillar width wp and pillar height hp are determined according to the characteristics of the photonic waveguide, the dependent fabrication parameters of hDRIE and tetch.

Figure 2-1 shows the detailed fabrication scheme of the photonic waveguide. To fabricate the photonic waveguide, the multistep process composed of oxide deposition, photolithography, dry etching and wet etching is used

Design of the photonic waveguide

In this case, it can be seen that the mode is well guided without mode radiation into the bulk silicon substrate. From this result, a ratio can be expected where the column width wp should be as narrow as the correct width and the column height hp as tall as the correct height. a) Imaginary part of the effective TE fundamental mode index calculated with respect to the pillar height hp by fixing the pillar width wp to 1 µm. As shown in Figure 2-5(a), (b), if the height of the hp pillar is higher than 2 µm, the imaginary part of the fundamental mode converges to zero.

The results of calculating the relationship between the optimal column width and optimal column height for which the imaginary part of the fundamental mode has zero are shown in Figure 2-6. The ratio between the optimal column width and optimal column height, that the effective index of the fundamental mode of the imaginary part has zero. In this case, the fundamental states are well controlled with no emission of states into the bulk silicon substrate.

From this result, it can be seen that the optimal condition of the pillar is wp=0.5 µm and hp= 1.3 µm. Finally, the defined design parameters are as follows: core width ws is 6 µm; core height hs is 3.9 µm; the width of the wp pillar is 0.5 µm; the height of the hp pillar is 1.3 µm.

Figure 2-4. (a) Cross-sectional structure of photonic waveguide where core width w s is 6 µm, core height h s is 3.5 µm, pillar width w p is 1 µm and pillar height h p is 3 µm

Characteristics of the designed photonic waveguide

Therefore, there is a possibility that the sidewall roughness of the waveguide silicon core can be caused by the wet etching process. After the fabrication of the designed photonic waveguide, the sidewall roughness can be reduced by a thermal oxidation process. Since the width of the core is wide, unlike the photonic waveguide structure, we get the pillarless input waveguide or output waveguide structures, as shown in Figure 3-3.(b). a) Top view of the entire structure of the mid-infrared device.

Next, the measured propagation loss result of the photonic waveguide fabricated using the truncation method at 1550 nm wavelength is discussed. In addition, coupling losses, which are caused when light from the small-core fiber (SCF) is coupled to the input waveguide and the fundamental mode of the input waveguide is coupled to the fundamental mode of the photonic waveguide in the depletion region, can be extracted through loss fiber to fiber insertion. Light with a wavelength of 1550 nm from a tunable laser source (TLS) was emitted into the input waveguide using single-mode fiber (SMF) and SCF.

The propagation loss of the fabricated waveguide was measured using the cut-back method at λ=1550 nm. The intensity profiles of fundamental TE mode and TM mode of the fabricated photonic waveguide are shown in Figure 4-3. a) Intensity profile of fundamental TE mode of fabricated photonic waveguide.

Figure 2-8. Intensity profiles for higher-order mode of designed photonic waveguide. (a) Intensity profile of TE 10 mode

Realization of the Photonic Waveguides

Sidewall roughness induced by wet etching

• Improvement of the wet etching condition
• Reduction of the sidewall roughness by oxidation process …

The sidewall roughness caused by the wet etching process allows light to scatter when light is propagated along the photonic waveguide. Therefore, since it affects the waveguide performance, it is very important to reduce the sidewall roughness. Here is how to reduce the sidewall roughness caused by the wet etching process.

To reduce the sidewall roughness caused by the wet etching process, there is a way to mix isopropyl alcohol (IPA) with KOH solution [18]. According to the literature, there is a mixing ratio of KOH + IPA solution that ensures good surface quality. Figure 3-1 shows the concentration of IPA required relative to the concentration of KOH to ensure good surface quality.

As shown in Figure 3-2, it can be seen that the sidewall roughness is reduced.

Figure 3-1. IPA concentration required according to KOH concentration for ensuring good surface quality

Actual waveguide structure used to improve mechanical stability

Fabrication results

The input waveguide was coupled to the photonic waveguide through the tapered region with a length of 100 µm. It is clear that the fundamental mode is well confined to the inverted triangle silicon structure. a) Structure of fibers with a small core. The coupling loss Lc was calculated as 2.72 dB in the case of TE mode and 3 dB in the case of TM mode. a) Intensity profile of the fundamental mode of the input waveguide or output waveguide.

The calculated coupling loss of the tapered region Lt is 1.6 dB in case of TE mode and 0.96 dB in case of TM mode. The combination of the coupling loss Lc between the SCF and the input waveguide and the narrowed region coupling loss Lt equals the total coupling loss Ltot on one side of the waveguide. The photonic waveguide has low transmission loss even at wavelengths longer than 3.6 µm, as it does not have SiO2 as a cladding material, which causes strong absorption in the mid-IR.

The total measured coupling loss is 15.4 dB, which is greater than the calculated coupling loss. A possible reason for the difference between the measured and calculated junction losses is the sidewall roughness of the silicon core, which is attributed to the wet etching process.

Measurement of the Propagation Characteristics of the Photonic Waveguides

Measurement setup

Again, the light from the output waveguide was coupled to another SCF, which was connected to a photo detector (PD) allowing measurement of the insertion loss between the optical fibers.

Propagation loss

Calculation of the mode characteristics

Then, the coupling loss between the SCF with a mode field diameter of 3.3 µm and the input waveguide or output waveguide was calculated using FIMMPROP.

Figure 4-4. (a) Structure of small-core fiber. (b) Intensity profile for single mode of small-core fiber

Coupling loss

Therefore, we have a plan to cooperate with the research institute that owns the mid-IR source and detector. Finally, chip-scale mid-IR chemical sensing will be performed, which detects molecules using bulk silicon-based photonic waveguide [19]. In conclusion, silicon-based photonic waveguides operating in the broad mid-IR have been investigated.

To optimize the photonic waveguides are theoretically designed and analyzed using FIMMWAVE and FIMMPROP simulation software. Green, “Mid-infrared to telecom-band supercontinuum generation in silicon-on-insulator highly nonlinear wire waveguides,” Optics Express , vol. Green, “Mid-infrared gap bridging in silicon nanophotonic spectral translation,” Nature Photonics, vol.

Holzwarth, et al., "Mid-infrared optical frequency combs at 2.5 μm based on crystalline microresonators," Nature communications, vol. Richardson, et al., "Demonstration of high-Q mid-infrared chalcogenide glass-on-silicon resonators," Optics letters, vol.

Figure 4-6. SEM images of the fabricated functional devices. (a) S-bend waveguides. (b) Y-branch beam splitters

Future Works

Conclusion