1. Introduction

A photonic crystal structure with a photonic bandgap (PBG) can propagate photons along an arbitrary direction and strongly confine photons in a region of sub-wavelength scale [1]. Photonic crystal-based devices such as waveguides and nanocavities can be used for various applications including ultrasmall optical filters [2], ultralow-power and ultrafast switches [3], nonlinear optics [4], and quantum processing [5]. In particular, two-dimensional (2D) photonic crystal slab structures are not only relatively easy to fabricate, but also serve as platforms of in-plane integration. Such 2D photonic crystal structures feature in-plane optical confinement by the 2D PBG effect and vertical confinement by the high contrast between the refractive index of the dielectric slab (e.g., silicon) and that of its surrounding medium (e.g., air). These high contrasts can reduce the leaky region of light—the so-called light-line in the waveguide or cavities; indeed, high-performance photonic crystal devices have been realized in air-membrane structures [1–5]. However, the mechanical and thermal instabilities of air-membrane structures have been ones of the obstacles to their application and heterogeneous integration. Furthermore, when air-membrane structures are exposed to air, continuous oxidation and unavoidable contaminations can affect the characteristics of photonic devices [6]. Although there have been some photonic crystal structures fabricated on supporting substrates with low refractive indices [7–11], the structures are not immune to the abovementioned problems because they are still exposed to air. More seriously, the performance of the photonic devices can degrade because of a loss of coupling between the transverse electric (TE) and transverse magnetic (TM) modes in asymmetric structures [9,10]. In this work, we investigate a glass-embedded photonic crystal device (Fig. 1(a) ) to improve the mechanical stability of the 2D photonic crystal slab structure and avoid the coupling loss between the TE and TM modes. We theoretically and experimentally demonstrate a single-mode waveguide with a band width of 90 nm and a high quality (Q) nanocavity with a Q factor of up to 160,000. This Q factor is the highest recorded value for a photonic crystal nanocavity with cladding material [12].

 

Fig. 1 (a) A schematic of a glass-embedded two-dimensional photonic crystal device consisting of a waveguide and a nanocavity. (b) The calculated photonic band diagram of the photonic crystal structure (t = 0.6a, d = 0.58a)

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2. Design of glass-embedded photonic crystal devices

Before designing the 2D photonic crystal waveguides and cavities, we calculated the PBG of a glass-embedded 2D photonic crystal slab structure using 3D finite-difference time-domain methods. The structure was assumed to be completely embedded in glass, with all holes infiltrated by the glass. The geometric structure had a hole diameter (d) of 0.58a and a slab thickness (t) of 0.6a (here, a is the lattice constant of the photonic crystal structure). The refractive indices (n) of the silicon and glass for a light with a wavelength of 1.55 μm were set to be n_si = 3.48 and n_glass = 1.45, respectively. Figure 1(b) shows the calculated band diagram of the structure. As seen in the figure, the PBG exists between 0.246 and 0.284 [c/a], where c is the velocity of light in vacuum. The PBG corresponds to a wide range of 1410~1630 nm at a =400 nm. We then introduced a point-defect cavity with three missing holes (L3) in the structure, as shown in Fig. 2(a) . The calculated resonant frequency of the cavity is 0.256 [c/a], which is within the PBG range. As shown in Fig. 2(b), the electric field at the resonant frequency is mainly confined within the cavity. The quality (Q) factor of the L3-type cavity is 1,100 and the modal volume is as small as 0.6(λ/n_si)³. To further increase the Q factor of the cavity, we calculated the shifted (s) L3 cavity [13] for neighboring holes in the glass-embedded structure. The Q factor of the cavity with s = 0.15a is three times higher than that of the L3 cavity. This implies that shifting neighboring holes is a valid approach for realizing high Q cavity in a glass-embedded photonic crystal structure. Furthermore, we investigated the optical coupling of a line-defect waveguide to the cavities in the glass-embedded structure, as shown in Fig. 2(c). The calculated dispersion of the waveguide is shown in Fig. 2(d). As seen in the figure, the waveguide with one row missing, denoted as W1( = $\sqrt{3}a$), has a narrow bandwidth of 0.003 [c/a], which corresponds to a bandwidth of 20 nm at 1.55 μm. The narrow bandwidth is mainly due to the effect of the glass light-line shown by the solid line in Fig. 2(d). To obtain a broader bandwidth, we adjusted the width (W) of photonic crystal waveguide because it strongly affects the dispersion of the photonic crystal waveguide. When W = 0.62W1, a broad bandwidth of 0.247-0.262 [c/a] is obtained. This corresponds to a bandwidth of 90 nm, which is comparable to that of air-membrane photonic crystal waveguides [14].

 

Fig. 2 (a) A schematic of a nanocavity with three missing holes and (b) the electric field distribution upon resonance. (c) A schematic of a waveguide with width (W), and (d) the dispersions of waveguides with W = W1 and 0.62W.

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3. Fabrication of glass-embedded photonic crystal devices

Next, we fabricated a glass-embedded silicon photonic crystal structure. A 2D photonic crystal structure was fabricated on a SOI wafer. The wafer consisted of a top layer of silicon 220 nm thick and a bottom layer of thermal silicon dioxide (SiO₂) 3μm thick. The photonic crystal patterns were formed in the silicon layer using electron beam lithography and plasma etching. Scanning electron microscope (SEM) images of the top surface of the photonic crystal structure, from above and in a cross-section, are shown in Figs. 3(a) and (b) , respectively. Subsequently, a SOG of hydrogen silsesquioxane with tunable and low refractive indices [15,16] was coated on the fabricated structure and cured at a temperature of 400°C for 1 h under nitrogen atmosphere. As seen in Figs. 3(c) and (d), the SOG uniformly coats the surface, and infiltrates the holes up to a depth of 530 nm from the silicon surface. According to our calculations, the thickness of the SOG layer does not affect the characteristics of the cavities considered here.

 

Fig. 3 (a) Top view and (b) cross-sectional SEM images of the silicon photonic crystal devices on an insulator before coating spin-on glass (SOG). (c) Optical microscope image of the photonic crystal device after coating SOG, and (d) cross-sectional SEM image of the glass-embedded photonic crystal structure.

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4. Results and discussion

In order to investigate the optical properties of the fabricated devices, light from a continuous waveguide laser (λ = 1510~1620 nm) was coupled to the cleaved facet of the waveguide, as shown in Fig. 4(a) . The transmission of the waveguide and emission from the cavity were observed (or measured) using a near-infrared camera (or a photoreceiver). Figures 4(b) and (c) show the measured spectra of the waveguide and the L3 cavity (a = 400 nm). A high transmission range exists in a frequency region from 1530 to 1620 nm, which is wider than those of previous waveguides with cladding materials [16–18]. At a point where the transmission intensity dips near 1568 nm (the transmittance T is estimated as 15~25%), the emission spectrum of the cavity shows a resonant peak. This implies that the light propagating through the waveguide is resonantly coupled to the cavity. As seen in the near-field pattern of the resonance (inset of Fig. 4(b)), the light is confined to, and emitted from, the cavity. The Q factor of the cavity is estimated as 370 by a Lorentzian fit to the spectrum. The intrinsic Q(Q _v) of the L3 cavity is 740~980 using Q _v = Q/T [13], which is in good agreement with the value calculated above. In order to investigate the refractive index of the SOG (n_SOG) and its dependence on the curing temperature, we prepared samples as shown in the schematic of Fig. 5(a) and cured them at different temperatures of 400, 600, and 800°C for 1 h under nitrogen atmosphere. Figure 5(b) shows the resonance spectra of cavities with the same geometric structure (a = 400 nm and s = 0.15a) at different temperatures. As the curing temperature increases, the resonant wavelengths are shifted toward longer wavelengths while maintaining nearly the same Q factor (2,200~2,800). Similar results are obtained in samples with different lattice constants, as shown in Fig. 5(c). In order to estimate the n_SOG, we calculated and comparing it to the experimental results. Here, we assume that n_Si and n_SiO2 are unchanged during the curing process because the nitrogen atmosphere does not oxidize these layers. Figure 5(d) shows the estimated n_SOG. The n_SOG increases from 1.37 to 1.57 as the curing temperature increases. This is because of the disassociation of Si-H bonds and film densification at high curing temperatures. The estimates are in good agreement with previously reported values [15]. This implies that the n_SOG of the top layer can be matched to the n_SiO2 of bottom layer by adjusting the curing temperature of the SOG. Finally, we applied the above result to a glass-embedded heterostructured cavity with a theoretical Q factor of 200,000 (n_glass = 1.45) and a small modal volume of 1.5 (λ/n_si)³. As shown in Fig. 6(a) , the heterostructured cavity consists of lattice structures with a ₁ = 406 nm, a ₂ = 410 nm, and a₃ = 415 nm. The glass-embedded cavity was fabricated at a curing temperature of 600°C to reduce the difference between n_SOG and n_SiO2 as well as the loss of TE-TM coupling. The measured resonant spectrum of the fabricated cavity is shown in Fig. 6(b). The line width of the spectrum is as narrow as 0.01 nm, which corresponds to a Q factor of 160,000. The Q factor is the highest recorded value for a nanocavity with cladding material [12]. The slight difference between the experimental and theoretical Q values may be due to the mismatch in the refractive index between the SOG and the bottom layers. If cavities with ultrahigh Q values are designed [19,20] by paying more careful attention to the curing process, it should be possible to achieve glass-embedded Q nanocavities comparable to air-membrane nanocavities with ultrahigh Q factors [21]

 

Fig. 4 (a) A schematic of coupling photons to the waveguide with the emission of photons from the cavity. (b) and (c) Transmission spectra of the waveguide and the L3 cavity, respectively. The insert in (c) represents the near-field image of the cavity upon resonance.

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Fig. 5 (a) A cross-sectional schematic of the photonic crystal structure with a top layer of SOG and a bottom layer of SiO₂. (b) The resonance spectra of the shifted (s) cavities of neighboring holes (a = 400 nm, s = 0.15) as a function of the curing temperature of SOG. (c) The resonant wavelengths of structures with a = 385, 400, and 415 nm as a function of curing temperature. (d) The estimated refractive index of the SOG (n_SOG) as a function of curing temperature.

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Fig. 6 (a) A schematic of a glass-embedded heterostructured cavity. (b) The measured resonant spectrum of the fabricated cavity (Q factor~160,000).

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5. Conclusion

In summary, to enhance the mechanical stability of a photonic crystal device and maintain its excellent performance, we investigated SOG-embedded silicon photonic crystal devices possessing waveguides and nanocavities on a SOI substrate. We experimentally demonstrated a transmission bandwidth of ~90 nm in a waveguide and a Q factor of 1,000 in L3 cavities. Furthermore, we showed that the refractive index of the SOG could be tuned from 1.37 to 1.57 by varying its curing temperature. Finally, we demonstrated a glass-embedded heterostructured cavity with an ultra-high Q factor of 160,000—the highest recorded value for a photonic nanocavity with cladding material. These results should stimulate further application and integration of mechanically stable photonic crystal devices with excellent properties.