Fabrication and characterization of chalcogenide glass photonic crystal waveguides.

We report on the fabrication of chalcogenide glass (Ag-As(2)Se(3)) photonic crystal waveguides and the first detailed characterization of the linear and nonlinear optical properties. The waveguides, fabricated by e-beam lithography and ICP etching exhibit typical transmission spectra of photonic crystal waveguides, and exhibit high optical nonlinearity. Nonlinear phase shift of 1.5pi through self-phase modulation is observed at 0.78 W input peak power in a 400 microm long device. The effective nonlinear parameter gamma(eff) estimated from this result reaches 2.6 x 10(4) W(-1)m(-1). Four-wave mixing is also observed in the waveguide, while two-photon absorption at optical communication wavelengths is sufficiently small and the corresponding figure of merit is larger than 11.


Introduction
Chalcogenide glasses (ChGs) are based on chalcogen elements (S, Se and Te), mixed with other elements such as As, Ge, P and Sb. They are transparent at optical communication wavelengths, and exhibit high optical Kerr nonlinearity as well as low two-photon absorption (TPA) [1,2]. The nonlinearity is non-resonant, and has a subpicosecond response time. ChGs also exhibit several photoinduced phenomena when illuminated with light near the band-gap. By the use of these phenomena, the formation of waveguides and gratings [3,4] and posttuning of photonic crystal waveguides [5] have been reported. In addition, integration with Siphotonics is also possible because ChGs can be formed and processed at low temperature (<400 °C). These characteristics make ChGs a promising platform for integrated all-optical devices [6]. Recently, it is found that Ag-doped As 2 Se 3 glass (i.e., Ag x (As 0.4 Se 0.6 ) 100-x ) has particularly high Kerr nonlinearity higher than As 2 Se 3 by around 2−4 times at wavelength λ = 1053 nm without increasing TPA coefficient [7], and low material absorption loss at infrared region [8]. We apply this glass in a guiding layer of our devices described below.
Photonic crystal waveguides (PCWs) consisting of photonic crystal slab and a line defect, are a useful platform for on-chip photonic devices and circuits. PCWs can enhance nonlinear effects by exploiting its small modal cross-section and slow light effect [9]. In SOI-based PCWs, self-phase modulation (SPM), two-photon absorption (TPA) and third harmonic generation have been observed [10][11][12]. In GaAs and AlGaAs-based PCWs, the enhancement of SPM and TPA is demonstrated [13,14]. PCWs have been finding applications for integrated nonlinear devices [15].
Further nonlinear enhancement is expected for the combination of the ChGs and PCWs. However, the difficulty in the fabrication has limited its experimental demonstration [5]. ChGs are much softer than Si, which makes difficult to obtain vertical sidewalls of air-holes and clean facets for efficient optical coupling to the waveguides. Hence, only their fundamental characteristics have been investigated under the limited condition [16], and their nonlinear characteristics have not been investigated yet.
In this study, we develop the fabrication technique of fine Ag-As 2 Se 3 ChG PCW using ebeam lithography and ICP etching, and characterize their detailed linear and nonlinear optical properties, for the first time. We successfully observed the light propagation characteristics of the waveguides. In addition, we confirmed the generation of SPM and the four-wave mixing (FWM) in the ChG PCW. Negligible TPA is also observed at optical communication wavelengths.

Design and fabrication
Figure 1(a) shows the schematic of ChG PCW, which consists of air-holes embedded in a Ag-As 2 Se 3 layer. An InP substrate is used for easy cleavage. A Ti layer is used for better adhesion between As 2 Se 3 and InP. Typical device length is 400 µm. To analyze waveguide modes, we used 3D-FDTD method. In this analysis, we assumed that refractive index of the slab is 2.85, the normalized air-hole diameter 2r/a is 0.52 and normalized slab thickness d/a is 0.63, where a is the lattice constant. Figures 1(b) and 1(c) show the three-dimensional photonic band diagram, which show the presence of guiding modes within the bandgap. Provided the light propagation at λ = 1550 nm, we obtain a = 480 nm, 2r = 250 nm and d = 300 nm. These structural parameters indicate that the fabrication of ChG PCW is practical. The transmission bandwidth of even guided mode sandwiched by the air light cone and the band-edge completely covers the C band of silica fiber communications.  Figure 2 shows a fabrication process of ChG PCW. First, Ti layer is thermally evaporated onto an InP substrate by 10 nm at 6.0 nm/min. Then, commercially available As 2 Se 3 , Furuuchi Chemical is thermally evaporated onto the Ti layer by 300 nm at a rate of 10 nm/min. The Ag-As 2 Se 3 layer cannot be obtained by direct evaporation of the ternary Ag-As-Se glass because of their phase difference. Therefore, Ag layer is evaporated on the As 2 Se 3 layer by 10 nm, and doped into As 2 Se 3 film using photodoping technique [17]. 10 nm thickness of Ag corresponds to an Ag content of 5 at.%. Next, standard e-beam lithography is used to pattern PCWs. An EB resist ZEP-520A, ZEON is spincasted onto the surface, and prebaked at 180 °C for 2 min. PCWs are patterned by e-beam accelerated by 50 kV. After development, the substrate is post-baked at 130 °C for 30 min. The pattern of PCW is transferred to Ag-As 2 Se 3 layer by ICP etching (RIE-10iP, Samco). The etching gas is a CHF 3 and CF 4 mixture. After the etching, a residual EB resist on the surface of the guiding layer is removed by a cleaner solution. Finally, Ti and InP underneath the PCWs are removed by HF and successive HCl solution to form an air-bridge structure. The cross-sectional image shows that the air-holes have sufficiently smooth and vertical sidewalls, and the cleaved facet looks clean enough to couple external light to the waveguide efficiently. From the top view, it is evaluated that the fluctuation of air-hole diameter is ± 10 nm.   Figure 4 shows the transmission spectrum of a fabricated ChG PCW. Here, tunable laser light was coupled to the waveguide from the cleaved facet through microscope objectives. Transmission drops caused by the light line and band edge are confirmed, indicating that the obtained spectrum is a typical one of single line defect PCW. The transmission bandwidth defined by the light line and band edge is in good agreement with photonic band analysis shown in Fig. 1(c). The transmitted power is ~5 dB lower than the typical one of our 400-µmlong Si-PCW at λ = 1550 nm (corresponding to group index n g ~5). In our Si-PCW, the propagation loss is typical 1 dB/mm, corresponding to 0.4 dB loss in a 400-µm-length

Self-phase modulation
We observed SPM by launching optical pulses in the transmission band near the band-edge. In the SPM and TPA measurements, we used another sample which has a band-edge at λ ~1550 nm. Figure 5 shows experimental setup. The optical pulse from passive mode-locked laser was expanded spectrally by SPM at the first erbium-doped fiber amplifier (EDFA). Then, the pulse was filtered at desired center wavelength λ p and pulse width τ p . We chose λ p = 1547 nm corresponding to the band edge of the sample (n g = 10), and τ p = 2.0 ps to suppress SPM in silica fibers of the setup. The filtered pulse was amplified by the second EDFA, and coupled to the PCW through the microscope objectives. The output pulse from the PCW was detected with a lensed fiber. The pulse spectrum was observed by an optical spectrum analyzer (OSA). Figure 6 shows spectra of output pulse at different launched input peak powers P in for 400µm-long device. The SPM-induced spread spectrum is clearly observed when the coupled pulse peak power is higher than 0.63 W. The asymmetrical spreading of the spectra is caused by the cut-off at the band-edge. When we coupled pulses of P in = 0.78 W to the PCW, the output pulse spectrum exhibited two peaks and center dip, indicating that the nonlinear phase change ∆φ NL = 1.5π [18]. This is the evidence of the large nonlinear enhancement of the ChG.
Let us compare the efficiency of the nonlinear phase change with Si-wire [19]. The Ag-As 2 Se 3 PCW requires 0.78 W input peak power, 230 µm effective length L eff (1/e power decay length for a propagation loss of 13.5 dB/mm, neglecting TPA) and 0.18 µm 2 cross-sectional area for 1.5π nonlinear phase shift. On the other hand, in Si-wire, the same phase shift is observed with 6.85 W peak power, 3400 µm effective length (propagation loss: 0.36 dB/mm) and 0.106 µm 2 cross-sectional area. That is, the Ag-As 2 Se 3 PCW only needs nine times lower peak power, and 15 times shorter effective length and 1.7 times broader cross-sectional area. In total, it is 79 times more efficient than Si-wire.
To confirm the result, we also estimate the efficiency from material and structural parameters. Although the nonlinear refractive index n 2 of Ag-As 2 Se 3 has not been measured at λ ~1550 nm, it has been reported in [7] that the Ag doping enhances n 2 to 2 − 4 times larger at λ = 1053 nm. Let us assume here that n 2 of Ag-As 2 Se 3 is three times larger than 2300 × 10 −20 m 2 /W for As 2 Se 3 [20], i.e. 6900 × 10 −20 m 2 /W, which is 26 times Si's (270 × 10 −20 m 2 /W) [21]. The nonlinear enhancement is proportional to n g 2 [10,14]. Since n g of the fabricated PCW and Si-wire are estimated to be 10 and 4.5, respectively, the enhancement is (10/4.5) 2 = 5 times. Regarding the cross-sectional area, it is the same as mentioned above. In total, the fabricated PCW should be more efficient than Si-wire by 26 × 5/1.7 = 76 times. This value is in good agreement with the experimental value.  In the same manner, we estimated the effective nonlinear parameter γ eff = ∆φ NL /P in L eff to be 2.6 × 10 4 W −1 m −1 . This value is much larger than a reported value for As 2 S 3 ChG ribwaveguide of 10 W −1 m −1 [22]. The above n 2 for Ag-As 2 Se 3 is already 23 times larger than 300 × 10 −20 W −1 m −1 for As 2 S 3 . Since n g of the Ag-As 2 Se 3 PCW and As 2 S 3 rib-waveguide are estimated to 10 and 2.4, respectively, the enhancement is (10/2.4) 2 = 17 times. As for A eff , we assumed that it is proportional to the cross-sectional area of the waveguide, i.e., 0.18 and 1.2 µm 2 for the PCW and rib-waveguide, respectively, corresponding to 7 times enhancement. In total, deduced γ eff of the ChG PCW is 10 × 23 × 17 × 7 = 2.7 × 10 4 W −1 m −1 . This also agrees well with the above value. Thus, all the comparisons are consistent with each other.

Two-photon absorption
We measured input/output characteristics of a ChG PCW to evaluate the TPA. The experiment setup was the same as for the SPM. Figure 7 shows the result. Note that the output power keeps linear when the input power is sufficiently high to generate the SPM, indicating that the TPA of Ag-As 2 Se 3 is negligible in the optical communication wavelengths.
Nonlinear switching devices are designed to have a large figure of merit (FOM), which is defined as the nonlinearity per unit nonlinear absorption (i.e. FOM = n 2 /(βλ), where β is the TPA coefficient). The requirement for FOM depends on switching mechanisms, e.g. ~5 for Mach-Zehnder switches [23]. In bulk As 2 Se 3, FOM is as high as ~11 [1]. Ag-doped As 2 Se 3 exhibits not only high nonlinearity but negligible TPA. As the nonlinearity of Ag-As 2 Se 3 is higher than As 2 Se 3 by Ag doping, FOM > 11 is expected.  Figure 8 shows the experimental setup. We launched two optical pulses at different wavelengths as pump lights to generate FWM. The mode-locked laser pulse was expanded spectrally at the first EDFA, then divided by 3 dB coupler into two arms. The divided pulses were filtered at each band-pass filters as pump lights. The center wavelengths of the two pulses λ 1 (ω 1 ) and λ 2 (ω 2 ) were 1556 nm (192.7 THz) and 1560 nm (192.2 THz), respectively, both corresponding to the fast group velocity regime of the PCW. Their spectral width was suppressed to less than 2 nm to avoid the FWM in silica fibers of the setup. Then, they were amplified by following EDFA. A delay line was introduced into one arm to alter the timing of the confluence at a following 3 dB coupler. The timing of the two pulses was adjusted so that their auto-correlation trace exhibits the clearest beat. The pulses were coupled to the PCW through the microscope objectives. The output pulse was detected by a lensed fiber, and its spectrum was observed by the OSA. Figure 9 shows spectra of the output pulse. Signal and idler light are observed around the two pumps when the input peak power increases. The center wavelengths of signal λ sig (ω sig ) and idler λ idl (ω idl ) light are 1552 (193.2 THz) nm and 1564 nm (191.7 THz), respectively, which is in good agreement with theoretically estimated values from frequency matching conditions (ω sig = 2ω 1 − ω 2 , ω idl = 2ω 2 − ω 1 ). Considering the short device length (400 µm) and pulse propagation at the fast group velocity regime, this is another evidence of the large nonlinearity in the ChG.

Four-wave mixing
In Si-wire [24], the FWM occurs with 0.2 W peak power (CW), 10200 µm effective length (propagation loss: 0.24 dB/mm) and 0.09 µm 2 cross-sectional area. In comparison with them, 1.7 times higher peak power, 1/44 times shorter effective length and a twice cross-sectional area give the similar FWM, corresponding to 13 times higher efficiency than Si-wire's.

Conclusion
We have fabricated Ag-As 2 Se 3 chalcogenide glass photonic crystal waveguides and demonstrated large self-phase modulation, and four-wave mixing, and negligible two-photon absorption for the first time. The large effective nonlinear parameter (2.6 × 10 4 W −1 m −1 ) is the clear evidence of the advantage of chalcogenide glass photonic crystal waveguides for alloptical switching devices. Further nonlinearity enhancement is expected by exploiting low dispersion slow-light structures.