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Abstract
We propose a reconfigurable and non-volatile Bragg grating in the telecommunication C-band based on the combination of novel low-loss phase-change materials (specifically Ge2Sb2Se4Te1 and Sb2S3) with a silicon nitride platform. The Bragg grating is formed by arrayed cells of phase-change material, whose crystallisation fraction modifies the Bragg wavelength and extinction ratio. These devices could be used in integrated photonic circuits for optical communications applications in smart filters and Bragg mirrors and could also find use in tuneable ring resonators, Mach–Zehnder interferometers or frequency selectors for future laser on chip applications. In the case of Ge2Sb2Se4Te1, crystallisation produces a Bragg resonance shift up to ∼ 15 nm, accompanied with a large amplitude modulation (insertion loss of 22 dB). Using Sb2S3, low losses are presented in both states of the phase change material, obtaining a ∼ 7 nm red-shift in the Bragg wavelength. The gratings are evaluated for two period numbers, 100 and 200 periods. The number of periods determines the bandwidth and extinction ratio of the filters. Increasing the number of periods increases the extinction ratio and reflected power, also narrowing the bandwidth. This results in a trade-off between device size and performance. Finally, we combine both phase-change materials in a single Bragg grating to provide both frequency and amplitude modulation. A defect is introduced in the Sb2S3 Bragg grating, producing a high quality factor resonance (Q ∼ 104) which can be shifted by 7 nm via crystallisation. A GSST cell is then placed in the defect which can modulate the transmission amplitude from low loss to below -16 dB.
Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.
1. Introduction
To date, one of the challenges faced by silicon photonics is the integration of non-volatile reconfigurable components such as switches, filters, memories, though some progress has been made [1]. Silicon nitride (SiNx), an associated CMOS photonics enabling material, provides a very promising and complementary photonic platform for the development of low cost CMOS compatible waveguides and related photonic components [2]. This is due to the flexibility of the material in terms of fabrication (low temperature), tuneability of the refractive index contrast, transparency, and low temperature sensitivity [3]. As a result, SiNx waveguides have been widely employed for light propagation in the near infrared and in the visible range of the electromagnetic spectrum [4,5]. Although Si waveguides provide more confinement than SiNx, leading to more compact devices, Si$_3$N$_4$ or non-stoichiometric SiNx is more cost effective, thermally stable, and provides greater freedom when constructing more complex multi-layer photonic circuitry.
Phase-change materials have been a mature technology for decades in optical storage, and are now seen as a promising CMOS compatible route to provide the much needed non-volatile reconfigurability in integrated photonic components [6]. The phase-change materials (PCMs) [7,8] are meta-stable (decades) in either amorphous or crystalline phases, which exhibit large contrast in optical properties [9,10]. Switching between phases can be very fast (sub ns [11,12]) and is achieved using low-power thermal excitations, with intermediate mixed amorphous/crystalline phases also possible. In-situ control of the PCMs could be achieved using electrical graphene or ITO micro-heaters placed below the PCM layers. This approach has recently been demonstrated in Refs [13,14]. The crucial feature of integrated PCMs compared with conventional thermo-optic or electro-optic based programmable circuits [15–17] is that energy is only consumed during the actual switching process (due to non-volatility of PCMs). Different technological applications such as switches [18,19], wavelength division multiplexers [9], directional couplers [20], and plasmonic memories [21,22] have been already demonstrated. The combination of the Si$_3$N$_4$ platform and phase-change materials enables a variety of fast, non-volatile, and re-configurable devices for a range of on-chip applications [23–25]. Most technologically useful PCMs are binary, ternary or quaternary chalcogenide alloys. These materials have been exploited for a variety of tunable photonic applications ranging from filters [26] to metamaterials [27–29]. Different periodic photonic structures have been demonstrated using the well-known PCM, GST-225 [30]. Recently, a contra-directional coupler switching enabled by a Si-GST grating has been demonstrated in Ref. [31] and a Bragg grating in photosensitive Ge$_{23}$Sb$_7$S$_{70}$ chalcogenide micro-ring resonators via a novel cavity-enhanced photo inscription process, in which injection of light at the targeted C-band resonance frequency induces a spatially varying refractive index change, has been demonstrated in Ref. [32]. Sb$_2$S$_3$ and Ge$_2$Sb$_2$Se$_4$Te$_1$ (GSST) are recently developed PCMs [33,34], they were selected for this work due to their particularly low losses in the C-band, whilst still maintaining a reasonably high contrast (between phases) in refractive index. The optical properties of the PCMs (Sb$_2$S$_3$ and GSST) in amorphous and crystalline states have been measured using ellipsometry. The samples were deposited using magnetron sputtering in an argon atmosphere at a pressure of $5\cdot 10^{-3}$ mbar and power of 15 W in the case of the GSST, and a pressure of $2.67\cdot 10^{-3}$ mbar and power of 35 W in the case of the Sb$_2$S$_3$. All samples were deposited on a silicon substrate and the thickness of the deposited PCMs were 50 nm. The experimental ellipsometry measurements from 300 nm up to 1700 nm in wavelength are presented in Fig. 1.
Fig. 1. (a) Refractive index, $n$, and (b) extinction coefficient, $k$, ellipsometry measurements in amorphous (solid lines) and crystalline (dashed lines) state for Sb$_2$S$_3$ (red) and GSST (green).
Among various silicon photonic device structures for filter applications, Bragg Gratings (BGs) are one of the most efficient and commonly used technologies in integrated photonics. BGs are structures with a periodic modulation of the effective refractive index in the direction of propagation of the mode. This modulation in the refractive index can be achieved by intercalating two different materials or by generating defects in the structure. In this work, we propose a reconfigurable BG for generating non-volatile reconfigurable smart filters on-chip. BG structures can also be used in the design of photonic FPGA (field programmable gate arrays) or as components to provide synaptic weight in neuromorphic systems (synthetic brains) [35–37]. Different reconfigurable BGs filters have been demonstrated and proposed using the well known p-n junctions on silicon [38] or more recently, using graphene [39]. These approaches are volatile, and their maximum shift is only in the order of 1 nm. The phase-change BG presented here (we believe for the first time) is non-volatile, and achieves a significantly greater shift of $\sim$ 7 nm using the low-loss Sb$_2$S$_3$, and $\sim$ 15 nm using the higher-loss GSST.
2. Bragg grating design
2.1 Mode analysis
The optical platform consists of a Si$_3$N$_4$ ($n=2.01$) ridge photonic waveguide with cross-section dimension of $1200$ nm width and $300$ nm thickness. This waveguide geometry enables mode propagation in the C-band (both TM and TE) with negligible losses. The BG is produced by periodically spaced cells of phase-change material along the length of the waveguide, as shown in Fig. 2. These cells are 20 nm thick, comprising 10 nm of PCM, capped with 10 nm of SiO$_2$ to prevent oxidation. Cell length (LPCM), period ($\Lambda$) and consequently fill-factor ($FF$) can be efficiently engineered in order to control the device performance.
Fig. 2. Schematic of two phase-change reconfigurable BGs, both with a period of $\Lambda = 500$ nm , and with a period number N=10. The cells consist of 10 nm of PCM, capped with 10 nm of SiO$_2$, (a) uses Sb$_2$S$_3$ as the PCM, with a fill-factor $FF$=0.5 (c) uses GSST2241 as the PCM, with a fill-factor $FF$=0.2. Internal reflections are illustrated in both figures using red arrows. (b-d) Mismatch in the effective refractive index between the bare waveguide region and the hybrid waveguide region is illustrated and the cell length (LPCM) for each phase-change material is indicated.
Finite element simulations ( COMSOL Multiphysics) were used to calculate the effective refractive indices of the bare and hybrid waveguide with cell structure. These effective refractive indices were then used in the transfer matrix method (TMM) to calculate the BG properties (discussed later). The material properties are summarised in Table 1. For this study only TE mode will be considered. The effective contrast (between PCM phases) in the waveguide’s effective refractive index is determined by the specific PCM alloy used. For Sb$_2$S$_3$ this contrast is $\Delta n^{Sb_2S_3}_{eff}=1.4 \cdot 10^{-2}$. GSST provides a significantly greater contrast of $\Delta n^{GSST}_{eff}= 7.4 \cdot 10^{-2}$, however, unlike Sb$_2$S$_3$, GSST crystalline phase is lossy and contributes to insertion losses depending on the total combined cell length ($\alpha =0.53$ dB/$\mu$m). Different thickness of phase-change material have been considered to see the effect in the absorption and in the propagation constant, see Fig. 3.
Fig. 3. Effective refractive index and cell absorption for different PCM thickness ranging from 8 nm up to 25 nm for both materials, Sb$_2$S$_3$ (red) and GSST (green) for amorphous (solid lines) and crystalline (dashed lines) states respectively.
Table 1. Material optical properties ($n$ and $k$) used in the finite element simulations, and the resulting effective refractive index values (all at 1.55 $\mu$m) used in the transfer matrix method calculations.
2.2 Transfer Matrix Method (TMM)
Since a BG can be considered to be a multi-layer structure, a 2 x 2 TMM [40,41] can be applied to simulate its reflection and transmission properties. The transfer matrix is defined as:
To describe the phase-change BG, we considered the following notation, $\beta _{PCM}^{a}$ refers to the propagation constant when the phase-change material is fully amorphous, and $\beta _{PCM}^{c}$ to the propagation constant in the fully crystalline phase. The phase-change BG can then be described as follows:
where $N$ is the number of periods ($\Lambda$) of the grating. The length of the BG is defined as $L_{BG}=N\Lambda$, the fill factor ($FF$) is defined as $L_{PCM}/\Lambda$ and $T_{PCM}^{a-c}$ is defined as the transfer matrix for an homogeneous section through the phase change material cell, the superscripts $a$ and $c$ represent the amorphous or crystalline state of the cell, using the propagation constant $\beta _{PCM}^{a}$ or $\beta _{PCM}^{a}$ in Eq. (2) respectively. The Bragg wavelength, at 1.55 $\mu$m, determines the periodicity, which affects the fill-factor, and in turn this affects the effective refractive index. The centre wavelength of the grating is known as the Bragg wavelength and is described by [42]: where $n_{eff}$ is the average effective index. $n_{eff}$ = (1-$FF$)$n_{eff_1}$+$FF$ $(n_{eff_2})$, in which, $n_{eff_1}$ corresponds to the effective refractive index of the bare waveguide (without PCM cell), $n_{eff}^{w}$, while $n_{eff_2}$ is the effective refractive index of the hybrid structures, as shown in Figs. 2(b-d). Consequently, $n_{eff_1}=n_{eff}^{w} = 1.5348$ and $n_{eff_2}= n_{eff} ^{a-c}$.3. Results and discussion
Two phase-change BGs were designed to operate at 1.55 $\mu$m with two different period numbers of $N$ = 100 and $N$ = 200 for each type of PCM. Both designs were studied for the amorphous and crystalline state of the PCM. All BGs used a periodicity $\Lambda$= 500 nm, with a PCM thickness of 10 nm, SiO$_2$ cap thickness of 10 nm and both with 1200 nm width, completely covering the waveguide width. The BGs employing Sb$_2$S$_3$ used a $FF$ = 0.5, and the GSST based BGs used a $FF$ = 0.2 (different fill factors were selected due to the difference in the loss between the two phase change materials). Figure 4 shows the calculated performance (normalized transmission) of these four different BG combinations.
Fig. 4. Normalized transmission spectra comparing BG filters using the two studied PCMs, each evaluated with two different number of periods ($N=100$ yielding LBG = 49.8761 $\mu$m, and $N=200$ yielding LBG = 99.75 $\mu$m). Red curves are amorphous state, black curves are crystalline state. Solid curves are for $N=100$, and dashed curves for $N=200$. (a) uses Sb$_2$S$_3$ as the PCM. (b) uses GSST2241 as the PCM.
Table 2 summaries the performance of each BG. In general, the number of periods ($N$) determines the bandwidth and the extinction ratio (ER) of the filter. Increasing the number of periods causes the ER and reflected power to increase and the bandwidth to reduce. Therefore, to improve selectivity and reflected power, the number of periods can be increased. However, this results in a trade-off between device performance (number of periods) and the total length of the BG (L$BG$). Internal reflection losses (IRL), scattering losses (SL) and absorptive losses in the cells (PL), all contribute to the insertion losses (IL=PL + IRL + SL). The Sb$_2$S$_3$ SLs produced by a unique PCM unit cell have been evaluated yielding to values of $1.6\cdot 10^{-2}$ dB and $7.3\cdot 10^{-3}$ dB in the crystalline and amorphous state respectively. In the case of the GSST phase change material, the SLs due to one unit cell have values of $1.5\cdot 10^{-2}$ dB and $1.6\cdot 10^{-3}$ dB in the crystalline and amorphous state respectively. Thanks to the near-zero losses of Sb$_2$S$_3$ in both phases, the BGs incorporating this PCM have negligible losses due to absorption within the cells (PL) and the only significant contribution is due to the scattering losses. The effective refractive indices for both phases (see Table 1) are also similar enough to that of the bare waveguide to produce near zero IRL. This results in signal strength being maintained outside of the stop band, whilst providing high extinction ratios (for both the $N$=100 and $N$=200 cases). The contrast in refractive index between PCM phases results in the crystalline phase red-shifting the Bragg wavelength by 7 nm.
Table 2. Bragg grating performance comparison between the two studied phase-change materials. GSST2241 and Sb$_2$S$_3$ for different number of periods ($N$) and different phase-change materials state (amorphous or crystalline)
The BGs incorporating GSST behave very similarly to Sb$_2$S$_3$ in their amorphous phases thanks to near-zero losses and similar effective refractive index of the waveguide and cell. Due to the relatively large contrast in the refractive index of GSST, the Bragg wavelength can be shifted by 15 nm. However, once the GSST is crystallised the BG exhibits significant insertion loss due to absorption in the cells 0.53 dB/$\mu$m, internal reflections from the effective index mis-match (see Table 1) and scattering losses. This causes the GSST BGs behaving more like an amplitude switch; in which the amorphous phase attenuates at just 1.55 $\mu$m, while the crystalline phase attenuates all wavelengths. The number of periods has negligible effect on amorphous losses, but significantly affects crystalline losses. Therefore, if employing this filter as an amplitude switch, increasing the period number greatly enhances the all-wavelength ER.
3.1 Fractional crystallization
So far we have only considered fully amorphous and fully crystalline PCM phases. Here we exploit the fact that intermediate crystalline phases are also accessible and stable. These intermediate phases form when insufficient time is available to complete crystallization. Properties of PCM intermediate phases (which crystallise as just described) can be evaluated. In this paper we followed the approach taken in Ref. [43]:
By controlling the fraction of crystallisation, intermediate BG filter states can be achieved, resulting in truly tuneable (rather than switchable) BG filters. In Fig. 5 BG performance for a range of PCM crystalline fractions has been plotted. Increasing the crystallization of the phase-change material cell results in an increase of both filter bandwidth and extinction ratio. Reliably and repeatably accessing intermediate crystallisation states is not straight forward, but has been previously demonstrated [44,45].
Fig. 5. Transmission spectrum of two reconfigurable BG filters for 6 different levels of phase-change material crystallisation. Both filters have a period of $\Lambda$=500 nm, and $N=100$ for a BG length of LBG = 50 $\mu$m. (a) uses Sb$_2$S$_3$ with a fill factor $FF=0.5$. (b) uses GSST2241 and a fill-factor $FF=0.2$.
This tuneabale functionality results in a fine control over the bandwidth, and these tuneable BGs could be a key component in smart filters, reconfigurable mirrors, or frequency selectors. A complete study of all the relevant parameters that comprises a Bragg grating (central wavelength, extinction ratio, bandwidth and insertion losses) has been made for both phase-change materials and different crystallization fractions and can be found in Fig. 6. The phase change process itself can be thermally, optically or electrically driven. For example, thermal switching of PCMs in integrated photonic devices has been demonstrated using embedded ITO or doped-Si heaters [46,47], optical switching has been achieved using light pulses sent down the waveguide itself (see e.g. [6,9,48]) and by top illumination [23], while direct electrical switching has also been demonstrated, albeit for rather small PCM volumes [21]. Crystallization is achieved by heating the material above the glass transition temperature long enough to allow for crystal nucleation and growth processes to switch the required volume of material. Returning to the amorphous state is more challenging, and requires the PCM to be molten and then very rapidly quenched [48]. Not having equal crystallization levels between different PCM cells would of course affect the performance of the grating, for example by changing the peak position and bandwidth. However, the methods described previously for switching the PCM cells should be able to provide a good degree of uniformity of crystallization levels.
Fig. 6. Performance comparison between the two used phase-change materials: Sb$_2$S$_3$ with a fill factor $FF=0.5$ and GSST2241 with a fill-factor $FF=0.2$ for different crystallization states. (a) Central wavelength shift (b) bandwidth (c) insertion loss and (d) extinction ratio.
3.2 Tuneable band-pass filtering
The BGs described previously can be used as a building block to produce a non-volatile tuneable band-pass filter. These filters are capable of dynamically controlling which wavelengths are transmitted along the waveguide. This is achieved by arranging multiple different BG filters in series, with the ability to independently tune each BG, as shown in Fig. 7(a).
Fig. 7. Multilevel tuneable filters consisting of an array of 3 BGs placed in series. Each grating has a different periodicity ($\Lambda _1$, $\Lambda _2$, $\Lambda _3$). (a) Schematic of the filter, showing just 4 periods per grating (actual device consists of $N$=100 with fill-factor of 0.5). (b) Transmission spectrum for different BG periodicity for different phase-change material state. amorphous (red line) and crystalline (black line): The length of the three different BGs are $L_{BG_1}$=48.5 $\mu$m, $L_{BG_2}$=50 $\mu$m and $L_{BG_3}$= 51.5 $\mu$m respectively and (c) Bragg wavelength for different periods of the grating.
The BG represented in Fig. 7(a) use a period of $N$=100 and a fill-factor of 0.5. Because performance of the BG filters can be controlled just by modifying their period and fill-factor, the specific PCM alloy and its thicknesses can be common across all BG filters. This means all the required filters can be simultaneously fabricated onto the waveguide in a single lithography step. The example device, whose performance is shown in Fig. 7(b), consists of three BG filters in series, with periods of $\Lambda _1$=485 nm (LBG1 = 48.5 $\mu$m), $\Lambda _2$= 500 nm (LBG2 = 50 $\mu$m), and $\Lambda _3$=515 nm (LBG3 = 51.5 $\mu$m). This results in three Bragg wavelengths positioned around $\lambda _{BG1}$=1.5 $\mu$m, $\lambda _{BG2}$= 1.55 $\mu$m, and $\lambda _{BG3}$=1.6 $\mu$m respectively. Each of the central wavelengths can be tuned by 7 nm by changing the state of the Sb$_2$S$_3$ from amorphous to crystalline; this means the width of the pass-bands can be controlled. The extinction ratio for all the gratings in the amorphous state has a value of $\sim$ 9.2 dB and can be increased up to $\sim$16.3 dB when crystallization occurs. A dependence between the Bragg wavelength ($\lambda _b$) and the period ($\Lambda$) of the grating is shown in Fig. 7(c), the ratio between the period and the wavelength is 3.1. This configuration of filters results in an in-plane on-chip metamaterial that can be used as part of micro-ring resonators or Mach-Zehnder interferometers for future non-volatile frequency selectors.
4. Combined frequency and amplitude tuning
We have demonstrated that incorporating Sb$_2$S$_3$ cells on a silicon nitride waveguide can provide spectral tuneability controlled by the phase of the material (crystalline fraction). We have also shown that a similar BG structure using GSST can provide amplitude tuneability (ON/OFF switching). Building a BG primarily from Sb$_2$S$_3$ cells, and including a single ‘defect’ cell of GSST (see Fig. 8(a)) produces two useful effects. Firstly, the defect introduced within the BG (due to the difference between the two effective refractive indices $n_{eff}$ between PCMs) creates an ultra-high quality factor transmission peak. Secondly, while GSST is near-lossless when amorphous, its crystallisation fraction controls the amplitude of the new peak via optical absorption, as shown in Figs. 8(b-c).
Fig. 8. (a) Schematic of a re-configurable, ultra-high quality factor filter based on phase-change materials. A defect in the Sb$_2$S$_3$ BG is created by replacing a Sb$_2$S$_3$ cell with a GSST cell. This generates an ultra-high quality factor peak in the Bragg resonance. Absorption within the defect (and therefore amplitude of high quality factor peak) can be controlled via the crystallisation fraction of the GSST. (b) Transmission response of the proposed device for both phases of both phase-change materials (c) Zoom in of the peak resonance produced by the defect cell.
Due to the low total volume of GSST, the absorption (even when fully crystallised) in the defect cell is typically negligible (PL= $0.53$ dB$/\mu m \cdot 0.25 \mu m= 0.13$ dB), however, it is greatly enhanced at the high quality factor peak’s resonance wavelength. This amplification is due to the Fabry-Pérot like cavity resonance present in this section of the waveguide due to the two Bragg grating reflectors. In the proposed device, the number of periods ($N$) for the BG is 200 in total, 100 placed on the left hand side of the defect and 100 on the right hand side of the defect. A fill-factor ($FF=0.5$) and a period of 500 nm have been selected, obtaining a total length for the device of Ldevice = 100 $\mu$m. The GSST cell is also 250 nm long, and is placed between the two BG reflectors. The resulting device produces a transmission peak which can be tuned from 1548 nm to 1555 nm ($\Delta \lambda$ = 7 nm) depending on the Sb$_2$S$_3$ crystallization fraction. For amorphous Sb$_2$S$_3$, the quality factor is $Q=609$, with an extinction ratio of ER = 17 dB. When crystallised the quality factor raises up to $Q=1,028$, and the extinction ratio increases up to ER = 30 dB. The normalized transmission of this peak is near 100 percent when the GSST is amorphous, and can be tuned down to -7 dB for a-Sb$_2$S$_3$, and -16 dB for c-Sb$_2$S$_3$ for crystalline GSST. Furthermore, this BG filter configuration is also appropriate as a building block to produce band-pass filters like those discussed in the previous section.
5. Conclusions
We have proposed and theoretically demonstrated a reconfigurable BG based on novel phase-change materials (PCMs) in a silicon nitride (Si$_3$N$_4$) platform for optical communications applications in the C-band, that can be extrapolated to different bands and different applications for smart nano-systems and smart communications. The difference in the real part between the amorphous and crystalline state of the phase-change material (Sb$_2$S$_3$ and Ge$_2$Sb$_2$Se$_4$Te$_1$ studied here) could provide a promising route reconfigurable optical communications devices for photonic integrated circuits. We have also demonstrated the potential for combining multiple phase-change materials with complimentary properties to provide both spectral and amplitude control.
Funding
Engineering and Physical Sciences Research Council (EP/L015331/1, EP/M015130/1).
Disclosures
The authors declare no conflicts of interest.
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