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Abstract
Active spectral tuning of nanophotonic devices offers many fascinating prospects for the realization of novel optical function. Here, switchable spectral response is enabled by the architecture of one-dimensional (1D) photonic crystal (PC) integrated with phase change material of the germanium antimony telluride (GST). Active and precise tuning of the bistable passband and central resonant frequency is demonstrated in the 1D PC composed of alternate SiN and GST nanofilms. An analytical model is derived to specify the tunable spectral features, including the band gap and resonant frequencies. Both the measured and calculated results show distinct red shifts of passband and the resonant minima (or maxima), well confirming theoretical predictions. This work demonstrates a route to construct active photonic devices with the electrically or thermally tunable spectra via 1D PC and potentially extends diverse applications based on the PC platform.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Owing to the unique features of high sensitivity, low complexity and the prominent photonic band gap (PBG), photonic crystal (PC) has provided a powerful platform for both frontier research and a diverse range of photonic applications including omnidirectional reflectors [1,2], super prisms [3], beam polarizer [4,5], photonic crystal fibers [6–8], high efficiency LEDs [9], optical switches [10], sensors [11,12], nonlinear optics devices [13–15] etc. However, most PC devices with fixed properties [16,17] are gradually impeded in many fields where active spectral control is essential, e.g., integrated photonics or optoelectronics, optical interconnect and circuitry etc. So great efforts have been made to introduce active features into current passive devices by integrating active materials or elements, such as MEMS based hierarchy [18], strain sensitive materials [19], electrically tunable semiconductors [20], liquid crystals [21] etc. The early concept of tunable PC based devices traces back to 1998, when ferroelectric materials with pronounced electric or magnetic dependence of permittivity were used [22]. Recent strategies to construct the tunable PC devices include a metal-organic framework tunable by isomerization [23], photoresponsive liquid crystals [24], polymer-dominant soft materials tunable via hydrostatic forces [25] etc. However, very few are optional for application in integrated photonics or optoelectronics, especially in terms of low-complexity and process compatibility in micro and nanofabrication.
Recently, phase change materials have emerged as an efficient type of active media with the prominent features of switchable properties and process compatibility, typically the chalcogenide alloy of GeSbTe (GST) [26]. Upon a reversible phase transition between the amorphous and crystalline states by electric, thermal or optical stimuli, the GST assumes a distinct switching and tuning of the refractivity, which promisingly enables a variety of active photonic devices with switchable and bistable performance and non-volatile control, e.g. the tunable perfect absorber [27], tunable metasurface for wavefront control [25], the THz switch [28] and so on.
Therefore, it is fundamentally feasible and promising to integrate the phase change medium of GST with the PC platform. As a proof of concept, this paper confirms a strategy to construct active photonic devices via one-dimensional (1D) PC with the tunable band gap and resonant frequencies. An analytical model is derived first to characterize the tunable spectral features of 1D PC upon phase transition of the GST. Then, the 1D PC consisting of alternate SiN/GST nanofilms is demonstrated with the measured tunable spectra and thus the great potential as platforms of photonic switches or sensitively tunable band-pass, or stop filters.
2. Fundamental and scheme
2.1. Analytical model of ideal 1D PC
Typically, the architecture of an ideal one-dimensional (1-D) photonic crystal (PC) is formed by two types of dielectric films with different refractive indices alternately arranged in an order of (AB)N, as shown in Fig. 1(a). Here, N is the number of cycles, and nA, nA, dA, and dB are the refractive indices and thicknesses of the alternate films of A and B. In our scheme, one dielectric film (e.g., A) is the phase change material of GST that assumes distinctly different refractive indices nA and nA’ upon phase transition between the amorphous and crystalline states. Also, the amorphous state of film A is initialized with the refractive index nA and the same effective optical thickness (deff) as that of film B, i.e., deff=nBdB=nAdA.
Fig. 1. (a) Schematic of the ideal 1D photonic crystal composed of multiple parallel dielectric films that are superposed alternately. (b) Illustration of reflections and transmissions at the two interfaces of one single dielectric film.
In principle, the 1D PC can be regarded as a combination of interfaces alternately separated by media with different refractive indices (nA and nB). To probe into the behaviors of 1D PC, the single dielectric film or cavity between two interfaces is first investigated as a unit and the simplified model, as shown in Fig. 1(b). Conventionally, the classical transfer-matrix method (TMM) [29] is employed to analyze and calculate the reflected and transmitted fields for wave propagation through multiple parallel interfaces, namely
Therefore, by taking into account the boundary conditions at the 1st and the 2Nth interfaces, the reflectance of 1D PC can be obtained. For a single film A, by substituting the boundary conditions at the interfaces I (1st) and II (2nd), i.e., EI=EiI+ErI and HI=Y1 (EiI-ErI), and n1=n3=nB, n2=nA into Eq. (1), the reflectance of a single film A sandwiched between two dielectric films of B can be derived as
Similarly, according to the effective medium theory, the whole 1D PC structure can be regarded as an equivalent dielectric film or cavity of (AB)N with the total effective optical thickness equal to 1D PC, namely 2Ndeff, which consequently determines its base resonant frequency fR/2N. As a result, the reflection spectra of the equivalent film (AB)N has a series of maxima and minima at odd and even multiples, respectively, of its base frequencies, i.e., (2m + 1)fR/2N and mfR/N, where m is a positive integer.
2.2. Spectral features: band gap and middle frequency
Therefore, the spectral behavior of 1D PC can be ideally interpreted as the integrated effect of single film resonances and the resonance of an equivalent single cavity of (AB)N with an effective optical thickness of 2Ndeff. Figure 2 illustrates the reflection spectra of the single film (A), the equivalent single cavity and the 1D PC calculated using TMM. Obviously, the maxima in the reflection spectrum of the equivalent film remain the resonant peaks in the final spectrum of 1D PC, and the minima remain as the resonant valleys except those ones coincidentally overlapping with resonant maxima of the single film at some special frequencies, i.e., fR and its odd multiples. In this situation, such minima of resonant valleys are cancelled out by the single film resonant peaks at special frequencies and the band-pass range in the reflection spectrum is extended. Shown in Fig. 2, the first band gap of the 1D PC appears and centers at the frequency fR between the (N)th and (N + 1)th maxima, i.e., (2N-1)fR/2N and (2N + 1)fR/2N. Therefore, the first reflection passband or the transmission forbidden band of the ideal 1D PC can be approximately written as
Fig. 2. Reflection spectrum of the single film A, equivalent film, and the ideal 1D PC composed of N pairs of alternate dielectric film (AB)N for N = 3, with the same effective optical thickness deff=nd=dBnB=dAnA and the refractive indices of nA=2 and nB=3.
In our scheme, by exerting a thermal or electric amorphous-to-crystalline phase transition to the GST of film A, the refractive index changes from nA to nA’. Similarly, the reflection passband or transmission forbidden band for the 1D PC of (AB)N with different effective optical thicknesses can be generally normalized as
As a result, the above derivations reveal the fundamental of our scheme. The band gap and the central location of the 1D PC is linearly proportional to the average base frequency fRAve or wavelength λRAve, which is tunable by changing the effective optical thickness of either film A or B. That is, upon phase transition of film A, the whole 1D PC undergoes a change of the base resonant frequency from fR/2N to fRAve/2N, the band gap or width from fR/N to fRAve/N and the center location from fR to fRAve or λR to λRAve. Specifically, when the refractive index of film A increases from nA to nA’ upon the amorphous-to-crystalline phase change, there exists an increase of the whole effective optical thickness and a reduction of the band gap as well as the central frequency. It is noteworthy that the shrunk band gap with lower central frequency in the frequency spectrum leads to an expanded band gap with higher central wavelength in the wavelength spectrum.
3. Results and discussions
In order to verify the feasibility of the proposed scheme, a device model of tunable 1D PC working in the visible and near-infrared range is constructed by directly integrating the phase change medium into the hierarchy of (AB)N, e.g., replacing film A by the phase change medium of GST, as shown in Fig. 1(a).
In the numerical simulations based on the finite element method (FEM), film B is initialized with a refractive index of 2.0 and a thickness of 100 nm, and film A with varied refractive indices and thicknesses but with the same equivalent thickness as film B in the (AB)N setup. Figure 3 shows the reflection spectra of 1D PC when the constituent film A undergoes a phase transition. Obviously, when the refractive index of film A (i.e., the GST) increases from 2.5 to 3 and 4, there exists distinct red shifts of the base resonant frequency, the whole passband as well as the central frequency. As denoted in Fig. 3, the band gap shifts from [317 THz, 433 THz] to [298 THz, 388 THz] and [243 THz, 327 THz], with the bandwidth reduced from 116 THz to 90 THz and 84 THz, respectively, in good agreement with the analytical value indicated by our approximate model in Eqs. (3)–(4).
Fig. 3. Simulated reflection spectrum of 1D PC composed of six alternative layers of dielectric A and B (AB)N for N = 3 and film B with fixed refractive index (2.0) and thickness (100 nm) and film A with varied refractive indices shown in the inset.
Meanwhile, results in Fig. 3 also agree with our theoretical derivations of band gap of 1D PC in Eq. (4). When the refractive index difference between A and B becomes larger, the original central minima in-between the Nth and (N + 1)th maxima are gradually uplifted and flattened or disappeared to become the central maxima, as shown in Fig. 3. So, in some sense, such effect expands the passband to form the band gap of 1D PC by producing three successive maxima adjacently.
Further, even though our model is explored based on finite pairs of alternate dielectrics, it also better fits the ideal 1D PC for increasing the number of films N, where the refractive indices of film A and film B are 2 and 3, respectively. Obviously, as shown in Fig. 4(a), a larger N leads to a more prominent band gap and steeper sideband. The Nth resonant minima of 1D PC turns out to be more obviously cancelled out with ultimately an increasing reflection at the middle frequency inside the band gap. In addition, also shown Fig. 3 above, a varied contrast of refractive indices leads to distinctly different spectra, especially at the top region of the band gap. For direct comparisons, 1D PC setups with the same number (N) of alternate films and effective optical thickness but varied refractive indices are also calculated, as shown in Fig. 4(b). Apparently, a higher refractive index contrast makes the spectral features closer to that of ideal 1D PC with a more prominent band gap and a flatter top band, i.e. the Nth resonant minima is more clearly cancelled out with ultimately higher bandpass reflection.
Fig. 4. Simulated reflection spectra of the 1D PCs with the same effective optical thickness (deff=200 nm); (a) varied pairs of the same constituent dielectric films in the (AB)N setup, where the refractive indices of A and B are 2 and 3, respectively; (b) the same pairs of films (N = 3) with varied refractive indices of film A by phase transition, and thickness of film A is adjusted keep the effective optical thickness deff unchanged.
As a proof-of-concept experiment, a tunable 1D PC composed of three pairs of alternate SiN (100 nm) and GST (50 nm) films is fabricated. Magnetron sputtering (ACS-4000, ULVAC) is used in six periods of alternate GST and SiN depositions with the rate of 5 nm/min for GST. The reflection spectra of this device are then characterized by spectrophotometry (Lambda 950 UV/VIS) before and after the phase transition of GST triggered by thermally heating up to 300 °C. Figure 5(a) shows measured spectra of the fabricated 1D PC before and after phase transition.
Fig. 5. (a) Measured and (b) calculated reflection spectra of the fabricated 1D PC composed of three alternate pairs of SiN/GST nanofilms at the amorphous (black curve) and crystalline (red curve) state of GST. The solid curves in (b) are the ideal spectra of 1D PC without extinction and the dotted curves are obtained by considering the extinction in the visible and near IR range with the measured dispersion relation cited from Ref. [30].
For comparisons, the reflection spectra are also calculated by FEM using refractive indices of the single layer amorphous and crystalline GST cited from Ref. [30]. For approximate calculation of the spectrum of an ideal 1D PC, refractive indices at the middle frequency (∼1100 nm) of the measured band gap in Fig. 5(a) are extracted, namely na=4.6 + 1.2i and nc=6 + 2.75i. Further, to take into account the strong extinction and dispersion in the visible and near infrared region, indices with imaginary parts at different wavelengths are sampled and used to fit the experimental spectra, as shown in Fig. 5(b).
Obviously shown in Fig. 5, upon the amorphous-to-crystalline phase transition of GST film, distinct red shifts of the band gap and the resonant minima (or maxima) can be observed from both the measured and the calculated spectra. For the amorphous state, according to the measured middle wavelength at 1100 nm around [Fig. 5(a)], the band gap can be analytically predicted to be around [942 nm, 1320 nm] according to our approximate model in Eq. (4), and the (N + 1)th minima that locates just beyond the bad gap can be estimated as ∼1460 nm from Eq. (4) as well. Both results are in good agreement with the measured spectra (solid black) in Fig. 5(a) and also the calculated spectra (solid black) in Fig. 5(b). After phase change of GST film to the crystalline state, the band gap of 1D PC shifts to [1280 nm, 1800 nm] with the original (N + 1)th minima of ∼1460 nm truncated in the measured spectra.
In addition, other than the obviously movable and shrinkable band gap mentioned above, reflectance at one specially selected minimal wavelength (i.e. 1460 nm) in the amorphous state also assumes switchable values between the minimum valley or the maximum peak in Fig. 5. Also, shown in Fig. 6, the electric field distributions before [Fig. 6(a)] and after [Fig. 6(b)] phase transition of GST film that are extracted from the same FEM-based simulation, reveal the tunable reflection/transmission amplitude at the wavelength of 1460 nm, namely the relative spectral location outside or inside the band gap, under destructive (minimum reflection or maximum transmission) or the constructive (maximum transmission or minimum reflection) resonance, as agrees well with the spectra response in Figs. 5(a) and 5(b).
Fig. 6. The simulated electric field distribution at the incident plane (xoz) under normal incidence along z axis at the wavelength of 1460 nm when GST film is in (a) the amorphous and (b) the crystalline state respectively.
As a result, the switchable or controllable spectral behaviors, including the movable resonant minima, the expandable and movable passband or band bap, enable diverse active photonic devices such as switches, tunable filters, modulators etc.
4. Conclusion
In conclusion, we have demonstrated theoretically and experimentally a strategy to simply construct active photonic devices via a spectrum-tunable 1D PC consisting of alternating layers of SiN and GST nanofilm. An approximate analytical framework has been derived to explicitly reveal the mechanism of the tunable spectral behaviors. Both numerical calculations by the classical transfer matrix method and the experimental results have verified the effectiveness of the analytical framework, which is general and applicable to the rapid customization of 1D PC with an actively tunable spectrum, thus simplifying the design procedure. The device architecture, simply constructed by the well-established materials that are microelectronic or CMOS compatible, facilitate low-cost, low-complexity and high-volume production and integration. It is noteworthy that the tunable spectral behaviors enabled by the electrically- or thermally-controlled phase transition of GST provide a promising platform to dynamically adjust the spectral response for a diverse range of active photonic devices. Further, the materials of the GST family have been widely used in phase change memory for electronic storage. One can therefore expect a single active platform where the functions of photonics, electronics, and memory can be integrated simultaneously.
Funding
Croucher Foundation (CAS18EG01); National Key Research and Development Program of China (2017YFA0206002); National Natural Science Foundation of China-Chinese Academy of Sciences Joint Fund of Research utilizing Large-scale Scientific Facilities (U1832217); Pearl River S and T Nova Program of Guangzhou (201710010058); the Opening Project of Key Laboratory of Microelectronic Devices Integrated Technology, Chinese Academy of Sciences; the Opening Project of State Key Lab of Optical Technologies on Nano-Fabrication and Micro-Engineering, Institute of Optoelectronics, CAS.
Disclosures
The authors declare no conflicts of interest.
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