A New Family of Ultralow Loss Reversible Phase‐Change Materials for Photonic Integrated Circuits: Sb₂S₃ and Sb₂Se₃

Abstract

Phase‐change materials (PCMs) are seeing tremendous interest for their use in reconfigurable photonic devices; however, the most common PCMs exhibit a large absorption loss in one or both states. Here, Sb2S3 and Sb2Se3 are demonstrated as a class of low loss, reversible alternatives to the standard commercially available chalcogenide PCMs. A contrast of refractive index of Δn = 0.60 for Sb2S3 and Δn = 0.77 for Sb2Se3 is reported, while maintaining very low losses (k < 10−5) in the telecommunications C‐band at 1550 nm. With a stronger absorption in the visible spectrum, Sb2Se3 allows for reversible optical switching using conventional visible wavelength lasers. Here, a stable switching endurance of better than 4000 cycles is demonstrated. To deal with the essentially zero intrinsic absorption losses, a new figure of merit (FOM) is introduced taking into account the measured waveguide losses when integrating these materials onto a standard silicon photonics platform. The FOM of 29 rad phase shift per dB of loss for Sb2Se3 outperforms Ge2Sb2Te5 by two orders of magnitude and paves the way for on‐chip programmable phase control. These truly low‐loss switchable materials open up new directions in programmable integrated photonic circuits, switchable metasurfaces, and nanophotonic devices.

Non‐Volatile Reconfigurable Integrated Photonics Enabled by Broadband Low‐Loss Phase Change Material

“…On the other hand, the ellipsometry does predict the imaginary part of refractive index accurately which implies that the material's optical loss does not vary significantly as the dimensions reduce. The extracted loss is higher than what was found in the previous work [ 34 ] where SbS was also capped on Si waveguides. This higher loss can be attributed to surface oxidation or Sulfur migration due to the thin (10 nm) capping layer used.…”

“…Pulses of higher voltages led to further blue shift and broadening of resonance dip, which implies that the PCM amorphization is accompanied by waveguide damage, as some local hotspots may have temperature exceeding the melting point of Si (See Figure S5, Supporting information). Since the cyclability of blanket film SbS has been recently reported using laser pulses, [ 34,53 ] we believe the low cyclability of our device is more of a heater design issue rather than SbS's material issue. Transparent conductors whose conductivity is thermally stable such as graphene [ 54 ] and FTO [ 55 ] could provide a better solution for operation with large number of cycles.…”

“…This is superior to the recently‐reported broadband transparent GSST [ 33,39,40 ] which has near‐zero loss at 1550 nm in amorphous state but suffers from non‐negligible loss in the crystalline state (see Table 2). Another emerging PCM Sb 2 Se 3 has near zero loss in both states near 1550 nm [ 34 ] but is plagued by the bandgap absorption in the visible range and hence exhibits measurable losses near 633 nm. One potential limitation for SbS, however, is the simultaneous reduction in both Δn and k , as a result of Kramers–Kronig relation, and hence smaller phase shift per unit length.…”

All-Chalcogenide Programmable All-Optical Deep Neural Networks

“…Analogous to a biological neuron, this artificial neuron acquires optical inputs and performs a weighted addition (linear MAC operation) using the photonic programmable phase-change array (P 3 A) consisting of a network of programmable photonic latches to control 800 nm signals (SiN waveguide platform). The synaptic weights, which are acquired by off-line training, are then programmed as refractive index changes in an ultra-low loss Sb2S3-based multistate photonic phase change memory material 22 . These Sb2S3 memory elements are embedded in asymmetric directional couplers and is programmed using electrical Joule heat or all-optically using a pump laser 15,16,25 (Fig.…”

“…This concept synergistically embeds the synaptic nonvolatile (programmed) weights and volatile NLAF with the same material (here GST) framework heterogeneously integrated atop a silicon photonics neural network platform. The photonic neuron exploits nonvolatile structural transition (Dn = unity) while exhibiting ultra-low-losses (k = 10 -4 at NIR) PCM 22 for programmable MVM operations, and experimentally demonstrates volatile rapid transitions 23 as NLAF. Exemplary, the quantized synapses are obtained through a cascade of ultralow-losses Sb2S3-based 22 broadband 4-bit switches operating as an optical FPGA and here referred to as photonic programmable phase-change array (P 3 A), while the NLAF is achieved by deploying a reversible and rapid (< 10 ps) non-thermal modulation of the dielectric function of a GST film.…”