Optical phase change materials (O-PCMs), a unique group of materials featuring exceptional optical property contrast upon a solid-state phase transition, have found widespread adoption in photonic applications such as switches, routers and reconfigurable meta-optics. Current O-PCMs, such as Ge–Sb–Te (GST), exhibit large contrast of both refractive index (Δn) and optical loss (Δk), simultaneously. The coupling of both optical properties fundamentally limits the performance of many applications. Here we introduce a new class of O-PCMs based on Ge–Sb–Se–Te (GSST) which breaks this traditional coupling. The optimized alloy, Ge2Sb2Se4Te1, combines broadband transparency (1–18.5 μm), large optical contrast (Δn = 2.0), and significantly improved glass forming ability, enabling an entirely new range of infrared and thermal photonic devices. We further demonstrate nonvolatile integrated optical switches with record low loss and large contrast ratio and an electrically-addressed spatial light modulator pixel, thereby validating its promise as a material for scalable nonvolatile photonics. Here, the authors introduce optical phase change materials based on Ge-Sb-Se-Te which breaks the coupling between refractive index and optical loss allowing low-loss performance benefits. They demonstrate low losses in nonvolatile photonic circuits and electrical pixelated switching have been demonstrated.

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Broadband transparent optical phase change materials for high-performance nonvolatile photonics.

Nanophotonics has garnered intensive attention due to its unique capabilities in molding the flow of light in the subwavelength regime. Metasurfaces (MSs) and photonic integrated circuits (PICs) enable the realization of mass-producible, cost-effective, and highly efficient flat optical components for imaging, sensing, and communications. In order to enable nanophotonics with multi-purpose functionalities, chalcogenide phase-change materials (PCMs) have been introduced as a promising platform for tunable and reconfigurable nanophotonic frameworks. Integration of non-volatile chalcogenide PCMs with unique properties such as drastic optical contrasts, fast switching speeds, and long-term stability grants substantial reconfiguration to the more conventional static nanophotonic platforms. In this review, we discuss state-of-the-art developments as well as emerging trends in tunable MSs and PICs using chalcogenide PCMs. We outline the unique material properties, structural transformation, electro-optic, and thermo-optic effects of well-established classes of chalcogenide PCMs. The emerging deep learning-based approaches for the optimization of reconfigurable MSs and the analysis of light-matter interactions are also discussed. The review is concluded by discussing existing challenges in the realization of adjustable nanophotonics and a perspective on the possible developments in this promising area.

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Tunable nanophotonics enabled by chalcogenide phase-change materials

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.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/adfm.202002447

A new family of ultralow loss reversible phase‐change materials for photonic integrated circuits: Sb 2 S 3 and Sb 2 Se 3

Ultrafast all-optical switching, possessing the unique function of light controlling light, is an essential component of on-chip ultrafast optical connection networks as well as integrated logic computing chips. Ultrafast all-optical switching has attracted enormous research interests, the latest great developments of which have also yield progress in nanophotonics, integrated optics, nonlinear optics, material science, and optical communications, and so on. This review summarizes the fundamental realization principles, novel configurations, fancy materials, improved performance indexes, and ameliorated trigger method (transitioning from a traditional impractical free-space-vertical trigger to a more practical on-chip trigger) of ultrafast all-optical switching. Not only a systematic discussion of the current state-of-the art is provided, but also a brief outlook on the remaining challenges in the pursuit of the application of a practical on-chip ultrafast all-optical switching is also afforded.

Ultrafast All-Optical Switching

Reconfiguration of silicon photonic integrated circuits relying on the weak, volatile thermo-optic or electro-optic effect of silicon usually suffers from a large footprint and energy consumption. Here, integrating a phase-change material, Ge2Sb2Te5 (GST) with silicon microring resonators, we demonstrate an energy-efficient, compact, non-volatile, reprogrammable platform. By adjusting the energy and number of free-space laser pulses applied to the GST, we characterize the strong broadband attenuation and optical phase modulation effects of the platform, and perform quasi-continuous tuning enabled by thermo-optically-induced phase changes. As a result, a non-volatile optical switch with a high extinction ratio, as large as 33 dB, is demonstrated.

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GST-on-silicon hybrid nanophotonic integrated circuits: a non-volatile quasi-continuously reprogrammable platform

We report a multi-mode interference-based optical gate switch using a Ge2Sb2Te5 thin film with a diameter of only 1 µm. The switching operation was demonstrated by laser pulse irradiation. This switch had a very wide operating wavelength range of 100 nm at around 1575 nm, with an average extinction ratio of 12.6 dB. Repetitive switching over 2,000 irradiation cycles was also successfully demonstrated. In addition, self-holding characteristics were confirmed by observing the dynamic responses, and the rise and fall times were 130 ns and 400 ns, respectively.

Ultra-small, self-holding, optical gate switch using Ge 2 Sb 2 Te 5 with a multi-mode Si waveguide

1000-times stable switching operation of an optical gate incorporating a Ge 2 Sb 2 Te 5 thin film with Si wire waveguides is reported. The phase of the Ge 2 Sb 2 Te 5 was reversibly changed from the amorphous state to the crystalline state by the application of pulsed laser irradiation. The extinction ratio was 9.7 dB on average, and did not decline throughout the entire sequence of 1000 switching events induced by irradiation. The wavelengths of the signal light and of the laser pulses were 1550 nm and 660 nm, respectively.

Demonstration of 1000-times switching of phase-change optical gate with Si wire waveguides

Fast and low power consumption optical switches are required for photonic networks. To this end, we proposed the
optical gate switch using phase change material (PCM) and silicon waveguides. This switch had low power consumption
because it consumed power only when the state was changed. Furthermore, the chip size is very small due to the large
refractive index change of PCM.
In this paper, we studied the phase-change characteristics of various kinds of thin GST film on SOI (silicon-oninsulator).
A laser diode (LD) with a wavelength of 660 nm was used to irradiate the material. We compared the optical
responses by laser pulse irradiation on GST films, and concluded GST-147 was the most suitable material for the optical
switch because it had the lowest phase change threshold.
The phase-change characteristics of GST-225 films with thickness of 25 nm, 50 nm and 75 nm were also examined.
Thicker GST films had lower phase change thresholds. However, thermal simulations showed that the phase of the
bottom part of thicker films may not be changed. Therefore, we concluded that GST films with thicknesses between 25
nm to 50 nm are the most suitable for optical switches.

Phase change characteristics of Ge2Sb2Te5 thin film for a self-holding optical gate switch

PROBLEM TO BE SOLVED: To reliably perform amorphization of a phase change material portion in a short time in a waveguide optical gate switch by enhancing the heat radiation efficiency of the phase change material portion. SOLUTION: The phase change material portion is provided in an optical waveguide composed of a single crystal core layer and a clad layer surrounding the single crystal core layer, the phase change material portion changing the transmission quantity of light propagating in the optical waveguide by changing complex refractive index. The phase change material portion is formed in a multilayer structure or in a film thickness of 100 nm or less. COPYRIGHT: (C)2011,JPO&INPIT

Waveguide optical gate switch

Optical gate switch that uses phase change material with self-holding characteristics was proposed and fabricated. The length of the switch was only 1 µm and the switching time was less than 100 ns.

Tatsuya Toyosaki

Papers

Ultra-small, self-holding, optical gate switch using Ge 2 Sb 2 Te 5 with a multi-mode Si waveguide

Demonstration of 1000-times switching of phase-change optical gate with Si wire waveguides

Phase change characteristics of Ge2Sb2Te5 thin film for a self-holding optical gate switch

Waveguide optical gate switch

Small-sized self-holding optical switch using phase-change material