Low-loss Phase Change Material Research Articles

On-chip reconfigurable diffractive optical neural network based on Sb2S3.

A Sb2S3-based reconfigurable diffractive optical neural network (RDONN) for on-chip integration is proposed. The RDONN can be integrated into standard silicon-on-insulator systems, offering a compact, passive, all-optical solution for implementing machine learning functions. The weights of the proposed optical chip are reconfigurable without the need to modify hardware structures or re-fabricate the chip. Its main structure consists of multilayer metalines made from Sb2S3, a low-loss phase change material. The RDONN architecture is constructed using the two-dimensional electromagnetic propagation model and implements the classification task on the Iris dataset with both intensity modulation and phase modulation inputs. This demonstrates its feasibility, with classification accuracies reaching 95.0% and 98.3%, respectively. Our model enables reconfigurable manipulation of the weights in the on-chip diffractive optical neural network, which can be used in the design and fabrication of real chips. This advancement holds significant promise for future all-optical in situ learning systems.

Ultrathin BIC Metasurfaces Based on Ultra-Low-Loss Sb2Se3 Phase-Change Material.

Ultrathin and low-loss phase-change materials (PCMs) are highly valued for their fast and effective phase transitions and applications in reconfigurable photonic chips, metasurfaces, optical modulators, sensors, photonic memories, and neuromorphic computing. However, conventional PCMs mostly suffer from high intrinsic losses in the near-infrared (NIR) region, limiting their potential for high quality factor (Q-factor) resonant metasurfaces. Here we present the design and fabrication of tunable bound states in the continuum (BIC) metasurfaces using the ultra-low-loss PCM Sb2Se3. Our BIC metasurfaces, only 25 nm thick, achieve high modulation depth and broad resonance tuning in the NIR with high Q-factors up to 130. Experimentally, we employ these BIC metasurfaces to modulate photoluminescence in rare earth-doped upconversion nanoparticles, reducing the excitation power for multiphoton photoluminescence and enabling emission polarization manipulation. This work offers a promising platform for developing active resonant metasurfaces, with broad applications including optical modulation, ultrafast switches, color filtering, and optical sensing.

Continuously-tunable, compact, freespace notch-filter design using an all-dielectric metagrating capped with a low-loss phase change material

Abstract Active metasurfaces utilizing phase change materials (PCMs) are currently under investigation for applications in free-space optical communication, optical signal processing, neuromorphic photonics, quantum photonics, and compact LiDAR. Attention has now turned towards novel PCM like Sb2S3 which exhibit lower optical absorption and reasonable values of refractive-index contrast in comparison to traditional data-storage PCM. We propose and numerically study the class of all-dielectric metagratings capped with low-loss PCM and predict the possibility of continuously tunable resonances whose quality factors degrade gracefully during the amorphous-to-crystalline phase transition of the PCM. Specifically, we consider the CMOS-compatible silicon-nitride on silica substrate material platform for simple and asymmetric metagratings (in particular, the symmetric-broken dimerization) and Sb2S3 capping. Our numerical study predicts that notch-filters operating around the 1550 nm NIR wavelength window can be achieved with tuning range of over 76 nm with Q-factors ranging from 784 (amorphous-phase) to 510 (crystalline-phase) (a degradation in Q of about 35%) and insertion loss of about 0.9 dB. These performance figures are a significant improvement over previously published designs utilizing data-storage PCMs and other traditional notch-filter mechanisms. We examine the influence of grating dimerization and geometrical parameters on performance metrics of the notch-filter and predicts the possibility to trade-off rejection-band and in-band spectral transmission properties. Lastly, we perform a study of all-optical phase change mechanism. Our study is promising for the miniaturization of tunable notch-filter based optical systems.

Electrically Reconfigurable Phase-Change Transmissive Metasurface.

Programmable and reconfigurable optics hold significant potential for transforming a broad spectrum of applications, spanning space explorations to biomedical imaging, gas sensing, and optical cloaking. The ability to adjust the optical properties of components like filters, lenses, and beam steering devices could result in dramatic reductions in size, weight, and power consumption in future optoelectronic devices. Among the potential candidates for reconfigurable optics, chalcogenide-based phase change materials (PCMs) offer great promise due to their non-volatile and analogue switching characteristics. Although PCM have found widespread use in electronic data storage, these memory devices are deeply sub-micron-sized. To incorporate phase change materials into free-space optical components, it is essential to scale them up to beyond several hundreds of microns while maintaining reliable switching characteristics. This study demonstrated a non-mechanical, non-volatile transmissive filter based on low-loss PCMs with a 200 × 200 µm2 switching area. The device/metafilter can be consistently switched between low- and high-transmission states using electrical pulses with a switching contrast ratio of 5.5 dB. The device wasreversibly switched for 1250 cycles before accelerated degradation took place. The work represents an important step toward realizing free-space reconfigurable optics based onPCMs.

Reconfigurable and polarization-dependent optical filtering for transflective full-color generation utilizing low-loss phase-change materials

All-dielectric metasurface, which features low optical absorptance and high resolution, is becoming a promising candidate for full-color generation. However, the optical response of current metamaterials is fixed and lacks active tuning. In this work, we demonstrate a reconfigurable and polarization-dependent active color generation technique by incorporating low-loss phase change materials (PCMs) and CaF2 all-dielectric substrate. Based on the strong Mie resonance effect and low optical absorption structure, a transflective, full-color with high color purity and gamut value is achieved. The spectrum can be dynamically manipulated by changing either the polarization of incident light or the PCM state. High transmittance and reflectance can be simultaneously achieved by using low-loss PCMs and substrate. The novel active metasurfaces can bring new inspiration in the areas of optical encryption, anti-counterfeiting, and display technologies.

Rewritable Photonic Integrated Circuit Canvas Based on Low-Loss Phase Change Material and Nanosecond Pulsed Lasers.

Programmable photonic integrated circuits (PICs) are an increasingly important platform in optical science and engineering. However, current programmable PICs are mostly formed through subtractive fabrication techniques, which limits the reconfigurability of the device and makes prototyping costly and time-consuming. A rewritable PIC architecture can circumvent these drawbacks, where PICs are repeatedly written and erased on a single PIC canvas. We demonstrate such a rewritable PIC platform by selective laser writing a layer of wide-band-gap phase change material (PCM) Sb2S3 with a low-cost benchtop setup. We show arbitrary patterning with resolution up to 300 nm and write dielectric assisted waveguides with a low optical loss of 0.0172 dB/μm. We envision that using this inexpensive benchtop platform thousands of PIC designs can be written, tested, and erased on the same chip without the need for lithography/etching tools or a nanofabrication facility, thus reducing manufacturing cost and increasing accessibility.

Sb2S3 as a low-loss phase-change material for mid-IR photonics

We study the mid-IR optical properties of Sb2S3 – a phase-change material (PCM) with high potential for mid-IR integrated photonics. We show that Sb2S3 exhibits a high refractive index contrast Δn ≈ 1 between its two phases and an extinction coefficient k below 10−2 at mid-IR wavelengths (up to ∼10 µm). We study the impact of 100 nm Sb2S3 cladding on SiGe-on-Si waveguides and experimentally show that it brings additional propagation loss below 1 dB/cm in the 3.3–3.9 µm wavelength range in both phases. These findings pave the way for new uses of Sb2S3 in mid-IR integrated photonics.

Inverse design of compact nonvolatile reconfigurable silicon photonic devices with phase-change materials.

In the development of silicon photonics, the continued downsizing of photonic integrated circuits will further increase the integration density, which augments the functionality of photonic chips. Compared with the traditional design method, inverse design presents a novel approach for achieving compact photonic devices. However, achieving compact, reconfigurable photonic devices with the inverse design that employs the traditional modulation method exemplified by the thermo-optic effect poses a significant challenge due to the weak modulation capability. Low-loss phase change materials (PCMs) exemplified by Sb2Se3 are a promising candidate for solving this problem benefiting from their high refractive index contrast. In this work, we first developed a robust inverse design method to realize reconfigurable silicon and phase-change materials hybrid photonic devices including mode converter and optical switch. The mode converter exhibits a broadband operation of >100 nm. The optical switch shows an extinction ratio of >25 dB and a multilevel switching of 41 (>5 bits) by simply changing the crystallinity of Sb2Se3. Here, we experimentally demonstrated a Sb2Se3/Si hybrid integrated optical switch for the first time, wherein routing can be switched by the phase transition of the whole Sb2Se3. Our work provides an effective solution for the design of photonic devices that is insensitive to fabrication errors, thereby paving the way for high integration density in future photonic chips.

Freeform direct-write and rewritable photonic integrated circuits in phase-change thin films.

Photonic integrated circuits (PICs) with rapid prototyping and reprogramming capabilities promise revolutionary impacts on a plethora of photonic technologies. We report direct-write and rewritable photonic circuits on a low-loss phase-change material (PCM) thin film. Complete end-to-end PICs are directly laser-written in one step without additional fabrication processes, and any part of the circuit can be erased and rewritten, facilitating rapid design modification. We demonstrate the versatility of this technique for diverse applications, including an optical interconnect fabric for reconfigurable networking, a photonic crossbar array for optical computing, and a tunable optical filter for optical signal processing. By combining the programmability of the direct laser writing technique with PCM, our technique unlocks opportunities for programmable photonic networking, computing, and signal processing. Moreover, the rewritable photonic circuits enable rapid prototyping and testing in a convenient and cost-efficient manner, eliminate the need for nanofabrication facilities, and thus promote the proliferation of photonics research and education to a broader community.

Arbitrary Programming of Racetrack Resonators Using Low-Loss Phase-Change Material Sb2Se3.

The programmable photonic integrated circuit (PIC) is an enabling technology behind optical interconnects and quantum information processing. Conventionally, the programmability of PICs is driven by the thermo-optic effect, free carrier dispersion, or mechanical tuning. These effects afford either high speed or a large extinction ratio, but all require constant power or bias to maintain the states, which is undesirable for programmability with infrequent switching. Recent progress in programmable PICs based on nonvolatile phase-change materials (PCMs) offers an attractive solution to a truly "set-and-forget" switch that requires zero static energy. Here, we report an essential building block of large-scale programmable PICs─a racetrack resonator with independent control of coupling and phase. We changed the resonance extinction ratio (ER) without perturbing the resonance wavelength, leveraging a programmable unit based on a directional coupler and a low-loss PCM Sb2Se3. The unit is only 33-μm-long and has an operating bandwidth over 50 nm, a low insertion loss (∼0.36 dB), high ER (∼15 dB), and excellent fabrication yield of over 1000 cycles endurance across nine switches. The work is a crucial step toward future large-scale energy-efficient programmable PICs.

Optical switching beyond a million cycles of low-loss phase change material Sb2Se3

The development of the next generation of optical phase change technologies for integrated photonic and free-space platforms relies on the availability of materials that can be switched repeatedly over large volumes and with low optical losses. In recent years, the antimony-based chalcogenide phase-change material Sb2Se3 has been identified as particularly promising for a number of applications owing to good optical transparency in the near-infrared part of the spectrum and a high refractive index close to silicon. The crystallization temperature of Sb2Se3 of around 460 K allows switching to be achieved at moderate energies using optical or electrical control signals while providing sufficient data retention time for non-volatile storage. Here, we investigate the parameter space for optical switching of films of Sb2Se3 for a range of film thicknesses relevant to optical applications. By identifying optimal switching conditions, we demonstrate endurance of up to 107 cycles at reversible switching rates of 20 kHz. Our work demonstrates that the combination of intrinsic film parameters with pumping conditions is particularly critical for achieving high endurance in optical phase change applications.

Nonvolatile phase change material based multifunctional silicon waveguide mode converters

Multimode photonics as a new research field of integrated photonics is closely relying on the waveguide mode, particularly for the higher-order waveguide mode. To efficiently generate higher-order waveguide modes in a single device, here we propose multifunctional silicon waveguide mode converters based on the nonvolatile and low-loss phase change material antimony triselenide (Sb2Se3). Such device is formed by two cascaded conversion regions along the silicon waveguide, where the first conversion region is formed by embedding a tapered Sb2Se3 layer into the silicon waveguide asymmetrically and the second one is formed by embedding two Sb2Se3 layers into the silicon waveguide symmetrically. When the embedded Sb2Se3 layers in two conversion regions undergo the phase transition between amorphous state and crystalline state, such device will achieve the mode conversions from input TE0 mode to output TE0, TE1, TE2, and TE1 + TE3 mode with mode conversion efficiency (CE) > 95%, mode crosstalk (CT) < −17 dB, and insertion loss (IL) < 0.4 dB at λ = 1.55 μm in a device length of only 10.3 μm where this multifunctional mode conversion in a single device has not been reported previously. To eliminate the output hybrid mode, we further revise the design and finally achieve the mode conversion from input TE0 mode to output TE0, TE1, TE2, and TE3 mode directly with a relatively good performance (CE > 85%, CT < −11 dB, and IL < 1 dB at λ = 1.55 μm). We believe the proposed multifunctional mode converters could be used as higher-order mode sources for the multimode photonics applications.

Wavelength-shift-free racetrack resonator hybrided with phase change material for photonic in-memory computing.

The photonic in-memory computing architecture based on phase change materials (PCMs) is increasingly attracting widespread attention due to its high computational efficiency and low power consumption. However, PCM-based microring resonator photonic computing devices face challenges in terms of resonant wavelength shift (RWS) for large-scale photonic network. Here, we propose a PCM-slot-based 1 × 2 racetrack resonator with free wavelength shift for in-memory computing. The low-loss PCMs such as Sb2Se3 and Sb2S3 are utilized to fill the waveguide slot of the resonator for the low insertion (IL) and high extinction ratio (ER). The Sb2Se3-slot-based racetrack resonator has an IL of 1.3 (0.1) dB and an ER of 35.5 (8.6) dB at the drop (through) port. The corresponding IL of 0.84 (0.27) dB and ER of 18.6 (10.11) dB are obtained for the Sb2S3-slot-based device. The change in optical transmittance of the two devices at the resonant wavelength is more than 80%. No shift of the resonance wavelength can be achieved upon phase change among the multi-level states. Moreover, the device exhibits a high degree of fabrication tolerance. The proposed device demonstrates ultra-low RWS, high transmittance-tuning range, and low IL, which provides a new scheme for realizing an energy-efficient and large-scale in-memory computing network.

Rewritable photonic integrated circuits using dielectric-assisted phase-change material waveguides.

Photonic integrated circuits (PICs) can drastically expand the capabilities of quantum and classical optical information science and engineering. PICs are commonly fabricated using selective material etching, a subtractive process. Thus, the chip's functionality cannot be substantially altered once fabricated. Here, we propose to exploit wide-bandgap non-volatile phase-change materials (PCMs) to create rewritable PICs. A PCM-based PIC can be written using a nanosecond pulsed laser without removing any material, akin to rewritable compact disks. The whole circuit can then be erased by heating, and a new circuit can be rewritten. We designed a dielectric-assisted PCM waveguide consisting of a thick dielectric layer on top of a thin layer of wide-bandgap PCMs Sb2S3 and Sb2Se3. The low-loss PCMs and our designed waveguides lead to negligible optical loss. Furthermore, we analyzed the spatiotemporal laser pulse shape to write the PICs. Our proposed platform will enable low-cost manufacturing and have a far-reaching impact on the rapid prototyping of PICs, validation of new designs, and photonic education.

On-chip beam steering through reprogrammable integrated coding metasurfaces

Coding metasurfaces have found many applications in free space optics, but have not been used for integrated photonic applications. This can partly be attributed to the lack of implements to usefully implement such a framework in practice. The advent of low-loss phase change materials such as Sb2S3 has changed this by enabling the implementation of reprogrammable integrated metasurfaces. This manuscript presents Sb2S3 based 1-bit and 2-bit coding metasurfaces that can potentially also be practically implemented. These metasurfaces can achieve functionalities such as beam focusing, beam splitting, and beam steering through the rearrangement of the material phase of Sb2S3. This can find potential applications in numerous fields including integrated photonic computational platforms.

On-Chip Reconfigurable Focusing through Low-Loss Phase Change Materials Based Metasurfaces.

Metasurfaces are useful subwavelength structures that can be engineered to achieve useful functionality. While most metasurfaces are passive devices, Phase Change Materials can be utilized to make active metasurfaces that can have numerous applications. One such application is on-chip beam steering which is of vital utility for numerous applications that can potentially lead to analog computations and non-Von Neumann computational architectures. This paper presents through numerical simulations, a novel metasurface that can realize beam steering through active phase switching of in-planted arrays of phase change material, Sb2S3. For the purpose of numerical demonstration of the principle, beam focusing has been realized, on-chip, through active switching of the Sb2S3 unit cell between the amorphous and crystalline phases. The presented architecture can realize on-chip transformation optics, mathematical operations, and information processing, thus opening the gates for future technologies.

Mid-Infrared Continuous Varifocal Metalens with Adjustable Intensity Based on Phase Change Materials

Metalenses can greatly reduce the complexity of imaging systems due to their small size and light weight and also provide a platform for the realization of multifunctional imaging devices. Achieving dynamic focus length tunability is highly important for metalens research. In this paper, based on single-crystal Ge and a new low-loss phase change material Ge2Sb2Se5 (GSSe), a tunable metalens formed by a double-layer metasurface composite was realized in the mid-infrared band. The first-layer metasurface formed by Ge nanopillars combines propagation and the geometric phase (equivalent to a half-wave plate function) to produce single- or multiple-polarization-dependent foci. The second-layer metasurface formed by GSSe nanopillars provides a tunable propagation phase, and the double-layer metalens can achieve the tunability of the focus length depending on the different crystalline fractions of GSSe. The focal length varies from 62.91 to 67.13 μm under right circularly polarized light incidence and from 33.84 to 36.66 μm under left circularly polarized light incidence. Despite the difference in the crystallographic fraction, the metalens’s focusing efficiency is maintained basically around 59% and 48% when zooming under RCP and LCP wave excitation. Meanwhile, the incident wave’s ellipticity can be changed to alter the relative intensity ratios of the bifocals from 0.03 to 4.26. This continuous varifocal metalens with adjustable intensity may have potential in practical applications such as optical tomography, multiple imaging, and systems of optical communication.

Numerical modeling of an integrated non-volatile reflector switch and mode converter switch based on a low loss phase change material (Sb2Se3) in SiN platforms

Programmable integrated photonics is an emerging research field due to its range of applications, from data processing to computing. Phase change materials (PCMs) on waveguides provide enormous flexibility for programmable integrated photonics. These materials show a large contrast in the optical properties (such as refractive index and optical loss) between the two stable states (i.e., amorphous to crystalline) of the PCM. These states are reversible and reproducible with an external stimuli which could be optical, thermal or electrical. Their non-volatile behavior allows PCMs to serve as an active layer for programmable photonics. In this work, we investigate hybrid device architectures utilizing the non-volatile properties of PCMs for integrated programmable photonics in a Si3N4 platform. FDTD modeling was carried out to design two configurations of non-volatile reconfigurable switches: (1) A non-volatile reconfigurable reflector switch consisting of a photonic crystal (PhC) slab waveguide and having a thin layer of Sb2Se3 on top of the waveguide and (2) 1×2 mode converter switch. In the case of a non-volatile reconfigurable reflector switch, the reflectivity of the waveguides is controlled by shifting the mode-gap of the photonic crystal slab waveguide by changing the phase of the PCM. It has applications as a reconfigurable reflective filter in the optical communication system and on-chip smart Bragg mirror. Non-volatile broadband directional switches provide a new paradigm for designing programmable multifunctional nanophotonics, which works in the same way as electronic field-programmable gate arrays. A 1×2 mode converter switch — based on Sb2Se3 as a clad layer on a coupling waveguide of a directional coupler — is proposed. The optical switches proposed in this paper offer very low optical insertion loss (∼ 0.5 dB), low coupling length (∼ 12 µm), broadband operation (∼ 80 nm), small cross talk (∼ 16 dB), and zero static power consumption.

Magnetron-sputtered and thermal-evaporated low-loss Sb-Se phase-change films in non-volatile integrated photonics

Chalcogenide phase change materials (PCMs), featuring a large contrast in optical properties between their non-volatile amorphous and crystalline states, have triggered a surge of interest for their applications in ultra-compact photonic integrated circuits with long-term near-zero power consumption. Over the past decade, however, PCM-integrated photonic devices and networks suffered from the huge optical loss of various commonly-used PCMs themselves. In this paper, we focused on the deposition, characterization, and monolithic integration of an emerging low-loss phase change material, Sb2Se3 on a silicon photonic platform. The refractive index contrast between the amorphous and crystalline phase of the evaporated Sb-Se thin film was optimized up to 0.823 while the extinction coefficient remains less than 10−5 measured by ellipsometry. When integrated on a silicon waveguide, the propagation loss introduced by the amorphous thin film is negligibly low. After crystallization, the propagation loss of a magnetron-sputtered Sb-Se patch-covered silicon waveguide is as low as 0.019 dB/µm, while its thermal-evaporated counterpart is below 0.036 dB/µm.

Wide-Angle Tunable Critical Coupling in Nanophotonic Optical Coatings with Low-Loss Phase Change Material.

Realizing perfect light absorption in stacked thin films of dielectrics and metals through critical light coupling has recently received intensive research attention. In addition, realizing ultra-thin perfect absorber and tunable perfect absorber in the visible spectrum is essential for novel optoelectronics applications. However, the existing thin film stacks cannot show tunable perfect absorption in a wide-angle range. Here, a tunable perfect absorption from normal incidence to a wide-angle range (0° to 70°) by utilizing a two-layer stack consisting of a high refractive index low-loss dielectric on a high reflecting metal is proposed. This is experimentally demonstrated by depositing a thin layer of low-loss phase change material such as stibnite (Sb2 S3 ) on a thin layer of silver. This structure shows tunable perfect absorption with large spectral tunability in the visible wavelength. Furthermore, the absorption enhancement in 2D materials by transferring monolayer molybdenum disulfide on the stack, which shows 96% light absorption with enhanced photoluminescence, is demonstrated. In addition, the thin film stack can work as a scalable phase modulator offering a maximum phase tunability of ≈140° by changing the structural state of Sb2 S3 from amorphous to crystalline.