Low-power 7-bit hybrid volatile / nonvolatile tuning of phase shifters

Abstract

Systems and methods for low-power 7-bit hybrid volatile / nonvolatile tuning of phase shifters are described. In one embodiment, an optical programmable unit includes a phase change material (PCM) element, and a microheater configured proximate to the PCM element. The microheater includes doped silicon. The microheater is configured for changing phase states of the PCM element based on electrical excitation applied to the microheater, where optical properties of the PCM element are changed in proportion to the electrical excitation applied to the microheater. The optical programmable unit also includes a photonic component configured to direct light to the PCM element. The optical properties of the PCM element modulate optical properties of the photonic component.

Description

[0001]LOW-POWER 7-BIT HYBRID VOLATILE / NONVOLATILE TUNING OF PHASE SHIFTERS CROSS-REFERENCE TO RELATED APPLICATION This application claims the benefit of U.S. Provisional Application No.63 / 695579, filed September 17, 2024, the entire disclosures of which are hereby incorporated by reference. STATEMENT OF GOVERNMENT LICENSE RIGHTS This invention was made with government support under Grant No. DARPA-YFA Award, awarded by the Defense Advanced Research Projects Agency (DARPA) and Grant Nos. NSF-1640986, NSF-2003509 and NSF-2003509, awarded by the National Science Foundation (NSF) and Grant No. ONR-YIP Award, awarded by the Office of Naval Research (ONR). The government has certain rights in the invention. BACKGROUND Programmable photonic integrated circuits play an increasingly important role to enable high-bandwidth optical interconnects and large-scale in-memory computing as needed to support the rise of artificial intelligence and machine learning technology. To that end, chalcogenide-based non-volatile phase-change materials (PCMs) present a promising solution due to zero static power. However, high switching voltage and small number of operating levels present serious roadblocks to widespread adoption of PCM- programmable units. Accordingly, systems and methods for improved programmable photonic integrated circuits are still needed. SUMMARY This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features of the claimed subject matter. To support the rising demands of compute for artificial intelligence (AI), improvements are needed in both computing and communication systems. On one hand, a large amount of data needs to move between electronic computers and servers. Optical interconnects present a promising solution thanks to their much lower latency and energy 3915-P1368WO.UW -1- consumption compared to the traditional copper-based interconnects. In fact, optical circuit switching systems are already being deployed in data-centers. These switches may be slow for some purposes (e.g., micro electro-mechanical switches), but they need to provide very large change in optical properties, with ultra-low loss. On the other hand, the growing demand for vector-matrix multiplication poses a serious challenge for traditional computing architectures, including graphical processing units. This has led to exploration of optical vector-matrix multiplication, which requires in-memory computing. Both applications, optical circuit switching and vector-matrix multiplication, require ultra- compact, low-loss programmable photonic units, with high endurance, compatibility with electronic integrated circuit (i.e., where the actuation electrical power can be provided by on-chip circuit) and high bit-precision. While large-scale photonic integrated circuits (PICs) have now become available, thanks to sophisticated fabrication technique, a truly programmable photonic circuits with the aforementioned properties remain missing. In practice, most of the existing reconfiguration methods in integrated photonics are volatile (thermo-optic, free-carrier dispersion and Pockels effect), and are essentially optimized for high-speed switching. However, both optical circuit switching and vector-matrix multiplication may benefit from programmable photonic units, which are changed infrequently, but, advantageously, have zero or very low static power. Chalcogenide phase-change materials (PCMs) offer a promising solution to more compact device size and zero static power owing to their strong optical index modulation( (1)) and zero static power consumption. PCMs possess two stable, reversiblyswitchable micro-structural phases namely amorphous phase (a-phase) and crystallinephase (c-phase), with significantly different optical refractive indices ( 1). Due to theirnon-volatile phase transition under the ambient environment, PCMs do not require static power to maintain their state once switched. Without being bound to theory, it is believed that for a given non-volatile state the orientation of the molecules remains fixed, thus beingnon-volatile in absence of additional voltage pulses. The large refractive index change ( )and non-volatility of the PCMs allow for the creation of compact reconfigurable devices with zero static energy consumption. Furthermore, PCMs are suitable for large-scale integration as they can be deposited by sputtering or thermal evaporation onto various integrated photonics material platforms, including silicon and silicon nitride. Traditional PCMs like Ge Sb Te (GST) and GeTe show strong optical absorption, while emerging 3915-P1368WO.UW -2- wide-bandgap PCMs, such as Ge Sb SeTe (GSST), Sb S , and Sb Se , may reduce the absorption loss. These wide bandgap PCMs have garnered significant interest due to potential for both large-area endurable switching and repeatable multilevel operation. Notably, Sb S has the widest bandgap among these emerging PCMs, providing transparency down to 600 nm in the amorphous phase. Earlier experiments with Sb S show low-loss (<1 dB), endurable (>1000 cycles) and 5-bit operation by tuning PCM Sb S and also its potential in high-volume manufacturing in 300-mm wafers. However, the switching energy of the devices remained high. Moreover, due to inherent stochastic switching behavior of the PCMs, the number of operation levels may be limited. With the inventive technology, we demonstrate both low-energy switching of PCMSb S and a large number of intermediate levels at optical telecommunicationwavelengths (C-band, 1500 nm –1560 nm). In some embodiments, the low switching energy is achieved by optimizing the p++-i-n++ (PIN) microheater geometry. Both numerical and experimental results show that the switching energy monotonically reduces with the decrease of the intrinsic region width of the PIN microheater, albeit at the cost ofa lower quality ( ) factor. An intrinsic region width of 0.9 may be selected in someembodiments to achieve a good tradeoff, showing low switching voltage less than 3 V (25.7 mA current and 500 ns pulse duration, 35.33 nJ) and 1.6 V ( 15 mA current and 20ms pulse duration, 0.48 mJ energy) for amorphization and crystallization respectively. This voltage and current can be conveniently provided by CMOS electronic integrated circuits. With the inventive technology, such low power operation does not degrade the cyclability by demonstrating >10000 switching events in total on a single device. Finally, the inventive technology demonstrates 7-bit operation (127 levels) using hybrid thermo-optic / PCM tuning, combining a coarse 5 bit (22 levels) non-volatile tuning by PCM changes with volatile finer tuning by thermo-optic. The CMOS compatible voltage / energy operation coupled with 7-bit operation marks important steps towards PCM-based large-scale PIC systems. Based on the above-described PCMs, we demonstrate electrically programmablewide bandgap Sb S -clad silicon ring resonator using silicon microheater at CMOScompatible voltage of < 3V. In some embodiments, the inventive device shows low switching energy of 35.33 nJ (0.48 mJ) for amorphization (crystallization) and reversible phase transitions with high endurance (> 2000 switching events) near 1550 nm. Combining volatile thermo-optic effect based on precisely controlled temperature of the PCM element 3915-P1368WO.UW -3- with non-volatile PCM crystallization / amorphization changes, the inventive technology demonstrates 7-bit (127 levels) operation with excellent repeatability and reduced power consumption. In many embodiments, this low-voltage and low-energy operation, combined with the hybrid volatile-nonvolatile approach, marks a significant step towards integrating PCM-based programmable units into large-scale optical interconnects. In one embodiment, an optical programmable unit includes: a phase change material (PCM) element; and a microheater configured proximate to the PCM element. The microheater includes doped silicon. The microheater is configured for changing phase states of the PCM element based on an electrical excitation applied to the microheater. Optical properties of the PCM element are changed in proportion to the electrical excitation applied to the microheater. The optical programmable unit also includes a photonic component configured to direct light to the PCM element, where the optical properties of the PCM element modulate optical properties of the photonic component. In one aspect, the photonic component is a waveguide. In one aspect, the photonic component is configured to operate as a resonator. In one aspect, the photonic component is configured for passing light through the PCM element. In one aspect, the microheater includes: a p++ doped region; a n++ doped region; and an intrinsic region without doping configured between the p++ doped region and the n++ doped region, where the PCM element is configured on the intrinsic region. In one aspect, the PCM element is chalcogenide based PCM element. In one aspect, the PCM element is configured for changing its state by a transition voltage that is lower than 3 V. In one aspect, the PCM element possesses a first plurality of optically non-volatile states and a second plurality of optically volatile states. In one aspect, the PCM element is placed from one non-volatile state to another by the electrical excitation applied to the microheater. In one aspect, the PCM element, once in a non-volatile state, remains in the non- volatile state until a next electrical excitation is applied to the microheater. In one aspect, the electrical excitation comprises a combination of 2 or more of: 3915-P1368WO.UW -4- electrical voltage; electrical current; electrical pulse shape; and electrical pulse duration. In one aspect, the PCM element is placed from one volatile state to another by a DC voltage applied to the microheater, and the DC voltage is configured to generate a target temperature of the PCM element. In one aspect, upon removal of the DC voltage, the PCM element is configured to revert to its prior volatile state. In one aspect, the optical programmable unit is tunable into at least 7-bit different non-volatile states. In one aspect, the optical programmable unit is tunable into a total of at least 127 non-volatile and volatile states in combination. In one aspect, the optical programmable unit further includes: an input grating coupler for incoming light; and an output grating coupler for outgoing light. In one aspect, the optical programmable unit further includes a Mach Zehnder interferometer. In one aspect, the photonic component is a micro ring resonator. In one embodiment, a method for programing an optical programmable unit includes: applying an electrical excitation to a microheater, where the microheater comprises doped silicon; and changing phase states of a phase change material (PCM) element by the microheater, where optical properties of the PCM element are changed in proportion to the electrical excitation applied to the microheater. The PCM element, once having achieved its optical property, remains in a non-volatile state with respect to its optical property till a next electrical excitation is applied to the microheater. DESCRIPTION OF THE DRAWINGS The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 3915-P1368WO.UW -5- FIGURE 1 is a partially schematic isometric view of a high Q ring resonator in accordance with an embodiment of the present technology; FIGURE 2 is a 2D schematic of the PCM microheater design in accordance with an embodiment of the present technology; FIGURE 3 is a scanning electron microscope (SEM) image of Sb S loadedmicroheater in accordance with an embodiment of the present technology; FIGURE 4 is a schematics of an optical system 1000 that includes an optical microscope image of the micro-ring resonator 100 in accordance with an embodiment of the present technology; FIGURE 5 is a graph of switching energy and Q factor for a resonator that switches from crystalline to amorphous state in accordance with an embodiment of the present technology; FIGURE 6 is a graph of switching energy and Q factor for a resonator that switches from amorphous to crystalline state in accordance with an embodiment of the present technology; FIGURE 7 is a graph of measured endurance for a resonator in accordance with an embodiment of the present technology; FIGURE 8 is a graph of light transmission for a resonator in accordance with embodiments of the present technology; FIGURE 9 is a graph of nonvolatile phase shifts for a resonator in accordance with embodiments of the present technology; and FIGURE 10 is a graph of nonvolatile and volatile phase shifts for a resonator in accordance with embodiments of the present technology. DETAILED DESCRIPTION While illustrative embodiments have been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention. FIG.1 is a partially schematic isometric view of a high Q ring resonator 100 in accordance with an embodiment of the present technology. Illustrated ring resonator 100 includes a phase change material (PCM) element 30 that can change its phase from the crystalline phase to an amorphous phase or from the amorphous phase to the crystalline 3915-P1368WO.UW -6- phase based on temperature and / or voltage pulse that the PCM element 30 is submitted to. In some embodiments, the PCM element 30 can be made an integral element of a ring resonator of 100 or another optical assembly (e.g., an optical switch, optical filter, etc.) by, for example, being in the optical path of such ring resonator. Since optical properties of the PCM can be changed (tuned), for example by tuning its refractive index "n" under different temperature / voltage conditions, the overall performance of the ring resonator or other optical assembly can also be tuned by bringing the PCM element 30 to its target crystalline / amorphous state and / or temperature. That is, in many embodiments, PCM element 30 possesses two stable, reversibly switchable micro-structural phases namely amorphous phase (a-phase) and crystalline phase (c-phase), with significantly different optical refractive indices ( 1). Due to its non-volatile phase transition under theambient environment, PCM element 30 does not require static power to maintain its state once switched. Without being bound to theory, it is believed that for a given non-volatile state the orientation of the molecules remains fixed, thus being non-volatile in absence of additional voltage pulses. Such relatively large refractive index change ( ) and non-volatility allow for the creation of compact reconfigurable devices with zero static energy consumption. The PCM element 30 may be disposed over or within a semiconductor ring 26, which can comprise a transparent semiconductor material, for example, silicon (Si). In some embodiments, the entire resonator 100 may be manufactured on a semiconductor For example, a silicon micro-ring resonator 100 (also referred to as a photonic component) may be cladded with 10- m-long, 0.45- m-width and 20-nm-thick layer of Sb S PCM. The ring resonator 100 can be fabricated on a standard silicon-on-insulator (SOI) wafer with a 220 nm silicon layer and a 2 buried oxide layer. In some embodiments, the ring resonators have a bus-ring gap of 270 nm to achieve a near-critical coupling condition. The 500 nm wide waveguides can be created by partially etching 120 nm of silicon. Next, PCM material is deposited as 450 nm wide Sb S stripes onto the SOI chip via sputtering and liftoff. The 50 nm smaller width compared to the waveguide compensates for the electron beam lithography (EBL) overlay tolerance and error from liftoff. The PCM elements 30 that are Sb S stripes can be encapsulated with 40 nm of thermal Al O grown by atomic layer deposition (ALD) at 150°C. This conformal encapsulation may be useful for preventing Sb S 3915-P1368WO.UW -7- from oxidation and thermal reflowing, which improves endurance of the PCM. In some embodiments, the photonic component 100 may be a waveguide. Temperature control of the PCM element 30 can be accomplished by operation of a microheater that includes a first electrode 12 (also referred to as a source or a positive voltage electrode) and a second electrode 14 (also referred to as ground or a negative voltage electrode). Operation of the micro heater is explained below with reference to FIG.2. FIG.2 is a 2D schematic of the PCM microheater design in accordance with an embodiment of the present technology. Illustrated microheater 50 may also be manufactured at a semiconductor scale using semiconductor manufacturing equipment and processes. Such microheaters 50 may be on-chip silicon PIN microheaters, with the intrinsic region width in a range of 0.9 to 1.9 m. In some embodiments, the first electrode 12 and the second electrode 14 are made of palladium (Pd) or other suitable conductor. The first electrode 12 may be deposited over a p++ doped semiconductor 22, and the second electrode 14 may be deposited over a n++ doped semiconductor 24. When the first electrode 12 and the second electrode 14 are subjected to a voltage pulse 40, electrical charges flow between the doped semiconductors 22 and 24, thus causing the semiconductor ring 26 to heat in a controlled manner. As a result, different voltage pulses 40 cause different predetermined temperatures, in turn causing crystalline or amorphous states of the PCM element 30. Due to a small scale (i.e., a semiconductor scale) of the PCM element 30, the target phase states of the PCM element can be achieved within a relatively short time and with high precision. Also, as further explained below, the microheater 50 is suitable for affecting both the non-volatile and volatile states of the PCM element 30. FIG. 3 is a scanning electron microscope (SEM) image of Sb S loadedmicroheater 50 in accordance with an embodiment of the present technology. Hatching is added to the image for better understanding of particular elements of the microheater. In operation, electrical current flows between the p++ doped semiconductor 22 and n++ doped semiconductor 24 through the semiconductor material 26 (e.g., undoped silicon), therefore thermo-optically and phase-change tuning the PCM element 30. FIG.4 is a schematics of an optical system 1000 that includes an optical microscope image of the micro-ring resonator 100 in accordance with an embodiment of the present technology. As explained above, the illustrated micro-ring resonator 100 (or 3915-P1368WO.UW -8- a photonic component in a more general sense) may be manufactured at a semiconductor scale, therefore being suitable for low-energy and fast tuning of its operating wavelength. In operation, the micro-ring resonator 100 may function as an optical switch or a resonator that is a part of a larger optical system 1000. In some embodiments, a source of light 15 (also referred to as a transmitter) emits light that enters an input grating coupler 16. The source of light 15 may be a tunable laser or a tunable light emitting diode (LED). Light that exits the input grating coupler 16 becomes an input signal for the illustrated micro-ring resonator 100. The PCM element 30 is characterized by the refractive index (n) coefficients that is different for different wavelengths of light. As further explained below, the refractive index of the PCM element 30 can be tuned through thermo-optic / PCM tuning by the microheater 50. Depending on the thermo-optic / PCM tuning of the PCM element 30, the wavelength of the input light that matches the preferred (resonant) wavelength of the resonator 100 as the matched light is optically coupled with the resonator. Conversely, the input light with a wavelength that is outside of the preferred wavelength will not optically couple with the resonator. These different matching / not-matching states can be sensed by a light sensor 18 (also referred to as a receiver) that receives the output light through an output grating coupler 17. In some embodiments, the light sensor 18 can be an optical diode or a Mach Zehnder interferometer that is sensitive to the wavelengths of interest. In some embodiments, a controller 90 can control operation of the optical system 1000, like for example operation of the microheater 50, tunable source of light 15, light sensor 18, etc. In different embodiments, the controller 90 can be a digital computer, a digital controller circuit, an analog controller, etc. FIG.5 is a graph of switching energy and quality (Q) factor for a resonator that switches from crystalline to amorphous state in accordance with an embodiment of the present technology. FIG.6 is a graph of switching energy and Q factor for a resonator that switches from amorphous to crystalline state in accordance with an embodiment of the present technology. The material of the sample PCM element is Sb2S3, but other materials are possible in different embodiments. The horizontal axis shows the width of the hand vertical axis shows the switching energy in nJ (FIG. 5) and mJ (FIG. 6) from one state to another, that is, the switching energy from crystalline state to amorphous state in FIG.5, and the switching energy from amorphous to crystalline state in FIG.6. The right hand vertical axis shows the quality factor of the 3915-P1368WO.UW -9- resonator. The illustrated changes take place over the width of the PCM element 30. In general, the switching energy increases with the increased width and the accompanying increase of the thermal mass of the PCM element. The heat transfer simulations were performed in COMSOL Multiphysics, where the intrinsic region width ( ) of the PIN microheater is varied to optimize the switchingenergy. The COMSOL simulation couples semiconductor and heat transfer modules, and therefore can capture physics of the device. The solid lines in FIGs. 5 and 6 show the simulation results for different thickness of the PCM element 30. We can observe a monotonic reduction of the switching voltage and energy with the decrease of . The increase of the amorphization voltage from 3 V at 0.9 m to 5V at 1.9 m shows the importance of careful design of the microheater geometry. The monotonic trends may be attributed to two factors: (1) the increase in increases the PIN diode resistance and reduces the current flow; (2) the overall heating volume of the PIN microheater increases, thus requiring more energy. Furthermore, the simulation shows that at = 0.9 m,pulses with 3 V amplitude (13.53 mA current) and 500 ns duration are already enough to amorphize the material. The Sb S is heated above its melting temperature (801 ±18 K), after which it is quenched below the glass transition temperature of 573 K in 200ns (quench rate 2 K / ns). Next, we experimentally validate our simulation results (the measurement results are shown by the circles in FIGs.5 and 6). The trend observed in the experiments agrees qualitatively well with the simulation. Experimentally, we have measured the switching conditions for partial amorphization for a fixed resonance wavelength shift of 0.12 nm (i.e, =0.12 nm) and not the maximum possible . This decision was made to ensure that the device is not damaged while finding the amorphization and crystallization conditions and all the devices with varying parameters can be measured. However, we should note that complete amorphization of Sb S does require higher switchingvoltage and energy. For p++ - i - n++ (PIN) microheater with = 0.9 m, a completeamorphization of Sb S may requires higher voltage and energy by 10% and 47% respectively. The switching energy requirement can be calculated as switching voltage multiplied by the estimated current from measured IV curve multiplied by applied pulse duration. Furthermore, the Q-factor decreases with the decrease in intrinsic region width and constitutes the design trade-off. As shown in FIGs. 5 and 6 the energy requirement 3915-P1368WO.UW -10-is higher by 15% ( 17%) for amorphization (crystallization) experimentally ascompared to switching conditions computed for the center of PCM via COMSOL simulation. The observed higher energy requirement can be attributed to four factors: (1) in simulation we have considered simplified 2D geometry to calculate the phase change requirement for the centre of PCM element but the actual geometry can be more complex, leading to non-uniform heat distribution and potentially higher energy requirements; (2) the actual kinetics of the phase-change process, including nucleation and growth rates, may be different in the experiment compared to the idealized rates assumed in simulations, requiring more energy to achieve a full phase transition in the experiment; (3) the interfaces between the PCM and surrounding materials can introduce additional thermal resistance or electrical contact resistance in reality as compared to the ones captured in simulation, leading to higher energy requirements in experiments; (4) in reality there can be material inhomogeneity in terms of crystalline structure, or defect density, which can lead to different thermal and electrical properties compared to the idealized conditions used in simulations. FIG.7 is a graph of measured endurance for a resonator in accordance with an embodiment of the present technology. The horizontal axis shows the number of switching events. For the illustrated experiment, 2000 switching events were observed. (1000 switching cycles) with a pulsing rate of 10 Hz. The illustrated endurance test was run by alternatively switching the device between the crystalline and amorphous state of the PCM element. The laser was tuned at about 1546.25 nm, which is on resonance with the micro-ring cavity for amorphous Sb S . N performance degradation was observed at the end of 2000 switching events, and the experiment was stopped only because of the long duration of the experiment. We noticed a sinusoidal modulating pattern, which can be attributed to the self-heating effect of the micro-ring resonator. To verify that this modulation is not related to PCM modulation, we repeated the cyclability test at a lower pulsing rate of 1 Hz. The frequency of the modulation remained the same, indicating that the modulation is not related to electrical pulses nor the PCMs. Looking into deeper details of the constant oscillation, in a doped silicon micro- ring resonator, light is absorbed due to free-carrier absorption or two photon absorption, producing extra heat. Such self-heating effect changes the effective refractive index of the ring by the thermo-optic effect, causing a shift of the ring resonance harnessing. During the cyclability test, the laser was initially tuned on resonance, leading to a strong 3915-P1368WO.UW -11- light field in the ring. Due to the self-heating effect, the resonance is gradually shifted until the laser becomes completely off-resonant when light inside the resonator becomes too weak to heat up the cavity. The resonator then gradually cools itself down to the room temperature, causing the laser on-resonance again and repeating the previous process. Such cyclic phenomenon results in the sinusoidal pattern seen in the endurance test in FIG.7. We note that this undesired effect can be weakened by reducing the laser power or by setting the laser slightly off-resonance, as demonstrated in other measurements. FIG.8 is a graph of light transmission for a resonator in accordance with embodiments of the present technology. The thickness of the PCM element (wi) is 0.9 from 1545.5 nm to 1550 nm. A resonance shift of 0.25 nm is obtained upon switching the Sb S from the amorphous phase to the crystalline phase. Considering the free spectral range of about 2.42 nm, this resonance shift amounts to 0.2 optical phase shift per round trip.As explained above, the crystalline-amorphous phase change is controllable by the microheater 50. In some embodiments, the observed condition for complete amorphization (RESET pulse) of Sb S is 500 ns pulse width at 2.75V whichcorresponds to a current of 25.7 mA and switching energy of 35.33 nJ. The crystallization condition (SET pulse) is achieved by a 20 ms pulse at 1.6 V corresponding to a current of 15 mA and switching energy of 0.48 mJ. The SET and RESET processes were repeated for 5 cycles and the slight variation is shown as standard deviation among the cycles, showing excellent repeatability for the binary representation. The applied amorphization pulse produces relatively high temperature which further activated the doped silicon. We observed that the condition became stable after around first ten pulses. It is also noted that device damage happens much more often during crystallization than amorphization due to the long pulse duration and low heater resistance. Even slight increase of the voltage can lead to significant increase in temperature, which may melt the silicon waveguides. The lower graph in FIG. 8 shows 22 individual transmission spectra corresponding to each resonant wavelength shift due to PCM tuning at different amorphization / crystallization levels within the 0.25 nm wavelength bend. These distinct amorphization / crystallization levels are also shown in FIG. 9 together with the accompanying phase shifts associated with the levels. As further explained with respect to FIG.10, the inherent stochasticity of the PCM switching limits the achievable number 3915-P1368WO.UW -12- of levels, which can be compensated for by adding thermo-optic fine tuning on top of the PCM coarse tuning. In such combined configuration, the PIN diode heaters are used for both volatile and non-volatile tuning. With this hybrid tuning approach, we demonstrate 7-bit multilevel operation (i.e., up to 127 levels). In some embodiments, we start the experiment by applying a 500 ns pulse at 2.45V to partially amorphized the crystalline Sb S device to demonstrate multiple levels. In some embodiments, after one partial amorphization pulse, the resonant wavelength shifts to the left by 0.01 nm (i.e. = 0.01 nm) thus corresponding to an intermediate PCM level. The resonance shifts due to PCM tuning enlarge as we increase the amorphization voltage from 2.45 to 2.65V, providing resonance shift of = 0.01 nm in each step and 22 operation levels. The partial amorphization pulse changes the Sb S optical phase in a coarse, however nonvolatile fashion. In some embodiments, even 350 cycles are achievable in total for seven intermediate amorphization levels with only minimal variation. Next, to provide precise thermal tuning, the inventive technology may apply DC voltage to the same microheater starting from, for example, 0.85 V up to 1.5 V. In consequence, the resonance wavelength shifts to left at the beginning because the free- carrier dispersion effect dominates the thermo-optic effect. When the applied voltage is larger than 1V, red shifts appear. This shift in is volatile and the resonance shifts back upon removal of the DC voltage. As explained above, FIG. 9 shows the nonvolatile switching levels and their associated phase shifts. In the illustrated embodiments, 22 such nonvolatile levels are obtainable within 0.25 nm span of wavelengths using the voltage pulses described above. A person of ordinary skill would understand that for different geometries (e.g., different thickness / shape of the PCM element 30), different materials of the PCM element, and / or different target nonvolatile levels different sets of voltage pulses may be used. FIG.10 is a graph of both nonvolatile and volatile phase shifts for a resonator in accordance with embodiments of the present technology. In particular, 127 switching levels are illustrated, with error bars indicating the range from the thermo-optic tuning and PCM tuning over 2 cycles. The thermo-optic levels were recorded by 5 repeated experiments in each cycle to ensure that the resonance shift is from the thermo-optic tuning (DC heating induced) and not from the PCM tuning (a voltage pulse induced). The phase shifts are shown for different switching levels via PCM tuning. In between two nonvolatile PCM tuned levels obtained by voltage pulses, approximately 5 thermo- 3915-P1368WO.UW -13- optic volatile levels are recorded in average by gradually increasing the DC voltage in the range of 0.85 to 1.5V. The phase shift ( ) can be calculated as:= ( / ) 2 (1)where corresponds to the resonant wavelength shift for a free spectral range (FSR) of 2.42 nm. It should be noted that the thermo-optic levels show a much higher loss in comparison to the PCM levels mainly due to free carrier effect. Further, inventors noted that in the process of doing additional multilevel endurance experiments on the same device, it was possible to switch the PCM for > 10K events combining partial and complete amorphization and crystallization cycles. In some embodiments, the switching voltage and energy can be further reduced by suspending the p-i-n microheater, which prevents the generated heat from dissipating to the buried oxide and silicon substrate. This technique may exhibit significant efficiency power reduction of > 98%. We note that one of the main challenges of this approach is engineering the structural stability to avoid deformation or thermal damage during the fabrication and operation. Furthermore, using multiple PCM "islands" instead of one single PCM patch might further improve the endurance of phase-change devices and enhance the granularity of the phase transitions, allowing for more intermediate levels. Each island confines the phase transition to a smaller region, thus reducing the cumulative thermal and mechanical stress during switching. This localized switching can minimize the risk of material fatigue, defect accumulation, and degradation of the PCM or surrounding layers, which are common causes of failure in continuous PCM segments. This approach may allow individual islands to be selectively switched, enabling partial operation and reducing the total number of switching events each island experiences, thereby prolonging the overall device lifetime. However, this may require more careful design and more complex fabrication processes to ensure uniform switching behavior across all PCM islands. To achieve more operation levels, the pulse shape can be optimized to allow more precise control of the PCM transition, such as multi-pulse techniques or pulse width modulation. Furthermore, increasing the capping layer thickness or employing other more inert capping materials such as silicon nitride can also increase the endurance of PCM. 3915-P1368WO.UW -14- In conclusion, the inventive technology enables a high number of states (e.g., 127- level), low-energy hybrid tuning of micro-ring resonators, marrying nonvolatile PCMs for coarse but energy efficient tuning with thermo-optic effect for precise tuning. It is known that PCM tuning is not entirely deterministic for intermediate levels, due to cycle- to-cycle structural differences. Bringing the deterministic thermo-optic tuning to fine tune the response can overcome this long-standing limitation. On the other hand, compared to a hypothetical pure thermo-optic approach, such hybrid approach has a much lower static power – the device is firstly brought closer to the desired operation point and then treated with only slight voltage / energy for thermal fine tuning. This can easily provide orders of magnitude lower average static power, depending on the levels achievable with PCMs and system's static time. We also note that since the applied electric current for thermo-optic tuning is relatively small, the loss from the free-carrier dispersion effect as well as the thermal crosstalk can be significantly reduced. As another advantage of the inventive technology, we achieved operation voltage compatible with CMOS driving voltages (< 3V) by optimizing microheater geometry, and yet allowing for integration with CMOS EICs using various integration techniques. In some embodiments, this is an important step for the further scale up of PCM-based programmable PICs for applications in data centers and AI. Therefore, the inventive technology demonstrates low-voltage and low-energy switching of PCMs by the PIN doped silicon microheaters. The heating performance of PIN diode heaters is numerically and experimentally confirmed, and a monotonically increasing switching voltage is observed with the increase in intrinsic region width. With a relatively small intrinsic region of 0.9 μm, some embodiments demonstrate CMOS- compatible switching voltage of 2.75V (1.6V), energy 35.33 nJ (0.48 mJ) for amorphization (crystallization) of a 10-μm-long Sb S on silicon waveguides. The inventive technology further demonstrates 127-level operation of PCM-clad Si micro- ring resonator and over 10 thousand cycles in total by combining PCM coarse tuning with thermo-optic fine tuning. In some embodiments, arbitrary intermediate levels can be repeatedly achieved by using the thermo-optic fine tuning to compensate for the randomness of PCM tuning, providing a more accurate and finer quasi-continuous tuning method. The inventive technology makes improvements with low-voltage, low-energy, hybrid tuning of PCM photonic devices that bridge the gap of CMOS compatible PCM 3915-P1368WO.UW -15- switching, paving the way for scalable, co-packaged PCM-based electro-optical systems that meet the demands of, for example, AI, data centers, and neuromorphic computing. It is to be understood that the presently disclosed and / or claimed inventive concept(s) is not limited in its application to the details of construction and the arrangement of the components or steps or methodologies set forth in the following description. The presently disclosed and / or claimed inventive concept(s) is capable of other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting. Unless otherwise defined herein, technical terms used in connection with the presently disclosed and / or claimed inventive concept(s) shall have the meanings that are commonly understood by those of ordinary skill in the art. Further, unless otherwise required by context, singular terms shall include pluralities, and plural terms shall include the singular. All patents, published patent applications, and non-patent publications mentioned in the specification are indicative of the level of skill of those skilled in the art to which the presently disclosed and / or claimed inventive concept(s) pertains. All patents, published patent applications, and non-patent publications referenced in any portion of this application are herein expressly incorporated by reference in their entirety to the same extent as if each individual patent or publication was specifically and individually indicated to be incorporated by reference. All of the articles and / or methods disclosed herein can be made and executed without undue experimentation in light of the present disclosure. While the articles and methods of the presently disclosed and / or claimed inventive concept(s) have been described in terms of preferred embodiments, it will be apparent to those skilled in the art that variations may be applied to the articles and / or methods and in the steps or in the sequence of steps of the method described herein without departing from the concept, spirit, and scope of the presently disclosed and / or claimed inventive concept(s). As utilized in accordance with the present disclosure, the following terms, unless otherwise indicated, shall be understood to have the following meanings. The use of the word "a" or "an" when used in conjunction with the term "comprising" may mean "one", but it is also consistent with the meaning of "one or more", 3915-P1368WO.UW -16- "at least one", and "one or more than one". The use of the term "or" is used to mean "and / or" unless explicitly indicated to refer to alternatives only if the alternatives are mutually exclusive, although the disclosure supports a definition that refers to only alternatives "and / or". Throughout this application, the term "about" is used to indicate that a value includes the inherent variation of error for the quantifying device, the method being employed to determine the value, or the variation that exists among the study subjects. For example, but not by way of limitation, when the term "about" is utilized, the designation value may vary by plus or minus twelve percent, or eleven percent, or ten percent, or nine percent, or eight percent, or seven percent, or six percent, or five percent, or four percent, or three percent, or two percent, or one percent. The use of the term "at least one" will be understood to include one as well as any quantity more than one, including but not limited to, 2, 3, 4, 5, 10, 15, 20, 30, 40, 50, 100, etc. The term "at least one" may extend up to 100 or 1000 or more, depending on the term to which it is attached; in addition, the quantities of 100 / 1000 are not to be considered limiting, as lower or higher limits may also produce satisfactory results. In addition, the use of the term "at least one of X, Y, and Z" will be understood to include X alone, Y alone, and Z alone, as well as any combination of X, Y, and Z. The use of ordinal number terminology (i.e., "first", "second", "third", "fourth", etc.) is solely for the purpose of differentiating between two or more items and is not meant to imply any sequence or order or importance to one item over another or any order of addition, for example. As used herein, the words "comprising" (and any form of comprising, such as "comprise" and "comprises"), "having" (and any form of having, such as "have" and "has"), "including" (and any form of including, such as "includes" and "include") or "containing" (and any form of containing, such as "contains" and "contain") are inclusive or open-ended and do not exclude additional, unrecited elements or method steps. The term "or combinations thereof" as used herein refers to all permutations and combinations of the listed items preceding the term. For example, "A, B, C, or combinations thereof" is intended to include at least one of: A, B, C, AB, AC, BC, or ABC and, if order is important in a particular context, also BA, CA, CB, CBA, BCA, ACB, BAC, or CAB. Continuing with this example, expressly included are combinations that contain repeats of one or more item or term, such as BB, AAA, AB, BBC, AAABCCCC, CBBAAA, CABABB, and so forth. The skilled artisan will understand 3915-P1368WO.UW -17- that typically there is no limit on the number of items or terms in any combination, unless otherwise apparent from the context. In the context of this disclosure, the terms "about," "approximately," "generally" and similar mean + / - 5% of the stated value. 3915-P1368WO.UW -18-

Claims

CLAIMS What is claimed is:

1. An optical programmable unit, comprising: a phase change material (PCM) element; a microheater configured proximate to the PCM element, wherein the microheater comprises doped silicon, and wherein the microheater is configured for changing phase states of the PCM element based on an electrical excitation applied to the microheater, and wherein optical properties of the PCM element are changed in proportion to the electrical excitation applied to the microheater; and a photonic component configured to direct light to the PCM element, wherein the optical properties of the PCM element modulate optical properties of the photonic component.

2. The optical programmable unit of claim 1, wherein the photonic component is a waveguide.

3. The optical programmable unit of claim 1, wherein the photonic component is configured to operate as a resonator.

4. The optical programmable unit of claim 1, wherein the photonic component is configured for passing light through the PCM element.

5. The optical programmable unit of claim 1, wherein the microheater comprises: a p++ doped region; a n++ doped region; and an intrinsic region without doping configured between the p++ doped region and the n++ doped region, wherein the PCM element is configured on the intrinsic region.

6. The optical programmable unit of claim 1, wherein the PCM element is chalcogenide based PCM element. 3915-P1368WO.UW -19-7. The optical programmable unit of claim 1, wherein the PCM element is configured for changing its state by a transition voltage that is lower than 3 V.

8. The optical programmable unit of claim 1, wherein the PCM element possesses a first plurality of optically non-volatile states and a second plurality of optically volatile states.

9. The optical programmable unit of claim 8, wherein the PCM element is placed from one non-volatile state to another by the electrical excitation applied to the microheater.

10. The optical programmable unit of claim 9, wherein the PCM element, once in a non-volatile state, remains in the non-volatile state until a next electrical excitation is applied to the microheater.

11. The optical programmable unit of claim 8, wherein the electrical excitation comprises a combination of 2 or more of: electrical voltage; electrical current; electrical pulse shape; and electrical pulse duration.

12. The optical programmable unit of claim 8, wherein the PCM element is placed from one volatile state to another by a DC voltage applied to the microheater, and wherein the DC voltage is configured to generate a target temperature of the PCM element.

13. The optical programmable unit of claim 12, wherein, upon removal of the DC voltage, the PCM element is configured to revert to its prior volatile state.

14. The optical programmable unit of claim 1, wherein the optical programmable unit is tunable into at least 7-bit different non-volatile states.

15. The optical programmable unit of claim 14, wherein the optical programmable unit is tunable into a total of at least 127 non-volatile and volatile states in combination. 3915-P1368WO.UW -20-16. The optical programmable unit of claim 1, further comprising: an input grating coupler for incoming light; and an output grating coupler for outgoing light.

17. The optical programmable unit of claim 16, further comprising a Mach Zehnder interferometer.

18. The optical programmable unit of claim 16, wherein the photonic component is a micro ring resonator.

19. A method for programing an optical programmable unit, the method comprising: applying an electrical excitation to a microheater, wherein the microheater comprises doped silicon; and changing phase states of a phase change material (PCM) element by the microheater, wherein optical properties of the PCM element are changed in proportion to the electrical excitation applied to the microheater, wherein the PCM element, once having achieved its optical property, remains in a non-volatile state with respect to its optical property till a next electrical excitation is applied to the microheater.

20. The method of claim 19, wherein the electrical excitation comprises a combination of 2 or more of: electrical voltage; electrical current; electrical pulse shape; and electrical pulse duration.

21. The method of claim 20, further comprising placing the PCM element from one volatile state to another by a DC voltage applied to the microheater, wherein the DC voltage is configured to generate a target temperature of the PCM element.

22. The method of claim 21, further comprising, upon removal of the DC voltage, reverting the PCM element is to its prior volatile state. 3915-P1368WO.UW -21-23. The method of claim 22, wherein the optical programmable unit is tunable into at least 7-bit different non-volatile states.

24. The method of claim 23, wherein the optical programmable unit is tunable into a total of at least 127 non-volatile and volatile states in combination.

25. The method of claim 19, further comprising: coupling incoming light with an input grating coupler; and coupling outgoing light with an output grating coupler.

26. The method of claim 19, wherein the PCM element is a chalcogenide based PCM element. 3915-P1368WO.UW -22-

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