Patent Description:
The advent of photonic technologies, in particular in the area of optical signalling, coupled with advances made in fabrication capabilities has created a growing need for practical photonic memories. Such memories are essential to supercharge computational performance in serial computers by speeding up the von-Neumann bottleneck, i.e. the information traffic jam between the processor and the memory. This bottleneck limits the speed of almost all processors today; it has already led to the introduction of multicore processor architectures and drives the search for viable on-chip optical interconnects. However, shuttling information optically from the processor to electronic memories is presently not efficient because electrical signals have to be converted to optical ones and vice-versa. Information transfer and storage exclusively by optical means is highly desirable because of the inherently large bandwidth, low residual cross-talk and high speed of optical information transfer. On a chip it has been challenging to achieve because practical photonic memories may need to retain information for long periods of time and achieve full-integration with the ancillary electronic circuitry, thus requiring compatibility with semiconductor processing.

A further performance limitation may occur between the rapid access volatile memory (e.g. DRAM) associated with the processor, and the longer term, non-volatile memory of a storage device (e.g. NAND based memory of a solid state device, or magnetic domain based storage of a hard disk). Storage class memory has been proposed as an intermediate class of memory that is both non-volatile, and with faster reading than existing storage devices.

Optical gate switches have been fabricated using Ge<NUM>Sb<NUM>Te<NUM> (GST), for example <NPL>. This device uses an external, free-space optical light path to provide an optical switching signal to the GST element to change the state of the switch. This requires precision alignment of bulk free-space optical elements to the GST element, and is impractical for controlling multiple GST elements, especially where the elements are densely packed (e.g. in an array), which may be useful in a practical memory module. It is also difficult to address a small GST element in this way (due to diffraction limits).

<CIT> discloses a phase change material-aid micro ring-based optical waveguide switch. The optical waveguide switch comprises a micro ring waveguide resonant cavity and two straight waveguides coupled with the resonant cavity; and a phase change material, the state change of the phase change material is controlled through a phase change control source to control the loss of the micro ring waveguide resonant cavity.

<CIT> discloses a display device comprising a plurality of pixels, each pixel having a portion of a solid-state, phase-change material such as germanium-antimonium-telluride (GST) or vanadium dioxide, wherein the phase-change material can be reversibly brought into an amorphous state or a crystalline state and has a refractive index that is reversibly, electrically controllable.

<NPL>) discloses a proposal of an all-photonic, non-volatile memory, and processing element based on phase-change thin-films deposited onto nanophotonic waveguides.

<CIT> discloses an optical switch composed of a quartz substrate, first and second input waveguides, a first directional coupler, first and second arm waveguides of a Mach-Zehnder interferometric circuit, a second directional coupler, first and second output waveguides, and a phase change material portion.

It is desirable to address at least some of the abovementioned problems.

In a first aspect of the invention, there is provided a photonic device according to claim <NUM>.

In a second aspect of the invention, there is provided a method of varying the transmission, reflection or absorption properties of a memory cell or system according to claim <NUM>.

Optional and/or preferable features are laid out in the dependent claims.

The term optical as used herein relates to electromagnetic wavelengths of between <NUM> and <NUM>.

The modulating element may comprise a phase change superlattice material.

The modulating element may comprise a material with a refractive index that is switchable between at least two stable values.

The modulating element may comprise a plurality of stable solid states, each corresponding with a different transmission, reflection or absorption characteristic of the waveguide.

A reflection characteristic of the waveguide may be modified by switching the state of the modulating element.

The modulating element may comprise a material comprising a compound or alloy of a combination of elements selected from the following list of combinations: GeSbTe, VOx, NbOx, GeTe, GeSb, GaSb, AgInSbTe, InSb, InSbTe, InSe, SbTe, TeGeSbS, AgSbSe, SbSe, GeSbMnSn, AgSbTe, AuSbTe, and AlSb.

The material may comprise a mixture of compounds of alloys or combinations of elements from the list.

A core material of the waveguide may comprise an insulator.

A core material of the waveguide may have an optical bandgap of at least <NUM>, <NUM>, <NUM>, <NUM> or 3eV.

A core material of the waveguide may comprise a material selected from: silicon, silicon nitride, gallium nitride, gallium arsenide, magnesium oxide, and diamond (single crystal or polycrystalline).

The modulating element may comprise a material having more than two states, between which the material is switchable, each state associated with a different transmission, reflection or absorption characteristic of the waveguide.

The modulating material may have a thickness of less than <NUM> or <NUM>.

The waveguide may comprise an optical structure configured to enhance interaction of the switching signal with the modulating element.

The optical structure may comprise at least one of: a photonic crystal, a cavity in a core of the waveguide, an antenna and a plasmonic antenna.

The waveguide may comprise an optical resonator, the modulating element being evanescently coupled to the optical resonator.

The waveguide may comprise a plurality of optical resonators, each having a different resonant frequency, the device comprising a modulating element evanescently coupled to each optical resonator, wherein the transmission, reflection or absorption properties of the waveguide at each of a plurality of wavelengths is independently modified depending on the state of the respective modulating element coupled to the resonator corresponding with the respective wavelength, the state of each modulating element being switchable by an optical switching signal carried by the waveguide.

The waveguide may be a coupling waveguide, and further comprising a first and second waveguide, the coupling waveguide optically coupling the first waveguide to the second waveguide, the degree of coupling depending on the state of the modulating element.

The optical resonator may comprise a ring resonator, disk resonator, race-track resonator, wheel resonator.

The device may comprise an optical cavity, and the modulating element may be used to tune a resonant frequency of the optical cavity. The transmission, reflection or absorption characteristic may comprise a resonant frequency or a transmission, reflection or absorption spectrum.

The electrical conductor may comprise a first electrode in contact with the modulating element, and a second electrode in contact with the modulating element, so that a conducting path is defined through the modulating element between the first and second electrode. The state of the modulating element may thereby be switchable by passing the electrical signal through the conducting path.

The modulating element may comprise a layer of material. The modulating element may be sandwiched between the first and second electrode, so that the conducting path is substantially normal to a plane of the modulating element. The first and second electrode may alternatively be arranged to define a lateral conducting path substantially parallel to a plane of the modulating element.

At least one of the first and second electrode may comprise a substantially transparent material (e.g. at the wavelength of an optical switching signal), such as indium tinoxide (ITO).

The resistor may comprise a metal or semiconductor element.

A Mach-Zehnder interferometer is provided that splits an input signal received at an input port between a first and second optical path, and then recombines the input signal after it has passed through the first and second optical path, wherein at least one of the first and second path comprise a device according to the invention.

A tunable grating is provided comprising a device according to an embodiment, wherein the grating is defined by a plurality of modulating elements disposed side-by-side on the waveguide.

The waveguide may be a planar waveguide, and the grating may be configured to define an out-of-plane coupler that couples light between the waveguide and free space in a direction at an angle to the plane of the waveguide, wherein the transmission, reflection or absorption property is resonant frequency of the coupler.

A tuneable optical filter is provided comprising a device according to the invention.

An optical switch is provided comprising a device according to an embodiment.

A switching fabric is provided comprising: a plurality of horizontal waveguides, and a plurality of vertical waveguides, and a device according to an embodiment optically coupling each horizontal waveguide to each vertical waveguide at each respective intersection therebetween.

In an example not within the scope of the claims, there is provided an optical system, comprising:.

The system may further comprise a light detector coupled to the waveguide and configured to determine the transmission, reflection or absorption characteristic by detecting a probe signal from the waveguide.

The controller may be configured to use the light source to provide the probe signal and the switching signal.

A further light source may be provided coupled to the waveguide for providing the probe signal; wherein the controller is configured to use the further light source to provide the probe signal.

The light source and the further light source may both be monochromatic, and have different wavelengths. The light source and further light source may be coherent, or incoherent.

In an embodiment, there is provided an optical system, comprising:.

An optical memory module is provided, comprising a plurality of memory cells according to an embodiment.

A computer comprising a processor and the memory module.

Each feature (or features) of each example or embodiment may be combined with each feature (or features) of any other example or embodiment.

Each and every embodiment and feature disclosed in the priority documents is optionally disclaimed from the scope of the present disclosure.

Embodiments will be described, by way of example only, with reference to the accompanying drawings, in which:.

A candidate technology for all-optical memories is to use phase-change materials (PCMs), which are already the subject of intense research and development over the last decade, in the context of electronic memories. A striking and functional feature of these materials is the high contrast between the crystalline and amorphous phase of both their electrical and optical properties. In particular, PCMs (e.g. chalcogenide-based PCMs) have the ability to switch between these two states in response to appropriate heat stimuli (crystallization) or melt-quenching processes (amorphization). These PCMs (which include tellurides and antimonides) can be switched on a sub-nanosecond timescale with high reproducibility which enables ultra-fast operation over switching cycles up to <NUM><NUM> times using current-generation materials. New and improved PCM materials, such as the so-called phase-change super-lattice materials, are expected to deliver even better performance in the future. In addition, at normal operating temperatures (e.g. NIST standard temperature and pressure conditions) the states may be stable for years, which may be appropriate for non-volatile memory.

Many PCMs show significant change in refractive index in the visible and even larger changes in the near-infrared wavelength regime, which is typically the spectral region of choice for telecommunication applications. According to embodiments, PCMs are embedded in photonic circuits, to provide fast and repeatable all-optical, multi-level, multi-bit, non-volatile memory. Wavelength division multiplexed (WDM) access can be achieved on a chip comprising at telecommunications wavelengths compatible with on-chip optical interconnects. In contrast to prior art free-space optical implementations where PCM cells are switched with a focused laser in the far-field, devices according to embodiments are operated in the optical near-field (by evanescent coupling).

Waveguide integrated modulating elements are not restricted in size by the diffraction limit of the input light and can hence be miniaturized to nanoscale dimensions. In the detailed exemplary embodiments described below, the well-studied alloy Ge<NUM>Sb<NUM>Te<NUM> (GST) is used because of its proven data retention capabilities and high state discrimination down to nanoscale cell sizes, which enables dense packaging and low-power memory switching.

Data may be stored in a modulating element comprising a PCM material. The modulating element may be placed directly on top of a nanophotonic waveguide core. This is a convenient way of coupling the waveguide to the modulating element in the nearfield. Both changing the state (writing/erasing) of the modulating element, and read-out of the current state of the modulating element is carried out via evanescent coupling between the waveguide and the modulating element and is thus not subject to the diffraction limit. The writing and readout of the modulating element is done directly within the waveguide, and may use nanosecond optical pulses. Embodiments therefore provide a promising route towards fast all-optical data storage in photonic circuits.

<FIG> shows a device <NUM>, comprising a waveguide <NUM> and a modulating element <NUM>. The modulating element <NUM> is evanescently (near field) coupled to the waveguide <NUM>, so that a light signal carried by the waveguide <NUM> interacts with the modulating element <NUM>.

The modulating element <NUM> may comprise any material that is switchable between different states, each different state corresponding with different optical properties of the modulating element. Preferably, the modulating element <NUM> comprises a phase change material such as GST. The modulating element <NUM> may comprise a further encapsulation layer, which may comprise ITO, for example to protect the PCM layer from oxidation.

The waveguide <NUM> may be, but is not limited to, a planar waveguide, for example a rib waveguide. The waveguide <NUM> comprises a core material that is capable of carrying an optical switching signal <NUM> to the modulating element <NUM> so as to switch the state of the modulating element <NUM>. In general, suitable materials for the waveguide core may have a bandgap of at least 1eV.

One example of a suitable material for the core of the waveguide <NUM> is silicon nitride. Alternative materials include silicon, gallium nitride, gallium arsenide, diamond (monocrystalline or polycrystalline) and magnesium oxide, but any material with a bandgap greater than 1eV may be suitable. The waveguide core may comprise an insulating material or a semiconductor.

An air cladding may be used around the waveguide core <NUM>. Other materials may be used. Solid phase cladding materials may be used to reduce a thermal time constant of the modulating element <NUM>.

The waveguide <NUM> comprises a first port <NUM> and a second port <NUM>. The transmission characteristics of the optical waveguide <NUM> may be inferred by applying a probe signal <NUM> at the first port <NUM>, and monitoring the resulting output probe signal <NUM> at the second port <NUM>. The probe signal <NUM> may be a pulsed signal or a continuous wave signal.

The state of the modulating element <NUM> is switchable by an optical switching signal <NUM> carried by the waveguide <NUM>. An optical switching signal <NUM> may be input to the waveguide at either of the first or second port <NUM>, <NUM>. The switching signal <NUM> may have a higher power than the probe signal <NUM>. The evanescent coupling of the optical switching signal <NUM> may result in the absorption of optical power by the modulating element <NUM>. The consequent heating of the modulating element <NUM> by the optical switching signal <NUM> may change the state of the modulating element <NUM>. Since the modulating element <NUM> is optically coupled to the waveguide <NUM>, changes to the optical properties of the modulating element <NUM> result in changes to the transmission, reflection or absorption characteristics of the optical waveguide <NUM>. The state of the modulating element <NUM> may be used to encode information.

<FIG> illustrates the operation of a device <NUM>. A probe signal <NUM> applied to a first port <NUM> may be used to monitor a transmission characteristic of the waveguide <NUM> by monitoring the output probe signal <NUM> at the second port <NUM>.

As illustrated in <FIG>, the crystalline state <NUM> (which may be assigned as Level <NUM>, <NUM>) exhibits higher optical attenuation and thus less optical transmission than the amorphous state (which may be assigned Level <NUM>, <NUM>). Therefore, stored data may be encoded in the amount of light <NUM> transmitted through (along) the waveguide <NUM> (i.e. exiting the second port <NUM> of the waveguide) and can be read-out with a low-power probe signal <NUM>, which may comprise a series of optical pulses <NUM>. The state (e.g. degree of amorphisation/crystallinity) of the modulating element <NUM> influences the optical properties of the waveguide <NUM> and therefore the waveguide mode profile, as illustrated for the simulated transverse-electric (TE) mode <NUM> in <FIG>.

In the crystalline state <NUM>, the PCM may be more absorptive, thus pulling the light towards the modulating element <NUM>, resulting in stronger attenuation of the passing optical signal. In the amorphous phase <NUM>, on the other hand, the absorption is reduced and therefore the modulating element <NUM> does not attenuate the waveguide transmission to the same degree.

Changing the state of the modulating element <NUM> may be achieved by inducing a phase-transition (or partial phase-transition) with an optical switching signal <NUM>, which may comprise a more intense light pulse <NUM> than the probe pulses <NUM> of the probe signal <NUM>. If the energy absorbed by the modulating element <NUM> is sufficient to heat it up to a transition temperature of the PCM, these pulses <NUM> can initiate either amorphization or crystallization. Referring to <FIG>, a transition <NUM> from a crystalline state <NUM> to an amorphous state <NUM> may be thought of as a write step, and a transition <NUM> from an amorphous state <NUM> to a crystalline state <NUM> may be thought of as an erase step.

For amorphization, the PCM (e.g. GST) may be melted and then cooled down rapidly to preserve this disordered state. On the other hand, heating the PCM above the crystallization temperature (but below the melting temperature) for a few nanoseconds may enable recovery of the atomic ordering and thus crystallization. Transmission properties of the waveguide <NUM> may therefore be modulated by varying the absorptive state of the modulating element <NUM>. This is different from the phenomenon employed in conventional optical storage (where reflectivity is instead modulated).

An all photonic nonvolatile memory element may be provided. Multiple modulating elements may be addressed using wavelength multiplexing, which may be applied to deliver multi-bit memory access. Multiple levels of information (i.e. multiple bits) may be encoded in a single modulating element <NUM> by using more than two states to encode information. A single shot read and write switching signal may be used to transition the modulating element between any of the more than two states.

Although a PCM with multiple stable states is particularly appropriate for non-volatile memory applications, different materials may be used that do not comprise states that are stable (e.g. at NIST STP). The state of the modulating element <NUM> may be volatile, and a controller may be provided to periodically refresh the state of the modulating so as to maintain desired state thereof (e.g. maintain information encoded by the state). The device may be temperature controlled (e.g. cooled), so that the PCM is stable, rather than operating at typical room temperatures.

The waveguide <NUM> may be fabricated from <NUM> Si<NUM>N<NUM> / <NUM> SiOz wafers using lithography. A soft reflow process may be employed to reduce intrinsic surface roughness of the photosensitive resist following exposure. Subsequently, reactive ion etching (RIE) may be used (e.g. in CHF<NUM>/O<NUM>) to pattern the Si<NUM>N<NUM> followed by the complete removal of the remaining resist under O<NUM> plasma. The depth of the etch (and of the starting layers) may be selected depending on the design of the device. In the present examples, an etch depth of <NUM> was used, except for the ring devices, where an etch depth of <NUM> was used. Example devices for which results are described herein are designed with a waveguide width of <NUM> or <NUM> (for ring devices).

The modulating elements <NUM> for which results are shown herein are fabricated by depositing GST and indium tin oxide (ITO) in a lift-off process. For this, a second lithography step is carried out using positive tone PMMA resist at <NUM>% concentration. Openings on top of the waveguides are defined, aligned to the previously written waveguide structures. Subsequently a <NUM> layer of GST is sputtered and capped with <NUM> of ITO to avoid oxidation. Finally, lift-off of the GST/ITO layers is done in hot acetone supported by soft sonication.

The features and steps of this fabrication process flow are merely exemplary, and embodiments may be fabricated in other ways.

The device <NUM> may be operated by using an optical switching signal <NUM> and an optical probe signal <NUM>. Both writing and erasing of the modulating element <NUM> may be performed with nanosecond light pulses that are generated off-chip. This is convenient in an experimental context, but it is also envisaged that a suitable light source may be included on-chip. An electro-optical modulator (EOM) may be used to modulate the optical switching signal <NUM>. The optical switching signal <NUM> may be coupled into the on-chip waveguides, for example using grating couplers. The device <NUM> may be tuned for operation in the optical telecommunications C and L band.

The read-out of the state of the modulating element <NUM> may be performed using a probe signal <NUM> having readout pulses (e.g. of <NUM> ps duration, generated with the EOM) or with a continuous wave (CW) probe signal <NUM>. In both cases, the probe signal <NUM> may have at least one order of magnitude less power than the switching signal <NUM>. The probe signal <NUM> may be spectrally separated from the switching signal <NUM>. A colour selective filter may be used to separate the probe signal <NUM> from the switching signal <NUM>. Further suppression of the switching signal <NUM> may be achieved by counter-propagation of the probe signal <NUM> and the switching signal <NUM> through the waveguide <NUM>.

This counter propagation of the probe and switching signal may be implemented with a set of two optical circulators (not shown) which direct light coming from a laser source onto the device <NUM> and subsequently onto the photodetectors. An optical band-pass filter may be used to suppress the influence of back-reflections. The filter may have a <NUM> dB insertion loss, a <NUM> dB bandwidth of <NUM> and an off-resonance suppression exceeding <NUM> dB for wavelengths further apart than <NUM>.

The switching signal <NUM> may be generated from a continuous wave (CW) diode laser in combination with an electro-optical modulator, controlled by an electrical pulse generator. Switching signal pulses <NUM> may be further amplified in power by a low-noise erbium-doped fiber amplifier (EDFA). Such an implementation may provide accurate control over both pulse length and power, enabling pulses as short as <NUM> ps, with pulse edges in the picosecond-regime and with peak powers up to <NUM> mW.

The probe signal <NUM> may comprise CW-light from a second laser source. After propagation through the device <NUM> and the band-pass filter, the light may be is split into two beams which are respectively detected by a fast photodetector and a low-noise photodetector. The signal of the fast detector enables recording a high-resolution time-trace of the response with a <NUM> oscilloscope while the overall device transmission may be monitored at all times with the low-noise detector.

Crystallization of amorphous PCM may enhance optical absorption in the modulating element <NUM> at telecommunication wavelengths, for instance by one order of magnitude. This increase in optical absorption results in an increase in the proportion of the energy of a switching signal <NUM> in the waveguide <NUM> that is absorbed by the modulating element <NUM>. This renders both amorphization and crystallization transitions possible with optical switch signal pulses <NUM> of comparable length and power. Full recrystallization with a single light pulse <NUM> may require optimisation of the pulse (e.g. duration and power).

If the phase transition occurs before the end of the pulse <NUM>, the continued optical energy supply may heat up the PCM further, to the melting temperature, and cause immediate reamorphization. Temperature variations across the memory cell may mean that this cannot be prevented completely since all parts of the PCM material may not crystallize simultaneously. To address this issue, an erase transition <NUM> (making the PCM material more crystalline) may be based on stepwise partial recrystallization using a train of consecutive pulses <NUM>. In order to prevent reamorphization of already crystallized regions, the individual pulse energies may be gradually decreased from pulse to pulse (e.g. by approximately <NUM> % of the initial pulse energy). The initial pulse energy may correspond with the pulse energy used for a write transition <NUM>. The energy of the final pulse determines which state is achieved, and therefore what transmission characteristic of the waveguide <NUM> is achieved. The final state can be fully crystalline or an intermediate state (i.e. partially amorphous and partially crystalline), for instance by stopping the erase transition <NUM> at the same energy required for a write transition <NUM> to that specific level.

High reproducibility of the transition operations may be achieved by an initial conditioning step. A conditioning step may comprise performing a write/erase switching cycle a few times on the as-deposited and subsequently annealed GST. Within the first few cycles the read-out transmissions, which initially vary slightly from cycle to cycle, stabilize to a fixed value.

<FIG> shows the change in the detected output signal <NUM> (which indicates a transmission characteristic of the waveguide <NUM>) upon repeated switching between the crystalline (low transmission) state <NUM> and amorphous (high transmission) state <NUM> of the PCM. Each transition <NUM>, <NUM> results in a change in the state of the modulating element <NUM> (which may be referred to as "switching"), resulting in a clear change in the output signal <NUM> between a high level <NUM> (corresponding with a more amorphous state) and a low level <NUM> (corresponding with a fully crystalline state). The results demonstrate unequivocal binary data storage, with good reversibility and high transmission contrast (around <NUM>% in this example). Switching over <NUM> cycles is shown in <FIG>. The write transition <NUM> was initiated by a single 100ns pulse, while the erase transition used a sequence of six consecutive 100ns with decreasing power. The time between each transition <NUM>, <NUM> was one minute.

<FIG> illustrates that the switching of the modulating element <NUM> is highly reproducible over fifty cycles with a measured confidence interval of ±<NUM>%. The first standard deviation <NUM>, <NUM> for the amorphous state <NUM> and crystalline state <NUM> are shown, along with the respective second standard deviations <NUM>, <NUM>.

The low-transmission state <NUM> is initially prepared from the fully crystallized phase in such a way that reversibility of the operation is ensured (as discussed above). On the other hand, the absolute transmission at the high level <NUM> is determined by the employed switching energy, which defines the final level of amorphization, and the GST dimension along the waveguide <NUM>, which defines the modulation depth.

To ensure high transmission contrast in the results of <FIG> a GST cell of <NUM> length was used. A change in read-out transmission of <NUM> % was observed using a single <NUM> ns write pulse of <NUM> pJ energy. Since the GST cell <NUM> absorbs nearly <NUM> % of the pulse in the crystalline state (derived from a measured optical attenuation of -<NUM> dB past the device), this corresponds to a switching energy of <NUM> pJ. Further demonstrations of binary operation were realized by employing devices with smaller GST lengths and lower write pulse energy, as described in the supplementary material. In particular, modulation depth up to <NUM> % and binary operation with pulses as short as <NUM> ns with switching energies of <NUM> pJ have been achieved. While the data in <FIG> demonstrates the nonvolatility of states encoded in a modulating element <NUM> comprising a PCM for several minutes, it has been confirmed that the phase-state may be preserved over a much longer times, up to a period of at least three months. Indeed, extrapolating from the well-studied data retention properties of GST an optical memory can be expected to remain non-volatile on a timescale of years.

<FIG> shows a transmission electro micrograph of a section through a device <NUM>. The TEM specimens were prepared by focused ion beam (FIB). The cross sectional lamellae were cut from a device along the waveguide and thinned to a thickness of less than <NUM> for TEM imaging. The silicon nitride waveguide <NUM> is visible, and the deposited GST layer <NUM> and the ITO layer <NUM> can also be seen. The ordered lattice structure of GST in the crystalline state is visible. Fourier analysis of the TEM image of <FIG> produces a diffractogram with clear features corresponding to the ordered lattice structure of cubic GST. <FIG> shows a Fourier analysis of similar data from a PCM material of device that has been optically switched into an amorphous state showing the pronounced halo expected for the amorphous phase.

Besides repeatability, speed may be an important factor for applications in which a device <NUM> is used as a memory element. In this context, the speed at which read, write and erase operations can be achieved in a device is important. Read-out may rely on photon absorption, with information encoded in the amount of signal (e.g. signal power) transmitted through the waveguide <NUM>. The read-out can therefore be performed on picosecond time scales and is not a bottleneck in achieving high speed operation.

On the other hand, write operations <NUM> and erase operations <NUM> are linked both to amorphization and crystallization times which are intrinsic properties of the modulating element <NUM>. In prior art GST cells, amorphization times in the picosecond range are reported, and crystallisation times in the nanosecond to sub-nanosecond and nanosecond.

In the case of a memory cell comprising a device, the writing speed (amorphization) may be considered to be the more stringent requirement since it determines how quickly information can be stored. As outlined above, in the initial prototype devices it has been shown that write operations are possible with pulses as short as <NUM> ns. To determine how fast a device <NUM> might be operated, the phase-transition of the modulating element <NUM> was monitored by performing time-resolved measurements during optical switching.

The observed transient behaviour of a device <NUM> showed that, besides the length of the write pulse, the speed of the device <NUM> is also limited by the post-excitation relaxation time (which may be termed dead time). For <NUM> ps write pulses, an operation speed of <NUM> was obtained (taking pulse length and dead time into account). Writing speeds of a few GHz can be expected using picosecond instead of nanosecond pulses to switch the state of the modulating element <NUM>.

<FIG> shows a device comprising a waveguide <NUM>, the waveguide <NUM> comprising a first, second and third ring resonator 160a-c. Each of the ring resonators 160a-c has a different diameter, and is thereby tuned to a different resonant frequency and corresponding wavelength. A grating coupler <NUM> is provided at a first and second port of the waveguide <NUM>, for coupling from free space into the waveguide <NUM>. Each of the ring resonators 160a-c comprises a modulating element <NUM>, the location of which is indicated by the dotted circles 170a-c. Since each of the ring resonators 160a-c admits only a specific wavelength of light (due to cavity internal interference preventing off-resonance wavelengths from entering a ring), only light with a wavelength close to resonance can be used to switch or read-out the respective memory cell.

Each of the modulating elements is therefore individually addressable, and the spectral transmission properties of the waveguide <NUM> is adjusted by switching the state of each modulating element. There may be a plurality of probe signals <NUM> at different wavelengths, each probe signal <NUM> having a wavelength tuned to address a specific modulating element <NUM> (which may encode data). In this way the state of a plurality of modulating elements <NUM> may be determined simultaneously, using a wavelength division multiplexed (WDM) signal.

This type of device may be used to implement a single memory element or cell that can store a plurality of bits of data, which can subsequently be read in parallel. The simplicity of devices make them fully compatible to on-chip nanophotonic circuitry, allowing for easy integration and exploitation of a wide range of commonly used optical signal processing techniques such as WDM. The device of <FIG> illustrates a wavelength-multiplexed integrated multi-bit architecture that is suitable for ultra-fast read-out with up to <NUM> dB modulation depth.

This approach uses the wavelength-filtering property of the on-chip optical cavities (in this example, ring resonators) which enables wavelength selective addressing of individual modulating elements <NUM>.

<FIG> shows a spectral transmission characteristic of the waveguide of <FIG>, in which the distinct resonances 230a-c, respectively corresponding with resonators 160a-c, are clearly visible (along with further such resonances <NUM>). The switching signal for the first, second and third modulating elements (of the first 160a, second 160b and third 160c resonators) comprise laser pulses at <NUM>, <NUM> and <NUM> respectively. While a write transition is carried out with a single <NUM> ns pulse, repeatability is again ensured by performing an erase transition with a train of consecutive <NUM> ns pulses of decreasing energy (at the appropriate wavelength).

In <FIG> the individual changes of the three resonances upon switching, resulting from the modified refractive index of the GST element, are shown. <FIG> shows a spectral response corresponding with the first resonance, showing a spectra <NUM> before a writing operation <NUM>, a spectra <NUM> after a writing operation <NUM>, and a spectra <NUM> after an erase operation <NUM> (returning the PCM to a crystalline state). It can be seen that the initial state is recovered after one write/erase cycle, since the spectra <NUM> and <NUM> are substantially identical.

<FIG> illustrates that each modulating element can be addressed individually. Spectra for both the second and third resonance are shown:.

Switching signals applied to address a single modulating element clearly do not affect the other modulating elements.

<FIG> shows that each modulating element can be read-out individually. In this example <NUM> ps probe pulses <NUM> were used, with a pulse energy of <NUM>±<NUM> pJ. Here, probe signals at three distinct wavelengths (<NUM>, <NUM> and <NUM>, respectively) are used to probe the transmission characteristics of the waveguide <NUM>, and hence the state of each modulating element (and any data encoded in the state thereof). <FIG> shows a time history <NUM>, <NUM>, <NUM> of a first, second and third probe signal respectively corresponding with each of the first, second and third modulating elements. In order to maximise the readout contrast on switching, probe pulses for each modulating element may be detuned (e.g. red-detuned) from the wavelength of the corresponding switching signal onto the slope of the cavity resonance. While a modulation depth of <NUM> dB is achieved upon switching each individual memory cell, the read-out level of the non-addressed memory elements does not change. With this approach modulation depths exceeding <NUM> dB are possible.

A plurality of states of the modulating element <NUM> may be used to encode more than one bit of information in each modulating element <NUM>. This may be combined with wavelength addressable multi element cell architectures, and may further increase the number of bits that can be encoded or stored in a device.

The invention may be applicable to future high-density data storage, where it may be desirable to reduce the overall dimensions of the device and to use multi-level access (i.e. storing multiple bits per modulating element) in a single cell. The smallest prototype memory element realised thus far has a footprint of <NUM><NUM>, but it is expected that smaller cells could be achieved in accordance with recent reports on electrical PCM-based devices.

<FIG> is a graph illustrating switching between more than two states of a modulating element <NUM>. A detected probe signal <NUM> is shown with respect to time, and the detected probe signal <NUM> is switched between four different levels (<NUM>, <NUM>, <NUM>, and <NUM>) by writing operations <NUM> and erase operations <NUM>. At a) the state of the modulating element <NUM> and hence the detected probe signal <NUM> is switched from <NUM>, to <NUM>, to <NUM>, to <NUM>, and then directly back to <NUM>. At b) the state of the modulating element <NUM> and hence the detected probe signal <NUM> is switched from <NUM>, directly to <NUM>, then to <NUM>, then directly to <NUM>, then to <NUM>, then to <NUM>, and then directly back to <NUM>. At c) the state of the modulating element <NUM> and hence the detected probe signal <NUM> is switched from <NUM>, directly to <NUM>, then directly to <NUM>, then to <NUM> and back directly to <NUM>, the to <NUM> and back to <NUM>.

The modulating element <NUM> may not only have a small footprint (smaller than can readily be addressed using free space optics) but may also be capable of encoding multiple levels in an element, using simple but extremely effective write/erase and read techniques. Using optical switching pulses <NUM> with varying pulse energy it is possible to move freely and reliably between more than two states with high repeatability. This multi-level operation relies on the freely accessible intermediate crystallographic states of the GST, i.e. states with a mixture of crystalline and amorphous regions. These mixed states exhibit optical transmission properties lying between those of the level <NUM> and level <NUM> shown in <FIG>.

The data of <FIG> was recorded using a <NUM> long PCM element, with each transition between levels being initiated by a single <NUM> ns light pulse. Four clearly distinguishable levels are reached with switching pulses Pi of level-specific energies in the range <NUM> to <NUM> pJ. The energy of pulse P<NUM> was <NUM>±<NUM> pJ, the energy of pulse P<NUM> was <NUM>±<NUM> pJ, and the energy of pulse P<NUM> was <NUM>±<NUM> pJ. In <FIG> these levels were reached in a serial manner and subsequently the erase operation R was carried out from level <NUM>. Furthermore, the same bit levels were also shown to be accessible in random order as shown in <FIG>. Here the erase operation R (i.e. a return to level <NUM>) was not only possible from the highest transmission state, but from any intermediate level as shown in <FIG>.

These results demonstrate that both write and erase operations, to and from any level, are possible with high accuracy allowing a reliable multi-bit memory operation. This is particularly attractive because such arbitrary transitions are very difficult to achieve in electronic memories employing phase change materials, where iterative write-and-erase algorithms involving multiple (typically <NUM> to <NUM>) write/read(/re-write) cycles are needed to achieve a pre-defined level, adversely affecting the overall write speed and power consumption.

The number of possible levels in a device is limited by the separation (difference in transmission) between the highest and lowest state and the required confidence interval of an intermediate level. The former can be increased by using either a larger modulating element or higher pulse energies. The confidence interval, on the other hand, is mainly limited by the minor variations in the switching and by the signal-to-noise ratio (SNR) of the read-out measurement. Therefore, the number of memory levels can be increased by just using a higher read-out power ensuring a better SNR (within limits).

This is illustrated in <FIG>, where <NUM> levels of state discrimination (i.e. <NUM> bits per cell) are demonstrated within a modulating element <NUM>. Each level corresponds to a partially crystalline state, presenting a specific change in transmission by applying pulses with varying energies as presented in <FIG>. The individual levels are reached with pulses Pi of level-specific energies in the range <NUM> to <NUM> pJ (the energies in pJ being approximately EP<NUM>=<NUM>, EP<NUM>=<NUM>, EP<NUM>=<NUM>, EP<NUM>=<NUM>, EP<NUM>=<NUM>, EP<NUM>=<NUM>, and EP<NUM>=<NUM>). In <FIG> it can also be observed that the difference between the transmissions of any two consecutive levels is much higher than the uncertainty marked by the a band <NUM> across each level. In <FIG> it is also demonstrated that each level can be reached from both directions, i.e. with an amorphization as well as a crystallization step. This implies that any level is accessible from all others, with very accurate control of the transmission levels and remarkable repeatability (as seen by the accurate re-writing of levels <NUM> to <NUM> in <FIG>), just by applying the appropriate write or erase pulse. This provides a significant progression in functionality and may be important for the realization of practicable photonic memories.

Another aspect of device performance for data storage applications is energy consumption per bit. In a memory cell comprising a device both writing and erasing may rely on changes in state of the modulating element material. For the example device comprising a GST modulating element <NUM>, the switching energy is given by the amount of energy that is required to heat the GST above the melting (amorphization) or glass-transition (crystallization) temperature, respectively. Therefore, the energy consumption is directly related to the volume of the memory element and read-out contrast. The relationship between switching energy and read-out contrast is shown in <FIG>. In binary operation, a read-out contrast of <NUM>% was demonstrated with <NUM> pJ switching energy. On the other hand, switching energies as low as <NUM> pJ for the same device are possible for a reduced contrast of <NUM>% which still enabled clear distinction of the two levels. In addition, it is estimated that energy consumption can be improved by up to one order of magnitude by operating the device with sub-nanosecond instead of tens of nanosecond pulses. A thermo-optical analysis has shown that the portion of absorbed energy that gets lost due to thermal diffusion increases significantly with increasing pulse length. Therefore, shorter and more intense pulses are beneficial in terms of energy requirement by quickly heating up the modulating element <NUM> to the required transition temperature while reducing thermal diffusion losses.

In this early prototype of a device, energy consumption and speed achieved compares well with pre-existing electrical counterparts. For example, current commercial PCM-based electrical memories (at the <NUM> node) typically require write pulses of <NUM> - <NUM> ns duration and read pulses of <NUM> ns (considerably longer than the 10ns/500ps write/read demonstrated), along with <NUM>-<NUM> pJ write energy (c. ~<NUM> pJ). Although research-level electrical PCM devices improve on such performance figures (e.g. <NUM>. 4pJ write energy and <NUM> ns write pulses), the performance can be further improved by operating them with shorter pulses and by moving to modulating elements with smaller footprint, as well as through the development of new materials with faster and lower temperature switching. Higher signal to noise ratio to improve the read-out contrast could also be obtained with the use of optical cavities, which would also reduce switching energies.

To reduce the device footprint, alternative architectures, such as plasmonic antennas could be explored. Alternatively, scaling down is plausible by using photonic circuitry operating at shorter wavelengths (therefore, narrower waveguides) or by using alternative phase-change materials with a higher difference in refractive index (e.g. in the C and L-band). This way, smaller devices may provide sufficiently good contrast. While multi-bit access has been demonstrated with micro-ring resonators with relatively large footprint, alternative technologies such as ultra- compact on-chip optical multiplexer / demultiplexers can be employed for size reduction. In addition, optical cavities with smaller mode volume such as photonic crystal devices may be used to localize the interaction volume of the optical mode with the memory element further and thus lead to a smaller system size for wavelength selective memory access.

<FIG> respectively illustrate examples of a device <NUM>, similar to that of <FIG>, in which photonic crystal structures <NUM> (<FIG>) and plasmonic antennas <NUM> (<FIG>) are used to enhance interaction with the modulating element <NUM>.

<FIG> illustrates a device in which the waveguide <NUM> comprises a first port <NUM> for receiving a probe signal <NUM>, a second port <NUM> for detecting the output probe signal <NUM>, and a separate control port <NUM> for receiving the switching signal <NUM>.

Although examples have been described with reference to memory applications, it will be understood that embodiments are not restricted to memory, and may also be used in optical switching. <FIG> illustrates an optical switch <NUM> comprising a first waveguide <NUM>, a second waveguide <NUM>, and a coupling waveguide <NUM>. The coupling waveguide comprises a resonant optical cavity <NUM> and a modulating element <NUM> evanescently coupled to the coupling waveguide <NUM>. The state of the modulating element <NUM> controls a transmission characteristic of the coupling waveguide <NUM>, so as to vary the degree to which light is coupled between the first and second waveguide <NUM>, <NUM>. An input light signal <NUM> can thereby be switched between output signal <NUM> and output signal <NUM>. The coupling may be directional, so that light launched in the first waveguide <NUM> is coupled and launched in a specific direction in the second waveguide <NUM>.

<FIG> illustrates a switching fabric using the same principles as the switch of <FIG>. A plurality of horizontal waveguides <NUM> each intersect a plurality of vertical waveguides <NUM>. At each intersection, at least one coupling waveguide <NUM> is provided, operating on the same principle as described with reference to <FIG>, controlling the degree to which the intersecting vertical and horizontal waveguides <NUM>, <NUM> are coupled. Two coupling waveguides may be provided at each intersection, for controlling coupling in each direction at each intersection. The coupling between each intersection need not be via a ring resonator, but may instead by via any optical arrangement comprising a device (e.g. an optical switch).

<FIG> illustrates a transistor like latch <NUM>, in which transmission, via a coupling waveguide <NUM>, between a first and second waveguide <NUM>, <NUM> is controlled based on the state of a modulating element <NUM> that is evanescently coupled to the coupling waveguide <NUM>. A control port <NUM> is provided in the coupling waveguide (analogous to a gate contact) for controlling transmission between the first and second waveguides <NUM>, <NUM>.

<FIG> illustrates a Mach-Zehnder interferometer <NUM> comprising a device <NUM>. The inteferometer receives an input signal <NUM> at an optical splitter, which divides the input signal <NUM> between a first path <NUM> and a second path <NUM> of the interferometer. Each of the first and second path <NUM>, <NUM> comprise a waveguide. The first optical path comprises a waveguide <NUM>, the transmission properties of which are varied by the state of a modulating element <NUM> evanescently coupled to the waveguide <NUM>. In this example, the modulating element <NUM> may adjust an optical path length of the first optical path <NUM> (for instance by varying refractive index). Signals of the first and second optical path are recombined at optical combiner <NUM>, so as to produce a first and second output signal <NUM>, <NUM>.

<FIG> illustrates a tuneable optical filter <NUM> comprising a device <NUM>. The filter <NUM> comprises a waveguide <NUM>, to which is evanescently coupled a plurality of modulating elements <NUM>, so as to define a grating, such as a Bragg grating <NUM>. The transmission characteristics of the grating <NUM> are varied depending on the state of the modulating elements, so as to vary the transmission and reflection properties of the filter <NUM>, thereby varying the degree to which an input signal <NUM> is reflected as return signal <NUM>, or transmitted as signal <NUM>.

<FIG> illustrates a grating coupler <NUM> comprising a device <NUM>. The coupler comprises a waveguide <NUM>, to which is evanescently coupled a plurality of modulating elements <NUM>, so as to define a grating. The transmission characteristics of the grating are varied depending on the state of the modulating elements, so as to vary the transmission and reflection properties of the grating coupler <NUM>, thereby varying the degree to which an input signal <NUM> is reflected as return signal <NUM>, or transmitted as signal <NUM>.

<FIG> illustrates a device <NUM> according to an embodiment in which the state of the modulating element <NUM> may be altered by an electrical signal. The device <NUM> comprises an optical waveguide <NUM>, with a modulating element <NUM> evanescently coupled to the waveguide <NUM>. The state of the modulating element <NUM> modifies the transmission, reflection or absorption characteristics of the waveguide <NUM>, depending on its state. A resistor <NUM> is provided in thermal contact with (e.g. on top of, or adjacent to) the modulating element <NUM>. When an electrical current is passed through the resistor <NUM>, it will heat up the modulating element <NUM>, which results in the modulating element <NUM> changing state (as previously described). As previously described, an optical probe signal may be used to determine the state of the modulating element <NUM>.

The resistor <NUM> may comprise part of a conducting track, for example a metal or semiconductor track. In some embodiments the resistor <NUM> may comprise a material that is substantially transparent at the optical probe signal wavelength. The resistor <NUM> may, for example comprise a resistor track patterned over the modulating element. A dielectric layer or insulating layer may be interposed between the resistor <NUM> and the modulating element <NUM>.

<FIG> illustrates an embodiment in which the state of the modulating element <NUM> can be read and/or written electrically (i.e. by an electrical signal). The state of the modulating element <NUM> may further be read and/or written optically, by optical signals carried by the waveguide <NUM> (as described above). The device <NUM> comprises a waveguide <NUM>, modulating element <NUM>, first electrode <NUM> and second electrode <NUM>.

The first electrode <NUM> and second electrode <NUM> are both in electrical contact with the modulating element <NUM>, so that a voltage difference applied between the first and second electrodes <NUM>, <NUM> results in a current through the modulating element <NUM>. Where the modulating element <NUM> comprises a layer of material, it may be convenient for the first electrode <NUM> to be disposed under the layer, and the second electrode <NUM> to be disposed on top of the layer.

Alternatively, a lateral arrangement of electrodes may be used, in which the flow of current through the layer of the modulating element <NUM> is substantially in the plane of the layer.

The resistance of the modulating element <NUM> may be inferred from its voltage-current characteristics via the first and second electrodes <NUM>, <NUM>. The state of the modulating element <NUM> may thereby be inferred from an electrical probe signal applied to via the first and second electrodes <NUM>, <NUM>. Furthermore, the state of the modulating element <NUM> can be varied by Joule heating the modulating element <NUM> by applying a voltage difference between the first and second electrodes <NUM>, <NUM>.

At least one of the first and second electrodes <NUM>, <NUM> may comprise an optically transparent material, such as ITO. The optical reading and/or writing of the modulating element <NUM> may be substantially as described above, with reference to other embodiments.

Features of the example embodiments described with reference to <FIG> and <FIG> may be combined. For example, an arrangement that includes both a resistor <NUM>, enabling heating of the modulating element <NUM> by thermal conduction (without passing current through the modulating element <NUM>) may be combined with first and second electrodes <NUM>, <NUM>. Such a device may be electrically written via the resistor and/or first and second electrodes <NUM>, <NUM>. Furthermore, the device may be read and/or written optically, by optical signals in the waveguide <NUM>.

The stacked arrangement of layers depicted in the drawings is merely schematic, and each layer may be partially embedded within another layer (e.g. by an patterning and planarization process), or may be conformal over the topography of other layers. For example, the upper surface of the lower electrode in <FIG> may be coplanar with the upper surface of the waveguide core. Other variations are possible.

Electrical reading and/or writing may be more straightforward to interface with an electrical controller. Optical reading and/or writing may offer faster speed. Depending on the application, different combinations of electrical and optical reading and writing may be appropriate.

A device has been described that is suitable for use as an integrated, all-photonic, truly-nonvolatile memory that provides multi-level (e.g. <NUM> level) storage in a single cell along with multi-bit (e.g. <NUM> bit) wavelength division multiplexed access (via a single waveguide). A low-dimensional (i.e. small) modulating element (e.g. phase-change elements) may be integrated with a suitable waveguide. The modulating elements are switched between states by evanescent coupling to light travelling along the waveguides and are thus not restricted in size by the diffraction limit. Furthermore, the ability to switch readily and directly between more than two levels has been shown, with accurate control of the readout signal and excellent repeatability (capabilities that requires complex iteration based algorithms in electronic phase change memories). The capability for fast (-<NUM> ps), low power (-<NUM> fJ), single shot readout of the modulator element state has been shown, along with repeated (x100) write/erase cycling while maintaining high readout contrast. Embodiments of the invention are fully scalable: large arrays of all-optical memory elements can be realised in accordance with embodiments which are conveniently addressed, using WDM techniques, through on-chip waveguides; such attributes are applicable for the realization of practical on-chip optical interconnects. Embodiments of the present invention may be used for storage class memory.

Hybrid circuits exploiting modulating elements (which may comprise PCMs) may be enabled in accordance with embodiments, leading to new forms of non-conventional (non-von Neumann) computation and processing.

Embodiments of the present invention may be suitable for use in a neuromorphic or synaptic-based processor. For example, the accumulation of phase change in the modulating element may be exploited to operate a device as an accumulator or adder, which is a basis computational element. A phase change switching element can be configured to exhibit spike-timing dependant plasticity, which relies on relative spike timings from either side of the synapse. The gradual programming of state of a PCM modulating element using optical (or electrical) signals may be used to emulate a synaptic connection.

It will be appreciated that the vast majority of applications embodiments may comprise all optical reading and writing, but some embodiments may comprise an optical/electrical interface, so that electrical signals are provided to the device for reading and/or writing.

From reading the present disclosure, other variations and modifications will be apparent to the skilled person. Such variations and modifications may involve equivalent and other features which are already known in the art of photonic devices, and which may be used instead of, or in addition to, features already described herein.

Although the appended claims are directed to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention.

Claim 1:
A photonic device comprising (<NUM>): an optical waveguide (<NUM>), and a modulating element (<NUM>) that is evanescently coupled to the waveguide (<NUM>), the modulating element (<NUM>) comprising a phase change material; wherein the modulating element (<NUM>) modifies a transmission, reflection or absorption characteristic of the waveguide (<NUM>) dependant on its state, and the device (<NUM>) comprises an electrical conductor configured to switch the state of the modulating element (<NUM>) using an electrical signal that heats the modulating element (<NUM>);
characterised in that the electrical conductor comprises a resistor (<NUM>) in thermal contact with the modulating element (<NUM>), so that the modulating element is switchable by passing the electrical signal through the resistor (<NUM>), and heating the modulating element (<NUM>) primarily by conduction.