In our simulations, we initialize a fixed amorphous region and monitor the crystal growth for different probe powers. Figure 2 e shows a partial cross section of the GST volume at four different instances in time for a fixed 1. As pointed out in the introductory section, coincidence detection has important applications in neuromorphic computing based on the spatiotemporal structure of neuron spiking patterns, 24 the training of neural networks 25 and machine learning. We set the optical power of the WRITE pulses such that a single pulse will heat the GST below this switching threshold, but two overlapping pulses will surpass this threshold as illustrated in Figure 3 a.
Figure 3 b shows the measured optical power of four sets of these pulses with different time delays. It is worth noting that although the response time of our device is long compared with the time scale of the WRITE pulses tens of milliseconds versus tens of nanoseconds , we are still able to clearly resolve a 5 ns difference in time delay between two WRITE pulses due to the high speed melting process by which GST reaches an amorphous state.
In practice, the accuracy of this technique is largely dependent on the bandwidth and SNR of the photodetector used. To demonstrate the temporal response of our coincident detector, we performed an additional experiment using signals consisting of multiple pulses. Although in practice the repetition rate could be much higher limited experimentally to less than 20 MHz by our 50 ns WRITE pulse width , it is instructive to see the response of our device between individual WRITE pulses in the pulse train.
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At 0 ns time delay, the transmission remains almost constant throughout the duration of the pulse train with small visible spikes in the transmission where the overlapping WRITE pulses occur. As the time delay between the two pulse trains increases, the amplitude and duration of the transmission spikes decrease until they are no longer overlapping.
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As the time delay increases further, the transmission response disappears indicating the two pulse trains are no longer correlated in time. In a final experiment, we examine the volatile response of a 0. In this configuration, five 1. From the perspective of volatility, integrating GST in an optical resonator has the added benefit of lowering the probe power required for initiating volatility in addition to providing greater contrast between two transmission states.
In Figure 4 c, we show that the probe power required to recrystallize the GST is reduced by approximately a factor of four compared to the GST on the waveguide see Figure 2 b. As discussed previously, higher probe powers increase the material absorption and cause the WRITE pulses to amorphize a larger area of the GST resulting in an increased maximum switching contrast. This can be attributed both to the smaller area of GST 0.
While these results are beneficial for applications requiring volatility, we note that for nonvolatile applications, such as optical routing 39 or multiplexed photonic memory, 10 one must be aware that optical power in the resonator can have this volatile effect on optical PCMs. Higher switching contrast again reduces the speed of recrystallization due to lower absorption of the probe. While these results already show significant improvement over the device used in Figures 1 - 3 in terms of contrast, speed, and energy efficiency, further optimization in the coupling between the bus waveguides and ring resonator to better match the loss of the GST i.
In conclusion, we have observed volatile behavior in a traditionally nonvolatile PCM, demonstrating that the best of both worlds can be attained with a single material. By varying the optical power of the probe, we observed an exponential decrease in the data retention time of our PCM memory cell, enabling retention times ranging from years nonvolatile operation to milliseconds volatile operation.
Crucially, by developing an advanced multiphysics model, we show that the recrystallization dynamics of the materials at higher probe powers determine its volatility, and therefore can be controlled at will. This was achieved in a fully integrated and optical platform and used for both multilevel data storage and detecting coincident events between two binary signals.
By operating our device in a volatile manner, we were able to resolve timing differences as low as 5 ns between two pulses. We also reduced the power and speed requirements for both recrystallization and optical switching by an order of magnitude by integrating GST in a ring resonator.
Ne for stimulating conversations. All authors thank the collaborative nature of European science for allowing this work to be carried out. Several authors have filed patent applications in the field of photonic memory and computing including in the use of tunable volatility. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors.
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Photonic Integrated Circuits 1
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This is very beneficial in realizing advanced photonic integrated circuits PICs.