Experimental Investigation of Refractory Time of Optically Induced Spiking in Resonant Tunnelling Diode Photodiodes

We report experimental investigation of time refractory time on optoelectronic neuromorphic receiver circuit built with a nanoscale resonant tunnelling diode-photodetector (RTD-PD), capable to produce optically induced spiking events with nanosecond repetition rate. RTDs are voltage-controlled nano-electronic semiconductor epilayer elements that includes a double barrier quantum well structure which due to the resonant tunnelling effect has an N-shaped current-voltage (IV) curve exhibiting wide-bandwidth negative differential conductance (NDC), which provides electrical gain and amplification [1]. The RTD-PDs employed consist of a InGaAlAs/InP based double barrier quantum well (DBQW) RTD that incorporates a 250 nm InGaAs layer adjacent to the DBQW for light absorption in the wavelength range ∼1000 nm to ∼1600 nm. An RTD-PD can be used to implement controllable excitability in RTD-PD driving circuits where the load line intersects the RTD-PD I-V just in one of the PDC regions not far from the NDC region, procuring an all-or-none response depending on the amplitude of the optical stimulus. Fig. 1(a) shows the equivalent lumped electrical schematics of RTD-PD based light-triggered spike firing circuit – the RTD-PD consist of a InGaAs/AlAs RTD with a 100 nm intentionally added InGaAs spacer layer for light absorption [2]; Fig. 1(b) describes the generic path of the circuit operating point when the optical pulse intensity overcomes the firing threshold; Fig. 1(c) presents the typical spike response to an optical stimulus of a regular train of optical pulses - the optical pulses are 1 ns long, being coupled to the RTD-PD via a single mode optical fiber, and the circuit response was measured using a high-bandwidth oscilloscope (for more details see [2]); and Fig. 1(d) show the circuit smallest possible separation between two consecutive spikes ≈2.5 ns, corresponding to the refractory or restoring time of the excitable circuit.