Fig. 4. Demonstration of SR in monolayer MoS₂ photodetector.

a White Gaussian noise of different standard deviations (σ) superimposed on 2.5 Hz periodic and subthreshold LED signal of amplitude V_LED = 2.4 V and b corresponding histogram of the LED voltage distribution. Each histogram shows two Gaussian curves of the same variance but different means, one centered at V_LED = 1 V (LED OFF) and the other one centered at V_LED = 2.4 V (Dim LED), marking the level difference between the two states of the LED signal. c PSD plots obtained from the FFT of the output current (I_DS) measured in the monolayer MoS₂ photodetector for a total duration of T_P = 204.8 s in response to the LED signals in (a). The MoS₂ photodetector was biased in the OFF-state (V_BG = −2 V). In the presence of a finite and appropriate amount of noise, the subthreshold LED signal is detected. (see Supplementary video files 1–3 for real-time observation of SR in the MoS₂ photodetector). d SNR as a function of σ for different sampling time T_P. For very low variance Gaussian noise (σ < 0.2 V), the signal hardly crosses the detection threshold of the MoS₂ photodetector, i.e., V_LED = 2.6 V. As σ increases, a larger fraction of the LED signal corresponding to the 2.4 V level crosses the detection threshold of the MoS₂ photodetector, increasing the SNR. For even larger variances, the SNR starts to decrease since the tail of the Gaussian distribution centered at V_LED = 1 V will also start to cross the detection threshold of the MoS₂ photodetector, obscuring the periodicity of the V_LED = 2.4 V signal. The SNR curves in this instance show the typical SR response found in biological species such as paddlefish, crayfish, etc. e Energy consumption by the MoS₂ photodetector is plotted as a function of σ for different T_P. Since the MoS₂ photodetector was operated in the deep OFF-state (V_BG = −2 V) to minimize the dark current, the operating power budget is drastically reduced, making the signal detection extremely energy efficient. For comparison, refer to Supplementary Note 9 for the exploitation  of SR using the Si photodiode (see Supplementary video files 4–6 for real-time observation of SR in the Si photodiode). f The energy consumption by the Si photodiode for detecting subthreshold optical signal, exploiting SR, is found to be in the range of a few micro-Joules, which is 1000X higher compared to the MoS₂ photodetector for sampling time T_P = 100 s.