Search

We present the first ultrafast optical pump–probe spectroscopic measurements, to the best of our knowledge, on silicon nanowires (SiNWs). In this study, we performed femtosecond pump–probe measurements on vapor–liquid–solid-grown SiNWs to investigate the influence of the NW diameter, pump and probe polarizations, and pump fluence on the observed dynamics while tuning the probe wavelength below and above the indirect bandgap in Si. For smaller NW diameters, carriers were found to relax more rapidly into both extended and localized states, indicating that a surface-mediated mechanism governs the observed dynamics. The magnitude of the photoinduced transmission change exhibited strong polarization dependence, showing that optical transitions in these quasi-1D systems are highly polarized along the NW axis. Finally, density-dependent experiments revealed that the relaxation time decreases with increasing photoexcited carrier density for an above bandgap probe; however, no significant density-dependent changes in the relaxation dynamics were observed when probed below the bandgap. In short, our experiments reveal the influence of diameter, polarization, and carrier density on carrier dynamics in SiNWs, shedding light on the phenomena that govern carrier relaxation in these important nanosystems and giving insight on their future use in nanophotonic applications. [ABSTRACT FROM PUBLISHER]

This article presents a digitally-assisted multi-channel neural recording system. The system uses a 16-channel chopper-stabilized Time Division Multiple Access (TDMA) scheme to record multiplexed neural signals into a single shared analog front end (AFE). The choppers reduce the total integrated noise across the modulated spectrum by 2.4 × and 4.3 × in Local Field Potential (LFP) and Action Potential (AP) bands, respectively. In addition, a novel impedance booster based on Sign-Sign least mean squares (LMS) adaptive filter (AF) predicts the input signal and pre-charges the AC-coupling capacitors. The impedance booster module increases the AFE input impedance by a factor of 39 × with a 7.1% increase in area. The proposed system obviates the need for on-chip digital demodulation, filtering, and remodulation normally required to extract Electrode Offset Voltages (EOV) from multiplexed neural signals, thereby achieving 3.6 × and 2.8 × savings in both area and power, respectively, in the EOV filter module. The Sign-Sign LMS AF is reused to determine the system loop gain, which relaxes the feedback DAC accuracy requirements and saves 10.1 × in power compared to conventional oversampled DAC truncation-error ΔΣ-modulator. The proposed SoC is designed and fabricated in 65 nm CMOS, and each channel occupies 0.00179 mm 2 of active area. Each channel consumes 5.11 μW of power while achieving 2.19 μV rms and 2.4 μV rms of input referred noise (IRN) over AP and LFP bands. The resulting AP band noise efficiency factor (NEF) is 1.8. The proposed system is verified with acute in-vivo recordings in a Sprague-Dawley rat using parylene C based thin-film platinum nanorod microelectrodes.