Electrically Tuneable Silicon-Based Photomodulator for THz Applications

Silicon is a well-established semiconductor which is extensively used in a wide variety of applications; it is the basic component for photovoltaic solar cells and is a widespread substrate in many photonic devices. More recently, silicon has found new use in the ever-growing research area of terahertz (THz) waves. For example, silicon can serve as a THz photomodulator i.e., when the silicon is illuminated by an above bandgap optical source which renders the silicon opaque to the THz radiation. The opacity level (modulation depth) and the switching speed between the transparent and opaque states of silicon (modulation speed) are determined by the lifetime of the photoexcited charge carriers in the silicon wafer. For some applications (such as in THz single-pixel imaging) the effective lifetime of silicon is a critical parameter, which must be carefully considered to reach the best trade-off possible between the modulation depth and the modulation speed. However, neither bare silicon (too low) nor routinely surface passivated silicon (too high) yields effective lifetimes that are best suited for such THz applications. Hence we suggest a novel device configuration featuring controlled carrier lifetime modulation of Si-based THz modulators via voltage application across a transparent gate electrode.<br/><br/>In this work, we report the fabrication, characterisation, and implementation in a THz imaging setup of such a photomodulator. We deposited hafnium oxide via atomic layer deposition as the passivation layer on both side of a high-resistivity silicon substrate. The hafnium oxide provides a sufficient electrical insulation and an adequate surface passivation level yielding lifetimes in the hundreds of microseconds. We then coated the hafnium oxide layers with poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) as the conductive layer for the electrical gating. The choice of PEDOT:PSS is particularly relevant for THz photomodulation as it is transparent to both the THz and the visible light. We performed an injection-dependent lifetime characterisation of the photomodulator using standard photoconductance decay (PCD) measurements while a bias voltage in a -10V to +10V range was applied between the PEDOT:PSS top layer and the silicon layer. We observed results for the effective lifetime as a function of the applied bias voltage that are consistent with similar devices previously reported in literature. We also confirmed the applied voltage homogeneity across the whole photomodulator surface by repeating a similar experiment using a photoluminescence imaging technique.<br/><br/>We implemented the electrically tuneable photomodulator in a THz time domain spectroscopy setup (THz-TDS) capable of single-pixel imaging. Similar to the PCD measurements, we first characterised the photomodulator effective lifetime by monitoring the THz radiation decay for a range of photoexcitation intensities and applied bias voltages. The lifetime curves extracted from this THz-TDS photomodulation configuration showed comparable trends with the lifetime curves produced by PCD. To demonstrate the potential of this electrically tuneable THz photomodulator, we performed single-pixel imaging where the photoexcitation source is now spatially structured so that the photomodulator acts as a THz spatial light modulator. We showed that tuning only the bias voltage, which controls the photomodulator effective lifetime, can massively impact the THz imaging. With such silicon-based THz photomodulator, the effective lifetime is not a constraining fixed factor but rather an additional degree of freedom for THz photomodulation.