# All-Optical Modulation and Ultrafast Switching in MWIR with Sub-Wavelength Structured Silicon
## 1 Introduction  
Silicon, as the most common semiconductor in electronic integrated circuits, has been considered as one of the best candidates for the natural hybrid of photonic and electronic devices [1]. In recent years, various silicon-based devices have been proposed for the generation, modulation and detection of light [2; 3; 4; 5]. The combination of silicon photonics with electronic devices not only brings the advantage of improved bandwidth and fundamental speed, but also shows the potential of massive production with low fabrication cost in comparison with other semiconductors. Among all the silicon-based photonics units, the optical modulator, as one of the most critical components in optical communication, thermal sensing and imaging systems, has drawn great research interest lately [6]. The conventional approach to optical modulation is based on the first order electro-optical effect (Pockels effect) of materials. That is, by applying an external electric field, the dielectric function of the material alters and results in a modification of the optical response and a change of the reflectance or transmittance. However, as a centrosymmetric semiconductor, intrinsic silicon has a weak electro-optical coefficient and therefore is not an ideal material for implementation in electro-optical modulation [7; 8]. As an alternative method, thermal-optical modulation was investigated by [PERSON] et al., but it is only suitable for low repetition rate applications due to its slow response [9; 10]. Currently, the free carrier plasma dispersion effect, where the material optical response varied with the free carrier density, is the most commonly-investigated modulation mechanism in silicon [11]. The free carrier density can be altered by the carrier depletion, accumulation or injection by either electronic or optical means [12]. The latter case is known as all-optical modulation,which has its primary advantage in ultrafast modulation speed, typically in the picosecond range [13]. As early as in 2004, [PERSON] et al. demonstrated a scheme of ultrafast all-optical modulation with silicon photonic structures in telecommunication wavelengths. A modulation depth of 90% with less than a 500-ps modulation speed was achieved [14]. Soon after, [PERSON]'s team reported an improved modulation speed of 70 ps by using ion-implanted silicon photonic crystal nanocavities [15]. So far, most of the research interests and achievements in silicon optical modulators (including the examples mentioned above) have explored the range of telecommunication wavelengths. The Mid- to Long Wavelength Infrared region (MWIR and LWIR, respectively) has so far attracted less attention, despite the enormous potential and high demand on the market [16]. The most demanding applications operating in MWIR, such as thermal imaging, security countermeasures and free space communication, exploit the high transmittance of the Earth's atmosphere in the wavelength range between 3.5 and 5 \(\upmu\)m. A longer wavelength assures that there is less scattering compared to the near-infrared and visible light, as well as no active vibrational transitions of H\({}_{2}\)O and CO\({}_{2}\) molecules, making it suitable for remote sensing and detection with little influence from weather conditions [17]. Despite the importance and maturity of the current MWIR detection and imaging technology, the present applications rely entirely on electronic and mechanical modulators, which limits the modulation speed to a few hundreds of nanoseconds. The development of all-optical modulators aims not only at improving the performance speed, but also at enabling new functionalities, such as ultrafast time-of-flight detection, anti-fluctuation atmosphere imaging and broadband free space communication, based on the current MWIR detection technologies [18]. In 2011, [PERSON] et al. theoretically demonstrated the feasibility of all-optical modulation with crystalline silicon in a broad infrared range (1-14 \(\upmu\)m) [19]. More recently, [PERSON] et al.