Abstract:The emergence of silicon photonics over the past two decades has established silicon as a preferred substrate platform for photonic integration. While most silicon-based photonic components have so far been realized in the near-infrared (near-IR) telecommunication bands, the mid-infrared (mid-IR, 2-20-μm wavelength) band presents a significant growth opportunity for integrated photonics. In this review, we offer our perspective on the burgeoning field of mid-IR integrated photonics on silicon. A comprehensive survey on the state-of-the-art of key photonic devices such as waveguides, light sources, modulators, and detectors is presented. Furthermore, on-chip spectroscopic chemical sensing is quantitatively analyzed as an example of mid-IR photonic system integration based on these basic building blocks, and the constituent component choices are discussed and contrasted in the context of system performance and integration technologies.

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Passive Q-switching or mode-locking by placing a saturable absorber inside the laser cavity is one of the most effective and popular techniques for pulse generation. However, most of the current saturable absorbers cannot work well in the visible spectral region, which seriously impedes the progress of passively Q-switched/mode-locked visible pulsed fibre lasers. Here, we report a kind of visible saturable absorber-two-dimensional transition-metal dichalcogenides (TMDs, e.g. WS2, MoS2, MoSe2), and successfully demonstrate compact red-light Q-switched praseodymium (Pr(3+))-doped all-fibre lasers. The passive Q-switching operation at 635 nm generates stable laser pulses with ∼200 ns pulse duration, 28.7 nJ pulse energy and repetition rate from 232 to 512 kHz. This achievement is attributed to the ultrafast saturable absorption of these layered TMDs in the visible region, as well as the compact and all-fibre laser-cavity design by coating a dielectric mirror on the fibre end facet. This work may open a new route for next-generation high-performance pulsed laser sources in the visible (even ultraviolet) range.

In this paper, both nonlinear saturable absorption and two-photon absorption (TPA) of few-layer molybdenum diselenide (MoSe2) were observed at 1.56 μm wavelength and further applied to mode-locked ultrafast fiber laser for the first time to our knowledge. Few-layer MoSe2 nanosheets were prepared by liquid-phase exfoliation method and characterized by x ray diffractometer, Raman spectroscopy, and atomic force microscopy. The obtained fewlayer MoSe2 dispersion is further composited with a polymer material for convenient fabrication of MoSe2 thin films. Then, we investigated the nonlinear optical (NLO) absorption property of the few-layer MoSe2 film using a balanced twin-detector measurement technique. Both the saturable absorption and TPA effects of the few-layer MoSe2 film were found by increasing the input optical intensity. The saturable absorption shows a modulation depth of 0.63% and a low nonsaturable loss of ∼3.5%, corresponding to the relative modulation depth of 18%. The TPA effect occurred when the input optical intensity exceeds ∼260 MW∕cm 2. Furthermore, we experimentally exploit the saturable absorption of few-layer MoSe2 film to mode lock an all-fiber erbium-doped fiber laser. Stable soliton mode locking at 1558 nm center wavelength is achieved with pulse duration of 1.45 ps. It was also observed that the TPA process suppresses the mode-locking operation in the case of higher optical intensity. Our results indicate that layered MoSe2, as another two-dimensional nanomaterial, can provide excellent NLO properties (e.g., saturable absorption and TPA) for potential applications in ultrashort pulse generation and optical limiting.