Wide-spectral saturable absorption (SA) has been experimentally demonstrated in two-dimensional (2D) nanomaterials with outstanding performance, such as low saturation intensity, deep modulation depth, and fast recovery time of excited carriers. Hence, 2D nanomaterials can be utilized as saturable absorbers for mode-locking or Q-switching to generate laser pulses with short duration and high repetition rate. Here, the SA properties of graphene, layered transition metal dichalcogenides, Group-V elements, and other 2D nanomaterials are reviewed by summarizing their slow-or fastsaturable absorption behavior using the modified Frantz-Nodvik model or the steady-state solution of Hercher's rate equations. The dependence of SA in 2D nanomaterials on excitation wavelength, linear absorption coefficient, and pulse duration is also explained. Finally, the applications of these 2D nanomaterials in a range of pulsed lasers are summarized. Figure 2. a-c) Schemes showing the principle of open-aperture (a), closed-aperture Z-scans (b), and I-scan (c).

Mid‐infrared ultrafast lasers have emerged as a promising platform for both science and industry because of their inherent high raw power and eye‐safe spectrum. 2D nanostructures such as graphene have emerged as promising photonic materials for laser mode‐locking to generate ultrashort pulses. However, there are still many unanswered questions about graphene's key advantages to be practical devices, especially over the matured semiconductor saturable absorber mirror (SESAM). In this work, we conducted systematic comparisons on the nonlinear optical properties of graphene and that of a commercial SESAM at 2 μm wavelength. Our results showed that graphene has significant advantages over the commercial SESAM, exhibiting ∼28% less absorptive cross‐section ratio of excited‐state to ground‐state and ∼50 times faster relaxation time. This implies that graphene can be exploited as a better mode‐locker than the current commercial SESAM for high power, high repetition rate and ultrafast mid‐infrared laser sources.

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We describe a nanopore-based optofluidic instrument capable of performing low-noise ionic current recordings of individual biomolecules under laser illumination. In such systems, simultaneous optical measurements generally introduce significant parasitic noise in the electrical signal, which can severely reduce the instrument sensitivity, critically hindering the monitoring of single-molecule events in the ionic current traces. Here, we present design rules and describe simple adjustments to the experimental setup to mitigate the different noise sources encountered when integrating optical components to an electrical nanopore system. In particular, we address the contributions to the electrical noise spectra from illuminating the nanopore during ionic current recording and mitigate those effects through control of the illumination source and the use of a PDMS layer on the SiN membrane. We demonstrate the effectiveness of our noise minimization strategies by showing the detection of DNA translocation events during membrane illumination with a signal-to-noise ratio of ∼10 at 10 kHz bandwidth. The instrumental guidelines for noise minimization that we report are applicable to a wide range of nanopore-based optofluidic systems and offer the possibility of enhancing the quality of synchronous optical and electrical signals obtained during single-molecule nanopore-based analysis.

Polyvinyl alcohol (PVA) was utilized as a matrix to host two-dimensional (2D) liquid-phase-exfoliated MoSe 2 nanosheets. These 2D MoSe 2 /PVA composite thin films were experimentally proven to be preferable for efficient nonlinear optical devices. Our nonlinear optical study shows that these composite thin films possess strong saturable absorption (SA) over a wide wavelength range from 400 nm to 800 nm under the irradiation of femtosecond and nanosecond lasers. The SA property of our films was measured for various laser pulse durations, wavelength and linear absorption. Moreover, employing pump-probe, excitonexciton annihilation was experimentally observed and studied at 800 nm. Our research gives clear insight into the photophysical properties of MoSe 2 /PVA thin films and shows the material's potential as a photonic device.