Layered PtSe2 for Sensing, Photonic, and (Opto-)Electronic Applications.

Since the first experimental discovery of graphene 16 years ago, many other 2D layered nanomaterials have been reported. However, the majority of 2D nanostructures suffer from relatively complicated fabrication processes that have bottlenecked their development and their uptake by industry for practical applications. Here, the recent progress in sensing, photonic, and (opto-)electronic applications of PtSe2 , a 2D layered material that is likely to be used in industries benefiting from its high air-stability and semiconductor-technology-compatible fabrication methods, is reviewed. The advantages and disadvantages of a range of synthesis methods for PtSe2 are initially compared, followed by a discussion of its outstanding properties, and industrial and commercial advantages. Research focused on the broadband nonlinear photonic properties of PtSe2 , as well as reports of its use as a saturable absorber in ultrafast lasers, are then reviewed. Additionally, the advances that have been achieved in a range of PtSe2 -based field-effect transistors, photodetectors, and sensors are summarized. Finally, a conclusion on these results along with the outlook for the future is presented.

Bacterially synthesized tellurium nanostructures for broadband ultrafast nonlinear optical applications.

Elementary tellurium is currently of great interest as an element with potential promise in nano-technology applications because of the recent discovery regarding its three two-dimensional phases and the existence of Weyl nodes around its Femi level. Here, we report on the unique nano-photonic properties of elemental tellurium particles [Te(0)], as harvest from a culture of a tellurium-oxyanion respiring bacteria. The bacterially-formed nano-crystals prove effective in the photonic applications tested compared to the chemically-formed nano-materials, suggesting a unique and environmentally friendly route of synthesis. Nonlinear optical measurements of this material reveal the strong saturable absorption and nonlinear optical extinctions induced by Mie scattering over broad temporal and wavelength ranges. In both cases, Te-nanoparticles exhibit superior optical nonlinearity compared to graphene. We demonstrate that biological tellurium can be used for a variety of photonic applications which include their proof-of-concept for employment as ultrafast mode-lockers and all-optical switches.

Ultrafast Carrier Dynamics and Bandgap Renormalization in Layered PtSe2.

Carrier interactions in 2D nanostructures are of central importance not only in condensed-matter physics but also for a wide range of optoelectronic and photonic applications. Here, new insights into the behavior of photoinduced carriers in layered platinum diselenide (PtSe2 ) through ultrafast time-resolved pump-probe and nonlinear optical measurements are presented. The measurements reveal the temporal evolution of carrier relaxation, chemical potential and bandgap renormalization in PtSe2 . These results imply that few-layer PtSe2 has a semiconductor-like carrier relaxation instead of a metal-like one. The relaxation follows a triple-exponential decay process and exhibits thickness-dependent relaxation times. This occurs along with a band-filling effect, which can be controlled based on the number of layers and may be applied in saturable absorption for generating ultrafast laser pulses. The findings may provide means to study many-body physics in 2D materials as well as potentially leading to applications in the field of optoelectronics and ultrafast photonics.

Intensity-dependent nonlinear refraction of antimonene dispersions in the visible and near-infrared region.

Antimonene is a stable 2D allotrope of antimony that is predicted to have a direct bandgap, high third-order optical nonlinear susceptibility, and high electron mobility. These properties give it huge potential applications in photonics and optoelectronics. However, the nonlinear refractive response of antimonene dispersions, which is the key to nonlinear refraction-based devices, has not been fully investigated. In this work, we investigated the optical nonlinearities of the antimonene dispersions by spatial self-phase modulation (SSPM) at 405, 785, and 1064 nm wavelengths. The SSPM rings were observed at 405, 785, and 1064 nm, implying the broadband nonlinear optical response of antimonene dispersions from visible to near-infrared. The effective nonlinear refractive index, n2, and the third-order susceptibility, χ(3), of the antimonene dispersion were measured to be ∼10-5 cm2 W-1 and ∼10-8 esu, respectively. Furthermore, the nonlinearity of antimonene was demonstrated to be tuneable by the laser intensities. The relative change of the nonlinear refractive index, Δn2e/n2e, was observed to range from 14% to 63% for different intensities. Our results will be helpful for the photonic applications of antimonene in a broadband wavelength range, such as optical modulators and switchers.

Ultrafast Nonlinear Excitation Dynamics of Black Phosphorus Nanosheets from Visible to Mid-Infrared.

The recent progress on black phosphorus makes it a promising candidate material for broadband nanophotonic devices, especially operating in the mid-infrared spectral region. Here, the excited carrier dynamics and nonlinear optical response of unoxidized black phosphorus nanosheets and their wavelength dependence were systematically studied from 800 nm to 2.1 µm. The wavelength-dependent relaxation times of black phosphorus nanosheets are determined to be 360 fs to 1.36 ps with photon energies from 1.55 to 0.61 eV. In a comparative study with graphene, we found that black phosphorus has a faster carrier relaxation in near- and mid-infrared region. With regard to nonlinear optical absorption, the response of black phosphorus significantly increases from near- to mid-infrared, and black phosphorus is also confirmed to be better as saturable absorber to MoS2 in infrared region.

Saturable absorption behavior of free-standing graphene polymer composite films over broad wavelength and time ranges.

A comparative research on saturable absorption (SA) behavior dependence on wavelength and pulse duration was performed for graphene polymer composites. Free-standing graphene-polyvinyl alcohol (PVA) composite films were fabricated by using solution cast method in combination of liquid phase exfoliation. SA responses were observed by using an open-aperture Z-scan technique for 340 fs pulses at 1030 nm and 515 nm from a mode-locked fiber laser, and 6 ns pulses at 1064 nm and 532 nm from a Q-switched Nd:YAG laser. The graphene films possess better SA property, i.e., larger SA coefficient and figure of merit (FOM), and lower saturation intensity I(s), for ns pulses than that for fs pulses at the similar near infrared (NIR) wavelength. For fs pulses, the films show better SA response at 1030 nm than that at 515 nm. By employing slow and fast SA modelling, the excited state and ground state absorption cross sections were estimated to be ~10(-17) cm(2), and the ratio was ~0.6 at NIR for both fs and ns pulses.