Context: There are many 2D materials with inherent non-linear optical properties. The interaction of high intensity lasers with such materials on a surface can switch on unique optical phenomena in the material. Such properties can in turn be exploited to make microstructures suitable for devices in applications like electronics, photonics and catalysis.
TL;DR: This study explores how intense light interactions can shape two-dimensional materials like black phosphorus into precise nanoscale structures beyond the optical diffraction limit. By leveraging strong light−matter interactions, the researchers demonstrate the creation of ultrafine nanostructures with unique properties, paving the way for advancements in nanotechnology and optical lithography techniques for 2D materials.
In this study conducted by an all Korean group of researchers from Ulsan National Institute of Science and Technology (UNIST), Yonsei University, Korea University, and the Institute for Basic Science delves into the transformative power of intense, ultrashort-pulsed light in shaping two-dimensional (2D) matter into intricate nanoscale architectures. This study explores the utilization of strong light−matter interactions to manipulate 2D materials, with a particular emphasis on black phosphorus (BP).
nanosculpting BP with femtosecond-pulsed light. Incident light induces self-organized optical fields in BP, resulting in highly regular nanostructures over micrometer area by structured periodic ablation with a width of approximately 50 nm. Inset: side view of experimental layout. (b) Selected-area diffraction pattern of BP flake in panel c; scale bar: 5 1/nm. Anisotropic crystal structure containing AC (010) and ZZ (100) lattices. (c) Optical micrograph of 30-layered single-crystalline BP flake(sample 1). Dotted circle indicates irradiated area. Scale bar: 20 μm. (d) Low-magnification bright-field TEM image of the BP nanoribbon array atmarked position (square) in panel c. Orientation (grating vector) of the array is parallel to light polarization (denoted above panel). Crystalline axes (AC and ZZ) are denoted. Scale bar: 1 μm. Inset: fast-Fourier transform of array; scale bar: 50 μm−1. (e) Enlarged view of marked region in paneld. Intensity profile across array (dashed line). Typical width of nanoribbon is 50 nm; scale bar: 200 nm. (f) AFM profile of the BP nanoribbon array(sample 2). (g) High-resolution bright-field TEM image of nanoribbon edge (sample 3); scale bar: 2 nm.
BP, known for its single-elemental composition and strong in-plane anisotropy, serves as an ideal candidate for exploring the potential of light-induced nano structuring. In-plane anisotropy in optical materials refers to the variation in optical properties, such as refractive index or absorption, depending on the direction of light propagation within the plane of the material. This characteristic can significantly influence the behavior of light within these materials, affecting phenomena like polarization and light dispersion. By leveraging the unique properties of BP, such as its optical, vibrational, electronic, thermal, and mechanical characteristics, the researchers aim to push the boundaries of nanoscale engineering.
One of the key highlights of this study is the ability to tailor 2D matter into one-dimensional (1D) and quasi-zero-dimensional (0D) nanostructures with remarkable precision. The researchers demonstrate the creation of nanostructures that are up to two orders of magnitude smaller than the wavelength of the incident light (used as the tool for nano structuring), approaching dimensions where quantum effects come into play. This level of control over nanoscale architectures opens up new possibilities for designing advanced devices and systems with tailored functionalities.
Exposure: The methodology employed in this study involves wide-field illumination using intense, ultrashort-pulsed light to induce laser-induced periodic surface patterns on BP. Unlike traditional bulk materials, 2D matter exhibits more intricate and controllable periodic structures under pulsed light, showcasing the enhanced light−matter interactions achievable in 2D systems.
The interaction of BP with the pulsed light leads to multiple effects (MI), which is refers to the complex nonlinear optical interactions that occur when intense, ultrashort-pulsed light interacts with a material surface. MI results in unique phenomena such as filamentation of light, self-trapping, and the formation of intricate nanostructures. A simulation of MI electric field on BP could be seen in Figure 1. The bright lines in the figure represent the 2D field intensity arising on the BP surface due to MI.
The visualization in Figure 2 vividly demonstrates how the intensity of the electric fields evolves with increasing laser fluence, showcasing the intricate patterns and distributions of the self-trapped light induced by MI on the BP surface. This simulation provides valuable insights into the complex interactions between light and matter at the nanoscale, offering a deeper understanding of the mechanisms driving the nanostructuring process in the study.
The laser fluences used in the simulations are set to 30, 50, 70, and 90 mJ/cm², progressing from panels a to d. The incident polarization for all panels is set as horizontal. To simulate the effects of both MI and Surface Waves (SW), periodic boundary conditions are applied in the vertical direction to create an edge-reflection free region, while the finite size effect is considered in the horizontal direction.
The laser is exposed to the sample using wide-field illumination by intense, ultrashort-pulsed light. The specimen is excited using an ytterbium-based amplifier with a repetition rate of 1 kHz and a pulse duration ranging from 550 fs to 10 ps. The femtosecond laser output is frequency-doubled or -quadrupled to 515 and 257 nm, respectively, and then incident onto the specimen. The excitation beam has a full width at half-maximum (FWHM) of 30 μm and an angle of incidence of 3−4°. By varying the excitation polarization using zero-order half-/quarter-waveplates, the researchers can control how the laser light interacts with the sample, leading to the formation of nanostructures with high precision and detail.
By conducting experiments on BP flakes transferred to TEM grids with SiO2 and Si3N4 substrates, the researchers were able to minimize surface degradation and optimize sample fabrication conditions within a controlled environment. To visualize the tailoring process of 2D materials in real-time, the researchers utilized in situ transmission electron microscopy (TEM) coupled with optical access to the specimen. This innovative approach allowed for the direct observation of the effects of intense light on BP nanostructures, providing valuable insights into the mechanisms underlying the transformation of 2D matter. By capturing micrographs and diffractograms using a CMOS-based retractable direct electron detector, the researchers obtained high-resolution images that shed light on the intricate nanostructuring process.
This work represents a significant advancement in the field of nanoscale architecture engineering. By harnessing the power of intense light interactions, the researchers have demonstrated the ability to sculpt 2D materials with unprecedented precision, opening up new avenues for the development of next-generation nanodevices and technologies. Formation of complex structures by this approach is not trivial and would require fundamental understanding of the materials optical properties. However once a materials is well understood it should be sufficiently easy to replicate. Different nanostructures fabricated by this laser exposure technique can be seen in Figure 3.
Reference: Tailoring Two-Dimensional Matter Using Strong Light–Matter Interactions
