Optical near-field imaging and nanostructuring by means of laser ablation

Scanning electron microscope (SEM) photos showing SLC of 0.1-µm alumina particles from a Si surface (with water film assistance). The KrF laser (pulse duration 16 ns, wavelength λ = 248 nm) had a fluence of 120 mJ/cm², and 20 pulses were used. Figure reproduced with permission from ref.³, American Institute of Physics.

SEM image (50 × 50 µm²) of 480 nm PS particles spin coated on a silicon wafer. The insert shows a blow-up demonstrating the spherical shape of these colloid particles. Figure reproduced with permission from ref.¹², Spring Nature.

Experimental SLC efficiency for various PS spheres vs. laser fluence. The number of cleaning pulses from a Nd:YAG laser (λ = 532 nm, FWHM = 8 ns) was 20 in each case. The cleaning threshold is size-independent, and the efficiency exhibits a steep increase above the onset of cleaning (110 mJ/cm²). The same threshold is obtained when only 1 cleaning pulse is applied. Figure reproduced with permission from ref.¹⁵, SPIE.

Cleaning efficiencies for ns DLC (40 laser shots each) and SLC (20 shots each). The DLC experiment was carried out in “normal” laboratory atmosphere, without the application of an additional vapor pulse as for the SLC experiment in Fig. 3. Figure reproduced with permission from ref.¹⁵, SPIE.

Scanning electron micrograph of a cleaned area (DLC, λ = 583 nm, FWHM 30 ps) in a sample contaminated with 800 nm PS spheres. Elliptical holes can be found over the illuminated region, and also a remaining particle can be seen. Figure reproduced with permission from ref.¹⁵, SPIE.

Mie calculation of the optical near field of a spherical transparent particle with a diameter d >> laser wavelength λ. The scale on the color code gives the enhancement factor.

Holes generated during DLC of a silicon surface with a fs laser pulse (150 fs, λ = 800 nm) by an aggregate of six 1700 nm PS spheres. Figure reproduced with permission from ref.¹⁵, SPIE.

Holes generated during DLC of a silicon surface with a fs laser pulse (150 fs, λ = 800 nm). (a) laser beam incident at an angle of 45° from the left side. (b) 45° incidence from the back. Figure reproduced with permission from ref.²⁷, John Wiley and Sons. A more detailed description of ablation effects upon inclined irradiation of nanospheres has been presented by Wang et al.¹³¹.

Calculated field intensities for spheres (PS, refractive index n = 1.6) that are irradiated at λ = 800 nm. The diameters of the spheres are 1700 nm (a,d), 800 nm (b, e) and 320 nm (c, f). Plotted is the intensity in a cross section as seen from the side (a–c) and in the plane of the substrate surface as seen from above (d–f). The light field enters from the top, and the polarization vector of the electric field is indicated by an arrow. The displayed areas are 5100 nm × 5100 nm, 2400 nm × 2400 nm, and 960 nm × 960 nm, respectively. Figure reproduced with permission from ref.¹⁵, SPIE.

Near-field of a colloidal sphere 320 nm in diameter. (a) Mie calculation of the optical near-field intensity in the substrate plane. (b) AFM picture of the silicon substrate under the colloidal sphere after irradiation with a femtosecond laser pulse (150 fs, λ = 800 nm). The particle itself has been removed due to material ablation. Figure reproduced with permission from ref.⁸⁵, AIP Publishing.

Femtosecond-resolved ablation dynamics of Si in the near field of a small dielectric sphere, appearing as a black disk in the top left frame. Shown are surface reflectivity images at the particle location for different time delays (indicated in the upper right corners) upon exposure to a 120 fs-laser pulse incident from the left at an angle of 54°. With the exception of the images corresponding to the surface before and after the arrival of the laser pulse, the images have been normalized to an image obtained blocking the pump pulse in order to minimize the scattering effects caused by the 7.9 μm-diameter particle. A zoom (2×) has been applied to the relevant part of the image and pasted as an inset. Figure reproduced from ref.²⁵, under a Creative Commons Attribution License 2.0.

Near-field induced holes created by irradiating clusters of 800 nm PS spheres on silicon. (a) Trimer irradiated with a 100 fs pulse. (b) Two dimers irradiated with an 8 ns pulse. The wavelength was in both cases λ = 800 nm. Figure reproduced with permission from ref.³³, SPIE.

(a) Nanohole pattern generated by illuminating an ordered array of PS microspheres with a single laser pulse (τ = 150 fs, λ = 800 nm). Some remaining spheres are also seen. (Sphere diameter 800 nm; similar images are shown in ref.³³). (b) Silicon nanoholes fabricated on a (100) Si surface by using a single-shot 265.7-nm Ti:sapphire laser radiation (300 fs, fluence 15 ± 3.5 mJ/cm²) together with a regular lattice of SiO₂ microspheres (r = 150 nm). The width of the holes at FWHM is 57 ± 6.5 nm and their depth with respect to the original silicon surface 6 ± 1 nm. Figure reproduced with permission from ref.⁴¹, Springer Nature.

(a) (i) and (ii) SEM image of a periodic pit array in a 35 nm thick Al film on silicon, formed after the illumination of 0.95 µm SiO₂ particles by a single pulse with a fluence of 300 mJ/cm² (KrF excimer laser, λ = 248 nm, pulse width 23 ns). (b) AFM image and depth profile of a pit. Figure reproduced with permission from ref.³⁶, AIP Publishing.

(a) Schematic diagram of the experimental configuration for direct laser writing of nanoline arrays on a Sb₇₀Tb₃₀ substrate surface. (b) SEM image of two hexagonal nanodot arrays ablated by a single KrF laser pulse (pulse width 15 ns) at an incident angle of 0° (with a fluence of 6.5 mJ/cm²) and 30° (with a fluence of 1.0 mJ/cm²), respectively. Figure reproduced with permission from ref.⁵², AIP Publishing.

AFM scan of a hole created in polycarbonate by irradiation of a 760 nm bead with a 15 ns laser pulse (λ = 355 nm) at 2.4 J/cm². A cross-profile of the central part is shown at the bottom. Figure reproduced with permission from ref.⁵⁵, Springer Nature.

(a) AFM image of a single triangular gold nanoparticle on glass before laser treatment. (b) and (c) AFM images of the sample after illumination with a single linearly polarized laser pulse with a pulse duration of 35 fs. The black arrows indicate the polarization direction of the laser light (note: according to the authors, the scale in plane has to be corrected from 2 µm/div to 0.2 µm/div). Figure reproduced with permission from ref.⁸⁸, Springer Nature.

(a) SEM image of a regular array of triangular nanostructures prepared by colloid lithography. The colloid monolayer used as an evaporation mask has only been partly removed in this case. (b) AFM image of one triangle. Note that the scale in vertical direction is enlarged compared to the scale in plane. Figure reproduced with permission from ref.¹⁰⁶, under the a Creative Commons Attribution License 2.0.

Gold triangles on silicon, side length 450 nm, 25 nm thick. (a) As prepared. (b) After irradiation with one laser pulse (laser source: Ti:sapphire laser, λ = 800 nm, pulse width 150 fs). (c) After chemical removal of the gold. Light polarization vertical (indicated by the arrow on the right side). In (b), the former positions of the gold triangles are outlined as contours, formed by gold clusters which developed in the evaporation process through the colloid mask. Figure reproduced with permission from ref.⁸³, AIP Publishing.

Field intensity distribution, as calculated by means of DDA, at the interface between a Si wafer and a 480 nm side length triangle (upper row) and a 240 nm side length triangle (lower row) of 30 nm height (λ = 800 nm, left side: polarization along y; right side: polarization along z). Figure reproduced with permission from ref.¹⁰³, Springer Nature.

Measured and calculated absorption spectra for (a) 85 nm and (b) 540 nm nanotriangles on glass with the corresponding calculated scattered field intensities (normalized to the incoming field) for 800 nm irradiation on silicon. Small inset: calculated field distribution for a single 540 nm triangle. The double-headed arrows give the direction of the polarization of the incident laser light. Scale bars: 100 nm (a) and 300 nm (b). False color rulers: 0 to 255 (a) and 0 to 30 (b). Figure reproduced with permission from ref.¹⁰⁴, Springer Nature.

AFM images of the ablation pattern for nanotriangles with a side length of 85 nm (left) and 540 nm (right) on a silicon substrate (laser wavelength 800 nm, pulse length 150 fs). The left sample had to be cleaned using the snow jet technique before the AFM scan, so the gold clusters around the contour of the original triangle positions are missing. The position of the height profile is depicted in the AFM image with a bar in the corresponding color. In the graph, the profiles are shown with an arbitrary offset. The double-headed arrows give the direction of the polarization of the incident laser light. Scale bars: 100 nm (left) and 300 nm (right). False color rulers: 0 nm to 10 nm (left) and 0 nm to 70 nm (right). Figure reproduced with permission from ref.¹⁰⁴, Springer Nature.

SEM micrographs of two different types of nanotriangles prepared by electron beam lithography, and their corresponding ablation patterns on a silicon substrate for two orientations with respect to the polarization of the incident light. The laser wavelength was 800 nm, the pulse width 150 fs, and the local incident fluence is indicated below each frame. Figure reproduced with permission from ref.¹⁰⁶, under a Creative Commons Attribution License 2.0.

FDTD calculations for the structures presented in Fig. 23. The field intensity enhancement was extracted in a plane between the SiO₂ layer (thickness 2.4 nm) and the Si substrate below the triangle. The simulation volume was 3 µm × 3 µm× 6 µm with “perfectly matched layer” boundary conditions. The meshing was set to automatic mode far from the triangle, and a manually refined mesh with a cell size of 2 nm × 2 nm × 0.5 nm was added around the particle. Figure reproduced with permission from ref.¹⁰⁶, under a Creative Commons Attribution License 2.0.

AFM images of nanopatterns generated on a fused silica substrate with a single pulse of a Ti:saphhire laser (λ = 790 nm, FWHM 35 fs) in a region of low (a), medium (b), and high (c) laser fluence. The applied pulse energy was E = 0.16 mJ. The red triangles (right image) indicate the original position of the triangular NPs on the substrate prior to irradiation. For clarity reasons, the red triangles have been drawn larger than the original NPs, which, in fact, have a tip to tip distance of approximately 100 nm. The black arrows indicate the polarization direction of the incoming laser light. Figure reproduced with permission from ref.¹¹², The Royal Society of Chemistry (https://doi.org/10.1039/c0jm03829f).

AFM images of nanopatterns generated like in Fig. 25, but with a different laser polarization, in a region of low (a), medium (b), and high (c) laser fluence. The applied pulse energy was E = 3.8 mJ. The red triangles (right image) indicate the original position of the triangular NPs on the substrate prior to irradiation. Again, for clarity reasons, the red triangles have been drawn larger than the original NPs, which, in fact, have a tip to tip distance of approximately 100 nm. The black arrow indicates the polarization direction of the laser light. Figure reproduced with permission from ref.¹¹², The Royal Society of Chemistry (https://doi.org/10.1039/c0jm03829f).

SEM image of modified nanotriangles (produced with 1740 nm colloids) after irradiation with a single ps laser pulse (λ = 800 nm, FWHM 300 ps). Scale bars: 300 nm, laser polarization in x - direction. Figure reproduced with permission from ref.¹⁰⁴, Springer Nature.

AFM images of nanocraters ablated by 150 nm gold nanoparticles on silicon (100) and corresponding cross section as found along the white dotted line. The laser pulse width was 220 fs, the wavelength 780 nm and the fluence 128 mJ/cm²; the light was s-polarized at 45° incident angle. The scale bar is 200 nm. Figure reproduced with permission from ref.¹¹⁰, Springer Nature.

Nanoscale ablation site and depth profile: SEM images of the nanorods before and after laser irradiation at a local effective fluence of (a) 54 mJ/cm², right at the ablation threshold, and (b) 218 mJ/cm². The scale bars correspond to 75 nm and the yellow arrows indicate the incident polarization. (c) The depth profile of the ablation site shown in (b), along the long axis shown with the dotted line as obtained using AFM. Note the different scales for the horizontal and vertical axes in (c). Figure with permission from ref.¹²⁹, The Optical Society.

Simulations of the near-field enhancement |E|² (a) and of the norm of the Poynting vector distribution |P| (c) for a round-edged triangle as prepared by e-beam lithography. The middle (b) shows the experimental ablation pattern. A comparison reveals that the experimental pattern agrees much better with (a) than with (c). Figure reproduced from ref.¹⁰⁵.