2.1.

Proposed System

For the excitation and inhibition beams, the polarization property of light is used to generate two focal spots by utilizing an SLM, corresponding to the s- and p-polarized components of the beam. Individual and dynamic on–off and aberration control can be realized for both the excitation and inhibition spots. Figure 1(a) provides a schematic of the proposed system, and Fig. 1(b) depicts its physical arrangement. The system includes eight modules numbered 1–8, as shown in Fig. 1(b). First, the s- and p-polarized components of the beam are split so that they can be individually controlled; then they are modulated by the left and right tilt phase masks (loaded on the SLM) so that the s- and p-components generate two separate spots after focusing. The distances between the spots and their positions can be adjusted using the tilt coefficient. For the inhibition beam, a vortex phase (loaded on the SLM) is utilized to generate a doughnut spot. The wavelength of the excitation beam is 780 nm (Coherent, Chameleon Vision II, 3 W, Glasgow, UK; pulsed 80-MHz repetition rate), and that of the inhibition beam is 532 nm (Laser Quantum, Opus 532-5W, Stockport, UK; continuous wave). Modules 1 and 2, 3 and 4, and 5 and 6 are the same; they differ only in having different optical elements fit for different wavelengths. Modules 1 and 2 (MRC, Heidelberg, Germany) are used for beam-pointing stabilization. Modules 3 and 4 first split the excitation/inhibition beam into two beams using a polarizing beam splitter (PBS), so that the individual on–off control of each beam can be realized using an acoustic-optical modulator (AOM). Then the two beams are recombined using another PBS. Modules 5 and 6 perform phase modulation using SLMs. The polarization property of SLMs is completely leveraged; the s- and p-components are modulated by different areas on the SLM screen, and a quarter-wave plate is used for the polarization shift.^45,46 In module 7, the excitation and inhibition beams are combined and then successively passed through a galvanometer scanner (CTI, 8310K, Lexington, Massachusetts) and a scan lens (Nikon, PROJ-XSD007835 A1, Tokyo, Japan). In module 8, a microscope with an objective lens (Nikon, NA 1.45, $100\times$, Tokyo, Japan) is used for beam focusing and photoresist-sample holding. Additional details regarding each module and the entire system are provided in Figs. S1–S5 in the Supplementary Material.

Fig. 1

Proposed ${\mathrm{P}}^3\mathrm{L}$ system and focal spots: (a) schematic of the ${\mathrm{P}}^3\mathrm{L}$ system and (b) physical arrangement with eight modules. (c), (d) Focal spots located near the $x-y$ and $y-z$ planes, respectively. The light blue and green spots were generated by the 532-nm beam, and the purple and red spots were produced by the 780-nm beam, forming two combinations. The combination of light blue and purple spots was named channel 1, and the other combination was named channel 2. (c) and (d) share the same scale bar.

The scanning imaging function was utilized to verify the quality of the spots (two excitation solid spots and two inhibition doughnut spots) by imaging 150-nm-diameter gold beads, as shown in Figs. 1(c) and 1(d), in which one excitation solid spot and one inhibition doughnut spot are superposed in three dimensions. The $0-2\pi$ vortex phase was used to modulate the inhibition light. The primary wavefront aberrations were corrected carefully by applying Zernike polynomials.^47,48 To present the features of our system concisely, a pattern containing two lines of the test is shown in Fig. S6 in the Supplementary Material. The final phase masks loaded on the SLMs are depicted in Figs. S7(a) and S7(b) in the Supplementary Material. The distances and relative positions of the two spots were quantitatively controlled by adjusting the tip and tilt coefficient through careful calibration. The system was also used to generate a traditional 3D dark spot (Fig. S8 in the Supplementary Material).

2.2.

PSF Verification and Image Acquisition

The galvanometer, AOM, piezostage, and trigger signals to the PMT were controlled by external voltages. An eight-channel analog-out card (National Instruments, NI PXIe-6733, Austin, Texas) provided these voltage signals, and another DAQ card (National Instruments, NI PXIe-6363, Austin, Texas) was used for data acquisition. The two cards were synchronized via a PXIe chassis (National Instruments, NI PXIe-1073, Austin, Texas), which was controlled through a manufacturer-supplied dynamic link library and C++ software developed in-house. Scanning could be conducted in any region of interest in the $x-y$, $x-z$, and $y-z$ planes, and images could be automatically acquired once the parameters were set.

2.3.

Printing Manner and Monitoring

Except for the cubic metamaterial sample, which was printed in a dip-in manner, all the samples in this study were printed in the conventional oil–substrate–photoresist manner: the objective lens on the inverted microscope was immersed in oil and the photoresist was presented on the substrate. We used a glass slide as a substrate (refractive index: 1.51) and fixed it on the piezostage using a specially designed adaptor holder. The widefield image was acquired by a charge-coupled device camera, and the software was provided by the manufacturer. Four instant snapshots taken during the parallel printing process for the two lines of text are shown in Fig. S7(c) in the Supplementary Material (Video 1, MP4, 6.995 MB [URL: https://doi.org/10.1117/1.AP.4.6.066002.s1]). The substrate was illuminated from above by a light-emitting diode light source (wavelength, 625 nm, Thorlabs, CHROLITS-C1, Newton, New Jersey; driver, Thorlabs, LEDD1B, Newton, New Jersey). All the laser powers were measured at the aperture before the objective lens on the microscope by a power meter (power sensor, S121C, Thorlabs; power and energy meter console, Thorlabs, Newton, New Jersey).

2.4.

Materials and Sample Processing

A formulation containing 0.5 g of 7-diethylamino-3-thenoylcoumarin, 8.6 g of tricyclodecane dimethanol diacrylate, 0.7 g of ethoxylated bisphenyl fluorene diacrylate, and 0.7 g of o-phenyl phenoxyethyl acrylate was used as the sample photoresist.⁴⁹ The exposed samples were developed in propylene glycol 1-monomethyl ether 2-acetate for 15 min, followed by rinsing in isopropanol for 5 min. A 2-nm-thick gold layer was evaporated on the samples prior to topography observation.

3.1.

2D Sub-40-nm Feature Size Verification

We printed a nanowire on a glass substrate ($170\text{-}\mu \mathrm{m}$-thick microscope coverslip) with and without PPI to verify the feature size of our system. Many previous works have shown that, for a certain focal-spot speed, the final feature size is sensitive to both the excitation and inhibition powers. The spot speed was set to $1\mathrm{mm}/\mathrm{s}$, and five excitation powers were chosen for each inhibition power, as shown in Fig. 2(a). Figure 2(b) shows a scanning electron microscopy (SEM) image of the nanowire at the excitation and inhibition powers indicated by the red box in Fig. 2(a), demonstrating that the line width is reduced from 138.8 to 39.8 nm. Suspended nanowires were also printed for further demonstration, and the experimental results are presented in Figs. S9 and S10 in the Supplementary Material.

Fig. 2

Two-dimensional feature size verification experiment results. (a) Feature size versus inhibition beam intensity under excitation beam exposure with different powers. (b) SEM image of the nanowire obtained at the point in the red box in (a). After the PPI is turned on, the line width is compressed from 138.8 to 39.8 nm. Scale bar: 100 nm.

3.2.

Bit Dot Pattern Printing

Because large data-recording systems are important potential applications of multifocal DLW,³³ we printed a bit pattern to verify the feasibility of our system. The pattern in Fig. S6 in the Supplementary Material was pixelated and reprinted using PPI-2PP and 2PP. The pitch of the bits was set to 200 nm. The dwell time of one dot was $8\mu \mathrm{s}$, the excitation spot power was 14.5 mW, and the inhibition power was 41.0 mW. The results are presented in Fig. 3: the bit dots printed with PPI [upper line in Fig. 3(a)] are clearly resolved with distinct gaps; by contrast, the bit dots printed without PPI [lower line in Fig. 3(a)] are blurred. The two lines of text were still printed simultaneously, whereas the two lines must be printed individually in a normal single-beam-path system. Consequently, the efficiency of our method is twice that of a normal single-beam-path system. Further experimental verification can be found in Fig. S11(a) in the Supplementary Material, which depicts two different bit dot patterns printed by two channels with PPI. In addition, two square-wave patterns with a 150-nm bit gap were printed, and the results are presented in Figs. S11(b) and S11(c) in the Supplementary Material.

Fig. 3

Bit-pattern-printing results. (a) Full-view SEM image of the printed pattern. The pattern shows two lines of text and is pixelated. Top line: Chinese words, printed by channel 1 with PPI; bottom line: English words (translation of the text in the top line), printed by channel 2 without PPI. Scale bar: $2\mu \mathrm{m}$. (b), (c) Enlarged views of the yellow and blue boxes in (a), respectively; the horizontal and vertical pitches of the bits are 200 nm. Scale bars: $1\mu \mathrm{m}$.

3.3.

Three-Dimensional Structure Printing

To test the parallel printing of an actual 3D nanostructure, a metamaterial architecture⁵⁰ (a cube) was printed. This cube was used as a bench sample in Ref. 42. Here we considered two cells as bench samples: one containing $2\times 2\times 2$ units and the other containing $3\times 3\times 3$ units. The unit 3D models and more relative results are shown in Fig. S12 in the Supplementary Material. Because of the individual control of the light beam, the distance between the two focal spots did not need to match the size of the unit to make one spot print one unit, which makes the system more flexible. In fact, we could also have generated more spots in the focal plane to print the units simultaneously using the SLM, but that was out of the scope of this experiment. For this micron-scale structure, a sub-100-nm feature size was not required; therefore, the cells were printed without PPI. The distance between the two spots was adjusted to $50\mu \mathrm{m}$, and the power of a single spot was 31.5 mW. The objective lens was changed to $40\times$ (Olympus, NA 1.40, Tokyo, Japan), and the focal-spot speed was $50\mathrm{mm}/\mathrm{s}$ [step size: $100\mathrm{nm}(x)\times 100\mathrm{nm}(y)\times 200\mathrm{nm}(z)$]. The corresponding results are presented in Fig. 4, where the size of each cell is $200\mu \mathrm{m}$. Three-fold stitching was required, and the areas printed by different spots are marked by different colors in Figs. 4(b) and 4(e).

Fig. 4

Metamaterial cubic-unit printing results: (a) SEM image of a $2\times 2\times 2$ unit cell, (b) top view of (a), (c) zoomed-in view of the yellow box in (a), (d) SEM image of a $3\times 3\times 3$ unit cell, (e) top view of (d), and (f) zoomed-in view of the yellow box in (d). In (b) and (e), the portions corresponding to the light-blue stripes were printed by channel 1, and those indicated by the dark-red stripes were printed by channel 2. Scale bars: $50\mu \mathrm{m}$.

To demonstrate the feasibility of the 3D fabrication method further, additional 3D structures were printed. First, two cubic frames were printed by the two spots simultaneously. The two frames were of the same size but had different patterns (squares and circles), as shown in Fig. 5(a). Each spot printed one frame, the separation of the two spots was $20\mu \mathrm{m}$, and the side length was $18\mu \mathrm{m}$. Second, three types of continuous 3D nanostructures were printed: periodic, wire, and spherical structures, and oblique-view SEM images of these structures are provided in Figs. 5(b)–5(d), respectively. Figure 5(b) shows a periodic fence array with 10 hexagonal cells; each cell has a side length of $8\mu \mathrm{m}$ and a height of $10\mu \mathrm{m}$. The photonic wire is a functional nanostructure used to connect integrated circuits across chip boundaries. We printed a photonic-wire structure model containing four wires crossing each other spatially [Fig. 5(c)]. A $32\text{-}\mu \mathrm{m}$-diameter buckyball model was also printed [Fig. 5(d)]. For these structures, the objective lens was changed back to $100\times$. The focal-spot power was 15.2 mW, and the speed was set to $30\mathrm{mm}/\mathrm{s}$ [step size: $100\mathrm{nm}(x)\times 100\mathrm{nm}(y)\times 200\mathrm{nm}(z)$]. A few 2.5D structures were also printed as additional evidence, and the results can be found in Fig. S13 in the Supplementary Material.

Fig. 5

Oblique-view SEM images of 3D nanostructures parallel-printed using two spots. (a) Two cubical box frames, (b) periodic structure: hexagonal fence, (c) wire structure: nanophotonic wires, and (d) spherical structure: buckyball model. Scale bars: $10\mu \mathrm{m}$.