Nanoscale multi-beam lithography of photonic crystals with ultrafast laser

The study addresses how to fabricate photonic crystal structures at the nanoscale inside transparent crystals to manipulate light propagation in the visible and near-infrared. Photonic crystals, consisting of periodic arrangements of materials with differing refractive indices (e.g., air holes, channels, or dielectric rods), enable a wide range of photonic devices such as emitters, receivers, switches, modulators, and resonators. While 1D and 2D photonic crystals are relatively easier to fabricate, 3D structures are challenging because refractive index modulation is required along all three spatial axes. Ultrafast laser lithography is promising for 3D structuring due to its high peak intensity, nonthermal nature, and capacity for localized modification inside transparent materials. However, conventional single-beam, multi-scan direct writing is limited by diffraction, spot overlap, and mechanical precision, constraining nanoscale feature size and throughput. Parallel processing approaches like microlens arrays and laser interference lithography offer some advantages but are typically limited in flexibility or to planar structures. The paper proposes an SLM-based multi-beam ultrafast laser lithography strategy enabling controllable nanoscale gaps between parallel channels inside YAG crystals, with post-etching to form hollow structures, improved efficiency via parallelization, and the capability to realize complex channel arrays and functional devices (e.g., diffraction gratings).

- Material and overall approach: Transparent YAG crystal was processed using tightly focused femtosecond laser pulses in a transverse writing configuration to inscribe modification tracks. Pulse energy per beam was set above the YAG modification threshold but below ablation, ensuring structural change without material removal. After laser writing, chemical etching preferentially removed the laser-modified regions to yield hollow channels (phosphoric acid etching). - Beam shaping: A spatial light modulator (SLM) imposed phase holograms to generate multi-beam focal arrays. A binary Dammann grating phase controlled the number of beams and their lateral spacing; adjusting the number of stripes (K) tuned inter-spot separation. An optimized blazed Fresnel phase altered axial focus position. Superimposing Dammann and Fresnel phases produced complex multi-spot light fields, including arrangements with multiple foci in the x–z plane (e.g., 8-focus distribution) and customizable 2D/3D patterns. - Scanning strategy: Multi-beam scanning simultaneously inscribed parallel modification tracks, increasing throughput by approximately N−1 for N beams. Transverse writing mitigated working-distance constraints of high-NA objectives. Although the focus is asymmetric in this geometry, it enables long channels; the resulting channel cross-sections are noncircular. - Optical modeling: Debye diffraction integral calculations were performed to simulate tightly focused intensity distributions for the shaped beams, predict the inter-beam gap at the focus, and guide hologram design. Simulations provided estimates such as the gap X_CD (~200 nm for double-beam case), matching experiments. - Parameter studies: The influence of SLM grating stripe number (K) and pulse energy on channel width and inter-channel gap was investigated. Larger K increases beam separation; energy tuning affects modification volume and etched channel dimensions. - Characterization: Post-etch structures were imaged from top and side views via optical microscopy. Structural modifications were analyzed by Raman spectroscopy (monitoring shifts, intensity changes, and FWHM) and X-ray photoelectron spectroscopy (XPS) of Al 2p to assess changes in site occupancy (AlIV vs AlV) and confirm similarity between single- and multi-beam modifications. - Device demonstration: Optical diffraction gratings with different periods were fabricated by programming channel arrays; their diffraction behavior was measured and compared to theoretical expectations.

- Nanoscale double-channel gaps: Two-beam writing in YAG followed by etching produced two uniform parallel channels with a measured inter-channel gap of 261 nm (top and side views). Simulations predicted ~200 nm, in good agreement considering modification thresholds. - Single- vs double-beam channels: A single-beam etched channel width was ~1 μm; in the double-beam case, each channel width was ~0.7 μm with the nanoscale gap between them. - Control via SLM grating stripes (K): Increasing K (2→16) increases the inter-channel gap; for K<8, gaps remain at the nanoscale. Only K=2 and K=4 yielded channel widths <1 μm; at K=6, channel width increased abruptly. Gap width shows an approximately linear increase with K. - Energy dependence: As pulse energy increases (50–300 nJ range), gap width decreases while channel width increases (negative correlation for gap vs energy; positive for channel width vs energy). - Larger separations: For higher K (18→72), adjacent channel spacing increased from ~6 μm to ~18 μm. An observed trend was increased gap width accompanied by decreased channel cross-section length with increasing stripe number, consistent with Fraunhofer diffraction expectations and Debye simulations. - Multi-beam scalability and efficiency: Using Dammann grating phases, 2-, 4-, and 8-beam light fields were realized, and corresponding parallel tracks were fabricated. After etching, uniform hollow channel arrays were obtained. Throughput increased by approximately N−1 for N beams. - Structural analyses: Raman spectra of modified regions showed increased intensity of the 403 cm−1 T2g mode, slight blue shift (403.5→404.1 cm−1), and FWHM broadening (e.g., 6.5→9 cm−1 at 263 cm−1), indicating lattice defects and partial bond breaking that enhance etch selectivity. XPS Al 2p revealed a shift in site occupancy toward octahedral Al (AlV) at the expense of tetrahedral Al (AlIV). Multi-beam and single-beam modifications exhibited similar spectroscopic signatures, supporting uniformity and stability of multi-beam processing. - Device functionality: Diffraction gratings with various periods were fabricated in crystal; measured diffraction behavior aligned with theoretical analysis. - Uniformity and minimal secondary effects: Multi-beam parallel scanning avoided secondary processing due to overlap and reduced sensitivity to mechanical fluctuations, producing uniform subwavelength structures without cracking.

The proposed SLM-enabled multi-beam femtosecond laser lithography addresses the challenge of fabricating nanoscale, 3D-capable photonic crystal structures inside transparent crystals. By shaping the focal field into multiple, precisely spaced spots and operating above the modification threshold yet below ablation, the method writes uniform, closely spaced modification tracks in a single pass, overcoming diffraction-limited single-beam constraints and reducing issues from scan overlap and mechanical precision. Controlling the binary grating stripe number (K) and pulse energy provides independent tuning of inter-channel gaps and channel widths from the nanoscale up to tens of micrometers. Debye diffraction simulations accurately predict the focused intensity and resulting gap, guiding hologram design and process parameters. Raman and XPS analyses confirm that the nature of laser-induced modification (defects, Al site transformation) is consistent between single- and multi-beam writing, ensuring that multi-beam parallelization does not degrade material quality. The capability to superimpose Dammann and Fresnel phases demonstrates axial control and more complex spatial arrangements, enabling the construction of functional photonic elements such as diffraction gratings whose performance matches theory. Overall, the method offers a programmable, efficient route to integrate nanostructured photonic crystal features within bulk crystals for integrated photonics.

This work introduces a multi-beam ultrafast laser lithography approach, implemented via SLM-programmed phase holograms (Dammann and Fresnel phases), to fabricate nanoscale and microscale channel arrays inside YAG crystals with high precision and efficiency. It achieves controllable inter-channel gaps down to 261 nm (with simulations predicting ~200 nm), scalable multi-beam parallelization (2, 4, 8 beams), and complex spatial arrangements through superimposed phase designs. Spectroscopic analyses (Raman, XPS) indicate uniform and consistent laser-induced modifications between single- and multi-beam processes, enabling selective chemical etching to form hollow channels and functional photonic structures such as gratings with expected diffraction properties. The approach provides a versatile platform for fabricating complex photonic crystal architectures for integrated photonics. Future work could extend the methodology to more intricate fully 3D lattices, broaden the range of materials and etchants, and further optimize beam shaping and energy delivery for finer feature control.

- Cross-section shape: Transverse writing yields an asymmetric (non-circular) focus and noncircular channel cross-sections. - Etch selectivity dependence: Formation of hollow channels relies on differential etch rates between laser-modified and unmodified regions; materials with insufficient selectivity may limit applicability. - Parameter sensitivity: Channel width and gap depend sensitively on SLM grating stripe number (K) and pulse energy; abrupt changes (e.g., at K=6) require careful calibration. - Material and scope: Demonstrations are performed in YAG; generalization to other crystals may require process tuning. While axial beam shaping is shown, fully realized complex 3D photonic crystal lattices are not comprehensively detailed in the excerpt. - Absolute date/time and some apparatus specifics (e.g., exact scan speed units) are not fully specified in the provided text.