PMOD: Photochemical Metal-Organic Decomposition

• Photochemical metal–organic decomposition (PMOD) is a bottom-up lithographic process that uses UV-induced crosslinking of a photosensitive precursor to form patterned lithium niobate films.
• The method integrates solution preparation, precise UV exposure through a photomask, solvent development, and thermal calcination to achieve polycrystalline structures with tunable nonlinear optical properties.
• Applications include cost-effective fabrication of devices like frequency doublers and modulators with feature sizes reaching down to 30 µm, offering an alternative to conventional microfabrication.

The photochemical metal–organic decomposition (PMOD) technique is a bottom-up lithographic procedure enabling direct patterning of lithium niobate (LiNbO₃, LN) thin films into scalable optical metasurfaces under entirely ambient conditions. PMOD proceeds through a sequence of solution preparation, UV-driven photochemistry, solvent development, and thermal crystallization, yielding polycrystalline LN structures with tunable nonlinear optical responses and feature sizes down to $30\,μ\mathrm{m}$, while circumventing the complexity and cost typical of conventional top-down microfabrication methodologies (Ali et al., 15 Nov 2025).

1. Custom Photosensitive Organometallic Precursor Chemistry

Central to PMOD is the synthesis of a UV-sensitive, negative-resist organometallic precursor. Commercial lithium niobium ethoxide, $\mathrm{LiNb(OC_2H_5)_6}$, is dissolved in ethanol at 5% w/v concentration. Benzoylacetone (BzAc, $\mathrm{C_6H_5}$–CO–CH₂–CO–CH₃), a β-diketone ligand, is introduced in a 1:2 molar ratio and chelates both Nb and Li. The reaction:

$\mathrm{LiNb(OC}_2\mathrm{H}_5)_6 + 2\,\mathrm{BzAcH} \xrightarrow[12\,\mathrm{h}]{70\,^\circ\mathrm{C}} \mathrm{LiNb(OC}_2\mathrm{H}_5)_4(\mathrm{BzAc})_2 + 2\,\mathrm{C}_2\mathrm{H}_5\mathrm{OH}$

produces a stable, negative photoresist complex. Its strong absorption at $\lambda \approx 365$ nm, attributed to $n\rightarrow\pi^*$ and $\pi\rightarrow\pi^*$ transitions within the Nb–BzAc chelate, renders the material suitable for UV crosslinking.

2. Photochemical Patterning Mechanism

UV exposure initiates decomposition and crosslinking within the precursor film. Irradiation at 365 nm (lamp intensity 21.1 mW/cm²) through a photomask imparts a dose $D \approx 63$ J/cm² for 50 minutes. The incident photons promote excited-state chemistry:

$$\mathrm{M}–\mathrm{O}–\mathrm{BzAc} + h\nu \rightarrow \mathrm{M}–\mathrm{O}^\bullet + \mathrm{BzAc}^\bullet \rightarrow \text{intermolecular}\; \mathrm{M}–\mathrm{O}–\mathrm{M} \text{ crosslinking}$$

This crosslinking yields an amorphous oxide network insoluble in ethanol at locations exposed to UV. Notably, patterning efficacy exhibits threshold behavior, confined between 50 and 90 minutes of exposure, suggestive of optimized photochemical kinetics; no quantum yield data were reported.

3. Solvent Development and Feature Formation

Post-irradiation, the substrate is immersed in anhydrous ethanol for 60 s. Unexposed precursor regions dissolve, while crosslinked regions adhere to the substrate, defining an array of amorphous Li–Nb–O patterns. A subsequent ethanol rinse ensures removal of residual organics, preserving the precise spatial correspondence to photomask geometry.

4. Thermal Calcination and Crystallization

Crystallization is effected by calcination in ambient air—ramping to 650 °C at 5 °C/min, holding for 45 minutes, then cooling naturally. The transformation:

$$[\mathrm{Li}–\mathrm{Nb}–\mathrm{O}]_{\text{amorphous}} + \tfrac{1}{2}\,\mathrm{O}_2 \xrightarrow{650^\circ\mathrm{C}} \mathrm{LiNbO}_3\,(\text{rhombohedral})$$

eliminates organics and establishes stoichiometric LN. Powder XRD and Raman spectra confirm phase purity and absence of secondary LiNb₃O₈ or Li₃NbO₄ phases. Film thickness post-calcination is approximately 300 nm.

5. Patterning Resolution and Metrology

PMOD achieves sub-50 µm patterning fidelity on 3″ × 3″ substrates under ambient conditions. Empirical results demonstrate:

• Lines: defined down to 30 µm width; edge roughness arises near this limit.
• Squares: crisp at ≥70 µm; smaller geometries exhibit incomplete development.
• Complex logos/interconnected shapes: resolved with uniform film thickness.

Performance is limited by photomask resolution, UV diffraction, and development kinetics.

Feature Type	Minimum Size Achievable	Observed Limitations
Line	30 µm	Edge roughness, discontinuities
Square	70 µm	Diffraction, incomplete removal
Complex shape	variable	Uniformity preserved

6. Nonlinear Optical Properties of Patterned Lithium Niobate

The PMOD-patterned LN exhibits robust, isotropic second harmonic generation (SHG) due to polycrystalline porosity, which obviates strict phase-matching requirements. Femtosecond excitation (680–1300 nm) with multiphoton microscopy generates bright SH at $\lambda = ½ \lambda_0$. SH intensity obeys the quadratic dependence:

$I_{2\omega} \propto |\chi^{(2)}|^2 I_\omega^2$

High $R^2 = 0.99$ fit values confirm true second-order nonlinearity. Tunable frequency-doubling is observed: SH spectral peaks shift from 420 nm (pump 840 nm) to 480 nm (pump 960 nm). Isotropic SHG response is evident in polarization-dependent measurements, consistent with randomly oriented domains.

7. Comparison with Top-Down Microfabrication and Generalization

PMOD avoids ion-beam, RIE/ICP, and e-beam processes characteristic of top-down fabrication. It utilizes inexpensive chemical reagents, UV illumination, and furnaces—dispensing with cleanroom infrastructure. While PMOD yields moderately porous, polycrystalline LN with high $\chi^{(2)}$, single-crystal quality and smoother facet edges remain exclusive to top-down procedures, albeit with lower throughput and higher cost.

The method generalizes to other non-centrosymmetric oxides; BaTiO₃ and LiTaO₃ can be synthesized by analogous precursor design—substituting barium or tantalum alkoxides and tuning ligand ratios and UV exposure. Calcination profiles (e.g., 700 °C for BaTiO₃) are optimized to stabilize respective crystal phases. A plausible implication is that PMOD may provide a universal route for ambient fabrication of nonlinear optical ceramics.

8. Applications and Future Directions

PMOD enables cost-effective fabrication of frequency doublers, modulators, metalenses, and quantum light sources, offering a blueprint for scalable manufacture of nanophotonic devices. The isotropic nonlinear response, ambient processing, and sub-50 µm resolution position PMOD as a methodological advance in patterning non-centrosymmetric materials for integrated optics. Extension to other chemistries and further refinement of precursor design are anticipated to broaden PMOD’s utility to alternative classes of nonlinear optical films (Ali et al., 15 Nov 2025).