Cascade upconversion: a strategy enabling four-photon lithography in week light

doi:10.21203/rs.3.rs-5239325/v1

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Cascade upconversion: a strategy enabling four-photon lithography in week light

https://doi.org/10.21203/rs.3.rs-5239325/v1

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Multiphoton lithography(MPL) offers the highest resolution available in submicron scale true 3D printing, but excessive femtosecond laser intensity prevents it from leading to higher photon counts. To circumvent away this effect, we present a cascade upconversion (CUC) strategy, which is a combination of two efficient two-photon upconversion processes to achieve four-photon photopolymerization. In order to demonstrate the advantages and feasibility of this approach, we combine excited state absorption upconversion using high concentration Ho³⁺/Yb³⁺ doped upconversion nanoparticles (UCNPs) with triplet-triplet annihilation (TTA) upconversion to fabricate 3D polymer structures by low-cost continuous wave 980 nm laser at 10⁵ W/cm². This method overcomes the diffusivity caused by isotropy of UCNPs luminescence, and achieves two orders of magnitude improvement in resolution while maintaining the advantages of using near infrared(NIR) light. This new strategy provides a general way for designing four-photon and even six-photon MPL in week light.

Physical sciences/Optics and photonics/Optical techniques/Lithography

Physical sciences/Optics and photonics/Optical materials and structures/Nanoparticles

Multiphoton lithography

cascaded upconversion

upconversion nanoparticles

triplet-triplet annihilation.

High-resolution true 3D lithography can be achieved using multiphoton optical upconversion, typical methods include instantaneous mutiphoton absorption, excited state absorption, and triplet-triplet annihilation [1–4]. In the instantaneous multiphoton absorption process, by increasing the number n of photon simultaneously absorbed, the resolution is $\:1/\sqrt{n}$ that of single photon [5, 6]. However, it is also accompanied by a sharp decrease in the absorption cross-section [7]. Table 1 shows the performance of lithography using the upconversion method described above. When n = 3, the femtosecond laser light intensity required for photopolymerization reaches 10⁹ W/cm², and the light intensity required for four-photon is at least 10¹³ W/cm² [13]. High-intensity lasers will adversely affect the lithography process, such as thermal polymerisation, material damage, expensive equipment [15]. Therefore, it is difficult to achieve lithography by one-step four-photon upconversion of a single molecule.

Table 1

performance of lithography using the upconversion method
Lithography method	Photon number	Resolution	Excitation light intensity	Absorption coefficient / Quantum efficiency	Functional materials
Single-photon absorption	1	~ 500 nm^[8]	< 10 W/cm²	> 20%	SU–8
Instantaneous absorption	2	~ 50 nm^[9]	> 10⁷ W/cm² (fs)	~ 10 cm/GW^[13]	ZrO₂–BTMST
	3	~ 30 nm^[5]	> 10⁹ W/cm² (fs)	~ 1 cm³/GW^2[13]	HSQ
	4	None	None	~ 10^− 2 cm⁵/GW^3[13]	None
Excited state absorption	2	1µm^[10] (clusters)	Not mentioned	~ 6%	Yb³⁺, Er³⁺ UCNPs
Excited state absorption	3	~ 50 µm^[11]	< 10 W/cm²	2%	Yb³⁺, Tm³⁺ UCNPs
Triplet-triplet annihilation	2	~ 2 µm ^[12]	∼10 W/cm²	> 10%^[14]	PdOEP + DPA
Note: Absorption coefficient of instantaneous absorption are derived from Z-scan tests on general multiphoton molecules.

Upconversion photopolymerization (UCPP) can use low intensity continuous light to achieve photopolymerization using UCNPs [16–18]. UCNPs exhibit the ability to emit UV light via an anti-Stokes process with a typical excited state absorption upconversion mechanism when excited by NIR radiation, such as wavelengths at 980 nm, 1064 nm, and 808 nm [19–21]. NIR photopolymerization technology holds distinct advantages over short-wavelength light sources for deep 3D printing and in vivo structural manipulations, owing to its superior penetration depth and reduced tissue damage [22–25]. However, the technology has yet to attain the requisite high resolution because of the diffusivity caused by isotropy of UCNPs luminescence, which significantly impedes its potential in wide domains such as biological 3D priting and data storage [26, 27]. Presently, UCPP is capable of generating rudimentary irregular structures with resolution exceeding 10 µm and more stable, albeit complex, structures with dimensions still above 50 µm [10, 28]. TTA upconversion also requires low excitation light intensity and achieves higher quantum efficiency [29–31]. TTA polymerisation only requires the use of low-intensity continuous light, but it is still a two-photon absorption method, and the resolution that can be achieved at present does not reach the level of instantaneous two-photon absorption lithography [12].

Cascaded upconversion photopolymerization(CUCPP) is a method of combining two efficient upconversions to achieve four and more photon lithography. The luminescence from the UCNPs can be used to trigger the TTA upconversion. The binary system composed of 2,3-butanedione and 2,5-diphenylazole (PPO) enables TTA upconversion, converting visible light into UV radiation. [32]. Since the absorption spectrum of the PPO system ranges from 520 nm to 720 nm [33], various rare earth elements such as Ho, Er, and Eu exhibit strong upconverted luminescence within this range. By combining Ho³⁺/Yb³⁺-doped UCNPs with PPO under low-cost continuous NIR excitation in week light. The internal light energy conversion mechanism of CUC system was analyzed by fluorescence spectroscopy, and the feasibility and advantages of the material coupling were analyzed by numerical simulation with state equations. With a homemade NIR laser direct writing system, submicron scale patterns as well as true 3D structures were realized with a minimum resolution of 290 nm.

The principles and comparison of Tm³⁺/Yb³⁺-doped UCPP and Ho³⁺/Yb³⁺-doped UCNPs-TTA CUCPP are illustrated in Fig. 1(a). The UCPP process occurs under NIR excitation, where Tm³⁺/Yb³⁺-doped UCNPs directly radiate UV light inside the photoresist. The upconverted UV light is directly used to initiate photopolymerization. The intensity of the UV light decreases with the distance from the Tm³⁺/Yb³⁺-doped UCNP light source, and above a certain light intensity threshold, the photoinitiator generates sufficient radicals to trigger the photopolymerization of monomers. The underlying challenge stems from the fact that, upon NIR excitation, UCNPs emit upconverted light isotropically inside the photoresist. Once the intensity of this upconverted light surpasses the intensity threshold for initiating photopolymerization, crosslinking reactions occur, leading to the formation of polymerized regions. However, this isotropic nature of the upconverted emission complicates the precise control of the photopolymerized resolution, resulting in gradual enlargement polymerized zone and pronounced edge roughness over time. In contrast, CUCPP is the combined effect of two upconversion processes: (1) the Ho³⁺/Yb³⁺-doped UCNPs upconvert the 980 nm excitation and emit light at 645 nm; (2) This upconverted 645 nm light is rather used to directly initiate photopolymerization but absorbed by the TTA molecules, which undergoes TTA upconversion process and emits UV light. Finally, Energy is transferred to UV photoinitiator, thus inducing it to generate free radicals to initiate the photopolymerization. Notably, the upconversion of TTA is a two-photon process, and the intensity of UV emission is approximately proportional to the square of the excitation light intensity, and only the TTA molecules in the region near the Ho³⁺/Yb³⁺-doped UCNPs can emit sufficiently strong UV light to initiate the photopolymerization. Therefore, the photopolymerization region is controlled in a very small range. Figures 1(b, c) demonstrate the actual luminescence effects of Tm³⁺/Yb³⁺ and Ho³⁺/Yb³⁺ -doped UCNPs within photoresist, as well as the changes in the photopolymerization region diameters of the two methods over time under the same 980 nm light intensity. The full video can be referred to in Supplementary Media 1. Post-development scanning electron microscopy (SEM) results, as shown in Fig. 1(e), allow for a more intuitive comparison. It can be observed that the line structure produced by UCPP exhibits uneven thickness and an apparent diffuse shape. In contrast, CUCPP demonstrates a clear advantage in overcoming dispersion. Videos of both photolithography processes are available in Supplementary Media 2.

Highly-concentration core-shell-shell β-NaYF₄: Ho³⁺/Yb³⁺@NaYbF₄@NaYF₄ UCNPs were selected as the internal red light emission sources in the photoresist. The energy level structure and entire energy transfer process of CUCPP is shown in Fig. 2(a). Yb³⁺ acts as a sensitizer with a large absorption cross-section at 980 nm, efficiently absorbing NIR light at 980 nm and transferring energy to Ho³⁺. Ho³⁺ undergoes excited state absorption and emits anti-Stokes light at 645 nm. The TTA system, which absorbs light at 645 nm and emits UV light, was chosen as 2,3-butanedione (biacetyl, sensitizer)/2,5-diphenylazole (PPO, annihilator). The electrons in the ground state of sensitizer absorb photons emitted by UCNPs, and enter an excited state. The excited electrons relax back to the ground state through nonradiative relaxation, spontaneous emission, or intersystem crossing (ISC). The ground state electrons of the annihilator are sequentially excited twice by absorbing the ISC energy of the photosensitizer. Similarly, the excited electrons relax back to the ground state through nonradiative relaxation or spontaneous emission. Due to the presence of similar energy gaps, the relaxation energy can be transferred to the photoinitiator. Under the same excitation and relaxation conditions, the excited photoinitiator is converted to a triplet state through ISC. The photoinitiator in the triplet state decomposes into free radicals, which initiate the photopolymerization of monomers.

Ho³⁺ exhibits distinct upconversion fluorescence peaks at 540 nm, 645 nm, and 750 nm. Figure 2(b) tested the TTA system under excitation with these three monochromatic wavelengths, and a blank sample containing only photoinitiator and monomer was prepared as a contrast at 645 nm. The breaks in the lines indicate the harmonic wavelengths. Only the 645 nm light can efficiently excite the production of UV light, confirming that the energy from Ho³⁺/Yb³⁺-doped UCNPs is transferred to TTA molecules via 645 nm light. Figure 2(c) shows the emission spectrum of the TTA system at 365 nm and the photoluminescence (PL) of core-shell-shell β-NaYF₄: Ho³⁺/Yb³⁺@NaYbF₄@NaYF₄ UCNPs. The TTA system can absorb light within the range of 600–760 nm and emit UV light. The peak at 645 nm for Ho³⁺ corresponds to anti-Stokes light emitted from the electronic transition ⁵F₅-⁵I₈, which falls within the absorption range of the TTA system. Figure 2(d) shows the absorption spectrum of the photoinitiator 184 and photoluminescence spectrum of the TTA system. Both have a favorable overlap region in the UV range, enabling efficient transmission of energy.

The intensity of anti-Stokes luminescence follows a power-law relationship with the excitation light intensity. Figures 3(b), (e), and (h) show double-logarithmic curves of excitation light intensity versus emission light intensity for three cases: 980 nm excitation of Ho³⁺/Yb³⁺-doped UCNPs producing 645 nm light, 645 nm excitation of the TTA system producing 365 nm light, and 980 nm excitation of the UCNPs-TTA hybrid system producing 365 nm light. Under 980 nm NIR light irradiation, the electron transition of Ho³⁺ from ⁵F₅ to ⁵I₈ can be described by a power index n = 1.406 of the excitation photon energy (980 nm), which is necessary for the electron to reach the ⁵F₅ excited state. That is to say, the intensity of the emission at 645 nm is approximately proportional to the 1.406th power of the excitation light intensity. Likewise, under 645 nm light irradiation, the emission intensity of the TTA system at 365 nm is approximately proportional to the 1.797th power of the excitation light intensity. When the Ho³⁺/Yb³⁺-doped UCNPs-TTA hybrid system is directly irradiated with 980 nm NIR light, the luminescence intensity at 365 nm is proportional to the 3.102nd power of the NIR excitation light intensity. Since 3.102 > 3, and the overall process's power index is approximately equal to the sum of the previous two process's power index, the entire process cannot be a three-photon process, but a four-photon process.

Numerical simulations of the photopolymerization area's evolution under laser irradiation over time further demonstrate the advantages of achieving low resolution via CUCPP. The detailed numerical simulations procedure can be found in the Supplementary Numerical Simulation Method. Figure 3(c) and 3(f) respectively show the double logarithmic relationships of 645 nm upconversion light from Ho³⁺ and 365nm light from the TTA system with the intensity of the 980 nm and 645 nm excitation light. Both processes are consistent with the results obtained from actual measurements. By combining the two processes, it is possible to derive the change in UV light emission from the Ho³⁺/Yb³⁺-doped UCNPs-TTA hybrid system with the intensity of the 980 nm excitation light, as shown in Fig. 3(i). The double logarithmic curve has a value of 3.2, which is close to the actual measurement results. Using a 980 nm Gaussian beam to irradiate a 60 µm × 60 µm area, and after obtaining the distribution of 645 nm light intensity, the point spread function is used to connect the luminescence processes of UCNPs and TTA. This enables determination of the polymerization area size at specific irradiation times and calculation of the relationship between the radius of the photopolymerization region and irradiation time, intensity of the 980 nm laser. As shown in Fig. 3(j), the UCPP region diameter continuously increases over time. In contrast, during the first second, the CUCPP region diameter increases with time and then stabilizes. This characteristic indicates that the UV light emitted by the Ho³⁺/Yb³⁺-doped UCNPs-TTA hybrid system is more concentrated at the irradiation center, effectively overcoming the diffusion. The numerical simulation results are in good agreement with the experimental measurements. By comparing Fig. 3(k), it can also be observed that the CUCPP diameter increases more slowly with the increase of excitation light intensity.

High-concentration doped core-shell-shell β-NaYF₄: Ho³⁺/Yb³⁺@NaYbF₄@NaYF₄ UCNPs were synthesized via two-step hydrothermal method, and successfully incorporated into the photoresist after surface modification. Other materials used include: dipentaerythritol pentaacrylate (SR399), 1-hydroxycyclohexyl phenyl ketone (photoinitiator 184), 2,3-butanedione (biacetyl), 2,5-diphenylazole (PPO), and 6-Mercapto-1-hexanol. The molecular structures of all materials are shown in Fig. 4(a, b). All materials were used as received without further purification.

The synthesis method for the β-NaYF₄:20%Yb,10%Ho cores can be found in the Supplementary Methods. Growing two shell layers outside the nanoparticle core significantly enhances the luminescence efficiency of Ho³⁺. Here is the method for growing the shell layers. Adding a water solution containing NaCF₃COO (4.0 mmol) and Yb(CF₃COO)₃ (4.0 mmol) to a mixture of 8 mL oleic acid (OA, 90%) and 10 mL 1-octadecene (ODE, > 90%), both of which were placed in a 50 mL flask. Nanoparticles were previously dispersed in cyclohexane. The mixture was then heated at 100°C for 20 minutes and further heated at 130°C for 30 minutes to form Yb-oleate complex. After cooling to room temperature, the NaYF₄:20%Yb,10%Ho cores were added to a flask, and the mixture was heated at 100 ℃ for 20 minutes to evaporate off cyclohexane. Finally, the mixture was heated to 320°C and maintained under vigorous stirring in Ar flow for 45 minutes. The core-shell UCNPs were precipitated by adding ethanol and collected by centrifugation at 10,000 rpm for 10 minutes. By repeating the aforementioned steps using freshly collected β-NaYF₄: Ho³⁺/Yb³⁺@NaYbF₄ UCNPs as cores and substituting Yb(CF₃COO)₃ with Y(CF₃COO)₃, an additional shell layer was grown to obtain core-shell-shell β-NaYF₄: Ho³⁺/Yb³⁺@NaYbF₄@NaYF₄ UCNPs.

As shown in Fig. 4(c, d), the UCNPs exhibit a hexagonal crystal structure with a diameter of 12 nm. The powder X-ray Diffraction (XRD) pattern of β-NaYF₄: Ho³⁺/Yb³⁺@NaYbF₄@NaYF₄ confirms the formation of hexagonal phase and is in good agreement with the JCPDS data (card no. 28–1192, a = 5.960 Å, c = 3.510 Å, Space group P63/m). Compared to core-shell structures, the spatial separation distribution of sensitizers Yb³⁺ and activators Ho³⁺ in the adjacent layers of the bilayer core-shell-shell UCNPs can significantly suppress their cross-relaxation, enabling efficient energy transfer at the interface and enhancing the emission efficiency of Ho³⁺ by 10 to 100 times. Furthermore, the core-shell-shell effectively suppresses concentration quenching. By experiment measurement, the upconversion luminescence efficiency of UCNPs is positively correlated with the concentration of Ho³⁺ in the core layer within the range of 0–20%, and obvious luminescence weakening occurs only when the concentration of Ho³⁺ in the core layer is increased above 30%. The sensitizers and activators used in the CUCPP were Yb³⁺ (20%) and Ho³⁺ (10%) in the core layer of UCNPs.

The surface modification of UCNPs was performed by mixing 6-Mercapto-1-hexanol into the cyclohexane solution containing UCNPs. The synthesized UCNPs have alkyl groups on their surface and are insoluble in acrylate-based photoresist monomers. 6-Mercapto-1-hexanol can break the alkyl groups at the carboxyl sites and replace them with thiol groups, resulting in high solubility of UCNPs in SR399. A mixture of surface-modified UCNPs solution and 184 was mixed with SR399 at room temperature for 1 hour, centrifuged to remove the precipitate, and then dried in a vacuum oven for 5 hours to obtain the polymer sample. As shown in Fig. 4(e), UCNPs exhibit excellent dispersion in the polymer, which facilitates uniform outward diffusion of upconverted luminescence upon NIR excitation.

CUCPP can approach the limit resolution resolution achievable with single-beam exposure. The device used for CUCPP is a self-assembled continuous laser direct writing system operating at 980 nm. The laser used is a continuous-wave 980 nm solid-state laser produced by Yuanming Laser in China, priced at $300, emitting a circular Gaussian beam. As shown in Supplementary Fig. 6, The laser beam is expanded and focused on the substrate. The substrate is controlled by custom-designed software to manipulate the stage movement and the opening, closing of the optical shutter. The software can slice 3D structures layer by layer and plan the path for each layer to achieve additive manufacturing of 3D structures. The limit resolution achievable by a single 980 nm excitation beam is approximately $\:\frac{\lambda\:}{2NA\bullet\:\sqrt{3.1}}\approx\:200\:\text{n}\text{m}$ . Figures 5(a) show SEM images of line patterned samples with an excitation intensity of 10⁵ W/cm² and a laser writing speed of 80 µm/s. The structural resolution is 290 nm, which approaches the limit resolution. During the CUCPP process, the emission of red light from the UCNPs remains a diffusion-based process, leading inevitably to some blurring of the structure, which is why reaching the ultimate resolution is challenging.

CUCPP can be used to print two-dimensional patterns and 3D structures. Figure 5(b, c) shows SEM images of the two-dimensional panda pattern polymer samples that emit upconversion red light upon irradiation with 980 nm light. Figure 5(d-f) presents oblique top-view SEM images of a 3D regular dodecahedron structure and its top-view luminescent image under 980 nm light illumination at different heights. The 3D regular dodecahedron structure is produced through layer-by-layer line scanning, with each edge length of 15 µm and lateral resolution of 3 µm. Figure 5(g, h) depicts top-view SEM images of a 3D woodpile structure, where the overall design dimensions are 40 µm × 40 µm, the width between lines is 3 µm, the layer height is 1 µm, and each line has a resolution of 480 µm. Laser fabrication is achieved through repeated motion to build up the layers.

CUCPP combines excited state absorption upconversion and TTA upconversion to achieve a four-photon process, and it was proven by analyzing the exponential relationship between upconversion light intensity and excitation light intensity. This strategy overcomes the diffusivity in UCPP while maintaining low excitation intensity and high writing speed, as well as the advantages of using NIR light. CUCPP can fabricate 3D structures, achieving a minimum linewidth of 290 nm under a 10⁵ W/cm² NIR laser intensity and 80 µm/s writing speed, which is two orders of magnitude improvement in resolution compared to UCPP. This approach provides a general method of implementing four-photon lithography in week light by combining two efficient upconversions, and the first upconversion step can be in a variety of forms. With the advantage of circumventing the low absorption cross section, it will be the path to higher photon number MPL.

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SUPPLEMENTARYINFORMATION.docx
SupplementaryMedia1.mp4
Luminescence and photopolymerization of Tm3+/Yb3+ and Ho3+/Yb3+ -doped UCNPs
SupplementaryMedia2.mp4
Line structure produced by UCPP and CUCPP

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Cascade upconversion: a strategy enabling four-photon lithography in week light

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