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Abstract
A triple-wavelength patterned quantum dot film was fabricated for the light source of digital holography to improve both the axial measurement range and noise reduction. The patterned quantum dot film was fabricated after optimizing the photolithography process condition based on the UV-curable quantum dot solution, which was capable of multiple patterning processes. In addition, an optimized pattern structure was developed by adding TiO2 nanoparticles to both the quantum dot and bank layers to increase the scattering effect for the improved photoluminescence intensity. Finally, the newly developed light source with the balanced spectral distribution was applied to the digital holography, rendering it applicable as an improved light source.
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1. Introduction
Digital holography (DH) can restore images in a three-dimensional (3D) form by reconstructing both the intensity and phase of objects, rendering it useful in many applications [1–4]. Recently, digital holographic systems also show a new trend toward more compact, low cost, field portable applications [5–8]. Especially, quantitative phase imaging (QPI) is one of the major DH applications [9,10]. Since QPI is based on phase data, it can achieve axial resolution at the nanometer scale. Moreover, compared with conventional 3D measurement methods, DH acquires object images faster and non-invasively. Therefore, QPI has been studied in various application fields [11–21]. However, there are technical issues in DH that generate phase ambiguities due to wave theory [22] and noise due to interferences effects [23–25]. After developing of multi-wavelength digital holography [26,27], there have been studies on improving the axial measurement range by using a synthetic wavelength with dual-wavelength lasers and LEDs as a light source [28–32]. Moreover, noise reduction has been addressed by applying noise reduction algorithms [33] and low coherent light sources [34–38], although improvements are still required. A study was conducted using quantum dots in our laboratory, which have the advantage of the desired wavelengths and low interference for fabricating a dual-wavelength quantum dot film through a micro-imprinting process to apply it as the light source in DH [39,40]. By utilizing a quantum dot film with bandpass filters and a synthetic wavelength, it was able to restore images with improved axial measurement range and noise reduction [40]. In addition, the DH optical system and alignment were simplified by using a single light source [40]. Although the dual-wavelength DH can improve the axial measurement range by using a synthetic wavelength, the noise amplification should be accompanied, which is a critical issue in DH [41–43]. Noise amplification can result in precision decreases by the same factor as the synthetic wavelength compared with a single wavelength [44]. Therefore, many studies have been conducted to optically unwrap the phase map with reduced noise by the dual-wavelength DH [28,45,46]. In addition, triple-wavelength DH has been introduced to reduce noise amplification based on the hierarchical phase imaging method for an enhanced axial measurement range [41,44]. To extract the real image of an object, however the off-axis optical setup causes the system to be complicated and bulky. Actually, the hierarchical phase unwrapping method can reveal the phase and reduce noise amplification [44]. With this method, both enhanced axial resolution and noise reduction can be realized by selecting the appropriate wavelengths. Therefore, a single multi-wavelength quantum dot film can be used as an improved light source for DH with enhanced axial measurement range, superior noise reduction, and a simplified optical system.
In this study, we designed and fabricated a triple-wavelength single quantum dot film to operate as a light source for DH by multiple patterning through the photolithography process. The light source with the resultant film can improve the axial measurement range with phase noise reduction by utilizing the hierarchical phase unwrapping method as well as simplified the optical setup with a single triple-wavelength light source. To fabricate quantum dot film as a compact multi-wavelength converter with desired wavelengths, the photolithography process conditions were optimized based on the negative photoresist, which included quantum dot particles with improved dispersibility. The photoluminescence intensity was also improved by optimizing the pattern structures with TiO2 nanoparticles in both quantum dot and bank layers, which can be able to increase the scattering effect. Moreover, the concentration of quantum dots was controlled separately in each wavelength to minimize the imbalanced intensity. To verify the noise reduction while maintaining an axial measurement range, in-line phase-shifting digital holography was evaluated experimentally for an object step height of 1.8 µm after applying a newly developed light source.
2. Optimization of the UV curable quantum dot solution and the photolithography process condition
2.1 UV curable quantum dot solution
To fabricate patterned quantum dot films using the photolithography process, a UV-curable quantum dot solution is initially required. Making a UV-curable quantum dot solution based on positive photoresist that have relatively low viscosity may have an advantage of dispersing the quantum dot particles. However, the positive photoresist based quantum dot solution could dissolve the previously cured patterns during the iteration patterning process due to the positive photoresist being chemically sensitive and unstable. Therefore, in this study, we made a UV-curable quantum dot solution based on negative photoresist. However, the dispersity of quantum dot particles in a negative photoresist with relatively high viscosity required determining, which was the main issue. Accordingly, we optimized a UV-curable quantum dot solution based on negative photoresist that was combined with the quantum dot solution, propylene glycol monomethyl ether acetate (PGMEA), polymer dispersant, and TiO2 nanoparticles (100 nm).
SU-8 2025 negative photoresist (Kayaku Advanced Materials) was used as a photocurable material, which has the features of heat-resistant durability, high resolution, and high optical transmittance, rendering it suitable for use as a light source. The fabrication process of a UV-curable quantum dot solution for the photolithography process was as follows. Firstly, the CdSe/ZnS quantum dot (Global ZEUS) was mixed with solvent at a rate of 100 mg/ml. Then, propylene glycol monomethyl ether acetate (PGMEA) was mixed under magnetic stirring to ensure uniform dispersion of quantum dot solution by exchanging the solvent. PGMEA is also suitable for use as a light source due to its high transparency and can be used as a compatible solvent while maintaining SU-8 photoresist features [47–50]. To improve the dispersibility of the quantum dot particles, AJISPER PB-824 (Ajinomoto) was added as a polymer dispersant. This polymer dispersant easily dissolves in an ether-based solvent such as PGMEA and is also compatible with epoxy-based negative photoresists such as SU-8. Thus, the polymer dispersant was mixed with PGMEA and dissolved until the polymer dispersant particles melted. The weight percentage of the polymer dispersant was 5wt% of the amount of quantum dot particles [51]. Then, both solutions were mixed in a closed vial for 24 h with a magnetic stirrer. Finally, a UV-curable quantum dot solution with improved dispersibility was fabricated by mixing the SU-8 2025 negative photoresist and the previously prepared quantum dot solution as a weight ratio of 6:1 in a closed vial for 24 h with a magnetic stirrer. The controlled amount of TiO2 nanoparticles, which are chemically stable with had high durability characteristics, were also mixed as the scattering particles to achieve higher photoluminescence intensity. The amounts of each material were calculated depending on the concentration of the quantum dots in the final UV-curable quantum dot solution.
2.2 Photolithography process condition
To fabricate the quantum dot film based on the UV-curable quantum dot solution developed previously, the photolithography process condition was optimized through experiments because it is a new solution that could not use the general SU-8 process conditions. The procedures and optimized process conditions for patterning the quantum dot films are as follows. The quantum dot film was formed by spin coating the UV-curable quantum dot solution onto a cleaned glass substrate. To evaporate the solvent for the soft baking process, the substrate was placed on a hot plate at 65 °C for 3 min, followed by 95 °C for 6 min. Next, it was exposed with the photo mask and UV exposure (LA-100, Lithotech) at 25 mW/cm2 UV energy. Here, the exposure time was increased from 10 s to 20 s since the amount of energy absorbed was increased due to the high dispersibility of the quantum dot particles and the scattering effect of the TiO2 nanoparticles. As a post-exposure baking process, the substrate was placed on a hot plate at 65 °C for 1 min, followed by 95 °C for 6 min. Finally, for the developing process, the unexposed areas were removed by leaving the substrate in the SU-8 developer. The developing time was also decreased from 180 s to 90 s due to the adding effect of quantum dots and TiO2 nanoparticles. Then, it is cleaned with deionized (DI) water as a final step in the patterning process. As a result, we were able to determine the optimized photolithography process conditions on the UV-curable quantum dot solution that are capable of multiple patterning processes.
Based on the UV-curable quantum dot solution and optimized photolithography process conditions, the dispersibility and thickness were evaluated with the fabricated quantum dot film. As depicted in Fig. 1(a), the uniform dispersion of quantum dot particles of 0.1wt% with the emission peak of 530 nm was confirmed using a confocal microscope (LSM 980, ZEISS). In addition, the thickness variation on newly prepared UV-curable quantum dot solution was measured using a field emission scanning electron microscope (JSM-7800F, JEOL) as a function of spin coating speed, as depicted in Fig. 1(b). In this study, it was decided to fabricate the patterned quantum dot film with a thickness of 18.75 µm by applying spin coating speed at 1000 rpm. This was because a thicker film could reduce the transmittance, while a thinner film would require a higher concentration of quantum dots.
Fig. 1. (a) A microscope image on the quantum dot film of 0.1wt% with the emission peak of 530 nm observed from the confocal microscope. (b) Graph on the film thickness variation that was fabricated from SU-8 2025 solution [52] and the UV-curable quantum dot solution as a function of spin coating speed.
3. Patterned quantum dot film of triple-wavelength for the light source of digital holography
3.1 Optimization of the pattern structures to improve the photoluminescence intensity
Based on the previous experiment, the triple-wavelength patterned quantum dot film was fabricated for the single light source of DH. Quantum dots with the wavelengths of 530, 570, and 620 nm were selected to make a higher synthetic wavelength by minimizing the spectrum overlap. To minimize reabsorption issue between the patterns in each wavelength, the pattern was designed with 150 µm spacing, as explained in Fig. 2. Furthermore, it was very important to improve the photoluminescence intensity of the patterned quantum dot film as much as possible to use it as a light source for DH by employing TiO2 nanoparticles as the scattering particles, as shown in Fig. 2(b) and 2(c). The pattern structure in Fig. 2(b) improved the photoluminescence intensity, even at a low quantum dot density, by the addition of TiO2 nanoparticles for the scattering effects in the quantum dot (QD) layer. In addition, the pattern structure in Fig. 2(c) was also revealed to change the optical path of the blue LED, improving the photoluminescence intensity and minimizing reabsorption by fabricating the bank layer with TiO2 nanoparticles. The microscope images of quantum dot patterns on three different structures are shown in Fig. 3, where all patterned QD films are prepared with 0.1 wt% of quantum dots based on the optimized photolithography process conditions.
Fig. 2. Schematic illustrations of three different pattern structures, including (a) only quantum dots in the QD layer, (b) quantum dots and TiO2 nanoparticles in the QD layer, and (c) quantum dots and TiO2 nanoparticles in the QD layer, and TiO2 nanoparticles in the bank layer.
Fig. 3. Images of fabricated triple-wavelength (530, 570, and 620 nm) patterned quantum dot film of three different pattern structures as shown in Fig. 2.
The density of the TiO2 nanoparticles was determined to improve the photoluminescence intensity in this study. Based on the results, the quantum dot film had the most improved photoluminescence intensity when the quantum dot layer contained 3wt% of TiO2 nanoparticles in the case of 0.1wt% of quantum dots. The spectral distribution and intensity of the fabricated three different quantum dot films were measured with an integrating sphere (QE-1000, Otsuka).
Then, the spectral distributions of the fabricated quantum dot films were evaluated by the measurement system to compare the photoluminescence intensities. As shown in Fig. 4, the pattern structure with TiO2 nanoparticles in both QD and bank layers resulted in the higher photoluminescence intensity compared to those from the pattern structure without TiO2 nanoparticles in both layers. The improved factors were 5.88 times in 524 nm, 5.26 times in 571 nm, and 5.05 times in 620 nm, respectively. The quantitative improved intensity values are also summarized in Table 1.
Fig. 4. Comparative graph on the photoluminescence intensity measured from the fabricated triple-wavelength patterned quantum dot films with three different structures.
Table 1. Normalized photoluminescence intensities from triple-wavelength patterned quantum dot films with three different structures
3.2 Balanced spectral distribution and digital holography evaluation
Since the triple-wavelength patterned quantum dot films were fabricated with the same concentration of 0.1wt% QD, they resulted in the imbalanced spectral distributions as shown in Fig. 4. We believe that this variation comes from mainly quantum efficiency difference in each wavelength quantum dot and slightly reabsorption issue. Therefore, the balanced spectral distribution should be realized by changing the concentration of quantum dots in each QD wavelength when used as a light source for DH. Based on the pattern structure with TiO2 nanoparticles in both QD and bank layers as shown in Fig. 2(c), an appropriate triple-wavelength single quantum dot film was fabricated with different concentrations of quantum dots in each wavelength. The concentration of quantum dots in each wavelength was fixed as 0.13wt% in the 530 nm wavelength QD layer, 0.1wt% in the 570 nm wavelength QD layer, and 0.16wt% in the 620 nm wavelength QD layer, respectively. Figure 5 shows FE-SEM microscope images at each patterning stage for the triple-wavelength patterned quantum dot films with 4 patterning iterations, confirming clear patterning of line patterns. Finally, the balanced spectral distribution was also confirmed after measuring the intensity from the triple-wavelength patterned quantum dot films with different concentrations of quantum dots, resulting in the intensity difference below 2.7%, as shown in Fig. 6.
Fig. 5. FE-SEM microscope images at each patterning stage for the triple-wavelength patterned quantum dot films with 4 patterning iterations, after (a) TiO2 bank patterning, (b) 620 nm QD patterning, (c) 570 nm QD patterning, and (d) 530 nm QD patterning.
Fig. 6. The balanced spectral distribution (indicated as (b)) measured from the final triple-wavelength patterned quantum dot films with different concentrations of quantum dots in each wavelength, such as 0.13wt% in the 530 nm wavelength QD layer, 0.1wt% in the 570 nm wavelength QD layer, and 0.16wt% in the 620 nm wavelength QD layer, respectively.
Finally, an in-line phase-shifting digital holography based on a Michelson interferometer was used to reduce complexity of optical system and evaluate the noise reduction effect of newly developed single triple-wavelength light source. Based on the optical setup of phase-shifting digital holography explained in the previous our report [40], we just replaced a light source with the fabricated triple-wavelength single patterned quantum dot film. The 7-step phase shifting method introduced by T. Tahara et al. [53] was used in this evaluation for the numerical process of reconstruction. By utilizing diffraction integrals, the complex amplitude distribution on object waves was extracted, allowing for the reconstruction of step-height images specified to each wavelength. Figure 7 shows the reconstructed images according to each single and synthetic wavelength. From the reconstructed image from Λ2-3 synthetic wavelength which means a dual-wavelength digital holography, we can obtain the cross-sectional profile of the object, as shown in Fig. 8(a). Thus, it was confirmed that the measured value is same as a step height of 1.8 µm. However, the amplified noise was also derived from the bottom surface profile, despite the enhanced effective range of the height measurement in dual-wavelength digital holography. Thus, we applied the hierarchical phase unwrapping to reduce this amplified noise [45], and it was measured that the noise level was suppressed from 88.5 to 19.4 nm after applying triple wavelengths unwrapping, as shown in Fig. 8(b). Here, the noise was quantified by calculating the standard deviation of the surface profile. This result means that the noise level decreases in triple-wavelength digital holography effectively while maintaining an effective range of height measurement. Thus, it was confirmed that the proposed triple-wavelength single light source could achieve high accuracy with suppressed noise as well as enhanced effective range of height measurements in a multi-wavelength digital holography.
Fig. 7. Reconstructed images according to each single and synthetic wavelength. The wavelength of Λ1-2, Λ1-3 and Λ2-3 means the synthetic wavelengths for 524, 571 and 620 nm.
Fig. 8. The measured cross-sectional profile on the object with step height of 1.8 µm from (a) dual-wavelength digital holography based on Λ2-3 synthetic wavelength and (b) triple- wavelength digital holography after applying the hierarchical phase unwrapping.
4. Conclusion
In this study, a triple-wavelength patterned quantum dot film was fabricated for the light source of digital holography to improve both the axial measurement range and noise reduction. We successfully fabricated the patterned quantum dot film after optimizing the photolithography process condition based on the UV-curable quantum dot solution, which was capable of multiple patterning processes. To improve photoluminescence intensity in the triple-wavelength patterned quantum dot film, an optimized pattern structure was developed by adding TiO2 nanoparticles to both quantum dot and bank layers to increase the scattering effect as well as reduce the reabsorption issue. In addition, the balanced intensity distribution was realized with the difference below 2.7% by adjusting the concentration of quantum dots in each wavelength layer, enabling the patterned structure to be used as a light source for triple-wavelength digital holography. Finally, the newly developed light source with the balanced spectral distribution was applied to the in-line phase-shifting digital holography, resulting in an effectively reduced noise level from 88.5 to 19.4 nm after applying triple wavelengths unwrapping. Thus, it was confirmed experimentally that the proposed triple-wavelength single light source could achieve high accuracy with suppressed noise as well as enhanced effective range of height measurements in a multi-wavelength digital holography.
Funding
Ministry of Science and ICT, South Korea (2023-22030004-20).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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