1. Introduction

An optical diffuser is a very critical optical component that can evenly scatter/diffuse light to fulfill a variety of practical applications, including hiding a light source, eliminating the image of a filament, broadening the angular range over a signal, making the appearance of a viewing screen more even, and spreading from the light source to a specified angle. Therefore, it has attracted extensive attention and has been widely used in light-emitting diode (LED) luminaires [1–4] and liquid crystal displays (LCDs) [5–7]. Laser light is known for its high spatial coherence. While this coherence is beneficial for many applications, it can also cause problems such as speckle, which can interfere with imaging and measurement. When a laser beam passes through a diffuser, the light waves become randomized in phase and direction, effectively disrupting the coherence of the laser beam. As a result, the scattered light produces a more uniform illumination or imaging pattern, reducing the occurrence of speckle. Diffusers can be made from a variety of materials, and can be designed to produce different levels of diffusion and scattering. By carefully selecting the type and design of the diffuser, the coherence of the laser beam can be controlled to achieve the desired level of speckle reduction.

The two most common types of optical diffusers are volumetric and surface-relief types [8,9]. The volumetric type depends on the dispersion of particles, since it is prepared by coating transparent substrates with a homogeneous solution made of organic [10–12] or inorganic particles [8,13–15]. Nevertheless, the preparation process for coating is complicated and light efficiency is relatively low. For the surface-relief type, special interfacial microstructures are used to obtain uniform light. The surface-relief diffusers are particularly appealing candidates for practical applications because of their wide-viewing, high contrast, and low ambient light reflection [15]. Surface-relief diffusers have been realized using delicate microfabrication methods such as lithography [16–18], embossing [11,19], self-assembly [20,21], isotropic etching [22] and laser ablation [23]. Despite intensive research efforts into the realization of the surface-relief diffusers, most of the manufacturing techniques are complex and expensive to implement and may not be feasible for high-throughput production [9,24,25]. Poor control over the microstructure [9] has also prevented any efficient application of production methods for high-quality optical diffusers.

In this article we propose a novel plasma-processing method for surface-relief diffuser films based on helium plasma surface treatment. This method provides an efficient route for the formation of special three-dimensional nanostructures that, after oxidation, can be used as optical diffusers. Metal surfaces exposed to high fluxes of helium plasma under specific conditions have been shown to form fiberform nanostructures (FNs) [26,27]. In a detailed TEM investigation, the thickness of a nanostructured layer can be plotted against the square root of the helium fluence [28]. An experimental procedure here uses a linear plasma irradiation device in an unconventional way, by irradiating a thin film of tungsten with helium plasma and varying the irradiation time.

2. Experiments

Quartz glasses (Labo-USQ, 1-mm thickness) were cut into 10$\times$10 mm$^2$ and cleaned with an ultrasonic washer. Radio frequency magnetron sputtering was used to deposit thin films of tungsten with a thickness of 100 nm on the quartz glass. Surface nanostructuring was performed with high fluxes of helium plasma in the linear plasma irradiation device NAGDIS-II [29,30]. Island-shaped nanostructures formed on the surface of tungsten thin films exposed to helium plasma [29]. It has been found that a fiberform nanostructure on tungsten surfaces is easy to form when the surface temperature is in the range of 1000–2000 K [26,31] and incident ion energy is greater than 20 eV [26]. Comprehensive information about FNs can be found, e.g., in the tutorial [32], which offers an in-depth examination of fiberform nanostructures, including their growth process, conditions, mechanisms, and applications. It is known that the FNs are made of pure tungsten without impurities and contain many helium bubbles inside the structure; almost all of the thin film was changed to a nanostructured layer when 3 min irradiation was performed [33], and nanostructured W is completely oxidized to WO$_3$ during the annealing process [34]. Helium plasma is generated by a direct current arc discharge using an LaB$_6$ cathode. Following the start of the plasma irradiation, the surface temperature was measured with a radiation pyrometer. In this study, the nanostructure was formed by exposing the tungsten thin film surface with a negative bias of -90 V to helium plasma for a varied duration. Nanostructured tungsten thin films were annealed in the air with the treatment at 600 $^{\circ }$C for 6 hours. The morphologies of the nanostructured thin flims after annealling were studied by a field emission scanning electron microscope (FESEM, JSM-7100F, JEOL).

Figure 1(a) shows a schematic of the setup for the measurement of the total transmittance with a spectrometer (AvaSpec-3648) with an integrating sphere. The experimental setup consisted of placing the sample in front of the integrating sphere, which collects all transmitted light and sends it to the spectrometer for analysis. The spectrometer measures the total amount of light that passes through the sample and records the data for analysis. A tungsten halogen light source (HL-200-HP, Ocean Optics) was used, and the total transmittance was calculated from a reduction of the light intensity measured with the spectrometer by placing a diffuser near the entrance of the chamber. An optical diffuser-free measurement value was used as a reference standard for the total transmittance measurement (shown in Fig. 1(b)).

 

Fig. 1. a) Schematic diagram of the total transmittance measurement setup, b) Schematic overview of the transmitted scatter distribution function (TDF). A sample is illuminated vertically, and the detector records the radiance exiting the sample at a specific azimuth and zenith angle.

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The optical diffusing properties of a sample were characterized using the transmitted scatter distribution function (TDF). The TDF is a measure of the amount of light scattered by a medium from one direction to another. A semiconductor laser emitting at 520 nm (Thorlabs, PL201) was used to illuminate the optical diffusers straight from the normal direction to the surface and the TDF was measured by changing the azimuth and zenith angles of the detector. The TDF provides information on how light is scattered and transmitted through the sample as a function of angle. The TDF was measured and analyzed to determine the scattering properties of the sample, which can be useful for a variety of applications in optics and photonics.

3. Results and discussion

After treatment with helium plasma, the tungsten thin film surfaces exhibited a nanostructured surface morphology. The nanostructured tungsten thin film samples were annealed under a flow of air at 600 $^{\circ }$C for 6 hours. Figure 2(a-c) illustrates the SEM micrographs of fiberform nanostructures formed by exposure to helium plasma for 5, 15, and 30 minutes respectively at a surface temperature of 1093-1099 K, followed by the annealing treatment. All samples were negatively biased, with a value of -90 V. As shown in Figure 2, an open interconnected nanometric fiberform structure is observed. Compared to the SEM observation before annealing shown in Ref. [33], it can be seen that nanostructured fibers remain after annealing. When the irradiation time is 5 minutes, uniform nano-sized protuberant fibers are formed and the nanostructured fiber is thicker than the fiber in Fig. 2(b-c). As the irradiation time increases to 15 minutes, the fiber continues to grow and starts to diverge into filament fibers. Studies have been conducted on the formation mechanism of fiberform nanostructures, which appears to be related to the creation and coalescence of helium bubbles, causing the surface to swell [26]. The surface shows more slender fibers at an irradiation time of 30 minutes. In Fig. 2, it can be seen that the fiber gradually increases in length and becomes slender with respect to the increased irradiation time. Moreover, the previous research [33] revealed that the 30-minute irradiation sample did not show significant differences in nanoscale features and structural characteristics compared to the longer irradiation times, indicating the limited impact of extended irradiation on the formation of fiberform nanostructures.

 

Fig. 2. FESEM images of (a) 5 minute, (b) 15 minute, (c) 30 minute irradiation tungsten thin film annealed at 600 $^{\circ }$C for 6 h.

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The total transmittances (T$_{{\rm Total}}$) of nanostructured films with different irradiation times are shown in Fig. 3. T$_{{\rm Total}}$ are plotted at wavelengths from 400 to 1000 nm, which covers the whole visible light region. An intensity without the optical diffuser is used as a reference. Based on the reference value, T$ _{{\rm Total}}$ can be derived by measuring the light intensity passing through the optical diffuser. T$_{{\rm Total}}$ of the non-irradiated sample is the highest in the visible region, close to 90$\%$. As the irradiation time is increased to 5 minutes, T$_{{\rm Total}}$ shows a sharp reduction. There may be light reflecting back in the direction of the incident. There is an obvious increment in T$_{{\rm Total}}$ when the irradiation time is increased from 5 to 15 minutes. There is almost no change from 15 to 30 minutes.

 

Fig. 3. Total transmittance (T$_{{\rm Total}}$) at wavelengths from 400 to 1000 nm.

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Figure 4 shows the measured TDF on the four samples. The vertical color bar from blue (low) to red (high) in Fig. 4(a-d) indicates the natural logarithm of the optical intensity. The nanostructured diffusers made with the plasma-processing method show significantly high diffusion ability. It can be clearly seen that the diffusion range gradually expands as the irradiation time increases. With an irradiation time of up to 30 minutes, the diffuser produces a super light diffusing effect. When compared to the diffusing abilities of the four samples, the 30 minute irradiation sample exhibits the most excellent diffusing ability, which is consistent with T$_{{\rm Total}}$ of the 30 minute irradiation sample. Furthermore, the 5 minute irradiation sample has a weaker diffusing ability, due to the lower T$_{{\rm Total}}$. Figure 5(a) presents an optical intensity distribution corresponding to Fig. 4 for a nanostructure-based diffuser. Figure 5(a) shows an offset caused by device adjustment. Figure 5(b) shows the relationship between diffusion efficiency and irradiation time. Total transmittance is subdivided into regular and diffuse transmittance, quantified by the ratios of directly and diffusely transmitted radiant power to the incident radiant power. The intensity in Figure 5(a) is a function of the detector’s zenith angle, which can be modeled as the sum of the cosine and the Gaussian functions. The cosine function represents the diffuse transmittance part and the Gaussian one represents the regular transmittance part. Here the diffusion efficiency in Fig. 5(b) is the diffusion percentage of the total sum of the peaked Gaussian and diffuse transmittance. It is the percentage of the area of the cosine function to the Gaussian and the cosine functions. The area is the double integral over the detector’s zenith angle of the Gaussian and the cosine functions respectively. From Fig. 5(a), there is a relatively sharp peak in the center for the 0 minute irradiated sample, and the area and intensity of the peak gradually become smaller as the irradiation time increases. It can be observed that the diffusion efficiency increases with the duration of the irradiation. The diffusion efficiency of the 30 minute irradiated sample is a thousand times higher than that of the unirradiated one. Through the novel optical diffuser, the incident light is scattered, all at different angles, due to the multi-scattering generated by the fiberform nanostructure. Figures 4 and 5 both indicate that the optical diffusion performance of the 30 minute sample is the best, which combines a high total transmittance and an excellent light diffusing ability.

 

Fig. 4. Optical intensity distribution of (a) 0 minute irradiation sample, (b) 5 minute irradiation sample, (c) 15 minute irradiation sample, and (d) 30 minute irradiation sample.

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Fig. 5. Optical intensity distribution for a) 0 and 30 minute irradiation sample and b) the relationship between irradiation time and diffusion efficiency based on novel optical diffusers with fiberform nanostructures.

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The optical diffusion performance of the 30 minute sample is compared with other published works on optical diffusers, as shown in Table 1. Kuo et al. developed a particle-coating bottom diffuser by mixing acrylic-styrene beads of various sizes with a thermosetting acrylic resin and applying them to PET substrates, resulting in an optical diffuser with a transmittance of 72$\%$ and a diffusion efficiency of 66$\%$ [7]. Zhong et al. successfully synthesized 3D flower-like hollow Mg-Al layered double hydroxides microspheres via a simple hydrothermal method, demonstrating their potential as effective light scattering materials for optical diffusers with suitable light transmittance, good diffusion capacity, and low sensitivity to incident angles [39]. Mahpeykar et al. developed a novel diffuser by embedding cellulose nanocrystals in a polydimethylsiloxane matrix, resulting in an optical diffuser with unique surface properties, mechanical flexibility, physical durability, and optical transparency [41]. In contrast, the FNs-based optical diffuser has a relatively low transmittance but a much higher diffusion efficiency. Though the characterization methods of the optical diffusers listed in Table 1 are different, the TDF results show that the optical diffusion performance of the 30 minute sample is excellent.

Table 1. Performance of optical diffusers (The definition of diffusion efficiency in this article is different from other reported works.)

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4. Conclusion

In this article, we have proposed a novel plasma-assisted processing technique to fabricate optical diffusers. Fiberform nanostructures formed by helium plasma irradiation and subsequent annealing can scatter light away and result in optical diffusion. Fiberform nanostructures were used as optical diffusers, and the optical properties were also demonstrated for the first time using the TDF and a spectrometer with an integrating sphere. The characteristics of optical diffusers fabricated using this method can be altered by changing the irradiation time. The 30 minute irradiation sample showed a diffusion efficiency of up to 95 $\%$, which is a thousand times higher than that of an unirradiated one. The novel diffuser based on fiberform nanostructures possessed good diffusion capacity and presented the advantages of wide-viewing. Despite the strong diffusion ability, the transmittance was relatively low when compared to others. Future studies will need to optimize the fabrication process to improve the transmittance. This excellent diffusion efficiency of the helium-plasma method offers an exciting new synthesis route for nanostructured materials that can be used to prepare a variety of multifunctional optical diffusers. In addition, it is expected that fiberform nanostructures on quartz glass have the potential for future use in other applications such as optical sensors [42,43], thus expanding the possibilities and applications.