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Abstract
The subwavelength microstructures (SWMS) on the surface of ZnS for antireflection in an infrared band have been theoretically designed and experimentally fabricated. The finite difference time domain (FDTD) simulation has been utilized to optimize geometry for obtaining high transmittance of SWMS. Then, during simulation for light field intensity distribution, the inner of SWMS emerges location and wavelength dependent light resonant region, which can be explained by Wood-Rayleigh (WR) law. Furthermore, according to refractive index gradient formation and light field coupling effect, the grating period and height are capable of regulating the band selection of antireflection and value of the transmittance, respectively. In addition, a rapid facile approach based on femtosecond laser parallel multi-beam has been proposed to experimentally realize the designed and optimal structures. The depth, period, and embedded nano-gratings of fabricated SWMS are tunable by controlling laser-processing parameters for antireflection in the wavelength of 8 μm-12 μm. Finally, the broadband and wide-angle antireflective SWMS on ZnS as well as robust mechanical strength and hydrophobicity have been achieved, expecting to be of great potential in an optoelectronic device application.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Zinc sulfide (ZnS) as one of critical infrared transmitting materials has a high refractive index (n = 2.2 at λ = 9 μm), which brings about large reflection losses in optical windows [1,2]. The conventional multi-layer thin films coated on the ZnS for antireflection exhibit good performance, but have disadvantages of low damage threshold, easy peeling off, narrow bandwidths, and poor acceptance angles [3,4]. Therefore, an alternative antireflection approach in military and aerospace applications is to build subwavelength microstructures (SWMS) on the surface of ZnS for creating a graded refractive index profile [5–7]. Owing to their suppressing surface Fresnel reflection and providing an effective refractive index gradient between the substrate and air [8,9], the transmission of light through optical windows could be effectively enlarged, resulting in improvement of infrared imaging quality [10]. The SWMS, inspired from moth eye with critical dimensions that are smaller than optical wavelength of incident light, is particularly appealing because it allows wide bandwidth, large angular response, high mechanical strength, and surface hydrophobicity [11–13]. However, current SWMS fabrication techniques such as e-beam lithography [14], photoetching lithography [15], reactive ion etching lithography [16] and nano-imprint lithography [17] are limited to particular material, organic or metal contaminations and complicated multistep time-consuming manufacturing, which is not beneficial for environment-friendly large area high-efficiency fabrication.
Due to its distinct advantages such as the programmable designability, high spatial resolution, mask-less and sub-diffraction feature size, femtosecond laser microfabrication is a promising method to form SWMS on various materials in one step processing [18–20]. Chen et al. has proposed a facile mask-less approach by femtosecond laser direct writing to produce SWMS on sapphire for mid-IR antireflection [21]. Hiroshi et al. has employed the line scan methods to fabricate reflection-free SWMS on silicon for highly sensitive optical sensors in infrared rays [22]. Li et al. also has fabricated SWMS on ZnS by scanning ultrafast pulse laser ablation for long-wave infrared (8-12 μm) window [23]. While the same obstacle as lithography, the employment of femtosecond laser single-point writing to generate large area SWMS is rather time-consuming [24]. For example, it almost needs 50 hours to form 2 cm × 2 cm square SWMS with a 2 μm period by the line scanning femtosecond laser methods. Hereby, it is the multi-scale challenge between micro-scale subwavelength period and macro-scale large area in SWMS fabrication.
In this study, firstly, the finite difference time domain (FDTD) simulation has been employed to design SWMS on ZnS for infrared transmittance enhancement. The optical field distribution, period, and height of SWMS are investigated to achieve high transmittance. Secondly, to overcome the disadvantages of traditional SWMS manufacturing process, the femtosecond laser parallel multi-beam method in experiment is proposed to high efficiently fabricate SWMS on the ZnS. With the assistance of a microlens array (MLA), the incident single-foci light field has been converted to periodic array of wavelets to form multi-foci pattern on the surface of species by objective lens. In addition, the influence of the distance between the objective lens and MLA, laser repetition rate, polarization of incidence laser, and laser power on the morphology evolution of SWMS have been studied. Finally, the infrared spectral character and surface wettability of fabricated SWMS also have been discussed.
2. Simulation and experimental details
In simulation, the SWMS on ZnS are designed to investigate the broadband antireflection performance in the spectral range of 1-13 μm by using FDTD method. The perfectly matched layer boundary condition is utilized in the direction of the beam propagation (z direction). Yet, the periodic boundary conditions are set in other two vertical direction of beam propagation (x and y direction). Additionally, the mesh size in simulation grid is chosen as 50 nm to achieve high resolution results. The sketch of the simulation SWMS model with a plane wave propagating from the back of the ZnS is exhibited in Fig. 1(b). Further, the refractive index of the ZnS is set in the model according to experimental measurement by Klein [25]. The initial values of height and period in SWMS model are optimized from the calculation of effective medium theory (EMT) [26]. Finally, the electric field component (Ex) distribution, transmittance, and reflectance of the SWMS can be stably obtained through simulation time of 500 fs. Therefore, the achieved optimal height and period in excellent antireflection performance might effectively guide the experimental setting.
Fig. 1 (a) The effective refractive index of SWMS with different period (p) versus filling factor from EMT calculation. (b) The schematic of the designed SWMS. (c)-(f) Electric field intensity distribution of the SWMS on ZnS at incident wavelength of 9 μm, 7 μm, 5 μm, 3 μm.
In experiment, the femtosecond laser with central wavelength of 800 nm is generated by a regeneratively amplified Ti:sapphire laser system (Spectra Physics) with a pulse duration of 120 fs and a tunable repetition rate ranging from 1 to 1000 Hz. Before being focused by the objective lens (20 × , Olympus, Numerical aperture = 0.45), the laser beam is split spatially by a MLA (Thorlabs, period = 150 μm, focal length = 5.3 mm), as shown in Fig. 4(a). Then, a number of multi-foci can be formed on the surface of sample, where 5 × 5 foci intensity distribution captured by a CCD camera displays in Fig. 4(c). Compared with single beam processing, the efficiency can be effectively improved by the parallel fabrication method, which results from full utilization of laser energy in multi beams process. Besides, the distribution of diffraction pattern through MLA has stability during all the sub-beams acting on the surface simultaneously. Consequently, parallel fabrication could effectively reduce the non-uniformity of the obtained period structures arising from stage vibration in moving and laser energy fluctuation. Furthermore, 2.3 mm thick poly-crystal ZnS mounted on a 3-axis linear translation stage with a precision of 20 nm in the X, Y and Z directions, the scanning speed of which could be adjusted. Then, a serial scanning process is employed to produce large-area structures. For example, as exhibited in Fig. 4(b), a typical 28 mm × 28 mm structure on the surface of ZnS with rainbow pattern has been completely scanned within 7 hour. The morphology of SWMS observed by a scanning electron microscope (SEM, Tescan, MIRA 3 LMU) are found composing of micro-nano hierarchical structures after being ultrasonically cleaned by deionized water for 30 minutes. The depth profile of fabricated SWMS can be obtained from atomic force microscopy (AFM, nanoIR, ANASYS). The optical properties and wettability of the SWMS on ZnS are characterized by Fourier-transform infrared spectrometry (FTIR, Nicolet 6700, ThermoFisher) and optical contact angle meter (HARKE, China), respectively.
3. Results and discussions
Figure 1(b) exhibits the schematic of the designed SWMS for simulation. For the one-dimensional subwavelength grating with period less than incident wavelength, the effective medium theory (EMT) could be utilized for SWMS layer calculation with TE mode and TM mode light. In order to obtain a more precise solution in EMT calculation, the second-order of effective dielectric permittivity (εTE and εTM) is expressed as [27]:
where ε0 and ε2 represent the dielectric permittivity of the air (n0 = 1) and bulk ZnS (n2 = 2.2), respectively. λ is the wavelength of incident light. f and p are the filling factor and period of the SWMS, respectively. Hereby, the relationship of effective refractive index at wavelength of 9 μm and filling factor for period p = 1 μm and p = 2 μm can be illustrated in Fig. 1(a). Obviously, the effective refractive indices of the SWMS for TE and TM mode are increasing from n = 1 (air) to n = 2.2 (bulk ZnS) with filling factor enlarging from 0 to 1. It is well known that the smoother refractive index gradient in SWMS layer could bring Δn between consecutive layer approaching to zero, resulting to sharp decline of surface Fresnel reflection [28]. Hence, the designed SWMS with graded refractive index interface profile will enhance the transmission intensity than bulk ZnS. Moreover, Fig. 1(a) also indicates that the SWMS with p = 1 μm for TE mode light possessing smoother profile has better antireflection performance than others. In addition, the subwavelength period restrictive condition for perfect antireflection can be deduced from grating diffraction equation:where θm, θ0, and m are diffraction angle, incident angle, and diffraction order, respectively. Because nonzero diffraction orders disappear in SWMS, the constrained equation of period with normal incidence light is derived from:Therefore, the max period of SWMS on ZnS with antireflection effect in the far-infrared wavelength range 8 μm-12 μm is 3 μm.Figures 1(c)-1(f) depict the FDTD simulated electric field intensity (Ex) distribution of the SWMS with 2 μm period and 2 μm height in the incident wavelength of 9 μm, 7 μm, 5 μm, 3 μm, respectively. The light field intensity () in the SWMS is symmetrically distributed along the direction of the incident light, which is derived from the periodic symmetrical structure of the one-dimensional grating. Moreover, the field intensity inside the grating structure is generally greater than that around the grating. This is due to the internal space of the SWMS compressing, causing the light waves to continuously collide and reflect on the inner wall of the structure, which can induce local electromagnetic field enhancement [29]. This local enhancement effect also representing light wave resonance in grating will effectively capture the incident light and reduce the reflected light, thereby achieving the anti-reflection effect. In addition, Figs. 1(c)-1(f) also have expressed that the light field in SWMS gathers at different position with different incident wavelength. When the incident wavelength is 9 μm, the light resonant region is located at the bottom of the grating. However, when the incident wavelength is 7 μm, the light resonant region is changed to the top of the grating. It can be explained by Wood-Rayleigh (WR) law that the resonance of the coupling light field is determined by both structure size and light wavelength [30].
In order to specifically design and optimize the geometry of ZnS SWMS for broadband and wide-angle antireflection, the transmission and reflection spectra as a function of structural parameters and optical parameters (grating height, period, incident light polarization, and incident angle) have been calculated by FDTD method. Figure 2 shows the relationship between the infrared band transmission and reflection spectra of SWMS with grating period of 2 μm and grating height beginning at 0.5 μm. In Fig. 2(a) and 2(c), when the incident wavelength is less than 4 μm, the spectral lines exhibit oscillation with wavelength varying. This is due to high order diffraction contribution on the surface of ZnS when the grating period is close to the incident wavelength [31]. However, the transmittance and reflectance of SWMS in the wavelength range of 4 μm-13 μm are regularly changing with grating height increasing from 0.5 μm to 3 μm. Therefore, the spectral curves corresponding to four specific grating height of 0.5 μm, 1 μm, 1.5 μm, and 2 μm have been selected and plotted in Fig. 2(b) and 2(d). The far-infrared transmittance of SWMS in Fig. 2(b) is obviously enhanced with grating height enlarging from 0.5 μm to 2 μm. Besides, the far-infrared reflectance in Fig. 2(d) is reduced with grating height increasing. Furthermore, the transmittance in the wavelength of 9 μm at the grating height of 0.5 μm and 1 μm are 64% and 70%, which is lower than that of bulk ZnS (72%). As the grating height reach to 1.5 μm and 2 μm, the transmittance have risen to 78% and 86%, respectively. In other words, the transmittance of ZnS SWMS with small grating height will decrease comparing to bulk ZnS. Only when the grating height reaches a certain value, the SWMS might exhibit an antireflection behavior. For example, the surface reflectance of SWMS with 2 μm height in the wavelength of 9 μm has dropped to 14%. These phenomena can be explained by linear smooth refractive index gradient formation on the surface of ZnS with larger grating height, resulting to suppression of surface Fresnel reflection [32].
Fig. 2 Contour plot of the FDTD simulated transmittance (a) and reflectance (c) of SWMS as a function of height and wavelength. Simulated height-dependent transmittance spectra (b) and reflectance spectra (d) of SWMS on ZnS in the wavelength range 7 μm-13 μm. The transmittance and reflectance at wavelength of 9 μm versus height for single-side (e) and double-side (f) SWMS.
In order to intuitively and intensively study the effect of grating height on antireflection, the transmittance and reflectance at wavelength of 9 μm for designed one side and double side SWMS on ZnS with grating height continuously changing from 0 to 4 μm have been displayed in Fig. 2(e) and 2(f), respectively. The grating height dependent transmittance and reflectance curves are not complete linear variation, but present quasi-periodic fluctuation with similar cosine function. Moreover, the curves also indicate that the grating height should be at least 1.2 μm on purpose of achieving antireflection with one side SWMS on ZnS. The average transmittance value of double side SWMS with large grating height is close to 90%, which is larger than that of one side SWMS. Hence, the double side SWMS can effectively suppress Fresnel reflection on the front and back surface of ZnS, showing better antireflection than one side SWMS.
Figures 3(a)-3(d) reveal the infrared transmittance and reflectance spectrum of ZnS SWMS with grating height of 2 μm for different period. The values of the transmittance in Fig. 3(a) are mostly greater than 50%. Especially, at the wavelength range from 4 μm to 13 μm, a majority of transmittance are more than 70%. This indicates that the ZnS transmission enhancement region originating from SWMS antireflection is mainly located at far-infrared band. In addition, the transmittance and reflectance in Fig. 3(a) and 3(c) on the condition of period changing have displayed a relatively uniform distribution comparing to that effect of grating height. Herein, the suppressing reflection of the SWMS will not be significantly declined because subwavelength period in surface microstructure is not the dominant factor affecting their refractive index gradient. Further, Fig. 3(b) illustrates that the transmittance of about 90% has been achieved in the wavelength of 8.8 μm for the SWMS with period of 1.5 μm. However, for the SWMS with period of 3 μm, the beginning wavelength corresponding to the transmittance of 90% is 11 μm. Accordingly, this tendency provides a possibility for band selection in ZnS antireflection simply by control the period of SWMS, which can be interpreted by WR equation in light field coupling effect [33]. As the light resonant has happened in structure, the array period (p) and incident wavelength (λ) must obey the following WR equation [34]:
where n, θ, and k are refractive index of optical transmission medium, incident angle, and integer for resonance order, respectively. In the case of light propagating at fixed angle of incidence in the air, the resonance wavelength corresponding to antireflection band could be positively tuned by array period of the SWMS.Fig. 3 Contour plot of the FDTD simulated transmittance (a) and reflectance (c) of SWMS as a function of period p and wavelength. Simulated period-dependent transmittance spectra (b) and reflectance spectra (d) of SWMS on ZnS in the wavelength range 8 μm-13 μm. The transmittance (e) and reflectance (f) of SWMS at wavelength of 9 μm with different incident angle θ versus incident polarization angle.
Figure 3(e) and 3(f) illustrate the calculated transmittance and reflectance curves of SWMS with 2 μm height and 2 μm period in the wavelength of 9 μm at different incident angle as a function of incident polarization angle. With the incident angle enlarging from 0° to 50°, the values of transmittance are apparently reduced with a small extent, and the values of reflectance are slightly enhanced. In addition, the transmittance at any incident angle will decrease from maximum to minimum with polarization angle increasing from 0° to 90°, where 0° and 90° polarization is corresponding to the incident light perpendicular (TE) and parallel (TM) to the grating direction, respectively. The polarization selection in the one dimension SWMS antireflection is devoted to the structured birefringence effect [35], which can be eliminated in two-dimensional grating structures. Additionally, in order to realize transmittance enhancement in maximum for polarization light, the micro-pillar structures are deservedly preferred, which is due to their nearly linear refractive index profile to suppress surface reflection [36]. However, the two-dimensional grating structures in large area are difficult to realize efficiently in experiment by femtosecond laser parallel processing, and so are not treated here.
On purpose of confirming experimentally the antireflection results of designed and optimized structure, the SWMS on the surface of ZnS have been fabricated by femtosecond laser parallel processing, as shown in Fig. 4. Firstly, to obtain the SWMS in design period, the array period of multi-foci light field in Fig. 4(c) formed by MLA and objective lens should be intensively investigated. Following the multiple slit diffraction theory [37], the angle distance Δθ of adjacent diffraction order in diffraction pattern of MLA can be defined as:
where N is the microlens number in a row of MLA, dMLA is the period of MLA, θ is the incident angle. Considering the small value of θ, the spot distance in diffraction pattern of D location is given by the following equation:Therefore, according to the Gauss law, the spot distance ΔL of adjacent diffraction order on surface of ZnS after light passing through MLA and objective lens can be given as [38]:where fMLA and fOL are the focal length of MLA and objective lens, respectively, m is distance between multi-foci plane and objective lens. The typical parameters of λ = 800nm, N = 40, dMLA = 150 μm, D = 100 mm, fMLA = 5.2 mm, fOL = 3.9 mm have been utilized to obtain multi-foci period of ΔL = 3.2 μm, which is close to the value of FDTD calculated subwavelength period. Hence, the femtosecond laser parallel process system in Fig. 4(a) has ability to fabricate FDTD designed SWMS for ZnS antireflection.Fig. 4 (a) Schematic diagram of the experimental setup for parallel femtosecond laser fabrication. MLA: microlens array; OL: objective lens; D: distance between MLA and OL; Sample: ZnS. (b) Photography of a SWMS sample fabricated by parallel femtosecond laser. (c) A typical optical field intensity distribution of 5 × 5 foci diffraction pattern.
Figures 5(a)-5(c) display the SEM images of the microstructures on ZnS fabricated by parallel femtosecond laser with different distance D. The obtained period of the SWMS is over 1/40 times of the initial period of the MLA (150 μm) by 20 × objective lens focusing. Moreover, the SWMS period will be slightly enlarging with the value of D increasing from 40 mm to 120 mm, which is in accord with results from Eq. (10). Additionally, Figs. 5(d)-5(f) show that the grating depth of SWMS obviously enhances with laser scanning speed decreasing from 1000 μm/s to 500 μm/s. In particular, the grating depth acquired from AFM profile in Figs. 5(g)-5(h) has increased from 700 nm to 1000 nm with scanning speed switching from 500 μm/s to 250 μm/s. The reasons of scanning speed dependent depth might be elaborated by that the pulse number of femtosecond laser will be soaring accompanying with decreasing of speed, resulting to enlargement of grating depth. On the other hand, the Bessel-like beam array of multi-foci created by spherical aberration in MLA enables further enhancement of grating depth [39]. Unfortunately, the SWMS with grating depth of 1 μm have not reach the optimal FDTD simulated model for antireflection, which is due to the limitation of initial beam profile of femtosecond laser with Gaussian energy distribution [40].
Fig. 5 SEM images of the microstructures with different distance D between MLA and OL: (a) D = 40 mm, (b) D = 80 mm, (c) D = 120 mm, and different laser scanning speed: (d) ν = 1000 μm/s, (e) ν = 800 μm/s, (f) ν = 500 μm/s. The AFM profile of fabricated SWMS with scanning speed of (g) ν = 500 μm/s and (h) ν = 250 μm/s. The laser pulse energy is 50 μJ, and the repetition rate is 1000 Hz.
After laser irradiation, the femtosecond laser-induced period surface structures (LIPSSs) with nano-scale period have emerged in the micro-grating groove [41]. The origination of the nano-gratings is attributed to the interference effect between the incident laser and excited surface wave during femtosecond laser-material interactions [42]. The MLA multi-beam direct interference or their interference with induced localized surface plasmon polariton (SPP) waves result in the interactions between laser wavelets and material. It has been found that the repetition rate, beam polarization, and laser power are crucial in forming the nanostructures’ morphology. Figures 6(a)-6(c) demonstrate the evolution of the fringes pattern on the surface of materials after parallel femtosecond laser processing with different repetition rate. When the repetition rate is 167 Hz, there is neither nano-scale periodic structure nor polarization-dependent ablation trace in columns of the micro-gratings, as presented in Fig. 6(a). While the repetition rate reaches to 200 Hz, nano-structures emerge and they are randomly spreading in the laser multi-beam ablated trace, as revealed in Fig. 6(b). Finally, the nano-scale periodic structures could be seen at 250 Hz repetition rate in Fig. 6(c). The nano-gratings embedded in the micro-gratings result to hierarchical structures on the surface of ZnS. For current three situations, the laser pulse energy and scanning speed are fixed at 45 μJ and 1 mm/s, respectively. It has been demonstrated that less laser pulse irradiation is hard to produce nano-grating structures [43]. During the formation of the structures, the standing plasmon wave is critical to modulate surface electromagnetic field. However, the less pulse ablation is easy to consume than enormous pulse for maintaining the standing plasmon in material modification. Therefore, high repetition rate could more easily generate nano-structures in the micro-gratings. Figures 6(d)-6(f) show that the orientation of the nano-gratings relative to the micro-gratings is freely tuned by adjusting incident laser bean polarizations, which is realized by rotating half wave plate in front of MLA. With different incident laser polarization, represented by yellow arrow in Figs. 6(d)-6(f), the orientation of the nano-grating is always perpendicular to laser polarization. This phenomenon is originated from multi-beam laser interference effect and laser local field enhancement [44]. During multi-pulse femtosecond laser ablation, the surface free electrons emerge following the SPP generation. Meanwhile, the movement of the surface free electrons is modulated by SPP. Therefore, the nano-gratings are less produced in the perpendicular laser polarization direction as the extinction of the SPP scattering. In addition, the nano-gratings can be largely formed along the laser polarization direction for boost of the SPP scattering. Consequently, The SPP mode in the formation of the nano-gratings reflects the polarization direction of the incident laser field.
Fig. 6 SEM images of the microstructures at different laser repetition rates: (a) 167 Hz, (b) 200 Hz, (c) 250 Hz. (d)-(f) are morphology evolution of the microstructures orientation adjusted by changing the incident laser polarization. The yellow arrows show different laser polarization direction. (g) Duty ratio of the nano-gratings’ area in the entire micro-grating region with various laser energies. The inset is the SEM image of fabricated surface corresponding to the laser energy. (h) Simulated nano-grating period versus excitation of electron. The inset is the SEM image of the typical nano-grating. The scale bars in (a)-(f) and (h) are 5 μm and 400 nm, respectively.
Next, the duty ratio of the nano-gratings relative to the micro-gratings in SWMS could be adjusted by controlling the laser energy, as illustrated in Fig. 6(g). For a low laser energy as 40 mW measured before the MLA, the width of the nano-gratings’ area is about 0.42 times of the entire micro-gratings. When the laser energy raises to 70 mW, the duty ratio reaches to 0.61. Obviously, at a fixed laser scanning speed of 1 mm/s, the duty ratio of the nano-gratings in the micro-gratings is greatly enlarging from 0.30 to 0.66 as laser energy increases from 30 mW to 80 mW. This phenomenon could be explained by the high laser energy induced extension of the multi-foci spot corresponding to the formation of nano-gratings. According to the SPP-laser coupling effect, the duty ratio enlargement originates in the admixture enhancement of the field-distribution effect and the grating-coupling effect. The inset in the Fig. 6(h) indicates that the period of the nano-gratings is about 180 nm, which is much smaller than the 800 nm wavelength of the incident laser. Such small period of the nano-structures can be elucidated by the Drude-like optical response of the free-carrier plasma mode [45]. Therefore, the relationship of the period Δ of nano-gratings on ZnS and excited electrons density neh could be illustrated in Fig. 6(h). With the increase of laser power, the electrons are also excited particularly violent. Then, the electrons are regulated to produce SPP propagating along the direction of the laser polarization. If the laser energy is exceeding the threshold, the period increases a little for the enhancement of SPP reflection. Finally, the period of the nano-gratings reaches to a steady-state value between 150 nm and 170 nm, which is similar to the 180 nm experimental period of the typical nano-gratings.
Figure 7(a) exhibits the measured and calculated transmittance spectra of flat ZnS and SWMS ZnS. Apparently, comparing to flat ZnS, the transmittance of SWMS is enlarging in the wavelength range of 3-12 μm. Especially, the value of SWMS transmittance is almost 74% at the wavelength of 9 μm, which is attributed to effectively elimination of Fresnel reflection loss on the surface of ZnS. Moreover, it can be clearly seen that the calculated transmittance spectra of flat ZnS and SWMS are also similar to the measured results except for considerable etaloning in FDTD simulated curves due to light interference between upper and lower ideal smooth surface of sample [46]. Figure 7(b) performs the abrasion tests of fabricated SWMS to character their mechanical robustness. It can be observed that the values of transmittance in measured wavelength range are slightly reduced with the friction circle numbers increasing from 5 to 40. Further, after 40 cycles in abrasion tests, the SWMS have still behaved a high transmittance value of 70.7% at the wavelength of 9 μm, which indicates their good mechanical stability. As depicted in Fig. 7(c), the values of transmittance in infrared band for SWMS are apparently decreasing with the duty ratio of nano-grating enlarging, when the incident laser power is increasing from 50 mW to 70 mW. This phenomenon is due to the great enhancement of scattering intensity induced by deep-subwavelength structures from multiple-pulse laser ablation. In addition, the evolution of the nano-structures’ orientation on transmitted spectrum for SWMS on ZnS has been displayed in Fig. 7(d). It can be clearly seen that the SWMS with nano-gratings perpendicular to micro-gratings behave better antireflection performance than those with other orientations. The possible reason is that vertical nano-gratings may relax the structured birefringence effect in one-dimensional micro-gratings, resulting in TE and TM incident light being responded by SWMS.
Fig. 7 (a) Experimented (symbol exp) and simulated (symbol sim) transmittance spectra of the flat ZnS and SWMS. (b) Transmittance spectra of fabricated SWMS after different circle of abrasion tests. (c) Transmittance spectra of SWMS fabricated by different laser power of 50 mW, 60 mW, and 70 mW. (d) Transmittance spectra of SWMS with different orientation angle β between nano-gratings and micro-gratings. The inset shows the SEM image of SWMS with angle β = 60°. The incident angle θ dependent measured transmittance spectra of ZnS with one side (e) and double side (f) SWMS.
Figure 7(e) and 7(f) illustrate the measured angle dependent transmittance spectra of one side and double side SWMS with period of 3.2 μm and depth of 1 μm. The SWMS have considerably reduced surface Fresnel reflection at double side surface of ZnS, expressing a high transmittance value of about 76.5% over a wide wavelength range of 4-10 μm. This transmittance value is much larger than that of the ZnS with one side SWMS (about 71%). Besides, as the incident angle increases from 0° to 40°, the transmittance of one side and double side SWMS are decreasing on account of the enhanced reflection loss, which is consistent with the calculated results in Fig. 3(e). Nevertheless, the values of transmittance of one side and double side SWMS at the wavelength of 9 μm for the incidence angle up to 40° are still over 72.2% and 77.7%, respectively. It implies that the fabricated SWMS on surface of ZnS could achieve broadband response and large acceptance angle in the infrared band for antireflection.
The wettability of the SWMS is also a vital factor for its wide application in harsh environment like rainy and foggy [47]. Figures 8(a)-8(c) give the photograph of the water droplets on the flat ZnS, SWMS ZnS with laser scanning speed of 0.5 mm/s and 2 mm/s, respectively. It can be clearly seen that the water contact angles θc of the ZnS with SWMS are higher than that of the flat ZnS (θc = 89°). Hence, the SWMS will sharply raise the hydrophobicity of the ZnS by introducing the surface roughness into the flat ZnS, resulting in surface energy dropping. Then the water droplets on SWMS will finally slide down by the gravity or evaporate by heat, thus enabling ZnS to function well for antireflection in wet environment. Moreover, the contact angles of SWMS are firstly rapidly enlarging with laser scanning speed increasing from 0 to 0.5 mm/s, then maintain stability after scanning speed varying between 0.5 mm/s and 3.5 mm/s, as exhibited in Fig. 8(d). Therefore, the fabricated SWMS on ZnS have high potential application in self-cleaning, anti-fog, and quick dry functions for various optical components.
Fig. 8 Photographs of water droplets on the surface of (a) flat ZnS and SWMS ZnS with laser scanning speed of (b) 0.5 mm/s and (c) 2 mm/s. (d) the contact angle and surface energy of SWMS as a function of laser scanning speed.
4. Conclusions
In summary, the SWMS on the surface of ZnS for antireflection in infrared band have been designed by optimizing their geometry to enhance the transmittance in FDTD method. Firstly, the initial antireflective period of SWMS in the wavelength of 8 μm-12 μm is deduced as 3 μm from EMT calculation. Then, the simulated transmittance of SWMS in far-infrared band is apparently increasing with enlarging of grating height. The phenomena can be explained by linear smooth refractive index gradient formation on the surface of ZnS with larger grating height, resulting to surface Fresnel reflection being greatly suppressed. Finally, the ZnS antireflection band is tunable by controlling the period of SWMS, which can be interpreted by WR equation in light field coupling effect. To prove the validity of FDTD simulation, the SWMS on ZnS with optimal geometry have been fabricated by using femtosecond laser parallel multi-beam. The depth and period of fabricated SWMS could be readily adjusted by laser scanning speed and distance D between MLA and objective lens. The nano-gratings with period of 180 nm embedded in micro-gratings hybrid structures with period of 3.2 μm have been found on the morphology of SWMS, which is due to interference effect between the incident laser and excited surface wave during femtosecond laser-material interactions. Ultimately, the obtained SWMS on surface of ZnS could achieve broadband response and large acceptance angle in the infrared band for antireflection, as well as robust mechanical strength and hydrophobicity. These results could shed light on promoting functional surface structures to broad applications in optical and optoelectronic industry.
Funding
National Key R&D Program of China (Grant No. 2017YFB1104300, 2017YFB1104800); National Natural Science Foundation of China (NSFC) (Grant No. 51505505).
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