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Abstract
A Yb femtosecond laser was used to fabricate antireflective microstructures on CdSSe single crystal samples. We investigated several microstructure fabrication methods, including direct single pulse ablation using 200 fs pulses, ablation with in-depth focusing, ablation in the presence of additional spherical aberration and ablation with obstruction of peripheral rays. We performed a comprehensive analysis of the implemented antireflection microstructure fabrication methods. The performances of the antireflection microstructures were measured using an infrared Fourier spectrum analyzer and the best samples demonstrated $>99\%$ transmission in the 4.5–6 µm range and average transmission near $97\%$ in range from 2.7 to 8 µm.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Surface antireflection (AR) techniques are highly important for improving the efficiency of key optical components for broadband mid-infrared (mid-IR, 2-10 µm) applications. Examples of such components include modern solid-state mid-IR tunable lasers [1,2] and flexible fibers for use in spectroscopy and IR radiation delivery [3]. These applications require materials with high refractive index values (average n of 2.4) and suffer from severe reflection losses of up to 31% per surface, so increasing the surface transmission is a crucial problem.
The conventional solution to the problem is to use single-layer or multilayer thin-film dielectric antireflection coatings (ARCs). Despite the impressive progress that has been made in ARC manufacturing in recent decades, fabrication of a coating that is capable of increasing transmission to up to 99% or more per surface for a wide range of wavelengths remains a complex task. While it is possible to achieve decent mid-IR transmission using precisely designed multilayer ARCs, development of coatings with more than 97% transmission over the whole mid-IR range that are simultaneously capable of working at high angles of incidence (up to $60^{\circ}$) is at least nontrivial and may be impractical [4]. Additionally, ARCs are vulnerable to surface contamination and thermomechanical damage [5] and have lower light-induced damage thresholds than the substrates on which they are deposited [6].
Antireflection microstructures (ARMs) offer another way to eliminate surface reflections. ARMs are surfaces that have been patterned using periodic structures with different cross-section profiles. The typical height of these structures is a few microns and their period is strongly dependent on the wavelength range in which increased transmission is required. The microstructure antireflection performance principle can be described using effective medium theory [7] for wavelengths longer than
where n is the refractive index of the material and p is the microstructure period.Therefore, for wavelengths greater than that determined using Eq. (1), the ARM acts like a gradient refractive index layer and that consequently leads to reduced reflection because of index-matching [8]. ARMs thus provide better performance than ARCs. In addition, they are resistant to surface contamination [9] and delamination due to mechanical or thermal stresses and have almost the same light-induced damage threshold as the untreated substrate, which means that they are suitable for high power applications [6,10,11]. Also they could be used in visible [12], infrared [13] and THz [14] wavelength ranges.
Two major issues that must be addressed in ARM development are precise modeling of the structures to obtain the required parameters and the complexity of pattern fabrication to ensure that the ARM is as close to the model pattern as possible. Several works have determined optimal combinations of the ARM cross-section profile and depth and they showed good agreement with experimental results [15–19]. In addition, different fabrication methods for ARMs on optical surfaces have been under development since the 1970s. Proposed fabrication solutions include leaching [20], etching [21], lithography [22,23], photopolymerization 3D printing [24], glancing angle deposition [25,26], light induced periodic surface structures formation [27,28] and biomimetic replication [29]. These methods are either unreliable or, despite their ability to produce ARMs with great AR performance, are difficult to implement and thus have major limitations that make them impractical.
As a result of the rapid development in ultra-short pulse laser technologies, femtosecond (fs) laser systems are now commonly available and there have been several reports of the ARM fabrication using fs laser systems with complex ablation techniques and post-processing [30]. These techniques require overly complex fabrication setups and lack both AR efficiency and sufficiently high production rates. We therefore focus on straightforward and efficient methods for ARM fabrication by single pulse direct fs laser ablation using different auxiliary techniques.
2. Materials
It is possible to treat any nonluminescent optical material by fs laser ablation because luminescence and fs laser ablation are concurrent processes. From the various materials available, we have practical interest in implementing ARMs for solid-state mid-IR active laser media such as II-VI chalcogenide compounds doped with Cr$^{2+}$ and Fe$^{2+}$ ions. Therefore, we mostly treated CdSSe single crystals, which have almost the same physical characteristics as common ZnSe, ZnS, CdS and CdSe materials. The refractive index of CdSSe is approximately 2.35 and the material is transparent over the entire mid-IR range. CdSSe is highly luminescent in green-red visible region under ultraviolet (UV) radiation [31] and thus cannot be treated using UV light. ARM samples were fabricated on CdSSe crystal surface as squares with a side of 2 mm.
3. Experimental setup
A Pharos PH1-SP Yb laser (Light Conversion, Lithuania) was used as the femtosecond pulse source. Yb lasers normally work at a wavelength of 1026 nm and are incapable of wavelength tuning, but in the given laser system, both second and third harmonic generation are implemented. This provides a choice of three operating wavelengths: 1026 nm, 513 nm and 342 nm. At all three wavelengths, the laser system can produce 200 to 1000 fs pulses at repetition rates of 1 to 200 kHz. The maximum available average power is 4 W at first harmonic, 1.5 W at the second, and nearly 550 mW at third harmonic. For sample positioning, we use Aerotech ANT-90 (Aerotech Incorporated, USA) three-axis nanopositioners to provide in-position stability of 2 nm and repeatability of 75 nm at velocities of up to 200 mm/s. At our disposal, there are Mitutoyo 100X objective (Mitutoyo Corporation, Japan) lens with numerical aperture (NA) of 0.5 and 50X lens with NA of 0.42. However, the 100X objective lens is used to focus the beam onto the sample surface and reach the material’s ablation threshold. The choice is due to the fact that an objective with higher NA should be more suitable for microstructure fabrication because of the reverse proportional dependence between the spot size and the focus depth (the ablation depth). So using a lower NA lens even with the same magnification will always lead to increase of the spot size.
Higher repetition rates are better because of the resulting increase in the production rate, although the rate does not affect the quality of the ARM at all. However, other fabrication characteristics do influence the ARM quality directly and it is important to minimize the pulse duration because nonthermal ablation typically only occurs at durations of less than 500 fs and the quality of the structure continues to improve with decreasing pulse duration [32], thus the shortest available pulse duration of 200 fs was used. In addition, it is important to use shorter wavelengths on purpose of minimize spot size to fabricate ARMs with smaller periods. The choice of the average power is dependent on both the material to be treated and the technique used, but we typically only use a small percentage of the available power from 1 to 100 mW. Therefore, for ARM fabrication 200 fs pulses at 200 kHz produced at the second harmonic wavelength of 526 nm were used, because the luminescence of CdSSe prevents treatment at 342 nm.
4. Measurement setup
Versa 3D (FEI Company, USA) scanning electron microscope (SEM) was used to investigate the morphology of ARM samples including the simple top-down observation view and cross-section imaging obtained using ion beam etching. The platinum mask was preliminary applied with ion beam assisted chemical deposition to protect ARM surface. Spectral characteristics of the samples were investigated with Fourier-transform infrared spectrometer Bruker Vertex 70v. Before the measurement samples were cleaned with isopropyl alcohol and wipes for optical elements from ablation residues and other contaminants. Standard measurement setup with vacuumized samples’ chamber and optics bench was used, the spectral resolution was set to 4 cm−1, the internal device aperture was set to 6 mm and an additional 1.38 mm aperture right before the sample was used to make the spot smaller than the structure size. Therefore clear aperture of each ARM sample under test was 1.38 mm in diameter. Each transmission spectrum was averaged by 40 scans.
5. Results and discussion
5.1 Direct single pulse ablation
In the simplest case, material ablation occurs directly under the high energy density of a single fs pulse (Fig. 1(a)). The sample’s movement velocity is synchronized with the repetition rate of the laser to fabricate ARMs with the desired period. The average power of laser is adjusted to be 0.36 mW at the second harmonic and the spot diameter (1/e$^2$) is approximately 0.9 µm. These figures give the maximum energy density 0.5 J/cm$^2$ at the sample's surface, which is sufficient to ablate CdSSe and form holes with a diameter of 1.1 µm, as shown in Fig. 1(b), and a depth of 0.7 µm (note the sample 52$^\circ$ tilt when by scanning electron microscopy (SEM))), as shown in Fig. 1(c). The transmission spectra of ARMs fabricated by the direct single pulse ablation method are also shown in Fig. 1(d). Despite vacuumization, there is a residual CO$_2$ absorption near 4.5 µm which affects on each sample transmission spectrum measurement. The diffraction wavelength is 2.6 µm. The maximum transmission is approximately $93\%$ at 5 µm and the average transmission is $89\%$ within the range from 4 to 10 µm. The level of transmission obtained is insufficient to allow the ARM to be used in solid-state mid-IR lasers and it is thus necessary to modify the proposed method using auxiliary techniques.
Fig. 1. Principle of direct single pulse ablation method (a), SEM top-down (b) and cross-section (c) image of an ARM sample fabricated using the method and the transmission spectra of the sample (d) in a comparison with the transmission of an untreated surface of CdSSe
One way to improve the ARM performance is to increase the structure’s depth, which should be 1–5 times the diffraction wavelength divided by refractive index [18,19]. We also had conduct modeling using COMSOL finite elements method software to estimate depth of the ARM which should provide decent anti-reflective performance. The infinite periodic lattice of parabolic conical holes in CdSSe material represent ARM in our model. Described setup has precision enough to estimate the behavior of transmission spectra and that is our purpose of modelling. For that we calculated ARM transmission spectra in range from 1 µm to 8 µm with 25 nm step for structures with depth varied from 0.5 µm to 5 µm with 100 nm step. Our modelling has given the similar to [18,19] results (Fig. 2). The reduced efficiency of our ARMs is caused by the structures being too shallow. A deeper structure provides a smoother refraction index gradient and thus provides better AR performance. An ARM fabricated by single pulse ablation without any auxiliary techniques has a depth of approximately 0.5–0.7 times the diffraction wavelength.
Fig. 2. Dependence of transmission on ARM depth (a), behavior of averaged (b) and maximum transmission over whole mid-IR (c) as calculated via finite element method simulations
In accordance with the results in the literature [18,19,33], reflection reduction only occurs in the range of wavelengths greater than the diffraction wavelength, which is determined using Eq. (1). Therefore, if we accept that the mid-IR is the range from 2 µm to 14 µm, the typical period of the corresponding ARM must be no more than 0.82 µm. Unfortunately, we cannot reliably fabricate an ARM with a period of less than 0.9 µm using the current setup because we are limited by the diffraction spot diameter and, in this case, the holes are placed to be as close as possible to each other with the aim of reducing undesirable reflections from the untreated gaps. To increase the transmission of the ARMs, it is necessary to increase the depth of the structure, which, in the constant diameter case, is only possible through use of the auxiliary techniques developed in this work; these techniques are described in the next section.
5.2 Direct single pulse ablation with obstruction of peripheral rays
The first technique used to change the laser beam’s longitudinal energy distribution involves obstruction of the peripheral rays before the lens and use of only the paraxial part of the Gaussian distribution. This technique would change the aspect ratio of the ablation volume because of the process discrimination induced by the peripheral rays and can be described as a concurrent process of Kerr lens self-focusing of the paraxial part of the beam while defocusing on the plasma stimulated by the peripheral rays [34]. We fabricated samples using aperture with a diameter of 0.5 mm aperture to obstruct the peripheral part of 3-mm-diameter Gaussian beam before the objective lens. Aperture blocked nearly 95% of incident power. In order to compensate that, we increased power up to 7,6 mW. The scheme used for the experiment is shown in Fig. 3(a). The SEM top-down view, cross-section and the transmission spectra of the samples are shown in Fig. 3(b), Fig. 3(c) and Fig. 3(d), respectively.
Fig. 3. Principle of direct single pulse ablation method with obstruction of peripheral rays (a), SEM top-down (b) and cross-section (c) image of an ARM sample fabricated by that method and the transmission spectrum of the sample (d) in a comparison with the transmission of an untreated surface of CdSSe
An ARM fabricated by direct single pulse ablation with peripheral ray obstruction exhibits maximum transmission of approximately 98% at 5 µm and and the average transmission within the 2.7 to 10 µm range is 94% due to depth increase from 0.7 to 0.9 µm. These results are much better than those that can be achieved by direct single pulse an ablation and could successfully compete with the performances of some dual-band multilayer ARCs [4].
5.3 Direct single pulse ablation in presence of spherical aberration
The second technique that we propose to use involves use of an optical element with additional spherical aberration inserted between the objective lens and the sample crystal. Some authors already had suggested to introduce optical delays into the beam path to influence the character of ablation volume [35]. In the simplest case, this element could be a plane-parallel plate. Because of the additional spherical aberration, the peripheral rays of the Gaussian beam will propagate further than the paraxial rays and will have a different focal point. This should therefore solve the problem of Kerr lens concurrence with plasma defocusing and will elongate the ablation volume. The distance between the paraxial focal point and the peripheral, i.e., the elongation of the longitudinal axis of the ablation volume ellipse, can easily be calculated using geometric optics as:
where NA is the objective lens numerical aperture, n is the medium-to-material refractive index ratio, and L is the thickness of the plane-parallel plate.From analysis of Eq. (2), we determined that the optimal refractive index of the plate is nearly 1.7 for an objective lens NA of 0.5 objective lens and propose Al$_2$O$_3$ for use as the plane-parallel plate material because it is commercially available and has a refractive index of 1.78. We fabricated ARM samples using plates with thicknesses of 0.5 and 2 mm (the experimental setup is shown in Fig. 4(a)) positioned at distance of 5 mm from the surface of sample, but it appears that 0.5 mm of Al$_2$O$_3$ was insufficient to affect the longitudinal energy distribution or to increase the depth of the ARM because no increase in transmission was observed at all when the results were compared with those of ARMs obtained by direct single pulse ablation with no auxiliary techniques. Also we again had to increase a power up to 8 mW because presence of spherical aberration induced severe dissipation of beam power.
Fig. 4. Principle of direct single pulse ablation method in presence of spherical aberration (a), SEM top-down view (b) and cross-section (c) image of an ARM sample fabricated by that method and the transmission spectrum of the sample (d) in comparison with the transmission of an untreated surface of CdSSe
The ARM that was fabricated using direct single pulse ablation in the presence of a spherical aberration with a 2-mm-thick Al$_2$O$_3$ has more debris and residues of ablated material on the surface as shown in (Fig. 4(b)). The depth of the ARM is 1.2 µm (Fig. 4(c)). Despite the lack of surface quality, maximum transmission is nearly 98%, and the average transmission is approximately 94% (Fig. 4(d)). Absorption near 3.5 µm corresponds to isopropyl alchohol used as cleaning solution for backface of samples. The fact that ARM with fairly poor surface quality has the same transmission as one fabricated with peripheral rays obstruction technique can be explained with increased depth and smoother refractive index gradient because of presence of randomly structured residues of ablated material agglomerated on the ARM surface.
5.4 Direct single pulse ablation with precise in-depth focusing
Finally, we decided try to increase the depth by precisely shifting the focal point deeper into the material bulk to use more of the ablation volume (Fig. 5(a)) formed in the Gaussian beam and attempt to achieve some filamentation [34]. Several samples fabricated with a shift of 2 µm below the surface were investigated. To compensate the losses on ablated material we had to slightly increase power to 0.5 mW. The SEM images and the transmission spectrum of the best of these samples are shown in Fig. 5(b), Fig. 5(c), and Fig. 5(d), respectively.
Fig. 5. Principle of direct single pulse ablation method with precise in-depth focusing (a), SEM top-down view (b) and cross-section (c) image of an ARM sample fabricated by that method and the transmission spectrum of the sample (d) in comparison with the transmission of an untreated surface of CdSSe
Surprisingly, this simple technique did indeed increase depth up to 1.5 µm and maximum transmission up to the theoretical maximum of $100\%$ at 5 µm, with average transmission of 97% within the range from 2.7 to 10 µm.
6. Conclusion
In our work, a simple and straightforward ARM fabrication method using some auxiliary techniques while involving no additional treatment steps for ARM processing was demonstrated. We implemented four different ARM fabrication methods and the best one of these methods was fs single-pulse direct ablation with precise in-depth focusing. The proposed method was used to fabricate an ARM with a performance that was very close to the theoretical maximum of 100% transmission at 5 µm. Transmission of 99% in the range from 4.5 to 6 µm and an average transmission of 97% in the range from 2.7 to 10 µm were obtained. A commercially available fs laser can be used with our method to fabricate ARMs for mid-IR applications in which highly refractive materials are used. With further improvement of the proposed method, it may be possible to fabricate ARMs for shorter wavelengths, if required.
Funding
Russian Science Foundation (RSF) (17-79-20431); Ministry of Education and Science of the Russian Federation (Minobrnauka) (3.5267.2017/8.9).
Acknowledgments
A. A. B., V. A. L., and M. K. T. acknowledge the Russian Science Foundation according to the research project No. 17-79-20431 for the support of experimental work with ARM fabrication setup. V. E. K. acknowledges the State Task of the Ministry of Education and Science of the Russian Federation No 3.5267.2017/8.9 for the support of transmission measurements and SEM imaging of fabricated ARM samples.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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