超快貝塞爾光束在硫化鋅晶體表面制備納米孔

常改艷，王玉恒，程光華

（1. 中國(guó)科學(xué)院 西安光學(xué)精密機(jī)械研究所 瞬態(tài)光學(xué)與光子技術(shù)國(guó)家重點(diǎn)實(shí)驗(yàn)室，陜西 西安 710119；2. 中國(guó)科學(xué)院大學(xué)，北京 100049；3. 華南理工大學(xué) 廣東省半導(dǎo)體照明與信息化工程技術(shù)研究中心，廣東 廣州 510641；4. 西北工業(yè)大學(xué) 電子信息學(xué)院 光學(xué)影像分析與學(xué)習(xí)中心，陜西 西安 710072）

1 Introduction

The mid-infrared photonics has attracted considerable attention because its wavebands cover thermal imaging bands, multiple atmospheric windows (3～5μm and 8～14μm) necessary for free space communication, as well as main absorption bands of most chemical and biological molecules. It is an inevitable trend for photonic integration to cut costs, improve performance and miniaturize special optical instruments, for example, micro-scale photonic circuits are manufactured in optical materials with a transparent window up to 20μm. The application of integrated photonics has promoted the rapid development of optical communication technology and the expansion of applied wavelength from near infrared to middle and far infrared. However, the infrared window materials in medium and long wave bands are very limited, mainly including GaAs, GaP, ZnSe, ZnS, InSb, etc.[1-4]. Considering the optical and mechanical properties of these materials, ZnS crystal has become one of the most promising infrared optical materials due to its advantages such as low absorption, high hardness,large Young"s modulus, wide transparent window(0.37～14μm) and good chemical stability[5]. The performance of photonic devices is related to nonlinear optics, the response of materials to the interaction between light and matter, and the balance between the relaxation processes of materials. This means that it is necessary to precisely control the interaction between light and matter and the size of photonic devices so as to realize effective optical transmission and control in complex optical systems.

Ultra-fast Bessel pulse laser is the combination of ultra-fast laser pulse and spatial beam shaping(zero-order Bessel beam) and is formed by symmetric interference between the plane wave and the wave vector distributed on the conic bus. It has a long non-diffraction transmission distance (focal depth DOF = W/2·tanθ, where W is the radius of incident Gaussian beam and θ is the beam half-cone angle in the Bessel region) and good transmission robustness (self-recovery)[6-8]. An ideal Bessel beam has a narrow and strong central interference region[9-10].This indicates that the Bessel beam is superior to the Gaussian beam in the micro-nano machining of extended micro-channels. For example, the Bessel beam has been used to successfully prepare the nanopore micro-channel structure with a aspect ratio of 10000:1 in borosilicate glass[11-13]. In addition, ultrafast Bessel laser pulses have shown strong applicability in the high-depth and large-area machining, the micro-nano welding, and the development of a mix of micro-nano scale features with new photonic functions[6,12-14]. Although ZnS crystal is one of the most promising infrared window materials, its high nonlinear coefficient can cause serious distortion of temporal and spatial distribution of laser pulses when the focused femtosecond laser is transmitted in this crystal. Moreover, the self-focusing and the self-defocusing of plasma can cause spatial beam splitting so that the laser energy cannot be effectively concentrated on the focus. Early experimental and theoretical studies showed that the peak power density could be effectively reduced by pulse broadening, thus weakening the nonlinear effect. 15ps-20ps Ti-sapphire pulsed laser can effectively absorb laser energy and write waveguide in sulfide glass. In the early stage, ultra-fast Bessel beam was used to fabricate the nanopores mainly in quartz glass, silicate glass and borosilicate glass, whose nonlinear absorption was different from that of ZnS by orders of magnitude. Secondly, the hardness, thermal diffusion coefficient and Young"s modulus of ZnS crystal are also significantly different from those of the above glass. Therefore, it is difficult to fabricate nanopores on ZnS crystal with ultrafast laser. So far,the preparation of periodic subwavelength microstructures on ZnS crystal has been reported[15].However, the use of ultrafast laser to process the nanopores with high aspect ratio on ZnS crystal is still in blank. In Ref. [16], we reported and discussed the influence of beam mode on the quality of drilling on ZnS crystal. The experimental results show that on ZnS crystal, the pore-forming quality of Bessel beam is better than that of Gaussian beam.Therefore, the use of Bessel laser to directly write nanoscale microstructures on ZnS crystal, to prepare various medium-infrared optical components and to improve or achieve one of their capabilities has become the current research hotspot of mediuminfrared integrated photonics, with a broad application prospect.

In this paper, a Yb:KGW femtosecond laser with the central wavelength of 1 030nm, the repetition frequency of 100kHz, and the pulse width of 223 fs was used to generate the Gauss-Bessel beam through an axicon. Then the parameter ranges of the nanopores fabricated on the surface of ZnS crystal were studied. The morphology of nanopores was analyzed by using a Focusing Ion Beam (FIB, Helios G4 CX) and a Scanning Electron Microscope(SEM, JEOL JSM-7500F). The dependence of the surface morphology, diameter and depth of nanopores on laser pulse energy and pulse width was determined. The potential applications of nanopores were discussed.

2 Experimental device

The Bessel beam generation device described in Ref. [8], as shown in Fig. 1, is used in this experiment. Firstly, the beam is expanded to 8 mm, and the beam diameter will affect the focusing depth.Applying a combination of an axicon with 1° base angle (Altechna, 1-APX-2-H254-P, n = 1.45), focusing lens L1with focal length f1= 400mm, and the 20× near-infrared focusing objective lens L2with f2= 10mm (Mitutoy NIR, numerical aperture NA =0.42, working distance L= 20mm), and using a spatial filtering system, we build an experimental device for generating zero-order Bessel beam laser direct-writing microstructure. A Yb:KGW modelocked regenerative laser amplification system(Pharos, Light Conversion) with a central wavelength of 1 030nm and adjustable repetition frequency is used to write nano-pore microstructures on ZnS crystal. The pulse width can be continuously adjusted within 223 fs～25 ps by a removable grating compressor in the laser system. The energy of laser pulse is regulated by a half-wave-plate Po-larized Beam Splitter (PBS). The sample where the nanopores are written is an 18 mm×8 mm×2mm four-sided polished ZnS crystal. It is fixed to a computer-controlled XYZ 3D high-precision air bearing platform (Aerotech, ANT130). A focusing lens is used to focus the beam on the sample. A high-precision (75 nm) NC system is used to move the sample at a constant speed during the irradiation with a continuous laser pulse. The nanopore spacing can be adjusted by the repetition frequency of the laser source and the translational velocity of the sample(Λ = v/f). During continuous laser irradiation, the Aerotech platform moves the sample at a constant speed (in the positive direction of X-axis) and simultaneously moves the relative movement with respect to the laser focus (in the negative direction of Z-axis) to ensure that a new exposure area is always placed in the laser action area. This writing method can help observe the dynamic evolution process of the pore diameter inside the polished sample.

Fig. 1 Experimental setup for microchannel writing with Bessel laser圖1 貝塞爾激光刻寫微通道結(jié)構(gòu)實(shí)驗(yàn)裝置圖

3 Experimental results and analysis

The focusing ultrashorter laser pulse has ultrahigh peak power and can selectively induce a series of nonlinear effects of transparent materials, such as multi-photon absorption, Kerr effect, plasma self-focusing and self-defocusing, so as to change the microregion morphology and molecular structure of the materials[17]. For the ultra-fast laser with a pulse width of 20ps and a wavelength of 1μm, its air ionization threshold is approximately 2.7×1013W/cm2[18].It can be calculated that, in this experiment, the laser power density range acting on ZnS crystal is 0.57×1013～6.02×1013W/cm2. When the laser power density is greater than the ionization threshold of air or ZnS crystal, the laser pulse will generate strong nonlinear absorption in the focus area and cause the formation of plasma, thus effectively absorbing the laser energy and forming a local deposition area with high energy density[19]. The extreme high temperature and pressure in the focus area leads to phase transition and micro-explosion, and the energy diffuses from the focus area to the surrounding material in the form of shock wave, so as to construct a 3D micro-nano structure inside the transparent material[20-22]. By discussing and analyzing the deposition energy from the nonlinear transmission of laser pulse inside transparent material as well as the possible thermal mechanism, the phase change and the formed microstructural morphology can be controlled more easily. For the laser parameters and air/ZnS crystal interface issue in this experiment, the nonlinear Schrodinger equation in common use can be used to calculate the concentration of plasma in the focus region. However, the mechanism of micro-explosion on the air/crystal interface is more complex. By referring to the parameters and law of femtosecond laser waveguide writing on quartz glass and sulfide glass[23-24], this experiment studied the features of the microstructure fabricated by Bessel beam on ZnS crystal surface when the pulse width changed from femtosecond to 20ps and the energy changed from damage threshold to 63μJ.The FIB, SEM and phase contrast microscope(PCM, Olympus BX51) were used to characterize the morphology and length of nanopores and microchannels on the sample surface and determine the dependence of surface morphology, diameter and depth of nanopores on laser pulse energy and pulse width.

3.1 Features of nanopores on ZnS crystal surface

Pulse energy and pulse width are the key parameters that affect the interaction between laser and matter. Pulse energy determines the effective radi-ation flux that the unit area of material is exposed to in unit time. Pulse width is related to the axial energy deposition efficiency of Bessel laser beam[6,25].The experimental and numerical results of the interaction between ultrafast beam and quartz glass and of micro-nano fabrication show that several picoseconds of pulses can effectively overcome the decrease of laser focus plasma density caused by strong nonlinear effects. Considering spatial and temporal focusing characteristics, the nonlinear numerical simulation results of this material show that the plasma concentration generated by the 4.7-ps pulse ionized at the focal point is one order of magnitude higher than that generated by the 60-fs pulse of the same energy[6]. However, the nonlinear absorption coefficient of ZnS is one order of magnitude higher than that of common glass, so in the process of pulse width optimization, the range of pulse variation needs to be increased. During the experiment, the maximum pulse width was adjusted to 25 ps by adjusting the compressor of the regenerative amplifier to cover the writing range of the sulfide waveguide[26]. When the laser pulse width was increased from 10ps to 20ps, the damage threshold range of the ZnS crystal used in this experiment was 6.51～8.32J/cm2. The detailed measurement and calculation methods have been reported in Ref. [27].

The Fig. 2is a picture of the nanopores on the sample surface, which was obtained from SEM characterization without any post-treatment. During the SEM morphology characterization, the ZnS surface was coated with a gold film about 50nm thick to become conductive. The black dots at the center of the white ring are nanopores, each of which is written with a single pulse. The white halo around the nanopores is formed by the sputtering or redeposition of the material removed during laser processing. Similar material deposition principles have been reported in Ref. [28]. ZnS crystal is an important wideband infrared window material with strong photosensitivity and inherent sub-stability. Its band gap width is 3.6～3.8 eV. Under the 1 030-nm infrared laser radiation, it performs the three-photon absorption. Its nonlinear absorption coefficient (a3=0.5×10?3cm3/GW2) is two orders of magnitude higher than quartz glass. These characteristics indicate the necessity of spatiotemporal envelope control of ultrafast laser pulses[15]. Pulse width is a parameter sensitive to ZnS crystal surface damage. The Fig. 2 shows that 12.5-ps pulse width is the critical value where a nanopore will occur. When the pulse width was less than 12.5 ps, the nanopores with a aspect ratio did not appear in the pulse energy range of 35.83～62.82μJ. Instead, the pits with certain depth appeared, as shown in the left column of Fig.2. This is because under short-duration pulses, the carrier generation speed is faster and the plasma defocusing efficiency is higher, so that the axial energy deposition density will be reduced and the thermodynamic conditions for micro-explosion can"t be met[29].

The Fig. 3 shows the dependence of nanopore diameter on the pulse width and energy of Bessel beam. The minimum nanopore diameter on ZnS crystal surface was measured by SEM to be 80nm,and was written by the laser with a pulse width of 12.5 ps and pulse energy of 48.67μJ. The curves in Fig. 3 show that when the pulse energy is fixed, the nanopore diameter will increase with the pulse width within the pulse width window of 12.5～20ps.When the pulse width is fixed, the nanopore diameter will increase with the pulse energy. Moreover, the greater the energy is, the more obvious this trend will be. However, this does not seem to be the case at the pulse width of 12.5 ps. Since pulse width is a parameter sensitive to crystal surface ablation, the laser near the critical pulse width 12.5 ps can cause various nonlinear effects to interact with each other and result in the instability of nanopore. All the experiments in this paper demonstrate the damage characteristics under monopulse action, which are significantly different from the results of multi-pulse action[30]. As far as monopulse is concerned, the selforganizing interference is difficult to occur, and the modulation of laser intensity in the focus area and of plasma intensity is not obvious.

Fig. 2SEM images of nanopores on ZnS surface at various pulse energies. (a) 35.83μJ; (b) 48.67μJ; (c) 62.82μJ. The laser pulse widths in columns 1 to 4 are 10ps, 12.5 ps, 15 ps and 20ps respectively圖2不同激光參數(shù)下ZnS 表面納米孔隙結(jié)構(gòu)的SEM 形貌。 (a) 35.83μJ；(b) 48.67μJ；(c) 62.82μJ。第1 到4 列的激光脈沖寬度分別是10ps, 12.5 ps, 15 ps 和20ps

Fig. 3 Nanopore diameter on ZnS crystal surface changes with pulse energy under different laser pulse widths圖3 不同激光脈沖寬度下ZnS 晶體表面納米孔隙直徑隨脈沖能量的變化情況

The dynamic process of nanopore formation is generally divided into three stages. The first stage is the nonlinear absorption of crystal and the distortion of pulse space-time. In this stage, different laser parameters directly affect the absorption and deposition of energy in the focus area. The second stage is plasma relaxation, which includes the release of backscattering pressure of high temperature electrons and the transfer of electron energy to the lattice to cause lattice heating and deformation. The third stage is mainly thermal process and lattice relaxation lasting from picoseconds to microseconds.The change of refractive index of quartz glass under the action of ultra-fast Bessel beam can be seen from time-resolved phase contrast photographs and plasma luminescence intensity. For example, the refractive index of fs time window decreases, that of ps window increases, and those of ns andμs time windows decrease. However, it is difficult to determine whether the nanopores are formed in the femtosecond stage or nanosecond stage. Both the backscattering pressure of the electrons and the rapid expansion caused by thermal effect may lead to micro-explosion. The self-focusing and self-defocusing effects in nonlinear absorption process and the Rayleigh Taylor instability in fluid dynamics process will lead to the irregular change of nanopore diameter[19].

3.2 Features of nanopores in ZnS

AFM, SEM and other surface analysis tools are powerless to characterize the 3D morphology of the nanopores with high depth-diameter ratio. Therefore, in order to obtain the longitudinal morphology features of nanopores, the characterization method of "FIB denudation +SEM imaging" was selected.The FIB etching process enables us to minimize the damage to one-dimensional Bessel pores so that we can analyze the longitudinal morphology of nanopores more intuitively. The results of SEM surface characterization showed that, under 20-ps pulse width and 62.82-μJ pulse energy, the nanopore structure on the surface of ZnS crystal was an oval rather than a regular circle occurring under other laser parameters. Under overhigh peak power density, the Bessel beam was subjected to unexpected nonlinear effects on the crystal surface, which affected the normal transmission of the beam. The possible influencing mechanisms include: Kerr selffocusing, plasma self-defocusing, multi-photon ionization, and plasma shielding. These effects work together to constrain the strength of the Bessel beam on the material surface. Therefore, 20-ps pulse width and 48.67-μJ pulse energy are selected as the laser parameters for follow-up study.

The Fig.4 shows the nanopore structure prepared under 20-ps pulse width and 48.67-μJ pulse energy. It can be seen that, the pore channel has a variable diameter along the laser transmission direction. This has something to do with the distribution of axially deposited energy density of Bessel pulse laser during the nonlinear optical transmission inside the material.[6]. The appearance of pore wall indicates that the material has undergone a process of melting and resolidification. The white flake on the microstructure surface is the residual of the gold film which was applied on the sample surface for conductive use during the SEM characterization and then ultrasonically cleaned. The chemical composition of the white flake was characterized by energy spectrometer (EDS). The results showed that the flake was gold. The lamination diagram along the FIB etching direction is helpful to observe the morphological changes of pore microchannels during the etching process. It can be seen that the pore microchannel transitions from about 3μm to a larger length as the FIB etching goes on. Because the FIB etching direction can"t be easily aligned to the extension direction of the pore, the information on nanopores isn"t exhaustive in this paper. To learn more about the pore depth, we observed the side of the nanopore by optical phase contrast microscope.

The Fig. 5(b) shows the side view of pore microchannels under a 20× phase contrast microscope(transmission type). In the PCM transmission-type microscope, the phase shift corresponding to negative refractive index change looks darker on the gray background. For quartz glass, the black traces indicate that the increase in refractive index, while the white traces is the nanopore or gratings with decreasing refractive index. But for sulfide glass with high refractive index, the change of refractive index cannot be judged by diffraction contrast. On the one hand, the refractive index varies greatly and the phase may be inverted; on the other hand, the indication left after laser damage has strong scattering[25].The above enlarged view shows that the microchannels have good uniformity without fault. The nanopore fabricated on ZnS crystal by 20ps/48.67μJ Bessel laser pulse is about 270μm long. The refractive index of ZnS crystal is higher than that of quartz glass. If there is no nonlinear effect, the effective length of Bessel beam focus in the ZnS crystal should be larger than that of quartz glass. However,both the simulation and experimental results show that the length of nanopore in ZnS is only half of that in quartz glass. The main reason is that the travel of a nanopore is a highly nonlinear process. It is difficult to derive the law of pore pattern change in the two materials from a single parameter. To explore the positive morphology and pore size change of nanopores inside the material, the sample surface was repeatedly polished, gold-plated and characterized by SEM. The Fig. 5(a) is a SEM image of the cross section of the polished micro-channel. The measurement results show that the pore diameter is about 180nm, 40nm smaller than the unpolished diameter. To verify the operability and repeatability of microchannels, several pore microchannels spaced 15μm apart were processed on the ZnS crystal by using the Bessel beams with the same pulse width and energy (20ps, 48.67μJ). Three of them are shown in 5(b). From Fig. 4 and Fig. 5, the accurate information on nanopore depth cannot be obtained,except for the information of 3μm ＜ pore depth ＜270μm, which is about half of the pore depth on the surface of optimized quartz glass. Even so, it is still of great significance to fabricate nanopores in midinfrared sulfide glass and sulfide crystal. This technique has laid an important technical foundation for the realization of mid-infrared waveguide/standingwave waveguide Fourier transform spectrometer[31-32].Its applications in mid-infrared photonic crystals,mid-infrared microfluidic devices and photonic screens are of great significance[33].

Fig. 4 SEM image of cross section of FIB-etched nanopore圖4 FIB 刻蝕納米孔隙橫截面處的SEM 形貌圖

Fig. 5 Characteristics of nanopore inside ZnS crystal. (a)SEM image of cross-section of polished nanopore;(b) side view of the nanopore under a 20× phase contrast microscope (transmission microscopy type). The upper inset is an enlarged view of a microchannel. Note that all the microstructures in the picture are written under the laser parameters of 20ps and 48.67μJ圖5 ZnS 晶體內(nèi)部納米孔隙的結(jié)構(gòu)特征。 (a)拋光后納米孔隙橫截面的SEM 圖；(b) 20×相襯顯微鏡下的納米孔隙結(jié)構(gòu)的側(cè)面圖(透射式顯微模式），上方插圖是微通道的放大圖，圖中的微結(jié)構(gòu)均是在20ps,48.67μJ 的激光參數(shù)下刻寫的

4 Conclusion

The nanopore structures with high aspect ratio were written in ZnS crystal with ultrafast Bessel laser. By adjusting the laser parameters, the optimal energy window (36～63μJ) and pulse width window (12.5～20ps) for nanopore writing on ZnS crystal were obtained to fabricate the nanopores with a diameter of 80～320nm. By using the scanning electron microscope, focused ion beam and phase contrast microscope, the morphology and length of nanopores and pore channels on the sample surface were characterized, and the dependence of surface morphology, diameter and depth of nanopores on laser pulse energy and pulse width was identified.This technique is of great significance to the fabrication of mid-infrared waveguide Fourier transform spectrometer and sulfide photonic crystal.

——中文對(duì)照版——

1 引 言

中紅外光子學(xué)因其波段覆蓋了熱成像、自由空間通信所必需的多個(gè)大氣窗口（3～5μm 和8～14μm），以及大多數(shù)化學(xué)和生物分子的主要吸收帶，而引起了廣泛關(guān)注。光子集成以降低成本、提高性能為目標(biāo)，專用光學(xué)儀器小型化是必然趨勢(shì)，例如在透明窗口高達(dá)20μm 的光學(xué)材料中制造微尺度光子電路。集成光子學(xué)的應(yīng)用促進(jìn)了光通信技術(shù)的快速發(fā)展，應(yīng)用波長(zhǎng)也由近紅外向中、遠(yuǎn)紅外擴(kuò)展。但是，中長(zhǎng)波紅外窗口材料十分有限，主要有GaAs, GaP、ZnSe、ZnS、InSb等[1-4]。綜合考慮材料的光學(xué)特性和機(jī)械性能，ZnS 晶體以其低吸收，高硬度，大的楊氏模量和透明窗口（0.37～ 14μm）以及良好的化學(xué)穩(wěn)定性等優(yōu)勢(shì)成為最具應(yīng)用前景的紅外光學(xué)材料之一[5]。光子器件的性能與非線性光學(xué)材料對(duì)光與物質(zhì)相互作用的響應(yīng)以及材料弛豫過(guò)程之間的平衡有關(guān)。這意味著必須精確控制光與物質(zhì)相互作用的過(guò)程及光子器件尺寸，以在復(fù)雜的光學(xué)系統(tǒng)中有效進(jìn)行光傳輸和控制。

超快貝塞爾脈沖激光將超快激光脈沖和光束空間整形（零階貝塞爾光束）相結(jié)合，由平面波與分布在圓錐母線上的波矢量對(duì)稱干涉形成，其無(wú)衍射傳輸距離（焦深DOF = W/2·tanθ, W 是入射高斯光束半徑，θ 是貝塞爾區(qū)的光束半錐角）長(zhǎng)，傳輸魯棒性（自恢復(fù)）好[6-8]。理想的貝塞爾光束具有窄而強(qiáng)的中心干涉區(qū)域[9-10]。這表明在擴(kuò)展微通道長(zhǎng)度微納加工方面，貝塞爾光束優(yōu)于高斯光束，如貝塞爾光束在硼硅酸鹽玻璃中成功制備了深徑比為10000:1 的納米孔微通道結(jié)構(gòu)[11-13]。同時(shí)，超快貝塞爾激光脈沖在高深度和大面積加工、微納焊接及開(kāi)發(fā)具有新的光子功能的微納尺度特征方面顯示了強(qiáng)大的適用性[6,12-14]。ZnS 晶體是最具前景的紅外窗口材料之一，由于其非線性系數(shù)高，聚焦后的飛秒激光在硫化鋅晶體中傳輸時(shí)，激光脈沖的時(shí)間和空間分布會(huì)發(fā)生嚴(yán)重畸變，自聚焦和等離子體產(chǎn)生的自散焦導(dǎo)致光束空間分裂，激光能量不能有效集中在焦點(diǎn)。前期的實(shí)驗(yàn)和理論研究表明，通過(guò)脈沖展寬的方法，能有效降低峰值功率密度，從而降低非線性效應(yīng)，15 ps～20ps 鈦寶石脈沖激光能夠在硫化物玻璃內(nèi)實(shí)現(xiàn)激光能量的有效吸收和波導(dǎo)刻寫。用超快貝塞爾光束制造納米孔的工作，前期主要集中在石英玻璃、硅酸鹽玻璃以及硼硅酸鹽玻璃，其非線性吸收與硫化鋅有數(shù)量級(jí)的差別；其次，硫化鋅晶體的硬度、熱擴(kuò)散系數(shù)、楊氏模量與上述玻璃也有明顯差別。因此，用超快激光在硫化鋅晶體上制作納米孔具有一定難度。到目前為止，在ZnS 晶體上制備周期性的亞波長(zhǎng)微結(jié)構(gòu)的研究雖有報(bào)道[15]，然而，利用超快激光在ZnS 晶體上加工高深徑比的納米孔仍處于空白。文獻(xiàn)[16] 報(bào)道了光束模式對(duì)ZnS晶體打孔質(zhì)量的影響，并進(jìn)行了討論。實(shí)驗(yàn)結(jié)果顯示，用貝塞爾光束在ZnS 晶體上打孔，其成孔質(zhì)量?jī)?yōu)于高斯光束。因此，利用貝塞爾激光在ZnS 晶體上直寫納米級(jí)微結(jié)構(gòu)，制備各種中紅外光學(xué)元器件，提高或?qū)崿F(xiàn)其某種性能已成為當(dāng)下中紅外集成光子學(xué)的研究熱點(diǎn)，并且具有廣闊的應(yīng)用前景。

本文使用中心波長(zhǎng)為1 030nm、重復(fù)頻率為100kHz、脈沖寬度為223 fs 的Yb:KGW 飛秒激光，用錐鏡產(chǎn)生高斯-貝塞爾光束，研究了在ZnS晶體表面制造納米孔的參數(shù)范圍，用聚焦離子束(FIB, Helios G4 CX)剝 蝕 和 掃 描 電 子 顯 微鏡(SEM, JEOL JSM-7500F)分析了納米孔隙的形貌，確定了納米孔隙表面形貌、直徑及深度與激光脈沖能量、脈沖寬度的關(guān)系，討論了納米孔的潛在應(yīng)用。

2 實(shí)驗(yàn)裝置

實(shí)驗(yàn)采用文獻(xiàn)[8] 中的貝塞爾光束生成裝置，如圖1 所示。首先將光束擴(kuò)束到8 mm，光束直徑將影響聚焦深度，采用1°底角軸棱錐（Altechna, 1-APX-2-H254-P, n = 1.45），焦距f1= 400mm的聚焦透鏡L1，f2= 10mm 的20×近紅外聚焦顯微物鏡L2（Mitutoyo NIR，數(shù)值孔徑NA = 0.42, 工作距離L = 20mm）的組合，利用空間濾波系統(tǒng)產(chǎn)生零階貝塞爾光束激光直寫微結(jié)構(gòu)的實(shí)驗(yàn)裝置。采用中心波長(zhǎng)為1 030nm、重復(fù)頻率可調(diào)的摻鐿鎢酸釓鉀（Yb:KGW）鎖模激光再生放大系統(tǒng)（Pharos, Light conversion）在ZnS 晶體上刻寫納米孔隙微結(jié)構(gòu)。通過(guò)激光系統(tǒng)中的可移動(dòng)光柵壓縮器可以使脈沖寬度在223 fs～25 ps 間連續(xù)調(diào)節(jié)。利用二分之一波片偏振分光棱鏡（PBS）來(lái)調(diào)節(jié)激光脈沖能量。用于刻寫納米孔隙的樣品是尺寸為18 mm×8 mm×2mm 的四面拋光的硫化鋅晶體，將其固定在計(jì)算機(jī)控制的XYZ 三維高精密空氣軸承平臺(tái)（Aerotech，ANT130）上，使用聚焦物鏡將光束聚焦到樣品上。利用高精度（75 nm）數(shù)控系統(tǒng)在連續(xù)激光脈沖的照射過(guò)程中以恒定速度移動(dòng)樣品。納米孔隙間隔可以通過(guò)激光光源的重復(fù)頻率和樣品的平移速度來(lái)調(diào)節(jié)（Λ = v/f）。連續(xù)激光輻照過(guò)程中，Aerotech 平臺(tái)以恒定速度同時(shí)移動(dòng)樣品（沿x 軸的正方向移動(dòng)）和激光焦點(diǎn)（沿z 軸的負(fù)方向移動(dòng)），使它們做相對(duì)運(yùn)動(dòng)，以確保在連續(xù)激光輻照過(guò)程中，樣品始終有一個(gè)新的曝光區(qū)置于激光作用區(qū)內(nèi)。這種刻寫方式，有利于觀測(cè)拋光后樣品內(nèi)部孔隙直徑的動(dòng)態(tài)演變過(guò)程。

3 實(shí)驗(yàn)結(jié)果及分析

聚焦的超短激光脈沖具有超高峰值功率，可以空間選擇性地誘導(dǎo)透明材料產(chǎn)生一系列非線性效應(yīng)，如多光子吸收、克爾效應(yīng)、等離子體自聚焦自散焦等，進(jìn)而實(shí)現(xiàn)材料微區(qū)形貌、分子結(jié)構(gòu)等的改變[17]。對(duì)于20ps 脈沖寬度,1μm 波長(zhǎng)的超快激光，空氣電離閾值約為2.7×1013W/cm2[18]。計(jì)算可得，本實(shí)驗(yàn)中作用在ZnS 晶體上的激光功率密度為0.57×1013～6.02×1013W/cm2。當(dāng)激光功率密度大于空氣或硫化鋅晶體的電離閾值時(shí)，激光脈沖將在焦點(diǎn)區(qū)域產(chǎn)生強(qiáng)烈的非線性吸收，并形成等離子體，從而有效吸收激光能量，形成局部高能量密度沉積區(qū)域[19]。焦點(diǎn)區(qū)域內(nèi)形成極端的高溫高壓條件會(huì)導(dǎo)致產(chǎn)生相變和微爆，并且能量以沖擊波的形式自焦點(diǎn)區(qū)域向周圍材料擴(kuò)散，從而在透明材料內(nèi)部構(gòu)建微納米尺度的三維結(jié)構(gòu)[20-22]。對(duì)透明材料內(nèi)部的激光脈沖非線性傳輸?shù)某练e能量和可能的熱機(jī)制進(jìn)行討論分析，有利于控制相變和產(chǎn)生的微結(jié)構(gòu)形態(tài)。針對(duì)本實(shí)驗(yàn)中的激光參數(shù)和空氣/硫化鋅晶體界面問(wèn)題，常用的非線性薛定諤方程可以用來(lái)計(jì)算焦點(diǎn)區(qū)域內(nèi)的等離子體濃度。但在空氣/晶體界面，微爆的產(chǎn)生機(jī)理更復(fù)雜。參考石英玻璃和硫化物玻璃的飛秒激光波導(dǎo)刻寫參數(shù)規(guī)律[23-24]，實(shí)驗(yàn)研究了脈沖寬度從飛秒到20ps、能量從損傷閾值到63μJ 變化范圍內(nèi)貝塞爾光在硫化鋅晶體表面制造微結(jié)構(gòu)的特征，利用FIB、SEM 和相襯對(duì)比顯微鏡(PCM, Olympus BX51)表征樣品表面納米孔及孔隙通道的形貌和長(zhǎng)度信息，確定了納米孔隙表面形貌、直徑及深度與激光脈沖能量、脈沖寬度的關(guān)系。

3.1 硫化鋅晶體表面納米孔隙特征

脈沖能量和脈沖寬度是影響激光與物質(zhì)相互作用的關(guān)鍵參數(shù)，脈沖能量影響材料在單位時(shí)間內(nèi)單位面積所經(jīng)歷的有效輻照通量。脈沖寬度與貝塞爾激光束軸向能量沉積效率有關(guān)[6,25]。超快光束與石英玻璃相互作用及微納制造的實(shí)驗(yàn)與數(shù)值分析結(jié)果顯示：幾個(gè)皮秒的脈沖能夠有效克服過(guò)強(qiáng)的非線性效應(yīng)導(dǎo)致的激光焦點(diǎn)等離子密度下降。考慮時(shí)空聚焦特性和材料的非線性數(shù)值模擬結(jié)果表明，4.7ps 的脈沖比60fs 相同能量的脈沖在焦點(diǎn)處電離產(chǎn)生的等離子體濃度高一個(gè)數(shù)量級(jí)[6]，然而硫化鋅的非線性吸收系數(shù)比常見(jiàn)的玻璃高一個(gè)數(shù)量級(jí)。因此在脈沖寬度優(yōu)化過(guò)程中，需要增加脈沖變化范圍。實(shí)驗(yàn)過(guò)程中，通過(guò)調(diào)節(jié)再生放大器的壓縮器，將脈沖寬度最大調(diào)節(jié)到25 ps，覆蓋了硫化物波導(dǎo)寫入范圍[26]。當(dāng)激光脈沖寬度由10ps 增加至20ps 的過(guò)程中，測(cè)量得到本實(shí)驗(yàn)中使用的ZnS 晶體的損傷閾值范圍為6.51～8.32J/cm2，詳細(xì)的測(cè)量及計(jì)算方法見(jiàn)文獻(xiàn)[27]。

在沒(méi)有任何后處理的情況下，通過(guò)SEM 表征得到的樣品表面納米孔如圖2 所示。在SEM形貌表征時(shí)，ZnS 表面被鍍約50nm 厚的金膜，以使樣品導(dǎo)電。白色圓環(huán)中心的黑點(diǎn)就是納米孔，每個(gè)孔都是由單發(fā)脈沖刻寫的，納米孔周圍的白色光暈是激光加工過(guò)程中去除的材料濺射或者再沉積而產(chǎn)生的，類似的材料沉積原理見(jiàn)文獻(xiàn)[28]。硫化鋅晶體具有很強(qiáng)的光敏性和固有的亞穩(wěn)定性，是重要的寬帶紅外窗口材料，禁帶寬度為3.6～3.8 eV, 在1 030nm 紅外激光作用下，其是三光子吸收，非線性吸收系數(shù)（a3= 0.5×10?3cm3/GW2）比石英玻璃高兩個(gè)數(shù)量級(jí)。這些特征表明了對(duì)超快激光脈沖時(shí)空包絡(luò)控制的必要性[15]。脈沖寬度是ZnS 晶體表面損傷的敏感參數(shù)，圖2 表明12.5 ps脈寬是納米孔隙出現(xiàn)的臨界脈寬值，當(dāng)脈寬小于12.5 ps 時(shí)，35.83 ～ 62.82μJ的脈沖能量并未出現(xiàn)具有深徑比的納米孔結(jié)構(gòu)，而是如圖（1）左側(cè)第一列中出現(xiàn)的具有一定深度的凹坑狀結(jié)構(gòu)。這是因?yàn)樵诙痰拿}沖寬度下，載流子生成速度較快，等離子體散焦效率相對(duì)較高，使得軸向能量沉積密度減少，不滿足微爆產(chǎn)生的熱力學(xué)條件[29]。

圖3 顯示了貝塞爾光束的脈沖寬度及脈沖能量對(duì)ZnS 晶體表面納米孔直徑的影響。用SEM測(cè)得ZnS 晶體表面納米孔隙的直徑最小可達(dá)80nm，是在脈沖寬度為12.5 ps，脈沖能量為48.67μJ的激光加工參數(shù)下刻寫的。圖中的曲線表明：固定脈沖能量，在12.5 ps ～ 20ps 的脈寬窗口內(nèi)，納米孔直徑隨著脈沖寬度的增加而增加；固定脈寬，納米孔直徑隨著脈沖能量增加而增加，且能量越大，這種變化趨勢(shì)越明顯。但在12.5 ps 時(shí)，不符合這一規(guī)律。這是由于脈沖寬度是晶體表面燒蝕的敏感參數(shù)，12.5 ps 為臨界脈寬，激光作用在晶體表面，產(chǎn)生的各種非線性效應(yīng)相互影響，導(dǎo)致孔徑變化不穩(wěn)定。本文所有實(shí)驗(yàn)均是單脈沖作用下的損傷特性，與多脈沖結(jié)果具有明顯差別[30], 對(duì)單脈沖而言，自組織干涉難以發(fā)生，焦點(diǎn)區(qū)域激光強(qiáng)度的調(diào)制和等離子體強(qiáng)度的調(diào)制不明顯。

納米孔形成的動(dòng)力學(xué)過(guò)程一般分為3 個(gè)階段：首先是晶體的非線性吸收與脈沖時(shí)空的畸變，不同激光參數(shù)直接影響焦點(diǎn)區(qū)域能量的吸收與沉積；其次是等離子體弛豫階段，包括高溫電子的背向散射壓力的釋放，電子能量傳遞到格子，導(dǎo)致晶格的升溫和形變；第三階段主要是熱過(guò)程和晶格的弛豫過(guò)程，從皮秒持續(xù)到微秒。從時(shí)間分辨的相位對(duì)比照片與等離子體發(fā)光強(qiáng)度可以看出石英玻璃在超快貝塞爾光束作用下的折射率改變：fs 時(shí)間窗口的折射率降低、ps 窗口的折射率升高，以及ns 和μs 時(shí)間窗口的折射率降低。然而難以判斷納米孔的形成是在飛秒階段還是納秒階段，電子背向散射壓力和熱效應(yīng)產(chǎn)生的快速膨脹均可能導(dǎo)致微爆的產(chǎn)生。非線性吸收過(guò)程中的自聚焦自散焦效應(yīng)、流體動(dòng)力學(xué)過(guò)程的瑞利泰勒不穩(wěn)定性都會(huì)導(dǎo)致孔徑變化的不規(guī)律[19]。

3.2 硫化鋅內(nèi)部納米孔隙特征

AFM、SEM 等表面分析工具無(wú)法表征高深徑比納米孔三維形貌，因此，為了獲取納米孔隙縱向形貌特征，本文選擇FIB 剝蝕+SEM 成像的表征方式。FIB 刻蝕工藝能夠?qū)?duì)一維貝塞爾孔隙的損害降到最低，有利于更直觀地分析納米孔的縱向形貌。SEM 表面表征結(jié)果顯示，在20ps 脈沖寬度，62.82μJ 脈沖能量的激光參數(shù)下，ZnS 晶體表面的納米孔隙結(jié)構(gòu)是橢圓形，而非像其他激光參數(shù)下出現(xiàn)的規(guī)則圓形。分析認(rèn)為在過(guò)高的峰值功率密度下，貝塞爾光束在晶體表面經(jīng)歷了非線性效應(yīng)，影響了光束的正常傳輸。可能的影響機(jī)理：克爾自聚焦、等離子體自散焦、多光子電離、等離子體屏蔽效應(yīng)。這些效應(yīng)共同作用，導(dǎo)致貝塞爾光束在材料表面的強(qiáng)度受到鉗制。因此，選定20ps 脈沖寬度，48.67μJ 脈沖能量的激光參數(shù)用于后續(xù)研究。

圖4 是在20ps 脈沖寬度、48.67μJ 脈沖能量下制備的納米孔隙結(jié)構(gòu)。由圖4 可見(jiàn)，該孔隙通道沿著激光傳輸方向具有可變的直徑，這與貝塞爾脈沖激光在材料內(nèi)部的非線性光傳輸過(guò)程中的軸向沉積能量密度分布有關(guān)[6]。孔壁表明材料經(jīng)歷了熔化和再凝固的過(guò)程。微結(jié)構(gòu)表面的白色片狀物是SEM 表征樣品時(shí)為了樣品導(dǎo)電噴在表面的金膜經(jīng)過(guò)超聲清潔后的殘留。利用能譜儀（EDS）對(duì)該白色片狀物進(jìn)行了化學(xué)成分的表征，顯示結(jié)果為金元素。沿FIB 刻蝕方向的層疊圖有利于觀察刻蝕過(guò)程中孔隙微通道的形態(tài)變化。可以看出，隨著FIB 刻蝕，孔隙微通道從大約3μm向更長(zhǎng)過(guò)渡。由于FIB 刻蝕方向很難與孔的延伸方向平行，因此，本文中未能獲得納米孔的全部信息。為了獲得更多的孔深信息，用光學(xué)相位對(duì)比顯微鏡觀察了納米孔的側(cè)面。

圖5(b)顯示了20×相襯對(duì)比顯微鏡下孔隙微通道的側(cè)面（透射式顯微模式）圖像。PCM 透射式顯微模式中，負(fù)的折射率變化對(duì)應(yīng)的相移在灰色背景上顯得較暗，對(duì)石英玻璃而言，黑色痕跡表示折射率增加，白色表示折射率降低的納米孔或者納米光柵。但高折射率的硫化物玻璃則不同。利用衍射襯度無(wú)法判斷折射率的變化情況：一方面折射率變化大，相位可能倒裝；另一方面激光損傷后的痕跡有強(qiáng)散射[25]。上方放大圖像顯示，微通道未出現(xiàn)斷層，具有良好的均勻性，用20ps、48.67μJ 的貝塞爾激光脈沖在硫化鋅晶體上制造的納米孔長(zhǎng)度約為270μm。硫化鋅晶體的折射率大于石英玻璃，如果沒(méi)有非線性效應(yīng)，貝塞爾光束焦點(diǎn)的有效長(zhǎng)度在硫化鋅晶體內(nèi)應(yīng)該大于石英玻璃，然而模擬和實(shí)驗(yàn)結(jié)果均顯示，硫化鋅內(nèi)孔長(zhǎng)度只有石英玻璃的一半。這主要因?yàn)榧{米孔行程是一個(gè)高度非線性的過(guò)程，難以從單一參數(shù)得到兩種材料孔型的變化規(guī)律。為了探究材料內(nèi)部納米孔的正向形貌及孔徑變化特征，對(duì)樣品表面進(jìn)行反復(fù)拋光，鍍金以及采用 SEM 表征。圖5(a)是拋光后微通道橫截面的SEM 圖像，測(cè)量結(jié)果顯示孔隙的直徑約為180nm。與拋光前的直徑相比，減小了40nm。為了驗(yàn)證微通道的可操作性和可重復(fù)性，采用同一脈寬和能量（20ps、48.67μJ）的貝塞爾光束在硫化鋅晶體上加工了間隔為15μm的多個(gè)孔隙微通道，圖5(b)是截取的其中3 個(gè)微通道。從圖4 和圖5 不能獲得納米孔深度的準(zhǔn)確信息，只能判斷孔深范圍大于3μm，小于270μm，是優(yōu)化后石英玻璃表面孔深的一半左右。即便如此，在中紅外硫系玻璃和硫系晶體中制造納米孔仍然具有重要意義，為中紅外波導(dǎo)駐波波導(dǎo)傅立葉變換光譜儀的實(shí)現(xiàn)奠定了重要的技術(shù)基礎(chǔ)[31-32]。對(duì)于中紅外光子晶體、中紅外微流器件、光子篩等應(yīng)用均有重要意義[33]。

4 結(jié) 論

利用超快貝塞爾激光在ZnS 晶體中刻寫了高深徑比的納米尺度的孔隙結(jié)構(gòu)，通過(guò)調(diào)控激光參數(shù)制造了直徑為80～320nm 的納米孔，獲得了在ZnS 晶體上刻寫納米孔的最佳能量(36μJ ～63μJ)及脈寬(12.5 ps ～ 20ps)窗口。利用掃描電子顯微鏡、聚焦離子束和相襯對(duì)比顯微鏡表征了樣品表面納米孔及孔隙通道的形貌和長(zhǎng)度信息，確定了納米孔隙表面形貌、直徑及深度對(duì)激光脈沖能量、脈沖寬度的影響。該技術(shù)對(duì)中紅外波導(dǎo)傅立葉變換光譜儀、硫化物光子晶體制作具有重要意義。