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Metallic photonic crystals (MPCs) and metamaterials operating in the visible spectrum are required for high-temperature nanophotonics, but they are often difficult to construct. This study demonstrates a new approach to directly write two-dimensional (2D) MPCs on tungsten surfaces through the cylindrical focusing of two collinear femtosecond laser beams with certain temporal delays and orthogonal linear polarizations. Results are physically attributed to the laser-induced transient crossed temperature grating patterns and tempo-spatial thermal correlations. Optical properties of the fabricated MPCs are characterized. Such a simple and efficient technique can be used to fabricate large-area, 2D microstructures on metal surfaces for potential applications.
© 2015 Optical Society of America
1. Introduction
Photonic crystals are designed periodic subwavelength structures with particular energy bandgaps, which offer the capability of controlling electromagnetic waves in a range comparable to the periodicity of structures. By exploiting dielectric materials, photonic crystals have been intensively investigated in several applications, such as filters, optical switches, all-optical integrated circuits [1,2 ], low-threshold lasers [3], and highly efficient light-emitting diodes [4]. However, metallic photonic crystals display large bandgaps because of dispersive and absorbing properties in the optical regime, which may help modulate intrinsic thermal emission behaviors [5–8 ]. Combined with the ability to operate at high temperatures (>1000 K), 2D MPCs are potential high-performance selective thermal emitters for the emerging field of energy conversion, including solar thermophotovoltaic and radioisotope thermophotovoltaic generators, as well as highly efficient solar absorbers and emitters [9–11 ]. Selective emitters can also be applied as highly efficient infrared radiation sources for infrared spectroscopy and sensing applications [12].
To date several methods, such as electron beam lithography and fast atom beam etching [13], chemical vapor deposition [7], atomic layer deposition [14], nano-imprinting [15], interference lithography and deep reactive ion etching (DRIE) [16] and laser interferometric ablation [17], have been developed to fabricate MPCs. For MPCs composed of multilayer structures and metal-dielectric composite coatings [15,18,19 ], their suitability for high-temperature applications are restricted by the thermal instability. As a commonly used technique, electron-beam lithography can produce well-defined structures; however, this technique often requires multiple complex fabrication procedures with costly sophisticated equipment and is also limited by low throughput, time consuming and small areas. Furthermore, through using interference lithography in a Mach-Zehnder setup in combination with DRIE, a large area and highly defined structures in metallic substrates can be easily achieved, but it does still require more steps than direct writing [16]. In other words, all these techniques naturally belong to an indirect method of microprocessing of metals. Recently, 2D MPCs have been also developed through interferometric ablation on metal targets with several spatially combing beams of laser pulses; as a result, structure periodicity reaches as large as several micrometers and bandgap is observed in the infrared spectrum [17,20,21 ]. In general, how to adopt a single step to fabricate large-area MPCs tuned for the visible and near-infrared regime remains a challenge.
This study proposes and demonstrates a novel method to fabricate MPC structures with a periodicity of approximately 560 nm on the hard metallic material of tungsten surfaces through the cylindrical focusing of femtosecond laser pulses. In contrast to traditional laser beam interferometry, the proposed method employs the collinear incidence of double femtosecond laser pulses with orthogonal linear polarizations and certian temporal delays. Each pulse successively introduces a transient subwavelength temperature grating perpendicular to the laser polarization direction on the metal surface because energy coupling occurs between the laser and its excited surface plasmon polariton (SPP). Therefore, the short time delay ablation of the two crossed temperature gratings initially generates the quadratic matrix of pristinemetallic islands on the target surface, which is then transferred into truncated cone-like microstructures via the correlated surrounding thermal effects of the two temperature patterns. Further experiments reveal that such 2D MPC structures disappear when the time delay of double laser pulses increases. This technique is the direct writing of two time-delay femtosecond laser pulses with orthogonal linear polarizations, especially exhibiting high reproducibility in a large area with a single step.
2. Experiment and results
A commercial Ti:sapphire femtosecond laser amplifier system (Spitfire, Spectra-physics, Inc.) was used as a light source, which delivers linearly polarized femtosecond laser pulse trains at a repetition rate of 1 kHz, with the central wavelength of 800 nm and the pulse duration of 50 fs. The schematic of the experimental setup is shown in Fig. 1 . After passing through a birefringent crystal of YVO4 with a thickness of 1.26 mm and a diameter of 10 mm, an incident laser pulse is temporally split into two pulses with a time delay of 1.2 ps. For the normal incidence on the crystal, the two obtained laser pulse beams that possess crossed linear polarizations collinearly propagate in space. A tungsten plate (Goodfellow, Co.) with a dimension of 25 mm × 25 mm × 1 mm serves as a material sample to be structured, whose surface was mechanically polished with a fine-grade emery paper and then degreased in acetone before the experiment was performed. Tungsten was selected because this metal does not degrade at high temperatures when used as the primary absorbing and thermal-emitting MPCs. The focusing element used in this study was a fused silica-based plano-convex cylindrical lens with a focal length of f = 50 mm; as a result, a line-shaped focal region with a length of approximately 4.5 mm was obtained. The sample was mounted on a computer-controlled x–y–z translation stage (Newport UTM100 PPE1) with a resolution of 1 µm, and its surface was perpendicularly oriented to the optical axis. The translation direction was also perpendicular to the line-shaped focal region. Femtosecond laser microstructuring processes were performed by translating the sample across the line focus at a speed range of 0.005–0.4 mm/s. The incident laser energy was adjusted using neutral-density filters and measured before the cylindrical lens. The experiments were performed in air. The morphological characteristics of the laser-exposed surfaces were examined through scanning electron microscopy (SEM) and atomic force microscopy (AFM).
Fig. 1 Schematic of the experimental setup for direct fabricating large-area 2D photonic crystal structures on tungsten surface by two cross-polarization femtosecond laser pulses. The double arrows represent the directions of the linear polarization of femtosecond laser pulses, and θ denotes the azimuth angle of the birefringent crystal.
Fig. 2(a) shows a typical SEM image of the surface morphological characteristics obtained at a total incident energy of approximately 0.21 mJ when the birefringent crystal was adjusted to an azimuth angle of θ = 49°; thus, the energy ratio of the two pulses is 1.3. The square lattices of metallic microbumps are regularly generated on the sample surface; as a result, the feature of 2D MPC structures is observed. The corresponding zoom-in image shows that these structures consisting of bumps with an average diameter of approximately 320 nm yield a spatial periodicity of Λ = 560 nm. The scattered nanoparticle debris around the microbumps indicates that thermal ablation may occur during structural formation. In order to obtain the smooth surface of the 2D MPC structures, the fabricating experiments can be carried out in vacuum condition, which is helpful to greatly reduce the debris production [22]. The AFM measurements of the periodic surface structures are shown in Figs. 2(b) (left) and 2(c). The topographic image reveals that each microbump exhibits a truncated cone-like shape; the cross-section profile line demonstrates that the modulation height of the microbumps is approximately 150 nm. The surface structures were also investigated with the mirror electron microscopy (MEM) mode of a non-scanning low energy electron microscope, and the measured result is shown in Fig. 2(b) (right). In contrast to SEM, MEM applies a negative bias voltage to a sample, and local microfields on the surface cause the probing electron beam to become reflected and detected [23]. In the MEM image obtained in our study, the bright and dark areas represent protrusions and depressions, respectively. The electric potential distribution differences on the sample surface can be analyzed on the basis of this result.
Fig. 2 (a) SEM images of the square lattices of metallic microbump structures on tungsten surface fabricated by two time-delay femtosecond laser pulses with orthogonal linear polarizations at the total laser energy of 0.21 mJ and the translation speed of 0.02 mm/s. (b) A typical AFM image (left) and MEM image (right) of the microbumped surface structures. (c) A cross-section profile line of the AFM image in Fig. 2(b).
In the experiments, the evolution of 2D surface structures was determined with varying incident laser energies. In Fig. 3(a) , the 2D periodic surface structures exhibit an evident orientation preference parallel to the top-left and bottom-right diagonal direction at a laser energy of approximately 0.13 mJ; the one-dimensional grating-like pattern is also predominantly formed along the particular direction with a spatial periodicity of approximately 600 nm. Furthermore, some irregularly shaped microbumps are generated on the grating ridges. When the total laser energy increases to 0.18 mJ, the orientation preference in the surface structures likely disappears and exhibits the uniform distribution of the microbumps in terms of size and periodicity; thus, the 2D square lattices of the microstructures are achieved. When the incident laser energy reaches approximately 0.29 mJ, the orientation preference in the surface structures appears again, but the microbump formation on the periodic ridges becomes indistinct. This result can be considered as one-dimensional periodic grating-like surface structures; this finding is similar to previous observations through the single-beam irradiation of femtosecond laser pulses [24–26 ]. However, the formation of regular square lattices of microbumps can be achieved within a range of 0.01–0.04 mm/s when the translation speed of the sample is altered at a specific laser energy [Fig. 3(b)]. The measured dependence of the structure features on translation speed is shown in Fig. 3(c). The spatial periodicity and the bump size gradually increase at high speeds.
Fig. 3 Evolution of the microstructures on tungsten surface with varying incident laser parameters. (a) Morphologies of the structures formed with several laser fluences. (b) Morphologies of the structures formed with several translation speeds. (c) The measured periodicity and feature size of the microbumps as a function of the translation speed.
We changed the azimuth angle θ of the birefringent crystal that corresponds to the alternative ratio of the double laser energies but maintained the orthogonal linear polarizations to investigate the formation conditions suitable for the square lattices of metallic microbumps. In these circumstances, different surface morphological characteristics can be obtained, but the regular 2D MPC structures are only determined within two regimes: 44° ≤ θ ≤ 56° and 124° ≤ θ ≤ 136° in the range of 180°, as shown by the shaded regions I and II in Fig. 4(a) , wherein the two laser pulse energies do not exhibit remarkable differences. These two shaded regions physically represent the processing windows to directly fabricate 2D periodic arrays of microbumped structures on the metal surface. For instance, one-dimensional periodic grating-like structures in regime I are generated on the sample surface when θ = 40° or 60° [Figs. 4(b) and 4(g)]. At θ = 46° or 58°, the 2D periodic microbump structures can be observed on the surface but with different patterns of orientation preferences [Figs. 4(c) and 4(f)]. At θ = 49° or 52°, the regular 2D square lattices of microbumps are evidently distributed [Figs. 4(d) and 4(e)]. This result indicates that the energy ratio of the double laser pulses is crucial to the formation of regular 2D square lattices of metallic microbumps; indeed, the structure orientation is determined on the basis of the polarization direction of the strong laser pulse.
Fig. 4 (a) Simulated variations of the double laser pulse energies with different azimuth angles of the birefringent crystal, where the shaded regions represent two available regimes for the formation of 2D square lattices of microbump structures; (b)–(g) SEM images of surface structures obtained at different azimuth angles of the birefrigent crystal, represent θ = 40°, 46°, 49°, 52°, 58° and 60°, respectively where the laser energy is 0.2mJ.
The square lattices of metallic microbumps can be fabricated in the 2D extended areas of the tungsten surface. Figure 5(a) demonstrates the large-area 2D MPC structures generated in one step at a laser pulse energy of 0.24 mJ and a translation speed of 0.02 mm/s. The 2D fast Fourier transformation is shown in Fig. 5(b); the distinct bright spots indicate the homogeneity of the microbump arrangement along different directions. The length of the region containing the microbump structures can be extended to millimeter scales depending on the laser energy and defocusing distance of the sample.
Fig. 5 (a) SEM image of the 2D microbumped MPC structures in an extended area on tungsten surface irradiated by two time-delay femtosecond laser pulses with orthognal linear polarizations. (b) An image of the fast Fourier transformation (FFT) of Fig. 5(a).
3. Proposed model and its confirmation
The phenomena described in preceding sections can be physically elucidated as follows. When a previous laser pulse irradiates the target, the free electrons near the metal surface are initially triggered into a collective motion mode of SPP, whose subsequent coupling with the incident laser modulates the beam energy into spatially periodic patterns on the subwavelength scale. i.e., a subwavelength temperature grating oriented perpendicularly to the polarization direction of the incident laser pulse is developed into the transient state [27,28 ]. The deposited energies in the periodic surface regions are then relaxed to the lattice through electron-phonon processes, which often last tens of picoseconds in metals [29,30 ]. In this period, if a second femtosecond laser pulse linearly polarized in another direction arrives, the induced transient temperature grating appears, and this grating is oriented perpendicularly to the previous one. The two transient temperature gratings with the same periodicity display mutually orthogonal alignments because the two laser pulses yield the same central wavelength. Therefore, their crossed ablations on the same area of the metal surface likely generate the quadratic matrix of cube-shaped metal islands. In the surrounding of each unablated island, the heating of the two orthogonal temperature gratings becomes correlated in spatial and temporal domains and eventually melts the metal island into a truncated cone-like shape because the thermal effects of these two gratings occur within a relatively short time interval [Fig. 6 ].
Fig. 6 An illustration of physical stages for the formation of 2D microbumped photonic crystal structures on tungsten surface by two time-delayed femtosecond laser pulse beams. (a) The prior laser pulse induces a transient subwavelength temperature grating-like pattern (yellow regions) within the beam spot, with orientation perpendicular to the incident polarization direction due to the laser-SPP coupling. (b) The delayed laser pulse associated with different linear polarization also induces its own transient subwavelength temperature grating-like pattern (blue regions). As a result, the short time delay between double laser pulses results in two orthogonal temperature grating-like patterns on the sample surface. (c) Quadratic matrix of pristine metallic islands are initially generated by the crossed ablations of two temperature gratings, and then they are sculptured into microbumps due to the surrounding correlated thermal effects.
If one of the laser strikings is slightly stronger than the others during physical processes, the induced periodic temperature grating causes a larger ablation depth; as a result, the 2D microbumped surface structures exhibit orientation preferences. When the two laser strikings become much different, the crossed ablation effect of the two laser pulses is greatly diminished. As such, the interaction result of the stronger laser pulse becomes predominant; thus, only one-dimensional periodic grating-like structures are observed on the sample surface [Fig. 4(b)]. However, 2D square lattices of microbumps are no longer formed when a traditional two-step sequential processing method is applied to the tungsten material, wherein the delayed laser pulse with different linear polarizations completely irradiates after the previous laser pulse undergoes physical interaction. This result is contrary to the previous observations on semiconductor surfaces [31,32 ]. Therefore, the formation of truncated cone-like bump microstructures on metal surfaces physically originates from the correlated thermal melting of the two laser pulses rather than from a simple overlap of the two laser ablations. In the two-step sequential processes, a tempo-spatial correlation between the two laser thermal meltings cannot be satisfied because of prolonged time delays.
We conducted further experiments by varying time delays between the two laser pulses by using a Michelson interferometer to confirm our assumptions. In this procedure, a horizontally polarized femtosecond laser pulse out of the laser amplifier was transformed into two pulses. The two laser pulses were set as 0.104 mJand 0.08 mJ to reach the energy ratio of 1.3 that could induce the regular microbump structures on the metal surface. The linear polarization direction of the lower-energy pulse was also changed by 90° to the vertical direction by using a half wave-plate. With certain time delays, the two pulses were collinearly focused onto the sample by using the cylindrical lens. The uniform distribution of 2D microbump structures appears on the metal surface when the time delay between the two laser pulses is 40 ps, as shown in Fig. 7 ; this result indicates that the two laser thermal melting behaviors can strongly correlate in the tempo-spatial domain. With a gradual increase in time delay, the formation of the surface microstructures changes. For example, the laser-exposed surface is developed into a new type of microstructure with a 2D matrix of cubiod-shaped pillars at a time delay of 130 ps; furthermore, the periodicity of the alignment in the vertical direction is decreased almost by half. This result suggests that the transient temperature grating formed by the delay-incident laser pulse is physically affected, although the correlation between the two thermal melting processes is much weakened. The irradiation of the previous laser pulse can modulate the optical properties of the metal surface to alter the transient grating periodicity of the delayed laser pulse. As the time delay increases to 160 ps, the surface morphological structure induced by the two laser pulses displays one-dimensional grating-like patterns; the vertical orientation of these patterns indicates the dominant role of the previous incident higher-energy laser pulse; this observation also reveals that the interaction of the delayed incident lower-energy laser pulse is nearly neglected. The experiment in Fig. 1 was repeated by replacing the YVO4 crystal with a zero-order quarter waveplate, which can yield a femtosecond laser pulse polarized in either a circular state or an elliptical state instead of two temporally separated laser pulses with othorgonal linear polarizations. Under these conditions, the uniform 2D square lattices of the microbump structures are no longer formed on the metal surfaces. These results provide supporting evidence for the physical correlations during the two laser-matter interactions.
Fig. 7 Evolution of microstructures on tungsten surface with different time delays between the two time-delay femtosecond laser pulses with orthogonal linear polarizations, where the two laser pulse energies are E1 = 0.104 mJ and E2 = 0.08 mJ, respectively, and their linear polarization directions are denoted by the red double arrows (time delays represent the first arrival of the higher-energy laser pulse E1).
Compared with other methods used to fabricate 2D MPC structures, the proposed technique provides several advantages. First, the direct writing approach is characterized by high-throughput and low-cost processes; for instance, multi-step fabrication processes or interference lithography is no longer involved. Thus, the fabrication complexity is greatly reduced and the reproducibility of the method is improved. Second, with the line focusing of the cylindrical lens, the construction of 2D MPC structures can be extended into a large area. Thus, processing speed is likely improved. Third, pre-patterned substrates are no longer needed. This technique can also be applied to other metals such as Molybdenum material, and the designs of 2D surface patterns become more flexible.
4. Optical characterization
In addition, we also characterized the optical properties of the fabricated 2D MPCs by measuring the spectral reflectivity with a Fourier transform infrared (FTIR) spectrometer (VERTEX-70, Bruker’s), in which an objective lens (36 × , NA = 0.5) was used to collect the reflected ray within about ± 30° from the center angle, and the corresponding results are shown in Fig. 8 . Compared to the polished tungsten surface, the reflectivity of the sample microstructured by two femtosecond laser beams is found to decrease drastically in particular for the wavelength less than 2.0 µm, and it tends to grow towards the high reflectivity at longer wavelengths. Local extrama are also observed on the spectrum, which might reveal the effect of the square lattices of metallic microbumps. Moreover, since our samples are the bulk material, the measured decrease tendency of the optical reflectivity can also indicates an increase of its absorptivity and even thermal emission properties.
Fig. 8 Measured spectral reflectivity of the 2D MPC structures and the polished tungsten at the normal incident angle at room temperature. The incident beam is randomly polarized.
5. Conclusions
In summary, a novel method was developed to directly fabricate large-area 2D photonic crystal structures on a bulk tungsten surface by using the collinear incidence of two time-delay femtosecond laser beams with mutually orthogonal linear polarizations and cylindrical lens for focusing. At a constant time delay of 1.2 ps between the two pulses obtained by the birefringent crystal, the formation of regular 2D arrays of surface structures with truncated cone-like bumps was determined at a diameter of 320 nm, a depth of 150 nm, and a periodicity of approximately 560 nm. The structural characteristics and homogeneity can be engineered by varying the laser parameters. The fabrication is physically attributed to the correlated thermal ablations of the two transient subwavelength temperature grating-like patterns with crossed orientations. Further experiments were conducted with variable time delays between the two orthogonally polarized femtosecond laser pulses, and the obtained results supporte our theoretical and analytical findings. Moreover, the 2D MPC structures fabricated by this method showe excellent spectral selectivity, displaying an obvious reduction in the optical reflectivity bleow the wavelength of 2.0 μm. As a direct and efficient 2D array structure fabrication technique, this method can be applied to other metals and larger areas and even be scalable to fit nano-features by using incident shorter laser wavelengths; the obtained structure can also be applied as thermophotovoltaic emitters and plasmonic devices.
Acknowledgments
We acknowledge financial supports from National Natural Science Foundation of China (grant no. 11274184), Natural Science Foundation of Tianjin (grant no. 12JCZDJC20200) and the Research Fund for the Doctoral Program of Higher Education of China (grant no. 20120031110032). The technical supports from Yong Yang and Jing Li are greatly appreciated.
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