Two dimensional (2D) periodic microstructures composed of short periodic ripples, long periodic ripples and micro-holes are fabricated on ZnO crystals via the interference of two femtosecond laser beams. The relative reflectivity and transmissivity of visible light of these 2D microstructures decrease to the values of 30% and 20%, respectively. Theoretical and experimental studies indicate that besides the effects of increased surface area, the decrease of reflectivity is influenced greatly by the Mie scattering of surface microstructures, and the transmissivity, by the damage of crystalline structures.
©2010 Optical Society of America
Periodic ripples induced by laser irradiation in semiconductors and dielectrics have been studied intensely in the last four decades. The ripple periods were usually close to the laser wavelength λ, and these long periodic ripples (LP ripples) were attributed to the interference between the incident laser and the surface scattered light field [1–3]. Recently, LP ripples and short periodic ripples (SP ripples, the periods were of 1/10-1/2 λ) were observed in semiconductors, dielectrics and metals after irradiation of linearly polarized femtosecond laser pulses [4–16]. The SP ripple periods change with laser fluences, wavelengths and pulse numbers, and their orientation was mostly perpendicular to the laser polarization. The formation mechanism of SP ripples is an interesting problem, and many experimental and theoretical studies have been reported [5,7,8,12]. However, only a few studies were reported about the optical absorption and its application of these LP and SP ripples .
Optical absorption is one of the key factors that determine the usage efficiency of solar energy. Texturing the front surface of semiconductors by femtosecond laser irradiation is an efficient method to improve the optical absorption [17–19]. Enhancement in optical absorption of silicon was observed on sample surface with micro spikes and fibrous nanostructures after femtosecond laser ablation. Additionally, Zhao et al  reported nanoripples on SiC surface fabricated by femtosecond laser irradiation, and the optical absorptivity increased by 40%.
Zinc oxide (ZnO) semiconductor has attracted a great deal of interests by its prospects in optoelectronics applications due to its direct wide band gap (Eg~3.3eV at 300K) and the large exciton binding energy (~60meV) . Femtosecond laser-induced nanostructures on ZnO have been studied intensively [21–26]. References [21–23] reported the formation of SP ripples induced by femtosecond laser pulses, and their Raman and luminescence properties. Huang et al studied the fabrication of uniform ZnO nanosquares by alternately ablation of two orthogonal polarized femtosecond laser beams [24,25]. Dufft et al reported the LP- and SP ripples formation on ZnO. They found the formation processes depend on the laser fluences and pulse numbers, and attributed the formation of SP ripples to the surface second harmonic generation (SHG) . Recently, we fabricated 2D complex nanostructures on ZnO crystal by the interference of two femtosecond laser beams . In this paper, we further studied the fabrication of different types of 2D micro-/nanostructures via two-beam interference by adjusting laser fluences and pulse numbers, and found the optical absorptivity increased to 90%. The mechanisms of the enhanced optical absorption were also studied theoretically.
The experiments were conducted on a commercial Ti: sapphire regenerative amplifier laser system (Legend Elite, Coherent). It generates laser pulses at center wavelength of 800nm with pulse duration of 50fs and pulse energy of 3.5mJ. The laser system operates at repetition rates of 1-1000Hz. The laser beam was split into two beams with the same intensity and polarization by a beam splitter. Figure 1(a) shows the sketch of two-beam interference. The two beams denoted by two red lines propagating in yz plane, and are focused on the same spot simultaneously with two lens of 250mm focal length. The correlation angle (2θ) is set as 13.9° unless otherwise noted. The two double arrows show the linear polarizations of two beams, which are parallel to x axis. The experimental set-up has been described in detail in reference . Figure 1(b) shows the calculated intensity distribution of two-beam interference in xy plane with grating period of λ/2sinθ, where the intensity of each beam was normalized.
We used commercial available c-cut ZnO (0001) single crystal with sizes of 10mm × 10mm × 1mm. The two surfaces were both optically polished with roughness less than 10 nm and normal to z axis. The sample was mounted on a XYZ translation stage controlled by a computer. After laser irradiation, the ablation areas were observed by scanning electron microscope (SEM, JEOL JSM-5600).
In order to study the optical absorption of the microstructures in ZnO crystal, we measured the reflection and transmission spectra before and after laser irradiation. Figure 1(c) shows the sketch of measurement of reflection and transmission spectra. The white light source was generated by focusing 800nm femtosecond laser into a water cell. Then, it was focused on the ablation craters through a 5 × Nikon objective lens at incident angle of 45°. The reflection and transmission light was focused into fiber optic spectrometer (USB2000, Ocean Optics) to measure the reflection and transmission spectra. The reflection and transmission light were scattered by structured surface, which called for a large collection angle to minimize the measurement error. The scattered light was collected by a silica lens with diameter of 25 mm. The lens was set at the position of 10 mm in front of the sample, and the collection angle 2α was 100°, which is large enough to gather in the majority of scattering light resulted from the structured surface. In addition, we cursorily measured the scattered light beyond the collection angle, and found they could be negligible compared with the collected light.
3. Results and discussion
Figure 2 shows the 2D periodic microstructures on ZnO crystal induced by the interference of two femtosecond laser beams. As reported in reference , the long periodic grating is determined by the interferential intensity pattern (see Fig. 1(b)). Besides, there are quasi-periodic SP ripples with periods of 180-250 nm embedding in the long periodic gratings (see Fig. 2(a)). If the laser fluence increases to 0.4 J/cm2, Fig. 2(b) shows that LP ripples formed on the sample surface after irradiation of 10 laser pulses. The period of the interferential pattern is 3.3 μm, and the LP ripples are of 1.5μm long. The rest parts of the sample surface are only slightly ablated, and very tiny wrinkles formed there. The formations of SP ripples and LP ripples in Figs. 2(a) and 2(b) depend on the laser fluences, which is coincident with the results in reference [22–26]. After 50 laser pulses irradiation, Fig. 2(c) shows that LP ripples shorten to 1.2 μm, and the grooves among them becomes wider and deeper. Meanwhile, SP ripples form on the interval stripes with lower light intensity. Finally, the grooves among LP ripples grow into 2D micro-holes array (see Fig. 2(d)). Meanwhile, part of the ablation plume deposited on the sample surface, and most of the SP ripples were covered.
In reference , only SP and LP ripples were observed at the ablation area, which is different from the results in Figs. 2(c) and 2(d). We proposed that two-beam interference played an important role in the formation of 2D micro-holes array. After formation of LP ripples shown in Fig. 2(b), the energy of subsequent pulses was mostly deposited in the grooves of LP ripples because of the inhomogeneous absorption. Simultaneously, the LP ripples cannot grow longer because of the restriction of interferential intensity distribution. Therefore, the grooves of LP ripples became deeper and wider. It is noted that the lengths of LP ripples decreased from 1.5μm to 1μm in Figs. 2(b)-2(d). Therefore, self-focus effect possibly takes place in the formation process of micro-holes array.
The laser fluences shown in this paper were the average value of single beam. We measured the 1/e-radius of the laser focus on the sample surface, and calculated the laser fluences by the quotient of pulse energies and spot areas [28,29].
Figures 3(a) and 3(b) show the reflection and transmission spectra of the unstructured surface and 2D periodic structures, respectively. Relative to the reflection and transmission spectra of unstructured surface, spectra of 2D structures decreased significantly with the irradiation of laser pulses. Figures 3(c) and 3(d) show the spectra of relative reflectivity and relative transmissivity of 2D microstructures in ZnO crystal after irradiation of 10, 30 and 70 laser pulses, respectively. Here the reflection and transmission spectra of the unstructured surface were normalized. The fluence of single laser beam was 0.4 J/cm2. The relative reflectivity decreased to 61% after irradiation of 10 laser pulses, and it reduced gradually to 29% for 70 pulses. At the same time, the relative transmissivity reduced to 25%. We further measured the relative reflection and transmission spectra for incident angles in the range of 20°-60°, and found they are nearly same as shown in Figs. 3(c) and 3(d).
We measured the spectra of reflectivity, transmissivity and absorptivity of unstructured ZnO surface in the range of 450nm-750nm, as shown in Fig. 4(a) . Hereafter, we can obtain the spectra of absolute reflectivity, transmissivity and absorptivity of 2D microstructures from the products of actual and relative spectra as shown in Fig. 4(a) and Figs. 3(c)-3(d). Then, we calculated the mean values of absorptivity in the range of 450nm-750nm for ablation spot irradiated by different laser conditions, as shown by the data in Fig. 4(b). It increases from 39% to 80-90% after irradiation of 50-100 laser pulses. After the sample was irradiated by single laser beam, Fig. 4(c) shows that the absorptivity is only 50-80% though the laser fluences are higher than the values in Fig. 4(b).
We proposed that Mie scattering effect of the surface microstructures played an important role in the reduced reflectivity for the sizes of surface structures were close to the wavelengths of white light. Figure 1 indicated the size “a” of 2D microstructures in the range of 0.2-3.3 μm. The character “a” represents the diameter of small sphere in Mie theory, which denotes the periods of surface structures in the simulations. The factor 2πa/λ is around 2-30 for visible light, and the Mie scattering effects (MS effects) of these microstructures are very significant . According to Mie theory, the back scattering light decreased with the increasing of forward one.
The numerical calculation model of finite-difference time domain (FDTD) method is shown in Fig. 5(a) . The Gaussian light beam with width of 4 μm, at a central wavelength of 550 nm transmitted normally into the sample. The calculation area is 10 × 5μm2 and the grid size is 5nm × 5nm. The refractive index n and extinction coefficient k of ZnO are taken as 2.3 and 0, respectively. The surrounding area is perfect match layer (PML boundary), which eliminated the influence of boundary reflection. The quasi-periodic ripples were considered as periodic gratings in the calculations. “w” and “d” in Fig. 5(a) represented the periods (widths) and depths of grooves, respectively. Their values were adjusted based on the experimental results shown in Fig. 2.
The commercial software OPTIFDTD was used to calculate the Fresnel diffraction light. The reflection and transmission light were collected on the two observation lines, and Poynting vectors were integrated there. In the last, the reflection and transmission spectra were obtained via Fourier transform. In order to study the influences of periodic surface structures on the reflectivity and transmissivity, the simulation results showed the relative reflection and transmission spectra of periodic surface structures in the range of 450-750nm, where the calculated spectra of unstructured surface were normalized.
In our simulation model, the sample is only 5μm thick. If the right observation line is set outside the sample, the front and back surfaces will form a cavity, which will introduce extra strong oscillations to the spectra. When the observation line lies within the sample, only the reflection of the back surface is ignored, which lead to an error less than 5%.
Simulations of three types of periodic structures were conducted: SP ripples, LP ripples and interference gratings induced by two-beam interference, which are shown in Figs. 5(b), 5(c) and 5(d), respectively. Figure 5(b) shows the spectra of relative reflectivity and transmissivity of SP ripples which appeared in the formation of 2D periodic structures (see Figs. 2(a)-2(c)). Relative to the unstructured surface, the reflectivity of SP ripples decreased significantly. The strong oscillations result from the periodicity of surface microstructures in the calculation model in Fig. 5(a), which is just like the spectral selectivity of gratings. The SP ripples are quasi-periodic. If quasi-periodic surface structures were used in the numerical simulation model, the oscillations were significantly weakened.
For LP ripples with period w of 850 nm and depth d of 250 nm, calculation results indicate the relative reflectivity decrease to about 60%, as shown in Fig. 5(c). If the depth d is 750 nm, the relative reflectivity decreases to 35%. For the 3300 nm gratings with depth of 400 nm, Fig. 5(d) shows that relative reflectivity decreases to 90%, while it decreases to 70% for the depth of 1200 nm.
The above calculation results can explain well the experimental results of the relative reflectivity shown in Fig. 3(c). After irradiation by 10 laser pulses, LP ripples formed on the ZnO crystal surface (see Fig. 2(b)). For LP ripples with period w of 850 nm and depth d of 250 nm, the relative reflectivity decreases to 60%. While for the 3300 nm gratings with depth of 400 nm, it decreases to 90%. The relative reflectivity of 2D periodic microstructures decreases to 54%. After 30 pulses irradiation, SP ripples formed and the grooves between LP ripples become deeper and wider. Calculation results indicate these factors decrease the reflectivity obviously. After micro-holes array formation, MS effects of the 850 nm ripples with depth of 750 nm induce the relative reflectivity to decrease to 40%, while it decreases to 70% for the 3300 nm gratings with depth of 1200 nm. The two factors reduce the reflectivity to 28%, which accords well with the experimental value (Fig. 3(c)). Compared with 1D microstructures, 2D microstructures reduce the relative reflectivity by 30-40%.
The values of dashed lines between 450nm and 750nm in Fig. 5 are generally more than 1 (relative to the transmissivity of unstructured ZnO surface), which indicates that the relative transmissivity was enhanced due to MS effect of surface structures. However, Fig. 2(d) shows clearly the relative transmissivity decreases significantly after 2D periodic structures formation. Many research groups indicated that enhancement of optical absorption came from increased surface area  and structural defects [18,19]. The modified crystalline of ZnO by laser irradiation may induce the enhancement of optical absorption .
In order to study the mechanisms of the enhancement of optical absorption, we conducted micro-Raman experiments with Raman spectrometer (Jobin Yvon T64000) excited by argon ion laser at wavelength of 514nm and measured in backscattering geometry. The results are shown in Fig. 6 . From unablated spot to the 2D micro-holes array, the main Raman-shift peak at 437 cm−1, namely, E2 mode, decreases to less than a half. Meanwhile, the peak at 572 cm−1, which attributes to A1 (LO) phonon, increases to 2500. These results indicate the crystalline structure is greatly damaged during the formation of 2D microstructures. The image of transmission electron microscopy indicated that the crystalline structure transmits to amorphous state after periodic ripples formed in the sample surface . The damage of crystalline structures in the structured surface layer leads to the formation of surface defect states which induces the enhancement of optical absorption coefficient . When the extinction coefficient k is taken as 0.02, numerical calculation results show that the relative transmissivity decreases to 20%.
We fabricated 2D microstructures composed of periodic interferential gratings, SP ripples, LP ripples and microholes by changing the laser fluences and pulse numbers, and measured the optical absorption of visible light. Compared with the crystal without laser irradiation, the relative reflectivity and transmissivity of 2D microstructures decrease to 30% and 20%, respectively. We conducted theoretical study by FDTD method to study scattering behaviors of structured surface, and found Mie scattering played an important role in the decrease of reflectivity. Micro-Raman spectra confirmed that besides the effects of increased surface area, the damaged crystalline structures also play an important role in the reduced transmissivity and the enhancement of optical absorption.
This work is supported by Shanghai Leading Academic Discipline Project (B408), National Key Project for Basic Research of China (2006CB806006, 2006CB921105 and 2010CB923203), National Natural Science Foundation of China (10874044 and 10904038), Ministry of Education of China (30809), and Shanghai Municipal Science and Technology Commission (09142200500, 09JC1404700, 08JC1408400, 09ZR1409300 and 08PJ1404800), and Twilight Project sponsored by Shanghai Education Committee (07SG25).
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