788 OPTICS LETTERS / Vol. 34, No. 6 / March 15, 2009

Complex periodic micro/nanostructures on 6H-SiCcrystal induced by the interference of three

femtosecond laser beams

X. Jia,1 T. Q. Jia,1,* L. E. Ding,1 P. X. Xiong,1 L. Deng,1 Z. R. Sun,1 Z. G. Wang,1 J. R. Qiu,2 and Z. Z. Xu2

1Department of Physics, State Key Laboratory of Precision SpectroscopyEast China Normal University, Shanghai 200062, China

2State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics,Shanghai 201800, China

*Corresponding author: tqjia@phy.ecnu.edu.cn

Received December 4, 2008; revised January 28, 2009; accepted February 4, 2009;posted February 12, 2009 (Doc. ID 104401); published March 11, 2009

We reported three types of complex micro/nanostructures on 6H-SiC crystal induced by the interferences ofthree femtosecond laser beams by arranging three types of laser polarization combinations. The micro/nanostructures are composed of two parts: two-dimensional long-periodic micropatterns determined by theinterferential intensity pattern and short-periodic nanopatterns determined by the interferential polariza-tion pattern. Theoretical calculation indicates that the different polarization combinations will lead to a dis-tinct complex interferential polarization pattern and intensity pattern, and they accord well with the ex-perimental results. © 2009 Optical Society of America

OCIS codes: 320.7130, 220.4241, 110.4235.

Surface nanopatterning has become increasingly im-portant owing to the rapid development of nanoelec-tronic, photonic crystals, and biomedical devices. Re-cently, various techniques, such as self-assembly ofcolloidal particles, direct laser writing, and holo-graphic lithography (HL), etc., were used to fabricatetwo-dimensional (2D) and three-dimensional periodicstructures [1–5]. Owing to its simple setup and lowcost, HL has become an important technology to fab-ricate two- and three-dimensional micro- and nano-structures by adjusting the number of laser beamsand their spatial arrangement [6–9]. However, thismethod is less flexible, and only specific types of pat-terns can be generated [9].

Laser-induced periodic surface structures havebeen intensely studied in the past four decades[10–13]. The periods were usually close to the laserwavelength, and they were explained by the interfer-ence between the incident laser and the surface scat-tered wave [12,13]. Besides, short-periodic nano-structures with periods much smaller than the laserwavelength have been observed on semiconductorsand dielectrics irradiated by femtosecond laserpulses [14–20]. Several explanations were proposedto study the formation mechanism of these short-periodic nanostructures, such as interference [16],self-organization [17], and Coulomb explosion [18].Recently, Shen et al. [19] and Juodkazis et al. [20],proposed that the refracted index of the inter-action region played an important role. The forma-tion mechanism of the short-periodic nanostructuresinduced by a single femtosecond laser beam is ahard job, and these formation mechanisms pro-posed by different research groups are different.However, there are some experimental resultsthat were nearly the same; namely, linearly polar-ized laser pulses induced nanoripples andcircularly polarized ones usually induced nanopar-

ticles [14,18].

0146-9592/09/060788-3/$15.00 ©

Combining the fabrication of the short-periodicnanostructures induced by a femtosecond laser withHL technology, we have achieved 2D complex nano-structures by two-beam interference [21]. In this Let-ter, we further conducted the experiments of three-beam interference and fabricated complex micro-/nanostructures by adjusting the laser polarization.The distribution of the laser intensity and the polar-ization was theoretically calculated, and it was vali-dated well by the experimental results.

Figure 1 shows the experimental setup of three-beam interference. A linearly polarized laser at awavelength of 800 nm with a pulse width of 50 fs anda pulse energy of 0.8 mJ was delivered from a com-mercial Ti:sapphire regenerative amplifier operatedat a 1–1000 Hz repetition rate (Hurricane, Spectra-

Fig. 1. Experimental setup for three-beam interference of800 nm femtosecond laser pulses. HF, half-wave plate; GZ,

Glan polarizer; BS, beam splitter; DL, delay line; L, lens.

2009 Optical Society of America

March 15, 2009 / Vol. 34, No. 6 / OPTICS LETTERS 789

Physics). The laser beam went through a half-waveplate (HF1) and a Glan polarizer to adjust the inten-sity and the polarization. Then it was split into threebeams with the same intensity by two beam splitters.Three half-wave plates (HF2, HF3, and HF4) wereused to rotate the polarization of the three beams, re-spectively. The inset in Fig. 1 shows the spatial ar-rangement of the three laser beams. A, B, and C rep-resent the three beams on the P plane, and O is thesuperposition point on the sample. The triangle ABCwas equilateral with a side length of 50 mm, and theP plane was 190 mm from the sample; hence theangle � between any two beams was about 14.8°. La-ser pulses of the three beams arrived at the samplesimultaneously, and the zero temporal delay pointwas determined by the signal of sum frequency via a�-barium borate (BBO) crystal.

Figure 2 shows the experimental results of three-beam interference for three types of polarization com-binations that are shown in the insets of Figs. 2(a),2(c), and 2(e), respectively. The 2D spots with a pe-riod of 3.6 �m formed hexagonal long-periodic micro-structures. Meanwhile, we surprisingly found peri-odic nanostructures on the microstructures. In Figs.2(a) and 2(b), the radially orientated nanorippleswere formed around each bulgy microspot. Besides,there were some nanoparticles formed on the six dotsaround a bulgy spot [see the white circle parts in Fig.2(b)]. The period of nanoripples and the diameter ofnanoparticles were about 150–200 nm. We rotated

Fig. 2. SEM images of the ablation areas induced by threelaser beams interference for three types of polarizationcombinations. The single laser beam parameters are (a)and (b) laser fluence F=0.14 J/cm2, repetition rate P=1 kHz, irradiation time t=0.03 s; (c) and (d) F=0.075 J/cm2, P=1 kHz, t=0.4 s; (e) and (f) F=0.05 J/cm2,P=10 Hz, t=4 s. The insets in (a), (c), and (e) show the po-

larization geometries of the three laser beams, respectively.

the three half-wave plates (HF2, HF3, and HF4) tochange the laser polarization by 90° and obtained thedifferent complex nanostructures shown in Figs. 2(c)and 2(d). The nanoripples were circularly distributedon each bulgy spot, and the nanoparticles also ap-peared on the same position. Figures 2(e) and 2(f)show the scanning electron microscope (SEM) imagesof the ablation area for parallel polarization of thethree laser beams. The short period of self-organizednanoripples was about 160 nm with an orientationperpendicular to the laser polarization [16,17]. Sur-face nanopatterning can improve the physical andthe chemical properties of some semiconductors[22,23] and biomaterials [24]. Through this methodwe can obtain periodic spots with nanostructuresthat periodically modify the material properties.

To understand the micro/nanopatterns describedabove, we calculated the interference patterns of thelaser intensity and the polarization. The three laserbeams were supposed as plane wave and expressedas Jones vector for different polarizations, Em�r�=E0m ·exp�i��t+kmrm+�0m��� cos �m

sin �m�, �m=A,B,C�,

where �m is the angle between the laser polarizationand the x axis. The three beams were normalized,and the initial phase �0m was set as zero. We con-ducted a vector synthesis of the three beams for a dif-ferent position on the interference area, E�r�=�mEm�r�. Consequently, we can calculate the distri-bution of the compound electric field and obtain thepatterns of polarization and intensity, I�r�= �E�r��2.

Figure 3 shows the experimental and the theoreti-cal results for the polarization combination shown inthe inset of Fig. 3(a). Figure 3(a) shows an SEM im-age of a 7 �m�7 �m ablation area. The laser condi-tions for one of the three laser beams are laser flu-ence F=0.13 J/cm2, repetition rate P=10 Hz, andirradiation time t=2 s. Figures 3(b)–3(d) were the

Fig. 3. (Color online) (a) SEM image of 7 �m�7 �m abla-tion area; (b)–(d) theoretical results of the patterns of lightintensity, polarization ellipticity, and polarization orienta-

tion, respectively.

790 OPTICS LETTERS / Vol. 34, No. 6 / March 15, 2009

theoretical calculation results for the same laser con-ditions as those in Fig. 3(a). Figure 3(b) is the inten-sity pattern with a period of 3.5 �m, which is in ac-cordance with the hexagonal long-periodic bulgyspots. The polarization was described by the polariza-tion ellipticity [Fig. 3(c)] and the polarization orien-tation [Fig. 3(d)]. Figure 3(c) shows the distributionof polarization ellipticity, where 0 represents the lin-ear polarization and 1 represents the circular one.The light field on the six dots around a bulgy mi-crospot are nearly circularly polarized, and nanopar-ticles formed on the corresponding positions [see thewhite circle parts of Fig. 3(a)] [18]. The light polar-ization is linear on the three symmetrical axes andelliptical elsewhere. The linearly polarized laserpulses can induce nanoripples [14–17] while ellipticalones induced short nanoripples [see Fig. 3(a)]. Theorientation of the nanoripples was usually perpen-dicular to the polarization or the major axis of ellipse.We calculated the polarization orientation denoted bythe angle � between the major axis of ellipse and thex axis [as shown in Fig. 3(d)]. On the vertical sym-metrical axes, the angles � are 0° (180° representsthe same polarization orientation). On the other twosymmetrical axes, they are 60° and 120°, respec-tively. The nanoripples formed there were all or-thogonal to the light polarization [see Fig. 3(a)]. Be-sides, the angle � rotated by 360° around a bulgymicrospot so that the radially orientated nanoripplesformed there. Theoretical and experimental resultsindicated clearly that the short-periodic nanopat-terns were determined by the interferential polariza-tion pattern.

Theoretical calculations for the polarization combi-nation shown in the inset of Figs. 2(a) and 2(c) werealso conducted, and the results explained well thecorresponding complex periodic micro/nanopatterns.

In summary, we carried out a three-beam interfer-ence experiment of 800 nm femtosecond laser pulsesand obtained complex micro/nanostructures on a6H-SiC crystal. The micro/nanostructures includedthe 2D long-periodic microspots and short-periodicnanoripples and nanoparticles. A theoretical calcula-tion of the interference patterns of the laser intensityand the polarization explained well the patterns ofcomplex periodic micro/nanostructures. Our experi-mental and theoretical results indicated that thelong-periodic micropatterns were determined by theinterferential intensity distribution and the short-periodic nanopatterns were determined by the inter-ferential polarization pattern. By adjusting the polar-ization of laser beams, we can change theinterferential intensity patterns and polarizationpatterns and obtain various types of complex micro/nanostructures, which have great potential applica-tions in the field of nanofabrication by a femtosecondlaser.

This work was supported by National Key Projectfor Basic Research of China (2006CB806006 and

2006CB921105), The Project of Shanghai Shuguang(07SG25), Shanghai Pujiang (08PJ1404800), ChineseNational Natural and Scientific Fundation(10874044), Ministry of Education of China (NCET-04-0420, 30800 and 20050269011), Shanghai PriorityAcademic Discipline (B408), and Shanghai MunicipalScience and Technology Commission (06DJ14008 and06QH14003).

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