Periodic nanoripples in the surface and subsurface layers in ZnO irradiated by femtosecond laser...
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Periodic nanoripples in the surface and subsurface layers in ZnO irradiated by femtosecond laser pulses X. Jia, 1 T. Q. Jia, 1, * Y. Zhang, 1 P. X. Xiong, 1 D. H. Feng, 1 Z. R. Sun, 1,3 J. R. Qiu, 2 and Z. Z. Xu 2,4 1 State Key Laboratory of Precision Spectroscopy, Department of Physics, East China Normal University, Shanghai 200062, China 2 State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Shanghai 201800, China 3 [email protected] 4 [email protected] * Corresponding author: [email protected] Received January 21, 2010; accepted February 26, 2010; posted March 15, 2010 (Doc. ID 123114); published April 15, 2010 We present the formation of periodic ripples in ZnO crystal irradiated by a wavelength-tunable femtosecond laser. The results indicate that in the surface thin layer, the periods change from 0.1 to with laser flu- ences and pulse numbers, and in the subsurface layer the periods are always /2n, where n is the refractive index. The formation processes and mechanisms are also discussed. © 2010 Optical Society of America OCIS codes: 320.7130, 220.4241, 110.4235. Compared with picosecond and nanosecond laser pulses, femtosecond pulse energy can be precisely and rapidly deposited in solid materials with fewer thermal effects [1,2]. Therefore, femtosecond lasers are widely used in microfabrications in transparent materials and metals [3–7]. An interesting phenom- enon is the formation of periodic nanoripples in semi- conductors and dielectrics irradiated by femtosecond laser pulses [4,7–13]. Surface periodic ripples with periods close to the laser wavelengths have been studied intensively over the past four decades. These long-period (LP) ripples were attributed to the interference between the inci- dent laser and the surface scattered light [14–16]. Recently, many references reported the formation of periodic nanoripples in semiconductors and dielec- trics irradiated by femtosecond laser pulses [17,18]. The nanoripple periods were much less than the la- ser wavelengths and thus were called short-period (SP) ripples. The periods of SP ripples in fused silica depended on the laser pulse energy and pulse num- bers, and they were caused by the interference be- tween the incident laser and plasma wave [4]. The formation of nanoripples in fused silica was further studied in [9], and it was attributed to the anisotropic local field enhancement in the nanoplasmas. LP and SP ripples have been observed in silicon surfaces ir- radiated by different laser fluences, and the ripples were induced by the capillary instability in the sur- face molten layer [10]. In this Letter, we report the formation of nan- oripples in ZnO crystal irradiated by femtosecond la- ser pulses at different wavelengths. We discuss in de- tail the formation processes of LP and SP ripples induced by laser pulses with different laser fluences and pulse numbers. The experiments were conducted with a regenera- tive amplified mode-locked Ti:sapphire femtosecond laser system (Legend Elite Coherent). It generated 800 nm, 50 fs laser pulses with pulse energy of 3.5 mJ. The 800 nm laser was used to stimulate an optical parametric amplifier system (TOPAS-A, Co- herent) to generate laser pulses for wavelengths tunable from 267 to 2600 nm. The pulse duration was slightly lengthened to the range of 60– 100 fs. We used commercially available ZnO crystal with sizes of 10 mm 10 mm 1 mm. The two surfaces were both optically polished with roughness less than 10 nm. The sample was mounted on a XYZ translation stage controlled by a computer. The laser beam was nor- mally focused on ZnO crystal surface. After irradia- tion, the sample was dipped in ethanol and cleaned for 10 min with an ultrasonic cleaner. The periodic ripples in the ablation spots were observed by scan- ning electron microscope (SEM). SEM images of ablation spots irradiated by 1300 nm laser pulses are shown in Fig. 1. For a laser fluence of 0.86 J/cm 2 , an ablation pit with a depth of tens of nanometers formed in the sample surface af- ter single-pulse irradiation. The successive laser pulses were intensely reflected and scattered in air by the ablation pit. The light field on the sample sur- face was modulated with periods nearly equal to the laser wavelength [15,16]. LP ripples began to emerge in the ablation pit after irradiation by 3–5 laser pulses. Figure 1(a) shows that regular LP ripples with period of 1150 nm were formed on the ablation spot after being irradiated by 20 laser pulses. After irradiation by 50 laser pulses, the surface thin layer was partly ablated away. LP ripples were split into two ripples, and SP ripples began to emerge in the subsurface layer [see Fig. 1(b)]. The surface thin layer was usually 100–1000 nm thick, and it de- pended on the laser fluences. It was totally ablated away after irradiation by 900 laser pulses, and regu- lar SP ripples with period of 340 nm formed in the subsurface layer [see Fig. 1(c)]. When laser fluences decreased to 0.19 J / cm 2 , Fig. 1(d) shows that several nanoripples with periods of 1248 OPTICS LETTERS / Vol. 35, No. 8 / April 15, 2010 0146-9592/10/081248-3/$15.00 © 2010 Optical Society of America
Transcript of Periodic nanoripples in the surface and subsurface layers in ZnO irradiated by femtosecond laser...
1248 OPTICS LETTERS / Vol. 35, No. 8 / April 15, 2010
Periodic nanoripples in the surface and subsurfacelayers in ZnO irradiated by femtosecond
laser pulses
X. Jia,1 T. Q. Jia,1,* Y. Zhang,1 P. X. Xiong,1 D. H. Feng,1 Z. R. Sun,1,3 J. R. Qiu,2 and Z. Z. Xu2,4
1State Key Laboratory of Precision Spectroscopy, Department of Physics, East China Normal University,Shanghai 200062, China
2State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics,Shanghai 201800, China3 [email protected] [email protected]
*Corresponding author: [email protected]
Received January 21, 2010; accepted February 26, 2010;posted March 15, 2010 (Doc. ID 123114); published April 15, 2010
We present the formation of periodic ripples in ZnO crystal irradiated by a wavelength-tunable femtosecondlaser. The results indicate that in the surface thin layer, the periods change from 0.1� to � with laser flu-ences and pulse numbers, and in the subsurface layer the periods are always � /2n, where n is the refractiveindex. The formation processes and mechanisms are also discussed. © 2010 Optical Society of America
OCIS codes: 320.7130, 220.4241, 110.4235.
Compared with picosecond and nanosecond laserpulses, femtosecond pulse energy can be preciselyand rapidly deposited in solid materials with fewerthermal effects [1,2]. Therefore, femtosecond lasersare widely used in microfabrications in transparentmaterials and metals [3–7]. An interesting phenom-enon is the formation of periodic nanoripples in semi-conductors and dielectrics irradiated by femtosecondlaser pulses [4,7–13].
Surface periodic ripples with periods close to thelaser wavelengths have been studied intensively overthe past four decades. These long-period (LP) rippleswere attributed to the interference between the inci-dent laser and the surface scattered light [14–16].Recently, many references reported the formation ofperiodic nanoripples in semiconductors and dielec-trics irradiated by femtosecond laser pulses [17,18].The nanoripple periods were much less than the la-ser wavelengths and thus were called short-period(SP) ripples. The periods of SP ripples in fused silicadepended on the laser pulse energy and pulse num-bers, and they were caused by the interference be-tween the incident laser and plasma wave [4]. Theformation of nanoripples in fused silica was furtherstudied in [9], and it was attributed to the anisotropiclocal field enhancement in the nanoplasmas. LP andSP ripples have been observed in silicon surfaces ir-radiated by different laser fluences, and the rippleswere induced by the capillary instability in the sur-face molten layer [10].
In this Letter, we report the formation of nan-oripples in ZnO crystal irradiated by femtosecond la-ser pulses at different wavelengths. We discuss in de-tail the formation processes of LP and SP ripplesinduced by laser pulses with different laser fluencesand pulse numbers.
The experiments were conducted with a regenera-tive amplified mode-locked Ti:sapphire femtosecondlaser system (Legend Elite Coherent). It generated
800 nm, 50 fs laser pulses with pulse energy of0146-9592/10/081248-3/$15.00 ©
3.5 mJ. The 800 nm laser was used to stimulate anoptical parametric amplifier system (TOPAS-A, Co-herent) to generate laser pulses for wavelengths �tunable from 267 to 2600 nm. The pulse duration wasslightly lengthened to the range of 60–100 fs. Weused commercially available ZnO crystal with sizes of10 mm�10 mm�1 mm. The two surfaces were bothoptically polished with roughness less than 10 nm.The sample was mounted on a XYZ translation stagecontrolled by a computer. The laser beam was nor-mally focused on ZnO crystal surface. After irradia-tion, the sample was dipped in ethanol and cleanedfor 10 min with an ultrasonic cleaner. The periodicripples in the ablation spots were observed by scan-ning electron microscope (SEM).
SEM images of ablation spots irradiated by1300 nm laser pulses are shown in Fig. 1. For a laserfluence of 0.86 J/cm2, an ablation pit with a depth oftens of nanometers formed in the sample surface af-ter single-pulse irradiation. The successive laserpulses were intensely reflected and scattered in airby the ablation pit. The light field on the sample sur-face was modulated with periods nearly equal to thelaser wavelength [15,16]. LP ripples began to emergein the ablation pit after irradiation by 3–5 laserpulses. Figure 1(a) shows that regular LP rippleswith period of 1150 nm were formed on the ablationspot after being irradiated by 20 laser pulses.
After irradiation by 50 laser pulses, the surfacethin layer was partly ablated away. LP ripples weresplit into two ripples, and SP ripples began to emergein the subsurface layer [see Fig. 1(b)]. The surfacethin layer was usually 100–1000 nm thick, and it de-pended on the laser fluences. It was totally ablatedaway after irradiation by 900 laser pulses, and regu-lar SP ripples with period of 340 nm formed in thesubsurface layer [see Fig. 1(c)].
When laser fluences decreased to 0.19 J/cm2, Fig.1(d) shows that several nanoripples with periods of
2010 Optical Society of America
April 15, 2010 / Vol. 35, No. 8 / OPTICS LETTERS 1249
120–240 nm were formed on the sites with surfacedefects, as the electrons in defect states are more eas-ily excited. The electron density wave evidently can-not modulate the laser field , but it can induce tinyripples. These tiny ripples are perpendicular to thelaser polarization for the longitudinal electron den-sity wave [4].
When the pulse number increased to 20,000, shal-low nanoripples formed over the whole ablation area[see Fig. 1(e)]. After being irradiated by 90,000 laserpulses with a fluence of 0.23 J/cm2, Fig. 1(f) showsthat the surface thin layer in the central part of ab-lation spot is peeled off, and regular SP ripples withperiods of 250 nm emerged in the subsurface layer.
The process of nanoripple formation was also stud-ied in ZnO irradiated by 800 nm laser pulses. Figures2(a) and 2(b) show the regular LP ripples and SPripples in the surface and subsurface layers after ir-radiation by 20 and 900 pulses, respectively. Whenthe laser fluence decreased to 0.27 J/cm2, tiny ripplesaround surface defects were observed after irradia-tion by 2000 laser pulses. After irradiation by 40,000pulses, Fig. 2(d) shows that regular SP ripplesformed in the subsurface layers. The process of nan-oripple formation induced by 800 nm laser pulses issimilar to that with the 1300 nm laser.
We further tuned the laser wavelengths to 650 and2100 nm and found the processes of nanoripple for-mation were both similar to the cases shown inFigs. 1 and 2. Figure 3 shows the dependences of na-
Fig. 1. (Color online) SEM images of ablation area irradi-ated by 1300 nm laser for (a) 0.86 J/cm2, 20 pulses;(b) 0.86 J/cm2, 50 pulses; (c) 0.86 J/cm2, 900 pulses;(d) 0.19 J/cm2, 2000 pulses; (e) 0.19 J/cm2, 20,000 pulses;and (f) 0.23 J/cm2, 90,000 pulses. The arrow in (d) showsthe laser polarization.
noripple periods � on laser pulse numbers, fluences,
and wavelengths � [4]. For lower laser fluences of0.26 J/cm2 �2150 nm� and 0.27 J/cm2 �800 nm�, nan-oripple periods increased from � /5n to � /2n with in-creasing laser pulses. However, for higher laser flu-ences of 0.86 J/cm2 �1300 nm�, 0.55 J/cm2 �800 nm�,and 0.6 J/cm2 �650 nm�, the periods decreased from �to � /2n. Here n is the refractive index of ZnO crystal.The periods trend toward the value of � /2n with in-creasing pulse number.
The results shown in Figs. 1–3 indicate clearly thatthe nanoripple periods in the surface thin layer de-pend greatly on the laser fluences and pulse num-bers. Periods change from the laser wavelength � to avalue much less than � /2n. However, when the sur-face thin layer is ablated away, the nanoripple peri-ods in the subsurface layer are always nearly equalto � /2n.
Figure 4 shows the surface structures irradiated by300 laser pulses at a wavelength of 1300 nm. Irregu-lar nanoripples were observed on the ablation spot,as shown in Fig. 4(a). After rubbing off the surfacethin layer with lens paper, we found that the periodic
Fig. 2. SEM images of ablation area irradiated by 800 nmlaser for (a) 0.55 J/cm2, 20 pulses; (b) 0.55 J/cm2, 900pulses; (c) 0.27 J/cm2, 2000 pulses; and (d) 0.27 J/cm2,40,000 pulses. The arrow in (b) shows the laserpolarization.
Fig. 3. (Color online) Dependences of nanoripple periodson laser pulse numbers, fluences, and wavelengths. Refrac-
tive index n is taken as 2 here.
1250 OPTICS LETTERS / Vol. 35, No. 8 / April 15, 2010
nanoripples in the subsurface layer were more regu-lar [shown in Fig. 4(b)]. These results further confirmthat the nanoripples in the surface thin layer and thesubsurface layer are different.
The processes of SP ripple formation in the subsur-face layer are very complex [4,7–13] and are brieflydiscussed here. The refracted and scattered lightcaused by nanoripples in the surface thin layer re-sults in the nonuniform distribution of the light fieldin the subsurface layer. In the sites with strongerlight, more laser energy is deposited by multiphotonabsorption, which leads to the change of dielectricconstants. The successive laser pulses are furtherscattered by the fluctuated dielectric constants. Thispositive feedback effect will rapidly increase the scat-tered light with the laser pulse irradiation and leadto the formation of a standing wave field. Thestanding-wave field of the scattered light plays animportant role in the formation of SP ripples in thesubsurface layer.
In summary, nanoripple formation is an accumula-tive process, and the initial ablation pit and ripplesgreatly influence the formation of regular LP and SPripples. The nanoripple periods are different in thesurface thin layer and subsurface layer. In the sur-face thin layer, they change within the range of0.1–1�, while in the subsurface layer, they are al-ways � /2n. In the future we will study experimen-tally and theoretically the mechanisms causing thedifference.
This work is supported by Shanghai LeadingAcademic Discipline Project (B408), National KeyProject for Basic Research of China (2006CB806006,2006CB921105 and 2010CB923203), NationalNatural Science Foundation of China (NSFC)(10874044 and 10904038), Ministry of Education ofChina (30809), and Shanghai Municipal Scienceand Technology Commission (09142200500,
Fig. 4. SEM images of nanoripples in (a) the surface th0.3 J/cm2 laser pulses.
09JC1404700, 08JC1408400, 09ZR1409300, and
08PJ1404800), and Twilight Project sponsored byShanghai Education Committee (07SG25).
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