ARTICLE IN PRESS

Optics and Lasers in Engineering 48 (2010) 940–949

Contents lists available at ScienceDirect

Optics and Lasers in Engineering

0143-81

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/optlaseng

Review

Self-organized micro/nano structures in metal surfaces byultrafast laser irradiation

Barada K. Nayak, Mool C. Gupta n

Department of Electrical & Computer Engineering, University of Virginia, Charlottesville, VA 22904, USA

a r t i c l e i n f o

Article history:

Received 26 February 2010

Received in revised form

25 April 2010

Accepted 26 April 2010Available online 8 June 2010

Keywords:

Ultrafast laser

Micro/nano structure

Laser processing

66/$ - see front matter & 2010 Elsevier Ltd. A

016/j.optlaseng.2010.04.010

esponding author. Tel.: +1 434 924 6167; fax

ail address: mgupta@virginia.edu (M.C. Gupta

a b s t r a c t

Ultrafast laser pulse interaction with matter, leading to formation of self-organized conical micro/nano

structures in various metals like Ti, Al, Cu, and stainless steel have been observed. Influence of laser

parameters such as fluence, number of shots, and gaseous environments on micro/nano structure

formation have been investigated. The critical fluence required for well-developed structure formation

is dependent on the optical and thermo-physical properties of the materials. By changing the number of

laser shots to generate micro/nano structures, surface reflectance of Ti surface could be tailored from

their original value (over 50%) to near zero over the wavelength range of 500–1000 nm. Also, we

have demonstrated that arrays of micro/nano holes could be formed in thin Ti foils by direct laser

treatment.

& 2010 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 940

2. Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 941

3. Results and discussion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 942

4. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 947

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 948

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 948

1. Introduction

Texturing surfaces to create micro/nanoscale surfacestructures has tremendous technological importance [1–19]. Theenhancements in surface area, resulting from formation of varioustypes of micro/nano structures have important applications in thefollowing broad areas of research and developments: (a) Biome-dical: improving cell growth and proliferation [1–9]; (b) Optical:improving light trapping properties [10]; (c) Surface character-istics: altering the surface wetting properties to make themsuperhydrophobic for self-cleaning applications [11–13];(d) Photovoltaics and light detection: efficient light trapping overa broad spectrum and improving photo responsivity [14–17];(e) Field emission: electron emission due to high field at the tipsof the microstructures [18]; (f) Tribology: obtaining desiredtribological properties of surfaces [19].

ll rights reserved.

: +1 434 924 8818.

).

Texturing of metal surfaces has been carried out using varioustechniques such as grit-blasting [2], chemical etching [3],electrochemical treatment [5], and laser treatment [4,6–9,12,13,20–23], etc. Studies have shown that laser processinghave advantage over other techniques in terms of morereproducibility and less contamination [4] due to its non-contactprocess. Surface texturing in metals has been reportedusing nanosecond Nd:YAG and excimer lasers [6,7], andpicosecond Nd:YAG and sub-picosecond excimer lasers [7].However, the structures formed by these lasers do not exhibitwell-defined features. Recently, femtosecond lasers havebecome an advanced tool for material processing and haveadvantages over nanosecond lasers in terms of high precision,reduced heat-affected zone, and minimum burr formation [24].More recently, we have demonstrated that femtosecondlaser pulses could be used to produce self-organized regulararrays of nano-textured conical microstructures in titaniumsurfaces [23].

In this work, we report the formation of micro/nano structuresin various metals and alloys of industrial importance. We haveinvestigated in detail the dependencies of micro/nano structures

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B.K. Nayak, M.C. Gupta / Optics and Lasers in Engineering 48 (2010) 940–949 941

on the experimental conditions such as laser fluence, number oflaser shots, gaseous environments. A possible mechanism re-sponsible for such micro/nano-structure formations has beendiscussed. We have shown that the surface reflectance of thesemetals and alloys could be controlled from their original value tonear zero by ultrafast laser texturing process. Also, we havedemonstrated that arrays of micro/nano holes could be formed inthin Ti foils by direct laser treatment.

2. Experimental

The titanium, aluminum, copper, and stainless steel sheets of99.6% purity are diced in to small square pieces of 1 cm2 in sizeand thickness in the range of 0.2–0.5 mm. The initial surface finishis as supplied (www.Goodfellow.com and shim stocks from Lyon

Fig. 1. SEM image of: (a) laser treated surface showing the formation of micro/nano s

1.2 J/cm2. Inset is the photograph of a control (gray color) and micro-structured tit

approximately 2000 shots showing the self-organized formation of micro/nano struct

ripples formed on the conical microstructures.

Industries) with no further polishing performed. The diced metalpieces are cleaned ultrasonically with acetone followed bymethanol. One such metal piece is then placed on a stage insidea vacuum chamber (base pressure �1 Torr) mounted on a highprecision computer controlled X–Y stage. The experiments arecarried out in vacuum (pressure �1 Torr), air or in the presence ofdifferent gases like He and SF6. The samples are exposed to �1 mJpulses of 800 nm wavelength and 130 fs pulse duration at arepletion rate of 1 KHz from a regeneratively amplified SpectraPhysics Ti-sapphire laser system. The laser beam is focused to aspot size of 200–800 mm along the normal onto the samplesurface by coated lenses of different focal lengths. The laserfluence is adjusted by using a Glan laser calcite polarizer. Thespatial profile of the laser pulse is nearly Gaussian and the fluenceis calculated using the spot size determined by exposing a pointon the sample surface to thousands of shots. The experiments are

tructures after exposing Ti surface with approximately 500 laser shots at fluence

anium piece (black in color), (b) laser-irradiated spot after being exposed with

ures, (c) cross-section of an individual conical microstructure and (d) nanoscale

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carried out either by single spot exposure or by making line scans.In order to scan an area bigger than the laser spot size, thesamples are translated using a computer controlled motorizedX–Y stage. By varying the scanning speed of the X–Y stage, theaverage number of laser pulses impinging on the sample surfaceat a particular spot is controlled.

After laser processing, the samples are analyzed for surfacemorphology using a scanning electron microscope (SEM) (ZeissSUPRA 40). Microstructure height variation measurements arecarried out using Dektak profilometer (from Vecco) and surfacenanostructures are characterized using a Digital InstrumentsNanoscope III atomic force microscope (AFM). Surface reflectancemeasurements on laser treated samples are carried out using anintegrating sphere.

Fig. 2. (a) AFM image of ripples on the titanium surface following 125 laser shots

at laser fluence 1 J/cm2 and (b) image of ripple profile (a) ripple height measured

with AFM.

3. Results and discussion

Fig. 1(a) shows the SEM image of a Ti surface after exposure tofemtosecond laser pulses viewed at an angle of 451 fromthe normal. The conical microstructures are formed when thesample is scanned under the laser beam (500 laser shots atfluence 1.2 J/cm2) in air. As evident from the image, thesestructures are nearly regular in position, 15–20 mm tall, andhave a base diameter in the range of 5–10 mm tapering down tofew microns at the tip. From the areal density calculation ofmicrostructures using an image processing program reveals themean spacing between the microstructures are about 13 mm forFig. 1(a). Fig. 1(b) shows the SEM image of a spot on the Ti surfacethat is exposed to around 2000 laser shots. It is clear fromthe image that micro/nano structures are self-organized insidethe irradiated spot. In order to investigate the sub-surfacemorphology of the micro/nano structures, cross-sectional SEMimaging is carried out. As shown in the Fig. 1(c), the morphologyresembles the initial structured material and no significant sub-surface laser damage is observed. A close look on the surfacemorphology reveals the formation of nanostructures (surfaceripples of wavelength �500 nm) over the microstructures asshown in the Fig. 1(d).

Laser-induced surface ripples and formation of conical micro/nano structures in semiconductors upon femtosecond andnanosecond laser irradiation have been extensively investigatedin the past [25–31]. Typically the observed ripple periods are ofthe order of incident laser beam wavelength and formationmechanisms of such surface ripples has been attributed tothe interference mechanism involving the incident and ascattered or stimulated wave at the surface [25–27]. However,observation of ripple wavelengths significantly higher or lowercompared to incident laser wavelength has lead to the investiga-tion of other possible mechanisms such as capillary waves [32], orinterference of surface plasmons and incident laser radiation [33].We have observed that for metallic targets, the surfaceripple wavelengths are smaller compared to the incidentradiation wavelength (800 nm) [see Fig. 2(a) and (b) forsurface topography analysis using AFM]; whereas forsemiconductors, our observation of ripple wavelength istypically in the order of wavelength of incident radiation. Onthe other hand, formation of conical microstructure formationoccurs with successive impingement of laser shots on targetsurface.

Our observations indicate that metal surfaces first undergonano-roughness the surface is covered with conical micro/nanostructures. For quantitative measurement of these nanoscalesurface features, AFM analysis is performed on laser treated Tisurfaces. Fig. 2(a) and (b) show respectively the AFM image of thenanoscale roughened surface and their height variations. It is clear

from these measurements that the heights of these nanoscalestructures are in the range of 50–100 nm. The period of thesenanoscale structures is dependent on several factors and warrantsa detailed investigation. Usually they are in the order of the laserwavelength for silicon and little smaller in metals under normallaser fluence condition (the fluence at which regular microconestructure formation occurs). However, we have observed that

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towards the edge of the laser spot, where fluence decreasesbecause of the Gaussian nature of our beam, the periodicity ofthese nanoscale roughness decreases. For example, in titanium,the period of nanoscale roughness is about 650 nm in the centralregion of spot. However towards the edge of the laser pulse, theperiod of nanosale roughness goes down to �400 nm. Furtherinvestigation is underway to understand the exact mechanismbehind conical micro/nano-structure formation and their self-organization.

Fig. 3 shows the laser fluence dependence of structureformation in Ti. The average number of laser shots in Fig. 3(a–j)is around 450. It is clear from the figure that microstructuredensity decreases with increasing laser fluence. Also, themicrostructures height and base width becomes larger for

Fig. 3. Formation of micro/nano structures with increasing laser fluence in J/cm2

for (a) 0.3 (b) 0.4 (c) 0.5 (d) 0.6 (e) 0.7 (f) 0.8 (g) 0.9 (h) 1 (i) 1.1 (j) 1.2. The number

of laser shots for all cases are kept at 450. Scale is 20 mm.

higher fluence conditions compared to low fluence conditions. Itis evident from the figure that conical microstructure formationstarts occurring for fluence around 0.5 J/cm2 (Fig. 1(c)). Forfluence below 0.5 J/cm2, the surface gets roughened, surfaceripples are generated, and except few sporadic microstructures,well-developed conical pillar formation does not occur. For pulseduration of 150 fs, laser multi-shot ablation threshold fortitanium is about 0.1 J/cm2 and ablation rate is about 50 nm/pulse at fluence 1 J/cm2 [34]. From Fig. 3(g–j), we see that whenthe fluence is about 1 J/cm2, the micro/nano-structure heights areabout 20 mm for about 450 laser shots, consistent with literature.However, in low fluence regime (fluenceo0.5 J/cm2), near regularstructure formation does not occur even with higher number oflaser shots (41500) exposures. It is possible that there is a criticalfluence necessary for conical pillar formation to occur and itsvalue is relatively higher from the ablation threshold.

Fig. 4(a) shows the variation of micro/nano-structure heightwith respect to the laser fluence for a given number of laser shots(5 0 0). The result shows a linear relationship betweenmicrostructure heights to laser fluence. While most of themicrostructure heights (maximum peak above mean line) are

0.20

5

10

15

20

25

30

35RMS meanMax peak above mean lineMax peak to max valley

Mic

rost

ruct

ure

heig

ht (µ

m)

0.44

8

12

16

20

24

Inte

r-co

ne d

ista

nce

(mic

ron)

Fluence (J/cm2)0.4 0.6 0.8 1.0 1.2

Fluence (J/cm2)0.6 0.8 1.0 1.2

Fig. 4. (a) Microstructure height variation with laser fluence. Approximate

number of laser shots is fixed to 500. Linear data fitting is shown by solid straight

lines and (b) dependence of inter-cone distance on laser fluence.

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200

0

10

20

30

40

50 RMS meanMax peak above mean lineMax peak to max valley

Number of laser shots

Mic

rost

ruct

ure

heig

ht (µ

m)

400 600 800 1000

Fig. 5. Microstructure height variation with number of laser shots. Laser fluence is

kept fixed at 1 J/cm2. Polynomial data fitting is shown by solid lines.

Fig. 6. Effect of gas environment on micro/nano structuring of stainless steel surface. S

normal. Laser fluence is 1.2 J/cm2, number of laser shots for each case is 500 and gas p

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consistent with the ablation rate (discussed above) but it showsthat there are some microstructures whose heights are muchhigher from the valleys (maximum peak to maximum valley).Also, RMS values of the microstructure heights are lowercompared to both ‘‘maximum peak above mean line’’ and‘‘maximum peak to maximum valley’’ indicating that all themicrostructures that are formed for a given experimentalcondition, do not have equal heights. The spacing between theconical microstructure (inter-cone distance) is observed to bedependent on the laser fluence. The mean inter-cone distance as afunction of laser fluence is shown in the Fig. 4(b). It is evidentfrom the figure that there are three regions of interest: (a) At lowfluence regime (0.5–0.7 J/cm2) the inter-cone distance increaseslinearly but slowly (b) At moderately higher fluence regimes(0.7–1.0 J/cm2), the inter-cone separation rapidly increases and(c) at high fluence regime (1.0–1.2 J/cm2) the inter-coneseparation does not seem varying much further with fluence.

We also examined the effect of number of laser shots onmicrostructure height for a given laser fluence (1 J/cm2) (seeFig. 5). As the number of laser shots increases, the RMS value,‘‘maximum peak above mean line’’ and ‘‘maximum peak to

EM images of laser structured stainless steel surface viewed 451 from the surface

ressure is 100 mbar. (a) He (b) SF6 (c) air and (d) vacuum (�1 mTorr).

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maximum valley’’ all goes up linearly until 500 shots. RMS valueand ‘‘maximum peak above mean line’’ flattens out after 500 shotsindicating that further exposure removes material uniformly.However, ‘‘maximum peak to maximum valley’’ keeps going upindicating some random occurrence of tall conicalmicrostructures and we have observed few such sporadic tallmicrostructures on the surfaces.

We have also studied the micro/nano-structure formation instainless steel. Fig. 6 shows the formation of conicalmicrostructures in stainless steel surface after irradiated withfemtosecond laser pulses in different gaseous environments. Themicrostructures are quasi regular in position. The pillars formedin SF6 shows tallest structure. Unlike Ti, however, theusual structure height is shorter in steel under similarconditions indicating less aggressive laser etching in case ofthe later. In our previous communication [23], we studied theeffect of gaseous environments on texture formation in Ti andfound that while texturing could occur in the presence of He, airor vacuum, the presence of SF6 was undesirable for textureformation in titanium. However in case of stainless steel,the presence of SF6 seems favorable to texture formation(Fig. 6(b)). It is possible that the reaction mechanism ofFe/F system is more favorable compared to Ti/F system.The pillars formed in SF6 are slim and taller (�20 mm)

Fig. 7. Effect of number of laser shots on the micro/nano structure formation. SEM im

Laser fluence is 1.2 J/cm2, the background is 100 mbar of air and number of laser shot

compared to other gaseous environments where pillar heightsare in the range of 10–15 mm. Although, He, SF6, air and vacuumproduces pillars, but in case of vacuum the structures are not asregular and pillar shape and size homogeneity is poor. Fig. 7shows the morphology evolution with number of laser shorts forstainless steel. It is clear from the figure that for a givenexperimental conditions, the pillar height and separationincreases with number of laser shots and pillar densitydecreases with it.

Next we tried texturing Al and Cu. Fig. 8 shows the micro/nano-structured Al and Cu surfaces prepared in air with 600 lasershots at a fluence of 0.16 J/cm2. Since the thermo-physicalproperties of Al and Cu are very different from Ti and stainlesssteel, we find that for fluence above 0.16 J/cm2 no textureformation is observed in Al due to excessive heat generationthat smears out the structures. On the other hand, under similarexperimental conditions, only rippled structure is observed for Cu(see Fig. 8 (b)). Although the ablation threshold and ablation ratesfor Al and Cu are very close (ablation threshold (Fth)¼0.105 J/cm2

and ablation rate of 2.2 nm/pulse for Al; Fth¼0.125 J/cm2 andablation rate of 2.5 nm/pulse for Cu) [35], but the reason why Cusurface did not develop microstructures could be due to thedifference in thermo-physical and optical properties of bothmetals. For instance, the melting temperature of Cu is about

ages of laser textured stainless steel surface viewed 451 from the surface normal.

s (a) 500 (b) 250 (c) 125 (d) 50.

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Fig. 8. Laser micro/nano structured surfaces prepared in air at a fluence of 0.16 J/cm2 and number of laser pulses �600 (a) Aluminum (b) Copper.

B.K. Nayak, M.C. Gupta / Optics and Lasers in Engineering 48 (2010) 940–949946

400 1C higher compared to Al. Therefore, it seems possible tocreate micro/nano structures in Cu by using higher laser fluenceconditions.

It is interesting to note here that femtosecond laser texturedmetal surfaces look complete black to bare eyes compared to theiroriginal luster. Inset in Fig. 1(a) shows a laser textured Ti piece(looks black) along with a control piece (silver grayish in color).So, we measured the optical reflectance as a functionof wavelength of laser-induced micro/nano-structured surfaces.Fig. 9 shows the total reflectance as a function of number oflaser shots for Ti samples prepared under different numberof laser shots while keeping the fluence fixed at 1 J/cm2. It isclear from Fig. 9 that it is possible to control the surfacereflectivity of Ti surfaces from its original values to near zero.

This remarkable light trapping properties of these metal surfacescould find many applications from photovoltaics to artificialcolorization.

Many applications, such as fluid sterilization, cell encapsula-tion, and drug delivery, require arrays of through poreswith nanometer dimensions. Various other techniques suchas ion beam lithography, anodization of aluminum, ionetching of polycarbonate films, focused ion beam drilling ofsilicon nitride, etc have been demonstrated over time butthey have their practical shortcomings like lack of robustness,fabrication time, and high device cost. Here we demonstratea simple, yet relatively fast process using ultrafast lasertreatment of titanium surface to produce arrays of nano/micropores.

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5000

10

20

30

40

50

60

N =1000

N =500

N =330

N =250

N =125

N =0

Tota

l Ref

lect

ion

(%)

Wavelength (nm)550 600 650 700 750 800 850 900 950 1000

Fig. 9. Controlling surface reflectivity of Ti surface by changing the average number of laser shots used to fabricate micro/nano structures. Laser fluence is kept fixed at 1 J/

cm2. In the figure, N stands for number of laser shots.

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Fig. 10(a) shows an SEM image of a titanium foil of thickness50 mm treated at laser fluence of �1.2 J/cm2 and scan speed isadjusted such that every spot receives about 1000 laser shots.Large area sample is prepared by raster scanning and about 25%overlapping between laser lines are maintained. It is clear fromthe figure that arrays of nano/micro pores have been formed onthe titanium surface. Fig. 10(b) is the higher magnification imageof Fig. 10(a) shows that the pores are of different diameters andthey are quite arbitrarily located. While some pores arefew hundred of nanometers wide (as shown in Fig. 10(c)), someothers are in the range of few microns. From Fig. 5 it is evidentthat for higher number of laser shots (in the �1000), themaximum peak to maximum valleys are around 50 mm.Since the titanium foil thickness in theses experiments are50 mm, therefore, there is always some probabilities that atsome spots the processing would produce pores. While amore controllable experimental window could be possible toreduce the hole diameters further and obtaining more uniformityamong the pore diameters, these experiments demonstratesan easy fabrication process of high aspect ratio holes inmetal foils like titanium and other materials. Alternatively,more uniform texture can be obtained with lower height andpores can be generated through chemical etching of texturedsurface.

Exact mechanism behind ultrafast laser-induced self-orga-nized conical micro/nano-structure formation in metals is not yetfully understood. Our observations indicate that ripple-likestructure formation is an ongoing process from early stages ofmicrostructure evolution and appear on the side surfacesof the fully developed conical microstructures. Therefore,initially formed ripples somehow get broken down during themicrostructure evolution to form self-organized conicalmicro/nano structures. Observation of ripples on various materialsurfaces upon laser (CW, nanosecond, and ultrashort pulse) radiation is a fascinating subject that has beenextensively studied in the past and also being investigated

today [26,27,23,36–42]. The ripple periods could be nearlyequal [26], or substantially smaller to laser wavelengths [23,37].Three main possible mechanisms have been proposed toexplain ripples: (a) optical effect (interference between incidentand scattered laser light at the surface) (b) capillarywave formation (hydrodynamic effects) (c) plasmonic effect(intereference between surface plasmons and incident laser).While an extensive study has been conducted to understandthe mechanism behind ripple formations, a further investigationis necessary to understand the exact mechanism behindconical micro/nano-structure formation in metals. The data anddiscussion provided in this study will be highly useful indevelopment of a mechanism for conical micro/nano-structureformation.

4. Conclusions

In conclusion, we report the formation of self-organized micro/nano structures due to ultrafast laser interaction with differentmetal surfaces. We have shown that ultrafast laser pulseinteraction can produce self-organized conical micro/nano struc-tures in various metals such as Ti, Al, and stainless steel.The thermophysical and optical properties of materials determinethe experimental conditions such as laser fluence, number oflaser shots, and gaseous environments necessary for formationof these conical micro/nano structures. We demonstrate thatreflectivity of metals can be controlled from their original luster tovery low reflectivity (near zero) by suitable experimentalconditions. Finally, we show that arrays of nano/micro porescould be produced on Ti foils by direct laser processing technique.This simple metal conical micro/nano structuring method couldopen up new areas of research and development activity inbiomedical, microelectronics, photovoltaics and other industrialapplications.

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Fig. 10. (a) Arrays of micro/nano pores formed in thin Ti foil by direct laser treatment; (b) higher magnification image of (a); and (c) image showing a single hole. Laser

fluence is 1.2 J/cm2 and approximate number of laser shots is 1000.

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Acknowledgements

We gratefully acknowledge the financial support of theNational Science Foundation under grant ECS-0100243, and NSFI/UCRC center grant. Also, financial support through NASA/NIALangley Professor award is highly appreciated.

References

[1] Zinger O, Zhao G, Schwartz Z, Simpson J, Wieland M, Landolt D, et al.Differential regulation of osteoblasts by substrate microstructural features.Biomaterials 2005;26:1837–47.

[2] Wennerberg A, Albrektsson T, Johansson C, Andersson B. Experimental studyof turned and grit-blasted screw-shaped implants with special emphasis oneffects of blasting material and surface topography. Biomaterials1996;17:15–22.

[3] Bathomarco R, Solorzano G, Elias C, Prioli R. Atomic force microscopy analysisof different surface treatments of Ti dental implant surfaces. Appl Surf Sci2004;233:29–34.

[4] Gaggl A, Schultes G, Muller W, Karcher H. Scanning electron microscopicalanalysis of laser-treated titanium implant surfaces—a comparative study.Biomaterials 2000;21:1067–73.

[5] Madore C, Piotrowski O, Landolt D. Through-mask electrochemical micro-machining of titanium. J Electrochem Soc 1999;146:2526–32.

[6] Peto G, Karacs A, Paszti Z, Guczi L, Divinyi T, Joob A. Surface treatment ofscrew shaped titanium dental implants by high intensity laser pulses. ApplSurf Sci 2002;186:7–13.

[7] Bereznai M, Pelsoczi I, Toth Z, Turzo K, Radnai M, Bor Z, et al. Surfacemodifications induced by ns and sub-ps excimer laser pulses on titaniumimplant material. Biomaterials 2003;24:4197–203.

[8] Stratakis E, Ranellaa A, Farsaria M, Fotakis C. Laser-based micro/nanoengi-neering for biological applications. Prog Quantum Electron 2009;33:127–63.

[9] Joshua R. Bush, Barada K. Nayak, Lakshmi S. Nair, Mool C. Gupta, Cato T.Laurencin, Improved bio-implant using ultrafast laser induced self-assemblednanotexture in titanium. Journal of Biomedical Materials Research: AppliedBiomaterials. Submitted for publication.

[10] Wu C, Crouch H, Zhao L, Carey J, Younkin R, Levinson J, et al. Near-unitybelow-band gap absorption by microstructured silicon. Appl Phys Lett2001;78:1850–2.

[11] Zorba V, Stratakis E, Barberoglou M, Spanakis E, Tzanetakis P, Anastasiadis S,et al. Biomimetic artificial surfaces quantitatively reproduce the waterrepellency of a lotus leaf. Adv Mater 2008;20:4049–54.

[12] Kietzig A, Hatzikiriakos S, Englezos P. Patterned superhydrophobic metallicsurfaces. Langmuir 2009;25:4821–7.

[13] Baldacchini T, Carey J, Zhou M, Mazur E. Superhydrophobic surfaces preparedby microstructuring of silicon using a femtosecond laser. Langmuir2006;22:4917–9.

[14] Nayak B, Iyengar V, Gupta M. Ultrafast laser textured silicon solar cells. MaterRes Soc Symp Proc 2009;1123:P07–09.

[15] Nayak B, Iyengar V and Gupta M, Efficient light trapping in silicon solar cellsby ultrafast-laser-induced self-assembled micro/nano structures, Progress inPhotovoltaics: Research and Applications. submitted for publication.

[16] Richard M, Richard F, Karger A, Carey J, Mazur E. Enhancing near-infraredavalanche photodiode performance by femtosecond laser microstructuring.Appl Opt 2006;45:8825–31.

[17] Carey J, Crouch C, Shen M, Mazur E. Visible and near-infrared responsivity offemtosecond-laser microstructured silicon photodiodes. Opt Lett 2005;30:1773–5.

ARTICLE IN PRESS

B.K. Nayak, M.C. Gupta / Optics and Lasers in Engineering 48 (2010) 940–949 949

[18] Zorba V, Tzanetakis P, Fotakis C, Spanakis E, Stratakis E, Papazoglou D, et al.Silicon electron emitters fabricated by ultraviolet laser pulses. Appl Phys Lett2006;88:081103–5.

[19] Li J, Xiong D, Wu H, Huang J, Dai J, Tyagi R. Tribological properties of lasersurface texturing and molybdenizing Duplex-Treated Ni-Base alloy. TribolTrans 2010;53:195–202.

[20] Dolgaev S, Fern�andez-pradas J, Morenza J, Serra P, Shafeev G. Growth of largemicrocones in steel under multipulsed Nd:YAG laser irradiation. Appl Phys A2006;83:417–20.

[21] Sano T, Yanai M, Ohmura E, Nomura Y, Miyamoto I, Hirose A, et al.Femtosecond laser fabrication of microspike-arrays on tungsten surface. ApplSurf Sci 2005;247:340–6.

[22] Voronov V, Dolgaev S, Lavrishchev S, Lyalin A, Simakin A, Shafeev G.Formation of conic microstructures upon pulsed laser evaporation of solids.Quantum Electron 2000;30:710–4.

[23] Nayak B, Gupta M, Kolasinski K. Formation of nano-textured conicalmicrostructures in titanium metal surface by femtosecond laser irradiation.Appl Phys A 2008;90:399–402.

[24] Chichkov B, Momma C, Nolte S, von Alvensleben F, Tunnermann A.Femtosecond, picosecond and nanosecond laser ablation of solids. Appl PhysA 1996;63:109–15.

[25] Guosheng Z, Fauchet P, Siegman A. Growth of spontaneous periodic surfacestructures on solids during laser illumination. Phys Rev B 1982;26:5366–81.

[26] Young J, Preston J, van Driel H, Sipe J. Laser-induced periodic surface structure.II. Experiments on Ge, Si, Al, and brass. Phys Rev B 1983;27:1155–72.

[27] Siegman A, Fauchet P. Stimulated wood’s anomalies on laser-illuminatedsurfaces. IEEE J Quantum Electron 1986;22:1384–403.

[28] Her T-H, Finlay R, Wu C, Deliwala S, Mazur E. Microstructuring of silicon withfemtosecond laser pulses. Appl Phys Lett 1998;73:1673–5.

[29] Pedraza A, Fowlkes J, Lowndes D. Silicon microcolumn arrays grown bynanosecond pulsed-excimer laser irradiation. Appl Phys Lett 1999;74:2322–4.

[30] Nayak B, Gupta M, Kolasinski K. Ultrafast-laser-assisted chemical restructur-ing of silicon and germanium surfaces. Appl Surf Sci 2007;253:6580–3.

[31] Nayak B, Gupta M, Kolasinski K. Spontaneous formation of nanospikedmicrostructures in germanium by femtosecond laser irradiation. Nanotech-nology 2007;18:195302.

[32] Gorodetsky G, Kanicki J, Kazyaka T, Melcher R. Far UV pulsed laser melting ofsilicon. Appl Phys Lett 1985;46:547–9.

[33] Huang M, Zhao F, Cheng Y, Xu N, Xu Z. Origin of laser-induced near-subwavelength ripples: interference between surface plasmons and incidentlaser. ACS Nano 2009;3:4062–70.

[34] Mannion P, Magee J, Coyne E, O’Connor G. Ablation threshold in ultrafastlaser micro-machining of common metals in air. Proc SPIE 2003;4876:470–5.

[35] Kim M, Takao T, Oki Y, Maeda M. Thin-layer ablation of metals and silicon byfemtosecond laser pulses for application to surface analysis. Jpn J Appl Phys2000;39:6277–80.

[36] Pedraja A, Fowlkes J, Jesse S, Mao C, Lowndes D. Surface micro-structuring ofsilicon by excimer-laser irradiation in reactive atmospheres. Appl Surf Sci2000;168:251–7.

[37] Crawford T, Haugen H. Sub-wavelength surface structures on siliconirradiated by femtosecond laser pulses at 1300 and 2100 nm wavelengths.Appl Surf Sci 2007;253:4970–7.

[38] Huang Y, Liu S, Li W, Liu Y, Yang W. Two-dimensional periodic structureinduced by single-beam femtosecond laser pulses irradiating titanium. OptExpress 2009;17:20759.

[39] Tsukamotoa M, Asukab K, Nakanoc H, Hashidad M, Kattoe M, Abea N, et al.Periodic microstructures produced by femtosecond laser irradiation ontitanium plate. Vacuum 2006;80:1346–50.

[40] Vorobyev A, Guo C. Femtosecond laser structuring of titanium implants. ApplSurf Sci 2007;30:7272–80.

[41] Oliveira V, Ausset S, Vilar R. Surface micro/nanostructuring of titanium understationary and non-stationary femtosecond laser irradiation. Appl Surf Sci2009;255:7556–60.

[42] Sakabe S, Hashida M, Tokita S, Namba S, Okamuro K. Mechanism for self-formation of periodic grating structures on a metal surface by a femtosecondlaser pulse. Phys Rev B 2009;79:033404–9.