Applied Surface Science 255 (2008) 2284–2289

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Applied Surface Science

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High throughput diffractive multi-beam femtosecond laser processing usinga spatial light modulator

Zheng Kuang a,*, Walter Perrie a, Jonathan Leach b, Martin Sharp a, Stuart P. Edwardson a,Miles Padgett b, Geoff Dearden a, Ken G. Watkins a

a Laser Group, Department of Engineering, University of Liverpool Brownlow Street, Liverpool L69 3GQ, UKb Department of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, UK

A R T I C L E I N F O

Article history:

Received 23 May 2008

Received in revised form 10 July 2008

Accepted 10 July 2008

Available online 25 July 2008

PACS:

42.40.Jv

42.62.Cf

81.20.Wk

Keywords:

Femtosecond laser

Spatial light modulator (SLM)

Computer generated holograms (CGH)

A B S T R A C T

High throughput femtosecond laser processing is demonstrated by creating multiple beams using a

spatial light modulator (SLM). The diffractive multi-beam patterns are modulated in real time by

computer generated holograms (CGHs), which can be calculated by appropriate algorithms. An

interactive LabVIEW program is adopted to generate the relevant CGHs. Optical efficiency at this stage is

shown to be �50% into first order beams and real time processing has been carried out at 50 Hz refresh

rate. Results obtained demonstrate high precision surface micro-structuring on silicon and Ti6Al4V with

throughput gain >1 order of magnitude.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction

Laser surface micro-structuring of materials (e.g. metals andsemi-conductors) with ultrashort laser pulses can demonstrate highprecision with little thermal damage due to the ultrafast timescaleon which energy is coupled to the electronic system [1–3]. In metals,this reduces heat diffusion during the temporal pulse length to thenanometer scale, comparable to the optical skin depth, provided thefluence F is kept in the low regime, where F < �1 J cm�2 [4–6].Consequently, the pulse energies used at low fluence for micro- andnano-structuring are often <10 mJ while high gain regenerativeamplifier systems running at n �1 kHz repetition rate provide mJlevel output pulse energies. The need to highly attenuate the outputbeam can have a severe limit on potential throughput. Thisrestriction may be one of the reasons why industrial uptake ofkHz femtosecond systems has been limited.

* Corresponding author. Tel.: +44 151 7944587.

E-mail address: z.kuang@liv.ac.uk (Z. Kuang).

0169-4332/$ – see front matter � 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2008.07.091

Spatial light modulators (SLM) are a dynamic diffractiveoptical element which can modulate the phase of an incomingwavefront. By applying a computer generated hologram (CGH)to the SLM, one can transform the incident beam intensitydistribution (e.g. Gaussian) into that desired for micro-manip-ulation, for example, in multi-trapping of micro-particles inoptical tweezers [7–12], or material micro-processing firstdemonstrated recently by Hayasaki et al. [13–16]. The abilityto address these devices in real time and synchronize withthe scanning method adds an additional flexibility for surfacemicro-structuring. Here, we demonstrate parallel processing ofa surface with arbitrary multiply diffracted spots through theapplication of CGHs.

2. Experimental

The experimental setup is shown in Fig. 1. The output from afemtosecond laser system (Clarke-MXR CPA2010, with 160 fspulsewidth, 775 nm central wavelength, 1 mJ pulse energy and1 kHz repetition rate) traverses a half wave-plate and glan laserpolariser and directed to a reflective SLM liquid crystal on silicon

Fig. 1. Experiment setup.

Fig. 2. (a) The interface of the LabVIEW program generated by Leach et al. [7] and (b) a CGH calculated by the program (left) and its computational reconstruction (right).

Z. Kuang et al. / Applied Surface Science 255 (2008) 2284–2289 2285

Fig. 2. (Continued ).

Z. Kuang et al. / Applied Surface Science 255 (2008) 2284–22892286

(LCoS) device with 1024 � 768 pixels (Holoeye LC-2500), orientedat 458 angle of incidence. A beam expanding telescope, withmagnification M � 2 helps to reduce the average intensity on theSLM. The diffracted spot pattern (zero and higher order) enter the10 mm aperture of a scanning galvo system and flat field lens withfocal length f = 100 mm. The proximity of the SLM to the inputaperture of the galvo ensures that diffracted beams are not clippedon the galvo mirrors and the flat field of the f-theta lens produces anear perfect focusing system. Substrates are mounted on aprecision 4-axis (x, y, z, u) motion control system (Aerotech)allowing accurate positioning of the substrate surface at the laserfocus. Appropriate holograms required to create arbitrary multi-beam patterns at the substrate surface are generated via an

Fig. 3. Optical images of two different multi-beam patterns. (Exposure time was�1 s, i.e.

pattern comprising 30 micro-sized holes.

interactive LabVIEW program using a number of algorithms tocalculate the CGHs [7]. Fig. 2(a) shows the interface of the LabVIEWprogram, and Fig. 2(b) is a CGH calculated by the program and itscomputational reconstruction.

3. Results and discussions

Fig. 3(a) shows optical micrographs of surface structuring onsilicon with an ‘LLEC’ pattern comprising 32 blind holes whileFig. 3(b) demonstrates a random spot pattern comprising 30 holes.Each hole pattern here was micro-machined simultaneously on thesilicon wafer by creating the spot patterns in LabVIEW thenapplying the calculated CGHs to create the desired geometries. The

1 k pulses.) (a) ‘LLEC’ pattern comprising 32 micro-sized holes and (b) random spots

Z. Kuang et al. / Applied Surface Science 255 (2008) 2284–2289 2287

incident pulse energy on the SLM was EP � 300 mJ. All diffractivespots have similar dimensions indicating accurate calculation ofthe CGHs using the ‘‘Grating and Lenses’’ algorithm [7,11]. Thelarge hole above is generated by the residual zero order reflectionhighlighting the loss of precision with excessive ultrashort pulseenergies.

In order to evaluate the pulse energy of each of the diffractedspots, we compared the size of hole produced to a calibrated seriesof drilled holes using a single beam of varying pulse energy. Whenusing the SLM, the diameter of each of the 30 diffractive spots wasmeasured to be �20 mm, which indicated that each of the spotshad a pulse energy of approximately 5 mJ. The total input pulseenergy was evaluated by measuring the power (to be �300 mJ)before the aperture of the scanning galvo. Consequently, thediffraction efficiency of the SLM is approximately 50%, which isfairly typical of commercially available units, especially when usedslightly away from their recommended operating wavelength (inthis case 400–700 nm)

Graphs of measured hole size and eccentricity versus distancefrom the zero order hole in both ‘LLEC’ and random spots patterns areshown in Fig. 4(a) and (b). The uniformity of the diffracted beams isshown by the measurements of ablated spot diameters using theWyko NT1100 optical surface profiler: F1 = 20.3� 1.2 mm (‘LLEC’pattern) and F2 = 21.7 � 1.1 mm (random spots pattern). Additionally,there is a modest increase in eccentricity with distance, which isprobably the result of the finite bandwidth of the laser source [16]. All

Fig. 4. Graphs of hole size and eccentricity vs. the distance from each diffracted spot to th

distance away from zeroth order hole and (b) eccentricity a/b vs. distance away from

diffractive optics suffer from the same limitation when used with abroadband source, namely for a given design of CGH, the shift of thefocused spot away from the zero order is proportional to thewavelength (l) of the spectral component. Accordingly, the spot iselongated by an amount equal to its fractional spectral bandwidthmultiplied by the number of equivalent grating line-pairs in the CGHdesign. For example, our 160 fs source has a fractional spectralbandwidth of approximately 0.7% (l = 775 nm, Dl = 5 nm). A spotshifted away from the zero order using a CGH with 25 line-pairs per mmwould result in an eccentricity of the diffraction limited focal spot ofapproximately 2:1, in agreement with the observed results. Lasersource bandwidth would therefore appear to set an upper limit on theuseful field of view of the system.

To avoid large variations in the required intensity distributionsbetween spots, it is advantageous to avoid patterns with a highdegree of symmetry, for example, straight lines, squares, or parallelgeometries [17].

While static holograms are useful and can be combinedwith the galvo system to demonstrate parallel processingwith fixed spot geometry, processing by real time control theCGHs is demonstrated in Fig. 5(a) and (b). Fig. 5(a) shows apattern comprising 121 holes completed by real time playingof 15 stored CGHs at 20 Hz refresh rate, and Fig. 5(b)demonstrates the formation of the pattern which was comple-ted within 0.75 s. The incident pulse energy on the SLM wasEP � 300 mJ.

e zero order hole in both ‘LLEC’ and random spots patterns. (a) Holes size (area) vs.

zeroth order hole.

Fig. 5. (a) Pattern completed by real time playing 15 series of CGHs at 20 Hz refresh rate (50 ms duration per CGH) and (b) the formation of the pattern.

Z. Kuang et al. / Applied Surface Science 255 (2008) 2284–22892288

By combining real time control of the CGHs with scanning,diffractive multi-beam processing has significant potential toproduce complex surface micro-machining patterns [18].Fig. 6(a) and (b) illustrates this on a polished Ti6Al4V substratewhere 6 micro-channels, a–f were generated by applying theappropriate CGHs at 50 Hz refresh rate while simultaneouslyscanning the diffracted spots at a speed of 1 mm/s. The resultingmicro-channels a–f are �40 mm wide and �10 mm deep.The large channel above, again is due to the zero order. Ti6Al4V,which is an metallic alloy widely used in aerospace andbioscience, has a relatively low ablation threshold (�0.1 J cm�2)[19] hence the advantages of diffractive multi-beam proce-

ssing are demonstrated as extensive attenuation was largelyavoided.

4. Conclusions

The results obtained demonstrate that precision diffractivemulti-beam ultrafast laser micro-structuring using an SLM is anovel method to increase processing throughput. Pulse energies upto 300 mJ at 1 kHz repetition rate have been diffracted into >30spots with 50% efficiency. This constitutes a throughput gain >1order of magnitude for precision micro-structuring tasks requiringmicro-joule level pulse energies. Future research will focus on full

Fig. 6. Micro-channels obtained by combining CGHs real time control with galvo

scanning. (a) Whole pattern view (5� objective); 6 channels a–f were generated by

the diffractive beams while the wider channel 0 above was formed by zero order

beam and (b) 3D larger zoom view by Wyko NT1100 optical surface profiler, where

�10 mm depth (10 times over scan) micro-channels are illustrated. (For

interpretation of the references to color in this artwork, the reader is referred to

the web version of the article.)

Z. Kuang et al. / Applied Surface Science 255 (2008) 2284–2289 2289

synchronizing action of hologram application with scanningtechniques hence producing more complex 2 and 3D structures.The zero order reflected light creates unwanted surface featuresbut can be removed by modifying the optical setup to include atelescope with unity magnification and blocking this component atthe Fourier plane.

Acknowledgements

The authors gratefully acknowledge the support of the NorthWest Development Agency (NWDA) in the UK, and the author

Zheng Kuang would like to thank the scholarship of OverseasResearch Students Award Scheme (ORSAS) funded by the UKGovernment which financially supports his PhD study at theUniversity of Liverpool.

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