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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 18 — Sep. 9, 2013
  • pp: 21198–21207
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Complete wavefront and polarization control for ultrashort-pulse laser microprocessing

O J Allegre, Y Jin, W Perrie, J Ouyang, E Fearon, S P Edwardson, and G Dearden  »View Author Affiliations


Optics Express, Vol. 21, Issue 18, pp. 21198-21207 (2013)
http://dx.doi.org/10.1364/OE.21.021198


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Abstract

We report on new developments in wavefront and polarization control for ultrashort-pulse laser microprocessing. We use two Spatial Light Modulators in combination to structure the optical fields of a picosecond-pulse laser beam, producing vortex wavefronts and radial or azimuthal polarization states. We also carry out the first demonstration of multiple first-order beams with vortex wavefronts and radial or azimuthal polarization states, produced using Computer Generated Holograms. The beams produced are used to nano-structure a highly polished metal surface. Laser Induced Periodic Surface Structures are observed and used to directly verify the state of polarization in the focal plane and help to characterize the optical properties of the setup.

© 2013 OSA

1. Introduction

Laser micromachining using ultrashort pulses with temporal pulselengths <10ps minimises thermal diffusion, lowers ablation threseholds and can generate highly reproducible micro- and nano-structures [1

1. R. Le Harzic, D. Breitling, M. Weikert, S. Sommer, C. Föhl, S. Valette, C. Donnet, E. Audouard, and F. Dausinger, “Pulse width and energy influence on laser micromachining of metals in a range of 100 fs to 5 ps,” Appl. Surf. Sci. 249(1-4), 322–331 (2005). [CrossRef]

4

4. D. Breitling, A. Ruf, and F. Dausinger, “Fundamental aspects in machining of metals with short and ultrashort laser pulses,” Proc. SPIE 5339, 49–63 (2004). [CrossRef]

]. As a result, industrial processes based on femtosecond and picosecond laser pulse durations are becoming increasingly widespread. Manufacturing applications include the very precise drilling of holes for fuel-injection nozzles [4

4. D. Breitling, A. Ruf, and F. Dausinger, “Fundamental aspects in machining of metals with short and ultrashort laser pulses,” Proc. SPIE 5339, 49–63 (2004). [CrossRef]

], the dicing of silicon wafers [5

5. N. Sudani, K. Venkatakrishnan, and B. Tan, “Laser singulation of thin wafer: die strength and surface roughness analysis of 80μm silicon dice,” Opt. Lasers Eng. 47(7-8), 850–854 (2009). [CrossRef]

], the scribing of thin film solar cells [6

6. G. Račiukaitis, S. Grubinskas, P. Gecys, and M. Gedvilas, “Selectiveness of laser processing due to energy coupling localization: case of thin film solar cell scribing,” Appl. Phys., A Mater. Sci. Process. 112(1), 93–98 (2013). [CrossRef]

] and the fabrication of volume Bragg gratings in transparent bulk materials [7

7. D. Liu, Z. Kuang, W. Perrie, P. J. Scully, A. Baum, S. P. Edwardson, E. Fearon, G. Dearden, and K. G. Watkins, “High-speed uniform parallel 3D refractive index micro-structuring of poly(methyl methacrylate) for volume phase gratings,” Appl. Phys. B 101(4), 817–823 (2010). [CrossRef]

]. Early ultrashort-pulse laser systems suffered from a comparatively low processing speed which slowed down their uptake in industry [3

3. D. Breitling, C. Föhl, F. Dausinger, T. Kononenko, and V. Konov, “Drilling of metals”, in Femtosecond Technology for Technical and Medical Applications, Topics Appl. Phys. 96, 131–156 (Springer-Verlag, 2004).

]. This has partly been addressed by the development of higher repetition laser systems from 200kHz to > 1MHz [8

8. Y. Bellouard, A. Champion, B. Lenssen, M. Matteucci, A. Schaap, M. Beresna, C. Corbari, M. Gecevicius, P. Kazansky, O. Chappuis, M. Kral, R. Clavel, F. Barrot, J. M. Breguet, Y. Mabillard, S. Bottinelli, M. Hopper, C. Hoenninger, E. Mottay, and J. Lopez, “The Femtoprint Project,” JLMN 7(1), 1–10 (2012). [CrossRef]

]. However as repetition rates increase above 200kHz, the required scan speeds from Galvo systems can be problematic, leading to a high pulse overlap where plasma absorption can produce wavefront distortions [4

4. D. Breitling, A. Ruf, and F. Dausinger, “Fundamental aspects in machining of metals with short and ultrashort laser pulses,” Proc. SPIE 5339, 49–63 (2004). [CrossRef]

].

This paper thus introduces a new technique for ultrashort-pulse laser microprocessing with complete control of the beam wavefront and polarization, using two SLMs in combination. We first give a brief mathematical description of this technique, using simple Jones vector formalism. We then describe how we implemented this experimentally in a picosecond-pulse laser microprocessing system. We detail how we produced a number of structured beams, including a linearly polarized vortex beam, a radially or azimuthally polarized vortex beam and multiple diffractive beams with a vortex wavefront and a radial or azimuthal polarization state. All these structured beams were used to process stainless steel samples. Laser Induced Periodic Surface Structures (LIPSS) were imprinted on the samples and used to visualize the electric field vector (polarization) structures at the focal plane, helping to characterize the optical properties of the setup.

2. Principle of wavefront and polarization control using Spatial Light Modulators

The wavefront and polarization of a laser beam can be fully controlled using two phase-only SLMs and a pair of waveplates. A phase-only SLM is made of a two-dimensional array of liquid-crystal pixels, to induce relative phase delays across the beam wavefronts [24

24. M. R. Beversluis, L. Novotny, and S. J. Stranick, “Programmable vector point-spread function engineering,” Opt. Express 14(7), 2650–2656 (2006). [CrossRef] [PubMed]

]. In the proposed setup illustrated in Fig. 1
Fig. 1 Schematic of the SLM Converter, which consists of two SLMs and two zero-order waveplates. SLM1 instigates a phase pattern designed to structure the beam wavefronts. After SLM1, the horizontal polarization is tilted to + 45 with a half-waveplate. SLM2 and the quarter-waveplate are used to convert the incident linear polarization into the desired state of polarization. This schematic represents the top view of the setup. For the Jones vector calculation, we define a coordinate system with a horizontal axis (x) and a vertical axis (y).
, SLM1 is used for structuring the beam wavefronts while SLM2, combined with a pair of waveplates are used for structuring both the beam wavefronts and polarization. This setup, referred to as an “SLM Converter” henceforth, can be represented using Jones vector formalism.

The Jones matrix representing the SLM Converter is noted T(x,y) henceforth, where x and y are the cross-sectional coordinates along the horizontal and vertical axis respectively. It is defined as:
T(x,y)=12Q×R×S2(x,y)×H×S1(x,y).
(1)
S1(x, y) and S2(x, y) are the Jones matrices of individual pixels of SLM1 and SLM2 respectively (see Table 1

Table 1. Jones matrices

table-icon
View This Table
). ϕ1(x,y)and ϕ2(x,y) are the relative phase delays induced by one pixel at location (x, y), on SLM1 and SLM2 respectively. H is the Jones matrix of a half-waveplate with its fast axis tilted by π8 radians from the horizontal. Q is the Jones matrix of a quarter-waveplate (see Fig. 1). It is noted that the quarter-waveplate has its fast axis tilted by π4 radians from the horizontal and we represent this by using a rotation matrix R in Eq. (1). This effectively changes the reference coordinate system so that the resulting Jones vectors after the quarter-waveplate are expressed in a coordinate system tilted byπ4 radians.

When a linearly polarized laser beam represented by the Jones vector Jin=(10) is incident on this setup, the resulting vector is Jout(x,y)=T(x,y)×Jin. The full derivation gives:

Jout(x,y)=eiπ2×eiϕ1(x,y)×eiϕ2(x,y)2×(sinϕ2(x,y)2cosϕ2(x,y)2).
(2)

The fourth term (sinϕ2(x,y)2cosϕ2(x,y)2) means that the resulting Jones vector Jout(x,y) represents a linear polarization, rotated by an angle ϕ22+π4 relative to the incident (horizontal) Jones vector Jin. The second and third terms mean Jout(x,y) has a relative phase delay which is set by both ϕ1 and ϕ2. As both ϕ1 and ϕ2are independently controllable for each SLM pixel (i.e. at various locations x, y), the proposed SLM Converter enables high-resolution spatial control of the laser beam wavefront and polarization.

3. Experimental details

To demonstrate how the wavefront and polarization of a laser beam can be controlled simultaneously, an SLM Converter is assembled as part of an ultrashort-pulse laser microprocessing system. A schematic of the experimental setup is shown in Fig. 2
Fig. 2 Schematic showing how the SLM Converter is used to control the wavefront and polarization of a picosecond-pulse laser microprocessing setup. The “polarization test components” are removed when the micro-processing tests are carried out.
.

3.1 Experimental setup

The laser source is a Coherent Talisker with a pulse width of 10ps, 532nm wavelength, M2 <1.3, average power of 8W, 200kHz maximum repetition rate and a horizontal linear polarization. A beam-expanding telescope (Jenoptic) with ×3 magnification is used to reduce the average intensity of the beam incident on the SLMs.

The SLM Converter (Fig. 1) consists of two Hamamatsu X10468-04 LCOS-SLMs (Liquid-Crystal On Silicon Spatial Light Modulators) and a pair of zero-order waveplates. The SLMs are of the phase-only, reflection type and consist of a 16x12mm, 800x600 pixel array of horizontally oriented liquid-crystal phase retarders.

After the SLM Converter, there is 4f optical system (Lens 1 and 2 in Fig. 2) which uses plano-convex lenses (f = 400mm) to re-image the surface of SLM2 (Fig. 1) to the 15mm input aperture of a scanning galvo. The beam is focused with a flat field f-theta lens (f = 250mm). Samples are mounted on a precision 3-axis (x, y, z) motion control system (A3200 Ndrive system, Aerotech) allowing accurate positioning in the focal plane.

3.2 Experimental procedure

To demonstrate the capabilities of the setup, we produce a number of optical field configurations. In each case, the produced beams are analysed at two places: immediately after the SLM Converter and at the focal plane of the microprocessing setup. After the SLM Converter, a polarizing filter and beam profiler (dotted components in Fig. 2) enable to analyse the collimated beams. To analyse the beams at the focal plane however, we use a different method since the high peak-intensity would damage any optics placed in this region. By focusing the beams on the surface of polished stainless steel samples, the focal intensity distribution is imprinted on the sample surface. Moreover, using a fluence near the ablation thresehold of the material leads to the formation of wavelength-sized LIPSS which typically develop orthogonally to the local electric field vectors [27

27. Z. Guosheng, P. M. Fauchet, and A. E. Siegman, “Growth of spontaneous periodic surface structures on solids during laser illumination,” Phys. Rev. B 26(10), 5366–5381 (1982). [CrossRef]

, 28

28. Q. Z. Zhao, S. Malzer, and L. J. Wang, “Formation of subwavelength periodic structures on tungsten induced by ultrashort laser pulses,” Opt. Lett. 32(13), 1932–1934 (2007). [CrossRef] [PubMed]

] and provide a direct method of analysing polarization in the focal region [17

17. O. J. Allegre, W. Perrie, K. Bauchert, D. Liu, S. P. Edwardson, G. Dearden, and K. G. Watkins, “Real-time control of polarisation in ultra-short-pulse laser micro-machining,” Appl. Phys., A Mater. Sci. Process. 107(2), 445–454 (2012). [CrossRef]

].

For all these tests, the laser output is attenuated to produce a pulse energy of 3μJ (~0.3J/cm2 at focal plane), and the samples are exposed for a duration of 5ms (~100 pulses at 20kHz pulse repetition rate) whilst the beam remains static with regards to the sample. After laser exposure, the produced focal spots are imaged with an optical microscope.

4. Results and discussion

The SLM Converter can produce individual as well as multiple first-order beams, each with the desired structured optical fields. A number of experimental configurations are demonstrated below.

4.1 Linearly polarized vortex beams

As a first case study, the SLM Converter was configured to produce a linearly polarized beam with a vortex wavefront (carrying an orbital angular momentum of l = 1). This was achieved by adjusting the phase delays at SLM1, whereϕ1(x,y) varied from 0 to 2π depending on the coordinates (x, y), producing a 2π pitch vortex wavefront overall, see Fig. 3(a)
Fig. 3 (a) Vortex wavefront induced at SLM1. (b) Incident beam profile before SLM1. (c) Resulting beam profile after SLM1. The colour coded scale represents intensity, in arbitrary units.
. In this case, SLM2 was configured so that it did not affect incident wavefronts (i.e. all the pixels were set at: ϕ2(x,y)=0). As a result, the linearly polarized Gaussian beam from the laser source had its wavefront structured into a phase vortex, whilst its polarization remained unchanged (i.e. it remained linear). The resulting beam was analysed after the SLM Converter. As expected, the beam profile, shown in Fig. 3(c), had the typical annular geometry associated with vortex beams [11

11. J. Hamazaki, R. Morita, K. Chujo, Y. Kobayashi, S. Tanda, and T. Omatsu, “Optical-vortex laser ablation,” Opt. Express 18(3), 2144–2151 (2010). [CrossRef] [PubMed]

, 12

12. M. Duocastella and C. B. Arnold, “Bessel and annular beams for materials processing,” Laser Photonics Rev. 6(5), 607–621 (2012). [CrossRef]

]. It is noted that the vortex structure at SLM1 had a central singularity where the rapid transverse phase variation induced significant diffraction effects, visible as diffraction rings around the beam in Fig. 3(c). It is also noted that the incident beam from the laser source, Fig. 3(b), had some residual structures which affected the wavefronts produced. These effects might be reduced by adding a spatial filter in the beam path after the SLM Converter.

The setup was used to imprint focal spots on the surface of a stainless steel sample, as detailed in Section 3.2 above. The produced focal spots had a ring-shaped structure (see optical micrographs in Fig. 4
Fig. 4 Optical micrographs showing the focal spot produced with ~100 pulses at 3μJ/pulse, with the SLM Converter producing a Gaussian beam (a) and a vortex beam (b). In each case, the LIPSS are perpendicular to the linear polarization, which is indicated with blue arrows.
), which is consistent with what is expected when focusing a vortex beam with a low NA lens [11

11. J. Hamazaki, R. Morita, K. Chujo, Y. Kobayashi, S. Tanda, and T. Omatsu, “Optical-vortex laser ablation,” Opt. Express 18(3), 2144–2151 (2010). [CrossRef] [PubMed]

]. As expected, LIPSS have formed within the produced laser spots, developing in a direction perpendicular to that of the incident polarization.

4.2 Radially and azimuthally polarized vortex beams

To further illustrate the capabilities of the SLM Converter, we show that both the polarization and wavefront structures of the beam can be controlled simultaneously. The SLM Converter was configured to produce radially and azimuthally polarized vortex beams. In this case, we leave ϕ1(x,y)=0 on SLM1 while inducing a vortex wavefront with a 4π pitch (i.e. carrying an orbital angular momentum of l = 2) at SLM2. As ϕ2(x,y) varied from 0 to 4π depending on the spatial coordinates (see wavefronts in Fig. 5
Fig. 5 Beam profiles produced by a (a) radially or (b) azimuthally polarized collimated beam, after transmission through a polarizing filter with its transmission axis oriented horizontally. The colour coded scale represents intensity in arbitrary units. The vortex wavefronts which are induced at SLM2 are shown in the top-left inlays.
), it produced a radial polarization overall as described in Eq. (2). In this configuration, the resulting beam has a 2π pitch residual phase vortex [26

26. O. J. Allegre, W. Perrie, S. P. Edwardson, G. Dearden, and K. G. Watkins, “Laser microprocessing of steel with radially and azimuthally polarized femtosecond vortex pulses,” J. Opt. 14(8), 085601 (2012). [CrossRef]

, 29

29. M. Beresna, M. Gecevicius, P. G. Kazansky, and T. Gertus, “Radially polarized optical vortex converter created by femtosecond laser nanostructuring of glass,” Appl. Phys. Lett. 98(20), 201101 (2011). [CrossRef]

], which is described by the phase term eiϕ2(x,y)2 in Eq. (2). This means that the resulting beam is radially polarized, with a vortex wavefront (i.e. it carries orbital angular momentum of l = 1). Furthermore, by adding a constant phase term π to the overall phase vortex at SLM 2, the output polarization can be converted simply from a radial to its orthogonal azimuthal state (Fig. 5).

The produced beams were analysed after the SLM Converter. The measured profiles shown in Fig. 5 are consistent with those typically expected from radially and azimuthally polarized beams, in line with [26

26. O. J. Allegre, W. Perrie, S. P. Edwardson, G. Dearden, and K. G. Watkins, “Laser microprocessing of steel with radially and azimuthally polarized femtosecond vortex pulses,” J. Opt. 14(8), 085601 (2012). [CrossRef]

, 29

29. M. Beresna, M. Gecevicius, P. G. Kazansky, and T. Gertus, “Radially polarized optical vortex converter created by femtosecond laser nanostructuring of glass,” Appl. Phys. Lett. 98(20), 201101 (2011). [CrossRef]

]. The profiles also reveal fringe structures within the beams. These structures were produced by interference effects at the centre of the vortex patterns induced by the SLMs [29

29. M. Beresna, M. Gecevicius, P. G. Kazansky, and T. Gertus, “Radially polarized optical vortex converter created by femtosecond laser nanostructuring of glass,” Appl. Phys. Lett. 98(20), 201101 (2011). [CrossRef]

, 30

30. U. Klug, J. F. Dusing, T. Sato, K. Washio, and R. Kling, “Polarization converted laser beams for micromachining applications,” Proc. SPIE 7590, 759006, 759006-8 (2010). [CrossRef]

].

To analyse the intensity profiles and polarization structures at the focal plane, focal spots were marked on the surface of stainless steel samples, as detailed in Section 3.2 above. Optical micrographs in Fig. 6
Fig. 6 Optical micrographs showing the focal spots produced with ~100 pulses at 3μJ/pulse, with the SLM Converter producing a vortex beam (orbital angular momentum l = 1) polarized radially (a) and azimuthally (b). The blue arrows indicate the polarization fields.
show the resulting focal spots. It can be seen that LIPSS have been produced around the edges, but not in the centre of the focal spots. As expected, the LIPSS produced with radially and azimuthally polarized beams were orthogonal to each other. The collimated radially polarized vortex beam from the SLM Converter produced a focal spot with LIPSS oriented in a radial pattern, see magnified area in Fig. 6(a). As LIPSS are orthogonal to the local electric field [27

27. Z. Guosheng, P. M. Fauchet, and A. E. Siegman, “Growth of spontaneous periodic surface structures on solids during laser illumination,” Phys. Rev. B 26(10), 5366–5381 (1982). [CrossRef]

, 28

28. Q. Z. Zhao, S. Malzer, and L. J. Wang, “Formation of subwavelength periodic structures on tungsten induced by ultrashort laser pulses,” Opt. Lett. 32(13), 1932–1934 (2007). [CrossRef] [PubMed]

], this suggests that the focal field was azimuthally polarized. Similarly, the LIPSS in Fig. 6(b) suggest that the collimated azimuthally polarized vortex beam produced a radially polarized focal field.

Radially/azimuthally polarized vortex beams such as those produced with our SLM Converter (Fig. 5) are expected to increase processing efficiency and speed, compared with conventional linearly or circularly polarized beams [22

22. M. Meier, V. Romano, and T. Feurer, “Material processing with pulsed radially and azimuthally polarized laser radiation,” Appl. Phys., A Mater. Sci. Process. 86(3), 329–334 (2007). [CrossRef]

]. It is noted that the SLM Converter can also produce radially/azimuthally polarized beams with a planar wavefront (i.e. without an orbital angular momentum) which typically produce a ring-shaped intensity structure in the focal plane [30

30. U. Klug, J. F. Dusing, T. Sato, K. Washio, and R. Kling, “Polarization converted laser beams for micromachining applications,” Proc. SPIE 7590, 759006, 759006-8 (2010). [CrossRef]

32

32. G. Wu, Q. Lou, J. Zhou, J. Dong, and Y. Wei, “Focal shift in focused radially polarized ultrashort pulsed laser beams,” Appl. Opt. 46(25), 6251–6255 (2007). [CrossRef] [PubMed]

]. The detail of these experiments will be published elsewhere.

4.3 Diffractive parallel processing with structured beams

Our earlier publications [7

7. D. Liu, Z. Kuang, W. Perrie, P. J. Scully, A. Baum, S. P. Edwardson, E. Fearon, G. Dearden, and K. G. Watkins, “High-speed uniform parallel 3D refractive index micro-structuring of poly(methyl methacrylate) for volume phase gratings,” Appl. Phys. B 101(4), 817–823 (2010). [CrossRef]

, 15

15. Z. Kuang, W. Perrie, D. Liu, S. Edwardson, J. Cheng, G. Dearden, and K. Watkins, “Diffractive multi-beam surface micro-processing using 10 ps laser pulses,” Appl. Surf. Sci. 255(22), 9040–9044 (2009). [CrossRef]

] detailed the concept of diffractive parallel processing with linearly polarized Gaussian beams, using a CGH to produce multiple first-order focal spots. Here we show that the SLM Converter enables diffractive parallel processing with spatially structured wavefront and polarization. The same optical field structures as detailed above (i.e. linear, radial and azimuthal polarization with a vortex wavefront) were combined with a CGH to produce multiple focal spots.

As an example, the CGH shown in Fig. 7(a)
Fig. 7 (a) CGH induced at SLM1 to produce three first-order focal spots. (b) Vortex wavefront induced at SLM1, in addition to the CGH. (c) Optical micrograph showing the three focal spots produced simultaneously with a vortex beam mode.
is designed to produce three first-order focal spots. The vortex wavefront shown in Fig. 7(b) was combined with the CGH and applied to the wavefront at SLM 1 (i.e. each pixel induced the appropriate value of ϕ1(x,y) to shape the wavefront). The samples were exposed to the laser beam as detailed in Section 3.2 above. Figure 7(c) shows an optical micrograph of a set of three focal spots imprinted simultaneously on the surface of a sample. The three spots produced had a ring shape and within each spot, LIPSS were oriented perpendicularly to the direction of the linear polarization (not visible in Fig. 7 due to the chosen magnification). The shape and LIPSS structures of each first-order spot are similar to those obtained when focusing a single linearly polarized vortex beam as described in Section 4.1 above, see Fig. 4(b). This confirms that the SLM Converter enables diffractive parallel processing with multiple first-order vortex beams. It is noted that the produced spots showed some distortion in their shape. This was due to the resolution limit of the SLMs, where the pixel size (~20μm) and discrete phase modulation induced some discontinuity in the produced wavefronts which affected the focal intensity distributions.

Finally, we show that the SLM Converter can generate diffractive parallel beams with a radial or azimuthal state of polarization. As before, a CGH designed to generate three first-order spots in the focal plane was applied to the wavefront at SLM1. SLM2 was used to structure polarization in the same way as in Section 4.2 (Fig. 5). The produced beams were first analyzed with the SPIRICON beam profiler and the resulting profiles are shown in Fig. 8
Fig. 8 Beam profiles produced by a (a) radially or (b) azimuthally polarized beam, after transmission through a polarizing filter with its transmission axis oriented horizontally. The colour coded scale represents intensity, in arbitrary units. Interference fringes from the first-order diffracted beams superimpose on the radial/azimuthal profiles. The vortex wavefronts which are induced at SLM2 are shown in the top-left inlays.
. As expected, diffraction fringes produced by the three first-order beams superimpose on the beam profiles produced by the radial and azimuthal polarization fields.

The focusing properties of these beams were investigated by marking focal spots on the surface of stainless steel samples, as detailed in Section 3.2 above. Optical micrographs in Fig. 9
Fig. 9 (a) Optical micrograph showing the multiple laser spots marked simultaneously at the focal plane, with ~100 pulses at 3µJ/pulse, when the SLM Converter produced three first-order beams polarized radially. (b) A magnification of one of three focal spots produced with a radially polarized beam. (c) One of three spots produced with an azimuthally polarized beam. The blue arrows indicate the direction of polarization, which is perpendicular to the LIPSS.
show the resulting focal spots. As expected, the laser exposure had simultaneously produced three first-order spots at the focal plane, see Fig. 9(a). LIPSS were produced around the edge but not in the centre of each focal spot, see Figs. 9(b) and 9(c), in the same way as with the single beams in Section 4.2 above. A detailed microscopic analysis confirmed that the LIPSS geometry within each spot was similar to that produced with the single beams in Section 4.2 (Fig. 6). The LIPSS geometry in Fig. 9 also indicated that an inversion in the dominant state of polarization had occurred at the focal plane compared to that of the collimated beam, in the same way as with the single beams (see Fig. 6). These results are consistent with multiple first-order focal spots with a radially or azimuthally polarized vortex field being produced at the focal plane. However, it is noted that the discrete phase modulation and pixel structure of the SLMs had induced some residual distortion which affected the polarization purity at the focal plane. This can be seen as a slight irregularity in the cylindrical symmetry of the LIPSS within the focal spots, see blue arrows in Figs. 9(b) and 9(c).

5. Conclusions

In this paper we present new developments in ultrashort-pulse laser microprocessing, using a technique which allows simultaneous control of the beam wavefront and polarization. This technique uses two phase-only liquid-crystal SLMs, where the first SLM structures the wavefront and the second SLM structures both the wavefront and polarization of the beam. As a proof of concept, we structured the wavefront and polarization of a picosecond-pulse laser beam and processed the surface of stainless steel samples. The produced beams had a vortex wavefront and various polarization states, including linear, radial and azimuthal. LIPSS are produced within the processed areas and are used as a direct method to verify the state of polarization in the focal plane. For the first time, diffractive parallel processing with a radial or azimuthal polarization and a vortex wavefront has been demonstrated. The advantage of this method is its flexibility, since it does not require any modification of the laser source and it allows the beam wavefront and polarization to be controlled simultaneously. Future work will further refine the technique, looking at how the polarization and wavefront structures can be produced and controlled dynamically for real-time process optimization. Control of wavefront as well as polarization may open up entirely new possibilities for material micro-structuring applications in the area of surface interactions and thin films.

Acknowledgments

The authors gratefully acknowledge the support of the EPSRC as well as the help of Prof. Miles Padgett who provided the SLM control software used in this research, of Jack Bennett at Hamamatsu for the loan of SLMs and of Dr Mary Erlund for her editorial contribution.

References and links

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7.

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J. Hamazaki, R. Morita, K. Chujo, Y. Kobayashi, S. Tanda, and T. Omatsu, “Optical-vortex laser ablation,” Opt. Express 18(3), 2144–2151 (2010). [CrossRef] [PubMed]

12.

M. Duocastella and C. B. Arnold, “Bessel and annular beams for materials processing,” Laser Photonics Rev. 6(5), 607–621 (2012). [CrossRef]

13.

N. J. Jenness, K. D. Wulff, M. S. Johannes, M. J. Padgett, D. G. Cole, and R. L. Clark, “Three-dimensional parallel holographic micropatterning using a spatial light modulator,” Opt. Express 16(20), 15942–15948 (2008). [CrossRef] [PubMed]

14.

Y. Hayasaki, T. Sugimoto, A. Takita, and N. Nishida, “Variable holographic femtosecond laser processing by use of a spatial light modulator,” Appl. Phys. Lett. 87(3), 031101 (2005). [CrossRef]

15.

Z. Kuang, W. Perrie, D. Liu, S. Edwardson, J. Cheng, G. Dearden, and K. Watkins, “Diffractive multi-beam surface micro-processing using 10 ps laser pulses,” Appl. Surf. Sci. 255(22), 9040–9044 (2009). [CrossRef]

16.

S. Hasegawa and Y. Hayasaki, “Polarization distribution control of parallel femtosecond pulses with spatial light modulators,” Opt. Express 21(11), 12987–12995 (2013). [CrossRef] [PubMed]

17.

O. J. Allegre, W. Perrie, K. Bauchert, D. Liu, S. P. Edwardson, G. Dearden, and K. G. Watkins, “Real-time control of polarisation in ultra-short-pulse laser micro-machining,” Appl. Phys., A Mater. Sci. Process. 107(2), 445–454 (2012). [CrossRef]

18.

S. Nolte, C. Momma, G. Kamlage, A. Ostendorf, C. Fallnich, F. Von Alvensleben, and H. Welling, “Polarization effects in ultrashort-pulse laser drilling,” Appl. Phys., A Mater. Sci. Process. 68(5), 563–567 (1999). [CrossRef]

19.

K. Venkatakrishnan, B. Tan, P. Stanley, and N. R. Sivakumar, “The effect of polarization on ultrashort pulsed laser ablation of thin metal films,” J. Appl. Phys. 92(3), 1604–1607 (2002). [CrossRef]

20.

V. G. Niziev and A. V. Nesterov, “Influence of beam polarization on laser cutting efficiency,” J. Phys. D Appl. Phys. 32(13), 1455–1461 (1999). [CrossRef]

21.

R. Weber, A. Michalowski, M. Abdou-Ahmed, V. Onuseit, V. Rominger, M. Kraus, and T. Graf, “Effects of radial and tangential polarization in laser material processing,” Phys. Procedia 12, 21–30 (2011). [CrossRef]

22.

M. Meier, V. Romano, and T. Feurer, “Material processing with pulsed radially and azimuthally polarized laser radiation,” Appl. Phys., A Mater. Sci. Process. 86(3), 329–334 (2007). [CrossRef]

23.

K. Venkatakrishnan and B. Tan, “Interconnect microvia drilling with a radially polarized laser beam,” J. Micromech. Microeng. 16(12), 2603–2607 (2006). [CrossRef]

24.

M. R. Beversluis, L. Novotny, and S. J. Stranick, “Programmable vector point-spread function engineering,” Opt. Express 14(7), 2650–2656 (2006). [CrossRef] [PubMed]

25.

J. Qi, W. Sun, J. Liao, Y. Nie, X. Wang, J. Zhang, X. Liu, H. Jia, M. Lu, S. Chen, J. Liu, J. Yang, J. Tan, and X. Li, “Generation and analysis of both in-phase and out-phase radially polarized femtosecond-pulse beam,” Opt. Eng. 52(2), 024201 (2013). [CrossRef]

26.

O. J. Allegre, W. Perrie, S. P. Edwardson, G. Dearden, and K. G. Watkins, “Laser microprocessing of steel with radially and azimuthally polarized femtosecond vortex pulses,” J. Opt. 14(8), 085601 (2012). [CrossRef]

27.

Z. Guosheng, P. M. Fauchet, and A. E. Siegman, “Growth of spontaneous periodic surface structures on solids during laser illumination,” Phys. Rev. B 26(10), 5366–5381 (1982). [CrossRef]

28.

Q. Z. Zhao, S. Malzer, and L. J. Wang, “Formation of subwavelength periodic structures on tungsten induced by ultrashort laser pulses,” Opt. Lett. 32(13), 1932–1934 (2007). [CrossRef] [PubMed]

29.

M. Beresna, M. Gecevicius, P. G. Kazansky, and T. Gertus, “Radially polarized optical vortex converter created by femtosecond laser nanostructuring of glass,” Appl. Phys. Lett. 98(20), 201101 (2011). [CrossRef]

30.

U. Klug, J. F. Dusing, T. Sato, K. Washio, and R. Kling, “Polarization converted laser beams for micromachining applications,” Proc. SPIE 7590, 759006, 759006-8 (2010). [CrossRef]

31.

K. Lou, S. X. Qian, X. L. Wang, Y. Li, B. Gu, C. Tu, and H. T. Wang, “Two-dimensional microstructures induced by femtosecond vector light fields on silicon,” Opt. Express 20(1), 120–127 (2012). [CrossRef] [PubMed]

32.

G. Wu, Q. Lou, J. Zhou, J. Dong, and Y. Wei, “Focal shift in focused radially polarized ultrashort pulsed laser beams,” Appl. Opt. 46(25), 6251–6255 (2007). [CrossRef] [PubMed]

OCIS Codes
(140.3300) Lasers and laser optics : Laser beam shaping
(140.3390) Lasers and laser optics : Laser materials processing
(140.7090) Lasers and laser optics : Ultrafast lasers
(230.6120) Optical devices : Spatial light modulators
(260.5430) Physical optics : Polarization
(080.4865) Geometric optics : Optical vortices

ToC Category:
Laser Microfabrication

History
Original Manuscript: July 22, 2013
Revised Manuscript: August 8, 2013
Manuscript Accepted: August 9, 2013
Published: September 3, 2013

Citation
O J Allegre, Y Jin, W Perrie, J Ouyang, E Fearon, S P Edwardson, and G Dearden, "Complete wavefront and polarization control for ultrashort-pulse laser microprocessing," Opt. Express 21, 21198-21207 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-18-21198


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References

  1. R. Le Harzic, D. Breitling, M. Weikert, S. Sommer, C. Föhl, S. Valette, C. Donnet, E. Audouard, and F. Dausinger, “Pulse width and energy influence on laser micromachining of metals in a range of 100 fs to 5 ps,” Appl. Surf. Sci.249(1-4), 322–331 (2005). [CrossRef]
  2. P. S. Banks, M. D. Feit, A. M. Rubenchik, B. C. Stuart, and M. D. Perry, “Material effects in ultra-short pulse laser drilling of metals,” Appl. Phys., A Mater. Sci. Process.69(7), S377–S380 (1999). [CrossRef]
  3. D. Breitling, C. Föhl, F. Dausinger, T. Kononenko, and V. Konov, “Drilling of metals”, in Femtosecond Technology for Technical and Medical Applications, Topics Appl. Phys. 96, 131–156 (Springer-Verlag, 2004).
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  7. D. Liu, Z. Kuang, W. Perrie, P. J. Scully, A. Baum, S. P. Edwardson, E. Fearon, G. Dearden, and K. G. Watkins, “High-speed uniform parallel 3D refractive index micro-structuring of poly(methyl methacrylate) for volume phase gratings,” Appl. Phys. B101(4), 817–823 (2010). [CrossRef]
  8. Y. Bellouard, A. Champion, B. Lenssen, M. Matteucci, A. Schaap, M. Beresna, C. Corbari, M. Gecevicius, P. Kazansky, O. Chappuis, M. Kral, R. Clavel, F. Barrot, J. M. Breguet, Y. Mabillard, S. Bottinelli, M. Hopper, C. Hoenninger, E. Mottay, and J. Lopez, “The Femtoprint Project,” JLMN7(1), 1–10 (2012). [CrossRef]
  9. M. Rioux, R. Tremblay, and P. A. Bélanger, “Linear, annular, and radial focusing with axicons and applications to laser machining,” Appl. Opt.17(10), 1532–1536 (1978). [CrossRef] [PubMed]
  10. N. Sanner, N. Huot, E. Audouard, C. Larat, and J. P. Huignard, “Direct ultrafast laser micro-structuring of materials using programmable beam shaping,” Opt. Lasers Eng.45(6), 737–741 (2007). [CrossRef]
  11. J. Hamazaki, R. Morita, K. Chujo, Y. Kobayashi, S. Tanda, and T. Omatsu, “Optical-vortex laser ablation,” Opt. Express18(3), 2144–2151 (2010). [CrossRef] [PubMed]
  12. M. Duocastella and C. B. Arnold, “Bessel and annular beams for materials processing,” Laser Photonics Rev.6(5), 607–621 (2012). [CrossRef]
  13. N. J. Jenness, K. D. Wulff, M. S. Johannes, M. J. Padgett, D. G. Cole, and R. L. Clark, “Three-dimensional parallel holographic micropatterning using a spatial light modulator,” Opt. Express16(20), 15942–15948 (2008). [CrossRef] [PubMed]
  14. Y. Hayasaki, T. Sugimoto, A. Takita, and N. Nishida, “Variable holographic femtosecond laser processing by use of a spatial light modulator,” Appl. Phys. Lett.87(3), 031101 (2005). [CrossRef]
  15. Z. Kuang, W. Perrie, D. Liu, S. Edwardson, J. Cheng, G. Dearden, and K. Watkins, “Diffractive multi-beam surface micro-processing using 10 ps laser pulses,” Appl. Surf. Sci.255(22), 9040–9044 (2009). [CrossRef]
  16. S. Hasegawa and Y. Hayasaki, “Polarization distribution control of parallel femtosecond pulses with spatial light modulators,” Opt. Express21(11), 12987–12995 (2013). [CrossRef] [PubMed]
  17. O. J. Allegre, W. Perrie, K. Bauchert, D. Liu, S. P. Edwardson, G. Dearden, and K. G. Watkins, “Real-time control of polarisation in ultra-short-pulse laser micro-machining,” Appl. Phys., A Mater. Sci. Process.107(2), 445–454 (2012). [CrossRef]
  18. S. Nolte, C. Momma, G. Kamlage, A. Ostendorf, C. Fallnich, F. Von Alvensleben, and H. Welling, “Polarization effects in ultrashort-pulse laser drilling,” Appl. Phys., A Mater. Sci. Process.68(5), 563–567 (1999). [CrossRef]
  19. K. Venkatakrishnan, B. Tan, P. Stanley, and N. R. Sivakumar, “The effect of polarization on ultrashort pulsed laser ablation of thin metal films,” J. Appl. Phys.92(3), 1604–1607 (2002). [CrossRef]
  20. V. G. Niziev and A. V. Nesterov, “Influence of beam polarization on laser cutting efficiency,” J. Phys. D Appl. Phys.32(13), 1455–1461 (1999). [CrossRef]
  21. R. Weber, A. Michalowski, M. Abdou-Ahmed, V. Onuseit, V. Rominger, M. Kraus, and T. Graf, “Effects of radial and tangential polarization in laser material processing,” Phys. Procedia12, 21–30 (2011). [CrossRef]
  22. M. Meier, V. Romano, and T. Feurer, “Material processing with pulsed radially and azimuthally polarized laser radiation,” Appl. Phys., A Mater. Sci. Process.86(3), 329–334 (2007). [CrossRef]
  23. K. Venkatakrishnan and B. Tan, “Interconnect microvia drilling with a radially polarized laser beam,” J. Micromech. Microeng.16(12), 2603–2607 (2006). [CrossRef]
  24. M. R. Beversluis, L. Novotny, and S. J. Stranick, “Programmable vector point-spread function engineering,” Opt. Express14(7), 2650–2656 (2006). [CrossRef] [PubMed]
  25. J. Qi, W. Sun, J. Liao, Y. Nie, X. Wang, J. Zhang, X. Liu, H. Jia, M. Lu, S. Chen, J. Liu, J. Yang, J. Tan, and X. Li, “Generation and analysis of both in-phase and out-phase radially polarized femtosecond-pulse beam,” Opt. Eng.52(2), 024201 (2013). [CrossRef]
  26. O. J. Allegre, W. Perrie, S. P. Edwardson, G. Dearden, and K. G. Watkins, “Laser microprocessing of steel with radially and azimuthally polarized femtosecond vortex pulses,” J. Opt.14(8), 085601 (2012). [CrossRef]
  27. Z. Guosheng, P. M. Fauchet, and A. E. Siegman, “Growth of spontaneous periodic surface structures on solids during laser illumination,” Phys. Rev. B26(10), 5366–5381 (1982). [CrossRef]
  28. Q. Z. Zhao, S. Malzer, and L. J. Wang, “Formation of subwavelength periodic structures on tungsten induced by ultrashort laser pulses,” Opt. Lett.32(13), 1932–1934 (2007). [CrossRef] [PubMed]
  29. M. Beresna, M. Gecevicius, P. G. Kazansky, and T. Gertus, “Radially polarized optical vortex converter created by femtosecond laser nanostructuring of glass,” Appl. Phys. Lett.98(20), 201101 (2011). [CrossRef]
  30. U. Klug, J. F. Dusing, T. Sato, K. Washio, and R. Kling, “Polarization converted laser beams for micromachining applications,” Proc. SPIE7590, 759006, 759006-8 (2010). [CrossRef]
  31. K. Lou, S. X. Qian, X. L. Wang, Y. Li, B. Gu, C. Tu, and H. T. Wang, “Two-dimensional microstructures induced by femtosecond vector light fields on silicon,” Opt. Express20(1), 120–127 (2012). [CrossRef] [PubMed]
  32. G. Wu, Q. Lou, J. Zhou, J. Dong, and Y. Wei, “Focal shift in focused radially polarized ultrashort pulsed laser beams,” Appl. Opt.46(25), 6251–6255 (2007). [CrossRef] [PubMed]

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