Optical-vortex laser ablation
Junichi Hamazaki1,*, Ryuji Morita1, Keisuke Chujo2,
Yusuke Kobayashi2, Satoshi Tanda1, and Takashige Omatsu2
1

2

Department of Applied Physics, Hokkaido University, Sapporo, 060-8628, Japan
Graduate School of Advanced Integration Science, Chiba University, Chiba, 263-5822, Japan* hamazaki@topology.coe.hokudai.ac.jp

Abstract: Laser ablation of Ta plates using nanosecond optical vortex
pulses was carried out, for the first time. It was suggested that owing toorbital angular momentum of optical vortex, clearer and smoother processedsurfaces were obtained with less ablation threshold fluence, in comparison
with the ablation by a nonvortex annular beam modified from a spatially
Gaussian beam.
©2010 Optical Society of America
OCIS codes: (140.3390) Laser materials processing; (140.3300) Laser beam shaping;(999.9999) Optical vortices;

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1. Introduction
The laser ablation technique is one of the most promising methods for material processing.
Since 1960’s, laser ablation processing has been developed, together with evolution of lightsources [1–4]. For industrial applications, high processing accuracy and low cost are required.
These requirements have motivated a lot of researches of laser ablation, mainly in terms ofmorphology of processed surface and drilling efficiency, and their pulse energy or duration
dependences [5–9]. Laser ablation dynamics using nanosecond (ns)-pulse in nontransparent
materials was interpreted so far, as follows [1,2,9–12]: (i) Energy of absorbed light is firsttransferred to the electron system, and then to the lattice system owing to electron-phonon
interaction. (ii) Lattice reaches equilibrium through phonon-phonon interaction and itstemperature increases. (iii) High-temperature-induced phase transition occurs from solid state
to liquid, gaseous, and plasma states. (iv) A pressure or a shock wave separates from thedense, hot focal volume, together with plasma plume expansion at high velocity (plasmaexpulsion).
In 1990’s, generation of an optical vortex (OV) was demonstrated [13]. Since OVs haveorbital angular momentum (OAM) of light owing to a phase distribution of mφ (m; integer,
called topological charge, φ; azimuthal angle) in transverse plane, OVs have been intensivelyinvestigated and attracted much attention [14,15]. Regarded as a new parameter of light,
OAM has recently begun to be employed in various experiments, for example, information
using multidimensional entangled states, especially trapping for Bose-Einstein condensates(BECs), microstructure rotation in laser tweezers and spanners etc [16–22]. In particular, in
optical tweezing, OVs are used because they can trap and manipulate particles with lessintensity than Gaussian beam (GB), which results in less absorption or photodamage due tomultiphoton processes. In addition, more efficient tweezing can be achieved thanks to the lossof the rays in the center of the beam. Analogously, efficient ablation may be expected withOVs. However, OVs have not been used in laser ablation and not studied their OAM effect on
the ablation processes and results.
In the present paper, we demonstrate the laser ablation using ns-optical vortex pulses. We
show that the ablation with OVs yields smoother processed surfaces and deeper drilling thanthat with the reference pulse, which has a modified Gaussian spatial profile with a center hole.
Moreover, we suggest an explanation of OAM effects on the laser ablation processes. Ourdemonstration represents a novel and better laser ablation technique using OV pulses and ourmodel provides another key to applications of laser plasma employing OVs.

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2. Experiments
2.1 Experimental setup
Figure 1(a) shows a schematic drawing of an experimental setup for ns-laser ablation systemusing OV pulses. The experiments were performed in atmospheric pressure and at room
temperature. Tantalum (Ta) plates were used as target samples. They exhibit low ablationthreshold in comparison with other metals [23–25], which enables us to perform ablation
experiment easily. Thickness of the Ta plates was ~1 mm and the plate surfaces were polishedby sandpaper. As a light source, we used a conventional Q-switched ns-Nd:YAG laser(Quanta-Ray GCR190) generating a spatially fundamental GB (HG00) with a wavelength of1064 nm and a pulse duration of ~40 ns. This laser can be operated in a single-shot mode.
A spiral phase plate (SPP) modulating spatial phase azimuthally (custom-made) was used toproduce OVs [26]. A combination of a telescope and a spatial filter (SF) containing 3-mmdiameter aperture was employed to improve spatial beam profiles. Figures 1(b) and 1(c) show
intensity patterns of OV (m = 2) and OV (m = 1) on the sample surface, respectively.
A typical self-shearing interference profile of the OV beam (m = 2) observed in a MachZehnder interferometer is shown in Fig. 1(d). Two-fork pattern is clearly seen, indicating onaxis phase singularity of the beam.
To make a clear difference between ablation results with and without OAM effects, weused a nonvortex annular beam (NVAB) modified from a spatially GB as a reference
incidence. NVAB was generated from a 3-mm-diameter GB by passing through a spatial
notch filter (SNF) which blocks 1-mm-diameter region at the beam center, as shown inFig. 1(e). Therefore, NVAB has no OAM (m = 0). NVAB also has doughnut-like intensity
profile on the sample surface similar to the OV profiles as shown in Fig. 1(f). The intensity
line profiles for NVAB and OV with m = 2 are quite similar even in the beam edges, as shownin Fig. 1(g).
Using a convex lens (L) with a focal length of 50 mm, the OVs and NVAB were roughly
focused. All the focal positions were at the same position. Sample surfaces were slightly
located off the focal planes (~2 mm away), where the beam spot sizes were ~130 µm. On thesample surfaces, the center dark spots were kept even for NVAB (Since NVAB is not aneigen-solution of the paraxial equation, the dark spot at the center of NVAB disappears in thefocal plane). Incident pulse energy was varied in the range of 0.3-3 mJ. After a single-shotpulse laser ablation, ablated surfaces of Ta plates were observed under a confocal laserscanning microscope (VK-9700/VK9710GS, Keyence Corp.), which measures depth as afunction of transverse position with an accuracy of ~0.02 µm in depth and a resolution of~0.02 µm in transverse displacement.
2.2 Experimental results
Figures 2(a)-2(c) show typical morphology of processed surfaces for (a) the OV (m = 2) and(b) OV (m = 1) pulses with incident pulse energy of 3 mJ, together with (c) NVAB as areference. The processed surface for NVAB (Fig. 2(c)) was rough and a lot of debris wereseen in the radial direction around the outer circumference. In good contrast to this, for theOV with m = 2 (Fig. 2(a)), smoother-processed surface was observed and numbers of debris
were significantly reduced in comparison with the case of NVAB. The processed surface for
the OV with m = 1 (Fig. 2(b)) showed an intermediate tendency between those for the OV
with m = 2 and NVAB.

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Fig. 1. (a) Experimental setup of laser ablation using OVs. SF is a spatial filter, SPP is a phase
plate to generate OVs, L is a convex lens with a focal length of 50 mm. Intensity profile of GB
is shown. (b) Intensity profiles of the OV (m = 2), (c) OV (m = 1) on the sample surface. (d)Shearing interference pattern of OV (m = 1) in the Mach-Zehnder configuration. Dotted-circles
indicate phase singularities. (e) Experimental setup of laser ablation using NVAB. SNF is
spatial notch filter. (f) Intensity profiles of NVAB on the sample surface. (g) Horizontal
intensity profiles of the OV (m = 2) and NVAB.

For high-numerical aperture (NA) configuration with NA~1, the beam polarization
dependences of ablation results as well as intensity profiles have been reported [27–30]. In ref[27], it was pointed out that these polarization dependences were due to the effects of angles
between electric-field vectors and processed surfaces, which govern the absorption efficiency
at processed surfaces. In contrast, in the present experiment where we employed a low NA
configuration (NA~0.03), beam polarization dependence may be small. Therefore, with only
linearly polarized beams, we performed the experiment to investigate OAM effects onablation results. Note that while incident beams were polarized in a horizontal direction, angle
dependences in processed surface morphology were not observed.

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Figures 3(a)-3(c) show distributions of the second-order derivative of the relative height ofprocessed surfaces with respect to the radial coordinate, indicating the surface curvature givenby ∂ 2 h(r , θ ) ∂ r 2 , where h , r and θ denote relative height, radius and azimuthal angle in the2-dimensional polar coordinates, respectively. The relative height is measured from the heightof non-processed surface. The origin of the polar coordinates is set to be the center position of
the processed surface, corresponding to the center of the incident beam. Generally, thesurfaces are drilled by the laser ablation process and some portion of the ablated material ispiled up outside (or around the vicinity of the center), as shown in Fig. 3(d). Therefore, thevalues of ∂ 2 h(r , θ ) ∂ r 2 become positive and negative inside and outside of the ablatedregion, respectively [26]. Thus, its sign change gives the measure of clearness of the outline
for ablated region. In the cases of OVs with m = 2 and 1 (Fig. 3(a) and 3(b)), the outline
structures were much clearer than that for NVAB (Fig. 3(c)). Moreover, the processed surface
for OV with m = 2 was smoother than that for OV with m = 1, implying that the larger
topological charge induces the clearer outline structure on the processed surface.
Features of processed surface roughness are obtained by spatial Fourier analysis. A
normalized spectrum of the relative height h for the processed surface directly shows theamount of undulation in a certain spatial frequency or a certain period, as illustrated in
Figs. 4(a)-4(d). Here, since areas of processed surfaces for OVs and NVAB are same, thenormalization was done with dividing amplitudes by average heights. For example, the spatial
Fourier spectra for near ideal ablation results show small Fourier-amplitude in the whole
spatial-frequency region except near zero-spatial-frequency.
Figure 5(a) shows the normalized spatial frequency spectra of r-averaged relative heighth1(θ) for the ablation results by OV and NVAB, together with that by a GB. Figure 5(b) shows
the normalized spatial frequency spectra of θ-averaged relative height h2(r) in the similar ma-Fig. 2. Laser microscope images of processed surfaces for (a) the OV with m = 2 (b) OV withm = 1, and (c) NVAB with incident energy of 3 mJ/pulse, respectively. Dotted circles denotedebris.

Fig. 3. Curvature distribution of the relative height h in radial direction for (a) OV with m = 2,(b) OV with m = 1, and (c) NVAB. (d) Cross-section illustration of processed surface along theblue line (x-axis) in the inset of figure.

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Fig. 4. Schematic cross-section of ablated zone with (a) smooth surface, (b) rough surface withlow- and high-spatial-frequency undulations, (c) rough surface with high-spatial-frequency
undulation, and (d) rough surface with low-spatial-frequency undulation. (a’)-(d’)
Corresponding schematic of spatial Fourier distributions for (a)-(d), respectively. Amplitudes atzero-spatial-frequency are excluded.

Fig. 5. Spatial Fourier spectra for (a) r- and (b) θ-averaged height profiles. Insets of (a) and (b)show r- and θ-averaged height distributions which are along pathways shown by blue lines in
the inset images. Red, black, and green lines are spatial frequency spectra and height
distributions for OV with m = 2, for NVAB, and GB, respectively.

-nner. Insets of Figs. 5(a) and 5(b) show relative height distributions h1(θ) and h2(r) in theazimuthal and radial directions, respectively, of which pathways were shown in thecorresponding images. In the azimuthal direction (Fig. 5(a)), it is clear that the amplitude for
the OV with m = 2 is smaller than those for NVAB and GB in the whole spatial-frequency
region. This indicates that the processed surface in azimuthal direction for OV with m = 2 wassmoother than those for NVAB and GB. In the radial direction (Fig. 5(b)), the amplitude for
the OV with m = 2 is smaller than those for NVAB and GB, in the whole spatial frequency
region. This manifests that undulations with any spatial in radical direction for the OV with m= 2 were less than those corresponding to NVAB and GB. Thus, it is clearly evaluated that theprocessed surface for the OV with m = 2 was smoother than those for NVAB and GB.
Figure 6 shows ablated depth D as a function of incident fluence F, for OV with m = 2 andNVAB. The depth D was evaluated as an absolute value of the minimum of h2(r)(see Fig. 3(d)). At any fluence, the ablated depth for the OV with m = 2 was deeper than thatfor NVAB. It is known the ablation depth dependence was described by a logarithmic low#120076 - $15.00 USD(C) 2010 OSA

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as D = ξ ln(F Fth ) , where ξ and Fth denote optical penetration depth and ablation-threshold
fluence, respectively [5,12]. Using this expression, the ablation-threshold fluence wasevaluated to be about 1.5 J/cm2 and 3.3 J/cm2 for the OV with m = 2 and NVAB, respectively,on the assumption of same values in optical penetration depth ξ for both cases. This is areasonable assumption since the optical penetration depth was determined by material
parameters and incident wavelength [25].
3. Discussion
Our experimental results as described in the previous section are summarized as follows:
1) The processed surfaces by OVs are clearer and smoother, having less debris, than that byNVAB. 2) The ablation-threshold fluence with OVs is less than that with NVAB. We discuss
here the differences between ablation results with OVs and NVAB.
In the present experiments, while NVAB and OVs are both horizontally-polarized andhave doughnut-beam intensity profiles with the similar-edge steepness (see Fig. 1(g)), theygave the different ablation results. It is considered that the differences between ablation resultswith OVs and NVAB can be due to (i) the OAM effect or (ii) the slight difference in spatial
profiles between OVs and NVAB. Spatial profiles of OVs and NVAB are quite similar but
only the spatial profile of NVAB has slight ring-shaped undulation owing to diffraction, as
shown in Fig. 1(f). This undulation may affect the smoothness of the processed surface.
Concerning spatial intensity profile of NVAB, while r-averaged intensity profile I1(θ) in theazimuthal direction was smoother, θ-averaged intensity profile I2(r) in the radial direction has
an undulation with a characteristic period of ~10 µm. For averaged height profiles h1(θ) andh2(r) in the corresponding processed surfaces, both amplitudes of their spatial Fourier spectra
were greater than those for OVs in whole spatial-frequency region. These worse ablation
results may be explained by the undulated intensity profile of NVAB, however, there was nocharacteristic spatial frequency in the Fourier spectra of h2(r) unlike that of I2(r). In addition,even spatial Fourier spectrum for h2(r) of the ablated surface by GB without any intensity
undulation was greater than that by OV in whole spatial-frequency region, indicating that theablated surface by GB was rougher than those by OVs. Moreover, ablated depth using OV
with m = 2 was deeper than that using NVAB and the evaluated ablation threshold fluence for
the OV with m = 2 was fairly less than that for NVAB. Theses quantitative differences may
not be explained by considering only undulation in intensity profile of the beams. Hence, as
an alternative explanation, we suggest that the differences between ablation results are mainlyascribed to the OAM effect.

Fig. 6. Ablated depth D as a function of incident fluence F. Red triangles and black squares
correspond to cases for OV with m = 2 and NVAB, respectively. Blue lines denote fitting
curves using the expression of D = ξ ln(F Fth ) (see the text).

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The OAM effect is described as follows. In the analogue of optical tweezing ormanipulation using OV, the OAM may cause the effects of rotational motion of moltenmaterial and/or laser-induced plasma through the OAM transfer by absorbing photons. The
former is that rotational motion of molten material to which OAM is transferred from photons
makes smoother processed surfaces. The latter is that rotational motion of laser-induced
plasma to which OAM is transferred assists the ablation processes, leading to smoothersurfaces. This is in an extension of the ablation assisted by laser-induced-plasmas, which wasreported in ref [31]. We cannot yet mention that those effects are dominant in the presentexperiments. This will be discussed elsewhere with new experimental results.
4. Conclusion
We performed laser ablation of Ta plates using nanosecond OV pulses, for the first time,observing the morphology of the Ta surface after single-shot laser irradiation. We found thatthe laser ablation by an OV pulse yielded (i) clearer outline of the ablated zone, (ii) lessdebris, (iii) less ablation threshold fluence, and (iv) smoother processed surface, than that by
NVAB modified from a HG00 beam. It was suggested that in comparison with ablation resultsby NVAB or GB, these improvements were attributed to the OAM effects.
Our demonstration using OV pulses represents a novel and better laser ablation technique,for example, in industries, fine, clear and smooth processing with less ablation threshold. Inaddition, it gives possibilities of annular ablation processing fabricating structures micropillar, pillar-based photonic crystals or meta-materials. Furthermore, our results provide
another key to applications of laser plasma employing OVs, such as spatial confinement andcontrol of plasmas.
Acknowledgment
This work was partially supported by Grant-in-Aid for the 21st Century COE program on“Topological Science and Technology” from the Ministry of Education, Culture, Sports,
Science and Technology of Japan, and Grant-in-Aid for Scientific Research (B), 2008-2010,
No. 20360025 from Japan Society for the Promotion of Science (JSPS).

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