## Focussing over the edge: adaptive subsurface laser fabrication up to the sample face |

Optics Express, Vol. 20, Issue 18, pp. 19978-19989 (2012)

http://dx.doi.org/10.1364/OE.20.019978

Acrobat PDF (4795 KB)

### Abstract

Direct laser writing is widely used for fabrication of subsurface, three dimensional structures in transparent media. However, the accessible volume is limited by distortion of the focussed beam at the sample edge. We determine the aberrated focal intensity distribution for light focused close to the edge of the substrate. Aberrations are modelled by dividing the pupil into two regions, each corresponding to light passing through the top and side facets. Aberration correction is demonstrated experimentally using a liquid crystal spatial light modulator for femtosecond microfabrication in fused silica. This technique allows controlled subsurface fabrication right up to the edge of the substrate. This can benefit a wide range of applications using direct laser writing, including the manufacture of waveguides and photonic crystals.

© 2012 OSA

## 1. Introduction

1. R. R. Gattass and E. Mazur, “Femtosecond laser micromachining in transparent materials,” Nat. Photonics **2**, 219–225 (2008). [CrossRef]

2. S. Wong, M. Deubel, F. Prez-Willard, S. John, G. A. Ozin, M. Wegener, and G. von Freymann, “Direct laser writing of three-dimensional photonic crystals with a complete photonic bandgap in chalcogenide glasses,” Adv. Mater. **18**, 265–269 (2006). [CrossRef]

3. R. Osellame, V. Maselli, R. M. Vazquez, R. Ramponi, and G. Cerullo, “Integration of optical waveguides and microfluidic channels both fabricated by femtosecond laser irradiation,” Appl. Phys. Lett. **90**, 231118 (2007). [CrossRef]

4. M. S. Rill, C. Plet, M. Thiel, I. Staude, G. Freymann, S. Linden, and M. Wegener, “Photonic metamaterials by direct laser writing and silver chemical vapour deposition,” Nat. Mater. **7**, 543–546 (2009). [CrossRef]

5. Y. Y. Cao, N. Takeyasu, T. Tanaka, X. M. Duan, and S. Kawata, “3d metallic nanostructure fabrication by surfactant-assisted multiphoton-induced reduction,” Small **5**, 1144–1148 (2009). [PubMed]

6. G. D. Marshall, A. Politi, J. C. F. Matthews, P. Dekker, M. Ams, M. J. Withford, and J. L. OBrien, “Laser written waveguide photonic quantum circuits,” Opt. Express **17**, 12546–12554 (2009). [CrossRef] [PubMed]

7. G. D. Valle, R. Osellame, and P. Laporta, “Micromachining of photonic devices by femtosecond laser pulses,” J. Opt. A, Pure Appl. Opt. **11**, 013001 (2009). [CrossRef]

8. P. Torok, P. Varga, Z. Laczik, and G. R. Booker, “Electromagnetic diffraction of light focused through a planar interface between materials of mismatched refractive indices: an integral representation,” J. Opt. Soc. Am. A **12**, 325–332 (1995). [CrossRef]

9. M. J. Booth, M. A. A. Neil, and T. Wilson, “Aberration correction for confocal imaging in refractive-index-mismatched media,” J. Microsc. **192**, 90–98 (1998). [CrossRef]

10. M. J. Booth, “Adaptive optics in microscopy,” Phil. Trans. R. Soc. A **365**, 2829–2843 (2007). [CrossRef] [PubMed]

11. M. J. Booth, M. Schwertner, T. Wilson, M. Nakano, Y. Kawata, M. Nakabayashi, and S. Miyata, “Predictive aberration correction for multilayer optical data storage,” Appl. Phys. Lett. **88**, 031109 (2006). [CrossRef]

13. B. P. Cumming, A. Jesacher, M. J. Booth, T. Wilson, and M. Gu, “Adaptive aberration compensation for three-dimensional micro-fabrication of photonic crystals in lithium niobate,” Opt. Express **19**, 9419–9425 (2011). [CrossRef] [PubMed]

12. C. Mauclair, A. Mermillod-Blondin, N. Huot, E. Audouard, and R. Stoian, “Ultrafast laser writing of homogeneous longitudinal waveguides in glasses using dynamic wavefront correction,” Opt. Express **16**, 5481–5492 (2008). [CrossRef] [PubMed]

14. R. R. Thomson, A. S. Bockelt, E. Ramsay, S. Beecher, A. H. Greenaway, A. K. Kar, and D. T. Reid, “Shaping ultrafast laser inscribed optical waveguides using a deformable mirror,” Opt. Express **16**, 12786–12793 (2008). [CrossRef] [PubMed]

17. P. S. Salter, A. Jesacher, J. B. Spring, B. J. Metcalf, N. Thomas-Peter, R. D. Simmonds, N. K. Langford, I. A. Walmsley, and M. J. Booth, “Adaptive slit beam shaping for direct laser written waveguides,” Opt. Lett. **37**, 470–472 (2012). [CrossRef] [PubMed]

## 2. The aberrated point spread function

*n*

_{2}from an immersion medium of RI

*n*

_{1}(Fig. 2). The substrate is bounded by two planes: T (the top surface) with normal parallel to the

*z*axis and Σ (the side surface) normal to the

*x*axis. The line of intersection of the two planes (the edge of the substrate) is parallel to the

*y*axis. Illumination is incident along the optic axis

*OPF*, which is also parallel to the

*z*axis.

*F*is the geometric, aberration free point of focus within the substrate, while

*P*is the intersection of the optic axis with the top surface T. The line

*O*′

*QF*is parallel to the

*x*axis and the point

*Q*designates the intersection of this line with the side surface Σ. The focussing depth

*PF*is

*d*, and the distance from the edge of the substrate

*QF*is

*g*.

*OPF*. The intersection of the focussing cone with the top surface T is a circular segment bounded by the chord

*CC*′, while the intersection with the side surface Σ is a hyperbolic segment likewise terminated by the chord

*CC*′ (Fig. 2). If the focussing objective lens has a numerical aperture

*NA*=

*n*

_{1}sin

*α*

_{1}, then the radius of the circular segment on T is given by

*R*=

*d*tan

*α*

_{1}. A general point

*A*within this circular segment on T is described by coordinates (

*r*,

_{t}*θ*) as shown in Fig. 2. The ray passing through point

_{t}*A*originates at a point in the pupil given by the coordinates (

*ρ*,

*θ*), where the pupil radius is normalised to one. Given that the ray passing through point

*A*makes an angle

*ϕ*with respect to the surface normal of T, and assuming the objective obeys the sine condition, the following transformation is valid:

*ρ*= sin

*ϕ*/sin

*α*

_{1}. Meanwhile the azimuthal angle must match:

*θ*=

*θ*. Simple geometry gives: Hence, using the above relationships, it is straightforward to transform the chord

_{t}*CC*′ described on the surface as:

*r*cos

_{t}*θ*=

_{t}*g*, into the pupil as: providing a boundary within the pupil discriminating between rays passing through either the top or side surface of the substrate. In Fig. 2(a), the rays passing through the top surface T are situated in the shaded part of the pupil inset, while those hitting Σ pass through the unshaded area of the pupil.

8. P. Torok, P. Varga, Z. Laczik, and G. R. Booker, “Electromagnetic diffraction of light focused through a planar interface between materials of mismatched refractive indices: an integral representation,” J. Opt. Soc. Am. A **12**, 325–332 (1995). [CrossRef]

*ρ*,

*θ*) which should be applied to the pupil in order to cancel such an aberration is given by [9

9. M. J. Booth, M. A. A. Neil, and T. Wilson, “Aberration correction for confocal imaging in refractive-index-mismatched media,” J. Microsc. **192**, 90–98 (1998). [CrossRef]

*NA*=

*n*

_{1}sin

*α*

_{1}=

*n*

_{2}sin

*α*

_{2}and

*λ*is the wavelength of light in vacuum.

*ξ*with the normal to Σ. The same ray will pass the plane of the top surface T (although not through T itself) at some point A and make angle

*ϕ*with the normal to T. It is convenient to introduce a virtual pupil, with optical axis

*O*′

*QF*and a numerical aperture

*NA*′ =

*n*

_{1}sin

*α*′

_{1}, where the

*α*′

_{1}is the maximum angle (relative to

*O*′

*QF*) of any ray that passes through the side face. If rays passing through the surface Σ can be considered as originating from the virtual pupil, then the aberration is equivalent to the form presented in Eq. (3), with the appropriate substitutions: and

*NA*′ =

*n*

_{1}sin

*α*′

_{1}=

*n*

_{2}sin

*α*′

_{2}. In order to determine the appropriate phase required in the real pupil, we must relate

*α*′

_{1},

*ρ*′,

*θ*′ to the variables

*α*

_{1},

*ρ*,

*θ*,

*g*and

*d*.

*C*Γ can be represented as

*C*Γ is also given by

*R*=

*d*tan

*α*

_{1}, and similarly

*R*′ =

*g*tan

*α*′

_{1}. Inserting these two relations into Eq. (5) allows us to determine the angle

*α*′

_{1}from:

*r*,

_{s}*θ*), as indicated by Fig. 2(d). Clearly

_{s}*θ*=

_{s}*θ*′, while

*r*and

_{s}*ρ*′ are connected by a similar expression to Eq. (1). Noting that

*ξ*and

*ϕ*are the angles that a particular ray forms with the

*x*and

*z*axes respectively, we introduce the quantity

*χ*to represent the angle for the ray with respect to the

*y*axis. A unit vector along

*FB*has a

*y*component cos

*χ*. Alternatively, by projecting onto the top surface T, the

*y*component can also be written as −sin

*ϕ*sin

*θ*. Thus using direction cosines and remembering that

_{t}*θ*=

*θ*, we can write: Again assuming that the objective obeys the sine rule

_{t}*ρ*= sin

*ϕ*/sin

*α*

_{1}, and by analogy in the virtual pupil

*ρ*′ = sin

*ξ*/sin

*α*′

_{1}, Eq. (7) leads to a definition for

*ρ*′ as: Hence the pupil correction phase for aberrations introduced by the refraction of rays at the side facet Σ, can be ascertained by combining Eq. (4), (6) and (9) to give: It is interesting to note that Eq. (10) is solely a function of

*ρ*cos

*θ*=

*x*and, hence, in this region of the pupil there are only variations in phase along the

*x*axis. As a demonstration, Fig. 3, shows typical pupil phase patterns for a 0.85

*NA*objective air lens (

*n*

_{1}= 1) focussing light of wavelength

*λ*= 790 nm at a depth of 50

*μ*m in a fused silica substrate (

*n*

_{2}= 1.45), for various distances from the edge (values of

*g*). Equation (2) gives the boundary

*CC*′ in the pupil, while Eq. (3) and (10) give the respective corrective phase distributions for the two pupil regions. There is necessarily a discontinuity in the pupil phase along the chord

*CC*′ when separating the pupil into rays passing through T and Σ. However, this can be easily accommodated by a liquid crystal phase-only spatial light modulator (SLM), which operates in the range [0, 2

*π*) radians.

18. A. Jesacher and M. J. Booth, “Parallel direct laser writing in three dimensions with spatially dependent aberration correction,” Opt. Express **18**, 21090–21099 (2010). [CrossRef] [PubMed]

## 3. Experimental layout

*f*system, composed of two achromatic doublet lenses. A 500

*μ*m diameter pinhole on an adjustable mount was inserted into the Fourier plane of the SLM and initially positioned to transmit all light incident on the SLM. The beam fully illuminated the back aperture of a Leitz NPl 50× objective with a 0.85 numerical aperture. The substrate was mounted on a three axis air-bearing translation stage, (Aerotech ABL10100 (

*x*,

*y*) and ANT95-3-V (

*z*)). An LED illuminated transmission brightfield microscope illuminated the specimen during fabrication.

*d*of 50

_{nom}*μ*m and 5

*μ*m from the side facet. The pulses were incident along the negative

*z*direction. There is clear void fabrication at depths

*Z*

_{1}and

*Z*

_{2}either side of

*d*due to the focal splitting discussed above, whereby rays passing through the side facet focus to a different point to those refracted at the top facet. Figures 4(c) and 4(d) are images showing the fabricated voids in the perpendicular plane, focused at depths

_{nom}*Z*

_{1}and

*Z*

_{2}respectively. Figure 4(d) clearly demonstrates the spread of fabrication further into the sample away from the optic axis due to the refraction of rays through the side facet. Overall, there is a good correspondence between the observed fabrication and the ray tracing shown in Fig. 1.

## 4. Plasma emission measurements and void fabrication

*z*position of the specimen which had the lowest threshold pulse energy for surface fabrication. The sample was then translated to a nominal depth

*d*and distance from the edge facet

_{nom}*g*. Using the values of

_{nom}*g*and

_{nom}*d*as a starting point, various phase patterns were applied to the SLM and the sample was irradiated with a continuous train of pulses with energy below that required for void formation. To measure the plasma emission intensity, the LED illumination (Fig. 4) was switched off and the focal volume was imaged onto the CCD. A typical image is shown in the inset of Fig. 5(b).

_{nom}*d*= 50

_{nom}*μ*m and distances from the substrate edge

*g*= 5, 10 and 15

_{nom}*μ*m. The sample was kept stationary and the SLM phase pattern was altered while monitoring the plasma intensity. In Fig. 5(a) the edge correction was varied while the depth correction remained constant. Note this corresponds to changing the value of

*g*in Eq. (2) and (10), which alters both the position of the chord

*CC*′ and the form of the phase modulation in region A of the SLM (as denoted in the inset of Fig. 5). The form of the phase pattern in region B of the SLM depends solely on

*d*, which remained constant as

*d*. Six sets of measurements were taken at different points along the substrate edge by both increasing and decreasing applied values of

_{nom}*g*either side of

*g*.

_{nom}*g*close to

*g*. This point corresponds to rays passing through both the top surface and side facet focussing to a common point, and hence generating the maximum plasma at the focus. This should also lead to fabrication of the tightest features at the lowest pulse energies. High repeatability in the peak position and relatively low error across the measurements sets (∼ 2%) indicate the reliability of the technique.

_{nom}*g*from the optimum value, it is obvious that the plasma emission should decrease, because the change in the position of the chord

*CC*′ renders a greater portion of the pupil subject to the correction function described by Eq. (10). Thus, rays actually passing through the top surface of the substrate are corrected as though they were passing through the side surface. This will lead to a decrease in focal intensity. However, increasing

*g*from the optimum value should have no effect on rays passing through the top surface. Hence, the sharp drop in plasma intensity for

*g*>

*g*demonstrates that Eq. (10) is appropriate for the phase correction of rays passing through the side surface and removes any focal splitting.

_{nom}*μ*m in fused silica at a distance of 10

*μ*m from the edge of the substrate. Sets of 3 voids, separated laterally by 5

*μ*m were generated by a burst of 100 pulses with energies of 90, 110 and 130 nJ as indicated in Fig. 6. Voids were generated firstly with aberration correction for rays passing through both the top and side surfaces (“Edge corrected”), and subsequently with aberration correction purely for the RI mismatch at the top surface neglecting the edge of the substrate (“No edge correction”). In the

*x*–

*y*plane (with the optic axis along

*z*) the void fabrication seems stronger for a given power with the edge correction applied. By viewing in the

*x*–

*z*plane, Fig. 6, it is clear that when the edge correction is applied, there is greater axial confinement in the void features. The lateral and axial dimensions of the ‘edge-corrected’ voids are ≲ 1

*μ*m and ∼ 1.8

*μ*m respectively, coinciding well with the expected dimensions considering the numerical aperture of the objective. Thus, we can see that the edge correction method minimizes the aberration to a level which is useful for fabrication. With no edge correction applied, there is axial elongation of the fabricated features and evidence of focal splitting due to rays passing through the side facet of the substrate.

## 5. Fabrication of tracks with power variation

*μ*m/s perpendicular to the edge at a depth of 50

*μ*m. Feedback from the translation stages was used to automatically update the amount of edge correction applied to phase patterns displayed on the SLM. The emission from the laser was a continuous pulse train (repetition rate 1 kHz) and the power was kept constant. The results are shown in Fig. 7(a). The SLM phase pattern was kept constant during fabrication of track (i), but track (ii) was written with automatically updated edge correction. It can be seen that the edge correction has a beneficial effect in maintaining fabrication up to the side surface for (ii) in contrast to track (i). However, it is clear that the degree of fabrication is still reduced as the focus nears the side surface. Such an effect is indeed to be expected since reflection of incident light from the side facet will be stronger than that from the top surface as the rays are incident at greater angles to the surface normal. Henceforth, to fabricate a spatially invariant structure some variation in the writing power is also required.

*I*

_{1}, and hence the fabrication beam, was maximum for a modulation depth (

*ϕ*) of the blazed grating equal to 2

_{o}*π*radians, and subsequent variation in

*ϕ*enabled adjustment of the power passing into the focus. The experimental dependence of

_{o}*I*

_{1}on

*ϕ*was found to show good correspondence to the theoretical prediction

_{o}*I*

_{1}∝ sinc

^{2}(1 −

*ϕ*/2

_{o}*π*) [21]. The blazed grating employed had a period of 0.6 mm (30 pixels), with an associated diffraction efficiency of 91% when

*ϕ*= 2

_{o}*π*. The grating was orthogonal to the tilt introduced due to the edge correction described by Eq. (10), causing the fabrication focus to be laterally shifted parallel to the substrate edge.

*ϕ*= 2

_{o}*π*(as near the substrate edge in Fig. 7(b) and 7(c)), the resulting phase pattern displayed on the SLM after implementing the modulo 2

*π*operation appears identical except for a uniform tilt across the pupil. This is expected since the light is efficiently directed into the first diffracted order. Away from the substrate edge in Fig. 7(b) and 7(c), where the blazed modulation depth is

*ϕ*< 2

_{o}*π*, the resultant phase pattern displayed on the SLM is easily recognisable as a grating and causes the light to be split into multiple orders, of which only the first is used for fabrication while all other light is blocked in the Fourier plane of the SLM.

## 6. Summary and discussion

*π*radians, we have not taken into account temporal effects of the pulse. However, if short pulsed lasers are employed, as in this article or for applications such as non-linear microscopy, there are some additional constraints that must be considered. It is clear that the rays passing through the top facet of the sample have to pass through a greater expanse of material than those incident on the side facet and, hence, are delayed in reaching the focus. This effectively limits the depth range over which the pulses from the two sets of rays will overlap temporally (

*d*<∼ 50

*μ*m in this present study), dependent on the refractive index of the substrate and the pulse duration. Nevertheless, at greater depths the phase correction employed here is still relevant. While the pulse might be temporally split, all components are focussed to the same point in the substrate and hence will only fabricate at that point even if the incident power were increased. This temporal splitting will not occur at all for continuous wave lasers or, in most practical situations, lasers with pulse lengths of a picosecond or longer.

## Acknowledgments

## References and links

1. | R. R. Gattass and E. Mazur, “Femtosecond laser micromachining in transparent materials,” Nat. Photonics |

2. | S. Wong, M. Deubel, F. Prez-Willard, S. John, G. A. Ozin, M. Wegener, and G. von Freymann, “Direct laser writing of three-dimensional photonic crystals with a complete photonic bandgap in chalcogenide glasses,” Adv. Mater. |

3. | R. Osellame, V. Maselli, R. M. Vazquez, R. Ramponi, and G. Cerullo, “Integration of optical waveguides and microfluidic channels both fabricated by femtosecond laser irradiation,” Appl. Phys. Lett. |

4. | M. S. Rill, C. Plet, M. Thiel, I. Staude, G. Freymann, S. Linden, and M. Wegener, “Photonic metamaterials by direct laser writing and silver chemical vapour deposition,” Nat. Mater. |

5. | Y. Y. Cao, N. Takeyasu, T. Tanaka, X. M. Duan, and S. Kawata, “3d metallic nanostructure fabrication by surfactant-assisted multiphoton-induced reduction,” Small |

6. | G. D. Marshall, A. Politi, J. C. F. Matthews, P. Dekker, M. Ams, M. J. Withford, and J. L. OBrien, “Laser written waveguide photonic quantum circuits,” Opt. Express |

7. | G. D. Valle, R. Osellame, and P. Laporta, “Micromachining of photonic devices by femtosecond laser pulses,” J. Opt. A, Pure Appl. Opt. |

8. | P. Torok, P. Varga, Z. Laczik, and G. R. Booker, “Electromagnetic diffraction of light focused through a planar interface between materials of mismatched refractive indices: an integral representation,” J. Opt. Soc. Am. A |

9. | M. J. Booth, M. A. A. Neil, and T. Wilson, “Aberration correction for confocal imaging in refractive-index-mismatched media,” J. Microsc. |

10. | M. J. Booth, “Adaptive optics in microscopy,” Phil. Trans. R. Soc. A |

11. | M. J. Booth, M. Schwertner, T. Wilson, M. Nakano, Y. Kawata, M. Nakabayashi, and S. Miyata, “Predictive aberration correction for multilayer optical data storage,” Appl. Phys. Lett. |

12. | C. Mauclair, A. Mermillod-Blondin, N. Huot, E. Audouard, and R. Stoian, “Ultrafast laser writing of homogeneous longitudinal waveguides in glasses using dynamic wavefront correction,” Opt. Express |

13. | B. P. Cumming, A. Jesacher, M. J. Booth, T. Wilson, and M. Gu, “Adaptive aberration compensation for three-dimensional micro-fabrication of photonic crystals in lithium niobate,” Opt. Express |

14. | R. R. Thomson, A. S. Bockelt, E. Ramsay, S. Beecher, A. H. Greenaway, A. K. Kar, and D. T. Reid, “Shaping ultrafast laser inscribed optical waveguides using a deformable mirror,” Opt. Express |

15. | A. R. de la Cruz, A. Ferrer, W. Gawelda, D. Puerto, M. G. Sosa, J. Siegel, and J. Solis, “Independent control of beam astigmatism and ellipticity using a slm for fs-laser waveguide writing,” Opt. Express |

16. | M. Pospiech, M. Emons, A. Steinmann, G. Palmer, R. Osellame, N. Bellini, G. Cerullo, and U. Morgner, “Double waveguide couplers produced by simultaneous femtosecond writing,” Opt. Express |

17. | P. S. Salter, A. Jesacher, J. B. Spring, B. J. Metcalf, N. Thomas-Peter, R. D. Simmonds, N. K. Langford, I. A. Walmsley, and M. J. Booth, “Adaptive slit beam shaping for direct laser written waveguides,” Opt. Lett. |

18. | A. Jesacher and M. J. Booth, “Parallel direct laser writing in three dimensions with spatially dependent aberration correction,” Opt. Express |

19. | N. T. Nguyen, A. Saliminia, W. Liu, S. L. Chin, and R. Vallee, “Optical breakdown versus filamentation in fused silica by use of femtosecond infrared laser pulses,” Opt. Lett. |

20. | A. Jesacher, G. D. Marshall, T. Wilson, and M. J. Booth, “Adaptive optics for direct laser writing with plasma emission aberration sensing,” Opt. Express |

21. | J. W. Goodman, |

**OCIS Codes**

(090.1000) Holography : Aberration compensation

(140.3390) Lasers and laser optics : Laser materials processing

(220.4000) Optical design and fabrication : Microstructure fabrication

(250.5300) Optoelectronics : Photonic integrated circuits

(130.2755) Integrated optics : Glass waveguides

(070.6120) Fourier optics and signal processing : Spatial light modulators

**ToC Category:**

Laser Microfabrication

**History**

Original Manuscript: July 5, 2012

Revised Manuscript: August 9, 2012

Manuscript Accepted: August 9, 2012

Published: August 15, 2012

**Citation**

P. S. Salter and M. J. Booth, "Focussing over the edge: adaptive subsurface laser fabrication up to the sample face," Opt. Express **20**, 19978-19989 (2012)

http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-18-19978

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### References

- R. R. Gattass and E. Mazur, “Femtosecond laser micromachining in transparent materials,” Nat. Photonics2, 219–225 (2008). [CrossRef]
- S. Wong, M. Deubel, F. Prez-Willard, S. John, G. A. Ozin, M. Wegener, and G. von Freymann, “Direct laser writing of three-dimensional photonic crystals with a complete photonic bandgap in chalcogenide glasses,” Adv. Mater.18, 265–269 (2006). [CrossRef]
- R. Osellame, V. Maselli, R. M. Vazquez, R. Ramponi, and G. Cerullo, “Integration of optical waveguides and microfluidic channels both fabricated by femtosecond laser irradiation,” Appl. Phys. Lett.90, 231118 (2007). [CrossRef]
- M. S. Rill, C. Plet, M. Thiel, I. Staude, G. Freymann, S. Linden, and M. Wegener, “Photonic metamaterials by direct laser writing and silver chemical vapour deposition,” Nat. Mater.7, 543–546 (2009). [CrossRef]
- Y. Y. Cao, N. Takeyasu, T. Tanaka, X. M. Duan, and S. Kawata, “3d metallic nanostructure fabrication by surfactant-assisted multiphoton-induced reduction,” Small5, 1144–1148 (2009). [PubMed]
- G. D. Marshall, A. Politi, J. C. F. Matthews, P. Dekker, M. Ams, M. J. Withford, and J. L. OBrien, “Laser written waveguide photonic quantum circuits,” Opt. Express17, 12546–12554 (2009). [CrossRef] [PubMed]
- G. D. Valle, R. Osellame, and P. Laporta, “Micromachining of photonic devices by femtosecond laser pulses,” J. Opt. A, Pure Appl. Opt.11, 013001 (2009). [CrossRef]
- P. Torok, P. Varga, Z. Laczik, and G. R. Booker, “Electromagnetic diffraction of light focused through a planar interface between materials of mismatched refractive indices: an integral representation,” J. Opt. Soc. Am. A12, 325–332 (1995). [CrossRef]
- M. J. Booth, M. A. A. Neil, and T. Wilson, “Aberration correction for confocal imaging in refractive-index-mismatched media,” J. Microsc.192, 90–98 (1998). [CrossRef]
- M. J. Booth, “Adaptive optics in microscopy,” Phil. Trans. R. Soc. A365, 2829–2843 (2007). [CrossRef] [PubMed]
- M. J. Booth, M. Schwertner, T. Wilson, M. Nakano, Y. Kawata, M. Nakabayashi, and S. Miyata, “Predictive aberration correction for multilayer optical data storage,” Appl. Phys. Lett.88, 031109 (2006). [CrossRef]
- C. Mauclair, A. Mermillod-Blondin, N. Huot, E. Audouard, and R. Stoian, “Ultrafast laser writing of homogeneous longitudinal waveguides in glasses using dynamic wavefront correction,” Opt. Express16, 5481–5492 (2008). [CrossRef] [PubMed]
- B. P. Cumming, A. Jesacher, M. J. Booth, T. Wilson, and M. Gu, “Adaptive aberration compensation for three-dimensional micro-fabrication of photonic crystals in lithium niobate,” Opt. Express19, 9419–9425 (2011). [CrossRef] [PubMed]
- R. R. Thomson, A. S. Bockelt, E. Ramsay, S. Beecher, A. H. Greenaway, A. K. Kar, and D. T. Reid, “Shaping ultrafast laser inscribed optical waveguides using a deformable mirror,” Opt. Express16, 12786–12793 (2008). [CrossRef] [PubMed]
- A. R. de la Cruz, A. Ferrer, W. Gawelda, D. Puerto, M. G. Sosa, J. Siegel, and J. Solis, “Independent control of beam astigmatism and ellipticity using a slm for fs-laser waveguide writing,” Opt. Express17, 20853–20859 (2009). [CrossRef]
- M. Pospiech, M. Emons, A. Steinmann, G. Palmer, R. Osellame, N. Bellini, G. Cerullo, and U. Morgner, “Double waveguide couplers produced by simultaneous femtosecond writing,” Opt. Express17, 3555–3563 (2009). [CrossRef] [PubMed]
- P. S. Salter, A. Jesacher, J. B. Spring, B. J. Metcalf, N. Thomas-Peter, R. D. Simmonds, N. K. Langford, I. A. Walmsley, and M. J. Booth, “Adaptive slit beam shaping for direct laser written waveguides,” Opt. Lett.37, 470–472 (2012). [CrossRef] [PubMed]
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