Adaptive optics for direct laser writing with plasma emission aberration sensing

Aberrations affect the focal spot quality in direct laser wr ite applications when focusing through a refractive index mism atch. Closed loop adaptive optics can correct these aberrations if a suit able feedback signal can be found. Focusing an ultrafast laser beam into tr ansparent dielectric material can lead to plasma formation in the foca l region. We report using the supercontinuum emitted by such a plasma to m easure the optical aberrations, the subsequent aberration correctio n using a spatial light modulator and the fabrication of nanostructures usin g the corrected optical system. © 2010 Optical Society of America OCIS codes: (090.1000) Aberration compensation; (140.3390) Laser mater ials processing; (130.2755) Glass waveguides; (250.5300) Photonic integra d circuits References and links 1. S. Wong, M. Deubel, F. Prez-Willard, S. John, G. A. Ozin, M. 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Kawata, “3D Metallic Nanostructure Fabrication by Surfactant-Assisted Multiphoton-Induced Reduction,” Sma ll 5, 1144–1148 (2009). 6. J. K. Gansel, M. Thiel, M. S. Rill, M. Decker, K. Bade, V. Sai le, G. v. Freymann, S. Linden, and M. Wegener, “Gold Helix Photonic Metamaterial as Broadband Circular Pol arizer,” Science325, 1513 (2009). 7. G. Della Valle, R. Osellame and P. Laporta, “Micromachining of photonic devices by femtosecond laser pulses,” J. Opt. A11, 013001 (18pp) (2009). 8. D. J. Little, M. Ams, P. Dekker, G. D. Marshall, J. M. Dawes, “ Femtosecond laser modification of fused silica: the effect of writing polarization on Si-O ring structure,” Opt. Express16, 20029–20037 (2008). 9. C. N. LaFratta, J. T. Fourkas, T. Baldacchini, and R. A. Far rer, “Multiphoton Fabrication,” Angew. Chem. Int. Ed.46, 6238–6258 (2007). 10. M. J. Booth, “Adaptive optics in microscopy,” Phil. Trans . R. Soc. A365, 2829–2843 (2007). 11. S. Campbell, S. M. F. Triphan, R. El-Agmy, A. H. Greenaway, a nd D. T. Reid, “Direct optimization of femtosecond laser ablation using adaptive wavefront shaping,” J. Op t A 9, 11001104 (2007). #120130 $15.00 USD Received 17 Nov 2009; revised 19 Dec 2009; accepted 20 Dec 2009; published 4 Jan 2010 (C) 2010 OSA 18 January 2010 / Vol. 18, No. 2 / OPTICS EXPRESS 656 12. C. Mauclair, A. Mermillod-Blondin, N. Huot, E. Audouard, and R. Stoian, “Ultrafast laser writing of homogeneous longitudinal waveguides in glasses using dynamic wave front correction,” Opt. Express 16, 5481–5492 (2008). 13. N. T. Nguyen, A. Saliminia, W. Liu, S. L. Chin, and R. Valle, “Optical breakdown versus filamentation in fused silica by use of femtosecond infrared laser pulses,” Opt. Let t. 28, 1591–1593 (2003). 14. M. A. A. Neil, M. J. Booth and T. Wilson, “New modal wave-fro nt sensor: a theoretical analysis,” J. Opt. Soc. Am. 17, 1098–1107 (2000). 15. M. J. Booth, M. A. A. Neil, R. Jǔ skaitis and T. Wilson, “Adaptive aberration correction in a confocal microscope,” PNAS99, 57885792 (2002). 16. M. A. A. Neil, R. Jǔskaitis, M. J. Booth, T. Wilson, T. Tanaka, and S. Kawata, “Ac tive aberration correction for the writing of three-dimensional optical memory devices,” App l. Opt. 41, 1374–1379 (2002). 17. P. T̈orök, P. Varga, Z. Laczik and G. R. Booker “Electromagnetic diff raction of light focused through a planar interface between materials of mismatched refractive indices : an integral representation,” J. Opt. Soc. Am. A 12, 325–331 (1995). 18. M. J. Booth, M. A. A. Neil and T. Wilson, “Aberration Corre ction for Confocal Imaging in Refractive Index Mismatched Media,” J. Microsc. 192, 90–98 (1998). 19. G. D. Marshall, M. Ams, and M. J. Withford, “Direct laser wr itten waveguide Bragg gratings in bulk fused silica,” Opt. Lett.31, 2690–2691 (2006). Laser material processing with short pulsed laser beams has been widely used for the fabrication of devices such as artificial bandgap materials [1], mic rofluidic devices [2, 3], metal nanostructures [4, 5, 6] and photonic devices based upon embedded waveguides [7]. Structures are created by focusing the laser into the material where multip hoton absorption and/or avalanche effects cause permanent material changes in the focal regio n [8, 9]. The fidelity of fabrication depends strongly on the quality of the focal spot. In many cas es, the quality of the focus is impaired by aberrations. A common problem is a mismatch betwee n the refractive indices of the processed material and the objective immersion medium. For example, nanophotonic devices are often fabricated in glasses that are not index matched wi th the focusing objective. While the aberration function caused by such a mismatch can be modelle d, other effects caused by nonlinearity or birefringence are much harder to derive. It is t herefore desirable to have a means of measuring aberrations directly. In contrast to microsco py, where they can be deduced from acquired images [10], alternative ways are required in nano fabrication applications, where the substrates are usually homogeneous and featureless. In rec e t publications it has been demonstrated that spatial characteristics of the fabricated str uctu es can be used to estimate aberrations [11, 12]. We propose an alternative way of measuring aberrat ions, which is based upon the supercontinuum emission of a plasma created in the beam focus. The plasma, which is generated by multiphoton absorption and avalanche effects, can usual ly be observed during nanofabrication processes and does not necessarily indicate the dest ruction of the glass matrix. Under strong focusing conditions, the plasma emission is mostly i otropic and unpolarized [13]. We show that an unaberrated focal spot corresponds to maximal p lasma emission and demonstrate the correction of system and sample-induced aberrations us i g a spatial light modulator (SLM). Figure 1 shows a sketch of the experimental set-up. The pulse s emitted from the regeneratively amplified titanium sapphire laser ( Solstice, Newport/Spectra Physics, 100 fs pulse duration, 1 kHz repetition rate, 790 nm centre wavelength) were a tt nuated using a rotatable half wave plate and a Glan-Taylor polarizer. A neutral density fil er was inserted if very low powers were required. The expanded beam was directed to a reflective liquid crystal SLM (X10468-02, Hamamatsu Photonics), which was used to shape the laser wave front. The SLM was imaged onto the objective pupil. The specimen was located on a 3D pie zo stage ( Tritor102SG, Piezosystem Jena) which provided up to 100 μm translation in all axes. The system incorporated a LED illuminated transmission microscope for observing the spe cim n. This microscope was also used to measure the intensity of the plasma emission from the laser focus. A figure inset shows the sample plane as it appeared on the CCD. The bright spot is t he light emitted by the plasma; the line below had been “written” into the specimen (borosil icate glass) by moving the sam#120130 $15.00 USD Received 17 Nov 2009; revised 19 Dec 2009; accepted 20 Dec 2009; published 4 Jan 2010 (C) 2010 OSA 18 January 2010 / Vol. 18, No. 2 / OPTICS EXPRESS 657 ple slowly whilst the laser was on. The laser exposure had cha nged the refractive index of the material. This was visible in the transmission microscope u nder slightly defocused imaging.

Laser material processing with short pulsed laser beams has been widely used for the fabrication of devices such as artificial bandgap materials [1], microfluidic devices [2,3], metal nanostructures [4,5,6] and photonic devices based upon embedded waveguides [7].Structures are created by focusing the laser into the material where multiphoton absorption and/or avalanche effects cause permanent material changes in the focal region [8,9].The fidelity of fabrication depends strongly on the quality of the focal spot.In many cases, the quality of the focus is impaired by aberrations.A common problem is a mismatch between the refractive indices of the processed material and the objective immersion medium.For example, nanophotonic devices are often fabricated in glasses that are not index matched with the focusing objective.While the aberration function caused by such a mismatch can be modelled, other effects caused by nonlinearity or birefringence are much harder to derive.It is therefore desirable to have a means of measuring aberrations directly.In contrast to microscopy, where they can be deduced from acquired images [10], alternative ways are required in nanofabrication applications, where the substrates are usually homogeneous and featureless.In recent publications it has been demonstrated that spatial characteristics of the fabricated structures can be used to estimate aberrations [11,12].We propose an alternative way of measuring aberrations, which is based upon the supercontinuum emission of a plasma created in the beam focus.The plasma, which is generated by multiphoton absorption and avalanche effects, can usually be observed during nanofabrication processes and does not necessarily indicate the destruction of the glass matrix.Under strong focusing conditions, the plasma emission is mostly isotropic and unpolarized [13].We show that an unaberrated focal spot corresponds to maximal plasma emission and demonstrate the correction of system and sample-induced aberrations using a spatial light modulator (SLM).
Figure 1 shows a sketch of the experimental set-up.The pulses emitted from the regeneratively amplified titanium sapphire laser (Solstice, Newport/Spectra Physics, 100 fs pulse duration, 1 kHz repetition rate, 790 nm centre wavelength) were attenuated using a rotatable half wave plate and a Glan-Taylor polarizer.A neutral density filter was inserted if very low powers were required.The expanded beam was directed to a reflective liquid crystal SLM (X10468-02, Hamamatsu Photonics), which was used to shape the laser wavefront.The SLM was imaged onto the objective pupil.The specimen was located on a 3D piezo stage (Tritor102SG, Piezosystem Jena) which provided up to 100 µm translation in all axes.The system incorporated a LED illuminated transmission microscope for observing the specimen.This microscope was also used to measure the intensity of the plasma emission from the laser focus.A figure inset shows the sample plane as it appeared on the CCD.The bright spot is the light emitted by the plasma; the line below had been "written" into the specimen (borosilicate glass) by moving the sam-ple slowly whilst the laser was on.The laser exposure had changed the refractive index of the material.This was visible in the transmission microscope under slightly defocused imaging.To verify that an unaberrated focus corresponds to the most intense plasma emission, the laser wavefront was deliberately distorted with the SLM and the corresponding plasma emission quantified.We represented the wavefront Φ within the objective pupil as a series of Zernike modes: Φ(ρ, θ ) = ∑ i a i Z i (ρ, θ ) with a i being the Zernike coefficients and Z i the modes, using the single index numbering scheme explained by Neil et al. [14].The normalized radial and angular coordinates are denoted by ρ and θ , respectively.Four different Zernike modes (astigmatism, coma, trefoil, tetrafoil) were subsequently applied to the wavefront and the beam was focused into lead glass (SF57, n=1.825).The plasma emission intensity was measured using the CCD camera.The objective was an Olympus PlanApo (60× 1.4 NA, oil immersion) and the pulse energy was 0.01 µJ.The results are summarized in Fig. 2. The graph shows the dependence of the plasma emission intensity on the applied rms aberration amplitude.The same aberration modes were applied in magnitudes from -0.8 to 0.8 rad with a step size of 0.4 rad.At each step, a defect was created by 100 laser pulses, each of 0.1 µJ energy.The images on the right are widefield images of the created defects, imaged with the fabrication objective.For all tested modes, the most isotropic defects corresponded to the maximal plasma emission.These results show that the plasma emission intensity is a valid metric for the focal spot quality.
Aberrations were corrected on a mode by mode basis using an iterative procedure [15].Bias aberrations ±b i Z i were introduced by the SLM and the corresponding plasma emission intensities I i± measured by integrating over the corresponding region on the CCD (see Fig. 1(b)).A phase update ∆Φ i = −g (I i+ − I i− ) Z i was then added to the SLM diffraction pattern and the whole process repeated until the aberration had been compensated, i.e.I i+ − I i− = 0.The gain factor g was experimentally determined for fast convergence.To expedite the process, both bias aberrations were simultaneously applied by displaying a binary pattern of the form where mod 2π symbolizes the "modulo-" operation that restricts the phase to an interval of [0, 2π).Equation (1) represents a binarized blazed grating with superimposed Zernike bias.A representative pattern is shown in Fig. 1(b).The grating constant k was chosen such that both diffraction orders were separated by about 5 µm in the specimen plane.As a consequence of using a binary phase pattern, the phases of the orders are naturally conjugated, i.e. one is distorted by +b i Z i and the other by −b i Z i .Similar binary patterns have been previously used Fig. 2. Graph: Plasma emission intensiy when different aberrations are applied using the SLM.Images: Defects created in the bulk of lead glass, with aberrations applied from -0.8 rad to 0.8 rad (rms value).For each mode, the most isotropic defect shape corresponds to the maximal plasma emission.The side length of each image corresponds to 5 µm.
to measure aberrations in light transmitted through a lithium niobate substrate [16].Using the plasma emission, the time required to correct one mode was typically in the range of a few seconds.In order to keep the plasma-induced material change as small as possible during the aberration sensing, the pulse energies were set to a level where they just generated sufficient plasma emission for detection on the CCD.We found that accurate measurements require the specimen to be moved during the process.Low speeds of around 1-2 µm/s were sufficient.To verify the aberration correction process, we introduced a known wavefront distortion using the SLM and measured it by using the plasma signal and the procedure explained above.The applied aberration magnitude of 1 rad rms was randomly distributed over seven low order Zernike modes.The modal coefficients are represented by the blue bar plot of Fig. 3(a).The remaining aberration magnitude after subsequent correction cycles is shown in Fig. 3(b).Each cycle involved measuring aberrations in the corresponding seven Zernike modes.Correction was complete to an accuracy of little more than 0.1 rad after two cycles.After three cycles, there remained a residual error of around 0.05 rad (< λ /100), which shows the accuracy limit of the method.The red bars in Fig. 3(a) show the Zernike coefficients obtained after six cycles.
Finally, the spherical aberration caused by a refractive index mismatch of objective immersion medium and sample was measured and corrected.The aberration function can be expressed Here, λ 0 denotes the vacuum wavelength, NA the numerical aperture of the microscope objec- tive and d nom the nominal focusing depth as indicated in Fig. 1. n 1 and n 2 are the refractive indices of immersion medium and substrate, respectively.The spherical aberration Φ SA causes not only an elongation, but also an axial translation of the focal spot.As this translation is easily removed by refocusing the stage, it is reasonable to define a modified function ΦSA , where the defocus has been removed to minimize the rms amplitude.Such a function will be easier to shape with adaptive optical elements.
The defocus function is defined as: Here, M is defined to remove the piston offset and N represents a normalization factor, such that the rms value of S is one: Spherical aberration was measured using the binary pattern of Eq. ( 1), with Z i replaced by ΦSA .To test our aberration correction procedure in a common nanofabrication set-up, we corrected for system and depth induced aberrations in fused silica using an air objective (Leitz 50× 0.85 NA, no coverslip correction).Figure 4 shows widefield transmission (a) and reflection confocal (b) images of defects, which have been produced by single laser pulses (E=3 µJ).For imaging, the fused silica block was turned by 90 degrees.The confocal images were taken at a wavelength of 405nm with the Olympus objective (1.4 NA, oil immersion) and appropriate aberration correction.Each row of images corresponds to a different focusing depth.The defects shown on the left were produced without any aberration correction.Clearly the quality, i.e. the local confinement of the defects, degrades quickly with increasing fabrication depth.At a nominal focus depth of 80 µm (corresponding to 130 µm fabrication depth) the affected region extends more than 30 µm in the axial direction.The white marks in the widefield images denote the respective axial positions of defects when aberration correction was applied.The defects shown in the right column have been fabricated using corrective phase patterns obtained by our aberration sensing technique.The resulting defect shapes are essentially independent of the focusing depth.At a nominal focus depth of 80 µm, we were able to produce defects extending as little as 1.5 µm in axial direction using a significantly lower laser pulse energy of 0.05 µJ (see Fig. 4(c)).Without application of the corrective SLM pattern, at this pulse energy the focal intensity was below the threshold of visible material modification.At a depth of 80 µm and when using the 50× 0.85 NA objective, the rms amplitude of the correction pattern Φ was 2.6 rad.Similar rms magnitudes with comparable peak to valley values have to be corrected when focusing 700 µm (nominal depth) deep into fused silica with an air lens of 0.45 NA.This much larger depth is due to the strong dependence of spherical aberration on the objective NA.It is therefore possible to fabricate aberration corrected nanostructures at large depths with low NA objectives, which are commonly used for writing waveguide based photonic devices.

Fig. 1 .
Fig. 1. a) Experimental set-up.The plasma emission is detected in backwards direction through a dichroic beamsplitter.The inset is a representative CCD image, showing the plasma emission and a written line of changed refractive index.b) Binary diffraction pattern for measuring trefoil.c) Laser intensity distribution in the focal plane, caused by b).The total signal in the marked regions is used as feedback for aberration estimation.

Fig. 3 .
Fig. 3. a) Blue bars: Zernike coefficients of a randomly chosen aberration, which was introduced to the wavefront to test the correction method.Red bars: Measured coefficients after six cycles.b) Remaining phase aberration after subsequent correction cycles.