1. Introduction

Femtosecond (fs) lasers are attractive tools in the field of laser material processing [1–6] based on nonlinear interactions induced by high-intensity laser pulses. An important technique of high resolution nonlinear processing is two-photon polymerization (2PP) of photosensitive polymer materials [7–15]. In the 2PP process, femtosecond laser pulses induce material modifications in an extremely localized focal volume leading to true three-dimensional micro- and nano-scale polymerized structures, which dimensions can be much smaller than the diffraction limit of the incident laser beam. For macroscopic objects the high process resolution of 2PP [16–18] can result in relatively long processing times unacceptable for practical applications. To solve this problem, several approaches to establish parallel processing with multiple beams were demonstrated such as applying a fly’s eye type lens [19], diffractive optical elements (DOEs) [20–23], or phase modulation technique with a spatial light modulator (SLM) [23–27]. Fly’s eye type lenses and DOEs provide an effective beam separation that can be used for efficient mass production of periodic structures, however both elements always generate a fixed beam pattern, which is not suitable for variable fabrication of single complex structures and rapid prototyping in the research and development field. Unlike these techniques, phase modulation with SLMs allows variable splitting of laser beams into multi-focus spots by means of computer generated holograms (CGH). Initially, SLMs were applied like switchable static DOEs to drill holes simultaneously [24], with the SLM simultaneously modifying the positions of multiple laser spots by changing the CGH. However, an SLM working as a static DOE still does not exhaust the full potential of SLMs for fabrication of complex structures in 2PP. Consequently, this technique has been developed further during the past three years to a dynamic process [28–31]. The dynamic process allows to display continuously different CGHs on the SLM, which can be even done synchronously to the motion of a positioning stage. In comparison to conventional 2PP processing with a single focus spot or multi-focus spots generated by a static DOE, this has following unique advantages: (i) Precise positioning of multi-focus spots can be realized without motion support using optics or mechanics. (ii) The positioning of each focus spot has a very good repeatability. (iii) Movement and light intensity of each focus spot can be controlled individually. (iv) Whereas only a small part of the laser power is used in the conventional one-focus 2PP technique for the processing with high-power laser systems, in a multi-focus 2PP technique the laser power is more efficiently used to multi-focus spots leading to decreased running costs in practical applications. These substantial progresses open up new vistas for the realization of fast multi-focus 2PP.

In this study, we demonstrate a novel multi-focus 2PP technique based on a dynamic DOE process with fast switching speed of CGHs on the SLM and simultaneous control of focus position and light intensity in each focus spot. Additionally, 2-D and 3-D 2PP structuring with a varying number of focus spots were carried out.

2. Experimental

Figure 1 shows a schematic illustration of the experimental setup. In this experiment, a high-repetition rate fs laser system (Chameleon Ultra II, Coherent GmbH; repetition rate: 80 MHz, wavelength: 780 nm, pulse width < 140 fs) was used. After passing a pulse energy controlling device and a beam expander, the laser pulse illuminates a reflection type liquid crystal spatial light modulator (LCSLM; LC-R2500, Holoeye photonics AG). This SLM displays a CGH that acts as a phase hologram with 256 gray levels. CGHs with 256 × 256 pixels are designed using an iterative 2-D Fourier transform calculation [32]. The first-order of the diffracted beam in the phase modulated laser pulse forms an arbitrarily designable pattern at the first Fourier plane P and the sample surface. The zero-order beam is eliminated at P. Finally, the phase modulated beam is projected into the sample with a 100 × immersion microscope objective lens (NA = 1.40) resulting in multiple focal spots. Inorganic/organic hybrid polymers (ORMOCER [33]) are used as sample material. To prepare a sample a droplet of this material is placed between two 150 µm thick glass plates with a Kapton frame of 100 µm thickness keeping the glass plates at constant distance. The space between the objective lens and the surface of the upper glass plate is filled with a refractive index matching oil (n_oil = 1.515) for better process resolution at high numerical aperture. The fabrication of 2PP structures is started at the surface of the bottom glass plate, which can be moved using a computer controlled XYZ positioning system. Stage motion is synchronized with CGH displaying by a control program developed in LabVIEW (National Instruments). After the polymerization process, all non-irradiated ORMOCER is removed by a dipping into 4-methyl-2-pentanone, and only the irradiated structures are obtained.

 

Fig. 1 Experimental setup for multi-focus two-photon polymerization with a spatial light modulator. Inset: sample preparation.

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3. Results and discussion

In a SLM based multi-focus 2PP process, the refresh rate of CGHs displayed on the SLM determines the process performance, which is given by the response time of the liquid crystal unit in the SLM. The maximum CGH refresh rate was determined as shown in Fig. 2(a) .

 

Fig. 2 Experimental setup for CGH refresh rate measurement (a) and detected signal (b).

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In this setup, the SLM alternatively displays 2 different CGHs on the SLM at different refresh rates. The controllability of the refresh timing was about 1 ms. One of the CGHs projects the focus point at the center of the iris diaphragm, the other one projects it out of the center, resulting in a periodic intensity sequence on the photo-detector. In this setup a CW laser diode (wavelength: 785 nm, average laser power: 40 mW) is used as a light source. Figure 2 (b) shows the photo detector signal at a CGH refresh rate of 10 and 20 Hz, respectively. Each signal is periodically corresponding to the CGH refresh rate. Note, that the falling time is shorter than the rising time. This is attributed to the principle of molecule-orientation control in the liquid crystal unit, which is controlled by an electric field that is applied to or released from the liquid crystal layer. Namely, a faster response of the SLM is obtained when the electric field is applied to the liquid crystal. However, the maximum refresh rate is given by the slower response that was found to be 20 Hz in this setup.

When writing complex structures with multiple foci it is crucial that each focus can be switched on and off individually. For a set of n foci, 2ⁿ switching states have to be realized. Figure 3 shows (a) all combinations of switching states for a set of four foci and (b) corresponding 2PP structures that were fabricated at a constant linear stage movement of 20 µm/s. The CGHs were changed at 8 Hz during stage movement. Note, that we do not observe disturbances on the sample at the transition between two different CGHs, e.g. lines are continued without break. This means that the transient duration in changing the CGHs on the SLM is negligibly short for this application.

 

Fig. 3 Combination of four focus spots (left) and fabricated 2PP line structures (right) (Media 1).

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To demonstrate writing of arbitrary 2D structures in a multi-focus 2PP process, the CGHs from Fig. 3 were used to write the logo of LZH. Figure 4 shows (a) the original design of the structure, and SEM images of 2-D 2PP structures consisting of (b) lines and (c) dots, respectively. In both cases, the structure was fabricated by a combination of linear stage motion and changing of the CGH corresponding to the original design shown in Fig. 4(a). Figure 4(b) was written in two passes. In this setup, a 2PP-linewidth slightly below 0.9 µm is typically realized. In order to fabricate an equidistant 2D line and space pattern, the distance between spots was set to 1.75 µm. However, at this small spacing interference starts to affect the stability and uniformity of the spots. To overcome this limitation a double-patterning approach was used: In the first pass, four lines were written simultaneously with individually controlled four focus spots at a spot to spot distance of 3.5 µm. After that, in a second pass four lines were generated in the gap at half line distance of 1.75 µm. During each pass the CGHs were changed 97 times at 20Hz refresh rate while the stage moved at a constant velocity of 35 µm/s. The two pass method helps to avoid unwanted interference between the focus spots on the sample surface and provides a narrow line distance of 1.75 µm. The input power in each focus spot is set at 11 mW, respectively. The SEM image shows a parallel line array with constant line width and space. Each line is positioned with high accuracy corresponding to the original design. In the case of the dot array structure shown in Fig. 4(c), each CGH was displayed for 100 ms while the stage stopped. The SEM image shows that this approach allows to fabricate 2-D dot array structures with good size uniformity. The minimum distance between these dots is again 1.75 µm.

 

Fig. 4 Design pattern of a 2-D structure (LZH logo) (a) and SEM images of 2PP structures consisting of lines (Media 2) (b) and dots (c). Both structures are fabricated by four focus spots.

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The multi-focus 2PP process can be even used to generate arbitrary curved lines from a simple linear stage motion. For this task the SLM has to control one focus spot continuously in a reciprocating motion between two different points at short distance. Figure 5(a) shows a SEM image of a sine curved line that was fabricated in this way by a set of 200 CGHs displayed at 20 Hz at a linear stage speed of 20 µm/s at 25 mW of input laser power. The CGH were calculated to represent a harmonic oscillation of the focus spot with an amplitude of 5 µm perpendicular to the direction of the stage motion. Since the SLM consists of discrete elements even the sine curve is reproduced discretely. Here, the full amplitude of 5 µm is composed of 240 steps of about 21 nm. Despite the frequent and complex CGH switching, the line is smooth, symmetrical and continuously polymerized. In addition, the multi-focus 2PP process can fabricate even multiple phase shifted sine curves with multi-focus spots, simultaneously. Figures 5(b) and 5(c) show three sine curves that were fabricated simultaneously by three oscillating foci at 40 mW and six foci at 48 mW with phase shifts of 60° and 120°, respectively. The resolution of the polymerized lines and the minimum curvature radius are limited by the refresh rate of the SLM, and they can be expected to become even better by development of a higher-speed phase modulation technique.

 

Fig. 5 SEM images of sine curve structures fabricated with a single focus spot (a) (Media 3), three focus spots (b), and six focus spots (c) at phase shifts of 120° or 60°, respectively.

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Substantial benefit in flexibility and processing time is expected for 3-D multi-focus structuring. A demonstration how this technique can be used to write asymmetric 3-D structures in parallel is shown in Fig. 6(a) : two different 3D objects are written simultaneously by individual control of two focus spots. Each object (convex and concave) consists of three different layer types, which are generated by three different CGHs (CGH1 – CGH3). CGH1 creates two focus spots spaced 26 µm on the sample surface, while CGH2 and CGH3 are used to generate only one focus spot: either the left or the right one. To produce the desired structures an laser input power of 42 mW is used and the spots are focused onto the sample surface with a 50 × microscope objective (NA = 0.7). Meanwhile the positioning stages are moved 100 µm in y-direction at 100 µm/s and 20 µm in x-direction in x-steps of 0.5 μm, and each of the three different z-levels is composed of three identical layers. Figure 6(b) shows a SEM image of the asymmetric 3-D structures resulting from this procedure. The SEM image illustrates that two different 3-D structures (concave and convex) can be built at the same time. Slight deformations observed in this image (e.g. at the edges of the side walls) can be attributed to shrinkage, which is characteristic for this sample material. It should be pointed out that this principle demonstration of parallel asymmetric 3-D fabrication, even shows that this technique can be applied to reduce the processing time of complicated large-scale structures.

 

Fig. 6 Asymmetric 3-D structures (convex and concave) in design (a) and in realization by parallelized multi-focus production (b).

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Beyond position control, the SLM can also be used to display CGHs that adjust the laser power at the sample surface. Figure 7(a) shows CGH images displayed on the SLM resulting in 10 and 30 mW of laser power, respectively. The CGHs only differ in contrast shape. The spatial frequency of these CGHs is identical and thus the focus position is not influenced. The SEM image in Fig. 7(b) illustrates how this can be used to write a sequence of lines by 2PP with varying laser power: While the incident laser power on the SLM is kept at a constant value, the focus power for each line is increased in steps of 0.5 mW up to 30mW by a sequence of corresponding CGHs. This becomes apparent in the written line pattern, which smoothly changes from thin traces to solid lines, attributed to a different height of the structures. This demonstrates that SLM can be used to control the number, position and individual power of focus spots.

 

Fig. 7 Calculated CGH patterns leading to laser powers of 10 and 30 mW at the sample surface, respectively (a) and a SEM image showing a 2PP line array fabricated at laser powers between 1 and 30 mW (b). The laser power is controlled with different CGHs.

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4. Conclusion

Multi-focus 2PP processing based on phase modulation by SLM has been developed. It has been demonstrated that this technique allows to control a set of focus spots individually in position and intensity. Parallel processing with multi-focus spots has been realized at a high CGHs refresh rates of up to 20 Hz, and curved and asymmetric 2-D structures have been produced. Furthermore, it has been demonstrated that this technique allows simultaneous fabrication of differently shaped 3-D structures using individually controlled focus spots. Beyond position control, this technique can be applied not only for switching focus spots (on/off) but also for adjusting of individual laser power in each focal spot. The presented SLM based multi-focus 2PP technique is very promising for high-speed, high-efficiency 2PP processing in industrial applications.