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Application of the two-photon polymerization (2PP) technique for the fabrication of submicron-size relief of radial binary diffractive optical elements (DOE’s) is studied. Binary DOE’s for the formation of special longitudinal intensity distribution (axial light segment) are realized. Interferometric investigations of the diffractive relief produced by the 2PP-technique and investigations of optical properties of the formed elements are presented. Results of computer simulations are in good agreement with the experimental observations.
©2010 Optical Society of America
1. Introduction
The two-photon polymerization (2PP) technique has been demonstrated as a powerful tool for the fabrication of high resolution 3D microstructures. By tightly focusing femtosecond laser pulses into the volume of a photosensitive polymer resin, 2PP can be produced in a tiny region of focal spot, where the accumulated energy is higher than the polymerization threshold. As a result, with the 2PP technique it is possible to produce arbitrary 3D structures with subwavelength resolution (see, for example [1–5], and references herein). It has been reported [6] that 2PP technique allows reaching the structuring resolution of up to 65 nm. The high resolution and relatively low costs make it reasonable to use this technique not only for 3D structuring but also for the fabrication of diffractive microrelief. Due to two-photon absorption, higher resolution than it could be possible with the well-known laser direct writing process [7,8] can be obtained.
The capability of the 2PP method for the fabrication of high quality diffractive optical elements (DOE’s) with continuous-like diffractive microrelief has been demonstrated in [9]. Application of the 2PP technique for realization of continuous-like diffractive microrelief on the top of optical fibers has been investigated in [10]. However, the fabrication of DOE’s with continuous-like diffractive microrelief has been restricted to small DOE aperture sizes (less than 300 microns) due to a relatively long 2PP fabrication time. Possibility of more effective use of laser radiation based on multi-focus 2PP with a special light modulator has been demonstrated in [11].
The present paper is devoted to the realization of binary (two-level) diffractive microrelief of radial DOE’s using the 2PP-technique. These DOE’s can be applied for the generation of any desired longitudinal intensity distributions (coaxial lines, sets of sequential axial focuses, etc.) required for many laser technologies. As important examples the following applications can be mentioned: laser alignment of different units at large distances [12], formation of images from extended or moving objects, which is also required for medicine [13,14], nondestructive investigations of materials [15] and devices [16], laser scanning [17,18] and interferometry [19,20], as well as, the fabrication of intraocular lenses [21].
In this paper, application of the 2PP technique for the realization of radial binary diffractive microreliefs (with a 2 mm aperture) is studied. To design the binary relief structures, a special numerical technique developed in [22] is applied. Note that the same structures can be produced by ordinary laser and electron beam writer systems. Compared to these systems, 2PP allows the fabrication of much more complex multilayer structures. That is why the DOE fabrication with 2PP technique has much better prospects.
2. Design of binary DOE’s for the forming of special longitudinal intensity distributions
Application of DOE for the transformation of a Gaussian laser beam into a required longitudinal intensity distribution is illustrated in Fig. 1 .
Using the Fresnel integral approach [7], the complex amplitude of the laser field at the axial distance z and the radial distance ρ can be written as:
where R is the radius of the DOE, is the zero-order Bessel function, is the phase of the DOE, σ is the radius of the illuminating Gaussian beam, and is the wavenumber.The laser intensity on the optical axis is given by where
For forming of the desired longitudinal distribution , a functional
should be minimized, where is the intensity generated by the DOE with phase function on an optical axis at a distance , N is the number of control points on the optical axis, is the energy coefficient defined by the amount of quantizing levels m (in our case m = 2). However, considering reduction of laser intensity along the optical axis due to the beam divergence, it is better to use the following functional [22]:leading to better algorithm convergence. Here μ is the coefficient chosen for the determination of proper relationship between the DOE diffraction efficiency and uniformity of laser intensity distribution. The direct search approach [22,23] was used to find a binary phase function (with the values of 0 or π) minimizing the functional (4). The binary DOE for the Gaussian beam focusing into six uniformly distributed focuses (N = 6) at a distance from DOE to F1 equal to 22.2 mm and to F2 = 23.5 mm (see Fig. 1) and forming the longitudinal light segment was designed. The following parameters were chosen: DOE radius R = 1 mm, number of radial pixels MR = 200, wavelength λ = 0.632 μm, and the Gaussian beam radius σ = 0.5 mm.Diffraction efficiency e and root mean square δ (RMS) of intensity were used as criteria for the DOE characterization. Diffraction efficiency is defined as the ratio between the DOE generated intensity and the desired intensity and is expressed by
RMS is given by
For the designed DOE the following parameters were realized: diffraction efficiency e = 22%, RMS = 4.3%. The phase of this DOE and the calculated longitudinal intensity distribution are shown in Figs. 2a, b .
Fig. 2 a) Phase function of the designed DOE and b) calculated longitudinal intensity distribution on the optical axis.
3. Experimental setup and materials
For the fabrication of DOE’s by the two-photon polymerization technique, we apply near-infrared femtosecond laser pulses. When tightly focused into the volume of a photosensitive material (or photoresist), they initiate 2PP process in a highly localized area around the focal spot. This allows the fabrication of any computer-generated 3D structure by direct laser ”recording” into the volume of a photosensitive material. The samples of polymer (mr-NIL 6000 photoresist from Microresist technology GmbH) were prepared by spincoating of glass substrates with the following dimensions 18 × 18 × 0.15 mm3. Thickness of the polymer layer was about 600 nm after deposition and drying at 100°C to increase the mechanical stability. The laser system used for two-photon polymerization, see Fig. 3 , is based on the mode-locked Ti:sapphire laser (pulse width <80 fs, λ = 780 nm, maximal power of 20 mW, and repetition rate of 73 MHz). The laser beam is focused by an objective lens (40X, 0.4 NA) to write the desired pattern. Fabrication was performed by translating the sample using a computer controlled 3D positioning stage. The accuracy of the scanner-based writing is ~100 nm, and of the positioning system over the complete travel range it is better than 400 nm.
Fig. 3 Schematical setup used for 2PP: S – shutter, λ/2 plate, P – polarizer, O1,O2 - objectives; L1–L4 - lenses; DM – dichroic mirror; LED – light-emitting diode, CMOS - camera; PM - power meter; and XYZ stages are used to position the sample.
The processing characteristics and writing laser power were controlled using A3200 AEROTECH software. The designed DOE’s were fabricated with the following parameters: DOE diameter is 2 mm; writing laser power is 3 mW; writing speed is 1 mm/s; number of horizontal slices is 1; photoresist thickness (slice step) is 0.6 µm; laser writing hatch step (inside the binary ring contours) is 0.5 µm; fabrication time of one DOE is 2 hours 20 minutes. After polymerization unexposed monomer was removed during 1 minute in a standard SU-8 developer.
4. Experimental results
The arrays of 2 mm diameter DOE’s were written and the realized diffractive reliefs were investigated by optical and SEM microscopy as well as by white-light interferometry. The results of DOE’s relief characterization are presented in Fig. 4 and 5 .
For investigation of focusing properties of the realized DOEs, the TEM00 mode of cw He-Ne laser with a beam diameter of 1 mm was used as an illuminating source. The laser intensity distributions formed at the different distances from the DOE plane were registered by uEye UI-1450C CCD camera. Intensity distribution in the focusing plane (with a maximum intensity at the optical axis) and longitudinal intensity distribution are presented in Fig. 6a and Fig. 6b, correspondingly. Minimum focal spot diameter is about 30µm. Experimental results are in good agreement with the DOE computer simulations (Fig. 2). The observed differences between the modeling results and experiments can be explained by the presence of higher order laser modes.
Fig. 6 Intensity distribution in the focusing plane with a maximum value at the optical axis (a) and longitudinal intensity distribution (b) generated by the 2PP fabricated DOE.
It should be noted that longitudinal intensity distribution in the formed light segment is practically uniform (Fig. 6b), which is important for applications. Development of fabrication technology for the realization of step-like relief with larger number of quantization levels will allow to increase diffractive efficiency of such elements (see (4), (5)) in the future. The capability of 2PP technique to create complex 3D structures with exceptionally high resolution (less than 100 nm) makes this technology advantageous for the fabrication of 3D microoptical devices. With this technique, one is able to introduce compact and relatively economic DOE’s focusing laser beam to desired areas, which is crucial for the practical applications, especially in metrology, biology and medicine.
The application of multibeam 2PP recording method is perspective for significant reducing of recording time of such DOE’s. In this work we fabricated the designed DOE with diameter 2 mm during 2 hours 20 min., using one-beam writing system. Application of multi-beam [11] system (which is planned to realize in future) can reduce the duration of the recording of such DOE’s in several times.
5. Conclusions
In this work, the ability of two-photon polymerization technique to fabricate high-quality 2 mm-size binary diffractive optical elements has been demonstrated. It has been shown that the fabrication of concentric binary rings with heights about 600 nm from commercially available photoresists is possible. The carried investigations of the optical properties of the fabricated DOE’s have demonstrated that experimental results are in good agreement with the numerical design.
It is particularly important that the 2PP technique may be used for the fabrication of diffractive optical elements forming desired longitudinal laser intensity distributions. Such DOE’s can be applied in metrology and medicine.
Acknowledgements
We would like to gratefully acknowledge EC FP7-Marie Curie-IIF-Program for support of this work (Proposal # 235969).
References and links
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