Flexible interference lithography using fibers as laser beam splitting and delivering system is demonstrated. A laser beam at 325 nm is used as the ultraviolet light source. Fiber bundles consisting of two, three, or four optical fibers have been utilized for the fabrication of two-dimensional photonic structures with different lattice configurations and different periods. The effective area of the fabricated waveguide grating structures is in the scale of centimeters in diameter with excellent homogeneity, which has much space for further improvement. This flexible interference lithography technique enables simple, compact, and efficient fabrication of photonic structures.
© 2014 Optical Society of America
Interference lithography plays an important role in nanophotonics [1–10], which is crucial for a number of nanofabrication techniques. Two- and three-dimensional photonic crystals have been produced using methods involving interference lithography. In particular, plasmonic nanodevices have been developed after metallization of the template structures patterned by interference lithography [11,12]. Multi-beam interference lithography enables achievement of a variety of photonic lattices [3, 5]. Although direct writing using two-photon processes [13–16] is advantageous in the construction of complex photonic structures with flexibly controlled shapes and dimensions, interference lithography is still indispensable in efficient and mass fabrication of large-area photonic devices.
Conventionally, the optical system of interference lithography requires a number of optical lenses, total reflection mirrors, beam splitters with precise adjustment design. In particular, the optical path length in each arm of the interference geometry needs to be strictly controlled, so that interference pattern with a large area and high contrast can be achieved, thus, the coherence length of the laser source can be utilized to its largest extent. Furthermore, the optical design becomes extremely complicated when more than three laser beams are involved in the interference geometry.
All of above problems can be solved if the conventional optical elements are replaced by optical fibers. In this work, we demonstrate interference lithography using fibers to achieve splitting, delivery, and orientation of the laser beams. Different photonic lattices with large effective areas are achieved using such a flexible interference lithography scheme.
2. Flexible interference lithography using optical fibers
Fused silica fibers are employed in the design of the flexible interference lithography scheme, as shown schematically in Fig. 1(a). These fibers are bundled together at one end and are loosen at the other. A 325-nm He-Cd laser (Kimmon Koha Co., Ltd.; model: IK3301R-G) with 30-mW output power was employed as the UV laser source, which was coupled directly into the common end of the fiber bundle. The diameter of each fiber is 400 μm and 4 fibers were bundled together as a common end. Thus, a minimum laser diameter of about 1.2 mm is required, which is approximately the diameter of the 325-nm laser beam at FWHM. This ensures homogeneous splitting of the laser beam. The bundled end of the fibers is mounted on a three dimensional stage, so that the laser beam can be aligned precisely with the fiber ends. Equal laser energy should be coupled into each fiber and the laser energy should be coupled into the fibers to the largest extent. At the free ends, each fiber is arranged such that the output coupling laser beams from the fibers can be overlapped on the recording medium with designed separation angles (α). The period of the fabricated grating structures can be evaluated by Λ = λ/[2sin(α/2)], where λ is the laser wavelength. In the practical experiments, it is easy to achieve double-, triple-, or quadruple-beam interference lithography. The fibers function both as a beam splitter and as a light-beam delivery system. At the output ends, the fibers are arranged such that the output divergent laser beams are overlapped symmetrically about the normal of the glass substrate onto the sample of spin-coated photoresist thin film.
The sample is prepared by spin-coating S1805 photoresist on to a glass substrate with an area of 20 × 20 mm2 and a thickness of 1 mm, where the glass substrate was initially coated with a layer of indium tin oxide (ITO) with a thickness of about 200 nm, which is used as a waveguide in the finished device. A spin-coating process with a speed of 2000 rpm and a duration of 30 seconds has been employed. The thickness of the photoresist layer was measured to be more than 300 nm using a dektak profilometer. The diameter of the effective area of the gratings is as large as 10 mm. Using different angles of α, gratings with different periods may be produced after the exposure and development processes. Photographs of the samples with fabricated photoresist gratings are shown in Fig. 1(b), where different colors show strong diffractions at different wavelengths.
3. Microscopic and spectroscopic characterization
Grating structures consisting of photoresist nanolines have been produced using the flexible optical system shown in Fig. 1(a). The atomic force microscopic (AFM) image of photoresist grating structures consisting of nanolines is shown in Fig. 2. The modulation depth of the grating structures is about 146 nm, which have a period of about 460 nm. Excellent homogeneity and clean edges can be observed with the AFM images, implying high contrast exposure of the photoresist film to the interference pattern. However, the total thickness of the spin-coated photoresist film is larger than 300 nm. Therefore, there is still some photoresist left between the grating and the substrate. The thickness of the remaining photoresist layer is also determined by the contrast of the interference pattern, the exposure and the development processes during interference lithography.
The gratings sitting on the ITO layer actually form waveguide grating structures (WGS), which are featured by narrow-band resonance modes. The quality of the grating structures can be evaluated by the strength of the waveguide resonance mode. Figures 3(a) and 3(b) show the optical extinction spectra of the waveguide resonance mode for TM and TE polarizations, respectively, with the incidence angle (θ) of the white light beam increased from 0 to 24 degrees. The insets of Fig. 3 show the geometric relationships between the polarization and incident directions of the light beam and the orientation of the grating structures. For TM polarization, where the incident light is polarized perpendicular to the photoresist grating lines, the waveguide resonance mode is located at about 686 nm at normal incidence and is split into two branches with increasing the incident angle, as shown in Fig. 3(a). The shorter-wavelength branch is stronger than the longer-wavelength on in amplitude of the extinction spectrum. The two branches are tuned to about 559 and longer than 800 nm, respectively, as the angle of incidence is increased to 24 degrees. For TE polarization, there is a red shift of about 6 nm for the whole spectroscopic response with respect to TM polarization, where the waveguide mode is resonant at about 693 nm at normal incidence and the shorter-wavelength branch is observed at about 566 nm for an incident angle of 24 degrees, as shown in Fig. 3(b).
The physics for the waveguide resonance mode has been investigated both theoretically and experimentally [17–20]. The diffraction of the incident light beam by the grating structures excites the propagation mode of the waveguide. The waveguide here is a combination of the ITO and the remaining photoresist layers. The propagating waveguide mode is diffracted by the top grating structures multiple times within the waveguide layer into the same direction as the directly transmitted beam. Destructive interference between these diffracted beams and the directly transmitted beam leads to a narrow-band extinction in the transmissive spectroscopic response of the waveguide-grating structures, which is defined as the waveguide resonance mode. The optical extinction spectrum of the resonance mode is degenerate at normal incidence and is split into two branches with increasing the incident angle. These two branches correspond to the + 1 and −1 orders of diffraction and evolve in opposite directions in the optical extinction spectrum with increasing the angle of incidence, as shown by the experimental results in Fig. 3. The resonance modes are located at different spectral positions at a same incident angle for TM and TE polarizations, as can also be observed in Fig. 3.
4. Fabrication of different photonic lattices
Different lattice structures may be produced using different arrangements of the fiber-based interference lithography geometry. Figures 4(a) and 4(b) demonstrate the fabrication of triangular lattices using three fibers to split the incident UV laser beam and to deliver the separated light beams. Figure 4(a) shows how the three fiber heads are arranged in an approximate equilateral-triangular geometry and Fig. 4(b) shows the AFM image of the triangular lattices made of photoresist, which has a modulation depth of about 34 nm. Such small modulation depth is important for achieving narrow-band filters using waveguide grating structures, as has been investigated in Ref [17–19]. The modulation depth is determined by the contrast of the interference pattern and by the exposure and development processes in interference lithography. Three periods may be resolved from Fig. 4(b) due to the slight deviation from equilateral geometry of the triangle, which are measured to be Λt1 = 560 nm, Λt2 = 606 nm, and Λt3 = 712 nm.
Figures 4(c) and 4(d) demonstrates the fabrication of square lattices using four fibers. However, due to the deviation from the “square” arrangement of the four fibers, the lattice squares are distorted. The grating periods are measured to be Λs1 = 504 nm and Λs2 = 585 nm. The modulation depth of the square lattices is as large as 166 nm. Thus, the square lattices in Fig. 4(d) have much higher contrast that the triangular lattices in Fig. 4(b). This is not only due to the number and the spatial arrangement of fibers, but also due to the corresponding polarization configuration .
The distortion of the square or triangular photonic lattices is due to the unsymmetrical arrangement of the fiber heads. This problem can be solved by mounting each mirror holder, to which each fiber head is fixed, as shown in Figs. 4(a) and 4(c), onto a three-dimensional translation stage. Thus, it will be very flexible to position each fiber head with a correct tilting angle into a more precisely square or triangular geometry.
Flexible interference lithography system using optical fibers enables stable, simple, low-cost, and easy-to-manage fabrication of photonic structures. The most important advantages involve: (1) Conventional optics for beam splitting and direction control has been replaced by flexible fibers; (2) No need to balance the optical path length, which has been done by using optical fibers with equal lengths; (3) Beam splitting through adjusting the coupling of the laser beam into the fiber bundle, instead through complicated and expensive multi-layer dielectric coatings; (4) Easy adjustment of the separation angle between laser beams and the overlapping area on the sample by simple arrangement of loosen fiber ends; (5) Easy achievement of different photonic lattices using different numbers and different geometric arrangement of the fibers.
We acknowledge the 973 program (2013CB922404) and the National Natural Science Foundation of China (11274031) for the support.
References and Links
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