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Multi-level diffractive optics for single laser exposure fabrication of telecom-band diamond-like 3-dimensional photonic crystals

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Abstract

We present a novel multi-level diffractive optical element for diffractive optic near-field lithography based fabrication of large-area diamond-like photonic crystal structure in a single laser exposure step. A multi-level single-surface phase element was laser fabricated on a thin polymer film by two-photon polymerization. A quarter-period phase shift was designed into the phase elements to generate a 3D periodic intensity distribution of double basis diamond-like structure. Finite difference time domain calculation of near-field diffraction patterns and associated isointensity surfaces are corroborated by definitive demonstration of a diamond-like woodpile structure formed inside thick photoresist. A large number of layers provided a strong stopband in the telecom band that matched predictions of numerical band calculation. SEM and spectral observations indicate good structural uniformity over large exposure area that promises 3D photonic crystal devices with high optical quality for a wide range of motif shapes and symmetries. Optical sensing is demonstrated by spectral shifts of the Γ-Z stopband under liquid emersion.

©2008 Optical Society of America

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Figures (7)

Fig. 1.
Fig. 1. The proposed single-surface three-level DOE (a) color-coded for each phase level as defined by orthogonal grooves of periodicities Λ x and Λ y, depths d 1 and d 2 and refractive indices n d1 and n d2 in a background medium of refractive index n b; and (b) laser exposure arrangement showing index matching medium (n i) between DOE substrate (n s) and photoresist (n r) layer which is spun onto a substrate of refractive index n s.
Fig. 2.
Fig. 2. Diffraction efficiency of a three-level DOE (Fig. 1a) as a function of groove depth d 1 with a fixed groove difference (d 2 - d 1)=331 nm for diamond-like structure. Inset (i) shows the unit cell with d 2 and d 1 phase elements of refractive index, n d=1.6, in air background (n b=1.0) and substrate refractive index n s=146.
Fig. 3.
Fig. 3. Near-field isointensity distribution computed by FDTD showing woodpile structure with clear offset S=c/4 between two orthogonally rotated logs as expected from the three-level DOE design of Λ=650 nm, d 1=1.13 µm, d 2=1.46 µm, n d=1.6 and n b=1.0. Inset (b) and (c) shows 2D intensity distribution (<I(x, y)>) of two planes separated axially by S=c/4=615 nm distance which show orthogonally rotated log-like intensity distributions as expected for a woodpile structure.
Fig. 4.
Fig. 4. Atomic force microscope image of the three-level DOE represented by the three different colors (heights). Enlarged section indentifies a unit cell abcd (iii) and the ideal height profile ABCD (iv) used in the FDTD simulation. Inset (i) and (ii) show single-line height profiles in orthogonal scan directions that define groove depths d 2 and d 1 (Length of scale bars as indicated).
Fig. 5.
Fig. 5. Top (a) and manually cleaved cross-sectional (b) SEM images of diamond-like woodpile structure in SU-8 photoresist showing 40 layers together with insets (i) and (iii), respectively, of predicted near-filed isointensity surfaces computed by FDTD. Inset (iv) shows enlarged view of cross-section of the actual structure and inset (ii) shows corresponding enlarged view of predicted isointensity surface of inset (iii).
Fig. 6.
Fig. 6. Band diagram (a) of the structure shown in Fig. 5 revealing a Γ-Z direction (normal incidence) (c-axis) stopband between the 5th and 6th band and corresponding normalized transmission spectrum (b) measured as normal angle of incidence through the structure in Fig. 5b showing a strong (-30 dB) stopband at 1.306 µm.
Fig. 7.
Fig. 7. Transmission recording of Γ-Z stopband during Ethanol emersion (t=0+) and evaporation (t>0+) and comparison with air-filled original photonic crystal spectrum (t=0-).

Equations (2)

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a = Λ ; c = Z T ( n r ) = ( λ d n r ) [ 1 1 λ d 2 ( n r Λ ) 2 ]
d 2 d 1 = Z T ( n b ) 4 = ( λ d n b ) 4 [ 1 1 λ d 2 ( n b Λ ) 2 ] ; S = c 4 = Z T ( n r ) 4
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