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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 1 — Jan. 3, 2011
  • pp: 241–246
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High phase retardation by waveguiding in slanted photonic nanostructures

Kalle Ventola, Jani Tervo, Pasi Laakkonen, and Markku Kuittinen  »View Author Affiliations


Optics Express, Vol. 19, Issue 1, pp. 241-246 (2011)
http://dx.doi.org/10.1364/OE.19.000241


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Abstract

We report a physical mechanism leading to high phase retardation in slanted photonic nanostructures. The phenomenon is based on the waveguiding of the transverse electric polarization component inside the slanted pillars, while the transverse magnetic component is not guided. Such a mechanism leads to very high phase retardation even with shallow structures that are suitable also for lithographical mass production. We present physical principle, numerical analysis of the phenomenon and designs for half-wave retarders. As an experimental result, a slanted grating producing 177 degrees retardation and 95.5% transmission is presented.

© 2011 Optical Society of America

1. Introduction

Controlling the polarization state of light plays a major role in many optical applications. One of the functions in this area is phase retardation, traditionally associated with quarter- and half-wave plates made from an anisotropic material, such as Calcite crystals. It is also well known that subwavelength (SWL) structures act as effective anisotropic medium [1

1. M. Born and E. Wolf, Principles of Optics, 7th ed. (Cambridge U. Press, 1999).

] with remarkably stronger birefringence than that of natural crystals. Unfortunately, the required depth of these structures is very large compared to the transverse detail size, which has hindered the use of SWL structures in practical applications [2

2. D. C. Flanders, “Submicrometer periodicity gratings as artificial anisotropic dielectrics,” Appl. Phys. Lett. 42, 492–494 (1983). [CrossRef]

5

5. T. Isano, Y. Kaneda, N. Iwakami, K. Ishizuka, and N. Suzuki, “Fabrication of Half-wave Plates with Subwavelength Structures,” Jpn. J. Appl. Phys. 43, 5294–5296 (2004). [CrossRef]

]. In this paper we introduce a novel physical principle concerning phase retardation and slanted SWL nanostructures which have been proven to be beneficial, for example, in improving the quality of light-incoupling in waveguide-optics [6

6. B. Wang, J. Jiang, and G. P. Nordin, “Compact slanted grating couplers,” Opt. Express 12, 3313–3326 (2004). [CrossRef] [PubMed]

] and for unidirectional surface plasmon excitation [7

7. N. Bonod, E. Popov, L. Li, and B. Chernov, “Unidirectional excitation of surface plasmons by slanted gratings,” Opt. Express 15, 11427–11432 (2007). [CrossRef] [PubMed]

]. In the following two sections this principle is studied analytically and numerically. In section 4, a lithographically fabricated example with optical measurement results is presented. Conclusions are given in section 5.

2. Physical principle

Consider a slanted structure with its refractive index denoted by ng on a substrate with refractive index nsub (see Fig. 1). The material on the top of the structures, as well as in the grooves, is air with n = 1. The slant angle, linewidth, structure depth, and the period are denoted by Θ, c, h, and d, respectively, while the fill factor is defined by f = c/d.

Fig. 1 Schematic cross-section of a slanted-binary structure. Figure illustrates the grating parameters: period d, linewidth c, depth h and slant angle Θ, and the principle of the large phase difference arising from qualitatively different propagation properties of TE and TM polarization components.

When the period is much smaller than the wavelength, the SWL structure acts as a negative uniaxial crystal, and hence the field inside the modulated layer is essentially an ordinary or extraordinary wave inside a crystal. In such a case the field is not coupled into the pillars, as they are seen only as an average, but the field propagates rectilinearly through the modulated layer. On the other hand, if the period is in the order of the wavelength, under optimal circumstances the pillars may act as planar waveguides and the light is coupled to propagate inside the pillars. In such a case, the optical path and the phase shift inside the element would significantly increase, of course depending on the structure parameters.

Since the behavior of light inside the modulated region is strongly dependent on the polarization of light, one may ask whether or not it is possible to design a structure in which either only the transverse electric (TE) or the transverse magnetic (TM) polarization state is coupled into the pillars, while the other polarization state propagates through the element rectilinearly. Figure 1 illustrates the idea with geometric optics which often gives a rough but qualitatively enlightening picture of light propagation in micro- and nano-optical systems. One can assume that such a phenomenon could appear and disappear as a function of, particularly, the structure period. Since the period must be in the order of the wavelength of the light, it is expected that the phase shift is affected, not only by the optical path length, but also by resonance effects, such as the waveguide mode resonating back and forth inside the pillar, which can further increase the phase difference.

3. Numerical design and field simulation

We performed parametric optimization of the structure parameters in order to find designs for half-wave retarders. In the first three cases, we assumed that the structure is made of titanium dioxide (TiO2) with its refractive index ng = 2. In the first two designs we fixed the slant angle Θ = 45°, and looked for a low-depth solution for two different periods. In the third design, we assumed smaller slant angle Θ = 33°. The fourth design example is for lines made of UV-curable material with ng = 1.704 and slant angle Θ = 45°. This case thus corresponds to a slanted structure that can be replicated [9

9. T. Levola and P. Laakkonen, “Replicated slanted gratings with a high refractive index material for in and outcoupling of light,” Opt. Express 15, 2067–2074 (2007). [CrossRef] [PubMed]

].

The optimized parameters for all four structures are given in Table 1. Examining the parameters, we immediately notice that the required structure depths are remarkably lower than with non-slanted phase retarding gratings, especially in the second case where depth is approximately 0.65 of the period. By comparison, with traditional gratings the corresponding depths are, at minimum, 4.5 times the period [5

5. T. Isano, Y. Kaneda, N. Iwakami, K. Ishizuka, and N. Suzuki, “Fabrication of Half-wave Plates with Subwavelength Structures,” Jpn. J. Appl. Phys. 43, 5294–5296 (2004). [CrossRef]

].

Table 1. Numerically designed grating parameters that lead to half-wave retardation between TE and TM polarizations. Refractive indices of the grating and the substrate are denoted with ng and nsub, respectively; d is the grating period, f is the fill factor (linewidth/period ratio), h is the grating depth while Θ is the slant angle. The transmittances for the TE and TM polarization states are given by TTE and TTM, respectively.

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Fig. 2 Relation between the phase shifts and the grating period. Phase shifts experienced by the zeroth-order TE and TM modes inside the grating layer, the difference between the phase shifts of the modes, as well as the total phase retardation between TE and TM polarized output fields as a function of the grating period. The other properties of the grating are given on the first row of Table 1.

Examining the figure, we can observe that the phase difference between TE and TM modes at small grating periods is rather low, as is expected since the grating acts as true effective medium. Further, the phase shifts experienced by both modes are radically increased when period is increased which supports the physical picture discussed above. However, the phase difference between the zeroth-order modes does not represent the full picture of the total phase retardation inside the structure. The correspondence between the prediction and the accurate result is indeed quite good up to 400 nm, after which the difference grows. This difference arises from various resonance phenomena that are very common in gratings when the period is close to the wavelength.

Further studying Fig. 2, it appears that, for example, with d = 200 nm, the qualitative behavior of both polarization components should be more or less the same, while at the design period d = 386 nm, the behaviors of the modes are completely different. On the other hand, at large periods the behaviors are again similar. In fact, already at d = 420 nm, also the TM mode seems to be coupled into the grating pillars. In order to ensure these effects, let us investigate more closely the field inside and in the close vicinity of the element at the three aforementioned periods. Figure 3 illustrates the phase of the field for these three periods. Examining the figure, we can see that with d = 200 nm the phase normals of both TE and TM polarization states are essentially planar inside the grating. The same holds true also for d = 386 nm for TM polarization, but not for TE state whose phase normals are roughly parallel to the grating pillars. At d = 420 nm the phase normals of both TE and TM polarization states are oriented along the pillars. Hence we may conclude that the qualitative picture is quite correct in this case. However, for larger periods, resonance effects change the picture, and phase normals cannot be easily distinguished. Hence the total phase shift is generally affected by many different sources, although the optical-path-effect discussed in Section 2 is usually the dominating source of phase retardation.

Fig. 3 Computed phase maps of the field with three different periods. The yellow outline marks the grating profile. One black–white–black sequence corresponds to π/4 phase shift. The propagation direction of the field is from left to right. The other grating parameters are taken from the first row of Table 1.

4. Experimental results

The restrictions set by our current lithography equipment prevented us from using the more optimal designs presented on first and second rows of table 1. Hence we fabricated an element with smaller slant angle Θ but with larger depth h (row three in table 1). Our goal was to experimentally prove the numerical predictions.

The example grating presented here was fabricated using the following process flow. First, a titanium dioxide layer was vacuum deposited on a fused silica substrate. A PMMA resist layer was then spincoated on top of the TiO2 layer. The resist layer was patterned using Vistec EBPG5000+ES HR Electron beam writer and developed in Methyl-isobutyl-ketone. The resulting resist grating was then used in standard chromium lift-off process to obtain a chromium grating on top of the TiO2 layer. The chromium grating was then used as a mask in reactive ion etching (RIE). In RIE, the sample surface was tilted to about 60 degrees to obtain slanted etching. Taking into account the properties of the plasma bombardment inside the etching chamber, 60 degrees tilting produces a slanted etching direction of 30 degrees, approximately. After the etching, the remaining chromium was removed by wet etching.

The cross section profile of the fabricated element can be viewed in the scanning electron microscope image in Fig. 4(a). First, it can be seen that the grating is over-etched and a sort of a bilayer structure is formed. The depth of the over-etched part in silicon dioxide is 180 nm, while the thickness of the TiO2 layer is 640 nm. It can also be seen that the sidewalls are not exactly parallel, a feature which is emphasized in the lower part of the grating. The slant angles of the left and right sidewalls are approximately 35 and 30 degrees. It is evident that this grating is not in total agreement with the design. However, in optical measurements it produced almost perfect half-wave retardation which was also verified numerically, using this actual grating profile in rigorous calculation.

Fig. 4 (a) The fabricated example grating in a cross section SEM image. The Grating period d =410nm and the slant angle is approximately 30 degrees. (b) Measured normalized analyzer transmission as a function of analyzer orientation. The black curve (substrate) represents the incoming linearly polarized beam. The red curve represents the beam altered by the half-wave retardation of the grating.

The phase retardation of the example grating was measured with quarter-wave-plate method, in which a HeNe-laser beam goes through an input polarizer, the grating element, a quarter-wave plate and an analyzer, in this order. The input polarizers transmission axis is oriented at 0 degrees. The grating is rotated so that the grating vector is at 45 degrees with respect to the input polarizer, whereas the quarter-wave plate is placed with its fast axis parallel to the input polarizer. Phase retardation of the grating is obtained from the analyzer angle corresponding to the minimum transmitted intensity (for more details see, for example, [10

10. B. Päivänranta, N. Passilly, J. Pietarinen, P. Laakkonen, M. Kuittinen, and J. Tervo, “Low-cost fabrication of form-birefringent quarter-wave plates,” Opt. Express 16, 16334–16342 (2008). [CrossRef] [PubMed]

]). The result for our example grating was 177 degrees phase retardation for λ =633 nm. The half-wave plate behavior was demonstrated simply by removing the quarter-wave plate from the above described setup, rotating the analyzer from 0 to 180 degrees and measuring the corresponding intensities, for both the grating and plain substrate. The resulting graphs are seen in Fig. 4(b). The black curve represents a laser beam going through the plain substrate while the red curve represents the same beam going through the grating. It can be seen that the beams are fully linearly polarized in orthogonal directions. The transmittance of the grating, compared to the plain substrate, is 95.5%.

5. Conclusion

A novel phenomenon concerning slanted dielectric subwavelength gratings was presented. A uniquely large phase retardation between TE and TM polarized waves occurs when light travels through a subwavelength grating with slanted sidewalls and correctly designed grating profile. With a certain grating period the TE polarization component is coupled into the slanted pillars of the grating while the TM polarization component travels through the grating rectilinearly. The phenomenon was explained analytically and with rigorous numerical calculations. This new property can be used to design grating-based wave plates with significantly lower grating depth than with traditional subwavelength gratings. The fabrication of a form-birefringent half-wave plate becomes significantly easier and also mass production of these elements is now possible. We have verified the theoretical predictions by fabricating an example grating in titanium dioxide. The phase retardation of the fabricated element was 177 degrees and its transmission was 95.5%. The future work in this topic could be investigating the mass production of the plastic structures (row four in table 1) and eventually their integration to micro-optical systems. We believe that these results are significant to all parties that are working with phase retardation issues.

Acknowledgments

The authors acknowledge the financial support from The Finnish Funding Agency for Technology and Innovation (TEKES), European Regional Development Fund (ERDF), and the Academy of Finland ( 118951).

References and links

1.

M. Born and E. Wolf, Principles of Optics, 7th ed. (Cambridge U. Press, 1999).

2.

D. C. Flanders, “Submicrometer periodicity gratings as artificial anisotropic dielectrics,” Appl. Phys. Lett. 42, 492–494 (1983). [CrossRef]

3.

C. W. Haggans, L. Li, T. Fujita, and R. K. Kostuk, “Lamellar gratings as polarization components for specularly reflected beams,” J. Mod. Opt. 40, 675–686 (1993). [CrossRef]

4.

N. Passilly, K. Ventola, P. Karvinen, P. Laakkonen, J. Turunen, and J. Tervo, “Polarization conversion in conical diffraction by metallic and dielectric subwavelength gratings,” Appl. Opt. 46, 4258–4265 (2007). [CrossRef] [PubMed]

5.

T. Isano, Y. Kaneda, N. Iwakami, K. Ishizuka, and N. Suzuki, “Fabrication of Half-wave Plates with Subwavelength Structures,” Jpn. J. Appl. Phys. 43, 5294–5296 (2004). [CrossRef]

6.

B. Wang, J. Jiang, and G. P. Nordin, “Compact slanted grating couplers,” Opt. Express 12, 3313–3326 (2004). [CrossRef] [PubMed]

7.

N. Bonod, E. Popov, L. Li, and B. Chernov, “Unidirectional excitation of surface plasmons by slanted gratings,” Opt. Express 15, 11427–11432 (2007). [CrossRef] [PubMed]

8.

L. Li, “Fourier modal method for crossed anisotropic gratings with arbitrary permittivity and permeability tensors,” J. Opt. A: Pure appl. Opt. 5, 345–355 (2003). [CrossRef]

9.

T. Levola and P. Laakkonen, “Replicated slanted gratings with a high refractive index material for in and outcoupling of light,” Opt. Express 15, 2067–2074 (2007). [CrossRef] [PubMed]

10.

B. Päivänranta, N. Passilly, J. Pietarinen, P. Laakkonen, M. Kuittinen, and J. Tervo, “Low-cost fabrication of form-birefringent quarter-wave plates,” Opt. Express 16, 16334–16342 (2008). [CrossRef] [PubMed]

OCIS Codes
(050.5080) Diffraction and gratings : Phase shift
(260.5430) Physical optics : Polarization
(050.6624) Diffraction and gratings : Subwavelength structures

ToC Category:
Diffraction and Gratings

History
Original Manuscript: September 14, 2010
Revised Manuscript: October 18, 2010
Manuscript Accepted: October 19, 2010
Published: December 22, 2010

Citation
Kalle Ventola, Jani Tervo, Pasi Laakkonen, and Markku Kuittinen, "High phase retardation by waveguiding in slanted photonic nanostructures," Opt. Express 19, 241-246 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-1-241


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References

  1. M. Born, and E. Wolf, Principles of Optics, 7th ed. (Cambridge U. Press, 1999).
  2. D. C. Flanders, “Submicrometer periodicity gratings as artificial anisotropic dielectrics,” Appl. Phys. Lett. 42, 492–494 (1983). [CrossRef]
  3. C. W. Haggans, L. Li, T. Fujita, and R. K. Kostuk, “Lamellar gratings as polarization components for specularly reflected beams,” J. Mod. Opt. 40, 675–686 (1993). [CrossRef]
  4. N. Passilly, K. Ventola, P. Karvinen, P. Laakkonen, J. Turunen, and J. Tervo, “Polarization conversion in conical diffraction by metallic and dielectric subwavelength gratings,” Appl. Opt. 46, 4258–4265 (2007). [CrossRef] [PubMed]
  5. T. Isano, Y. Kaneda, N. Iwakami, K. Ishizuka, and N. Suzuki, “Fabrication of Half-wave Plates with Subwavelength Structures,” Jpn. J. Appl. Phys. 43, 5294–5296 (2004). [CrossRef]
  6. B. Wang, J. Jiang, and G. P. Nordin, “Compact slanted grating couplers,” Opt. Express 12, 3313–3326 (2004). [CrossRef] [PubMed]
  7. N. Bonod, E. Popov, L. Li, and B. Chernov, “Unidirectional excitation of surface plasmons by slanted gratings,” Opt. Express 15, 11427–11432 (2007). [CrossRef] [PubMed]
  8. L. Li, “Fourier modal method for crossed anisotropic gratings with arbitrary permittivity and permeability tensors,” J. Opt. A, Pure Appl. Opt. 5, 345–355 (2003). [CrossRef]
  9. T. Levola, and P. Laakkonen, “Replicated slanted gratings with a high refractive index material for in and outcoupling of light,” Opt. Express 15, 2067–2074 (2007). [CrossRef] [PubMed]
  10. B. Päivänranta, N. Passilly, J. Pietarinen, P. Laakkonen, M. Kuittinen, and J. Tervo, “Low-cost fabrication of form-birefringent quarter-wave plates,” Opt. Express 16, 16334–16342 (2008). [CrossRef] [PubMed]

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