High efficiency, high selectivity ultra-thin resonant diffractive elements Svetlen Tonchev,1,2 Thomas Kämpfe,1 and Olivier Parriaux1,* 1
University of Lyon, Laboratoire Hubert Curien UMR CNRS 5516, 18 rue du Professeur Benoît Lauras, 42000 SaintEtienne, France 2 Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee Blvd., 1784 Sofia, Bulgaria *
[email protected]
Abstract: Resonant diffractive elements as the association of a surface corrugation with a surface wave exhibit boosted diffraction efficiency and high selectivity properties under the effect of ultra-shallow subwavelength surface reliefs. This is demonstrated by four examples of resonant functional structures made of very different material systems over the optical spectrum. All four structures are fabricated by slow wet etching as the inherent lateral broadening in corrugations of very small aspect ratio can be neglected. ©2012 Optical Society of America OCIS codes: (310.0310) Thin films; (050.1950) Diffraction gratings; (050.5745) Resonance domain; (050.6624) Subwavelength structures.
References 1. 2. 3. 4. 5.
6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.
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18. I. F. Salakhutdinov, V. A. Sychugov, A. V. Tishchenko, B. A. Usievich, O. Parriaux, and F. A. Pudonin, “Anomalous light reflection at the surface of a corrugated thin metal film,” IEEE J. Quantum Electron. 34(6), 1054–1060 (1998). 19. F. Garrelie, J.-P. Colombier, F. Pigeon, S. Tonchev, N. Faure, M. Bounhalli, S. Reynaud, and O. Parriaux, “Evidence of surface plasmon resonance in ultrafast laser-induced ripples,” Opt. Express 19(10), 9035–9043 (2011). 20. D. Basting, K. Pippert, and U. Stamm, “History and future prospects of excimer laser technology,” Riken Review no. 43, focused on 2nd International Symposium on Laser Precision Microfabrication (LPM2001), Jan. 2002. 21. H. Ridaoui, F. Wieder, A. Ponche, and O. Soppera, “Direct ArF laser photopatterning of metal oxide nanostructures prepared by the sol-gel route,” Nanotechnology 21(6), 065303 (2010). 22. Y. Jourlin, S. Tonchev, A. V. Tishchenko, C. Pédrix, O. Parriaux, D. Jamon, and F. Lacour, “Wideband, wide angular spectrum resonant reflection by mode coalescence in dual-mode slab waveguide,” presented at the 8th EOS Topical Meeting on Diffractive Optics, Delft, Netherlands, 27 Feb.-1 Mar. 2012.
1. Introduction Resonant diffractive optical elements, like standard DOEs, consist of a surface corrugation or index modulation modifying and shaping the wave front in the objective of achieving a desired optical function. The specific characteristic of a resonant DOE structure is the fulfillment of the optogeometrical conditions for the existence of a surface wave whose field has a substantial overlap with the corrugation, and that the latter can excite under definite synchronism conditions. This association between a surface wave and the surface or index modulation boosts the diffractive effect of the latter and gives it the selectivity properties of the former. The selectivity which the surface wave confers to the diffraction event concerns the polarization, the wavelength as well as the local wave vector. The examples given in the present paper will be listed according to the type of optical surface wave that concentrates the field in the corrugation region: true guided mode [1], leaky mode [2] and non-localized plasmon mode [3]. These three basic resonant grating structures are illustrated symbolically in Fig. 1 with the related surface wave field responsible for the incident wave field accumulation. Figure 1(a) corresponds to resonant reflection mediated by the coupled TE0 mode with the transverse electric field represented. Figure 1(b) is for –1st order resonant diffraction canceling the Fresnel reflection in a metal mirror based corrugated dielectric layer with the transverse electric field of the fundamental leaky-mode represented. The figure of 100% is the theoretically achievable efficiency in a single order. Figure 1(c) represents the effect of resonant light transmission through a continuous undulated metal film embedded in a homogeneous dielectric medium via the excitation of a TM plasmon mode; the field profile is that of the longitudinal electric field of the low-loss long range plasmon mode exhibiting a zero modulus at the middle of the metal film; the figure of 90% corresponds to the expectable transmission maximum in the red part of the spectrum with a silver or gold film of about 30 nm thickness.
Fig. 1. (a) True guided mode-, (b) leaky mode- and (c) plasmon mode-mediated resonant grating structures with modal field sketch.
So far the association between a surface microstructure and an optical resonance has been studied and applied essentially to periodic DOEs, and the present paper will limit its scope to resonant gratings although much is still to be explored in the objective of generating more #164762 - $15.00 USD Received 14 Mar 2012; revised 15 May 2012; accepted 15 May 2012; published 12 Nov 2012 (C) 2012 OSA 19 November 2012 / Vol. 20, No. 24 / OPTICS EXPRESS 26715
complex optical functions. As far as the fabrication issue is concerned, the lithography of resonant structures does not differ notably from that of standard DOEs. It is at the level of the etching that there are great differences as the surface wave structure may be made of (multi)layer materials of a wide diversity of refractive index and chemical composition. This is the reason for the next section devoted to the etching problematics. Sections 3 to 6 give examples of resonant functional structures all fabricated by wet etching processes. In section 3 is described a resonant grating mirror permitting to filter out transverse modes of order larger than the dominant mode. Section 4 gives two examples of polarizing laser mirrors generating a circularly symmetrical polarized mode by grating coupling to a leaky mode of the mirror’s multiplayer. In section 5 another wet etching process is applied for the definition of an ultra-shallow seed-grating at a metal surface for plasmon excitation, and, finally, section 6 shows that wet etching can even be applied to diffractive polarizing elements operating in the 193 nm ArF laser wavelength range. 2. Microetching of optical material surfaces There are strong incentives in planar photonics to borrow ready-developed microstructuring processes of microelectronics [4]. One problem with this manufacturing trend is that the standards are not - and cannot be - generalized to the same extent that the photonics market is very diverse with a plurality of 3D elements and modules, and thus the mechanism of economy of scale in microoptics is in many cases far from being effective. This problem is faced in photolithography, but it is particularly acute in etching where mainly silicon-based compounds (silica, SiON, silicon nitride) can borrow the well developed reactive ion etching (RIE) and related equipments provided the substrate size matches that of microelectronic wafers. The etching of high index metal oxide layers, of fluoride layers, or simply of a glass surface, requires some variant of reactive ion beam etching (RIBE) where a kinetic energy component is needed for evacuating the non-volatile decomposition products [5]. RIBE-like equipments are less standardized and do accept non-standard substrate sizes and materials. Dry etching is a passage obligé for corrugations of high aspect ratio. For resonant diffractive elements, it is not: whereas in a transmission grating of wavelength-scale period a typical aspect ratio (ridge height/width) is about 2 for the cancellation of the 0th order, that of a resonant grating can be notably smaller than 1/10 as will be shown in the examples hereunder. This means that the emerging domain of resonant diffraction may advantageously resort to wet etching technologies which permit non-vacuum batch processing at room temperature, chemical surface smoothing, and low investment costs. The chemical processes used here are toxic for microelectronics – so are they for optoelectronics too - but for passive element photonics they aren’t, and may therefore represent new reliable and low-cost manufacturing possibilities for this field. Some known generalities about spatially resolved surface wet etching will now be reminded and some hypotheses regarding the selection of the adequate chemistry and microstructuring process steps of optical material will be made. Passive microoptic permits to broaden the spectrum of acceptable chemical reactions beyond acidic solutions. These have the inherent tendency to create bubbles and exhibit a nonlinear etching rate at the beginning of the reaction which causes non-uniformity and nonreproducibility in the fabrication of very shallow microstructures. HF can etch SiO2, glasses and can even decompose some other metal oxides as well like Ta2O5, HfO2, but its use requires much care and it has the property of easily creeping between a substrate/photoresist interface which is a practical handicap for microstructuring objectives. Acids tend to render oxide surfaces hydrophilic, therefore the spreading of photoresist is often difficult and the resist adhesion is weak. Unlike microelectronics, photonics may opt for basic chemistry which offers a very wide range of possible solutions. Whereas the wet etching of metal layers relies upon oxidation-reduction reactions, inorganic dielectric layers (oxides, sulphides, fluorides, etc.) can be wet etched by designing reactions of the exchange type: it is well
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known that an exchange reaction can take place and be completed only if one of the compounds leaves the reaction [6]. To that end, the participation of a properly chosen complex-forming agent is required. A qualitative estimate of the electrochemical process direction in the substitution of ions is made by using the reactivity series [7]. The lower the electrode potential, the higher the reduction activity of the element and therefore the lower the oxidizing activity of its ions. The use of reactions with the participation of sodium and potassium ions in the role of strong reducers substantially increases the possibility to choose the most appropriate etching reaction; since most sodium and potassium compounds are water soluble, the exchange product dissolves in the often used water based solution. Using photoresist as a microstructuring etch-mask with a basic etchant should a priori be ruled out. However, there are baking and chemical passivation processes which render the photoresist etch-mask immune while still allowing an easy hot acetone removal without resist-rest at the surface [7]. This is an important asset because the advantages of basic etching should not be offset by the resort to an intermediate mask etching step if for instance a silica or metal etch mask would be required. Some specific precautions must be taken when using a basic wet etchant for high spatial resolution microstructuring: the photoresist layer must have a strong adhesion on the substrate to prevent non-uniformities and edge roughness. This can be solved by using standard adhesion promoters or some non-acidic surface treatment rendering the surface to be etched hydrophobic. Another very important precaution is to ensure the wettability of the groove bottom between photoresist walls. The above requirements of hydrophobicity of the surface for strong resist adhesion and wettability of the groove bottom after resist development are contradictory when water-based etch-solutions are concerned; this can be solved by adding a wetting agent to the etchant. Wet etching processes are very sensitive to the surface cleanliness and to residual nanolayer at the surface. This nanolayer can be the native oxide grown at a silicon or aluminum surface or a photoresist rest. Whereas dry etching is less vulnerable to a residual nanolayer than wet etching and permits to perform a short plasma cleaning prior to the actual dry etching process, wet etching can also be preceded by a very short wet treatment which dissolves for instance a native oxide nanolayer. The next section will describe a number of examples of high efficiency resonant structures of low aspect ratio and subwavelength dimension. 3. True-mode field enhancement and transverse laser-mode filtering The resonant structure concerned here is a slab waveguide with a grating coupler between an incident collimated free-space wave and a waveguide mode corresponding to Fig. 1(a). There are two main configurations as suggested by Fig. 2: oblique incidence for beam filtering, and normal incidence when the selectivity of resonant reflection is applied to intra-cavity laser emission control. When the coupling synchronism condition is fulfilled, the field in the grating region is very large and a corrugation depth of a small fraction of the wavelength is efficient enough to couple an incident beam of usual submillimeter diameter and to exploit the properties of this 0th order diffraction effect, for instance resonant reflection which theoretically reaches 100%. Since its experimental discovery [8] and its explanation as a waveguide grating feature in 1985 [9] the effect of resonant reflection has been mainly used as a narrow band reflection filter in biosensor [10] or in laser mirror applications [11]. The remarkable feature of resonant reflection is its polarization, wavelength and angular selectivity related with the grating excitation of a mode of a slab waveguide.
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Fig. 2. Grating coupling of a free space wave to a waveguide mode with associated wavelength, angular and polarization selective resonant reflection, (a) as a free space-wave filter, (b) as a laser mirror for emission control.
Wet etching is applied here in a 2D waveguide grating structure aimed at ensuring lossless single transverse mode filtering in a wide surface emitting Yb-, Er-doped microchip laser in order to achieve all at once high power and high brightness emission [12]. The angular width of a resonant grating mirror is set between the angular width of the fundamental and of the second order transverse mode. Such application requires a very shallow depth of hardly 30 nm and an extremely high uniformity of the depth and diameter of the holes in a high index Ta2O5 waveguide layer with a 2D period of 1100 nm in a hexagonal hole distribution meaning a groove aspect ratio of about 1/15. This demand can actually hardly be matched by RIBE, whereas a soft wet etching only can achieve it precisely with high uniformity within 1 or 2 nanometers. Figure 3(a) is an AFM scan of a wet etched hole in a Ta2O5 layer deposited by ion plating showing a bottom surface as smooth as that of the resist-protected top. Figure 3(b) is the top view of a hexagonal set of circular holes wet-etched in a Ta2O5 layer showing good uniformity. The etched Ta2O5 layer is only the waveguide part of the complete transverse mode selective element which also comprises a standard multilayer providing a non-selective reflection offset, and also a SiO2 overlay to decrease the modal field confinement in the Ta2O5 waveguide.
Fig. 3. (a) AFM scan of wet etched holes at the surface of an ion plated Ta2O5 layer. (b) SEM top view of the same with holes of 550 nm diameter.
The grating is transferred photolithographically by hard contact. The alkaline wet etching is made with an etching rate of 10 nm per hour. 4. Leaky-mode mediated polarization selection in laser mirrors The surface wave of a resonant structure does not have to be a true guided mode and the waveguide does not have to be a single slab layer. The waveguide can be a multilayer - as that of a highly reflective laser mirror for instance - and the resonance can be that of a leaky mode which permits one of the incident polarizations to tunnel through the multilayer mirror into a high index substrate by grating coupling to this mode, thus to degrade the reflection coefficient for this polarization, leaving the reflection of the non-coupled polarization unaffected, thus imposing the lasing of this polarization. Such a polarization selective mirror has been developed and fabricated [13] in the objective of checking the theoretical prediction of a possible 50 to 100% increase of laser machining efficiency [14] in a CO2 laser emitting #164762 - $15.00 USD Received 14 Mar 2012; revised 15 May 2012; accepted 15 May 2012; published 12 Nov 2012 (C) 2012 OSA 19 November 2012 / Vol. 20, No. 24 / OPTICS EXPRESS 26718
the radially polarized mode in comparison with the currently used circular polarization. A circular line resonant grating achieving high reflection coefficient for the local TM polarization and close to zero reflection for the local TE polarization reflects above 99% of the radially polarized laser mode and almost suppresses the reflection of the azimuthally polarized mode. Thanks to its resonant character, the almost 100% diffractive transmission through the highly reflective multilayer mirror is obtained by a corrugation grating of hardly 200 nm deep grooves in a thin germanium layer on top of a multilayer mirror. The radial period is about 6 µm for the 10.6 µm wavelength of a CO2 laser, meaning that a corrugation aspect ratio of about 1/15 suffices to almost cancel the TE reflection. Both dry and wet etchings have been used in the objective of comparing the profile roughness, the reproducibility as well as the fabrication cost. The dry process was argon ion beam etching whereas the wet chemistry was an alkaline solution with baked photoresist as an etch-mask. This is very well suited for wet etching, the more so as the polarizing function of the element is little dependent on the duty cycle. Figure 4(a), resp. (b) are the AFM picture of Ge grooves made by dry and wet microstructuring.
Fig. 4. AFM pictures of 200 nm deep, 3 µm wide grooves etched in a layer of amorphous Germanium. (a) Dry RIBE etching. (b) Alkaline wet etching.
The wet etched grooves are somewhat smoother; the groove bottom is flat because of the presence of an etch-stop layer guaranteeing a prescribed depth. The reflection spectrum of Fig. 5 is the result of the exact optimization of the resonant polarizing mirror. It exhibits a double TE dip whereas the typical signature of leaky-mode mediated transmission is a single dip while the TM polarization still experiences quasi 100% reflection.
Fig. 5. (a) Symbolic representation of leaky mode mediated TE tunneling into the substrate through the multilayer of a CO2 laser mirror. (b) Double-dip TE and TM reflection spectra of (a)
#164762 - $15.00 USD Received 14 Mar 2012; revised 15 May 2012; accepted 15 May 2012; published 12 Nov 2012 (C) 2012 OSA 19 November 2012 / Vol. 20, No. 24 / OPTICS EXPRESS 26719
This double-dip character illustrates how interestingly the resonant coupling mechanism can be tailored: in order to broaden the tolerances on the ZnSe/ThF4 multilayer, the multilayer was engineered so as to bring two TE leaky modes in coalescence which considerably widens the reflection dip. As a matter of fact, the reproducibility in index and thickness of the multilayer is much more critical than the corrugation profile; the latter is very tolerant and essentially relies upon the amplitude of its first Fourier harmonic. Figure 5(a) is a symbolic representation of the resonant leak of the TE polarization through the multilayer mirror; in a radial polarization generating mirror the grating shown has circular lines. Experimental results and donut beam profiles can be found in [13]. An alternative polarizing principle was also developed which is more appropriate for obtaining the lasing of the azimuthally polarized mode whereby the local TM polarization leaks through the multilayer mirror and the TE polarization is highly reflected. This polarization distribution has been shown to permit the machining of deep uniform diameter holes in metals whereas the radial polarization is better suited for cutting thick metal plates [15]. This new scheme was also demonstrated at the CO2 laser wavelength, but it is its implementation in the 1.0 to 1.1 µm range which is particularly interesting here. As shown in the reflection spectra of Fig. 6, the reflection differential between TE and TM is very wide band and extends over about 70 nm.
Fig. 6. TE and TM reflection spectra in the near IR of a wide band polarization selective laser mirror with moderate reflection differential for a high Q laser resonator. Inset: experimentally obtained donut mode with radial polarization, resulting in the typical bow-tie shape after a 45° linear analyzer.
The laser mirror consists of a SiO2/HfO2 multilayer with a last high index thin layer of hydrogenated amorphous silicon of about 50 nm thickness which, unlike single crystal silicon, is highly transparent in this wavelength range. With a radial period of 900 nm, the requested aspect ratio is about 1/10 which is again very well suited for wet silicon etching. The etching was made by a basic solution (30% water-diluted KOH at room temperature) with a baked photoresist etch-mask at a rate of 6 nm per minute. Figure 7 is the AFM scan of a few grating lines showing high smoothness and uniformity.
#164762 - $15.00 USD Received 14 Mar 2012; revised 15 May 2012; accepted 15 May 2012; published 12 Nov 2012 (C) 2012 OSA 19 November 2012 / Vol. 20, No. 24 / OPTICS EXPRESS 26720
Fig. 7. AFM picture of a small aspect ratio wet etched amorphous silicon grating showing the 450 nm wide, 50 nm deep grooves of the polarization selective laser mirror of Fig. 6.
The experimental TE and TM reflection spectra of Fig. 6 are measured under close to normal incidence to shun a beam splitter. The inset is the picture of the characteristic donut beam of azimuthal polarization distribution emitted by a Nd:YAG laser equipped with a circular line grating mirror. 5. Non-localized plasmon field enhancement The surface wave used in a resonant grating can also be the surface plasmon propagating at the interface between a metal surface and a dielectric overlay, or simply air. The remarkable features of a resonant grating relying upon the electromagnetic field concentration in a nonlocalized plasmon wave are total absorption of an incident TM wave [16] and resonant transmission through a continuous undulated thin metal film [17] as illustrated in Fig. 1(c). A weak TM resonant reflection can also be observed in symmetrical structures in the presence of the long range surface plasmon mode [18]. The example chosen here of wet etched subwavelength resonant grating is not application driven: it is part of a scientific endeavor to identify and analyze the role of surface plasmons in the formation of ripples at the surface of a metal submitted to high energy femtosecond laser pulses [19]. A number of very shallow sinusoidal undulations (about 10 nm depth) of different period (from 440 to 800 nm every 10 nm) were made on a nickel surface to act as a seed for plasmon excitation, the aim being to find out the period at which the ripple formation under normal incidence of femtosecond pulses is enhanced.
Fig. 8. (a) Picture under white light illumination of the Ni test-nanogratings of differing periods. (b) AFM scan of a 560 nm period, 10 nm deep undulation at the surface of a Nickel plate. (c) Histogram of ripple formation versus the period of the seed grating.
So shallow a corrugation cannot easily be made uniformly by dry etching. The alternative wet process was preceded by a resist photochemistry adapted to permit grating exposure on a high-reflectivity metallic substrate. A difficulty faced with a metal surface is that the electric field at the surface is close to zero since the TE polarization must be used to create a high contrast interferogram. The risk is therefore high that there still are some nanometers of unexposed and undeveloped photoresist at the groove bottom after development which prevents the wet etching of the metal substrate in the grooves to take place. To get round this
#164762 - $15.00 USD Received 14 Mar 2012; revised 15 May 2012; accepted 15 May 2012; published 12 Nov 2012 (C) 2012 OSA 19 November 2012 / Vol. 20, No. 24 / OPTICS EXPRESS 26721
difficulty one first makes a preliminary uniform exposure with non-structured light to deliver small but non-zero energy dose everywhere within the resist layer. This dose offset is adjusted to remain just below the photomodification threshold of the resist. The sample is then placed in the interferogram of a conventional Mach-Zehnder scheme. In the presence of the initial dose threshold all points of the resist layer located where an interference fringe is created get photomodified however small the dose delivered by the interference fringe. Thus all grooves are open down to the nickel substrate. The physical transfer of the grating from the resist layer to the nickel substrate was made by wet etching using an acidic solution. Nickel requires an acid plus an oxidizer to etch properly. Diluted nitric acid (HNO3) contains both. A 10:1 dilution gives an etching speed of 1 nm/sec. Figure 8(a) is the picture of the complete set of etched Nickel test samples with differing periods under white light illumination. Figure 8(b) is the AFM profile of one of the shallow Nickel surface corrugations; the relatively large roughness is an effect of the Nickel surface being unpolished as laminated. Figure 8(c) shows the scientific result obtained with single 150 fs pulse of 0.97 J/cm2 fluence in the form of an histogram demonstrating first that the surface plasmon does mediate the formation of ripples, and revealing secondly that a seed of ca 750-760 nm period enhances the ripple formation instead of the 790 nm expected from the bulk permittivity of Nickel which means that a femtosecond pulse modifies the electron density of the electrons participating in the plasmonic collective oscillation. The interested reader is invited to refer to Ref [19]. 6. Resonant grating for the control of deep-UV laser sources The wide application field of DUV is still an observation and exploration field for diffractive optics technologies. The required periods are well below 200 nm and the materials to be microstructured are difficult to etch. Some are aluminum-based like the high index LuAG, and most of them used in multilayers are fluorides such as LaF3, MgF2. One application that attracts industrial interests is certainly the control of the spatial and temporal coherence of KrF, and especially ArF excimer lasers, for instance the polarization control which is so far made by means of a cascade of intra-cavity prismatic Brewster elements [20]. In principle the type of solutions described above at a larger wavelength could be implemented. One of the problems is the etching of, e.g., 150 nm period gratings at a depth of hardly 10 nm. The use of standard RIBE is not appropriate since the first nanometers would be etched during the first seconds where a stable plasma regime is not established yet. Again, wet etching offers its solution which might even be here a passage obligé. Figure 9 shows 11 nm deep grooves obtained by wet etching in a LaF3 layer at a rate of 20 nm per minute. Here an exchange reaction was used with a complex-forming agent for taking off the fluorine from the reaction as a soluble compound. The etchant is basic (30% water-diluted NaOH at room temperature): Na combines with fluorine to form NaF which is soluble in water. During the present phase of etch-process setting up, the period is here larger than that required for a functional element at 193 nm wavelength.
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Fig. 9. AFM scan of an 11 nm deep wet etched corrugation in a layer of LaF3.
Figure 10 gives the reflection spectra of the grating mirror structure under development. The element is a shallow grating etched into the last LaF3 layer of a AlF3/LaF3 multilayer mirror. The period is 136 nm with a corrugation depth of 15 nm. It is difficult to achieve a diffractive polarization dichroism in a multilayer system in the deep UV as there are no high index materials available in a layer form. In the present AlF3/LaF3 system the index contrast is 1.39/1.66. This limits the possibilities of obtaining a high reflection differential between the TE and TM polarizations. The principle which is most adequate in this low index contrast system is to combine a standard quarter wave submirror composed here of 17 pairs of low and high index with a superstructure of a few layers comprising a corrugated last high index LaF3 layer playing the role of a close to 100% reflective grating waveguide of 52 nm thickness. This resonant grating is the second submirror of the complete mirror structure; it is a mirror for the TE polarization only since the waveguide thickness is adjusted to propagate the fundamental TE0 mode at the wavelength of 193 nm. The TM polarization does not couple to the TM0 mode of this waveguide whose resonance is located elsewhere in the spectrum, and only “sees” the multilayer submirror. Below the last high index waveguiding layer are a few layers whose role is to define a spacing between the multilayer submirror and the resonant TE submirror corresponding to a Fabry-Perot filter in its first resonant transmission peak at 193 nm wavelength. As a result, the TE reflection spectrum exhibits a deep reflection dip whereas the TM reflection spectrum is hardly affected and remains close to 100%. The reason why the ± 1st orders of the grating do not lead to diffraction losses for the TM polarization is that with a grating period of 136 nm and a wavelength of 193 nm the field of the 1st order in the low index layers is evanescent which forbids their transmission through the multilayer.
#164762 - $15.00 USD Received 14 Mar 2012; revised 15 May 2012; accepted 15 May 2012; published 12 Nov 2012 (C) 2012 OSA 19 November 2012 / Vol. 20, No. 24 / OPTICS EXPRESS 26723
Fig. 10. Reflection spectra of a polarizing grating mirror for an ArF excimer laser
The development of the technology of this deep UV polarizing laser mirror is still in progress. The optical lithography used for the definition of the needed 136 nm period grating pattern is the 272 nm period phase-mask transfer under normal incidence exposure of a TEpolarized ArF laser beam at 193 nm wavelength [21]. The main technical message of the present example is that the wet etching of such non-conventional layer material as LaF3 is under control. 6. Conclusion The present paper illustrates phenomenologically, technologically and experimentally that resonant gratings can be defined all through the optical spectrum and perform optical functions which conventional elements or modules don’t, or only with difficulties. The association of a corrugation and a surface wave gives the diffraction event a high contrast and high selectivity with surface reliefs having an aspect ratio of a small fraction only of the wavelength. While such characteristic does not necessarily ease the microstructuring by high etching rate standard dry etching technologies, it permits to resort to the very wide and diverse potential of well known and documented wet chemistry solutions. The examples shown are limited to periodic gratings. This is however not a technological limitation; it is rather an indicator of the present development stage of R&D on resonant diffraction. Besides, the application examples given above mainly refer to the processing of highly coherent light waves and beams. This is not a limitation either. The availability of high and very high index layer materials permits an extent of the applicability domain of resonant functional elements to light beams of wider wavelength and angular spectra [22]. Acknowledgment The authors are grateful to Mrs. S. Reynaud for the AFM scans. They thank Mr. Deyan Gergov of Laserproduct company, Sofia, Bulgaria, for making the CO2 and Nd:YAG lasers available for the testing of the grating mirrors.
#164762 - $15.00 USD Received 14 Mar 2012; revised 15 May 2012; accepted 15 May 2012; published 12 Nov 2012 (C) 2012 OSA 19 November 2012 / Vol. 20, No. 24 / OPTICS EXPRESS 26724
