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Applied Optics, 50, 9, pp. C396-C402, 2011-02-18

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Implementation of long-wavelength cut-off filters based on critical

angle

Guo, Yanen; Dobrowolski, J. A.; Li, Li; Poitras, Daniel; Tiwald, Tom

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Implementation of long-wavelength cut-off

filters based on critical angle

Yanen Guo,

1

J. A. Dobrowolski,

1,

* Li Li,

1

Daniel Poitras,

1

and Tom Tiwald

2

1National Research Council of Canada, 1200 Montreal Road, Ottawa, Ontario, Canada K1A 0R6 2J. A. Woollam Co., Inc., 645 M Street, Suite 102, Lincoln, Nebraska 68508, USA

*Corresponding author: dobrowolski@magma.ca Received 4 August 2010; accepted 1 September 2010;

posted 16 September 2010 (Doc. ID 132815); published 18 February 2011

Our first attempts at the fabrication of long-wavelength infrared cut-off filters with extended transmis-sion and rejection regions that are based on the use of the critical angle, the dispertransmis-sion of refractive indices, and on thin-film interference were not very successful. The design of the filter consisted of layers placed at the interface between two high-index prisms. Using the available deposition equipment, the layers produced were porous and very rough. The pores adsorbed water vapor, which resulted in absorp-tion. The roughness made the process of optical contacting very difficult. In this paper we describe the adjustments in the design and deposition processes that allowed us to obtain filters with a better and more stable performance. © 2011 Optical Society of America

OCIS codes: 120.2440, 260.2030, 310.6860, 260.6970, 310.1860, 240.1485.

1. Introduction

In 1999 we described a new type of long-wavelength cut-off filter that was based on thin film interference and critical angles as well as on the use of at least one material with a rapid variation of the refractive in-dex with wavelength [1]. Examples of materials with such behavior are, for example, some transparent conducting coatings and Reststrahlen materials. Layers of these materials need to be embedded be-tween two high-refractive-index prisms, together with layers made of a second material whose purpose is to suppress reflection within the pass band of the filter. The rejection region is caused by a combination of reflection, absorption, and critical angle effects. Because of the high-refractive index of the prisms, the best method for combining the two prisms is through optical contacting. A detailed description of the theory of such filters and some numerical re-sults for devices based on indium tin oxide (ITO) and magnesium oxide layers can be found in [1,2].

A tolerance analysis of our original ITO-based sys-tem indicated that the performance of the designs based on this principle would be very insensitive to film thickness variations but quite sensitive to var-iations in the optical constants [1]. Published data showed that the optical constants of ITO layers de-pend critically on the doping level and on the process used for their production. We therefore felt that it would be easier to verify experimentally our numer-ical results by constructing a device based on MgO, a material with a simple stoichiometry and a Rest-strahlen reflection peak in the 22 μm spectral region and a refractive index that increases significantly at shorter wavelengths.

In Section 2 of this paper we describe our first attempt at producing such a filter. In Section3, the results of our second attempt are presented. This is followed by some conclusions in Section 4.

2. First Deposition Attempt

The refractive index profile of the long-wavelength cut-off filter that we decided to produce is shown in Fig. 1(a). As already mentioned in the Introduc-tion, the material in which the critical angle occurs is MgO. The high-refractive index layers are made of

0003-6935/11/09C396-07$15.00/0 © 2011 Optical Society of America

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zinc sulfide, and in combination with the low index layers they acted as antireflection coatings for the pass band region. The multilayer consists of 21 layers with a total metric thickness of 10:3 μm and is sandwiched between two right-angled Cleartran prisms. Cleartran is a chemically vapor-deposited ZnS material that is transparent throughout the visi-ble and near-infrared parts of the spectrum [3]. To reduce the deposition time, the thin-film design was intentionally symmetrical, so that half the sys-tem could be deposited in one run onto two identical prisms that were later contacted to each other. The optical constants of the coating materials used in the calculations are given in Fig.2(a). They were ob-tained from measurements with visible and infrared spectrophotometic ellipsometers of single layer samples.

The calculated average transmittance of this filter for both polarizations is shown in Fig. 1(b). It was shown in our previous paper that there is very little polarization dependence of these curves in the cut-off and rejection regions [1]. Also indicated in this dia-gram is the expected transmittance of the system when the outer surfaces of the prisms are not pro-vided with antireflection coatings. Figure 3 shows the calculated error corridors resulting from 10% re-lative and 10 nm absolute random thickness

varia-tions in all the layers [Figs. 3(a) and 3(b)] and for 0.1% and 0.01% absolute random variations in the refractive indices of the ZnS and MgO materials [Figs.3(c)and3(d)]. These corridors were calculated from 50 randomly generated layer systems with the above errors and indicated that with such random er-rors, one would expect the transmittance curves of 66% of the experimentally produced coatings to fall within the corridors. These results confirm for the MgO case once again the conclusions we drew in [1] concerning the sensitivity of the designs to thickness and optical constant variations.

In Fig.4(a)we show the dependence of the design depicted in Fig.1(a)on the angle of incidence. As can be seen from this diagram, a limited tuning of the cut-off wavelength is achieved by adjusting the angle of incidence of the light on the coatings. However, this also results in the deterioration in the perfor-mance of the filter if convergent light is used [see Fig. 4(b)].

Because of the use of ZnS layers, magnetron sput-tering, which usually results in very accurate and dense layers, could not be used for this project be-cause our sputtering systems cannot deposit ZnS layers. The only equipment available in our labora-tory in which MgO and ZnS could be deposited in one run was an electron beam evaporation system

1.0 1.5 2.0 2.5 0 5 10 15 N = 21 Σ(d ) = 10.3 µm Refractive index Metric thickness (µm) MgO ZnS (a) Cleartran 0.0 0.2 0.4 0.6 0.8 1.0 0 2 4 6 8 10 12 14 16 18 20 Tavg e (b) calculated calc., no AR's Wavelength (µm) R T nm 450

Fig. 1. (Color online) Long-wavelength infrared cut-off filter based on 21 layers of MgO and ZnS placed between two Cleartran prisms. (a) Refractive index profile, (b) calculated transmittance for light of average polarization. Also shown is the effect of non-antireflection-(AR-) coated prism surfaces on the transmittance.

0.0 0.5 1.0 1.5 2.0 2.5 n MgO k MgO n ZnS k ZnS 0 2 4 6 8 10 12 14 16 18 20 O p ti ca l co nst a n ts n, k (a) Wavelength (µm) Wavelength (µm) k ZnS n ZnS n MgO k MgO 0 2 4 6 8 10 12 14 16 18 20 (b) n Al 2O3 n SiO 2 k Al 2O3 k SiO 2

Fig. 2. (Color online) Optical constants of the coating materials used in the calculations in this paper. (a) Experimentally determined optical constants of MgO and ZnS, (b) optical constants of Al2O3and SiO2taken from Palik’s books [10,11].

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equipped with an ion source (Balzers BAK 760 box coater with a 980 cm diffusion pump). ZnS is quite dense when evaporated at room temperature but MgO is not [4]. Raising the substrate temperature to 250° was not very effective in reducing the porosity of the MgO layers and, in addition, it reduced the sticking coefficient for the ZnS layers [5]. To obtain nonabsorbing MgO films, a residual atmosphere of 6× 10−5Torr of O

2 was required. Unfortunately,

the lifetime of the ion gun filaments under these con-ditions was shorter than the time required for the de-position of this thick-layer system, and so the MgO layers had to be evaporated without ion-assist.

Our first experimental results presented at the 2007 Optical Interference Coatings conference were

rather disappointing [6]. One reason was that the rough surfaces of the resulting multilayer made it difficult to optically contact the two coated prisms afterwards. Figures5(a)and5(b)show the rms sur-face roughness measured for the sursur-faces of BK7 wit-ness glasses without and with the multilayer that was deposited onto the Cleartran prisms during the first attempt. The witness glass was placed next to the prisms, so that it received the same coating. One can see that after the coating the RMS rough-ness increased from 14:8nm to a value of 28:6nm. Even after some light polishing the two coated prisms could not be contacted without the use of a few drops of liquid acetone.

0.0 0.2 0.4 0.6 0.8 1.0 theoretical error corridor Tav g e Tav g e (a) 10% thickness error theoretical error corridor (b) 10nm thickness error 0.0 0.2 0.4 0.6 0.8 1.0 theoretical error corridor 0 2 4 6 8 10 12 (c) Wavelength (µm) Wavelength (µm) 0.1 ZnS index error theoretical error corridor 0 2 4 6 8 10 12 (d)

0.01 MgO index error

Fig. 3. (Color online) Error corridors resulting from 10% and 10 nm random variations in the thicknesses of all the layers (a), (b) and for 0.1 and 0.01 random variations in the refractive indices of ZnS and MgO, respectively (c), (d).

0.0 0.2 0.4 0.6 0.8 1.0 44o angle 45o angle 46o angle 47o angle 0 2 4 6 8 10 12 Tavge (a) Wavelength (µm) Wavelength (µm) 0o cone 1o cone 2o cone 3o cone 4o cone 5.0 cone 0 2 4 6 8 10 12 (b)

Fig. 4. (Color online) Angular performance of the design of Fig.1. (a) Variation of cut-off wavelength with angle of incidence, (b) effect of convergent light.

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In addition, the MgO layers that made up most of the layer system were porous and adsorbed water va-por. This gave rise to a significant absorption in the near infrared spectral region. The average transmit-tances for s- and p-polarized light of the filter pro-duced under these conditions were measured for several angles of incidence both on a Nicolet FTIR Magna IR 550 series II spectrophotometer with a di-vergent beam and on a Woollam IR variable angle spectrophotometric ellipsometer with a collimated beam. However, here we only present the ellips-ometer data because they are more accurate for our angle sensitive filters (see Fig.4). In Fig. 6we com-pare the expected performance (a) at the design angle with the ellipsometric measurement (b). As mentioned before, this performance was less than satisfactory.

3. Second Deposition Attempt

It followed from the above that the two problems to overcome were the absorption of the water vapor in the MgO films and the excessive roughness that pre-vented an easy optical contacting of the coated prisms. Ritter in 1976 reported that Al2O3 forms

dense, amorphous coatings even when evaporated onto room temperature substrates [7]. In the past, Al2O3 has been used for enhancing the adhesion of

evaporated films to substrates, as well as smoothing

layers and, more recently, to prevent the breakup of thick MgF2 layers [8]. We found that when Al2O3

layers of approximately 80 nm thicknesses were de-posited on porous coatings in the presence of an ion beam, they sealed the pores against moisture pene-tration as well as smoothed the surface of the coating and thereby interrupted the conical growth within the MgO layer. This resulted in much smoother surfaces. Such thin thicknesses of Al2O3layers were

relatively small compared to the total thickness of the MgO layer so that their absorption in the long-wavelength region did not seriously impact the performance of the system.

The ease with which flat surfaces can be optically contacted also depends to a certain extent on the ma-terials of the surfaces that are to be joined. We there-fore also investigated numerically the effect on the performance of the layer system with three different thin layers of different materials placed at the center of the MgO layer. The materials chosen for these cal-culations were ZnS, air, and SiO2. In each case, it was

assumed that the total metric thickness of the layers in the contacted systems was 60 nm. In the case of the air gap, the optical contacting would take place on special small contact areas made of silica layers and placed at the edges of the prisms (see [9]). The refractive index profiles and the calculated

4000 nm 4000 nm 4000 nm

(a) (b) (c)

Fig. 5. (Color online) Surface roughnesses of multilayer coatings. (a) Witness glass without any coatings (RMS roughness 14:8nm), (b, c) witness glasses with the coatings deposited during first and second manufacturing attempts had RMS roughnesses of 28.6 and 2:96nm, respectively. 0.0 0.2 0.4 0.6 0.8 0 2 4 6 8 10 12 14 16 18 20 Tav g e (a) Wavelength (µm) Wavelength (µm) 0 2 4 6 8 10 12 14 16 18 20 (b)

Fig. 6. (Color online) Comparison of (a) the calculated expected and (b) the ellipsometrically measured performances of the first attempt of the manufacture of the IR long-wavelength cut-off filter.

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performances of a layer system containing the above three layers are shown in Fig.7. It can be seen from this figure that the best performance is obtained when the central layer is made of SiO2. Fortunately

this is also a material that can be easily optically con-tacted. In these calculations, the optical constants for Al2O3 and SiO2 [Fig. 2(b)] were taken from Palik’s

books [10,11]. Both of these materials have Rest-strahlen peaks at shorter wavelengths than MgO but do not significantly absorb at wavelengths short-er than the cut-off wavelength of the filtshort-er to be con-structed, and so they can be used in this application.

In view of the above experiments and calculations, we modified the 21-layer system shown in Fig.1 ac-cordingly, and the final design consisted of a total of 67 layers. With the exception of the central SiO2

layer, all the other additional layers were made of Al2O3. The refractive index profile and the calculated

performance of this new system are shown in Fig.8; the overall thickness of the system increased from 10.3 to 12:3 μm, with a slight deterioration in the per-formance. Since the total thickness of the Al2O3

layers is only a small fraction of the total thickness of the MgO layer, it was possible to use the ion gun during their deposition.

1.0 1.5 2.0 2.5 0 5 10 15 N = 21 Σ(d ) = 10.3 µm Refractive index Metric thickness (µm) MgO ZnS (a) Cleartran SiO 2 air 0.0 0.2 0.4 0.6 0.8 1.0 0 2 4 6 8 10 12 14 16 18 20 Tav g e Tav g e (b) Wavelength (µm) Wavelength (µm) Wavelength (µm) 60 nm of ZnS at center 0.0 0.2 0.4 0.6 0.8 1.0 0 2 4 6 8 10 12 14 16 18 20 Tavge (c) 60 nm of air at center 0.0 0.2 0.4 0.6 0.8 1.0 0 2 4 6 8 10 12 14 16 18 20 (d) 60 nm of SiO 2 at center

Fig. 7. (Color online) The effect on the calculated performance of the multilayer of Fig.1of placing thin layers of ZnS, air, and SiO2at the

center of the 21-layer system. (a) Refractive index profiles, (b), (c), (d) calculated transmittances for light of average polarization for central layers made of ZnS, air, and SiO2, respectively.

1.0 1.5 2.0 2.5 0 5 10 15 N = 67 Σ(d) = 12.3 µm Re fr a c ti ve i n de x

Metric thickness (µm) Wavelength (µm)

MgO ZnS (a) Cleartran SiO 2 Al 2O3 0.0 0.2 0.4 0.6 0.8 1.0 0 2 4 6 8 10 12 14 16 18 20 Tavg e (b) calculated calc., no AR's

Fig. 8. (Color online) Long-wavelength infrared cut-off filter based on MgO with thin Al2O3and SiO2layers. (a) Refractive index profile of

the 67 -layer system, (b) calculated transmittance for light of average polarization. Also shown is the effect of non-AR coated prism surfaces on the transmittance.

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Coatings according to this design were produced on two Cleartran prisms and on a witness glass. The surface roughness of one-half of the layer system deposited in this way is shown in Fig.5(c). It can be seen that the outermost surface now has an RMS roughness of only 2:96nm, which is not only smooth-er than the surface of the coating produced at the first attempt but also is even smoother than the sur-face of the uncoated witness glass. The two coated prisms produced during this second deposition at-tempt could be readily optically contacted.

The transmittance of the modified cut-off filter was measured soon after its manufacture at J. A. Woollam Co. Ltd. on an IR variable angle spectrophotometric ellipsometer. The results of these measurements are compared to the calculated transmittance in Fig.9(a). Clearly, the calculated and measured trans-mittances are now in much better agreement. One year later the cut-off filter was remeasured at the "Na-tional Research Council of Canada (NRC) on its newly acquired Woollam IR variable angle spectrophoto-metric ellipsometer. The agreement between these two measurements is very good (see Fig.9(b)), espe-cially if one considers that two different operators performed the measurements on two different instru-ments with different setups, likely on different parts of the filter.

4. Conclusions

Our first attempts at producing critical angle long-wavelength infrared cut-off filters made of MgO and ZnS layers were not very successful because the deposition process available to us resulted in por-ous layers with high RMS surface roughness. Because of the roughness, optical contacting of the two coated prisms was very difficult. In addition, the pores filled up with water vapor and the filters showed strong ab-sorption in the transmission band. We modified the design and the deposition process and used thin Al2O3layers to seal the pores in the MgO layer and

to interrupt the growth of a columnar structure with-in the MgO and thus produced smooth with-interfaces. A thin SiO2layer was used to facilitate the optical

con-tacting. This is a prime example of how frequently a

designer needs to incorporate layers in a thin film de-sign not for their optical effect but to meet other non-optical requirements. We think that some of the techniques that we have used and described in this paper might be useful in other application areas such as, for example, broadband, wide-angle polarizers.

Measurements performed on the cut-off filter after 1 yr did not reveal any significant ageing of the trans-mittance curve. We conclude that it is possible to pro-duce reasonable long-wavelength cut-off filters based on the above principle. However, filters of this type will be useful only when used with well-collimated light.

We believe this same principle could be used to produce cut-off filters operating at other wave-lengths. On the basis of the behavior of the optical constants of Al2O3 and SiO2 depicted in Fig.2(b) it

would appear that thick layers of these materials would also appear to be good candidates for cut-off filters effective at somewhat shorter wavelengths. Further tuning of the cut-off wavelength could prob-ably be achieved by mixing MgO or one of the above compounds with other transparent materials.

We would like to acknowledge the generous help of Alexei Bogdanov and Guy Parent with the atomic force microscopy roughness measurements, and Ian Miller and Chris Wimperis of LightMachinery with optical contacting.

References

1. J. A. Dobrowolski, L. Li, and J. Hilfiker, “Long-wavelength cut-off filters of a new type,” Appl. Opt. 38, 4891–4003 (1999). 2. J. A. Dobrowolski and L. Li, “Cut-off filters,” U.S. patent

6,271,968 (August 7, 2001).

3. Advanced Materials’ CLEARTRAN. See, for example,http:// www.lightmachinery.com/Cleartran.pdf.

4. A. P. Bradford, G. Hass, and M. McFarland, “Optical proper-ties of evaporated magnesium oxide films in the 0:22–8 μ wavelength region,” Appl. Opt. 11, 2242–2248 (1972). 5. M. Y. Znamenskii and V. A. Martsinovskii, “Study of the

de-pendence of the condensation factors of the films Ge, SiO, ZnS, ZnSe, and PbTe on the temperature of the covered substrate and their effect on the spectral characteristics of narrow-band interference filters,” J. Appl. Spectrosc. 46, 70–72 (1987). 0.0 0.2 0.4 0.6 0.8 Calcuated Tavge Tavgeon manufacture 0 2 4 6 8 10 12 14 16 18 20 Tav g e (a) Wavelength (µm) Wavelength (µm) Tavge on manufacture Tavge one year later

0 2 4 6 8 10 12 14 16 18 20

(b)

Fig. 9. (Color online) Comparison of (a) calculated and the measured average transmittances Tavge of the second attempt of the manufacture of the IR long-wavelength cut-off filter, and (b) transmittance spectra measured at J. A.Woollam Co. Inc. immediately after the manufacture and 1 year later at the NRC.

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6. J. A. Dobrowolski, Y. Guo, L. Li, and T. Tiwald, “Implementa-tion of long-wavelength cut-off filters based on critical angle,” in Optical Interference Coatings, OSA Technical Digest Series (Optical Society of America, 2007), paper TuD2.

7. E. Ritter, “Properties of optical thin film materials,” Appl. Opt. 20, 21–25 (1981).

8. J. A. Dobrowolski, Y. Guo, T. Tiwald, P. Ma, and D. Poitras, “Toward perfect antireflection coatings. 3. Experimental

re-sults obtained with the use of Reststrahlen materials,” Appl. Opt. 45, 1555–1562 (2006).

9. L. Li and J. A. Dobrowolski, “Optical coatings with an integral FTIR air layer,” Opt. Express 18, 3784–3792 (2010). 10. E. D. Palik, Handbook of Optical Constants of Solids, Vol. 1

(Academic, 1985), pp. 749–763.

11. E. D. Palik, Handbook of Optical Constants of Solids, Vol. 2 (Academic, 1991), pp. 761–776.

Figure

Fig. 1. (Color online) Long-wavelength infrared cut-off filter based on 21 layers of MgO and ZnS placed between two Cleartran prisms.
Fig. 4. (Color online) Angular performance of the design of Fig. 1. (a) Variation of cut-off wavelength with angle of incidence, (b) effect of convergent light.
Fig. 6. (Color online) Comparison of (a) the calculated expected and (b) the ellipsometrically measured performances of the first attempt of the manufacture of the IR long-wavelength cut-off filter.
Fig. 7. (Color online) The effect on the calculated performance of the multilayer of Fig
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