Thin film polarizing device having metal-dielectric films

A thin film polarizing device has a functional core consisting of at least two alternating thin film layers of metal and dielectric sandwiched between a pair of optical media providing input and output ports. The thicknesses of said layers and the optical constants of said layers and said optical media are selected such that the equivalent admittance of said functional core substantially matches the admittance of said optical media for one plane of polarization, thus allow light with said polarization to be transmitted, and has substantially only an imaginary part for the other plane of polarization, thus allows light with the other polarization to be reflected, at predetermined wavelengths and angles of incidence.

FIELD OF THE INVENTION
 This invention relates to the field of optics, and more particularly to
 thin film polarizing devices.
 BACKGROUND OF THE INVENTION
 Polarizing devices, including polarizers and polarizing beam-splitters
 (PBS), are essential optical components and are currently widely used in
 optical instruments, lasers, electro-optic displays, optical recording,
 etc. In polarizers, only the transmitted or reflected light is used, the
 other beam is of no essence. In PBSs, both the transmitted and the
 reflected beams are utilized and are equally important. Several parameters
 are often used to describe the performance of a polarizer or PBS: 1) the
 wavelength range over which the polarizer or PBS is operating; 2) the
 angular field of the incident light in which the polarizer or PBS is
 effective; 3) the extinction ratio of the desired to the unwanted a
 polarized light after the light passes through or is reflected by the
 polarizer or PBS; and 4) the transmittance or reflectance for the desired
 polarization.
 Commonly available polarizers and polarizing beam-splitters can be divided
 into several types that depend upon different physical principles:
 pile-of-plates polarizers, reflection polarizers, Polaroid sheet
 polarizers, polarizers based on birefringent crystals, metallic grid
 polarizers, and thin film interference polarizers or PBSs.
 Since the present invention is related to a thin film polarizing device,
 the prior art in this field is reviewed in the following section.
 Thin film polarizers or PBSs are based on the light interference in thin
 films, sometimes also in combination with other physical phenomena.
 Conventional thin film polarizers or PBSs are relatively versatile in
 terms of design and fabrication; they are not limited by size and they can
 be manufactured on large scale and at a low cost. However, they are also
 limited in performance.
 The most commonly used wide band thin film polarizer was invented in 1946
 by MacNeille (U.S. Pat. No. 2,403,731). It is based on the Brewster angle
 phenomenon and on light interference in thin films. Because of its
 importance, the theory on which it is based and its performance will be
 compared at some length in Section 6 with those of the PBS operating at
 angles greater than the critical angle proposed in this paper. Here it is
 sufficient to say that the MacNeille polarizer operates over a broad band
 of wavelengths, but is very sensitive to the angle of incidence. Once the
 incident angle moves away from the Brewster angle by .+-.2.degree., the
 performance of the polarizer deteriorates dramatically. The device can be
 used as a PBS, but the extinction ratio for the reflected beam in the
 conventional configuration of the device is rather low.
 Another thin film PBS is based on the separation that occurs between the
 edges for s- and p-polarized light of a cut-off filter that has been
 deposited onto a parallel plate or prism cubes and that is illuminated at
 an oblique angle. In the region between the two edges, s-polarized light
 is reflected and p-polarized light is transmitted. The angular field of a
 plate polarizer is relatively large compared to that of a MacNeille
 polarizer. The extinction ratio for the transmitted beam can be high if a
 large number of layers is used to reflect the s-polarized light. However,
 it is harder to achieve a high rejection ratio for the reflected beam. The
 plate polarizer has a very small bandwidth. It is often used in laser
 systems.
 For many applications, there is a need for low-cost and easily-producible
 non-absorbing, broadband, wide-angle polarizing beam-splitters (PBS). For
 example, a high efficiency back-lighting system for direct-view LCDs,
 disclosed in our co-pending patent application derived from US provisional
 application no. 60/110,166, requires the use of non-absorbing PBS to
 recover the polarization loss. The light loss in current LCDs is more than
 50%. However, none of the currently available polarizers or PBS meet the
 display requirements either because of their high absorption, or limited
 bandwidth, small angular field, limited size or high cost.
 The only thin film polarizing beam-splitter that meets the display
 requirements is the novel PBS that was described in the U.S. Pat. No.
 5,912,762 by Li Li and J. A. Dobrowolski. This PBS is broad-band,
 wide-angle, has high extinction ratios for both transmitted and reflected
 beams. It is based on the effects of frustrated total internal reflection
 and light interference in thin films. It works at angles larger than the
 critical angle. The refractive index of the substrate has to be larger
 than that of the low-index material. The higher the refractive index of
 the substrate, the better the performance is.
 The above patent describes a new concept of designing polarizing
 beam-splitters by using the effects of frustrated total internal
 reflection (FTIR) and interference. The PBS consists of low and high
 refractive index layers. The refractive indices of the low or high index
 layers are lower or higher than that of the substrate, respectively. The
 incident angle upon the low index layers is larger than the critical
 angle. As a result, the admittances of these low index layers have only
 imaginary parts. Therefore, they behave like perfect metals and they only
 attenuate light but do not absorb light. If these low index layers are
 thin, then frustrated total internal reflection will occur inside them,
 therefore, we can also call them FTIR layers. By combining the low and
 high index layers in a symmetrical structure and by carefully choosing the
 right layer thicknesses, the equivalent admittance of the symmetrical
 structure can have very different values for s-and p-polarized light over
 a range of angles and wavelength. Therefore, the symmetrical structure can
 be used to design very broad-band and wide-angle polarizing
 beam-splitters.
 Although the above-mentioned PBS has much better performance than
 traditional devices, it has some disadvantages for direct-view LCD
 back-lighting systems. In these systems, plastic substrates are preferred
 because low cost and low weight are essential factors in this application.
 Unfortunately, the highest refractive of indices of optical plastics is
 about 1.60, corresponding to that of polycarbonate, lower than that of
 some high-index glasses (for example 1.75). Therefore, the performance of
 the PBS is less satisfactory. In addition, the PBS requires accurate
 thickness control that is not desirable for large-scale production.
 Furthermore, for some cases, the PBS requires optical contacts in
 cementing two substrates together. This may limit the size of the PBS and
 also result in a high manufacturing cost.
 An object of the invention is to address this problem.
 SUMMARY OF THE INVENTION
 According to the present invention there is provided a thin film polarizing
 device comprising a functional core consisting of at least two alternating
 thin film layers of metal and dielectric sandwiched between a pair of
 transmissive optical media providing input and output ports, the
 thicknesses of said layers and the optical constants of said layers and
 said optical media being selected such at predetermined wavelengths and
 angles of incidence the optical admittance of said functional core
 substantially matches the optical admittance of said optical media for one
 polarization and has substantially an imaginary part for the other
 polarization, whereby light with said one polarization is transmitted, and
 light with the other polarization is reflected.
 The invention provides a simple polarizing beam-splitter with a
 metal-dielectric thin film coating. Such a polarizing beam-splitter can
 have a broadband and a wide angular field, reasonable extinction ratios.
 The thin film coating can have as few as three layers (typical five
 layers) and can be deposited on optical glasses or plastic substrates by
 conventional thin film deposition process in large scale, resulting in a
 lower manufacturing cost. The simple thin film polarizing device can be
 used in many potential applications. For example, it can be used in the
 proposed high efficiency back-lighting systems. They can also be used as
 pre-polarizer for many applications. To reduce the size of the PBSs, they
 can also be deposited onto thin micro-prisms.
 The metal-like material can be any material that has optical properties
 similar to a metal, i.e. with a small real refractive index and large
 extinction coefficient at the predetermined wavelengths. Such non-metal
 materials include Si or Ge or oxides thereof.
 Special metals such as silver or gold that has large extinction
 coefficients and low refractive indices are employed. Unlike conventional
 all-dielectric thin film PBSS, the performance of the metal-dielectric PBS
 depends on the ratio of the extinction coefficient of the metals and the
 refractive index of the dielectric material. The higher the ratio, the
 better the performance and broader as well.
 The invention also provides a method of making a thin film polarizing
 device comprising the steps of depositing at least two alternating thin
 film layers of metal-like material and dielectric on a first substrate;
 and providing a second substrate so that said two alternating thin film
 layers are sandwiched between said substrates providing input and output
 ports, and wherein the thicknesses of said layers and the optical
 constants of said layers and said optical media are selected such that at
 predetermined wavelengths and angles of incidence the optical admittance
 of said functional core substantially matches the admittance of said
 optical media for one polarization and has substantially only an imaginary
 part for the other plane of polarization, whereby light with said one
 polarization is transmitted and light said other polarization is
 reflected.

DETAILED DESCRIPTION OF THE INVENTION
 Theory
 The thin film polarizing device in accordance with the present invention is
 based on the use of light interference in thin metal and dielectric
 layers. The theory of such a device is explained in the following text.
 Mathematically, a thin film metal-dielectric symmetrical structure
 S.vertline.(DMD).vertline.S can be replaced by a single equivalent layer
 (FIG. 1). Here D, M and S stand for a dielectric layer, a metal layer and
 a substrate respectively. The equivalent admittance E and the equivalent
 phase thickness .GAMMA. of the single equivalent layer can be expressed
 as:
 ##EQU1##
 where .eta..sub.0, .eta..sub.1 and .eta..sub.2 are given by Equation (2a)
 and .delta..sub.1 and .delta..sub.2 are given by Equation (2b),
 ##EQU2##
 Here, n.sub.1 is the refractive index of the dielectric layer, n.sub.2
 -k.sub.2 i is the complex refractive index of the metal layer (n.sub.2 is
 the refractive index and k.sub.2 is the extinction coefficient), and
 n.sub.0 is the refractive index of the substrate. d.sub.1 and d.sub.2 are
 the thickness of the dielectric layer D and the metal layer M,
 respectively. .theta..sub.0 is the incident angle in the incident medium
 and the substrate and .lambda. is the wavelength. For a metal-dielectric
 symmetrical structure with the same basic structure (DMD) but with N
 periods, the equivalent admittance is also E but the equivalent phase
 thickness is .GAMMA.*N. The above equations and the following results also
 apply to a (MDM).sup.N metal-dielectric symmetrical structure.
 The general requirement for forming a polarizing beam-splitter is that the
 equivalent admittance of the symmetrical structure matches the admittance
 of the substrate for one polarization. As a result, all the light in this
 polarization state is transmitted. However, for the other polarization,
 the equivalent admittance has only the imaginary part, therefore, the
 symmetrical structure acts like a perfect metal. As long as the total
 phase thickness is thick enough, the light in this polarization is
 completely reflected.
 Based on the above general requirement, the exact conditions for forming a
 PBS have been derived from the above equations and will be described in
 detail. For simplicity, the following assumptions are made:
 1. k.sub.2 &gt;&gt;n.sub.2 in the metal layers, or n.sub.2.apprxeq.0;
 2. the layers are rather thin, therefore,
 cos(.delta..sub.1)=cos(.delta..sub.2)=1 and
 sin(.delta..sub.1)=.delta..sub.1, sin(.delta..sub.2)=.delta..sub.2 ;
 The equivalent admittance, E.sub.s and E.sub.p, and the phase thickness,
 .GAMMA..sub.s and .GAMMA..sub.p, of the single equivalent layer for both
 s- and p-polarized light can be simplified as:
 ##EQU3##
 In order to transmit p-polarized light, E.sub.p should be equal to the
 admittance of the substrate .eta..sub.0p,
 ##EQU4##
 From equation (5), one obtains:
 ##EQU5##
 In order to have a positive d.sub.2, any one of the following groups of
 conditions must be met:
 ##EQU6##
 From conditions c3 and c4, one obtains:
 ##EQU7##
 Since for a real incident angle .theta..sub.0, sin.sup.2 (.theta..sub.0) is
 always between 0 and 1. Therefore, conditions c3 and c4 can not be
 satisfied and thus are eliminated. Conditions c1 and c2 can be further
 simplified:
 ##EQU8##
 It is clear that as long as condition c1 or c2 is satisfied and d.sub.2
 satisfies equation (6) for an incident angle .theta..sub.0, the
 metal-dielectric symmetrical structure will transmit p-polarized
 independent of wavelengths.
 To demonstrate this, a first example with a thin metal-dielectric symmetric
 structure is calculated. Here, n.sub.0 =1.52, n.sub.1 =1.45, k.sub.2 =3.5,
 d.sub.1 =20.0 nm, and N=60. The design angle of incidence .theta..sub.0 is
 specified to be 67.0.degree. according to condition c2. d.sub.2 is
 calculated to be 1.4228 nm. FIGS. 2a and 2b show the calculated
 transmittance and reflectance of both s- and p-polarized light from 400 nm
 to 2000 nm at the design angle .theta..sub.0 =67.0.degree.. As expected,
 the reflectance of p-polarized light is very low, close to zero, and all
 p-polarized light is transmitted over the whole spectral region.
 In order to reflect s-polarized light, E.sub.s should only have the
 imaginary part. Substitute d.sub.2 with equation (6), E.sub.s can be
 simplified as:
 ##EQU9##
 In order to have imaginary E.sub.s, the above equation should be negative.
 Therefore, the conditions are:
 ##EQU10##
 From condition c6, one obtains:
 ##EQU11##
 It is clear that there is no real .theta..sub.0 that can satisfy condition
 c6, therefore, condition c6 is eliminated from the consideration.
 Condition c7 can be further simplified as:
 ##EQU12##
 To verify this, the first example with a thin metal-dielectric symmetric
 structure is calculated too. From condition c7, .theta..sub.0 must be
 larger than 61.23.degree.. The design angle of 67.0.degree. meets this
 condition. FIGS. 2a and 2b show the calculated transmittance and
 reflectance of both s- and p-polarized light from 400 nm to 2000 nm at the
 design angle .theta..sub.0 =67.01. As expected, the transmittance of
 s-polarized is very low, close to zero, and all s-polarized light is
 reflected over the whole spectral region. Therefore, the above coating is
 a polarizing beam-splitter.
 For a polarizing beam-splitter to transmit p-polarized light, conditions c1
 or c2 must be satisfied, to reflect s-polarized light, condition c7 must
 be satisfied. These three conditions are very useful. They give general
 guidelines on how to select coating materials and design angles and also
 on how to design the metal-dielectric polarizing device in accordance with
 the present invention. A normal design procedure is to use these
 conditions to generate a starting design. Then the starting design is
 always optimized with a computer program to obtain a final design. In the
 final design, the symmetry of the layer structure may not be preserved. In
 addition, the layer thicknesses in the final design could be rather thick
 for the dielectric layers. Furthermore, metal materials with considerately
 large refractive index n can also be used. In this case, the absorption is
 rather high.
 PREFERRED EMBODIMENTS
 In general, the thin film polarizing device in accordance with the present
 invention in FIG. 3 is comprised of first and second light transmissive
 substrates 230 and 231 serving as input and output ports, and a plurality
 of thin film layers 238 disposed between the first and second substrates.
 The thin film layers 238 consist of alternating dielectric layers 232,
 234, etc., and metal layers 233, 235, etc. Each dielectric layer can
 include a number of dielectric sub-layers 236, 228 etc., having one or
 more different refractive indices. The dielectric layers can be selected
 from transparent materials such as MgF.sub.2, SiO.sub.2, A1.sub.2 O.sub.3,
 Nb.sub.2 O.sub.5, TiO.sub.2, ZrO.sub.2, HfO.sub.2, Si, Ge, or mixtures of
 two or more transparent materials, etc. Each metal layer can include a
 number of metal sub-layers 237, 229, etc., each having one or more
 different complex refractive indices. The metal layers can be selected
 from metal materials such as Ag, Au, Al, nickel, Cu, etc., or metal
 alloys, or any material that has an extinction coefficient larger than
 that of the real refractive index, such as Si, Ge, or oxides in the
 selected spectral regions. Although the term "metal layer" is used in the
 description, it is clear that it can be extended to any material that has
 the required property of that the real refractive index n is small and the
 extinction coefficient k is large. The first and second substrates are
 made of transparent materials such as optical glasses, plastic,
 semiconductors, etc. The two prisms can be made of the same material. The
 thicknesses of the metal layers are small enough so that light incident
 upon the thin film layers at an oblique angles can be partially
 transmitted through the metal layers. This permits interference to take
 place between the light reflected at the interfaces of all thin film
 layers. In addition, the thicknesses of all layers are selected such that
 the optical admittance of the plurality of the thin film layers for
 polarized light is substantially the same as the optical admittance of the
 substrate for polarized light for a wide range of angles of incidence and
 a broad band of wavelengths. This permits substantially all incident
 p-polarized light to be substantially transmitted. The plurality of the
 thin film layers have an admittance for s-polarized light that is
 substantially different from the optical admittance of the substrate for
 s-polarized light for a wide-range of angles of incidence and a broad-band
 of wavelengths and thus they substantially reflect incident s-polarized
 light. The substrates are in the form of prisms that are shaped in such a
 manner as to allow the incident light to be incident upon the thin film
 layers at a plurality of angles that permits the above phenomenon to
 occur.
 Several embodiments of the thin film metal-dielectric polarizing device in
 accordance with the present invention have been designed. Conditions c1,
 c2 and c7 are used to find the starting designs. The thin film coatings
 and their performances are summarized in Table 1.
 TABLE 1
 The layer structures and performances of the
 five embodiments
 Embodiments
 2 3
 4 5
 1 Thick. Thick.
 Thick. Thick.
 Layer Structure Mat. Thick. (nm) Mat. (nm) Mat.
 (nm) Mat. (nm) Mat. (nm)
 Sub. 1.52 -- 1.52
 1 (1.45)/0-3.50 i/1.45).sup.60 1.45 49.73 1.52
 -- BK7 -- BK7 --
 2 d.sub.1 = 20.00 nm 0-3.50 i 6.64 0-3.50 i 6.74
 Ag 5.25 SiO.sub.2 37.36
 3 d.sub.2 = 1.4228 nm 1.45 215.09 1.45 210.43
 SiO.sub.2 217.58 Nickel 4.63
 4 0-3.50 i 11.05 0-3.50 i
 13.19 Ag 10.05 SiO.sub.2 183.77
 5 1.45 248.98 1.45 210.43
 SiO.sub.2 216.15 Nickel 8.45
 6 0-3.50 i 11.77 0-3.50 i 6.74
 Ag 5.22 SiO.sub.2 220.70
 7 1.45 254.6
 Nickel 8.27
 8 0-3.50 i 12.17
 SiO.sub.2 128.59
 9 1.45 254.61
 10 0-3.50 i 11.77
 11 1.45 248.98
 12 0-3.50 i 11.05
 13 1.45 215.09
 14 0-3.50 i 6.64
 15 1.45 49.73
 Sub. 1.52 -- 1.52 -- 1.52 --
 BK7 -- BK7 --
 No. of layers 121 15 5
 5 7
 Total metric 1285.4 1607.9 447.5
 454.3 591.8
 thickness (nm)
 Angle in prism 60.degree.-70.degree. 62.degree.-72.degree.
 65.degree.-75.degree. 70.degree.-80.degree. 66.degree.-76.degree.
 Angle in air .+-.7.6.degree. .+-.7.6.degree. .+-.7.6.degree.
 .+-.7.6.degree.
 Wavelength 400-2000 400-2000 400-700
 400-700 400-700
 (nm)
 Extinction 100:1 100:1 80:1
 50:1 50:1
 Ratio (T)
 Extinction 100:1 100:1 80:1
 50:1 50:1
 Ratio (R)
 The first embodiment has the same layer structure as the first example.
 n.sub.o =1.52, n.sub.1 =1.45, k.sub.2 =3.5. The layer thicknesses are
 obtained from the equations in conditions c7 and c1 or c2. No optimization
 is applied. FIGS. 4a and 4b show the calculated transmittance and
 reflectance of both s- and p-polarized light from 400 nm to 2000 nm at
 different angles of incidence. The thin film polarizing coating is
 original designed to work at an angle of incidence .theta..sub.0
 =67.0.degree., however, for nearby angles, the conditions c7 and c1 or c2
 are also approximately satisfied, therefore, the coating also works for a
 range of angles. This is general applied to other embodiments as well. In
 this case, the angles calculated are from 60.degree.-70.degree. in glass
 (.+-.7.60.degree. in air). The minimum extinction ratios for both
 transmitted and reflected beams are better than 100:1. It is clear that
 this thin film polarizing device has a broad band, wide angle and
 reasonably high extinction ratios.
 The second embodiment is similar to the first embodiment with the same
 substrate and coating materials, however, the number of layers is 15,
 compared to 121 in the first embodiment. In practice, it is very difficult
 to make a thin metal-dielectric thin film coatings with more than 100
 layers. To solve this problem, a starting design with a small N and thick
 layers was chosen. The starting design was then optimized. FIG. 5a and 5b
 show the calculated transmittance and reflectance at different angles of
 incidence. The angular field is between 62.degree.-72.degree. in glass
 (+7.6.degree. in air). The wavelength range is also 400 to 2000 nm. It is
 clear that the second embodiment has a performance very similar to the
 first embodiment except that the extinction ratios are little worse.
 The third embodiment consists of the same substrate and coating materials
 as the first and second embodiments. However, the bandwidth is smaller
 from 400-700 mn, compared to 400-2000 nm. As a result, the layer structure
 is significantly simple too. It consists of only 5 layers. FIGS. 6a and 6b
 show the calculated of transmittance and reflectance of the third
 embodiment for different angles of incidence. The angular field is between
 65.degree. and 75.degree. in glass and .+-.7.6.degree. in air. As the
 number of layers decreases, the layer thicknesses increase as well, the
 performance of the thin film polarizing device tends to have a better
 performance better at higher incident angles. The extinction ratios are
 better than 100:1 for most of angles and wavelengths.
 The fourth embodiment consists of SiO.sub.2 and Ag coating materials on BK7
 glasses substrates. Unlike the above three embodiments which do not
 consider the dispersion of coating materials, the fourth embodiment uses
 the measured optical constants at different wavelengths, these measured
 optical constants are closed to the published data shown in the books of
 "Handbook of Optical Constants of solids" and "Handbook of Optical
 Constants of solids II", both edited by E. D. Palik. In practice, the
 dispersion of some materials is large enough to be ignored. Therefore, it
 is necessary to replace the materials used in the above examples with the
 real optical constants of deposited films. The fourth embodiment is
 essentially the same as the third embodiment. The total number of layers
 is also 5. FIGS. 7a and 7b show the calculated transmittance and
 reflectance at different angles of incidence. Since the extinction
 coefficient of silver film change with wavelength from 2.0 at 400 nm to
 4.7 at 700 nm, therefore, the lower-limited angle is different for
 different wavelengths. As a result, the working angles are higher than
 that of the third embodiment, between 70.degree. and 80.degree.. In
 addition, there is absorption in the coating as well because the small
 real refractive indices in the silver layers. The extinction ratio of the
 thin film polarizing coating gets better when the angle of incidence
 increases. In addition, the absorption becomes smaller as well because the
 metal layer thicknesses are thinner.
 The fifth embodiment consists of SiO.sub.2 and Nickel coating materials on
 BK7 glasses substrates. As mentioned, the metal layers in the present
 invention are not necessary to have very small real refractive index n,
 metals with higher n can also be used such as Nickel. In this case, the
 absorption is large. So the transmittance and reflectance for the desired
 polarization are less than 100%. The optical constants of Nickel is taken
 from the books of "Handbook of Optical Constants of solids" and "Handbook
 of Optical Constants of SolidsII", both edited by E. D. Palik. The total
 number of layers is 7. FIGS. 8a and 8b show the calculated transmittance
 and reflectance at different angles of incidence. The wavelength range is
 from 400-700 nm and the angular field is between 66.degree.-76.degree. in
 glass (.+-.7.6.degree. in air). As expected, the absorption in this
 embodiment is rather larger because the real refractive index n of the
 Nickel is about 1.6 to 2.0. The transmittance and reflectance for the
 desired polarization is only about 50%. The extinction ratios are about
 100:1.
 Without departing from the spirit of the present invention, other
 embodiments of the thin film polarizing device having different coating
 and substrate materials working at different wavelength regions and
 different angles of incidence can be designed.
 The thin film polarizing device in accordance with the present invention is
 broad-band and wide angle. Although the extinction ratios are not as high
 as that of the thin film polarizing beam-splitter disclosed in the U.S.
 Pat. No. 5,912,762, it has a very simple structure, it can consist of only
 3-7 layers and can be produced in large scale at low cost. As a result,
 they have a lot of potential applications. For example, it can be used in
 the proposed high-efficiency back-lighting systems for LCD displays. They
 can also be used as pre-polarizers in many applications.
 The thin film polarizing device in accordance with the present invention
 can be manufactured with conventional physical vapor thin film deposition
 systems, such as resistance-heated evaporation, e-beam evaporation,
 ion-assisted evaporation, sputtering, ion-beam sputtering, etc., or
 chemical vapor deposition systems.