JP2013073047A - Broadband 1/4 wavelength plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a broadband 1/4 wavelength plate having a periodic structure with a high aspect ratio, achieving a large dimensional margin.SOLUTION: A 1/4 wavelength plate comprises a transparent material having a patterned periodic structure. The TE polarized beams and the TM polarized beams after transmitted through the periodic structure have a phase difference of 75° to 105° therebetween, each having an average transmittance of 90% or higher, at the 780 nm wavelength. The TE polarized beams and the TM polarized beams have a phase difference of 84° to 96° therebetween, each having an average transmittance of 90% or higher, at the 650 nm wavelength. The TE polarized beams and the TM polarized beams have a phase difference of 87° to 93° therebetween, each having an average transmittance of 90% or higher, at the 405 nm wavelength. The periodic structure portion includes a periodic structure having a trapezoidal cross section. The bottom length of the trapezoid is equal to the periodic length of the periodic structure. The tapered portion on one side of the trapezoid has a width of 60 nm or less.

Description

The present invention relates to a broadband quarter-wave plate using structural birefringence.
In particular, the present invention relates to a quarter-wave plate compatible with a wide band, which is used in an optical disc apparatus commonly used for three wavelengths (for example, 780 nm, 650 nm, and 405 nm) of a CD, DVD, and blue light (Blu-ray) disc.

The quarter wave plate is an optical element for efficiently separating the light projecting path and the light receiving path of the laser in an optical pickup function such as an optical disk device, and between the TE polarized light and TM polarized light transmitted through the wave plate. A phase difference of 90 ° (= 1/4 wavelength) is generated to convert circularly polarized light into linearly polarized light and linearly polarized light into circularly polarized light.
In particular, with the advent of optical discs that use blue light in recent years, there is a demand for a quarter-wave plate for broadband that can be used in common with three wavelengths (for example, 405 nm, 650 nm, and 780 nm) together with DVD and CD.

A quarter-wave plate is usually fabricated using structural birefringence.
That is, a fine periodic structure (for example, a lattice structure as shown in FIG. 7) is formed on the surface of the transparent material, and TE polarized light (the direction of the electric field vector is orthogonal to the periodic direction) and TM polarized light (of the electric field vector) transmitted through the periodic structure. A phase difference is generated between the two polarized lights by giving a difference in refractive index between the two directions. The phase difference due to structural birefringence can be changed depending on the dimensions (period, line width, height, etc.) of the periodic structure.

As such a wave plate, in a concavo-convex periodic structure in which a columnar portion and a groove portion are periodically repeated, the columnar portion is formed in a trapezoidal shape, thereby improving the 0th-order light transmittance. Has been proposed (see, for example, Patent Document 1).
Further, as a method for producing a quarter wavelength plate, since it is advantageous in terms of cost, application of an imprint method using a template called a mold has been proposed in recent years. The imprint method is a method in which a mold is pressed against an applied polymer resin (PMMA, polycarbonate, or the like), the resin is cured by light or heat, and then the mold is separated and transferred. (Refer nonpatent literature 1).

JP 2006-323059 A

Masahiro Morikawa, “Application of Nanoimprint Technology to Optical Devices”, Surface Technology, Surface Technology Association, 2008, Vol. 59, No. 10, p. 18

  In order to realize a TE-TM phase difference of about 90 ° and a high transmittance over a wide wavelength region (405 nm, 650 nm, 780 nm) by structural birefringence, a structural period of several hundred nm is required. Therefore, it is necessary to make a periodic structure with a high aspect ratio with high dimensional accuracy. In particular, in the imprint method, there is a problem of deformation or shape deterioration when separating the mold from the resin, that is, releasability, and it is difficult to produce a broadband quarter-wave plate that requires a fine structure. It was.

By forming the columnar part so that the cross-sectional shape becomes a trapezoidal shape, it is possible to produce a wave plate with good releasability. However, in order to achieve high transmittance over a wide wavelength region, it is necessary to appropriately set a periodic structure having a high aspect ratio with strict dimensional accuracy.
Therefore, the present invention has been made by paying attention to the above-mentioned conventional unsolved problems, and has a periodic structure with a high aspect ratio, relaxes dimensional accuracy and has a large dimensional margin, and has a broadband 1/4. The object is to provide a wave plate.

  In order to solve the above-described problem, in the broadband quarter-wave plate according to claim 1, the transparent material is patterned into a periodic structure in which columnar portions are periodically formed, and is transmitted through the periodic structure. Between TE polarized light and TM polarized light, it has a phase difference of 75 ° to 105 ° at a wavelength of 780 nm, an average transmittance of TE polarization and TM polarized light of 90% or more, and 84 ° to 96 at a wavelength of 650 nm. Having a phase difference of 90 ° and an average transmittance of TE polarized light and TM polarized light of 90% or more, and further having a phase difference of 87 ° to 93 ° and an average transmittance of TE polarized light and TM polarized light of 90% or more at a wavelength of 405 nm The cross-sectional shape of the columnar part is trapezoidal, the length of the base of the trapezoid is equal to the period length of the periodic structure, and the width of the tapered portion on one side of the trapezoid is 60 nm or less. With features To have.

  Further, in the broadband quarter-wave plate according to claim 2, the cross-sectional shape of the columnar portion is a trapezoid, the length of the base of the trapezoid is equal to the length of the period of the periodic structure, and one side of the trapezoid The width of the tapered portion is 60 nm or less, the height of the columnar portion is 2180 nm or more and 2220 nm or less, the period length of the periodic structure is 330 nm or more and 400 nm or less, and further, The average line width is 270 nm or more and 350 nm or less.

  In the present invention, the periodic structure portion has a trapezoidal periodic cross-sectional shape, and the length of the base of each trapezoid is equal to the length of the structural period. Therefore, the cross-sectional shape of the recessed portion of the periodic structure becomes a triangular shape, and as a result, the cross-sectional shape of the mold in the case of producing by imprinting also becomes a triangular shape, so deformation and shape deterioration when separating the mold from the resin Is unlikely to occur. As a result, even with a fine, high aspect ratio structure, it is possible to accurately produce a broadband quarter-wave plate by an imprint method that is advantageous in terms of cost, and to achieve the target of phase difference and transmittance. There is also an effect that the periodic accuracy and line width accuracy of the broadband quarter-wave plate for achieving can be relaxed.

  Furthermore, between the TE polarized light and TM polarized light after passing through the periodic structure, at a wavelength of 780 nm, it has a phase difference of 75 ° to 105 ° and an average transmittance of TE polarized light and TM polarized light of 90% or more, Further, at a wavelength of 650 nm, it has a phase difference of 84 ° to 96 ° and an average transmittance of TE polarization and TM polarization of 90% or more, and further, at a wavelength of 405 nm, a phase difference of 87 ° to 93 ° and 90% or more. The average transmittance of TE-polarized light and TM-polarized light, and the cross-sectional shape of the periodic structure portion is a trapezoidal periodic structure, and the length of the base of the trapezoid is equal to the period length of the periodic structure, and the trapezoid Since the periodic structure is produced so that the width of the taper portion on one side of this is 60 nm or less, a broadband quarter-wave plate can be easily realized.

It is a cross-sectional schematic diagram which shows the structure of embodiment of the broadband 1/4 wavelength plate of this invention. It is explanatory drawing which shows the target performance of a broadband quarter wavelength plate. It is explanatory drawing which shows the optical constant of typical polycarbonate. It is a cross-sectional schematic diagram which shows the parameter when calculating the phase difference and the transmittance | permeability of the broadband quarter wave plate of this invention. It is a characteristic view which shows an example of the result of having calculated the phase difference and the transmittance | permeability by using the polycarbonate film | membrane as a transparent film | membrane material for the broadband quarter wavelength plate of this invention. It is the table | surface which put together the result of having calculated the phase difference and the transmittance | permeability and calculating the wideband quarter wavelength plate of this invention by using the polycarbonate film as a transparent film material. It is a cross-sectional schematic diagram which shows the structure of the broadband 1/4 wavelength plate of a conventional structure. It is a cross-sectional schematic diagram which shows the parameter when calculating the phase difference and the transmittance | permeability of the broadband quarter wavelength plate of a conventional structure. It is the table | surface which put together the result which calculated the phase difference and the transmittance | permeability, and calculated the wideband quarter wavelength plate of the conventional structure by using the polycarbonate film as a transparent film material.

The broadband quarter-wave plate according to the embodiment of the present invention will be described below.
FIG. 1 is a configuration diagram showing a schematic configuration of a quarter-wave plate 1 in an embodiment of the present invention.
In FIG. 1, 2 is a periodic structure and 3 is a transparent substrate. The periodic structure 2 is configured by arranging a plurality of columnar portions 2a having the same shape adjacent to each other. The columnar portion 2a has a trapezoidal shape with a narrow upper side, and is formed so that the columnar portions 2a are in contact with each other at the bottom of the trapezoid. The periodic structure 2 is formed on the transparent substrate 3 to constitute the quarter wavelength plate 1.

This quarter-wave plate 1 has a phase difference of 75 ° to 105 ° at a wavelength of 780 nm between the TE polarized light 5 and the TM polarized light 6 after the incident light 4 has passed through the periodic structure 2 and the transparent substrate 3. In some cases, the average transmittance of the TE polarized light 5 and the TM polarized light 6 is 90% or more.
Further, at a wavelength of 650 nm, there is a phase difference of 84 ° to 96 ° between the TE-polarized light 5 and the TM-polarized light 6 after the incident light 4 has transmitted through the periodic structure 2 and the transparent substrate 3, and the average transmission. The rate is 90% or more. Further, at a wavelength of 405 nm, there is a phase difference of 87 ° to 93 ° between the TE polarized light 5 and the TM polarized light 6 after the incident light 4 has passed through the periodic structure 2 and the transparent substrate 3, and an average transmittance. Is over 90%.

  Further, in the trapezoidal shape in the cross-sectional shape of the periodic structure 2, the length of the base of each trapezoid is equal to the length of the structural period. At the same time, the period Λ (equal to the line width wb of the bottom side), the line width wa, and the height h of the upper side are determined so as to satisfy the phase difference and transmittance conditions at wavelengths of 780 nm, 650 nm, and 405 nm. ing.

Hereinafter, an example in which the quarter-wave plate 1 of the present invention is designed, that is, selection of the shape (dimension) of the periodic structure 2 will be described in detail with reference to the drawings.
The transparent material which becomes the periodic structure 2 will be described by taking polycarbonate (hereinafter abbreviated as PC) as an example. In addition to the same material as the periodic structure 2, the transparent substrate 3 may be synthetic glass, quartz, alumina, or the like. However, an extinction coefficient that is an index representing light absorbance is practically used over a wavelength range of 400 to 780 nm. What is necessary is just 0 (zero). Here, a design example in which the same PC as the periodic structure 2 is used as the transparent substrate 3 will be described.

FIG. 2 shows a general target performance of the broadband quarter-wave plate. As shown in FIG. 2, high transmittance and a phase difference of around 90 ° are required in a wide wavelength range of wavelengths 405 nm, 650 nm, and 780 nm.
The phase difference between the TE-polarized light 5 and the TM-polarized light 6 is such that the incident light 4 is transmitted through the periodic structure 2 formed on the transparent substrate 3, so that the refractive index for the TE component and the refractive index for the TM component can be different. Caused by. Here, when the refractive index for the TE component is nTE and the refractive index for the TM component is nTM, the refractive index difference Δn is
Δn = | nTE−nTM | (1)
When the height of the periodic structure 2 is h and the target wavelength is λ, the phase difference PS is
PS (°) ≒ 360 ° · Δn · h / λ (2)
It is represented by

The refractive index difference Δn corresponds to the optical constant (refractive index n, extinction coefficient k) of the original transparent material that becomes the periodic structure 2, the period Λ, the line width w, and the wavelength λ of the incident light 4. It depends on you.
Therefore, in the broadband quarter-wave plate 1, the period Λ, the line width w, and the height of the periodic structure 2 are set so that simultaneously PS = 90 ° at the required wavelengths (for example, 405 nm, 650 nm, and 780 nm). It is necessary to select h.

Here, considering the optical constants of the transparent material that becomes the periodic structure 2, the refractive index difference Δn after passing through the periodic structure 2 increases as the refractive index of the original transparent material increases. When the refractive index difference Δn increases, the height h of the periodic structure for obtaining a phase difference of 90 ° can be reduced from the equation (2).
That is, in order to keep the aspect ratio of the periodic structure 2 small, it is desirable to use a transparent material having as large a refractive index as possible. However, if the refractive index is too large, the difference from the refractive index of the space (= 1) becomes large, the reflectance increases, and the transmittance decreases. It is desirable to use it.

Next, considering the extinction coefficient of the transparent material, even if the aspect ratio of the periodic structure 2 is suppressed, a height h of at least several hundred nm is necessary. The extinction coefficient of the transparent material needs to be practically 0 (zero) in the necessary wavelength region.
The PC film is suitable as a transparent material used in the present invention because it has a relatively high refractive index among the transparent polymer resins and can be used as an imprinting resin.

FIG. 3 shows optical constants of typical PC films when the wavelength λ is 405 nm, 650 nm, and 780 nm.
The phase difference at wavelengths of 405 nm, 650 nm, and 780 nm of the periodic structure 2 in which the base of the trapezoidal shape of the cross section of the PC film having the characteristics of FIG. The transmittance was calculated and the broadband quarter-wave plate 1 was designed.

As shown in FIG. 4, the calculation was performed using parameters such as the period Λ, the average line width w corresponding to the middle height of the cross section, and the height h of the columnar portion 2a. As can be seen from FIG. 4, the difference per one side between the average line width w and the width of the upper side of the trapezoidal shape (wa in FIG. 1), and the average line width w and the width of the base of the trapezoidal shape (wb in FIG. 1). )), The taper width per side of the trapezoidal shape is 2a.
Λ = w + 2a
This relationship holds. The refractive index n of the transparent material was calculated as shown in FIG. 3, and the refractive index n ₀ around the broadband quarter-wave plate 1 was calculated as 1.0.

An example in which a favorable calculation result is obtained is shown in FIG.
FIG. 5 shows a case where the structure period Λ = 330 nm, the average line width w = 270 nm, and therefore the wideband quarter-wave plate 1 in the case where the taper width per side is 60 nm. The phase difference is calculated. 5A shows the case of a wavelength of 405 nm, FIG. 5B shows the case of 650 nm, and FIG. 5C shows the case of 780 nm.

In each of FIGS. 5A to 5C, the belt-like portions described at the upper and right ends represent the allowable range of the phase difference and the allowable range of the height h associated therewith in the general target performance of FIG. ing.
Furthermore, when the transmittance at a wavelength of 405 nm with respect to the height h in the above structure is calculated, it becomes 90% or more as shown in FIG.

In addition, when the transmittance is 90% or more at a wavelength of 405 nm, the wavelength 650 nm and 780 nm include a possible range at a wavelength of 405 nm due to the effects of a longer wavelength and a smaller refractive index. Since the transmittance is 90% in a wide height range, the drawing is omitted.
Thus, from the result of FIG. 5, in the structure of the present invention at this time, the target of the phase difference and the transmittance with respect to the wavelengths of 405 nm, 650 nm, and 780 nm is achieved in the range of the height h from 2180 nm to 2220 nm. It can be seen that the quarter-wave plate 1 according to the embodiment of the present invention can be manufactured.

Similar to FIG. 5, using a PC film as a transparent material, with the period Λ, the average line width w corresponding to the intermediate height of the cross section, and the height h of the columnar part 2a as parameters, the wavelength of 405 nm, FIG. 6 shows a table summarizing the results of calculating the phase difference and transmittance at 650 nm and 780 nm.
In addition, in the determination of “◯” and “×” in FIG. 6, the phase difference is a portion where the height h that satisfies the target range of the phase difference in FIG. 2 overlaps at wavelengths of 405 nm and 650 nm, or 405 nm and 780 nm, respectively. If there is, “○” is indicated. Further, the transmittance of 90% or more of the target is “◯”. Furthermore, “optimum height” in FIG. 6 represents the median value of “height (h) range satisfying phase difference specifications”.

(Comparative example)
As a comparative example, the same calculation as in the case of the present invention was performed for the periodic structure shown in FIG. That is, the phase difference and transmittance at wavelengths of 405 nm, 650 nm, and 780 nm were calculated using the PC film as a transparent material and using the period Λ, the line width w, and the height h of the periodic part as parameters as shown in FIG. A table summarizing the results is shown in FIG.

When the result of FIG. 6 of the present invention is compared with the result of FIG. 9 of the conventional structure, the following can be understood.
That is, in the conventional periodic structure having a vertical cross section, the target performance of the broadband quarter-wave plate can be satisfied if the space width (Λ-w) is about 77 nm or less. However, even if the cycle (line width) is the same, if the line width (cycle) is different by 5 nm, the target specification of phase difference or transmittance cannot be satisfied. Therefore, the required accuracy of the period or line width of the conventional structure is strict and is about 5 nm.

On the other hand, in the trapezoidal periodic structure 2 in which the base of the present invention is equal to the periodic length, a taper width (2a) of approximately 60 nm or less is required, but a wide range of 330 nm to 400 nm in period and 270 to 350 nm in average line width. Can clear the target of a broadband quarter-wave plate.
This means that the dimensional accuracy, which is the biggest problem in fine processing, can be relaxed, that is, the dimensional margin can be increased. One reason for this is that the trapezoidal cross section makes the change in the refractive index in the vertical direction of the processed portion more gradual, and the effect of reducing the reflectivity is produced. Also, with respect to the aspect ratio, there is no significant difference between the conventional structure and the structure of the present invention, and the method of the present invention is not particularly disadvantageous in the aspect ratio.

  INDUSTRIAL APPLICABILITY The present invention is expected to be used as a wideband quarter phase plate that efficiently separates a light projecting path and a light receiving path of a laser in an optical pickup function such as an optical disk device and improves the light utilization efficiency. Particularly in recent years, it is expected to be used as a quarter-wave plate compatible with a wide band that can be used in common with three wavelengths (405 nm, 650 nm, and 780 nm) together with a next-generation optical disk, DVD, and CD that use blue light.

DESCRIPTION OF SYMBOLS 1 Broadband quarter wave plate 2 Periodic structure 2a Columnar part 3 Transparent substrate 4 Incident light 5 TE polarized light 6 TM polarized light

Claims (2)

A transparent material is patterned into a periodic structure in which columnar portions are periodically formed, and a phase difference of 75 ° to 105 ° is obtained at a wavelength of 780 nm between TE polarized light and TM polarized light after passing through the periodic structure. And 90% or more of TE-polarized light and TM-polarized light with an average transmittance of 84 ° to 96 ° at a wavelength of 650 nm and 90% or more of TE-polarized light and TM-polarized light with an average transmittance. Furthermore, at a wavelength of 405 nm, it has a phase difference of 87 ° to 93 °, an average transmittance of TE polarized light and TM polarized light of 90% or more,
A cross-sectional shape of the columnar part is a trapezoid, a length of a base of the trapezoid is equal to a period length of the periodic structure, and a width of a tapered portion on one side of the trapezoid is 60 nm or less. Broadband quarter wave plate. The cross-sectional shape of the columnar part is a trapezoid, the length of the base of the trapezoid is equal to the length of the period of the periodic structure, and the width of the tapered portion on one side of the trapezoid is 60 nm or less,
Furthermore, the height of the columnar part is 2180 nm or more and 2220 nm or less, the period length of the periodic structure is 330 nm or more and 400 nm or less, and the average line width of the columnar part is 270 nm or more and 350 nm or less. 2. The broadband quarter wave plate according to claim 1, wherein

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