JP5217666B2 - Wavelength selective filter and optical instrument - Google Patents

Description

  The present invention relates to a wavelength selection filter and an optical apparatus.

The speed of light is 3 × 10 ¹⁰ cm / sec, ie 3 × 10 ¹⁴ μm / sec, and the tera is 10 ¹² . Therefore, the wavelength order of terahertz is around “10 ² μm”.
Recently, electromagnetic waves in the terahertz band, particularly “0.3 to 3 terahertz” (hereinafter referred to as “light” instead of electromagnetic waves) have attracted attention as a new imaging light source in the fields of biotechnology, medical care, security, and the like. Combined with advances in the generation and detection technology of “terahertz-band light”, research into application fields is progressing.

  For light in the terahertz band, it is often difficult to use various optical elements for normal visible light as they are, and it is necessary to realize a wavelength selection filter or the like that is compatible with the terahertz band.

There are many requests for realizing a “wavelength selective filter for a narrow wavelength band” that selectively filters light in a very narrow wavelength region as a wavelength selective filter.
As a wavelength selection filter having wavelength selectivity with respect to light in the terahertz wavelength band, the filter described in Patent Document 1 is known, and its filtering characteristic is “a considerably wide wavelength band”.

  On the other hand, as wavelength selection filters having a bandpass characteristic in a narrow wavelength band with respect to light in the visible wavelength band, those described in Patent Documents 2 to 4 are known. In particular, those described in Patent Documents 3 and 4 use resonance reflection in the periodic structure, similarly to the wavelength selective filter of the present invention. In the visible light region, “very narrow wavelength of several nm or less” It has a narrow-band bandpass characteristic that reflects only the “band light”.

Japanese Patent No. 3904029 JP 2005-275089 A JP 2007-156254 A JP 2008-008990 A

  As described above, the conventionally known wavelength selection filter for the narrow wavelength band is in the “visible wavelength range”, and the one for the terahertz wavelength band is not necessarily sufficient as the “wavelength selection filter for the narrow wavelength band”.

The present invention has been made in view of the above-described circumstances, and can select a good “narrow wavelength band wavelength” for terahertz band light, and “change the selected wavelength for terahertz band light. An object of the present invention is to realize a wavelength selective filter that can be made to perform.
Furthermore, this invention makes it a subject to implement | achieve the optical instrument which uses the said wavelength selection filter.

The wavelength selective filter of the present invention is a “wavelength selective filter that selectively reflects light in a desired narrow wavelength band out of incident light beams in a terahertz wavelength band in a linearly polarized state by resonant reflection”.
In this specification, “the terahertz wavelength band refers to a 0.3 THz to 3.0 THz band and a wavelength range of 100 μm to 1000 μm”.

  As described above, the wavelength selection filter of the present invention can select desired “narrow wavelength band light for reflection” from light in the terahertz wavelength band of the incident light flux.

The wavelength selection filter according to claim 1 includes first and second flat plate elements and a spacing variable means.
The “first flat plate element” has a parallel flat plate shape that is transparent to light in the terahertz wavelength band.

  The “second flat plate element” is a parallel flat plate shape that is transparent to light in the terahertz wavelength band, and is disposed in parallel with the first flat plate element.

  “Distance changing means” is means for changing the minute interval between the first and second flat plate elements. That is, the distance varying means maintains the parallel state of these elements from “0 (close contact state) to a finite minute distance” between the first and second flat plate elements arranged opposite to each other in parallel. Change.

  The first and second flat plate elements arranged in parallel to each other are arranged so as to be inclined by a predetermined angle with respect to the incident light beam.

  The “tilt angle” with respect to the incident light beam can be appropriately set according to the practical form of use of the wavelength selection filter as an optical element.

Of the first and second flat plate elements, at least one surface of the first flat plate element has “a fine groove structure in which a large number of grooves are formed at a predetermined pitch”.
The reason why the first and second flat plate elements are “inclined by a predetermined angle with respect to the incident light beam” is the normal line of the plane of these flat plate elements (normal line when the fine groove structure is considered to be a flat surface) And in a plane parallel to the groove arrangement direction in the fine groove structure.

The incident light beam is incident from the “first flat plate element side”.
Then, “the wavelength of the reflected light in the narrow wavelength band to be resonantly reflected is selected by changing the minute interval between the first and second flat plate elements by the interval variable means”. Here, “the wavelength of reflected light in a narrow wavelength band that is resonantly reflected” refers to the peak wavelength of the reflected light.

  “Narrow wavelength band” means that the reflection wavelength band width (hereinafter also referred to as “half-value width”) having an intensity that is ½ of the peak wavelength intensity of reflected light is several μm or less.

  As described above, the wavelength selective filter according to claim 1 has a fine groove structure on one side of “at least the first flat plate element” of the first and second flat plate elements. The flat plate element may be formed only on the surface on the side where the incident light beam is incident, and the second flat plate element may be “a flat surface on both sides”.

Further, in the wavelength selective filter for the terahertz wavelength band according to claim 1, “a fine groove structure is formed only on the surfaces of the first and second flat plate elements facing each other (that is, only each surface forming a minute interval). It can also be formed in the same direction ”(Claim 3).
“Forming the fine groove structure in the same direction” means that the longitudinal directions of the fine groove structure formed on these surfaces are parallel to each other.

  As described above, the wavelength selective filter according to claims 1 and 2 has a “fine groove structure” for the first flat plate element, and the wavelength selective filter according to claims 1 and 3 has a “fine groove structure” for the first and second flat plate elements. However, the “fine groove structure portion and the portion where the fine groove structure is formed (portion holding the fine groove structure)” in the flat plate element may be a different material. You can also.

From the viewpoint of manufacturing the flat plate element, it is preferable that the fine groove structure part and the “part where the fine groove structure is formed” are made of the same material.
The first and second flat plate elements can be made of the same material, including the case where the fine groove structure part and the “part where the fine groove structure is formed” are made of different materials and the same material. .

  In the case of claim 2, the material of the second flat plate element may be the same as that of the “part where the fine groove structure is formed” of the first flat plate element, but the fine groove structure portion may be different. In addition, the fine groove structure portion of the first flat plate element, “the portion where the fine groove structure portion is formed”, and the second flat plate element may be made of different materials, or “ For the portion where the fine groove structure is formed, the fine groove structure portion and the second flat plate element may be made of the same material.

  As described in claim 3, when both the first and second flat plate elements have a fine groove structure, the fine groove structure portion in each flat plate element and the “part where the fine groove structure is formed” It can be realized with a combination of up to four materials.

  However, from the viewpoint of ease of manufacturing the first and second flat plate elements, it is preferable to form them with the same material.

  The thickness of the flat plate element is “the thickness including the fine groove structure when the flat plate element has a fine groove structure”, that is, the thickness of the fine groove structure portion and the thickness of the “part where the fine groove structure is formed”. It is sum.

  The thicknesses of the first flat plate element and the second flat plate element may be the same or different from each other.

  The above-mentioned “phenomenon in which the peak wavelength of the resonant reflected light changes due to the change in the distance between the first and second flat plate elements” is a new knowledge obtained by the inventors through research.

  The “wavelength selection filter for the terahertz wavelength band” according to claim 4 includes a flat plate element and a rotation tilt driving means.

The “flat plate element” is a parallel flat plate shape transparent to light in the terahertz wavelength band, and has a fine groove structure in which a large number of grooves are formed at a predetermined pitch on a surface on which an incident light beam is incident.
“Rotating and tilting driving means” is means for rotating and tilting the flat plate element about an axis parallel to the grooves of the fine groove structure. In other words, the plate element is rotated and tilted with respect to the incident light beam by the rotation tilt driving means.

  “Selecting the wavelength of reflected light in a narrow wavelength band to be resonantly reflected by changing the incident angle of the incident light beam into the fine groove structure by the rotational inclination of the flat plate element by the rotational inclination driving means”.

  In other words, the wavelength selective filter according to any one of claims 1 to 4 can “change the wavelength of light in a narrow wavelength band to be reflected (light in a terahertz region)”.

  An optical element of the present invention is an optical apparatus having the wavelength selection filter for a terahertz wavelength band according to any one of the first to fourth aspects (Claim 5).

  As described above, the present invention uses the resonance reflection by the flat plate element formed with the fine groove structure, and therefore, the resonance reflection by the flat plate element having the fine groove structure and its characteristics will be briefly described below. .

FIG. 1 schematically illustrates a “flat plate element having a fine groove structure”.
The flat plate element has a parallel plate shape, and has a “homogeneous structure of the same material” as a whole, and has a portion indicated by reference numeral GD and a portion indicated by reference numeral FS.
A portion indicated by reference numeral GD is a “waveguide layer” and has a thickness t2 as shown in the figure.
The portion indicated by the symbol FS is a portion forming a “fine groove structure” and is called a “grating layer” as shown in the figure. The grating layer FS has a thickness: t1, as shown in the figure.
The waveguide layer GD corresponds to “a portion where a fine groove structure is formed” in the above description.

  In the flat plate element of FIG. 1, “thickness of waveguide layer GD: t2 and grating layer FS thickness: t1: (t1 + t2)” is the thickness of the flat plate element.

  The grating layer FS has a “fine groove structure” in which the illustrated cross-sectional shape (rectangular wave-shaped cross-sectional shape) is uniformly continuous in a direction orthogonal to the drawing, and grooves are formed at a constant pitch in the horizontal direction of the drawing.

In relation to this fine groove structure, the arrangement pitch of the fine grooves is “P”, and the width of the convex portion (referred to as land width) is “L”.
At this time, the ratio of L to P: L / P is called “filling factor (abbreviated as FF)” and is used as a parameter for “calculation of reflectance” described later.

The characteristic of resonant reflection when light in a linearly polarized state is incident on the flat element shown in FIG. 1 differs depending on the polarization direction.
Here, as two typical examples of the polarization direction, polarized light in the horizontal direction of the drawing, that is, the “groove pitch direction” in the fine groove structure is referred to as TM polarized light, and polarized light in a direction orthogonal to the drawing is referred to as TE polarized light. At this time, the reflected wavelength that is resonantly reflected differs depending on the polarization direction of the incident light.

  Generally speaking, TM-polarized light has a narrower half-value width of light that is resonantly reflected than light in the TE-polarized direction. Hereinafter, a case where TE-polarized light is incident from above as shown in FIG. 1 will be described.

The TE-polarized light incident on the flat plate element of FIG. 1 from the grating layer FS side is diffracted by the periodicity of the grating layer FS.
When the diffracted diffracted wave satisfies the “guided condition for propagating in the waveguide layer GD”, the diffracted wave recombines with the grating layer FS, and the diffracted wave (reflected in the “specular direction”) with respect to the incident light. Wave).

On the other hand, “a diffracted wave that does not satisfy the waveguiding condition” cannot be guided through the waveguiding layer GD, and passes through the flat plate element in the thickness direction. The “reflected wave by the diffracted wave that satisfies the waveguide condition” has a reflection efficiency of about 100%.
That is, the flat plate element of FIG. 1 functions as a “wavelength selection filter that selectively reflects light having a wavelength satisfying the waveguide condition with high efficiency”.
At this time, the “wavelength of light reflected with high efficiency” is determined for each flat plate element depending on the material and form of the flat plate element.

  The wavelength and half-value width of reflected light by the flat plate element shown in FIG. 1 (reflected wavelength band having an intensity half the peak wavelength intensity of the reflected light) are determined by the “structural parameters of grating layer FS and waveguide layer GD (structural parameter ( The pitch can be controlled by adjusting “Pitch: P, Land width: L, Thickness: t1, t2, Refractive index)”.

Hereinafter, among these parameters, changes in reflected light when pitch: P, land width: L, and thickness: t1 are individually changed will be specifically described by simulation.
As shown in FIG. 1, a “fine groove structure with a rectangular cross section in cross section” is formed on one side of a “Zeonor film (trade name: resin film manufactured by Nippon Zeon Co., Ltd.)” having a refractive index of 1.52 in the terahertz band and a thickness of 20 μm. Thus, a flat element was obtained.

  The parameters are as follows.

Pitch: P = 180 μm
Filling factor: FF = 0.5
Grating layer FS thickness: t1 = 10 μm
Waveguide layer GD thickness: t2 = 10 μm.

  As the incident light beam, light in the terahertz band was defined as a “parallel light beam”. As shown in FIG. 1, the parallel light beam was subjected to orthogonal incidence in the TE polarization state from the grating layer FS side. The above parameters are designed to “resonate and reflect at a wavelength of 188 μm”.

  The simulation calculation was performed under the above conditions using a well-known calculation algorithm “RCWA (Rigorous Coupled Wave Analysis)”. This calculation algorithm is an “electromagnetic exact calculation method” for a diffraction grating, and has been conventionally used as a method for accurately obtaining diffraction efficiency.

FIG. 2 is a “spectral reflectance characteristic” obtained as a calculation result. The horizontal axis represents wavelength in μm, and the vertical axis represents reflectance.
As apparent from FIG. 2, the reflected light has a wavelength of 188 μm, a reflectance peak value of about 1, and a half-value width of about 1 μm. That is, the full width at half maximum is as narrow as about 0.5% of the peak wavelength (wavelength giving the peak of reflectance): 188 μm, and the reflectance for wavelengths other than the vicinity of the peak wavelength is as small as 0.1 or less.
From this result, it is understood that the flat plate element functions as a “narrow band wavelength selection filter” and the reflectance of non-resonant reflected light is low.

The peak wavelength of the reflected light depends on the pitch P of the grating layer.
FIG. 3 shows spectral reflection characteristics when the pitch P of the grating layer FS is 180 μm (this is the case described above), 200 μm, and 220 μm. As shown in this figure, when the pitch P is different, the “peak wavelength of reflected light” is different.

  FIG. 4 is a graph showing the dependency of the “peak wavelength of reflected light” shown in FIG. As is apparent from this figure, the peak wavelength of the reflected light can be shifted to the longer wavelength side by increasing the pitch: P.

The thickness t1 of the grating layer FS as a parameter affects the peak wavelength and the half-value width of the reflected light of resonance reflection.
FIG. 5 shows the spectral reflection characteristics when the thickness of the grating layer FS: t1 is 10 μm (this is the case described above), 20 μm, and 30 μm. FIG. 5 shows how the peak wavelength and the half-value width of the reflected light change as the thickness of the grating layer: t1 changes.

  FIG. 6 is a graph showing the “dependence of the half-value width on the grating layer: t1” in the case of FIG. 5, and the half-value width (vertical axis) increases as the thickness: t1 (horizontal axis) increases. I understand that.

  Although not shown, it is known that when the thickness of the waveguide layer GD: t2 is changed, “the half width becomes narrower as the thickness of the waveguide layer increases”.

  In the example described above, a parallel light beam in the terahertz band is orthogonally incident on the flat plate element, but the peak wavelength of the reflected light by resonance reflection is the incident angle of the incident light beam (parallel light beam) to the flat plate element. It also changes depending on.

With respect to a flat plate element having the above parameters, the case of orthogonal incidence is used as a reference (incident angle: 0 degree), and the incident angle is 5 degrees and 10 degrees in the direction of pitch: P (in the plane shown in FIG. 1). FIG. 7 shows the spectral reflection characteristics when changed.
When the incident angle is increased, two peaks are generated on each of the long wavelength side and the short wavelength side, centering on the peak wavelength when the incident angle is 0 degree.
FIG. 8 shows a change according to the incident angle of the peak wavelength of the reflected light, and shows that “the peak wavelength changes linearly as the incident angle is increased”.
In this change, the “half-value width of the reflected light giving the peak wavelength” is constant.
The invention according to claim 4 utilizes the above-mentioned property. The principle of wavelength selection in the wavelength selective filter according to claims 1 to 3, that is, the principle of changing the peak wavelength is different from the “wavelength selection mechanism” of the wavelength selective filter of claim 4.

In addition, in FIG. 1, “the fine groove structure constituting the grating layer FS” has a rectangular wave shape in cross section, but the cross-sectional shape of the fine groove structure is not limited to the rectangular wave shape and is “thinning toward the light beam incident side”. It may be a trapezoidal shape or a triangular shape.
In such a groove structure with a trapezoidal or triangular cross-section, the “refractive index change when the light beam is incident” becomes gradual, and Fresnel reflection can be effectively reduced, further reducing the “reflectance of non-reflective wavelengths”. It is.

The influence of the polarization direction of the incident light beam on the resonance reflection will be described.
In the case where “linearly polarized parallel light flux” is incident on a flat plate element having the above parameters at right angles, the groove direction of the grating layer (the direction perpendicular to the direction of incidence and pitch: P, orthogonal to the drawing in FIG. 1) Direction) and the polarization direction of the incident light beam (the state where the incident light beam is TE-polarized with respect to the grating layer FS) is set to 0 °, and the polarization direction is 90 ° in 15 ° counterclockwise increments. FIG. 9 shows the reflection spectral characteristics when rotated up to.

  As the rotation angle in the deflection direction: β (shown as “polarization direction β degree” in the figure) increases, the peak intensity of the reflected light gradually decreases. FIG. 10 shows how the peak intensity is gradually reduced by increasing the rotation angle.

  The vertical axis in FIG. 10 shows the change in peak intensity with respect to the polarization direction (corresponding to the rotation angle: β on the horizontal axis) with respect to the groove direction, and “reflectance when β = 0 °” at the peak wavelength of reflected light is 100. It is shown as% and decreases with an increase in the rotation angle: β (horizontal axis “polarization direction”).

  As described above, according to the present invention, a novel wavelength selection filter capable of performing “narrowband wavelength selection” in the terahertz wavelength band and changing the wavelength to be selected can be realized.

  In addition, various optical devices suitable for the terahertz wavelength band can be realized using this wavelength selection filter.

Hereinafter, embodiments will be described.
FIG. 11 is a diagram for explaining an embodiment of the wavelength selection filter according to claims 1 and 2. This wavelength selection filter is a “wavelength selection filter that selectively reflects light in a desired narrow wavelength band out of incident light beams in a terahertz wavelength band in a linearly polarized state by resonant reflection”.

  As shown in FIG. 11A, the wavelength selection filter includes a first flat plate element 10A and a second flat plate element 10B. Reference numeral 10 denotes a composite of the first and second flat plate elements 10A and 10B.

These flat elements 10A and 10B are transparent to light in the terahertz wavelength band, have a parallel flat plate shape, and are provided to face each other in parallel.
The first and second flat plate elements 10A and 10B are arranged so as to be inclined by a predetermined angle (45 degrees in this embodiment) with respect to an incident light beam (“incident light” in the figure), and are terahertz wavelengths in a linearly polarized state. Of the incident light flux in the band, light in a desired narrow wavelength band is selectively reflected as resonance reflected light by resonance reflection.

  The first flat plate element 10A has a fine groove structure in which a number of grooves are formed at a predetermined pitch on one surface (the incident light incident side), and the incident light flux is from the fine groove structure side of the first flat plate element 10A. Incident. Both surfaces of the flat element 10B are flat surfaces.

  The interval between the first and second flat plate elements 10A and 10B shown in FIG. 11A is variable, and the flat plate element is changed from “0 (contact state) to finite minute interval” by the interval variable means. It can be changed while maintaining the parallel state of 10A and 10B.

FIGS. 11B to 11D show an example of the interval varying means.
The interval variable means includes holding frames 12 </ b> A and 12 </ b> B, a piezoelectric element 14, and a drive circuit 16.
The holding frame 12A has a flat plate shape, and as shown in FIGS. 11B and 11C, is formed with a rectangular opening 12A1 that matches the fine groove structure formed in the first flat plate element 10A. One flat element 10A is held on the surface opposite to the incident light incident side with the fine groove structure side being the incident light incident side.

  The holding frame 12B also has a flat plate shape, and as shown in FIGS. 11B and 11D, a rectangular opening 12B1 is formed in accordance with the fine groove structure formed in the first flat plate element 10A. The two flat plate elements 10A are held on the incident light incident side.

  The piezoelectric element 14 is provided so as to be held between the holding frames 12A and 12B, the operation surface is fixed to the holding frames, and is driven by the drive circuit 16 to change the interval between the holding frames 12A and 12B.

  In this change in interval, when the interval is minimum, the flat surfaces of the first and second flat plate elements 10A and 10B are in close contact with each other.

  The wavelength of the reflected light in the narrow wavelength band to be resonantly reflected is selected by changing the minute interval: t between the first and second flat plate elements 10A, 10B by the interval changing means.

  Since the first flat element 10A is structurally the same as that described with reference to FIG. 1, the structural parameters are the same as those in FIG. 1, and the pitch: P, land width: L, thickness: t1. , T2, and refractive index.

Calculation was performed using the calculation algorithm “RCWA”.
For each of the first flat plate element 10A and the second flat plate element 10B, the above-described “Zeonor film” having a refractive index of 1.52 in the terahertz wavelength band was used.
The thickness of this film was 20 μm, and this film was used as it was (thickness: 20 μm) for the second flat plate element 10B.

  In the first flat plate element 10A, a fine groove structure was formed on one side (incident light incident side) of the film. The structural parameters are as follows:

Pitch: P = 180 μm
Filling factor: FF = 0.5
Grating layer thickness: t1 = 10 μm
Waveguide layer thickness: t2 = 10 μm.

Spectral reflection characteristics were calculated when the interval t between the first and second flat plate elements 10A and 10B was changed to 0 μm, 10 μm, 20 μm, and 30 μm.
The result of the calculation is shown in FIG. It can be seen that the peak wavelength of the resonance reflected light shifts to the short wavelength side as the interval: t (indicated as “air layer” in FIG. 12) increases.

  FIG. 13 is a diagram in which the peak wavelength of the resonance reflected light accompanying the change of the interval: t is plotted with the interval: t (indicated as “air layer thickness” in FIG. 11) as the horizontal axis. It can be seen that the peak wavelength shifts to the short wavelength side almost linearly as t increases.

  With this action, the peak wavelength of the resonance reflected light of the wavelength selection filter can be adjusted, and a desired wavelength in the terahertz wavelength band can be selected.

  In the embodiment of FIG. 11, an example is shown in which piezoelectric elements are used as the interval variable means. However, the present invention is not limited to this, and the interval between the holding frames may be changed using mechanical variable means. Needless to say, it's good.

  Further, the shape of the openings 12A1 and 12B1 of the holding frames 12A and 12B is not limited to a rectangular shape, and may be an elliptical shape, a long hole shape, a circular shape, and the like. Can be matched.

  For example, when the shape of the holding frame and the opening is circular and the holding frame is ring-shaped, the flat plate elements are “tensioned uniformly from the center to the outside” when the flat plate elements 10A and 10B are bonded to the holding frame. Therefore, the flat element can be bonded without causing unnecessary distortion.

FIG. 14 is a diagram for explaining an embodiment of the wavelength selective filter according to claims 1 and 3.
This wavelength selection filter is a “wavelength selection filter that selectively reflects light in a desired narrow wavelength band out of incident light beams in a terahertz wavelength band in a linearly polarized state by resonant reflection”.
As shown in FIG. 14A, the wavelength selection filter includes a parallel flat plate-like first flat plate element 20A transparent to light in the terahertz wavelength band, and a parallel flat plate shape transparent to light in the terahertz wavelength band. The second flat plate element 20B is disposed in parallel with the first flat plate element 20A. Reference numeral 20 denotes a composite of the first and second flat plate elements 20A and 20B.

  The first and second flat plate elements 20A and 20B are arranged so as to be inclined at a predetermined angle with respect to the incident light beam, and the fine groove structure is “formed in the same direction” only on the surfaces facing each other. That is, the “pitch directions” of the fine groove structures of the flat plate elements 20A and 20B are parallel to each other.

The incident light beam (“incident light” in the figure) enters from the “side of the flat surface where the fine groove structure is not formed” of the first flat plate element 20A.
The minute interval between the first and second flat plate elements 20A and 20B is changed by the interval variable means.
The distance varying means is the same as that in the embodiment of FIG. 9, and has holding frames 20A and 20B, a piezoelectric element 24, and a drive circuit 26 as shown in FIGS. 14 (c) to 14 (e). .

  The holding frame 22A has a flat plate shape, and is formed with a rectangular opening 22A1 in accordance with the fine groove structure formed in the first flat plate element 20A. The first flat plate element 10A is connected to the incident light incident side. On the opposite surface, the fine groove structure side is held with the flat element 20B side.

  The holding frame 22B also has a flat plate shape, and is formed with a rectangular opening 22B1 in accordance with the fine groove structure formed in the second flat plate element 20B. The second flat plate element 20A is connected to the incident light incident side. On the opposite surface, the fine groove structure side is held with the incident light incident side.

  Accordingly, the fine groove structures of the first and second flat plate elements 20A and 20B held by these are close to each other.

  The piezoelectric element 24 is provided so as to be held between the holding frames 22A and 22B, the operation surface is fixed to the holding frames, and is driven by the drive circuit 26 to change the interval between the holding frames 22A and 22B.

  In this change in interval, when the interval is minimum, the tops of the fine groove structures of the first and second flat plate elements 20A and 20B are in close contact with each other.

  By changing the minute interval between the first and second flat plate elements 20A and 20B by the interval changing means, the wavelength of the reflected light in the narrow wavelength band that is resonantly reflected is selected.

Hereinafter, the “calculation performed on the wavelength selection filter of FIG. 14” by the above-described calculation algorithm will be described.
As shown in FIG. 14B, as the structural parameters, the thickness of the grating layer is t11, the thickness of the waveguide layer is t12, the pitch of the fine groove structure is P1, and the land width is L1. To do. At this time, the filling factor is FF1 = L1 / P1.

  For the flat element 20B, the thickness of the grating layer is t21, the thickness of the waveguide layer is t22, the pitch of the fine groove structure is P2, and the land width is L2. Filling factor: FF2 = L2 / P2.

  The interval between the flat plate elements 20A and 20B is t30.

  The structural parameters were specified as follows for the flat elements 20A and 20B.

"Flat plate element 20A"
Pitch: P1 = 180 μm
Filling factor: FF1 = L1 / P1 = 0.5
Grating layer thickness: t11 = 10 μm
Waveguide layer thickness: t12 = 10 μm.

"Flat plate element 20B"
Pitch: P2 = 180 μm
Filling factor: FF2 = L2 / P2 = 0.5
Grating layer thickness: t21 = 10 μm
Waveguide layer thickness: t22 = 10 μm.

  Both the flat elements 20A and 20B were assumed to have a fine groove structure formed on one side of a Zeonor film having a refractive index of 1.52 in the terahertz band and a thickness of 20 μm. As described above, the flat elements 20A and 20B have the same composition and the same shape.

FIG. 15 shows the calculation results for the spectral reflection characteristics when the interval between the flat plate elements 20A and 20B: t30 is changed to 0 μm, 5 μm, 10 μm, 15 μm, and 20 μm.
In FIG. 15, the horizontal axis represents the wavelength (μm), the vertical axis represents the reflectance, and the “interval” is indicated in the drawing as the interval: t30.

As shown in FIG. 15, it can be seen that the peak wavelength of the reflected light shifts to the short wavelength side when the interval t30 (air layer thickness) increases.
FIG. 16 is a diagram in which the peak wavelength of reflected light is plotted against the air layer thickness (t30), and it can be seen that the peak wavelength shifts substantially linearly to the short wavelength side as the interval: t30 increases.

  Therefore, the desired peak wavelength of the resonant reflected light can be selected by changing the interval t30. FIG. 17 shows alone the spectral reflection characteristics shown in FIG. 15 at an interval of t30 = 15 μm. If this figure is compared with the case of FIG. 2, it can be seen that the reflectance of non-reflected light is effectively reduced.

  In the embodiment of FIG. 14, the example in which the piezoelectric element is used as the interval variable means has been described. However, the present invention is not limited to this, and the interval between the holding frames may be changed using a mechanical variable means. Needless to say.

  Further, the shape of the openings 22A1 and 22B1 of the holding frames 22A and 22B is not limited to the rectangular shape, and may be an elliptical shape, a long hole shape, a circular shape, or the like. The shapes of the holding frame holding frames 22A and 22B themselves may be The shape can be appropriately adjusted.

  For example, when the shape of the holding frame holding frames 22A and 22B and the opening is circular, and the holding frame holding frames 22A and 22B are ring-shaped, the flat plate elements 20A and 20B are bonded to the holding frame holding frames 22A and 22B. In addition, since “a uniform tension from the center toward the outside” acts on the flat plate element, the flat plate element can be bonded without causing unnecessary distortion.

  FIG. 18 is a diagram for explaining one embodiment of the wavelength selective filter according to the fourth aspect.

  The wavelength selective filter of FIG. 18 is a “wavelength selective filter that selectively reflects light in a desired narrow wavelength band out of incident light beams in a terahertz wavelength band in a linearly polarized state by resonant reflection”. And a flat plate element 40 having a fine groove structure in which a large number of grooves are formed at a predetermined pitch on a surface on which incident light flux (indicated as “incident light” in FIG. 15) is incident. Rotating and tilting driving means for rotating and tilting the flat plate element 40 around “an axis parallel to the groove of the fine groove structure” is provided.

  For example, as shown in FIGS. 18B and 18C, the rotation / inclination driving means includes a holding frame 42A for holding the flat plate element 40 and a rotation driving means 44 for rotating the holding frame 42A.

The rotation driving means 44 is, for example, a “stepping motor”.
As shown in FIGS. 18B and 18C, the holding frame 42A has a flat plate shape and has a rectangular opening 42A0. The holding frame 42A is rotatably supported on a support body (not shown) by rotating shafts 42A1 and 42A2. The rotation drive means 44 can be rotated around the rotation axis.

  The flat plate element 40 is held by being bonded and fixed to the holding frame 42A with the longitudinal direction of the grooves in the fine groove structure (the direction perpendicular to the arrangement direction of the grooves on the element surface) being parallel to the rotation shafts 42A1 and 42A2. . That is, the directions of the rotating shafts 42A1 and 41A2 are parallel to the “longitudinal direction of the groove” of the fine groove structure. In FIG. 18B, the concave and convex portions of the fine groove structure are drawn in order to “clarify the surface on which the fine groove structure is formed”, but actually, as shown in FIG. The longitudinal direction of the groove of the structure is the left-right direction of FIG.

As the flat plate element 40, as in the flat plate element 10A described above, as structural parameters,
Pitch: P = 180 μm
Filling factor: FF = 0.5
Grating layer thickness: t1 = 10 μm
Waveguide layer thickness: t2 = 10 μm
In the case of using a “Zeonor film having a refractive index in the terahertz band: 1.52 and a thickness: 20 μm”, the peak wavelength of the resonance reflected light is as described above with reference to FIG. Furthermore, it changes according to the change of the incident angle of the incident light.

  Accordingly, the wavelength of the reflected light in the narrow wavelength band to be resonantly reflected can be selected by changing the incident angle of the incident light beam into the fine groove structure by the rotational inclination of the flat element 40 by the rotational inclination driving means.

  The shape of the opening 42A0 of the holding frame 42A is not limited to the rectangular shape, but may be an elliptical shape, a long hole shape, a circular shape, or the like, and the shape of the holding frame 42A itself can be appropriately matched to these shapes.

  For example, the shape of the opening of the holding frame holding frame 42A is circular, and the holding frame 42A is ring-shaped so that when the flat plate element 40 is bonded to the holding frame 42A, the flat plate element is “uniformly from the center toward the outside. Since the “tension” works, it is possible to bond the flat element without causing unnecessary distortion.

  The wavelength selective filter manufacturing method described above based on the embodiment is, for example, that a part to be a master is processed by “cutting” or “photolithography” to form an uneven pattern with a fine groove structure on the surface thereof. The concavo-convex pattern can be transferred to the aforementioned Zeonor film.

“A method in which a transfer mold (master) is produced by plating by electroforming and then transferred to a resin (Zeonor film) using this mold is suitable as a transfer method.
Further, instead of transfer, a “method of directly cutting a Zeonor film” may be used.
FIG. 19 is a diagram for explaining an optical apparatus using the wavelength selection element of the present invention.
The calculations described above were performed with the incident light flux as the “TE polarization state”.

  As shown in the figure, the wavelength selective filter 52 of the present invention (specifically, any one of the embodiments described above) is disposed after the terahertz light source 50 that emits a beam having a wide wavelength half width. Thus, it is possible to take out light in a narrow band having a desired wavelength, and to make this light incident on the optical element 54 in the subsequent stage. If the optical element 54 is, for example, a “sensor that receives light in the terahertz wavelength band”, only light of a specific wavelength can be detected.

It is a figure for demonstrating the flat element which has a fine groove structure. It is a figure for demonstrating the resonance reflection characteristic of a flat element. It is a figure for demonstrating the influence of the magnitude | size of the pitch with respect to a resonance reflection characteristic. It is a figure for demonstrating the influence of the magnitude | size of the pitch with respect to a resonance reflection characteristic. It is a figure for demonstrating the influence of the thickness of a grating layer with respect to a resonance reflection characteristic. It is a figure for demonstrating the influence of the thickness of a grating layer with respect to a resonance reflection characteristic. It is a figure for demonstrating the influence of the incident angle with respect to a resonance reflection characteristic. It is a figure for demonstrating the influence of the incident angle with respect to a resonance reflection characteristic. It is a figure for demonstrating the influence of the polarization direction with respect to a resonance reflection characteristic. It is a figure for demonstrating the influence of the polarization direction with respect to a resonance reflection characteristic. It is a figure for demonstrating one Embodiment of a wavelength selection filter. It is a figure for demonstrating the characteristic of the wavelength selection filter of FIG. It is a figure for demonstrating the characteristic of the wavelength selection filter of FIG. It is a figure for demonstrating another form of implementation of a wavelength selection filter. It is a figure for demonstrating the characteristic of the wavelength selection filter of FIG. It is a figure for demonstrating the characteristic of the wavelength selection filter of FIG. It is a figure for demonstrating the characteristic of the wavelength selection filter of FIG. It is a figure for demonstrating the other form of implementation of a wavelength selection filter. It is a figure for demonstrating the structure of an optical instrument.

Explanation of symbols

10A First flat plate element 10B Second flat plate element 12A Holding frame 12B Holding frame 14 Piezoelectric element 16 Drive circuit

Claims (5)

A wavelength selective filter that selectively reflects light in a desired narrow wavelength band by resonant reflection among incident light beams in a terahertz wavelength band in a linear polarization state,
A parallel plate-shaped first flat plate element transparent to light in the terahertz wavelength band;
A parallel flat plate that is transparent to light in the terahertz wavelength band, and a second flat plate element that is disposed in parallel with the first flat plate element;
An interval variable means for changing the minute interval between the first and second flat plate elements,
The first and second flat plate elements are disposed at a predetermined angle with respect to the incident light beam,
Of the first and second flat plate elements, at least one surface of the first flat plate element has a fine groove structure in which a large number of grooves are formed at a predetermined pitch,
Incident light flux is incident from the first flat plate element side,
A wavelength selection filter for a terahertz wavelength band, wherein a wavelength of reflected light in a narrow wavelength band that is resonantly reflected is selected by changing a minute interval between the first and second flat plate elements by the interval variable means. . The wavelength selective filter for a terahertz wavelength band according to claim 1,
A wavelength selective filter for a terahertz wavelength band, wherein the first flat plate element has a fine groove structure only on the surface on which incident light flux is incident, and the second flat plate element is a flat surface on both sides. . The wavelength selective filter for a terahertz wavelength band according to claim 1,
A wavelength selective filter for a terahertz wavelength band, having a fine groove structure formed in the same direction only on surfaces of the first and second flat plate elements facing each other. A wavelength selective filter that selectively reflects light in a desired narrow wavelength band by resonant reflection among incident light beams in a terahertz wavelength band in a linear polarization state,
A parallel flat plate that is transparent to light in the terahertz wavelength band.
A flat element having a fine groove structure in which a large number of grooves are formed at a predetermined pitch;
Rotation plate driving means for rotating and tilting the flat plate element about an axis parallel to the groove of the fine groove structure,
The wavelength of the reflected light in the narrow wavelength band to be resonantly reflected is selected by changing the incident angle of the incident light beam to the fine groove structure by the rotational inclination of the flat element by the rotational inclination driving means. A wavelength selective filter for the terahertz wavelength band.   An optical apparatus having the wavelength selection filter for a terahertz wavelength band according to any one of claims 1 to 4.

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