Title: Optical wavelength converting device and process for producing the same
Abstract: A photosensitive resist layer is formed on one surface of a single-polarized ferroelectric substance having nonlinear optical effects. The resist layer has properties such that, when light is irradiated to the resist layer, only exposed areas of the resist layer or only unexposed areas of the resist layer become soluble in a developing solvent. The resist layer is then exposed to near-field light in a periodic pattern with a device, which receives exposure light and produces the near-field light in the periodic pattern. The resist layer is then developed to form a periodic pattern. A periodic electrode is then formed on the one surface of the ferroelectric substance by utilizing the periodic pattern of the resist layer as a mask, the periodic electrode being formed at positions corresponding to opening areas of the mask. An electric field is applied across the ferroelectric substance by utilizing the periodic electrode to set regions of the ferroelectric substance, which stand facing the periodic electrode, as domain inversion regions.
Patent Number: 6,998,223 Issued on 02/14/2006 to Nihei, et al.
| Inventors:
|
Nihei; Yasukazu (Kaisei-machi, JP);
Naya; Masayuki (Kaisei-machi, JP)
|
| Assignee:
|
Fuji Photo Film Co., Ltd. (Kanagawa-ken, JP)
|
| Appl. No.:
|
649013 |
| Filed:
|
August 28, 2000 |
Foreign Application Priority Data
| Aug 27, 1999[JP] | 1999/241062 |
| Oct 15, 1999[JP] | 1999/293802 |
| Current U.S. Class: |
430/321; 430/312; 385/2; 385/5; 385/129; 385/122 |
| Current Intern'l Class: |
G02F 1/37 (20060101) |
| Field of Search: |
385/2,5,129,122
430/321,312
|
References Cited [Referenced By]
U.S. Patent Documents
| 3615470 | Oct., 1971 | Singletary.
| |
| 4006438 | Feb., 1977 | Bennett.
| |
| 4564576 | Jan., 1986 | Saigo et al.
| |
| 4707059 | Nov., 1987 | Ogura et al.
| |
| 4799750 | Jan., 1989 | Miyawaki et al.
| |
| 5131068 | Jul., 1992 | Kanarian et al.
| |
| 5384464 | Jan., 1995 | DeFornel et al.
| |
| 5568308 | Oct., 1996 | Harada.
| |
| 5594746 | Jan., 1997 | Harada.
| |
| 6013221 | Jan., 2000 | Byer et al.
| |
| 6198197 | Mar., 2001 | Yamaouchi et al.
| |
| 6344367 | Feb., 2002 | Naya et al.
| |
| 6497996 | Dec., 2002 | Naya et al.
| |
| Foreign Patent Documents |
| 4-335620 | Nov., 1992 | JP.
| |
| 06-027671 | Feb., 1994 | JP.
| |
| 06-043655 | Feb., 1994 | JP.
| |
| 06-095385 | Apr., 1994 | JP.
| |
| 06-202338 | Jul., 1994 | JP.
| |
| 7-72521 | Mar., 1995 | JP.
| |
| 07-114188 | May., 1995 | JP.
| |
| 08-179493 | Jul., 1996 | JP.
| |
| 10-170966 | Jun., 1998 | JP.
| |
| 2000/-221685 | Aug., 2000 | JP.
| |
| 2002/-020224 | Jan., 2002 | JP.
| |
| 99/15933 | Apr., 1999 | WO.
| |
Other References
Elliott, D.J., "Integrated Circuit Fabrication Technology", (1982), pp. 188-191.
Thompson, L.F., et al., "Introduction to Microlithography", pp. 288-335 (1983).
Patent Abstract of Japan 4335620 A Nov. 24, 1992.
Patent Abstract of Japan 10170966 Jun. 26, 1998.
|
Primary Examiner: Angebranndt; Martin
Attorney, Agent or Firm: Sughrue Mion, PLLC
Claims
What is claimed is:
1. A process for producing an optical wavelength converting device having a periodic
domain inversion structure, in which a periodic electrode is formed on one surface
of a single-polarized ferroelectric substance having nonlinear optical effects,
and an electric field is applied across the ferroelectric substance by the utilization
of the periodic electrode in order to set regions of the ferroelectric substance,
which stand facing the periodic electrode, as local area limited domain inversion
regions, the process comprising the steps of:
i) forming a first resist layer and a second resist layer in this order on the
one surface of the ferroelectric substance, the first resist layer being removable
by etching, the second resist layer being photosensitive and having properties
such that, when light is irradiated to the second resist layer, only exposed areas
of the second resist layer or only unexposed areas of the second resist layer become
soluble in a developing solvent,
ii) exposing the second resist layer to near-field light in a periodic pattern
with means, which receives exposure light and produces the near-field light in
the periodic pattern,
iii) developing the second resist layer, which has been exposed to the near-field
light, to form a periodic pattern in the second resist layer,
iv) etching the first resist layer by utilizing the periodic pattern of the second
resist layer as an etching mask to form a periodic pattern composed of the first
resist layer and the second resist layer, and
v) forming the periodic electrode on the one surface of the ferroelectric substance
by utilizing the periodic pattern, which is composed of the first resist layer
and the second resist layer, as a mask, the periodic electrode being formed at
positions corresponding to opening areas of the mask,
wherein the second resist layer has a film thickness of at most 100 nm.
2. A process for producing an optical wavelength converting device having a periodic
domain inversion structure, in which a periodic electrode is formed on one surface
of a single-polarized ferroelectric substance having nonlinear optical effects,
and an electric field is applied across the ferroelectric substance by the utilization
of the periodic electrode in order to set regions of the ferroelectric substance,
which stand facing the periodic electrode, as local area limited domain inversion
regions, the process comprising the steps of:
i) forming an electrode material layer on the one surface of the ferroelectric substance,
ii) forming a first resist layer and a second resist layer in this order on the
electrode material layer, the first resist layer being removable by etching, the
second resist layer being photosensitive and having properties such that, when
light is irradiated to the second resist layer, only exposed areas of the second
resist layer or only unexposed areas of the second resist layer become soluble
in a developing solvent,
iii) exposing the second resist layer to near-field light in a periodic pattern
with means, which receives exposure light and produces the near-field light in
the periodic pattern,
iv) developing the second resist layer, which has been exposed to the near-field
light, to form a periodic pattern in the second resist layer,
v) etching the first resist layer by utilizing the periodic pattern of the second
resist layer as an etching mask to form a periodic pattern composed of the first
resist layer and the second resist layer, and
vi) etching the electrode material layer by utilizing the periodic pattern, which
is composed of the first resist layer and the second resist layer, as an etching
mask, such that portions of the electrode material layer at positions corresponding
to opening areas of the mask are removed by the etching, whereby the periodic electrode
is formed,
wherein the second resist layer has a film thickness of at most 100 nm.
3. A process as defined in 2 wherein the exposure light has a wavelength falling
within the range of 250 nm to 450 nm.
4. A process as defined in 2 wherein the means, which receives the exposure light
and produces the near-field light in the periodic pattern, is a mask comprising
a light-transmitting member, which is capable of transmitting the exposure light,
and a metal pattern, which has opening areas and is formed on the light-transmitting
member, the near-field light being radiated out from the metal pattern, and
the mask comprising the light-transmitting member and the metal pattern is located
such that the metal pattern is in close contact with the resist layer, which is
laid bare and provided on or above the ferroelectric substance, or the metal pattern
is located close to the resist layer, which is laid bare and provided on or above
the ferroelectric substance, such that the near-field light reaches the resist
layer, which is laid bare and provided on or above the ferroelectric substance,
the exposure light being irradiated to the mask comprising the light-transmitting
member and the metal pattern in this state.
5. A process as defined in 2 wherein the means, which receives the exposure light
and produces the near-field light in the periodic pattern, is an optical stamp
constituted of a light-transmitting member, which is capable of transmitting the
exposure light and has a concavity-convexity pattern formed on one surface, the
optical stamp operating such that, when the exposure light is guided from within
the light-transmitting member to the one surface of the light-transmitting member
and is caused to undergo total reflection, the near-field light in a pattern in
accordance with the concavity-convexity pattern formed on the one surface of the
light-transmitting member is radiated out, and
the optical stamp is located such that the one surface of the optical stamp provided
with the concavity-convexity pattern is in close contact with the resist layer,
which is laid bare and provided on or above the ferroelectric substance, or the
one surface of the optical stamp provided with the concavity-convexity pattern
is located close to the resist layer, which is laid bare and provided on or above
the ferroelectric substance, such that the near-field light reaches the resist
layer, which is laid bare and provided on or above the ferroelectric substance,
the exposure light being irradiated to the optical stamp in this state.
6. A process as defined in 2 wherein the means, which receives the exposure light
and produces the near-field light in the periodic pattern, is a probe provided
with an opening having a diameter shorter than a wavelength of the exposure light,
the probe being caused to scan on the resist layer, which is laid bare and provided
on or above the ferroelectric substance, the exposure light being irradiated in
this state.
7. A process as defined in 2 wherein the ferroelectric substance is LiNbO3 doped
with MgO.
8. A process as defined in 2 wherein the periodic electrode has an electrode
line width of at most 0.3 μm.
9. The method according to 2, wherein a ratio (A/Λ) of an electrode line
width of said periodic electrode (A) and a period of inversion regions of the periodic
domain inversion structure (Λ) is less than 0.15.
10. The method according to 2, wherein a period of inversion regions of the periodic
domain inversion structure falls in a range between approximately 1.0-4.6 micrometers.
11. The method according to 2, further comprising subjecting ferroelectric substance
having the periodic electrode formed thereon to an electric field to form said
periodic domain inversion structure.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an optical wavelength converting device for converting
a fundamental wave into its second harmonic, or the like. This invention particularly
relates to an optical wavelength converting device having a periodic domain inversion
structure. This invention also relates to a process for producing the optical wavelength
converting device. This invention further relates to a solid laser for converting
a produced laser beam into its second harmonic by the utilization of the optical
wavelength converting device and radiating out the second harmonic.
2. Description of the Related Art
A technique, wherein a fundamental wave is converted into its second harmonic
by
the utilization of an optical wavelength converting device having a periodic domain
inversion structure has been proposed by Bleombergen, et al. in Phys. Rev., Vol.
127, No. 6, 1918 (1962). The periodic domain inversion structure is provided with
regions, in which spontaneous polarization (domain) of a ferroelectric substance
having nonlinear optical effects is inverted periodically. With the proposed technique,
phase matching between a fundamental wave and its second harmonic can be effected
by setting such that a period Λ of the domain inversion regions may be integral
multiples of a coherence length ΛC, which may be represented by Formula (1)
shown below.
Λ
c=2π/{β(2ω)-2β(ω)} (1)
in which β(2ω) represents the propagation constant of the second harmonic,
and β(ω) represents the propagation constant of the fundamental wave.
In cases where wavelength conversion is performed by using a bulk crystal of a
nonlinear optical material, the wavelength at which the phase matching is effected
is limited to a specific wavelength that is inherent to the crystal. However, with
the proposed technique, the phase matching can be effected efficiently by selecting
the period Λ of the domain inversion regions, which period satisfies Formula
(1), with respect to an arbitrary wavelength.
One of techniques for forming the periodic domain inversion structure described
above has been proposed in, for example, Japanese Unexamined Patent Publication
No. 7(1995)-72521. With the proposed technique for forming the periodic domain
inversion structure, after a periodic electrode in a predetermined pattern is formed
on one surface of a single-polarized ferroelectric substance having nonlinear optical
effects, an electric field is applied through corona charge across the ferroelectric
substance by the utilization of the periodic electrode and a corona wire, which
is located on the surface side of the ferroelectric substance opposite to the one
surface of the ferroelectric substance, and regions of the ferroelectric substance
which stand facing the periodic electrode are thereby set as local area limited
domain inversion regions.
A different technique for forming the periodic domain inversion structure described
above has been proposed in, for example, Japanese Unexamined Patent Publication
No. 4(1992)-335620. With the proposed technique for forming the periodic domain
inversion structure, an entire-area electrode is formed on a surface of a ferroelectric
substance on the side opposite to a surface on which a periodic electrode in a
predetermined pattern is formed, an electric field is applied across the ferroelectric
substance by the utilization of the entire-area electrode and the periodic electrode,
and local area limited domain inversion regions are thereby formed.
As a technique for forming the periodic electrode, a technique, wherein ridge
regions having predetermined shapes in a predetermined pattern are formed on one
surface of a ferroelectric substance, and electrode fingers of a periodic electrode
are formed on the surfaces of the ridge regions, has been proposed in, for example,
Japanese Unexamined Patent Publication No. 10(1998)-170966.
In cases where the periodic domain inversion structure is formed by the utilization
of the periodic electrode in the manner described above, particularly as for a
Z-cut ferroelectric substance plate, there is a strong possibility that, as the
period of the periodic electrode is set to be short in order for a second harmonic,
or the like, having a short wavelength to be generated, domain inversion regions,
which are adjacent to each other and extend through the ferroelectric substance
from the areas corresponding to electrode fingers of the periodic electrode, will
become connected with each other.
The problems described above will be described hereinbelow with reference to
FIG. 7. FIG. 7 is a graph showing results of evaluation of periodicity of various
bulk-form periodic domain inversion structures, each of which is formed in LiNbO
3
doped with MgO (hereinbelow referred to simply as MgO—LN) by the utilization
of a periodic electrode having an electrode line width (i.e., the line width of
each of the electrode fingers of the periodic electrode) A, the evaluation being
made with respect to various different values of a period Λ of domain inversion
regions and various different values of a duty ratio D (D=A/Λ). In FIG. 7,
the "◯" mark indicates that the periodicity is good over a length of at
least 1 mm. The "Δ" mark indicates that the periodicity is good only over
a length of less than 1 mm or that the regions in which the periodicity is good
occur sporadically. The "X" mark indicates that few regions in which the periodicity
is good occur.
As shown in FIG. 7, in order for good periodicity of the periodic domain inversion
structure to be obtained, it is efficient to set the duty ratio D at a small value,
i.e. to set the electrode line width A at a small value. Also, in cases where the
period Λ of the domain inversion regions is at most 7 μm, it is necessary
for the duty ratio D to be set at a value of at most 0.15. In cases where the domain
inversion length is approximately 1 mm, the duty ratio D should thus be set at
a value of at most 0.15. In the cases of large areas (in cases where the domain
inversion length is approximately 3 mm to 4 mm), such that the inversion periodicity
may be enhanced, the duty ratio D should be set at a value smaller than the value
of at most 0.15.
In cases where the periodic domain inversion structure is formed by the utilization
of the periodic electrode, each of the domain inversion regions is formed over
a region slightly wider than the region corresponding to the electrode line width
A due to the spread of the electric field. Therefore, even if the duty ratio D
is set at a value smaller than 0.15, the periodic domain inversion structure can
be formed, in which the ratio between the width of each domain inversion region
and the width of each non-inversion region is approximately equal to 1:1.
In view of the above circumstances, in cases where a second harmonic, or the
like,
having a short wavelength falling within, for example, the blue region or the ultraviolet
region is to be generated, it is necessary for a periodic electrode having a markedly
small electrode line width A to be formed. However, heretofore, it was difficult
to form a periodic electrode having a markedly small electrode line width A. Particularly,
with respect to the optical wavelength converting device in which the periodic
domain inversion structure is formed in the bulk form in a crystal of a Z-cut plate
of MgO—LN, an example in which a second harmonic having a wavelength falling
within the wavelength region of at most 470 nm has not heretofore been reported.
The term "periodic domain inversion structure in a bulk form in a crystal of a
Z-cut plate" as used herein means the periodic domain inversion structure in which
the domain inversion regions are formed over a range extending from a position
in the vicinity of a +Z surface of the plate to a position in the vicinity of a
-Z surface of the plate.
In cases where a second harmonic having a wavelength falling within the wavelength
region of at most 470 nm is to be generated with the aforesaid type of the optical
wavelength converting device, if the electrode line width A of the periodic electrode
employed for the formation of the periodic domain inversion structure is set at
a value of at most 0.3 μm, a periodic domain inversion structure reliably
having good periodicity over a wide area can be formed.
As techniques for forming a periodic electrode having a small electrode line
width
A, an EB drawing technique, a FIB deposition technique, and the like, have heretofore
been known. However, the conventional techniques for forming a periodic electrode
having a small electrode line width A are not appropriate for large-area patterning
and have a low throughput and a productivity markedly lower than the level of productivity
required for mass production.
As a technique capable of coping with large-area patterning, a technique utilizing
a contraction exposure apparatus has heretofore been known. However, the technique
utilizing the contraction exposure apparatus has the drawbacks in that the cost
of the contraction exposure apparatus is markedly high and it is difficult to obtain
an electrode line width A shorter than the wavelength of exposure light.
SUMMARY OF THE INVENTION
The primary object of the present invention is to provide a process for producing
an optical wavelength converting device, wherein a periodic electrode having a
markedly small electrode line width is capable of being formed, and a bulk-form
periodic domain inversion structure, in which domain inversion regions are formed
with a markedly short period that has heretofore been impossible, is thereby capable
of being formed.
Another object of the present invention is to provide an optical wavelength
converting device having a bulk-form periodic domain inversion structure, in which
domain inversion regions are formed with a markedly short period that has heretofore
been impossible.
A further object of the present invention is to provide a solid laser, wherein
the optical wavelength converting device is utilized, a produced laser beam is
capable of being converted into its second harmonic having a markedly short wavelength,
and the second harmonic is radiated out from the solid laser.
The present invention provides a first process for producing an optical wavelength
converting device having a periodic domain inversion structure, in which a periodic
electrode is formed on one surface of a single-polarized ferroelectric substance
having nonlinear optical effects, and an electric field is applied across the ferroelectric
substance by the utilization of the periodic electrode in order to set regions
of the ferroelectric substance, which stand facing the periodic electrode, as local
area limited domain inversion regions, the process comprising the steps of:
- i) forming a photosensitive resist layer on the one surface of the ferroelectric
substance, the resist layer having properties such that, when light is irradiated
to the resist layer, only exposed areas of the resist layer or only unexposed areas
of the resist layer become soluble in a developing solvent,
- ii) exposing the resist layer to near-field light in a periodic pattern
with means, which receives exposure light and produces the near-field light in
the periodic pattern,
- iii) developing the resist layer, which has been exposed to the near-field
light, to form a periodic pattern in the resist layer, and
- iv) forming the periodic electrode on the one surface of the ferroelectric
substance by utilizing the periodic pattern of the resist layer as a mask, the
periodic electrode being formed at positions corresponding to opening areas of
the mask.
The present invention also provides a second process for producing an optical
wavelength converting device having a periodic domain inversion structure, in which
a periodic electrode is formed on one surface of a single-polarized ferroelectric
substance having nonlinear optical effects, and an electric field is applied across
the ferroelectric substance by the utilization of the periodic electrode in order
to set regions of the ferroelectric substance, which stand facing the periodic
electrode, as local area limited domain inversion regions, the process comprising
the steps of:
- i) forming an electrode material layer on the one surface of the ferroelectric substance,
- ii) forming a photosensitive resist layer on the electrode material
layer, the resist layer having properties such that, when light is irradiated to
the resist layer, only exposed areas of the resist layer or only unexposed areas
of the resist layer become soluble in a developing solvent,
- iii) exposing the resist layer to near-field light in a periodic pattern
with means, which receives exposure light and produces the near-field light in
the periodic pattern,
- iv) developing the resist layer, which has been exposed to the near-field
light, to form a periodic pattern in the resist layer, and
- v) etching the electrode material layer by utilizing the periodic pattern
of the resist layer as an etching mask, such that portions of the electrode material
layer at positions corresponding to opening areas of the mask are removed by the
etching, whereby the periodic electrode is formed.
The present invention further provides a third process for producing an optical
wavelength converting device having a periodic domain inversion structure, in which
a periodic electrode is formed on one surface of a single-polarized ferroelectric
substance having nonlinear optical effects, and an electric field is applied across
the ferroelectric substance by the utilization of the periodic electrode in order
to set regions of the ferroelectric substance, which stand facing the periodic
electrode, as local area limited domain inversion regions, the process comprising
the steps of:
- i) forming a first resist layer and a second resist layer in this order
on the one surface of the ferroelectric substance, the first resist layer being
removable by etching, the second resist layer being photosensitive and having properties
such that, when light is irradiated to the second resist layer, only exposed areas
of the second resist layer or only unexposed areas of the second resist layer become
soluble in a developing solvent,
- ii) exposing the second resist layer to near-field light in a periodic
pattern with means, which receives exposure light and produces the near-field light
in the periodic pattern,
- iii) developing the second resist layer, which has been exposed to the
near-field light, to form a periodic pattern in the second resist layer,
- iv) etching the first resist layer by utilizing the periodic pattern
of the second resist layer as an etching mask to form a periodic pattern composed
of the first resist layer and the second resist layer, and
- v) forming the periodic electrode on the one surface of the ferroelectric
substance by utilizing the periodic pattern, which is composed of the first resist
layer and the second resist layer, as a mask, the periodic electrode being formed
at positions corresponding to opening areas of the mask.
The present invention still further provides a fourth process for producing an
optical wavelength converting device having a periodic domain inversion structure,
in which a periodic electrode is formed on one surface of a single-polarized ferroelectric
substance having nonlinear optical effects, and an electric field is applied across
the ferroelectric substance by the utilization of the periodic electrode in order
to set regions of the ferroelectric substance, which stand facing the periodic
electrode, as local area limited domain inversion regions, the process comprising
the steps of:
- i) forming an electrode material layer on the one surface of the ferroelectric substance,
- ii) forming a first resist layer and a second resist layer in this order
on the electrode material layer, the first resist layer being removable by etching,
the second resist layer being photosensitive and having properties such that, when
light is irradiated to the second resist layer, only exposed areas of the second
resist layer or only unexposed areas of the second resist layer become soluble
in a developing solvent,
- iii) exposing the second resist layer to near-field light in a periodic
pattern with means, which receives exposure light and produces the near-field light
in the periodic pattern,
- iv) developing the second resist layer, which has been exposed to the
near-field light, to form a periodic pattern in the second resist layer,
- v) etching the first resist layer by utilizing the periodic pattern
of the second resist layer as an etching mask to form a periodic pattern composed
of the first resist layer and the second resist layer, and
- vi) etching the electrode material layer by utilizing the periodic pattern,
which is composed of the first resist layer and the second resist layer, as an
etching mask, such that portions of the electrode material layer at positions corresponding
to opening areas of the mask are removed by the etching, whereby the periodic electrode
is formed.
In the third and fourth processes for producing an optical wavelength converting
device in accordance with the present invention, the second resist layer should
preferably have a film thickness of at most 100 nm. Also, the third and fourth
processes for producing an optical wavelength converting device in accordance with
the present invention should preferably be modified such that the first resist
layer is formed from a non-photosensitive material, and the etching performed for
the first resist layer is dry etching.
In the first, second, third, and fourth processes for producing an optical wavelength
converting device in accordance with the present invention, the exposure light
should preferably have a wavelength falling within the range of 250 nm to 450 nm.
Also, the first, second, third, and fourth processes for producing an optical
wavelength converting device in accordance with the present invention should preferably
be modified such that the means, which receives the exposure light and produces
the near-field light in the periodic pattern, is a mask comprising a light-transmitting
member, which is capable of transmitting the exposure light, and a metal pattern,
which has opening areas and is formed on the light-transmitting member, the near-field
light being radiated out from the metal pattern, and
- the mask comprising the light-transmitting member and the metal pattern
is located such that the metal pattern is in close contact with the resist layer,
which is laid bare on the ferroelectric substance, or the metal pattern is located
close to the resist layer, which is laid bare on the ferroelectric substance, such
that the near-field light reaches the resist layer, which is laid bare on the ferroelectric
substance, the exposure light being irradiated to the mask comprising the light-transmitting
member and the metal pattern in this state.
Further, the first, second, third, and fourth processes for producing an
optical wavelength converting device in accordance with the present invention should
preferably be modified such that the means, which receives the exposure light and
produces the near-field light in the periodic pattern, is an optical stamp constituted
of a light-transmitting member, which is capable of transmitting the exposure light
and has a concavity-convexity pattern formed on one surface, the optical stamp
operating such that, when the exposure light is guided from within the light-transmitting
member to the one surface of the light-transmitting member and is caused to undergo
total reflection, the near-field light in a pattern in accordance with the concavity-convexity
pattern formed on the one surface of the light-transmitting member is radiated
out, and
- the optical stamp is located such that the one surface of the optical
stamp provided with the concavity-convexity pattern is in close contact with the
resist layer, which is laid bare on the ferroelectric substance, or the one surface
of the optical stamp provided with the concavity-convexity pattern is located close
to the resist layer, which is laid bare on the ferroelectric substance, such that
the near-field light reaches the resist layer, which is laid bare on the ferroelectric
substance, the exposure light being irradiated to the optical stamp in this state.
Furthermore, the first, second, third, and fourth processes for producing
an optical wavelength converting device in accordance with the present invention
should preferably be modified such that the means, which receives the exposure
light and produces the near-field light in the periodic pattern, is a probe provided
with an opening having a diameter shorter than a wavelength of the exposure light,
the probe being caused to scan on the resist layer, which is laid bare on the ferroelectric
substance, the exposure light being irradiated to the probe in this state.
Also, in the first, second, third, and fourth processes for producing an optical
wavelength converting device in accordance with the present invention, the ferroelectric
substance should preferably be LiNbO
3 doped with MgO (MgO—LN).
In such cases, the periodic electrode should preferably have an electrode line
width of at most 0.3 μm.
The present invention also provides a first optical wavelength converting device,
comprising a crystal of a Z-cut plate of LiNbO
3 doped with MgO, domain
inversion regions being formed periodically in a bulk form in the crystal,
- wherein the domain inversion regions are formed with a period falling
within the range of 1.0 μm to 4.6 μm.
The present invention further provides a second optical wavelength converting
device, comprising a crystal of a Z-cut plate of LiNbO
3 doped with MgO,
domain inversion regions being formed periodically in a bulk form in the crystal,
- wherein the optical wavelength converting device is constituted to radiate
out a wavelength-converted wave having a wavelength falling within the range of
320 nm to 470 nm.
The present invention still further provides a third optical wavelength converting
device, comprising a crystal of a Z-cut plate of LiNbO
3 doped with MgO,
domain inversion regions being formed periodically in a bulk form in the crystal,
- wherein the domain inversion regions are formed with a period falling
within the range of 1.0 μm to 4.6 μm, and
- the optical wavelength converting device is constituted such that, when
a fundamental wave having a wavelength falling within the range of 640 nm to 940
nm impinges upon the optical wavelength converting device, the optical wavelength
converting device radiates out a second harmonic having a wavelength falling within
the range of 320 nm to 470 nm with the period of the domain inversion regions acting
as a first-order period for pseudo-phase matching.
The present invention also provides a solid laser, comprising the first, second,
or third optical wavelength converting device in accordance with the present invention,
the solid laser being constituted to covert a produced laser beam into its second
harmonic and to radiate out the second harmonic.
With the processes for producing an optical wavelength converting device in
accordance with the present invention, the photosensitive resist is exposed to
the near-field light, which oozes from the periodic pattern having a line width
shorter than the wavelength of the exposure light, and the exposed resist is then
developed. Therefore, a periodic electrode having an electrode line width of at
most 100 nm, i.e. a period of at most 200 nm, can be formed. Thus a periodic electrode
having a short electrode line width, which was impossible with conventional lithography,
can be obtained.
Specifically, in cases where the periodic electrode is formed on the
one surface of the ferroelectric substance by utilizing the periodic pattern of
the resist layer as a mask, the periodic electrode being formed at the positions
corresponding to the opening areas of the mask, the line width of each of the opening
areas of the mask may be set at a value of at most 100 nm.
In cases where the electrode material layer is formed on the one surface of the
ferroelectric substance, the electrode material layer is etched by utilizing the
periodic pattern of the resist layer as the etching mask, such that the portions
of the electrode material layer at the positions corresponding to the opening areas
of the mask are removed by the etching, and the periodic electrode is thereby formed,
the line width of each of the areas other than the opening areas of the mask (i.e.,
the line width of each of the areas remaining as the resist layer) may be set at
a value of at most 100 nm.
With the third and fourth processes for producing an optical wavelength converting
device in accordance with the present invention, wherein the double-layered resist
comprising the first resist layer and the second resist layer is employed, in cases
where the ferroelectric substance has a step-like area and an area, to which the
near-field light cannot reach if only one resist layer is formed, occurs, the first
resist layer can act to form a flat surface, and therefore the film thickness of
the second resist, which is photosensitive and is formed on the first resist layer,
can be uniformized. Accordingly, the near-field light can be radiated out uniformly
even in a large-area pattern, and a fine pattern of the second resist layer, which
is photosensitive, can be formed. The first resist layer is then patterned with
a conventional etching technique by utilizing the pattern of the photosensitive
second resist layer as the mask. In this manner, a fine pattern can be formed easily
and at a low cost.
With the processes for producing an optical wavelength converting device in
accordance with the present invention, wherein the mask provided with the metal
pattern or the optical stamp provided with the concavity-convexity pattern is employed
as the means, which receives the exposure light and produces the near-field light
in the periodic pattern, the advantages over the scanning exposure can be obtained
in that the exposure of a large-area periodic pattern can be performed instantaneously,
and therefore the optical wavelength converting device can be produced with a high
throughput and at a low cost.
With the processes for producing an optical wavelength converting device in
accordance with the present invention, wherein the periodic electrode having a
markedly small line width is capable of being formed in the manner described above,
the optical wavelength converting device comprising a crystal of a Z-cut plate
of LiNbO
3 doped with MgO, in which the domain inversion regions are
formed periodically in a bulk form in the crystal, can be obtained, wherein the
domain inversion regions are formed with a period falling within the range of 1.0
μm to 4.6 μm, and wherein the optical wavelength converting device
is constituted such that, when a fundamental wave having a wavelength falling within
the range of 640 nm to 940 nm impinges upon the optical wavelength converting device,
the optical wavelength converting device radiates out a second harmonic having
a wavelength falling within the range of 320 nm to 470 nm with the period of the
domain inversion regions acting as the first-order period for the pseudo-phase matching.
As the optical wavelength converting device comprising a crystal of a Z-cut plate
of LiNbO
3 doped with MgO, in which the domain inversion regions are
formed periodically in a bulk form in the crystal, an optical wavelength converting
device capable of radiating out a second harmonic having a wavelength of at most
470 nm has not heretofore been furnished. Since the absorption end of MgO—LN
is 320 nm, it is practically impossible to radiate a second harmonic having a wavelength
shorter than 320 nm from the optical wavelength converting device.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A to 1G are schematic views showing steps in a first embodiment of the
process for producing an optical wavelength converting device in accordance with
the present invention,
FIG. 2 is a schematic view showing a final step in the first embodiment of the
process for producing an optical wavelength converting device in accordance with
the present invention,
FIG. 3 is a side view showing a solid laser, in which an optical wavelength
converting device obtained with the first embodiment of the process for producing
an optical wavelength converting device in accordance with the present invention
is employed,
FIGS. 4A to 4F are schematic views showing steps in a second embodiment of
the process for producing an optical wavelength converting device in accordance
with the present invention,
FIG. 5 is a schematic view showing a step in a third embodiment of the process
for producing an optical wavelength converting device in accordance with the present invention,
FIG. 6 is a schematic view showing a step in a fourth embodiment of the process
for producing an optical wavelength converting device in accordance with the present
invention, and
FIG. 7 is a graph showing results of evaluation of periodicity of various bulk-form
periodic domain inversion structures, each of which is formed in a ferroelectric
substance by the utilization of a periodic electrode having an electrode line width
A, the evaluation being made with respect to various different values of a period
Λ of domain inversion regions and various different values of a duty ratio
D (D=A/Λ).
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will hereinbelow be described in further detail with reference
to the accompanying drawings.
FIGS. 1A to 1G and FIG. 2 show steps of producing an optical wavelength converting
device in a first embodiment of the process for producing an optical wavelength
converting device in accordance with the present invention. FIGS. 1A to 1G show
steps of forming a periodic electrode. FIG. 2 shows a step of inverting a spontaneous
polarization (domain) of a ferroelectric substance by utilizing the periodic electrode
having been formed with the steps shown in FIGS. 1A to 1G.
How the periodic electrode is formed will be described hereinbelow with reference
to FIGS. 1A to 1G. In this embodiment, MgO—LN is employed as the ferroelectric
substance having nonlinear optical effects. Firstly, as illustrated in FIG. 1A,
a Z-cut MgO—LN plate
10 is prepared. The MgO—LN plate
10
is subjected to single polarization, and the two Z surfaces of the MgO—LN
plate
10 are subjected to mirror polishing. The thickness of the MgO—LN
plate
10 is thus set at 0.3 mm.
Thereafter, as illustrated in FIG. 1B, a resist layer
11 constituted
of a photosensitive material is formed with a spin coating technique or a spraying
technique on one surface (a+Z surface)
10a of the MgO—LN plate
10. The thickness of the resist layer
11 is set at a value approximately
equal to or smaller than the oozing depth of near-field light, which oozing depth
is ordinarily approximately equal to 50 nm.
Thereafter, as illustrated in FIG. 1C, a mask
12 for generating
the near-field light in a periodic pattern is located such that the mask
12
is in close contact with the resist layer
11. The mask
12 comprises
a mask substrate, which is constituted of a dielectric material, such as glass,
and a lattice-like metal pattern, which has fine opening areas
12a,
12a, . . . and is formed on the mask substrate. In this embodiment,
as will be clear from the explanation made later, each of the opening areas
12a,
12a, . . . of the metal pattern corresponds to one of electrode fingers
of the periodic electrode to be formed, and each of metal areas
12b,
12b, corresponds to one of spaces between adjacent electrode fingers.
The mask
12 is located such that the opening areas
12a,
12a, . . . of the metal pattern stand side by side with respect to
a X axis direction of the MgO—LN plate
10. Also, a period Λ
of the metal pattern of the mask
12 is set at a value of 2.1 μm so
as to act as the first-order period with respect to a wavelength of 380 nm of a
second harmonic, which will be described later.
As illustrated in FIG. 1D, exposure light L, such as i-rays (having a wavelength
of 365 nm), is then irradiated from the rear side of the mask
12 (i.e.,
from the upper side in FIG. 1D) to the mask
12. As a result, near-field
light Ln oozes from the opening areas
12a,
12a, . .
. of the metal pattern, and the resist layer
11 is exposed to the near-field
light Ln.
Thereafter, the resist layer
11 is developed with a developing
solution, and the portions of the resist layer
11, which were exposed to
the near-field light Ln, become soluble in a developing solvent. In this manner,
as illustrated in FIG. 1E, a positive type of periodic pattern
11a of
the resist layer
11 is formed. Thereafter, as illustrated in FIG. 1F, the
periodic pattern
11a is utilized as a mask, and chromium (Cr)
13
acting as an electrode material is deposited to a thickness of, for example, 20
nm by vacuum evaporation. As a result, Cr
13 is deposited on a one surface
10a of the MgO—LN plate
10 and only at positions corresponding
to opening areas of the periodic pattern
11a of the resist layer
11. In lieu of Cr
13 being deposited by vacuum evaporation, tantalum
(Ta) may be deposited by a sputtering technique, or the like.
Thereafter, as illustrated in FIG. 1G, the positive type of periodic
pattern
11a of the resist layer
11 is removed by a lift-off
technique, and a periodic electrode
13a having a period Λ of
2.1 μm is thereby formed on the one surface
10a of the MgO—LN
plate
10. Since the mask
12 was located as described above such that
the opening areas
12a,
12a, . . . of the metal pattern
stand side by side with respect to the X axis direction of the MgO—LN plate
10, the electrode fingers constituting the periodic electrode
13a
stand side by side with respect to the X axis direction of the MgO—LN
plate
10.
In this embodiment, the width of each of the opening areas
12a,
12a, . . . of the metal pattern is set at a value of 0.2 μm,
and therefore an electrode line width A of the periodic electrode
13a
is set at a value of 0.2 μm. Accordingly, in this case, a duty ratio
D (D=A/Λ) of the periodic electrode
13a is equal to 0.1. The
value of the duty ratio D is lower than the value of 0.15 described above.
How the spontaneous polarization (domain) of the MgO—LN plate
10
is inverted by the utilization of the periodic electrode
13a will
be described hereinbelow with reference to FIG. 2. As illustrated in FIG. 2, the
MgO—LN plate
10 is located on an electrically conductive jig
1
such that the periodic electrode
13a is in contact with the electrically
conductive jig
1. The electrically conductive jig
1 is formed from
an electrically conductive material, such as copper or stainless steel, and is
grounded through a grounding wire
2.
Also, a corona wire
3 is located above a -Z surface
10b of
the MgO—LN plate
10, and a high voltage electric source
4 is
connected to the corona wire
3. In this state, an electric field is applied
through corona charge across the MgO—LN plate
10 by the utilization
of the corona wire
3 and the high voltage electric source
4. At this
time, the temperature of the MgO—LN plate
10 is set at 100° C.,
and the distance between the corona wire
3 and the MgO—LN plate
10
is set at 10 mm. In this state, an electric voltage of 5 kV is applied for one
second from the high voltage electric source
4 via the corona wire
3.
After the electric field has been applied, the periodic electrode
13a
is removed from the MgO—LN plate
10.
A test was made for confirming the formation of domain inversion regions in the
MgO—LN plate
10. In the test, the Y surface of the MgO—LN plate
10 was cut and polished. Thereafter, selective etching was performed by
use of a mixed etching solution containing hydrofluoric acid and nitric acid. When
the cross-section (the Y surface) of the MgO—LN plate
10 was observed,
it was confirmed that periodic domain inversion regions were formed at positions
corresponding to the positions of the electrode fingers of periodic electrode
13a
and with the predetermined period corresponding to the period of the periodic
electrode
13a. It was also confirmed that each of the periodic domain
inversion regions was formed uniformly to extend from the -Z surface to the +Z
surface and had uniform shape in the Y surface.
An optical wavelength converting device constituted of the MgO—LN plate
10 having been obtained in the manner described above will be described
hereinbelow with reference to FIG. 3. In the manner described above, periodic domain
inversion regions
21,
21, . . . are formed, which stand side by side
with respect to the X axis direction of the MgO—LN plate
10. Thereafter,
the +X surface and the -X surface of the MgO—LN plate
10 are polished.
Non-reflection coating layers are then formed on the +X surface and the -X surface
of the MgO—LN plate
10, and light passage surfaces
20a and
20b are thereby formed. In this manner, a bulk crystal type of optical
wavelength converting device
20 shown in FIG. 3 is obtained.
As illustrated in FIG. 3, the bulk crystal type of optical wavelength converting
device
20 having the periodic domain inversion structure is located on an
output side of an Ar laser pumped titanium sapphire laser
22. A laser beam
23 is produced by the Ar laser pumped titanium sapphire laser
22,
converged by a converging lens
24, and caused to impinge upon the bulk crystal
type of optical wavelength converting device
20. In this case, such that
phase matching may be effected with respect to the fundamental wave having a wavelength
of 760 nm and the second harmonic having a wavelength of 380 nm, with dispersion
due to variation of the refractive index of the MgO—LN for different wavelengths
being taken into consideration, the period Λ of the periodic domain inversion
regions
21,
21, . . . (which period is equal to the period of the
periodic electrode
13a) is set at a value of 2.1 μm.
The Ar laser pumped titanium sapphire laser
22 produces the laser beam
23 having a wavelength of 760 nm as the fundamental wave. The output power
of the Ar laser pumped titanium sapphire laser
22 is 400 mW. The laser beam
23 impinges upon the bulk crystal type of optical wavelength converting
device
20 and is converted into a second harmonic
25 having a wavelength
of 380 nm, which is one-half of the wavelength of the laser beam
23. The
second harmonic
25 undergoes phase matching (i.e., the pseudo-phase matching)
in the periodic domain inversion regions. As described above, the periodic domain
inversion regions
21,
21, . . . have good periodicity. Therefore,
the phase matching is effected appropriately, and the second harmonic
25
with power of 0.5 mW is obtained.
Steps in a second embodiment of the process for producing an optical wavelength
converting device in accordance with the present invention will be described hereinbelow
with reference to FIGS. 4A to 4F. In FIGS. 4A to 4F, similar elements are numbered
with the same reference numerals with respect to FIGS. 1A to 1G.
Firstly, as illustrated in FIG. 4A, the MgO—LN plate
10, which
is of the same type as that employed in the first embodiment described above, is
prepared. A Cr layer
30 having a thickness of 20 nm, which acts as an electrode
material layer, a first resist layer
31 constituted of an organic high-molecular
weight material, and a second resist layer
32 constituted of a photosensitive
material are formed in this order on the one surface (+Z surface)
10a
of the MgO—LN plate
10 and with a spin coating technique or a
spraying technique. The first resist layer
31 and the second resist layer
32 constitute a double-layered resist
33.
Thereafter, as illustrated in FIG. 4B, the mask
12, which is of
the same type as that employed in the first embodiment described above, is located
such that the mask
12 is in close contact with the double-layered resist
33. As in the first embodiment described above, the mask
12 having
the metal areas
12b,
12b, . . . and the opening areas
12a,
12a, . . . of the metal pattern are located such
that the opening areas
12a,
12a, . . . stand side by
side with respect to the X axis direction of the MgO—LN plate
10.
Also, as in the first embodiment described above, as will be clear from the explanation
made later, each of the opening areas
12a,
12a, of
the metal pattern corresponds to one of electrode fingers of the periodic electrode
to be formed, and each of metal areas
12b,
12b, . .
. corresponds to one of spaces between adjacent electrode fingers.
As illustrated in FIG. 4C, the exposure light L, such as i-rays (having a wavelength
of 365 nm), is then irradiated from the rear side of the mask
12 (i.e.,
from the upper side in FIG. 4C) to the mask
12. As a result, the near-field
light Ln oozes from the opening areas
12a,
12a, . .
. of the metal pattern, and the second resist layer
32 is exposed to the
near-field light Ln.
Thereafter, the second resist layer
32 is developed with a developing
solution, and the portions of the second resist layer
32, which were exposed
to the near-field light Ln, become soluble in a developing solvent. In this manner,
as illustrated in FIG. 4D, a negative type of periodic pattern of the second resist
layer
32 is formed. Thereafter, as illustrated in FIG. 4E, the periodic
pattern of the second resist layer
32 is utilized as an etching mask, and
the first resist layer
31 and the Cr layer
30 are subjected to dry
etching with an O
2 plasma.
Thereafter, as illustrated in FIG. 4F, the second resist layer
32
and the first resist layer
31 are removed, and a periodic electrode
30a
constituted of Cr is thereby formed on the one surface
10a of
the MgO—LN plate
10. Since the mask
12 was located as described
above such that the opening areas
12a,
12a, . . . of
the metal pattern stand side by side with respect to the X axis direction of the
MgO—LN plate
10, the electrode fingers constituting the periodic electrode
30a stand side by side with respect to the X axis direction of the
MgO—LN plate
10.
The quality of the first resist layer
31 does not deteriorate due to exposure
to light. Therefore
