JP3940553B2 - Upconversion optical element - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an up-conversion optical element, and more particularly to an up-conversion optical element such as a laser that can generate short-wavelength light with high efficiency using excitation light.
[0002]
[Prior art]
In recent years, in various application fields such as high-density optical recording and magneto-optical recording, it is required to shorten the wavelength of a semiconductor laser for use as a light source. There are several methods for shortening the wavelength of the laser, and these are roughly divided into the following three.
[0003]
(1) Direct oscillation using a semiconductor with a wide band gap.
(2) The wavelength of near-infrared or red laser light is converted to generate the second harmonic.
(3) By up-conversion, short wavelength oscillation is generated by excitation with red or infrared light.
[0004]
FIG. 7 is a conceptual diagram for explaining the principle of these three methods.
That is, FIG. 9A shows a method for obtaining the emitted light hν when electrons injected into the excited state ES by the injection process IJ transition from the excited state ES to the ground state GS. Regarding small lasers, semiconductor lasers and light-emitting diodes having a wavelength of around 410 nm using gallium nitride semiconductors are currently being developed.
[0005]
However, wide gap semiconductors have an essential problem that, in principle, the relaxation constant is large and it is difficult to form an inversion distribution, and it is difficult to reduce the size without using current injection excitation. Furthermore, there is a problem that doping is difficult and it is difficult to form a pn junction.
[0006]
FIG. 7B shows a mechanism for generating the second harmonic. In this mechanism, the second harmonic generation medium has a virtual level VS between the ground state GS and the excited state ES. By irradiating the excitation light hν1 having energy up to the virtual level VS, electrons can be excited stepwise from the ground state GS to the excited state ES via the virtual level VS. When the electrons excited in this way transition from the excited state ES to the ground state GS, light hν2 having a wavelength half that of the excitation light is emitted.
[0007]
However, since the generation of the second harmonic by such a mechanism uses a virtual transition state, the conversion efficiency is not high. Moreover, it is limited to the thing with the high power of excitation light. Furthermore, there is a problem that phase matching is difficult when the laser is of a waveguide type structure with the aim of miniaturization, while the bulk type structure inevitably increases the size of the element.
[0008]
FIG. 7C shows a mechanism for generating short wavelength light by up-conversion. That is, the up-conversion medium has a real level RS, not a virtual level, between the ground state GS and the excited state ES. Then, by irradiating excitation light hν1 and hν2 having energy from the ground state GS to the level RS and from the level RS to the excited state ES, electrons are transferred from the state GS to the state ES via the real level RS. When the electrons excited in steps and thus excited electrons transition from the excited state ES to the ground state GS, light hν3 having a wavelength half that of the excitation light is emitted. In such up-conversion, there is no virtual transition in the excitation process, and only a realistic transition is present, so that the efficiency is theoretically higher than that of the second harmonic. In FIG. 7C, for the sake of simplicity, only one real level RS is shown. However, as will be described in detail later, there are a plurality of ranks RS between the ground state GS and the excited state ES. May be present.
[0009]
As an up-conversion medium used for laser elements, Ho ³⁺ , Tm ³⁺ , Pr ³⁺ Or Pr ³⁺ / Yb ³⁺ Zirconium fluoride glass containing the above. All of these media have been confirmed to oscillate in a single mode fiber. In addition, AB _X Ln _1-X AlO ₄ (Where A is Ca ²⁺ Or Sr ²⁺ And B is Tm ³⁺ , Pr ³⁺ Or Er ³⁺ And Ln is Gd ³⁺ Or La ³⁺ And X is in the range of 0.001 ≦ X ≦ 0.2). JP-A-5-90693 can be cited as a document disclosing these media. Nd ³⁺ An example of using is described in Japanese Patent Laid-Open No. 10-41577.
[0010]
As an upconversion laser using an upconversion medium, initially Tm ³⁺ For the 1D2-3H4 transition (wavelength 455 nm) and 1G4-3H6 transition (wavelength 480 nm) of ⁺ The laser oscillated at 77 K by two-wavelength simultaneous excitation of a laser wavelength of 647.1 nm and a wavelength of 676.4 nm. At room temperature, Ho ³⁺ Laser oscillation was achieved by 5S2-5I8 transition (wavelength 550 nm) and 5S2-5I7 transition (wavelength 753 nm). For this excitation, Kr ⁺ A laser of 647.1 nm is used.
[0011]
[Problems to be solved by the invention]
Further classifying the up-conversion, a case where the ground state GS is excited to the final excited state ES through one excitation level RS as shown in FIG. There is a case where excitation is performed from the excitation RS1 to the final excitation state ES via the second excitation level RS2. In practice, there are many cases where more excitation levels exist.
[0012]
However, in the up-conversion, there is a case in which a relaxation process from these intermediate excitation levels to the ground state may occur, which causes a problem that the efficiency tends to decrease.
[0013]
FIG. 9 is a conceptual diagram illustrating a relaxation process in up-conversion. That is, in the example shown in FIG. 5A, a relaxation process RL occurs in which electrons excited to the excitation level RS fall into the ground state GS. In the example shown in FIG. 9B, the relaxation process RL from the second excitation level RS2 to the ground state GS occurs. When such a relaxation process occurs, the efficiency of excitation to the final excited state ES or the second excitation level RS2 is lowered, and as a result, there is a problem that the upconversion efficiency is lowered.
[0014]
The present invention has been made on the basis of recognition of such a problem, and an object thereof is to provide an up-conversion optical element having a novel configuration that suppresses a relaxation process from an intermediate excitation level to a ground state in up-conversion. There is to do.
[0015]
[Means for Solving the Problems]
According to an aspect of the present invention, an upconversion medium having a real level excited by one photon as an energy state, and the upconversion medium, and having a periodic refractive index distribution on the order of the wavelength of light. A photonic structure, the photonic structure corresponding to a wavelength of light generated by a relaxation process from the real level to the ground state of the upconversion medium, and corresponding to the refractive index distribution An upconversion optical element having a photonic band gap is provided.
[0016]
Here, the space may be formed of a photonic structure.
[0017]
The photonic structure has a photonic band gap corresponding to the refractive index distribution, and the photonic band gap corresponds to a wavelength of light generated by a relaxation process that may occur in the up-conversion medium. Can be.
[0018]
The up-conversion medium has a plurality of excitation levels between a ground state and an excited state, and the relaxation process corresponds to a transition from any one of the plurality of excitation levels to the ground state. Can be.
[0019]
Further, at least a part of the photonic structure may be made of the up-conversion medium.
[0020]
The up-conversion medium may include a dendrimer.
[0021]
Further, by further providing a means for applying a stress to the photonic structure or deforming it by changing the temperature, the photonic band gap can be adjusted to improve efficiency, or light modulation and switching can be performed. .
[0022]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
When a relaxation process from the excited level to the ground state occurs in up-conversion, light emission accompanying the transition occurs. Therefore, if this relaxation light emission can be forcibly suppressed, it is possible to prevent a decrease in efficiency due to the relaxation process.
[0023]
The present inventor has focused on this point and has come up with the idea of providing the up-conversion medium in a field where light having the relaxation light emission wavelength cannot exist. Such a “field” can be formed by a photonic structure.
[0024]
FIG. 1 is an explanatory diagram conceptually showing a main part configuration of an up-conversion optical element of the present invention. That is, this figure shows the wavelength conversion unit of the upconversion optical element of the present invention, and this wavelength conversion unit has a configuration in which the main part of the upconversion medium UC is enclosed inside the photonic structure PS. Alternatively, the up-conversion medium UC itself may be all or part of the constituent elements of the photonic structure PS.
[0025]
Here, the “photonic structure” means that the spatial distribution of the refractive index has a one-dimensional to three-dimensional periodicity on the order of the wavelength of light. A typical example is a “photonic crystal” in which two or more types of media having different refractive indexes are periodically arranged one-dimensionally or three-dimensionally in the order of the wavelength of light.
[0026]
In photonic structures, the relationship between the wave number of light and the frequency, ie photon energy, according to its periodic refractive index distribution shows a band structure (E. Yablonovitch, Phys. Rev. Lett. 58 (20), 2059). (1987)). This phenomenon is similar to a phenomenon in which the energy of electrons in a semiconductor shows a band structure in a periodic potential. In the photonic structure, a wavelength region called “photonic band gap” in which light does not propagate in any direction can appear. That is, in the photonic structure, light having an energy (wavelength) within the range of the photonic band gap cannot exist.
[0027]
FIG. 2 is a graph illustrating the light transmission spectrum in the photonic structure. That is, in the example of the figure, the light transmittance is almost zero in the wavelength range of about 550 to 650 nm, indicating that there is no light in this photonic band range.
[0028]
Some specific examples of the photonic structure will be described below.
(1) As a first example, there is a photonic crystal obtained by crystallizing silicon oxide microspheres by removing a solvent from a colloidal solution containing silicon oxide microspheres. This formation method utilizes the self-alignment of silicon oxide microspheres, and the resulting photonic crystal is called an opal type. In this method, crystals with a large repetition frequency can be obtained relatively easily (H. Miguez et al., Appl. Phys. Lett. 71 (9), 1148 (1997)).
[0029]
(2) As a second example, there is a photonic crystal obtained by the Wood-Pile method (S. Noda et al., Jpn. J. Appl. Phys., 35, L909 (1996)). In this method, a structure in which square members are arranged on two substrates using a semiconductor microfabrication technique is formed, and the two substrates are bonded to each other so that the square member portions are orthogonal to each other. By removing this substrate by etching, a structure in which two “square members” are stacked is formed. Similarly, a board with “square bars” arranged on the surface is prepared, and the square bars are stacked one by one by repeating adhesion and etching by precise alignment. This method can produce a diamond structure with a photonic band gap open in all directions.
[0030]
(3) As a third example, there is a photonic crystal obtained by a method called auto-cloning method (Kawakami et al., JP-A-10-335758). In this method, a two-dimensional periodic uneven pattern is formed on a quartz or semiconductor substrate by lithography, and thin films are stacked in multiple layers while reproducing the underlying uneven pattern by a bias sputtering method. In this way, a three-dimensional periodic structure is produced in the direction of the inner surface of the substrate on which the irregular pattern is first engraved and in the stacking direction perpendicular to the surface. This method is more reliable and reproducible than the opal type photonic crystal manufacturing method, and does not require a complicated and time-consuming microfabrication process as much as the woodpile method. A photonic crystal with a period can be produced.
[0031]
(4) As a fourth example, there is a photonic crystal obtained by utilizing an interference pattern of light (Tsunotomo, Koyama, JP-A-10-68807). In this method, a laser beam is irradiated so as to burn an interference pattern onto a multi-layered thin film that is one-dimensionally stacked, so that the film surface of the multi-layered film is utilized by using melting / evaporation or ablation at a portion where the light intensity is strong. Periodic cuts are made in the vertical direction to produce a photonic crystal. This method is considered to be an efficient method because many periods can be formed at a time when a periodic structure is formed by a laser interference pattern.
[0032]
Returning to FIG. 1 again, the up-conversion optical element of the present invention has the main part of the up-conversion medium UC enclosed in the photonic structure PS as shown in FIG. Specifically, as will be described later in detail as an example, for example, a configuration in which the “gap” of the photonic structure is filled with an upconversion medium can be used. Alternatively, the upconversion medium may form part of the photonic structure, or the upconversion medium may form the photonic structure. For example, if two types of up-conversion media having different refractive indexes are arranged on the order of the wavelength of light, this constitutes the photonic structure PS as it is.
[0033]
As the photonic structure PS, one having a photonic band gap corresponding to the transition of the relaxation process that can occur in the up-conversion medium UC is used.
[0034]
FIG. 3 is a conceptual diagram showing the transition that occurs in the up-conversion optical element of the present invention.
[0035]
That is, as shown in FIG. 5A, when excited from the ground state GS to the final excited state ES via one excited level RS, the electrons excited to the excited level RS are in the ground state. The upconversion medium UC is confined by the photonic structure PS having a photonic band corresponding to the wavelength of light generated by the relaxation process RL falling into the GS. Then, since there is no light corresponding to the relaxation process RL in the up-conversion medium, the relaxation process is limited, and the electrons stay in the excitation level RS for a long time, and are excited in the excited state ES with high efficiency.
[0036]
Similarly, even when a transition through two excitation levels RS1 and RS2 as shown in FIG. 4B occurs, the photonic structure having a photonic band gap corresponding to the relaxation process RL is included. By confining the up-conversion medium UC, the electron relaxation process can be suppressed and the excitation efficiency to the excited state ES can be greatly improved.
[0037]
The effects described above can be obtained in the same manner even when excited from the ground state GS to the excited state ES via three or more excited levels. That is, the up-conversion medium UC may be confined in a photonic structure having a photonic band gap corresponding to a possible relaxation process. Here, when a plurality of relaxation processes can occur, it is desirable to select a photonic structure having a photonic band gap corresponding to the relaxation process with the highest probability of occurrence.
[0038]
In the case of the transition shown in FIG. 3A, the relaxation process RL has the same or approximate wavelength as the excitation light hν1. In this case, if the photonic band gap is selected so as to suppress the relaxation process RL, the excitation light hν1 is also limited, and it may be difficult to excite the photonic medium.
[0039]
Therefore, in the present invention, as illustrated in FIG. 3B, a more remarkable effect can be obtained by using an upconversion medium having a transition in which the relaxation process RL and the pumping light have different wavelengths.
[0040]
As the up-conversion medium UC, glass containing neodymium ions or the like can be used as described above. In addition, when a “Dendrimer” having a large number of branch structures is used, since the branch functions as an antenna that collects energy, multi-photon excitation can be efficiently generated and the efficiency of up-conversion can be improved. .
[0041]
Furthermore, the band gap of the photonic structure PS can be adjusted by changing its periodicity, that is, the lattice constant. Therefore, by adding a mechanism that can change the periodicity of the photonic structure mechanically, electrically, or temperature, the photonic band gap can be adjusted to accurately match the relaxation process RL of the upconversion medium. Can do.
[0042]
【Example】
Hereinafter, embodiments of the present invention will be described in more detail with reference to examples.
[0043]
(First embodiment)
First, a laser device in which an upconversion medium is confined in a photonic structure having a one-dimensional laminated structure will be described as a first embodiment of the present invention.
[0044]
FIG. 4 is a conceptual diagram showing a cross-sectional configuration of a main part of the up-conversion optical element of this example. That is, the optical element 10A of the present example forms a one-dimensional photonic structure, and therefore has a BaTiO layer thickness of 80 nm. ₃ Layer 12 and Nd with a layer thickness of 80 nm _0.2 Y _0.8 Ba ₂ Cl ₇ A laminate in which 100 microcrystal-containing glass layers 14 are alternately laminated is provided. That is, this laminate constitutes a one-dimensional photonic structure, and a part thereof is constituted by the glass layer 14 that is an upconversion medium. This one-dimensional photonic structure has a photonic band gap in which light cannot be transmitted in a wavelength band from 580 nm to 640 nm.
[0045]
A coating 18 reflecting almost 100% at a wavelength of at least 380 nm to 420 nm was formed on the rear surface of the laminate. A coating 16 having a reflectance of 80% in the same wavelength range was formed on the front surface of the laminate. Since this optical element 10A transmits light having a wavelength of 803 nm, the excitation light L ₁ As a result, a Ti: sapphire laser having a time width of 10 nanoseconds and a wavelength of 803 nm was incident. Excitation light L ₁ The incident peak power density is about 1 MW / cm ² It was.
[0046]
As an example of materials for upconversion lasers that oscillate at room temperature, Nd _0.2 Y _0.8 Ba ₂ Cl ₇ Is disclosed in Japanese Patent Laid-Open No. 10-41577. This material is excited by a near-infrared semiconductor laser of 806 nm and oscillates at around 390 nm and 415 nm through three up-conversion processes. However, large light emission is observed near 600 nm. This is because some excited electrons are relaxed by light emission in the up-conversion process, and this relaxation process lowers the oscillation efficiency. Therefore, it is possible to increase the oscillation efficiency by suppressing this light emission.
[0047]
As a sample for comparison, the composition of FIG. ₃ A laser element having a structure excluding the layer 12 was prepared. Both laser oscillation lights were monitored with a photodiode, and the oscillation intensity signals were compared.
[0048]
FIG. 5 is a graph showing the relationship of the oscillation power intensity with respect to the excitation power intensity in this example and the comparative example. That is, both the samples of this example and the comparative example oscillated laser, but it was found that the slope efficiency (oscillation power change / excitation light power change) was overwhelmingly higher in this example.
[0049]
(Second embodiment)
Next, a laser element in which an upconversion medium is confined in a photonic structure using a grating (diffraction grating) will be described as a second embodiment of the present invention.
[0050]
FIG. 6 is a conceptual diagram showing a cross-sectional configuration of a main part of the up-conversion optical element of this example. That is, the optical element of this example has a grating pair type photonic structure having a structure in which the gratings face each other. JP-A-10-83005 can be cited as a document disclosing this photonic structure. Such a structure also has the same action as the one-dimensional photonic crystal.
[0051]
That is, a resist layer 23 was formed on the surface of the glass substrate 22, and a grating 24 having a pitch of about 160 nm was formed on the surface. Prepare two of these, each with BaTiO ₃ The layer 25 is sputtered to a thickness of 100 nm, and Nd is used as an upconversion medium. _0.2 Y _0.8 Ba ₂ Cl ₇ The microcrystal-containing glass layer 26 was sputtered with a thickness of 100 nm. Next, as shown in FIG. 6, they were bonded face to face.
[0052]
As a comparative example, a sample in which the grating 24 was not formed on the surface of the resist layer 23 was produced. As a result of performing the same evaluation as that of the first example described above, it was found that although all of the optical elements oscillated with laser, the optical element of this example had an overwhelmingly higher oscillation slope efficiency than the comparative example. .
[0053]
(Third embodiment)
Next, an optical element using a dendrimer as an upconversion medium will be described as a third embodiment of the present invention. Aida et al., “Chemical” Vol. 53, No3, (1998) report the presence of dendrimer five-photon excitation. Therefore, in the configuration of the first embodiment shown in FIG. _0.2 Y _0.8 Ba ₂ Cl ₇ Instead, an upconversion optical element using dendrimer L5AZO and laser dye IR-5 was prepared. At this time, the thickness of each layer is as follows: dendrimer / dye layer, BaTiO ₃ Both layers were 1.65 μm. 1600cm generated with an infrared monochromator ^-1 (6.25 μm) excitation light L ₁ Was irradiated. As a result, the light L in the vicinity of the wavelength 1.3 μm from this optical element. ₂ Was observed. This is considered to be the result of the emission of light by moving the energy of electrons excited by the dendrimer to five photons to the dye IR-5. Observation of an optical element to which up-conversion of five-photon excitation has contributed has not been obtained so far, and is considered to be obtained as a result of greatly suppressing the electron relaxation process according to the present invention.
[0054]
(Fourth embodiment)
Next, as a fourth embodiment of the present invention, an up-conversion optical element in which the photonic band gap can be adjusted by changing the photonic structure will be described.
[0055]
Yoshino et al. (JJA P, 38A, L786 (1999)) report that the photonic band of an organic opal photonic crystal is changed by applying pressure. In the same way, Nd _0.2 Y _0.8 Ba ₂ Cl ₇ A polymer opal was prepared. Its crystal structure is F.I. C. C. (Face-centered cubic) and spherical Nd _0.2 Y _0.8 Ba ₂ Cl ₇ (250 nm diameter) is contained in poly (2-methoxy-5-dodecyloxy-p-phenylenevinylene).
[0056]
In this photonic structure, an absorption region that hardly transmits light corresponding to the photonic band gap appears in the light transmission spectrum with a half width of about 20 nm. When a uniaxial pressure is applied to this structure so that the total length is 1 and the length is reduced to 0.38 minutes, the peak wavelength of the absorption region ranges from a wavelength of 570 nm to a wavelength of 700 nm. Changed.
[0057]
In addition, as in the first embodiment, the excitation light L of 803 nm ₁ , 390nm and 415nm emission L ₂ Was monitored. Then, it was confirmed that the amount of emitted light changed when a uniaxial pressure was applied to the optical element.
[0058]
On the other hand, organic materials generally have a large coefficient of thermal expansion. The thermal expansion coefficient of the organic opal of this example is also about 1 × 10. ^-4 / K. Therefore, when the Peltier element is brought into contact with the photonic structure and the temperature is increased from 20 ° C. to 120 ° C., the lattice constant can be increased by about 1%. When the temperature was raised at 120 ° C. with no uniaxial pressure applied, the peak wavelength of the absorption region shifted from 570 nm to 574 nm. Therefore, after adjusting the pressure to give the maximum light emission amount, the temperature was further raised, and the light emission amount changed slightly, but the light emission amount could be further increased by 1 to 2 percent.
[0059]
In the present embodiment, the photonic band can be changed by changing the photonic structure, and the photonic band gap can be matched or shifted with respect to the wavelength of light by the relaxation process. When this phenomenon is applied, a light modulation function or optical switching function for changing the emission intensity by changing the photonic structure is added, or as a stress sensor or a temperature sensor in which the emission intensity changes depending on stress or temperature. You can also get functionality.
[0060]
The embodiments of the present invention have been described above with reference to specific examples. However, the present invention is not limited to these specific examples. For example, examples of the photonic structure that can be used in the present invention include various structures including those conventionally referred to as “photonic crystals”. Further, the “gap” of the conventional photonic structure made of such a conventional material may be filled with an upconversion medium, or a part of these photonic structures may be replaced with the upconversion medium. Alternatively, all of these photonic structures may be replaced by an upconversion medium.
[0061]
Further, the configuration of the optical element may be one having a built-in excitation light source or emitting light upon receiving excitation light from the outside. Furthermore, you may have a light modulation element, a light receiving element, etc. together.
[0062]
【The invention's effect】
As described in detail above, according to the present invention, the upconversion medium is confined in the photonic structure having a photonic band corresponding to light emission by the relaxation process, thereby causing the upconversion with extremely high efficiency. A high-performance up-conversion optical element can be provided.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram conceptually showing a main part configuration of an up-conversion optical element of the present invention.
FIG. 2 is a graph illustrating a transmission spectrum of light in a photonic structure.
FIG. 3 is a conceptual diagram showing transitions that occur in the up-conversion optical element of the present invention.
FIG. 4 is a conceptual diagram showing a cross-sectional configuration of a main part of the up-conversion optical element according to the first embodiment of the present invention.
FIG. 5 is a graph showing the relationship of the oscillation power intensity to the excitation power intensity in the first example and the comparative example.
FIG. 6 is a conceptual diagram illustrating a cross-sectional configuration of a main part of an up-conversion optical element according to a second embodiment of the present invention.
FIG. 7 is a conceptual diagram for explaining the principle of three methods for obtaining a short wavelength.
FIG. 8A shows a case where the ground state GS is excited to the final excited state ES via one excitation level RS, and FIG. 8B shows the second excitation level RS2 from the first excitation RS1. In this case, the final excited state ES is excited.
FIG. 9 is a conceptual diagram illustrating a relaxation process in up-conversion.
[Explanation of symbols]
PS photonic structure
UC upconversion medium
GS ground state
RS excitation level
ES excited state
RL relaxation process
L ₁ Excitation light
L ₂ Emitted light
12 BaTiO ₃ layer
1 4 Nd _0.2 Y _0.8 Ba ₂ Cl ₇ Glass layer containing microcrystals
16, 18 coating
22 Glass substrate
23 resist layer
24 grating
25 BaTiO ₃ layer
26 Upconversion media

Claims (4)

An upconversion medium having, as an energy state, a real level excited by one photon;
A photonic structure comprising the upconversion medium and having a periodic refractive index distribution on the order of the wavelength of the light;
The photonic structure has a photonic band gap corresponding to a wavelength of light generated by a relaxation process from the real level to the ground state of the up-conversion medium and corresponding to the refractive index distribution. Characteristic up-conversion optical element. The up-conversion medium has a plurality of excitation levels between a ground state and an excited state, and the relaxation process corresponds to a transition from any one of the plurality of excitation levels to the ground state. optical device according to claim 1, wherein. The up-conversion medium, light element according to claim 1 or 2, characterized in that it comprises a dendrimer. Optical device according to any one of claims 1-3, characterized in further comprising a means for deforming by changing the adding stress to the photonic structure or temperature.

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