WO2021172393A1

WO2021172393A1 - Laser element

Info

Publication number: WO2021172393A1
Application number: PCT/JP2021/006969
Authority: WO
Inventors: サンガランゲドンアトゥラサンダナヤカ; アディカリムディヤンセラゲチャトゥランガニーセネヴィラッネ; コポナパハラワラワェブディカサンジーワバンダラカルナティラカ; 敏則松島; 安達　千波矢
Original assignee: 国立大学法人九州大学; 株式会社ＫＯＡＬＡＴｅｃｈ
Priority date: 2020-02-26
Filing date: 2021-02-25
Publication date: 2021-09-02
Also published as: TW202139553A

Abstract

This laser element, which has a two-dimensional DFB diffraction grating structure provided with at least two types of linear protruding section arrays that have different array directions, has a low laser oscillation threshold value. The DFB diffraction grating structure is preferably a two-dimensional DFB diffraction grating structure that has a concentric rectangular shape such as a concentric square shape, and an organic layer is preferably formed directly on the DFB diffraction grating structure.

Description

Laser element

The present invention relates to a laser device having a low laser oscillation threshold.

An organic laser device that uses an organic compound as an active layer can obtain a high photoluminescence (PL) quantum yield and a wide emission spectrum over the visible region, and its characteristics can be controlled by the molecular design of the organic compound, and the device is flexible. Since it has the advantage of being able to grant, research is being actively conducted. As a result of vigorous research toward the development of a current-excited type, a current-excited laser element using an organic compound has been realized for the first time (see Patent Document 1).
This current excitation type laser element has a structure in which a DFB (distributed feedback) diffraction grating structure as an optical resonator is formed on the surface of an ITO electrode, and an organic gain layer is provided on the DFB (distributed feedback) diffraction grating structure. In the DFB diffraction grating structure here, the first region in which the band-shaped convex portions are arranged in one direction at a fixed period and the band-shaped convex parts are arranged in the same arrangement direction as the first region in a lattice period different from that of the first region. It has a second region arranged, and each of these regions has a mixed periodic arrangement arranged alternately. The document describes that laser oscillation with a narrow full width at half maximum (FWHM) has been realized in an organic laser device having such a DFB diffraction grating structure.

International Publication No. 2018/043763 Pamphlet

In order to put an organic laser element into practical use, it is necessary to provide an energy-efficient organic laser element. Specifically, it is desired to provide an element that has a low laser oscillation threshold and oscillates with low energy. However, previous studies have not clarified under what conditions the energy-efficient organic laser device can be provided.

Under such circumstances, the present inventors have made diligent studies for the purpose of providing a laser element having a low laser oscillation threshold.

As a result of diligent studies by the present inventors in order to solve the above problems, a two-dimensional DFB diffraction grating structure having at least two types of linear convex arrangements having different arrangement directions is provided as an optical resonator. , It has been found that a laser element having an extremely low laser oscillation threshold can be realized. The present invention has been proposed based on these findings, and specifically has the following configuration.

[1] A laser element having a two-dimensional DFB diffraction grating structure having at least two types of linear convex arrangements having different arrangement directions.
[2] The laser element according to [1], wherein the DFB diffraction grating structure has a concentric rectangular shape.
[3] The laser element according to [1], wherein the DFB diffraction grating structure has a concentric square shape.
[4] The laser element according to [1], wherein the arrangements of the two types of linear convex portions have the same lattice period.
[5] The laser element according to any one of [1] to [4], which has a pair of electrodes and oscillates a laser when energized.
[6] The laser device according to any one of [1] to [5], wherein an organic layer is directly formed on the DFB diffraction grating structure.
[7] The laser device according to [6], wherein the organic layer contains an organic compound having at least one stilbene unit.
[8] The laser device according to [6] or [7], wherein the organic layer contains 4,4'-bis [(N-carbazole) styryl] biphenyl (BSBCz).
[9] The organic layer contains 4,4'-bis [(N-carbazole) styryl] biphenyl (BSBCz) and 4,4'-bis (N-carbazolyl) -1,1'-biphenyl (CBP). , [8].
[10] The laser device according to any one of [6] to [9], wherein the organic layer contains a compound having at least one fluorene unit.
[11] The laser device according to any one of [6] to [10], wherein the organic layer has a thickness of 80 to 350 nm.
[12] The laser element according to any one of [1] to [11], wherein the height of each linear convex portion constituting the DFB diffraction grating structure is less than 75 nm.
[13] The laser element according to any one of [1] to [12], wherein the DFB diffraction grating structure is _{composed of SiO 2.}
[14] The laser device according to any one of [6] to [13], wherein the DFB diffraction grating structure is provided between the organic layer and the transparent electrode.
[15] The laser device according to any one of [1] to [14], which does not contain a triplet quencher.

According to the present invention, it is possible to realize a laser element having a low laser oscillation threshold.

It is a schematic plan view which shows an example of the DFB diffraction grating structure provided in the laser element of this invention. It is a schematic cross-sectional view which shows the layer structure example of the laser element of this invention. It is a schematic perspective view which shows the manufacturing process of the laser element performed in Example 1. FIG. It is an SEM photograph of the DFB diffraction grating structure of the laser element manufactured in Example 1 and the laser element manufactured in Comparative Examples 2-7. It is a graph which shows the current density-voltage characteristic at the time of DC drive of the laser element which has a concentric square DFB diffraction grating structure manufactured in Example 1, and the laser element which does not have a DFB diffraction grating structure manufactured in Comparative Example 1. .. It is an emission spectrum at the time of pulse driving measured with various current densities about the laser element having a concentric square DFB diffraction grating structure manufactured in Example 1. 6 is a log-log graph in which the emission peak intensities in FIG. 6 are plotted with the current density on the horizontal axis. It is a graph which shows the angle dependence of the emission intensity measured at ^110Acm-2 about the laser element which has the concentric square DFB diffraction grating structure manufactured in Example 1. FIG.

Hereinafter, the contents of the present invention will be described in detail. The description of the constituent elements described below may be based on typical embodiments and specific examples of the present invention, but the present invention is not limited to such embodiments and specific examples. The numerical range represented by using "-" in the present specification means a range including the numerical values before and after "-" as the lower limit value and the upper limit value. In addition, the term "main component" in the present specification means the component having the highest content among the constituent components. Further, the isotope species of the hydrogen atom existing in the molecule of the compound used in the present invention is not particularly limited, and for example, all the hydrogen atoms in the molecule ^{may be 1} H, or part or all of them may be ² H. (Duterium D) may be used.

<Laser element>
The laser element of the present invention is characterized by having a two-dimensional DFB (distributed feedback) diffraction grating structure (distributed feedback type diffraction grating structure) having at least two types of linear convex arrangements having different arrangement directions.
Hereinafter, the DFB diffraction grating structure of the laser element of the present invention and a configuration example of the laser element will be described.

[DFB diffraction grating structure]
The DFB diffraction grating structure used in the present invention includes at least two types of linear convex arrangements having different arrangement directions.
Here, the "linear convex portion" is a line segment of the convex portion, and two or more linear convex portions are arranged at intervals from each other in a direction orthogonal to the extending direction of the linear convex portion. Therefore, the "arrangement direction" corresponds to the direction orthogonal to the extending direction of the linear convex portion. In the DFB diffraction grating structure used in the present invention, there are at least two groups consisting of linear convex portions arranged at intervals in this way, and these groups have different arrangement directions of the linear convex portions. There is. In the DFB diffraction grating structure used in the present invention, it is preferable that 2 to 6 groups of linear convex portions having different arrangement directions are present, and more preferably 2 to 4 groups are present. For example, two groups of linear convex portions having different arrangement directions may form a concentric rectangular DFB diffraction grating structure, or three groups of linear convex portions having different arrangement directions may be concentric. A hexagonal DFB diffraction grating structure may be formed, or four linear convex portions having different arrangement directions may be formed to form a concentric octagonal DFB diffraction grating structure. The linear protrusions constituting each group preferably include regions that are periodically arranged. Here, "periodic" means that the linear convex portions are arranged at substantially constant intervals, and in the present specification, the outer peripheral edges of adjacent linear convex portions or adjacent rectangular rings are arranged with each other. The interval Λ of is called the "lattice period".
The DFB diffraction grating structure used in the present invention is preferably formed concentrically.
The "concentric shape" in the present invention means a first lattice convex portion forming an annular shape (a shape obtained by cutting out a substantially similar figure one size smaller than the ring from the ring) in a plan view, and the first lattice convex portion and the plan view. Refers to a shape in which the second lattice convex portion to the nth lattice convex portion, which have substantially similar figures, are arranged so that their centers overlap. Here, the second lattice convex portion to the nth lattice convex portion (n is an integer of 3 or more) are also substantially similar to each other, and the ring of the first lattice convex portion is the smallest, and the larger the number of n, the more the ring. Will be larger. n may be selected from, for example, a range of 3 or more, a range of 10 or more, a range of 50 or more, a range of 100 or more, a range of 200 or more, a range of 500 or more, and a range of 1000 or more. Further, n may be selected from a range of less than 2000, a range of less than 800, a range of less than 400, and a range of less than 150. The ring referred to here does not have to be a perfect ring, for example, a part may be missing, or the rings may be connected to each other by adjacent lattice convex portions. Also, the width of the ring may be narrower or thicker than the region that occupies most of the area.
The center of the ring overlapping between the convex portion of the first lattice and the convex portion of the nth lattice means the center of gravity of the ring. For example, in the case of a ring having rotational symmetry, it is the center of rotational symmetry. Here, in this DFB diffraction grating structure, the annular convex portion may be provided in the region inside the first lattice convex portion and including the center of the ring. It is preferable that the annular convex portion has a shape substantially similar to the ring of the first lattice convex portion in a plan view.
The rings formed concentrically are preferably polygonal rings. At this time, a curved portion may be included in a part of the ring of the polygon. For example, the portion (corner) where the sides forming the polygon intersect may be formed in a curved shape. Further, the polygonal ring does not have to include a curved portion. Further, the polygonal ring may also be partially missing, for example, the corners may be missing, or the middle of the side may be missing. Polygonal rings may have all the same internal angles, but they may be different. The ring of the polygon may be a regular polygon. Examples of polygons include rectangles, hexagons, and octagons.
In a preferred embodiment of the present invention, the DFB diffraction grating structure is formed in a concentric rectangular shape. A concentric rectangular "rectangle" means a quadrangle. That is, the shape may be any shape surrounded by four straight lines. Specific examples of the "rectangle" include a square, a rectangle, a rhombus, a parallelogram, and the like, preferably a square and a rectangle, and more preferably a square. For example, in the concentric square shape as shown in FIG. 1, the arrangement directions of the adjacent sides (adjacent linear convex portions) of the rectangular ring shape are formed in different directions in the X direction and the Y direction.
By having such a concentric two-dimensional DFB diffraction grating structure in the laser element of the present invention, it is presumed that light feedback occurs in each arrangement direction to efficiently form a population inversion, and an extremely low laser oscillation threshold is set. It can be realized.

In the DFB diffraction grating structure used in the present invention, the lattice period Λ of the arrangement of each linear convex portion is Λ = λ / 2 n _eff (λ: wavelength of light causing Bragg reflection, n _eff : effective average refractive index of the diffraction grating). Is selected as an index.
The height h of each linear convex portion is preferably less than 75 nm, more preferably 20 to 70 nm.
Here, the lattice period Λ of the arrangement of the linear convex portions may be the same or different from each other in each arrangement, but it is preferable that they are substantially the same. , The width w and the height h of each linear convex portion may be the same or different from each other in each arrangement, but are preferably substantially the same. Further, the width w and the height h of each linear convex portion may be the same or different from each other in each arrangement, but it is preferable that they are substantially the same.

Examples of the constituent material of the DFB diffraction grating structure (each lattice convex portion and annular convex portion) include a material that can be used as a DFB diffraction grating and a material that has been used. It is preferable to use SiO _2.

[Construction example of laser element]
The laser device of the present invention may be a photoexcited laser device that oscillates by laser irradiation when the active layer is irradiated with excitation light, or holes and electrons are injected into the active layer and they are recombined to be generated. It may be a current excitation type laser element (semiconductor laser element) that oscillates a laser by the energy generated. The photoexcited laser element has a structure in which at least a DFB diffraction grating structure and an active layer are formed on a substrate. Further, the current excitation type laser element has at least an anode and a cathode (a pair of electrodes) and a structure in which a DFB diffraction grating structure and an active layer are formed between the anode and the cathode, and a voltage is applied between the pair of electrodes. The laser oscillates by energizing the active layer. In each laser element, the DFB diffraction grating structure is concentric.

The active layer is a layer that receives energy to generate excitons, forms a population inversion, and then causes stimulated emission due to the incident of light.
Here, in the current excitation type laser element, it is preferable that the excitons generated in the active layer by the current excitation do not substantially disappear (disappearance due to collision between excitons). Specifically, the loss due to exciton annihilation is preferably less than 10%, more preferably less than 5%, even more preferably less than 1%, even more preferably less than 0.1%, particularly preferably less than 0.01%. , Most preferably 0%.
Further, it is preferable that the current excitation type laser element does not show a substantial polaron absorption loss at the laser oscillation wavelength, that is, there is no substantial overlap between the polaron absorption spectrum and the emission spectrum. Specifically, the loss due to polaron absorption is preferably less than 10%, more preferably less than 5%, even more preferably less than 1%, even more preferably less than 0.1%, particularly preferably less than 0.01%. Most preferably 0%.
Further, it is preferable that the oscillation wavelength of the laser element does not substantially overlap the absorption wavelength region in the excited state and the absorption wavelength region of the radical cation or the radical anion. The loss due to absorption in the excited state is preferably less than 10%, more preferably less than 5%, even more preferably less than 1%, even more preferably less than 0.1%, particularly preferably less than 0.01%, most preferably less than 0.01%. It is 0%.
Further, the laser device of the present invention preferably does not contain a triplet quencher.

In the laser device of the present invention, the active layer is preferably an organic layer containing an organic compound, and more preferably an organic layer composed of an organic compound. Further, the active layer may form a light emitting portion together with other layers. Here, the light emitting portion has a laminated structure composed of an active layer and another layer laminated on the active layer. For example, in a current excitation type laser element, the entire light emitting portion is arranged between the anode and the cathode. .. The other layer constituting the light emitting portion is preferably an organic layer containing an organic compound, but may contain an inorganic layer in addition to the organic layer. Here, the organic layer may be composed of only an organic compound, or may be a layer containing an organic compound and an inorganic substance. However, in the case of a layer containing an organic compound and an inorganic substance, it is preferable that the organic compound is the main component (60% by weight or more of the components). In the present specification, each organic layer constituting the light emitting portion may be referred to as an "organic layer", and when the organic layers are continuously laminated (laminated without sandwiching the inorganic layer in between). In some cases), the entire laminated structure of the organic layer may be referred to as an "organic layer". Further, the entire laminated structure of the organic layer (active layer) when only the active layer is provided as the organic layer and the active layer and another organic layer are continuously laminated is "integrated". It is sometimes called "organic layer as".

It is preferable that an organic layer is directly formed on the DFB diffraction grating structure. This makes it possible to directly introduce the Bragg-reflected light in the DFB diffraction grating structure into the organic layer and contribute to stimulated emission in the active layer. The refractive index of the organic layer directly formed on the DFB diffraction grating structure is preferably different from the refractive index of the DFB diffraction grating structure.
The organic layer directly formed on the DFB diffraction grating structure may be an active layer, or may be an organic layer arranged between the DFB diffraction grating structure and the active layer. In this case, the organic layer between the DFB diffraction grating structure and the active layer may have a single-layer structure or a multi-layer structure.
The organic layer (integral organic layer) formed directly on the DFB diffraction lattice structure preferably contains an organic compound having at least one stilbene unit, preferably 4,4'-bis [(N-carbazole) styryl]. More preferably, it contains biphenyl (BSBCz), 4,4'-bis [(N-carbazole) stilbene] biphenyl (BSBCz) and 4,4'-bis (N-carbazolyl) -1,1'-biphenyl (CBP). ) Is more preferably included. Further, the organic layer (organic layer as a unit) directly formed on the DFB diffraction grating structure preferably contains an organic compound having at least one fluorene unit. An organic compound having at least one stilbene unit and an organic compound having at least one fluorene unit can easily form a population inversion and can efficiently cause stimulated emission. Therefore, it is preferable that these compounds are contained in the active layer. Further, in particular, a laser device containing BSBCz and CBP in the organic layer is characterized in that it exhibits a PL quantum yield of almost 100%, a short PL lifetime of about 1 ns, a large radiation decay constant (Kr), and a low ASE threshold.

The thickness of the organic layer directly formed on the DFB diffraction grating structure is preferably 80 to 350 nm, more preferably 100 to 300 nm, and further preferably 150 to 250 nm. Here, when the organic layer is continuously laminated on the organic layer directly formed on the DFB diffraction grating structure, the thickness of the entire laminated structure of the organic layer, that is, the organic layer as an integral body. It is assumed that the thickness is in the above thickness range. For example, when the layer farthest from the DFB diffraction grating structure is an inorganic layer and the other layers are organic layers, the thickness obtained by subtracting the thickness of the inorganic layer from the thickness of the light emitting portion is the above. It shall be in the thickness range of.

Further, it is preferable that the DFB diffraction grating structure is provided between the electrode and the organic layer. For example, the DFB diffraction grating structure may be provided between the cathode and the organic layer, may be provided between the anode and the organic layer, or may be provided in both of them. In this case, the electrode that sandwiches the DFB diffraction grating structure with the organic layer is preferably a transparent electrode. Further, the DFB diffraction grating structure is preferably formed directly on the electrodes.

Examples of other layers constituting the light emitting portion include a hole transport layer, a hole injection layer, an electron blocking layer, a hole blocking layer, an electron injection layer, an electron transport layer, and an exciton blocking layer. The hole transport layer may be a hole injection transport layer having a hole injection function, and the electron transport layer may be an electron injection transport layer having an electron injection function. A specific structural example of the current-excited laser device is shown in FIG. In FIG. 2, 1 is a substrate, 2 is a cathode, 3 is a DFB diffraction grating structure, 4 is an electron injection layer, 5 is an active layer, 6 is a hole injection layer, and 7 is an anode. Here, as an example of the laser device shown in FIG. 2, a configuration in which the hole injection layer 6 is a metal oxide layer such as molybdenum oxide and the electron injection layer 4 and the active layer 5 are organic layers can be mentioned. Further, an organic layer made of HAT-CN or the like may be inserted between the metal oxide layer and the anode 7. Further, the organic layer between the metal oxide layer and the DFB diffraction grating structure 3 may have a single-layer structure or a multi-layer structure of three or more layers in addition to the two-layer structure shown in FIG. ..
In the current-excited current-excited laser element, the laser light generated in the active layer may pass through the anode and be taken out to the outside, or may pass through the cathode and be taken out to the outside, and pass through the anode and the cathode. And may be taken out to the outside. Further, the laser beam generated in the active layer may be taken out from the end face of the light emitting portion.
Hereinafter, each member and each layer of the current excitation type laser element will be described. The description of the substrate and the active layer also applies to the photoexcited laser device and the active layer. For a description of the DFB diffraction grating structure, the description in the section [DFB diffraction grating structure] can be referred to.

(substrate)
The current-excited laser device of the present invention is preferably supported by a substrate. When the current excitation type laser element is configured to extract the laser light from the substrate side, a substrate having translucency to the laser light is used as the substrate, and a transparent substrate made of glass, transparent plastic, quartz, or the like is used. Is preferably used. On the other hand, when the current excitation type laser element has a configuration in which the laser beam is extracted from the side opposite to the substrate, the substrate is not particularly limited, and a substrate made of silicon, paper, or cloth may be used in addition to the above transparent substrate. can.

(anode)
As the anode in the current excitation type laser element, a metal having a large work function (4 eV or more), an alloy, an electrically conductive compound, or a mixture thereof as an electrode material is preferably used. Specific examples of such electrode materials include metals such as Al and Au, and conductive transparent materials such as _{CuI, indium zinc oxide (ITO), SnO 2, ZnO, and TiN.} Further, a material capable of producing an amorphous transparent conductive film such as IDIXO (In ₂ O _{3-ZnO) may be used.} The anode can be formed by forming a film of these electrode materials by a method such as vapor deposition or sputtering. Further, a pattern having a desired shape may be formed on the formed thin film by a photolithography method to serve as an anode, or when pattern accuracy is not required so much (about 100 μm or more), it is desired at the time of vapor deposition or sputtering of the electrode material. The pattern may be formed through a mask having the shape of. Alternatively, when a coatable material such as an organic conductive compound is used, a wet film forming method such as a printing method or a coating method can also be used.
However, when the current excitation type laser element is configured to transmit the laser light through the anode, the anode needs to have translucency with respect to the laser light, and the transmittance of the laser light is 1. It is preferably configured to be greater than%, and more preferably configured to be greater than 10%. Specifically, it is preferable to use the above-mentioned conductive transparent material for the anode, or to use a thin film formed of a metal or alloy having a thickness of 10 to 100 nm for the anode.
The sheet resistance as an anode is preferably several hundred Ω / □ or less. Further, the film thickness depends on the material, but is usually selected in the range of 10 to 1000 nm, preferably 10 to 200 nm.

(cathode)
On the other hand, as the cathode, a metal having a smaller work function than the material used for the anode (referred to as an electron-injectable metal), an alloy, an electrically conductive compound, or a mixture thereof is used as an electrode material. Specific examples of such electrode materials include sodium, sodium-potassium alloy, magnesium, lithium, magnesium / copper mixture, magnesium / silver mixture, magnesium / aluminum mixture, magnesium / indium mixture, aluminum / aluminum oxide (Al ₂ O). ₃ ) Examples thereof include a mixture, an indium, a lithium / aluminum mixture, and a rare earth metal. Among these, from the viewpoint of electron injectability and durability against oxidation and the like, a mixture of an electron injectable metal and a second metal which is a stable metal having a larger work function value than this, for example, a magnesium / silver mixture. Magnesium / aluminum mixture, magnesium / indium mixture, aluminum / aluminum oxide (Al ₂ O ₃ ) mixture, lithium / aluminum mixture, aluminum and the like are suitable. The cathode can be formed by forming a film of these electrode materials by a method such as vapor deposition or sputtering.
However, when the current excitation type laser element is configured to transmit the laser light through the cathode, the cathode needs to have translucency with respect to the laser light, and the transmittance of the laser light is 1. It is preferably configured to be greater than%, and more preferably configured to be greater than 10%. Specifically, it is preferable to use a thin film formed by forming the above electrode material with a thickness of 10 to 100 nm for the cathode, or to use a conductive transparent material such as ITO exemplified in the above (anode) section for the cathode. ..
The sheet resistance as a cathode is preferably several hundred Ω / □ or less, and the film thickness is usually selected in the range of 10 nm to 5 μm, preferably 50 to 200 nm.

(Active layer)
The active layer is a layer in which excitons are generated by recombination of holes and electrons injected from the anode and cathode, respectively, an inverted distribution is formed, and then stimulated emission is induced by light incident.
The active layer preferably contains an organic compound, and is preferably composed of an organic compound. Further, the active layer preferably contains an organic compound as a light emitting material for a laser. As the light emitting material for a laser, any known material can be used, but an organic compound having at least one stilbene unit and an organic compound having at least one fluorene unit are preferable. BSBCz can be mentioned as a specific example of an organic compound having at least one stilbene unit.
Further, in order to exhibit high luminous efficiency, the energy supplied to the active layer is efficiently converted into excited singlet energy and transferred to the luminescent material, and the singlet exciter generated in the luminescent material is used as the luminescent material. It is important to keep it inside. Therefore, the active layer preferably contains a host material as well as a light emitting material. As the host material, an organic compound having a lowest excited singlet energy level higher than that of the light emitting material can be used. As a result, the excited singlet energy generated in the host material can be easily transferred to the light emitting material, and the singlet exciter generated in the light emitting material can be confined in the molecule of this light emitting material, and the light emitting efficiency can be improved. It will be possible to fully withdraw. However, even if the singlet excitons cannot be sufficiently confined, it may be possible to obtain high luminous efficiency. Therefore, any host material capable of achieving high luminous efficiency is used in the present invention without particular limitation. be able to. The host material can be appropriately selected depending on the light emitting material. For example, when BSBCz is used as the light emitting material, CBP can be preferably used as the host material.
When a host material is used, the amount of the light emitting material for laser contained in the active layer is preferably 0.1% by weight or more, more preferably 1% by weight or more, and 50% by weight or less. It is preferably 20% by weight or less, and further preferably 10% by weight or less.
The host material in the active layer is preferably an organic compound having a hole transporting ability and an electron transporting ability, preventing a long wavelength of light emission, and having a high glass transition temperature.
The thickness of the active layer is preferably 5 to 500 nm, more preferably 50 to 300 nm.

(Injection layer)
The injection layer is a layer provided between the electrode and the active layer for lowering the driving voltage and improving the emission brightness. There are a hole injection layer and an electron injection layer, and between the anode and the active layer or the hole transport layer, And may be present between the cathode and the active layer or electron transport layer. The injection layer can be provided as needed. As the hole injection material and the electron injection material, a known organic hole injection material or a known organic electron injection material can be used, but a metal oxide such as molybdenum oxide can also be used as the hole injection material. An organic material to which an alkali metal such as Cs is added can also be used as an electron injection material.

(Blocking layer)
The blocking layer is a layer capable of blocking the diffusion of charges (electrons or holes) and / or excitons existing in the active layer to the outside of the active layer. The electron blocking layer can be placed between the active layer and the hole transporting layer to prevent electrons from passing through the active layer towards the hole transporting layer. Similarly, the hole blocking layer can be placed between the active layer and the electron transporting layer, blocking holes from passing through the active layer towards the electron transporting layer. The blocking layer can also be used to prevent excitons from diffusing outside the active layer. That is, the electron blocking layer and the hole blocking layer can also function as exciton blocking layers, respectively. The electron blocking layer or exciton blocking layer referred to in the present specification is used in the sense that one layer includes a layer having the functions of an electron blocking layer and an exciton blocking layer.

(Hole blocking layer)
The hole blocking layer has a function of an electron transporting layer in a broad sense. The hole blocking layer has a role of blocking the holes from reaching the electron transporting layer while transporting electrons, which can improve the recombination probability of electrons and holes in the active layer. As the material of the hole blocking layer, a material of the electron transport layer described later can be used as needed.

(Electronic blocking layer)
The electron blocking layer has a function of transporting holes in a broad sense. The electron blocking layer has a role of blocking electrons from reaching the hole transporting layer while transporting holes, which can improve the probability of recombination of electrons and holes in the active layer. ..

(Exciton blocking layer)
The exciton blocking layer is a layer for preventing excitons generated by recombination of holes and electrons in the active layer from diffusing into the charge transport layer, and the excitons are inserted by inserting this layer. It is possible to efficiently confine it in the active layer, and it is possible to improve the light emission efficiency of the element. The exciton blocking layer can be inserted into either the anode side or the cathode side adjacent to the active layer, and both can be inserted at the same time. That is, when the exciton blocking layer is provided on the anode side, the layer can be inserted between the hole transport layer and the active layer adjacent to the active layer, and when inserted on the cathode side, the active layer and the cathode can be inserted. The layer can be inserted adjacent to the active layer between and. Further, a hole injection layer, an electron blocking layer, or the like can be provided between the anode and the exciton blocking layer adjacent to the anode side of the active layer, and the cathode and the excitation adjacent to the cathode side of the active layer can be provided. An electron injection layer, an electron transport layer, a hole blocking layer, and the like can be provided between the child blocking layer and the electron blocking layer. When the blocking layer is arranged, it is preferable that at least one of the excitation singlet energy and the excitation triplet energy of the material used as the blocking layer is higher than the excitation singlet energy and the excitation triplet energy of the light emitting material.

(Hole transport layer)
The hole transport layer is made of a hole transport material having a function of transporting holes, and the hole transport layer may be provided with a single layer or a plurality of layers.
The hole transporting material has either injection or transport of holes or an electron barrier property, and may be either an organic substance or an inorganic substance. Known hole transporting materials that can be used include, for example, triazole derivatives, oxadiazole derivatives, imidazole derivatives, carbazole derivatives, indolocarbazole derivatives, polyarylalkane derivatives, pyrazoline derivatives and pyrazolone derivatives, phenylenediamine derivatives, arylamine derivatives, etc. Amino-substituted chalcone derivatives, oxazole derivatives, styrylanthracene derivatives, fluorenone derivatives, hydrazone derivatives, stilben derivatives, silazane derivatives, aniline copolymers, conductive polymer oligomers, especially thiophene oligomers, etc., but porphyrin compounds, aromatics, etc. It is preferable to use a group tertiary amine compound and a styrylamine compound, and it is more preferable to use an aromatic tertiary amine compound.

(Electronic transport layer)
The electron transport layer is made of a material having a function of transporting electrons, and the electron transport layer may be provided with a single layer or a plurality of layers.
The electron transporting material (which may also serve as a hole blocking material) may have a function of transferring electrons injected from the cathode to the active layer. Examples of the electron transporting layer that can be used include nitro-substituted fluorene derivatives, diphenylquinone derivatives, thiopyrandioxide derivatives, carbodiimides, freolenidenemethane derivatives, anthracinodimethane and anthrone derivatives, and oxadiazole derivatives. Further, among the oxadiazole derivatives, a thiadiazole derivative in which the oxygen atom of the oxadiazole ring is replaced with a sulfur atom, and a quinoxalin derivative having a quinoxalin ring known as an electron-withdrawing group can also be used as an electron transport material. Further, a polymer material in which these materials are introduced into a polymer chain or these materials are used as a polymer main chain can also be used.

The film forming method of each layer constituting these laser elements is not particularly limited, and may be produced by either a dry process or a wet process.

In the current excitation type laser element as described above, when a current equal to or higher than the threshold current density is passed between the anode and the cathode, holes and electrons are injected into the active layer to generate excitons, and a population inversion is formed. Later, stimulated emission occurs and stimulated emission light is emitted from the active layer. The light radiated from the active layer is Bragg-reflected by the DFB diffraction grating structure, so that the light of a specific wavelength strengthens each other and propagates horizontally to the main surface of the substrate and returns to the active layer. Induces stimulated emission in the active layer. By repeating such light feedback, laser oscillation in a single longitudinal mode occurs. At this time, in the laser element of the present invention, since the DFB diffraction grating structure has at least two types of linear convex arrangements having different arrangement directions, light feedback occurs in each arrangement direction to efficiently form a population inversion. At the same time, the intensification of light by Bragg reflection and the return of light to the active layer occur efficiently. As a result, the laser oscillation threshold value can be significantly reduced, and good laser oscillation characteristics can be obtained.

The features of the present invention will be described in more detail with reference to examples below. The materials, treatment contents, treatment procedures, etc. shown below can be appropriately changed as long as they do not deviate from the gist of the present invention. Therefore, the scope of the present invention should not be construed as limited by the specific examples shown below. The emission spectrum during pulse drive is measured using a multi-channel spectrometer (Hamamatsu Photonics: PMA-50), and the element characteristics during pulse drive are measured with an amplifier (NF: HSA4101) and photomultiplier tubes. This was performed using a tube (manufactured by Hamamatsu Photonics Co., Ltd .: C9525-02). In addition, electron beam lithography for forming a diffraction grating is performed using (JBX-5500SC system manufactured by JEOL Ltd.), and film thickness is measured using a profileometer (DektakXT manufactured by Bruker Co., Ltd.) and SEM. The photograph was taken using a scanning electron microscope (manufactured by Hitachi High-Tech Co., Ltd .: SU8000).

(Example 1) Manufacture of a laser element having a concentric square DFB diffraction grating structure The manufacturing process of the laser element performed in this example is shown in FIG. Here, as the diffraction grating structure, the concentric square DFB diffraction grating structure shown in FIG. 1 was formed. The lattice period Λ of each linear convex portion constituting the concentric square shape was 280 nm, the width w was 140 nm, and the height h was 60 nm.
First, as shown in FIG. 3A, a glass substrate on which a cathode made of indium tin oxide (ITO) having a film thickness of 30 nm was formed was prepared. Then, as shown in FIG. 3B, silicon oxide (SiO ₂ ) was formed into a film having a thickness of 60 nm in _{the central region (10 × 10 mm) of the ITO by a sputtering method to form a SiO 2 film.} .. Subsequently, as shown in FIG. 3C _{, four concentric square patterns were formed on the SiO 2} film by using an electron beam lithography method and a dry etching method to form a DFB diffraction grating structure. The patterning of this SiO ₂ film was performed as follows.
First, _{HMDS treatment was performed by spin-coating the surface of the SiO 2} film with hexamethyldisilazane (HMDS) and annealing at 120 ° C. for 120 seconds. Next, a resist solution (manufactured by Zeon Co., Ltd .: ZEP520A-7) was spin-coated and baked to form a resist layer having a thickness of 70 nm. This resist layer was irradiated with an electron beam beam by an electron beam lithography apparatus and treated with a developing solution (ZED-N50) to obtain a resist mask having a pattern corresponding to the DFB diffraction grating structure. .. _{Subsequently, plasma etching with CHF 3} was performed on the resist mask, _{and SiO 2} in the region where the mask was not formed was etched until the ITO surface was exposed. Then, _{plasma etching with O 2} was performed to remove the resist mask to form a concentric square DFB diffraction grating structure shown in FIG.
Next, as shown in FIG. 3D, each thin film and anode constituting the light emitting portion were laminated ^{on the DFB diffraction grating structure at a vacuum degree of 1.5 × 10 -4 Pa by a vacuum deposition method.} First, cesium (Cs) and CBP were co-deposited from different vapor deposition sources on the DFB diffraction grating structure to form a layer having a thickness of 50 nm. At this time, the concentration of cesium was set to 20% by weight. Next, BSBCz and CBP were co-deposited from different vapor deposition sources to form a layer having a thickness of 160 nm, which was used as an active layer. At this time, the concentration of BSBCz was set to 6% by weight. Next, molybdenum trioxide (MoO ₃ ) was formed to a thickness of 10 nm, and aluminum (Al) was further vapor-deposited to a thickness of 100 nm to form an anode to form a laser device.

(Comparative Example 1) Manufacture of a laser element having no DFB diffraction grating structure Same as Example 1 except that a light emitting portion is formed directly on ITO without forming a DFB diffraction grating structure and an Al anode is formed on the light emitting portion. A laser element was manufactured in the same manner.

(Comparative Examples 2 to 7) Manufacture of Laser Elements Having Other DFB Diffraction Grating Structures Except that the plan view shape of the DFB diffraction grating structure was changed to the shape shown in the SEM photographs of FIGS. 4 (a) to 4 (f). , A laser element was manufactured in the same manner as in Example 1. FIG. 4 (g) is an SEM photograph of the DFB diffraction grating structure formed in Example 1.

FIG. 5 shows the current density-voltage characteristics when a DC voltage is applied to each of the laser elements of Example 1 and Comparative Example 1. When the external quantum efficiency was measured in the range of 100 to 150 Acm- ² , the laser element of Example 1 showed higher external quantum efficiency than the laser element of Comparative Example 1.
The emission spectrum when pulse voltage is applied to the laser element of Example 1 at various current densities is shown in FIG. 6, and the result of plotting the emission peak intensity with respect to the current density is shown in FIG. The pulse voltage used here has a pulse width of 400 ns and a pulse frequency of 1 kHz.
As shown in FIG. 6, by applying a pulse voltage, a sharp peak derived from laser oscillation was observed from the laser element of Example 1. The laser oscillation threshold obtained from FIG. 7 was 20 Acm ^-1 , which was an extremely low value.
FIG. 8 shows the results of measuring the angle dependence of the emission intensity by driving the laser element of Example 1 at 110 Acm- ^{2 exceeding the threshold value.} The horizontal axis in the graph of FIG. 8 indicates the measurement angle of the emission intensity, which is 90 ° when measured from the normal direction with respect to the substrate surface and 0 ° or 180 ° when measured from the horizontal direction with respect to the substrate surface. .. As shown in FIG. 8, in the laser element of Example 1, the emission intensity was the highest in the normal direction with respect to the substrate surface, and directivity was recognized in the emission.

Also, each laser element of Comparative Example 2-7 also determines the lasing threshold _{J th} in the same manner as in Example 1, was measured emission wavelength lambda _DFB and the half-value width FWHM of its threshold _{J th.} The results of these measurements are shown in Table 1.

As shown in Table 1, the laser oscillation threshold J _th of the laser device of Example 1 was much lower value than the laser oscillation threshold J _th of Comparative Examples 2-7. Therefore, by forming the DFB diffraction grating structure with at least two types of linear convex arrangements having different arrangement directions, the laser oscillation threshold can be effectively reduced and the laser is oscillated at a relatively low current density. It was found that a current-excited organic laser element can be realized.
In the laser element of Example 1, a layer made of HAT-CN may be inserted between the _{Al anode and the MoO 3 layer.}

The laser element of the present invention has an extremely low laser oscillation threshold value, and even when configured as a current-pumped organic laser element, laser oscillation can be performed with a relatively low current density. Therefore, according to the present invention, it is possible to realize a highly practical current-excited organic laser element. Therefore, the present invention has high industrial applicability.

1 Substrate 2 Cathode 3 DFB diffraction grating structure 4 Electron injection layer 5 Active layer 6 Hole injection layer 7 Anode

Claims

A laser element having a two-dimensional DFB diffraction grating structure having at least two types of linear convex arrangements having different arrangement directions.
The laser element according to claim 1, wherein the DFB diffraction grating structure has a concentric rectangular shape.
The laser element according to claim 1, wherein the DFB diffraction grating structure has a concentric square shape.
The laser element according to claim 1, wherein the array of the two types of linear convex portions has the same lattice period.
The laser element according to any one of claims 1 to 4, which has a pair of electrodes and oscillates a laser when energized.
The laser device according to any one of claims 1 to 5, wherein an organic layer is directly formed on the DFB diffraction grating structure.
The laser device according to claim 6, wherein the organic layer contains an organic compound having at least one stilbene unit.
The laser device according to claim 6 or 7, wherein the organic layer contains 4,4'-bis [(N-carbazole) styryl] biphenyl (BSBCz).
Claim that the organic layer comprises 4,4'-bis [(N-carbazole) styryl] biphenyl (BSBCz) and 4,4'-bis (N-carbazolyl) -1,1'-biphenyl (CBP). 8. The laser element according to 8.
The laser device according to any one of claims 6 to 9, wherein the organic layer contains a compound having at least one fluorene unit.
The laser device according to any one of claims 6 to 10, wherein the organic layer has a thickness of 80 to 350 nm.
The laser element according to any one of claims 1 to 11, wherein the height of each linear convex portion constituting the DFB diffraction grating structure is less than 75 nm.
The DFB grating structure is composed of Si0 2, laser device according to any one of claims 1 to 12.
The laser element according to any one of claims 6 to 13, wherein the DFB diffraction grating structure is provided between the organic layer and the transparent electrode.
The laser element according to any one of claims 1 to 14, which does not contain a triplet quencher.