Surface-emitting quantum cascade laser

Abstract

To provide a surface emitting quantum cascade laser that is different from conventional surface emitting quantum cascade lasers and is capable of implementing excellent beam quality as a surface emitting quantum cascade laser.SOLUTION: A surface emitting quantum cascade laser includes a semiconductor layer other than a laser active layer and the laser active layer and includes a square lattice or rectangular lattice photonic crystal on the laser active layer. A unit lattice of the square lattice or rectangular lattice photonic crystal is made of a composition A and a composition B having a refractive index differing from that of the composition A. The composition A is a compound semiconductor composition or metal composition. The composition B is a compound semiconductor composition. The unit lattice of the square lattice or rectangular lattice photonic crystal has a structure comprising a columnar structure, which is made of the composition B and has a pentagon bottom surface, at the center part of a columnar structure, which is made of the composition A and has a square or rectangular bottom surface.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a surface-emitting quantum cascade laser. Specifically, it relates to a surface-emitting quantum cascade laser having a photonic crystal with a structure that improves the radiation loss ratio of the resonance mode, and also to a surface-emitting quantum cascade laser having a photonic crystal with a structure that can reduce the absorption of laser light in the laser element.

Background Art

[0002] The quantum cascade laser is a relatively new semiconductor laser whose oscillation was first confirmed in 1994, and it is the only small laser light source that can cover the wavelength band from mid-infrared to far-infrared including wavelengths of 3 to 5 μm, and even up to the terahertz region. Since there are absorption peaks of various gas species in the wavelength band of 3 to 5 μm, it is possible to measure the concentrations of various gases by using a quantum cascade laser.

[0003] In addition, the quantum cascade laser can measure the isotope ratio by utilizing the single-wavelength property of the laser, and since there is no other simple alternative measurement method, it has attracted great attention.

[0004] Moreover, the quantum cascade laser can enhance the sensitivity by the multipass mirror method of passing gas between opposing mirrors by utilizing the straightness of the laser, so that trace measurements at the ppb level are also possible. By further applying this straightness, the creation of a new technology that enables the detection of dangerous gases in remote areas such as volcanic eruption gases is also expected.

[0005] However, when applying the quantum cascade laser to high-sensitivity applications such as those using the above-described multipass method and detecting dangerous gases in remote areas, a high-quality beam that can maintain the beam shape even with a long optical path length is required. In addition, when applying it to the detection of dangerous gases in remote areas, since it is necessary to propagate the laser light over a long distance, high output is also required. Therefore, in order to realize these characteristics, research and development of a surface-emitting quantum cascade laser with high output power and high beam quality, which incorporates a photonic crystal structure at a position close to the active layer (i.e., the light-emitting layer) of the laser element, have been carried out (Patent Documents 1 and 2).

[0006] For example, Patent Document 1 discloses that a surface-emitting quantum cascade laser having a so-called composite structure composed of two types of two-dimensional lattices (i.e., two types of photonic crystals) suppresses the leakage of laser light from the side surface of the laser.

[0007] Also, Patent Document 2 discloses that a surface-emitting semiconductor laser element having a diffraction grating of a square lattice (i.e., a photonic crystal) and having a shape of a triangular prism with a substantially right-angled triangle as the bottom surface of its unit structure improves the laser beam quality.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0009] The photonic crystal used in the surface-emitting quantum cascade laser is a structure in which a plurality of materials with different refractive indices are periodically arranged two-dimensionally, and is incorporated using nanofabrication techniques such as electron beam lithography so as to be close to the active layer of the laser element. At this time, by appropriately designing the optical resonance mode of the photonic crystal and matching the resonance frequency to the gain frequency band of the laser, laser oscillation is achieved and surface emission of the laser light in the direction perpendicular to the emission surface is realized. The emission surface can be made into a large area of 100 μm square or more, and since the spread of the laser beam due to diffraction can be suppressed, it is possible in principle to achieve excellent beam quality when using the surface-emitting quantum cascade laser. In addition, since it is also possible to increase the area of the active region of the laser, it can be expected to obtain a large laser output according to the surface-emitting quantum cascade laser.

[0010] However, generally, in order to supply power to the quantum cascade laser, it is necessary to energize the laser element. Therefore, in the quantum cascade laser, it is necessary to add (i.e., dope) impurities (i.e., dopants) to the semiconductor layer constituting the laser element to increase the electrical conductivity of the semiconductor layer. However, increasing the electrical conductivity of the semiconductor layer in the surface-emitting quantum cascade laser causes absorption of the laser light and also reduces the radiative loss ratio of the optical resonance mode of the photonic crystal, which becomes a factor for reducing the extraction efficiency of the laser light. Here, the radiative loss ratio means the ratio of the laser output to the total loss including absorption loss.

[0011] Thus, in the surface-emitting quantum cascade laser, since light absorption in the laser element is inevitably caused in principle, there is a problem that the extraction efficiency of the laser light is low in the surface-emitting quantum cascade laser. This causes a decrease in the conversion efficiency from electricity to light and a decrease in the laser output, and directly leads to various problems such as deterioration of the characteristics of the surface-emitting quantum cascade laser, such as an increase in the element temperature due to the influence of the laser power absorbed inside the laser element.

[0012] For these reasons, in a surface-emitting quantum cascade laser, it is important to improve the extraction efficiency of the laser light. However, in the conventional surface-emitting quantum cascade lasers as described above, the extraction efficiency of the laser light is not yet sufficient. Also, in a surface-emitting quantum cascade laser, it is also important to be able to achieve excellent beam quality as a surface-emitting quantum cascade laser. Therefore, there is a demand for the development of a novel surface-emitting quantum cascade laser different from the conventional surface-emitting quantum cascade lasers.

[0013] In the present invention, in order to solve the above problems, there is provided a novel surface-emitting quantum cascade laser different from the conventional surface-emitting quantum cascade lasers, and an object thereof is to provide a surface-emitting quantum cascade laser capable of achieving excellent beam quality as a surface-emitting quantum cascade laser.

[0014] Alternatively, in the present invention, an object is to provide a surface-emitting quantum cascade laser having a photonic crystal with a structure capable of improving the radiation loss ratio of the resonance mode. The purpose is to increase the extraction efficiency of the laser light by improving the radiation loss ratio.

[0015] Alternatively, in the present invention, an object is to provide a surface-emitting quantum cascade laser having a photonic crystal with a structure capable of reducing the absorption of laser light in the laser element. The purpose is to increase the extraction efficiency of the laser light by reducing the absorption of the laser light.

[0016] Alternatively, in the present invention, an object is to provide a surface-emitting quantum cascade laser capable of achieving excellent beam quality as a surface-emitting quantum cascade laser even when reducing the amount of impurities doped in the semiconductor layer constituting the laser element.

Means for Solving the Problems

[0017] As a result of intensive studies, the present inventors have found that a surface-emitting quantum cascade laser having a square lattice or rectangular lattice photonic crystal on a laser active layer can solve the above problems when the square lattice or rectangular lattice photonic crystal is composed of a composition A of a compound semiconductor composition or a metal composition and a composition B of a compound semiconductor composition having a refractive index different from that of the composition A, and the unit lattice of the square lattice or rectangular lattice photonic crystal has a columnar structure having a pentagonal bottom surface made of the composition B at the center of a columnar structure having a square or rectangular bottom surface made of the composition A.

[0018] In addition, the present inventors have found that a surface-emitting quantum cascade laser having a square lattice or rectangular lattice photonic crystal on a laser active layer can solve the above problems when the square lattice or rectangular lattice photonic crystal is composed of a composition A of a compound semiconductor composition or a metal composition, a composition B of a compound semiconductor composition having a refractive index different from that of the composition A, and a composition C of a dielectric composition having a refractive index different from both of the compositions A and B. The unit lattice of the square lattice or rectangular lattice photonic crystal includes a columnar structure having a square or rectangular bottom surface made of the composition A on a layer made of the composition B, and a columnar structure having a pentagonal bottom surface made of the composition C at the center of the columnar structure having a square or rectangular bottom surface made of the composition A. The bottom surface of the columnar structure having the pentagonal bottom surface is located on the layer made of the composition B, and the columnar structure having the pentagonal bottom surface made of the composition C is embedded in the columnar structure having a square or rectangular bottom surface made of the composition A.

[0019] In addition, the present inventors have developed a surface-emitting quantum cascade laser having a square lattice or rectangular lattice photonic crystal on a laser active layer, wherein the square lattice or rectangular lattice photonic crystal is composed of a composition A of a compound semiconductor composition or a metal composition and a composition B of a compound semiconductor composition having a refractive index different from that of the composition A, and the unit cell of the square lattice or rectangular lattice photonic crystal includes a columnar structure having a square or rectangular bottom surface made of the composition A on a layer made of the composition B, and a columnar void structure having a pentagonal bottom surface as a void is provided at the center of the columnar structure having the square or rectangular bottom surface made of the composition A, the bottom surface of the void structure is located on the layer made of the composition B, and the void structure is embedded in the columnar structure having the square or rectangular bottom surface made of the composition A. The present inventors have found that the above problems can also be solved by the cascade laser having such a structure.

[0020] Based on these findings, the present inventors have completed the present invention.

[0021] Specifically, the present invention has the following aspects [1] to

[15] . [1] A surface-emitting quantum cascade laser having a semiconductor layer other than the laser active layer and the laser active layer, wherein the laser active layer has a square lattice or rectangular lattice photonic crystal thereon, the unit cell of the square lattice or rectangular lattice photonic crystal is composed of a composition A and a composition B having a refractive index different from that of the composition A, the composition A is a compound semiconductor composition or a metal composition, the composition B is a compound semiconductor composition, the unit cell of the square lattice or rectangular lattice photonic crystal has a structure including a columnar structure having a pentagonal bottom surface made of the composition B at the center of a columnar structure having a square or rectangular bottom surface made of the composition A, the cascade laser. [2] The bottom surface of the square or rectangle has a shape with the length of the horizontal side being a1 and the length of the vertical side being a2, and the ratio (a2 / a1) of the length of the vertical side (a2) to the length of the horizontal side (a1) is in the range of 1 or more and 2 or less. The pentagon has a shape obtained by removing a right triangle from one corner of a square or rectangle with the length of the horizontal side being b1 and the length of the vertical side being b2, and the ratio (b2 / b1) of the length of the vertical side (b2) to the length of the horizontal side (b1) is in the range of 1 or more and 2 or less. The cascade laser according to [1]. [3] The right triangle removed from one corner of the square or rectangle with the length of the horizontal side being b1 and the length of the vertical side being b2 has a shape having a base with the length of the missing part of the horizontal side having the length b1 being b1´ and a height with the length of the missing part of the vertical side having the length b2 being b2´. When the photonic crystal is a rectangular lattice, The ratio (b1´ / b1) of the length of the base (b1´) to the length of the horizontal side (b1) is 0.1 or more and 0.9 or less. The ratio (b2´ / b2) of the length of the height (b2´) to the length of the vertical side (b2) is 0.3 or more and 0.9 or less. When the photonic crystal is a square lattice, The ratio (b1´ / b1) of the length of the base (b1´) to the length of the horizontal side (b1) is 0.1 or more and 0.5 or less. The ratio (b2´ / b2) of the length of the height (b2´) to the length of the vertical side (b2) is 0.3 or more and 0.9 or less. The cascade laser according to [2]. [4] The ratio of the columnar structure having a pentagonal bottom surface made of the composition B in the unit cell of the square lattice or rectangular lattice photonic crystal is 20% or more and 80% or less. The cascade laser according to any one of [1] to [3]. [5] The cascade laser according to any one of [1] to [4], wherein the laser active layer is a multiple quantum well composed of two or more quantum well layers, and each quantum well layer contains any one of a III-V compound semiconductor composition, a compound semiconductor composition composed of ZnO and ZnMgO, or a compound semiconductor composition composed of Si and SiGe. [6] The cascade laser according to [5], wherein the III-V semiconductor composition is at least one selected from the group consisting of a compound semiconductor composition composed of InGaAs and AlInAs, a compound semiconductor composition composed of GaAs and InGaAs, a compound semiconductor composition composed of GaAs and AlGaAs, a compound semiconductor composition composed of InAs and AlGaSb, a compound semiconductor composition composed of GaN and AlGaN, and a compound semiconductor composition composed of GaN and InGaN. [7] The doping density of the laser active layer is 1×10 18 cm -3 or less, and the doping density of the semiconductor layer excluding the active layer is 1×10 19 cm -3 or less, the cascade laser according to any one of [1] to [6]. [8] The cascade laser according to any one of [1] to [7], wherein the laser oscillation wavelength is 3 μm or more and 9 μm or less. [9] The cascade laser according to any one of [1] to [8], wherein the composition A and / or the composition B contains a III-V compound semiconductor composition.

[10] The cascade laser according to [9], wherein the III-V compound semiconductor composition is at least one compound semiconductor composition selected from the group consisting of InP, InGaAs, GaAs, AlGaAs, GaInP, InAs, AlInAs, and GaP.

[11] The cascade laser according to any one of [1] to

[10] , wherein the composition A is an InP compound semiconductor composition or a metal composition, and the composition B is an InGaAs compound semiconductor composition.

[12] The cascade laser according to any one of [1] to

[11] , wherein the composition A is a metal composition.

[13] The cascade laser according to any one of [1] to

[12] , wherein the metal composition contains gold as a main component.

[14] Having a semiconductor layer other than the laser active layer and the laser active layer, A surface-emitting quantum cascade laser having a square lattice or rectangular lattice photonic crystal on the laser active layer, The unit cell of the square lattice or rectangular lattice photonic crystal is composed of a composition A, a composition B having a refractive index different from that of the composition A, and a composition C having a refractive index different from both the composition A and B. The composition A is a compound semiconductor composition or a metal composition, The composition B is a compound semiconductor composition, The composition C is a dielectric composition, The unit cell of the square lattice or rectangular lattice photonic crystal is Providing a columnar structure having a square or rectangular bottom surface made of the composition A on a layer made of the composition B, At the center of the columnar structure having a square or rectangular bottom surface made of the composition A, a columnar structure having a pentagonal bottom surface made of the composition C is provided, and the bottom surface of the columnar structure having the pentagonal bottom surface is located on a layer made of the composition B. The columnar structure having a pentagonal bottom surface made of the composition C is embedded in the columnar structure having a square or rectangular bottom surface made of the composition A. Having a structure The cascade laser.

[15] Having a semiconductor layer other than the laser active layer and the laser active layer, A surface-emitting quantum cascade laser having a square lattice or rectangular lattice photonic crystal on the laser active layer, The unit cell of the square lattice or rectangular lattice photonic crystal is composed of a composition A and a composition B having a refractive index different from that of the composition A. The composition A is a compound semiconductor composition or a metal composition, Composition B is a compound semiconductor composition, The unit cell of the square lattice or rectangular lattice photonic crystal is, A columnar structure having a square or rectangular bottom surface made of Composition A is provided on a layer made of Composition B, At the center of the columnar structure having a square or rectangular bottom surface made of Composition A, a columnar void structure having a pentagonal bottom surface as a void is provided, and the bottom surface of the void structure is located on a layer made of Composition B, The void structure is embedded in the columnar structure having a square or rectangular bottom surface made of Composition A, Having a structure, The cascade laser.

Advantages of the Invention

[0022] According to the present invention, there is provided a novel surface-emitting quantum cascade laser different from the conventional surface-emitting quantum cascade laser, which can realize excellent beam quality as a surface-emitting quantum cascade laser.

[0023] Alternatively, according to the present invention, there can be provided a surface-emitting quantum cascade laser having a photonic crystal with a structure capable of improving the radiation loss ratio of the resonance mode. By improving the radiation loss ratio, the extraction efficiency of the laser light can be increased.

[0024] Alternatively, according to the present invention, there can be provided a surface-emitting quantum cascade laser having a photonic crystal with a structure capable of reducing the absorption of laser light in the laser element. By reducing the absorption of the laser light, the extraction efficiency of the laser light can be increased.

[0025] Alternatively, according to the present invention, there can be provided a surface-emitting quantum cascade laser capable of realizing excellent beam quality as a surface-emitting quantum cascade laser even when the amount of impurities doped in the semiconductor layer constituting the laser element is reduced. For example, the doping density of the laser active layer is 1×10 18 cm-3 as follows, and having a doping density of the semiconductor layer excluding the active layer of 1×10 19 cm -3 it is possible to provide a surface-emitting quantum cascade laser capable of achieving excellent beam quality even when it is 1×10

[0026] As described above, according to the present invention, it is possible to provide a surface-emitting quantum cascade laser having a photonic crystal with a structure capable of improving the radiation loss ratio. Here, by designing the structure of the photonic crystal to have an effective radiation loss ratio (specifically, a large radiation loss ratio), it becomes possible to achieve a high extraction efficiency of laser light. For example, an extraction efficiency of the laser light of more than 40% can be achieved. Further, according to the present invention, even when the unit lattice of the photonic crystal is a square lattice, a higher extraction efficiency of laser light (specifically, approximately 15% or more) can be achieved than that of a conventional surface-emitting semiconductor laser element using a photonic crystal of the same square lattice as the unit lattice. According to the present invention, since it becomes possible to achieve a high extraction efficiency of laser light, an improvement in laser output, an improvement in the conversion efficiency from electricity to light, and suppression of the temperature rise of the laser element can also be expected. By suppressing the temperature rise, simplification of the element cooling mechanism can also be expected.

[0027] According to the present invention, since a photonic crystal having a structure capable of improving the radiation loss ratio is used, the Q value of the resonance mode at the Γ point, which is an important index of the surface-emitting laser, in the band structure of the light of the photonic crystal can be precisely designed. Therefore, it can be expected to achieve a high extraction efficiency of laser light. The photonic crystal having a structure capable of improving the radiation loss ratio used in the present invention can have characteristics that are qualitatively much superior to those of the conventionally used square lattice photonic crystal and triangular lattice photonic crystal. BRIEF DESCRIPTION OF THE DRAWINGS

[0028]

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[0029] Hereinafter, the embodiments for carrying out the present invention will be described in detail. It should be noted that the present invention is not limited to the following embodiments, and various modifications can be made within the scope of the gist thereof.

[0030] One aspect of the present invention is a surface-emitting quantum cascade laser having a semiconductor layer other than the laser active layer and the laser active layer, and having a square lattice or rectangular lattice photonic crystal on the laser active layer. The unit cell of the square lattice or rectangular lattice photonic crystal is composed of a composition A and a composition B having a refractive index different from that of the composition A. The composition A is a compound semiconductor composition or a metal composition, and the composition B is a compound semiconductor composition. Further, the unit cell of the square lattice or rectangular lattice photonic crystal has a structure including a columnar structure having a pentagonal bottom surface made of the composition B at the center of a columnar structure having a square or rectangular bottom surface made of the composition A.

[0031] In the present invention, the laser active layer is a so-called light-emitting layer. Specifically, it is composed of a so-called "multiple quantum well (MQW)" in which a plurality of quantum well layers are stacked, and is a layer that emits laser light by intersubband transition. The laser active layer may also be simply referred to as the "active layer", and may also be simply referred to as the "active layer" in the present application.

[0032] In the present invention, the semiconductor layer other than the laser active layer refers to all semiconductor layers (for example, cladding layers and photonic crystals) other than the laser active layer constituting the surface-emitting quantum cascade laser which is one aspect of the present invention.

[0033] Each quantum well layer constituting the laser active layer is preferably a layer containing any one of a III-V compound semiconductor composition, a compound semiconductor composition composed of ZnO and ZnMgO, or a compound semiconductor composition composed of Si and SiGe. Here, "containing" means that a dopant (that is, an impurity) may be doped (that is, added) as necessary. From the viewpoint of reducing the doping amount to the semiconductor layer constituting the laser element, it is desirable to make it as small as possible. Specifically, the doping density of the entire laser active layer is 1×10 18 cm -3 It is preferably set to the following amount.

[0034] As the III-V compound semiconductor composition, it is preferably at least one selected from the group consisting of a compound semiconductor composition composed of InGaAs and AlInAs (this composition may also be denoted as "InGaAs / AlInAs"), a compound semiconductor composition composed of GaAs and InGaAs (this composition may also be denoted as "GaAs / InGaAs"), a compound semiconductor composition composed of GaAs and AlGaAs (this composition may also be denoted as "GaAs / AlGaAs"), a compound semiconductor composition composed of InAs and AlGaSb (this composition may also be denoted as "InAs / AlGaSb"), a compound semiconductor composition composed of GaN and AlGaN (this composition may also be denoted as "GaN / AlGaN"), and a compound semiconductor composition composed of GaN and InGaN (this composition may also be denoted as "GaN / InGaN"). In this case, when the III-V compound semiconductor composition is "InGaAs / AlInAs", an InP substrate is preferred; when it is "GaAs / InGaAs" or "GaAs / AlGaAs", a GaAs substrate is preferred; when it is "InAs / AlGaSb", an InAs substrate is preferred; and when it is "GaN / AlGaN" or "GaN / InGaN", a GaN substrate is preferred.

[0035] When using a compound semiconductor composition composed of ZnO and ZnMgO (this composition may also be denoted as "ZnO / ZnMgO"), a ZnO substrate is preferred. When using a compound semiconductor composition composed of Si and SiGe (this composition may also be denoted as "Si / SiGe"), a Si substrate is preferred.

[0036] A photonic crystal is an artificial crystal with a nano-periodic structure in which substances with different refractive indices are arranged at intervals on the order of the wavelength of light. Since phenomena such as light being confined inside or unable to penetrate occur, it is used to confine light in a small area and enhance the interaction between light and matter.

[0037] The unit cell of the square lattice or rectangular lattice photonic crystal used in the present invention is composed of a compound semiconductor composition or a metal composition (this composition is referred to as "Composition A" for convenience in this application) and a compound semiconductor composition having a refractive index different from that of Composition A (this composition is referred to as "Composition B" for convenience in this application).

[0038] As Composition A and Composition B, it is preferable to include a group III-V compound semiconductor composition as the compound semiconductor composition. Representative examples include, for example, InP, InGaAs, GaAs, AlGaAs, GaInP, InAs, AlInAs, GaP, and it is preferable to use at least one selected from the group consisting of these compound semiconductor compositions. Here, "include" means that a dopant (i.e., an impurity) may be doped (i.e., added) as necessary, and from the viewpoint of reducing the doping amount to the semiconductor layer constituting the laser element, it is desirable to make it as small as possible. Specifically, the doping density of the entire semiconductor layer other than the laser active layer is 1×10 19 cm -3 It is preferably set to the following amount.

[0039] When using a group III-V compound semiconductor composition as Composition A and Composition B, it is necessary to select group III-V compound semiconductor compositions having different refractive indices from each other. Although it depends on the structure of the photonic crystal, generally, it is preferable to select a high refractive index one and a low refractive index one, and it is more preferable to select them so as to have a large refractive index difference.

[0040] Regarding the combination of Composition A and Composition B, for example, it is preferable to use an InP compound semiconductor composition or a metal composition as Composition A and an InGaAs compound semiconductor composition as Composition B.

[0041] When using a metal composition as Composition A, the metal composition is a composition containing a metal. Typical examples include compositions containing gold, copper, nickel, titanium, or combinations thereof, and these metals may be used alone or as the main component. When using a metal composition as Composition A, it is preferable to use a metal composition containing gold as the main component (for example, a metal composition composed of gold and Ti with gold as the main component). In the present application, the metal composition used as Composition A may also be simply referred to as "metal".

[0042] The unit cell of the photonic crystal is a square lattice or a rectangular lattice. From the viewpoint of increasing the radiation loss ratio, a rectangular lattice is preferable.

[0043] In the surface-emitting quantum cascade laser which is one aspect of the present invention, the structure of the unit cell of the square lattice or rectangular lattice photonic crystal has a structure in which a columnar structure having a pentagonal bottom surface made of Composition B is provided at the center of a columnar structure having a square or rectangular bottom surface made of Composition A. That is, the square lattice or rectangular lattice photonic crystal is two-dimensionally periodically arranged with a structure in which a columnar structure having a pentagonal bottom surface made of Composition B (that is, a pentagonal prism structure) is provided at the center of a columnar structure having a square or rectangular bottom surface made of Composition A as a basic unit.

[0044] The schematic of the structure of the unit cell of the square lattice or rectangular lattice photonic crystal is shown in FIG. 1 as a plan view seen from the laser light emission direction when a surface-emitting quantum cascade laser is fabricated using the photonic crystal.

[0045] As shown in FIG. 1, when looking at the unit cell of the square lattice or rectangular lattice photonic crystal two-dimensionally, the structure of the unit cell of the photonic crystal has a square or rectangular shaped portion made of Composition A and a pentagonal shaped portion made of Composition B at the center thereof.

[0046] In the fabrication of a unit cell of a square lattice or rectangular lattice photonic crystal, first, a pentagonal-shaped portion is fabricated by thin film fabrication using molecular beam epitaxy (MBE) method and metalorganic chemical vapor deposition method with Composition B (for example, InGaAs with a refractive index n1 of 3.40 in the figure) and nano-processing using electron beam lithography. The fabricated pentagonal-shaped portion is made of Composition B and corresponds to the photonic crystal. Next, by filling the gaps between the pentagons made of Composition B arranged in a two-dimensional periodic manner with Composition A (for example, InP with a refractive index n2 of 3.07) without gaps, a square or rectangular-shaped portion made of Composition A is fabricated. As a result, as shown in FIG. 1, the structure of the unit cell of the photonic crystal has a square or rectangular-shaped portion made of Composition A and a pentagonal-shaped portion made of Composition B at the center thereof.

[0047] As shown in FIG. 1, assuming that the length of the side in the horizontal direction of the square or rectangular-shaped portion made of Composition A (that is, the unit cell of the square lattice or rectangular lattice photonic crystal) is a1 and the length of the side in the vertical direction is a2, the value of the ratio of the length of the side in the vertical direction (a2) to the length of the side in the horizontal direction (a1) (a2 / a1) is preferably in the range of 1 or more and 2 or less, more preferably in the range of 1 or more and 1.8 or less, and even more preferably in the range of 1 or more and 1.5 or less. The length of the side a1 in the horizontal direction and the length of the side a2 in the vertical direction may be adjusted based on the calculation results by the finite element method according to the target laser oscillation wavelength. However, usually, when the average refractive index of the photonic crystal is n and the laser oscillation wavelength is λ (unit: nm), the length of the side a1 in the horizontal direction is preferably a value that satisfies na1 / λ = 0.8 or more and 1.2 or less, and more preferably a value that satisfies na1 / λ = 0.9 or more and 1.1 or less. In this case, the length of the side a2 in the vertical direction is preferably a value that satisfies na2 / λ = 0.8 or more and 2.4 or less, and more preferably a value that satisfies na1 / λ = 0.9 or more and 2.0 or less.

[0048] As shown in FIG. 1, the pentagonal-shaped portion (i.e., the photonic crystal portion) made of the composition B preferably has a shape in which a right-angled triangle is missing (i.e., removed) from one corner of a square or rectangle having a horizontal side length of b1 and a vertical side length of b2. In this case, the value of (b2 / b1), which represents the ratio of the vertical side length (b2) to the horizontal side length (b1), is preferably in the range of 1 or more and 2 or less, more preferably in the range of 1 or more and 1.8 or less, and even more preferably in the range of 1 or more and 1.5 or less. Here, the horizontal side length (b1) is preferably determined such that the proportion of the composition B in the unit cell is in the range of 20% or more and 80% or less. Therefore, the value of (b1 / a1), which represents the ratio to the horizontal side length (a1), is preferably in the range of b1 / a1 = 0.2 or more and 0.8 or less, more preferably in the range of 0.3 or more and 0.7 or less, and even more preferably in the range of 0.4 or more and 0.6 or less.

[0049] As shown in Fig. 1, assuming that the right-angled triangle has a shape with a base having a length of b1´ which is the length of the portion missing from the horizontal side having a length of b1, and a height having a length of b2´ which is the length of the portion missing from the vertical side having a length of b2, the value of (b1´ / b1) indicating the ratio of the length of the base (b1´) to the length of the horizontal side (b1) is preferably in the range of 0.1 or more and 0.9 or less, more preferably in the range of 0.2 or more and 0.9 or less, and even more preferably in the range of 0.3 or more and 0.9 or less when the photonic crystal is a rectangular lattice. Also, the value of (b2´ / b2) indicating the ratio of the length of the height (b2´) to the length of the vertical side (b2) is preferably in the range of 0.3 or more and 0.9 or less, more preferably in the range of 0.5 or more and 0.9 or less, and even more preferably in the range of 0.6 or more and 0.9 or less when the photonic crystal is a rectangular lattice. When the photonic crystal is a square lattice, the value of (b1´ / b1) is preferably in the range of 0.1 or more and 0.5 or less, more preferably in the range of 0.2 or more and 0.5 or less. Also, when the photonic crystal is a square lattice, the value of (b2´ / b2) is preferably in the range of 0.3 or more and 0.9 or less, more preferably in the range of 0.5 or more and 0.9 or less.

[0050] In the surface-emitting quantum cascade laser which is one aspect of the present invention, the ratio of the columnar structure having a pentagonal bottom surface made of the composition B occupying the unit cell of the square lattice or rectangular lattice photonic crystal is preferably 20% or more and 80% or less, more preferably in the range of 30% or more and 70% or less, and even more preferably in the range of 40% or more and 60% or less.

[0051] In the surface-emitting quantum cascade laser which is one aspect of the present invention, although a dopant (i.e., impurity) may be doped (i.e., added) as the laser active layer, from the viewpoint of reducing the doping amount to the semiconductor layer constituting the laser element, it is desirable to make it as small as possible. Specifically, the doping density of the laser active layer is 1×10 18 cm -3is as follows, and the doping density of the semiconductor layer other than the laser active layer is also 1×10 19 cm -3 or less, which is preferable. The doping density of the laser active layer is more preferably 1×10 17 or less, and even more preferably 5×10 16 or less. Also, the doping density of the semiconductor layer other than the laser active layer is more preferably 1×10 18 or less, and even more preferably 5×10 17 or less.

[0052] In the surface-emitting quantum cascade laser according to one aspect of the present invention, from the viewpoints of utilization for various gas species analysis and excellent beam quality, etc., the laser oscillation wavelength is preferably 3 μm or more and 9 μm or less, and more preferably 4 μm or more and 8 μm or less.

[0053] Next, regarding the present invention, another aspect different from the surface-emitting quantum cascade laser according to one aspect of the present invention will be described. In this case, the explanations already described in the surface-emitting quantum cascade laser according to one aspect of the present invention are applied in the same manner unless otherwise specified.

[0054] Another aspect of the present invention is a surface-emitting quantum cascade laser having a semiconductor layer other than the laser active layer and the laser active layer, and having a square lattice or rectangular lattice photonic crystal on the laser active layer. The unit cell of the square lattice or rectangular lattice photonic crystal is composed of a composition A, a composition B having a refractive index different from that of the composition A, and a composition C having a refractive index different from both the composition A and B. The composition A is a compound semiconductor composition or a metal composition, the composition B is a compound semiconductor composition, and the composition C is a dielectric composition. And the unit cell of the square lattice or rectangular lattice photonic crystal includes a columnar structure having a square or rectangular bottom surface made of the composition A on a layer made of the composition B, and a pentagonal bottom surface made of the composition C is provided at the center of the columnar structure having a square or rectangular bottom surface made of the composition A. The bottom surface of the columnar structure having the pentagonal bottom surface is located on the layer made of the composition B, and the columnar structure having the pentagonal bottom surface made of the composition C is embedded in the columnar structure having the square or rectangular bottom surface made of the composition A.

[0055] The unit cell of the photonic crystal used in the surface-emitting quantum cascade laser, which is another aspect of the present invention, is composed of a compound semiconductor composition or a metal composition (this composition is referred to as "composition A" for convenience in the present application), a compound semiconductor composition having a refractive index different from that of the composition A (this composition is referred to as "composition B" for convenience in the present application), and a dielectric composition having a refractive index different from both the composition A and B (this composition is referred to as "composition C" for convenience in the present application).

[0056] As the composition C, it is preferable to use SiO2 as the dielectric composition. Representative examples include, for example, Si3N4, ZrO, TiO2, and it is preferable to use at least one selected from the group consisting of these dielectric compositions.

[0057] The schematic diagrams of the unit cell structure of a square lattice or a rectangular lattice photonic crystal are shown in FIGS. 11(a) and 11(b), respectively, as a plan view and a side view seen from the laser light emission direction when a surface-emitting quantum cascade laser is fabricated using the photonic crystal.

[0058] As shown in FIG. 11(a), when the unit cell of a square lattice or a rectangular lattice photonic crystal is viewed two-dimensionally, the structure of the unit cell of the photonic crystal has a square or rectangular-shaped portion made of composition A and a pentagonal-shaped portion made of composition C at its center. And as shown in FIG. 11(b), the unit cell of a square lattice or a rectangular lattice photonic crystal has a columnar structure having a square or rectangular bottom surface made of composition A and a columnar structure having a pentagonal bottom surface made of composition C provided (i.e., arranged) at its center, and the two are located on a layer made of composition B. And the columnar structure having a pentagonal bottom surface made of composition C has a structure of being embedded in the columnar structure having a square or rectangular bottom surface made of composition A.

[0059] In the unit cell of the photonic crystal used in the surface-emitting quantum cascade laser, which is yet another aspect of the present invention, the portion composed of the composition C is a void. In this case, the schematic of the structure of the unit cell of the square lattice or rectangular lattice photonic crystal is as shown in FIGS. 11(a) and 11(b), and the structure is such that the portion composed of the composition C is replaced with a void. Therefore, the unit cell of the square lattice or rectangular lattice photonic crystal used in the surface-emitting quantum cascade laser, which is another aspect of the present invention, has a columnar structure having a square or rectangular bottom surface made of the composition A and a columnar void structure having a pentagonal bottom surface provided (i.e., disposed) in the central portion thereof, and is located on a layer made of the composition B. The columnar void structure having a pentagonal bottom surface exists as a void in the columnar structure having a square or rectangular bottom surface made of the composition A without penetrating the columnar structure. Therefore, the columnar void structure having a pentagonal bottom surface has a structure embedded in the columnar structure having a square or rectangular bottom surface made of the composition A as shown in FIG. 11(b).

[0060] Regarding conditions not defined in the present application, there are no particular limitations as long as the object of the present invention can be achieved.

Examples

[0061] Next, embodiments of the present invention will be described more specifically with reference to examples. However, the embodiments of the present invention are not limited to the following examples as long as the gist thereof is not exceeded.

[0062] Example 1-1 <Manufacture and Effects of Surface-Emitting Quantum Cascade Laser (Part 1)> Two types of surface-emitting quantum cascade lasers shown in FIGS. 2(a) and 2(b) were fabricated using the molecular beam epitaxy (MBE) method. FIGS. 2(a) and 2(b) are side views of the fabricated surface-emitting quantum cascade lasers. In the two types of surface-emitting quantum cascade lasers of FIG. 2, the active layers (which are laser active layers) 14 and 24, which are the light-emitting layers, are composed of multiple quantum wells (MQWs) mainly composed of InGaAs thin films and AlInAs thin films. The lower cladding layers 15 and 25 are composed of an N-type InP substrate and an InP thin film formed on the substrate. The lower cladding layers 15 and 25 are also referred to as InP cladding layers 15 and 25. The lower cladding layers 15 and 25 were also fabricated using the MBE method. In the said multiple quantum wells (MQWs), Si was doped as another component, and the doping amount was such that the doping density in the active layers 14 and 24 was 1×10 18 cm -3 or less. Photonic crystals 12 and 22 mainly composed of InGaAs were incorporated on the upper surfaces of the active layers 14 and 24. In FIG. 2(b), an InP upper cladding layer 27 (this layer is also referred to as the "InP cladding layer 27") was provided on the upper surfaces of the active layers 14 and 24. The fabrication of the photonic crystals 12 and 22 was carried out as follows: in FIG. 2(a), on the upper surface of the active layer 14, and in FIG. 2(b), on the upper surface of the InP cladding layer 27, first, an InGaAs thin film with a thickness of 1 μm was fabricated by the MBE method. After further applying an electron beam resist, it was processed into a two-dimensional periodic array of pentagonal prisms by electron beam exposure and dry etching according to the design shown in FIG. 1. In order to efficiently dissipate the heat generated by the laser element and suppress the temperature rise of the element, in Fig. 2(a), the cavities in the photonic crystal 12 generated by etching (i.e., the gaps between adjacent pentagonal prisms) are filled with InP by growing an InP film by metalorganic chemical vapor deposition, and an InP film with a thickness of approximately 3 μm is further formed on top thereof to serve as a cladding layer (this layer is referred to as the "embedded InP cladding layer 13"). Similarly, in Fig. 2(b), the cavities in the photonic crystal 22 generated by etching (i.e., the gaps between adjacent pentagonal prisms) are filled with a metal composed of gold and Ti with gold as the main component (i.e., Ti / Au metal), and a Ti / Au metal film with a thickness of approximately 1 μm is further formed on top thereof to serve as a Ti / Au metal layer (this layer is referred to as the "embedded Ti / Au metal layer 23"). The formation of the two-dimensional periodic array of pentagonal prisms and the fact that the cavities (i.e., the gaps between adjacent pentagonal prisms) are filled without gaps (i.e., filled) were confirmed by observation with an electron microscope. When filling with InP, a metal electrode composed of Ti and Au (i.e., Ti / Au electrode 11) was formed on the upper surface thereof as an electrode for current injection. Although not shown, a similar metal electrode was also formed in Fig. 2(b). In addition, an insulating film of SiO2 was provided. Further, metal electrodes 16, 26 for current injection (specifically, Ti / Au electrodes) were also attached to the lower surface of the InP substrate. An opening for extracting the laser light was provided at the center (not shown). In addition, the amount of dopant doped other than the active layers 14, 24 was such that the doping density excluding the active layers 14, 24 was 1×10 19 cm -3 or less. FIG. 1 is a plan view showing unit cells of photonic crystals 12 and 22 as viewed from the laser light emission direction in a surface-emitting quantum cascade laser. The structures of the photonic crystals 12 and 22 designed in this embodiment will be described with reference to FIG. 1. The unit cells of the photonic crystals 12 and 22 are designed such that the length of a1 is 1.37 μm, the length of a2 is 1.644 μm, the length of b1 is 1.123 μm, the length of b2 is 1.348 μm, the length of b1' is 0.786 μm, and the length of b2' is 0.943 μm in the figure. That is, the photonic crystals 12 and 22 have a unit cell of a rectangular lattice. Therefore, the unit cells of the photonic crystals 12 and 22 have a two-dimensional structure having a rectangular shape with the longitudinal length being a1 and the transverse length being a2, and also have a pentagonal shape made of a material different from the material constituting the rectangular shape at the center thereof. Here, the square or rectangular shape portion corresponds to the embedded InP cladding layer 13 in FIG. 2(a) and the embedded Ti / Au metal layer 23 in FIG. 2(b), so the material is InP in FIG. 2(a) and Ti / Au metal in FIG. 2(b). Also, the pentagonal shape portion corresponds to the photonic crystal 12 in FIG. 2(a) and the photonic crystal 22 in FIG. 2(b), so the material is InGaAs in both cases except for the doping component. In FIG. 2(a), the refractive index n1 of InGaAs, which is the material of the pentagonal shape portion, is 3.40, and the refractive index n2 of InP, which is the material of the rectangular shape portion, is 3.07. In FIG. 2(b), the refractive index n1 of InGaAs, which is the material of the pentagonal shape portion, is 3.40, and the refractive index n2 of the Ti / Au metal, which is the material of the rectangular shape portion, is 3.53. In both FIGS. 2(a) and (b), since the unit cells having the above structure were periodically fabricated in an amount of about 360×300 for the laser element, the area of the entire photonic crystal was about 500 μm×500 μm. Regarding the effects of the two types of surface-emitting quantum cascade lasers shown in FIG. 2 fabricated, it was confirmed that laser oscillation by current injection was carried out after cooling to liquid nitrogen temperature, and laser oscillation could be achieved as a surface-emitting quantum cascade laser, and excellent beam quality could be realized (not shown).

[0063] Example 1-2 <Manufacture and Effects of Surface Emitting Quantum Cascade Laser (Part 2)> The surface emitting quantum cascade laser shown in Fig. 3 was fabricated in the same manner as in Example 1-1. Fig. 3 is a side view of the fabricated surface emitting quantum cascade laser. Specifically, in the surface emitting quantum cascade laser of Fig. 3, the active layer 34, which is a light emitting layer, is composed of a multiple quantum well (MQW) mainly consisting of an InGaAs thin film and an AlInAs thin film. The lower cladding layer 35 is composed of an N-type InP substrate and an InP thin film formed on the substrate, and is also referred to as the InP cladding layer 35. The lower cladding layer 35 was also fabricated using the MBE method. In the said multiple quantum well (MQW), Si was doped as another component, and the doping amount was such that the doping density in the active layer 34 was 1×10 18 cm -3 and below. A photonic crystal 32 mainly composed of InGaAs was incorporated on the upper surface of the active layer 34. The photonic crystal 32 was fabricated by first fabricating an InGaAs thin film with a thickness of 1 μm on the upper surface of the active layer 34 using the MBE method, further applying an electron beam resist, and then processing it into a two-dimensional periodic array of pentagonal prisms by electron beam exposure and dry etching according to the design shown in Fig. 1. To efficiently dissipate the heat generated by the laser element and suppress the temperature rise of the element, the cavities (i.e., the gaps between adjacent pentagonal prisms) in the photonic crystal 32 formed by etching are filled with InP by growing an InP film by metalorganic chemical vapor deposition, and an InP film 37 with a thickness of approximately 3 μm is further formed on top of it to serve as a cladding layer (this layer is also referred to as the "embedded InP cladding layer 33"). The formation of the two-dimensional periodic array of pentagonal prisms and the fact that the cavities (i.e., the gaps between adjacent pentagonal prisms) are filled without gaps (i.e., are filled) were confirmed by observation with an electron microscope. An InGaAs layer 37 with a thickness of approximately 0.1 μm was further formed on the embedded InP cladding layer 33. A metal electrode composed of Ni and Au (i.e., the Ni / Au electrode 31) was fabricated as an electrode for current injection on the upper surface of the InGaAs layer 37. Also, an insulating film 39 of SiO2 was provided. Further, a metal electrode 36 for current injection (specifically, a Ni / Au electrode) was attached to the lower surface of the InP substrate 38. An opening for extracting the laser light was provided at the center (not shown). In addition, the amount of the dopant doped other than the light-emitting layer (i.e., the active layer) 34 was set to an amount such that the doping density excluding the light-emitting layer 34 is 1×10 19 cm -3 or less. FIG. 1 is a plan view showing a unit cell of the photonic crystal 32 as viewed from the laser light emission direction in the surface-emitting quantum cascade laser. The structure of the photonic crystal 32 designed in this embodiment will be described with reference to FIG. 1. The unit cell of the photonic crystal 32 is designed such that the lengths of a1 and a2 are both 1.355 μm, the lengths of b1 and b2 are both 1.065 μm, the length of b1' is 0.426 μm, and the length of b2' is 0.852 μm in the figure. That is, the photonic crystal 32 has a unit cell of a square lattice. Therefore, the unit cell of the photonic crystal 32 has a two-dimensional structure having a square shape with the same length a1 in the vertical direction and the same length a2 in the horizontal direction, but also has a pentagonal shape made of a material different from the material constituting the square shape at its center. Here, the square-shaped portion corresponds to the embedded InP cladding layer 33, so its material is InP. Also, the pentagonal-shaped portion corresponds to the photonic crystal 32, so its material is InGaAs. The refractive index n1 of InGaAs, which is the material of the pentagonal-shaped portion, is 3.40, and the refractive index n2 of InP, which is the material of the square-shaped portion, is 3.07. Since the unit cells having the above structure were periodically fabricated in the laser element in an amount of about 370×370, the area of the entire photonic crystal was about 500 μm×500 μm. Regarding the effect of the surface-emitting quantum cascade laser shown in FIG. 3 fabricated, as in Example 1, it was confirmed by cooling to liquid nitrogen temperature and performing laser oscillation by current injection. FIG. 4 shows the results of the output characteristics of the surface-emitting quantum cascade laser fabricated in this embodiment. From these results, it was confirmed that the laser oscillation threshold current was about 2.8 A and the maximum output was about 50 mW. Also, FIG. 5 shows the results of the far-field profile of the output laser beam of the surface-emitting quantum cascade laser fabricated in this embodiment. From these results, it was confirmed that a single-peak output beam with a small beam divergence angle was obtained. From these results, it was found that according to the surface-emitting quantum cascade laser fabricated in this embodiment, laser oscillation can be achieved as a surface-emitting quantum cascade laser, and excellent beam quality can be realized.

[0064] Comparative Example 1-1 <Manufacture of Surface-Emitting Quantum Cascade Laser for Comparison> A general surface-emitting quantum cascade laser having a structure in which only the shape of the bottom surface of the pentagonal prism portion in the unit cell constituting the photonic crystal 32 in the surface-emitting quantum cascade laser fabricated in Example 1-2 was replaced with a circular shape with a diameter of 1.102 μm and replaced with a cylinder was fabricated as Comparative Example 1-1. The manufacturing method is the same as that of Example 1-2 except that only the shape of the pentagonal prism portion in the unit cell is replaced with the shape of a cylinder. The results of the output characteristics of the surface-emitting quantum cascade laser of Comparative Example 1-1 are shown in Fig. 8 together with the results of the output characteristics of the surface-emitting quantum cascade laser of Example 1-2. From these results, it was confirmed that the surface-emitting quantum cascade laser of Comparative Example 1-1 has a smaller laser output than the surface-emitting quantum cascade laser of Example 1-2. In addition, the results of the far-field profile of the laser beam by the surface-emitting quantum cascade laser of Comparative Example 1-1 are shown in Fig. 9. This far-field profile is different from the far-field profile of Fig. 5 showing the far-field profile of the laser beam by the surface-emitting quantum cascade laser of Example 1-2, shows a doughnut shape, and it was confirmed that the beam divergence angle is also large. From these results, it was found that the surface-emitting quantum cascade laser of Comparative Example 1-1 has low beam quality, and excellent beam quality such as that of Example 1-2, which is a surface-emitting quantum cascade laser according to an embodiment of the present invention, cannot be obtained.

[0065] Example 2-1 <Influence of the Structure of the Unit Cell of the Photonic Crystal on the Laser Oscillation Wavelength> The surface-emitting quantum cascade lasers shown in Fig. 3 fabricated in Example 1-2 were newly fabricated in three types by the same method as in Example 1-2. These three types of surface-emitting quantum cascade lasers were fabricated such that, regarding the structure of the unit cell of the photonic crystal 32 that constitutes the surface-emitting quantum cascade laser, only the length of the unit cell of the square lattice of the photonic crystal 32 in Example 1-2 (specifically, the lengths of a1 and a2 in Fig. 1 are both the same 1.355 μm) differed within the range of 1.355 μm to 1.370 μm. Specifically, in the surface-emitting quantum cascade laser shown in Fig. 3, there are three types: a surface-emitting quantum cascade laser in which the lengths of a1 and a2 in Fig. 1 are both the same 1.360 μm, a surface-emitting quantum cascade laser in which the lengths of a1 and a2 are both the same 1.365 μm, and a surface-emitting quantum cascade laser in which the lengths of a1 and a2 are both the same 1.370 μm. Regarding the effects of the three fabricated surface-emitting quantum cascade lasers, similar to Example 1-2, they were cooled to liquid nitrogen temperature and laser oscillation by current injection was carried out for confirmation. Fig. 6 shows the results of the laser oscillation spectra of the three fabricated surface-emitting quantum cascade lasers together with the results of the laser oscillation spectra of the surface-emitting quantum cascade laser in Example 1-2. From these results, it was confirmed that as the length of the unit cell of the square lattice of the photonic crystal 32 increases, the laser oscillation wavelength (μm) increases. Therefore, it was found that the laser oscillation wavelength can be adjusted by adjusting the unit cell structure of the photonic crystal 32. Also, from the results of Fig. 6, it was confirmed that all the laser oscillation wavelengths (μm) are in the range of about 4.3 to 4.4 μm and within the range of 3 μm or more and 9 μm or less. Thus, according to the surface-emitting quantum cascade laser fabricated in this example, it was found that laser oscillation can be achieved as a surface-emitting quantum cascade laser and excellent beam quality can be realized.

[0066] Example 2-2 <The Influence of the Structure of the Unit Cell of the Photonic Crystal on the Extraction Efficiency of Laser Light (Part 1)> Regarding the case where a surface-emitting quantum cascade laser shown in Fig. 3 was fabricated in the same manner as in Example 1-2, the influence of the structure of the unit cell of the photonic crystal 32 constituting the surface-emitting quantum cascade laser on the extraction efficiency of the laser light was investigated. Specifically, regarding the extraction efficiency of the laser light of the surface-emitting quantum cascade laser, the dependence on the value of (b1' / b1) indicating the ratio of the length of b1' to the length of b1 in Fig. 1, and the value of (b2' / b2) indicating the ratio of the length of b2' to the length of b2 was calculated by numerical calculation using the finite element method. The extraction efficiency of the laser light is generally defined as the ratio of the laser output to the total energy loss occurring in the surface-emitting quantum cascade laser. However, in this example, for the sake of simplicity of calculation, the energy loss due to spontaneous emission, which has a relatively small contribution and does not affect the extraction efficiency of the laser light, was not considered, and the total energy loss was defined as the sum of the reabsorption of the laser light inside the element and the laser output radiated outside the element. Therefore, the extraction efficiency of the laser light is also the average value of the extraction efficiency of the laser light with respect to a plurality of electromagnetic resonance modes included in the gain spectrum of the active layer. Note that the unit cell of the photonic crystal 32 is the same square lattice as in Example 1-2 (that is, the lengths of a1 and a2 in Fig. 1 are both the same 1.355 μm). The results are shown in Fig. 7. From these results, it was found that regarding the extraction efficiency of the laser light, there are optimal values for the values of (b1' / b1) and (b2' / b2). Specifically, when the value of (b1' / b1) satisfies 0.1 ≤ (b1' / b1) ≤ 0.5 and the value of (b2' / b2) satisfies 0.3 ≤ (b2' / b2) ≤ 0.9, an extraction efficiency of approximately 15% or more can be obtained. It was also confirmed that this tendency also applies to the case where a surface-emitting quantum cascade laser shown in Fig. 2 was fabricated in the same manner as in Example 1-1 (not shown).

[0067] Example 2-3 <Influence of the Structure of the Unit Cell of the Photonic Crystal on the Extraction Efficiency of Laser Light (Part 2)> The extraction efficiency of the laser light from the surface-emitting quantum cascade laser was also calculated by numerical calculation using the finite element method. At that time, only the main factors that determine the extraction efficiency of the laser light (specifically, only the diffraction efficiency of the electromagnetic mode and the reabsorption by the semiconductor layer and the top electrode) were considered, and the reabsorption by the back electrode and the dissipation of the laser light in the horizontal direction, which have relatively small contributions and do not substantially affect the extraction efficiency of the laser light, were ignored. Regarding the surface-emitting quantum cascade laser fabricated by the same method as in Example 1-2, the influence of the structure of the photonic crystal constituting the surface-emitting quantum cascade laser on the extraction efficiency of the laser light was investigated. Specifically, the unit cells of the photonic crystals 12, 22, and 32 constituting the surface-emitting quantum cascade laser have a rectangular lattice in which the length of a1 in FIG. 1 is 1.380 μm and the length of a2 is 1.2×a1 (=1.656 μm). In addition, the pentagon, which is the bottom surface of the pentagonal prism located at the center of the unit cell of the rectangular lattice, is designed such that the length of b2 in FIG. 1 is 1.2×b1 and the length of b1 is such that the area of the pentagon occupies 50% of the area of the unit cell. Regarding the surface-emitting quantum cascade laser in which the photonic crystal has such a structure, the influence of the value of (b1' / b1), which is the ratio of the length of b1' to the length of b1, and the value of (b2' / b2), which indicates the ratio of the length of b2' to the length of b2, on the extraction efficiency of the laser light was investigated. Here, the doping densities of the embedded InP cladding layers 13 and 33 and the InGaAs photonic crystals 12, 22, and 33 are 4×10 16 cm -3 and 1×10 17 cm -3 respectively, and the doping density of the multiple quantum well layer is 5×10 16 cm -3 The results are shown in FIG. 10. From these results, when a rectangular lattice with a ratio of the length of a2 to the length of a1 of 1:1.2 is adopted (that is, when a rectangular lattice in which the value of (a2 / a1), which indicates the ratio of the length of a2 to the length of a1 in FIG. 1, is 1.2 within the range of 1 or more and 2 or less), the extraction efficiency of the laser light improves dramatically, and it was confirmed that when (b1' / b1)=(b2' / b2)=0.9, an extremely high extraction efficiency of the laser light of 61.5% can be obtained. It was also confirmed that this tendency also applies to the case where a surface-emitting quantum cascade laser shown in FIG. 2 was fabricated in the same manner as in Example 1-1 (not shown).

[0068] Example 2-4 <Influence of Dopant Amount> Regarding the surface-emitting quantum cascade laser fabricated by the same method as in Examples 1-1 and 1-2, the influence on the reabsorption of laser light was investigated. Here, the amount of reabsorption of laser light inside the surface-emitting quantum cascade laser element was calculated by numerical calculation using the finite element method. At that time, only the main factors that determine the reabsorption of laser light (specifically, only the reabsorption of light by carriers generated by doping dopants into the semiconductor layer and free electrons in the upper metal electrode) were considered, and the intersubband transition in the multiple quantum wells and the reabsorption of light by free electrons in the back electrode, which have relatively little contribution and do not substantially affect the reabsorption of laser light, were ignored. The surface-emitting quantum cascade laser used in the numerical calculation has a structure in which the unit cells of the photonic crystals 12, 22, and 32 are square lattices with the lengths of a1 and a2 in FIG. 1 being the same at 1.395 μm. In addition, the pentagon that is the bottom surface of the pentagonal prism located at the center of the unit cell which is a square lattice has a value of (b1' / b1) indicating the ratio of the lengths of b1' and b1 in FIG. 1 being 0.4, a value of (b2' / b2) indicating the ratio of the lengths of b2' and b2 being 0.8, and the lengths of b1 and b2 being the same, and the lengths of b1 and b2 are determined such that the pentagon becomes 50% of the unit cell (square lattice). The doping densities of the InP layer and the InGaAs layer in the surface-emitting quantum cascade laser are 4×10 16 cm -3 Let it be. Here, when the doping density of the multiple quantum well layer in the surface-emitting quantum cascade laser is reduced from 2.5×10 17 cm -3 to 5×10 16 cm -3 by one-fifth, it was confirmed that the ratio of reabsorption in the total laser light generation amount is reduced from 94.6% to 92.5% (not shown).

Industrial Applicability

[0069] According to the present invention, it can be used as a small laser light source that can cover the mid-infrared to far-infrared wavelength band including wavelengths from 3 to 5 μm, and even up to the terahertz region, and has the possibility of being used for various gas concentration measurements. Moreover, it is possible to measure the concentration of trace gases at the ppb level. Also, because it has the characteristic of straight-line propagation as a laser, it has the possibility of being used for detecting dangerous gases in remote areas such as volcanic eruption gases. In addition, it has the possibility of being used for measuring isotope ratios by utilizing the single-wavelength property of the laser. Therefore, great expectations can be placed on its applicability in various fields (for example, environment, food, bio, chemistry, medicine, electronic devices, automobiles, etc.).

Explanation of Symbols

[0070] 11, 31 Metal electrodes (Ti / Au electrodes or Ni / Au electrodes) 12, 22, 32 Photonic crystals 13, 23, 33 Embedded InP cladding layers or embedded Ti / Au metal layers 14, 24, 34 Active layers (light-emitting layers) 15, 25, 35 Lower cladding layers (InP cladding layers) 16, 26, 36 Metal electrodes (Ti / Au electrodes or Ni / Au electrodes) 27 Upper cladding layer (InP cladding layer) 37 InGaAs layer 38 InP substrate 39 Insulating film

Claims

1. A surface-emitting quantum cascade laser having a semiconductor layer other than the laser active layer and the laser active layer, wherein the laser active layer has a square lattice or rectangular lattice photonic crystal, the unit cell of the square lattice or rectangular lattice photonic crystal is composed of a composition A and a composition B having a refractive index different from that of the composition A, the composition A is a compound semiconductor composition or a metal composition, the composition B is a compound semiconductor composition, the unit cell of the square lattice or rectangular lattice photonic crystal has a structure in which a columnar structure having a pentagonal bottom surface made of the composition B is provided at the center of a columnar structure having a square or rectangular bottom surface made of the composition A, the cascade laser.

2. The square or rectangular bottom surface has a lateral side length of a １ and a longitudinal side length of a ２ such that the ratio of the longitudinal side length (a １ ) to the lateral side length (a ２ ), i.e., (a ２ / a １ ), is in the range of 1 or more and 2 or less. The pentagon has a horizontal side length of b １ , a vertical side length of b ２ , and is a shape lacking a right triangle from a corner of a square or rectangle with the horizontal side length (b １ ) and the vertical side length (b ２ ) such that the ratio (b ２ / b １ ) is in the range of 1 or more and 2 or less. The cascade laser according to claim 1.

3. Let the length of the horizontal side be b １ and the length of the vertical side be b ２ The right triangle cut off from one corner of the square or rectangle thus formed has a base with a length of b １ minus the length b １ ´ of the missing part of the horizontal side having a length of b, and a height with a length of b ２ minus the length b ２ ´ of the missing part of the vertical side having a length of b, and has a shape When the photonic crystal is a rectangular lattice, The length of the side in the lateral direction (b １ ), and the ratio (b １ ´) of the length of the base (b １ ´ / b １ ) is 0.1 or more and 0.9 or less, The length of the height (b ２ ') with respect to the length of the vertical side (b ２ ) has a ratio (b ２ ' / b ２ ) that is 0.3 or more and 0.9 or less, When the photonic crystal is a square lattice, The length of the side in the lateral direction (b １ ), and the ratio (b １ ´ / b １ ´) of the length of the base (b １ ) is 0.1 or more and 0.5 or less, The length of the height (b ２ ') with respect to the length of the vertical side (b ２ ) has a ratio (b ２ ' / b ２ ) that is 0.3 or more and 0.9 or less, The cascade laser according to claim 2.

4. The cascade laser according to any one of claims 1 to 3, wherein the ratio of the columnar structure having a pentagonal bottom surface made of the composition B in the unit cell of the square lattice or rectangular lattice photonic crystal is 20% or more and 80% or less.

5. The cascade laser according to any one of claims 1 to 4, wherein the laser active layer is a multiple quantum well composed of two or more quantum well layers, and each quantum well layer contains any one of a group III-V compound semiconductor composition, a compound semiconductor composition composed of ZnO and ZnMgO, or a compound semiconductor composition composed of Si and SiGe.

6. The cascade laser according to claim 5, wherein the group III-V semiconductor composition is at least one selected from the group consisting of a compound semiconductor composition composed of InGaAs and AlInAs, a compound semiconductor composition composed of GaAs and InGaAs, a compound semiconductor composition composed of GaAs and AlGaAs, a compound semiconductor composition composed of InAs and AlGaSb, a compound semiconductor composition composed of GaN and AlGaN, and a compound semiconductor composition composed of GaN and InGaN.

7. The doping density of the laser active layer is 1×10 １８ cm －３ or less, and the doping density of the semiconductor layer excluding the active layer is 1×10 １９ cm －３ or less. The cascade laser according to any one of claims 1 to 6.

8. The cascade laser according to any one of claims 1 to 7, wherein the laser oscillation wavelength is 3 μm or more and 9 μm or less.

9. The cascade laser according to any one of claims 1 to 8, wherein the composition A and / or the composition B contains a group III-V compound semiconductor composition.

10. The cascade laser according to claim 9, wherein the III-V compound semiconductor composition is at least one compound semiconductor composition selected from the group consisting of InP, InGaAs, GaAs, AlGaAs, GaInP, InAs, AlInAs, and GaP.

11. The cascade laser according to any one of claims 1 to 10, wherein the composition A is an InP compound semiconductor composition or a metal composition, and the composition B is an InGaAs compound semiconductor composition.

12. The cascade laser according to any one of claims 1 to 11, wherein the composition A is a metal composition.

13. The cascade laser according to any one of claims 1 to 12, wherein the metal composition contains gold as a main component.

14. It has a semiconductor layer other than the laser active layer and the laser active layer, A surface-emitting quantum cascade laser having a square lattice or rectangular lattice photonic crystal on the laser active layer, The unit cell of the square lattice or rectangular lattice photonic crystal is composed of a composition A, a composition B having a refractive index different from that of the composition A, and a composition C having a refractive index different from both the compositions A and B. The composition A is a compound semiconductor composition or a metal composition. The composition B is a compound semiconductor composition. The composition C is a dielectric composition. The unit cell of the square lattice or rectangular lattice photonic crystal is provided with a columnar structure having a square or rectangular bottom surface made of the composition A on a layer made of the composition B. A columnar structure having a pentagonal bottom surface made of the composition C is provided at the center of the columnar structure having a square or rectangular bottom surface made of the composition A, and the bottom surface of the columnar structure having the pentagonal bottom surface is located on a layer made of the composition B. The columnar structure having a pentagonal bottom surface made of the composition C is embedded in the columnar structure having a square or rectangular bottom surface made of the composition A. having a structure the cascade laser.

15. It has a semiconductor layer other than the laser active layer and the laser active layer, A surface-emitting quantum cascade laser having a square lattice or rectangular lattice photonic crystal on the laser active layer, The unit cell of the square lattice or rectangular lattice photonic crystal is composed of a composition A and a composition B having a refractive index different from that of the composition A. The composition A is a compound semiconductor composition or a metal composition. The composition B is a compound semiconductor composition. The unit cell of the square lattice or rectangular lattice photonic crystal is provided with a columnar structure having a square or rectangular bottom surface made of the composition A on a layer made of the composition B; a columnar void structure having a pentagonal bottom surface as a void is provided at the center of the columnar structure having a square or rectangular bottom surface made of the composition A, and the bottom surface of the void structure is located on a layer made of the composition B; the void structure is embedded in the columnar structure having a square or rectangular bottom surface made of the composition A; having a structure; the cascade laser.

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