LASER MODULE

The laser module includes a QCL element and a light source. The QCL element includes a substrate, a lower clad layer provided on the substrate, an active layer that is provided on an opposite side of the lower clad layer from the substrate and generates a first terahertz wave, an upper clad layer provided on an opposite side of the active layer from the lower clad layer, and a first electrode provided on an opposite side of the upper clad layer from the active layer. The second terahertz wave from the light source enters the active layer through the substrate, is reflected by the first electrode, and is amplified or wavelength-converted. The third terahertz wave amplified or wavelength-converted in the active layer is emitted to the outside through the substrate.

TECHNICAL FIELD

The present disclosure relates to a laser module.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of priority from Japanese Pat. Application No. 2022-053451, filed on Mar. 29, 2022, the entire contents of which are incorporated herein by reference.

BACKGROUND

Conventionally, a difference frequency generation type terahertz quantum cascade laser (DFG-THz-QCL: Difference Frequency Generation THz-Quantum Cascade Laser) has been known. For example, non-patent document 1 (Dominic Bachmann, Norbert Leder, Markus Rosch, Giacomo Scalari, Mattias Beck, Holger Arthaber, Jerome Faist, Karl Unterrainer, and Juraj Darmo, “Broadband terahertz amplification in a heterogeneous quantum cascade laser”, February 2015, Vol. 23, No. 3, OPTICS EXPRESS, pp. 3117-3125) discloses a method of amplifying a terahertz wave by causing the terahertz wave (pulse wave) to be incident on an end surface of an active layer of the terahertz quantum cascade laser.

SUMMARY

However, since a thickness of the active layer is usually only about several µm to several tens of µm, it is not easy to cause a sufficient amount of terahertz wave to be incident on the end surface of the active layer. In addition, when the terahertz wave is incident on the end surface of the active layer along a resonance direction (a direction in which the pair of end surfaces of the active layer face each other), the absorption amount of the terahertz wave in the active layer becomes relatively large, and the efficiency of amplification (or wavelength conversion) of the terahertz wave is suppressed to be low.

Accordingly, it is an object of an aspect of the present disclosure to provide a laser module capable of improving efficiency of amplification or wavelength conversion of a terahertz wave.

A laser module according to an aspect of the present disclosure includes: a quantum cascade laser element configured to generate a first terahertz wave having a difference frequency between a first frequency and a second frequency; and a light source configured to emit a second terahertz wave that is different from the first terahertz wave to the quantum cascade laser element. The quantum cascade laser element includes: a substrate; a first clad layer provided on the substrate; an active layer that is provided on an opposite side of the first clad layer from the substrate, constitutes a resonator that oscillates a light of the first frequency and a light of the second frequency, and generates the first terahertz wave; a second clad layer provided on an opposite side of the active layer from the first clad layer; and a metal electrode provided on an opposite side of the second clad layer from the active layer. The second terahertz wave enters the active layer through the substrate, is reflected by the metal electrode, and is amplified or wavelength-converted. A third terahertz wave, which is the second terahertz wave after being amplified or wavelength-converted in the active layer, is emitted to the outside through the substrate.

According to the laser module, a second terahertz wave emitted from a light source may be amplified or wavelength-converted using a quantum cascade laser element that generates a terahertz wave (first terahertz wave) by generating a difference frequency. In the laser module, the second terahertz wave emitted from the light source is incident on the substrate instead of the end surface of the active layer. In other words, the second terahertz wave enters the active layer (that is, the surface of the active layer facing the substrate) via the inside of the substrate. According to this method, compared to a method in which the second terahertz wave is incident on the end surface of the active layer, the second terahertz wave can be easily incident on the active layer, and absorption of the second terahertz wave by the active layer can be suppressed. In addition, by reflecting the second terahertz wave by the metal electrode located on the opposite side of the active layer from the substrate, the second terahertz wave (i.e., the third terahertz wave) amplified or wavelength-converted in the active layer can be easily extracted to the outside via the substrate again. As described above, according to the laser module, it is possible to improve efficiency of amplification or wavelength conversion of a terahertz wave.

The substrate may be formed of InP or Si. According to the above configuration, since the substrate is formed of a material (InP or Si) having a small absorption coefficient for the terahertz wave, loss (attenuation) of the terahertz wave passing through the substrate can be suppressed, and the light amount of the third terahertz wave extracted to the outside can be improved.

A length of the active layer in a first direction which is a resonance direction of the quantum cascade laser element may be 100 µm to 3 mm. According to the above configuration, it is possible to suitably realize a configuration in which the third terahertz wave can be extracted to the outside by reflecting the second terahertz wave once by the metal electrode. That is, it is possible to prevent the second terahertz wave from being multiply reflected in the quantum cascade laser element. Accordingly, it is possible to reduce the loss of the second terahertz wave in the quantum cascade laser element due to the multiple reflection can be reduced, and as a result, the light amount of the third terahertz wave can be improved.

The substrate may be formed of Si. The substrate may include: a first main surface facing the first clad layer; and a second main surface located on an opposite side of the first main surface. After the second terahertz wave is first reflected by the metal electrode and before the second terahertz wave is emitted to the outside as the third terahertz wave, the second terahertz wave may be reflected by the second main surface of the substrate at least once or more and re-enters the active layer. According to the above-described configuration, since the second terahertz wave can be incident on the active layer a plurality of times by causing the second terahertz wave to undergo multiple reflection in the quantum cascade laser element, the efficiency of amplification or wavelength-conversion can be effectively improved by increasing the number of times of amplification or wavelength-conversion of the second terahertz wave. When the second terahertz wave is multiply reflected in the quantum cascade laser element, the loss of the second terahertz wave in the quantum cascade laser element becomes larger than when the second terahertz wave is not multiply reflected. However, by forming the substrate with Si, it is possible to effectively suppress the loss of the second terahertz wave (absorption into the substrate). That is, according to the above-described configuration, it is possible to obtain a merit of multiple reflection (improvement in efficiency of amplification or wavelength conversion of the second terahertz wave) while suppressing a demerit of multiple reflection (loss of the second terahertz wave).

The substrate may include: a first end surface located on a first side in a first direction which is a resonance direction of the quantum cascade laser element; and a second end surface located on a second side opposite to the first side in the first direction. The active layer may include: a third end surface located on the first side in the first direction; and a fourth end surface located on the second side in the first direction. The second terahertz wave may be incident on the first end surface along a direction inclined with respect to the first direction so as to approach the active layer from the first side toward the second side in the first direction. According to the above configuration, the second terahertz wave can be made incident from the first end surface of the substrate and reliably guided to the active layer, and can be reliably reflected by the metal electrode and extracted to the outside.

The third terahertz wave may be emitted from the second end surface along a direction inclined with respect to the first direction so as to move away from the active layer from the first side toward the second side in the first direction. According to the above configuration, since it is possible to extract the output light (third terahertz wave) from the second end surface opposite to the first end surface which is an incident surface of the second terahertz wave, it is possible to easily avoid interference between the light source which outputs the second terahertz wave and the member which captures the third terahertz wave, and it is possible to improve design flexibility regarding arrangement of each member.

The substrate may include: a first main surface facing the first clad layer; and a second main surface located on an opposite side of the first main surface. The first end surface may be inclined with respect to a second direction orthogonal to the first main surface so as to approach the second end surface from the first main surface toward the second main surface along the second direction. According to the above configuration, an incident angle of the second terahertz wave with respect to the first end surface can be small, and reflection (loss) of the second terahertz wave on the first end surface can be suppressed.

An inclination angle of the first end surface with respect to a plane orthogonal to the first direction may substantially coincide with an angle at which an emission direction of the first terahertz wave is inclined with respect to the first direction. According to the above configuration, by causing the second terahertz wave to be substantially perpendicularly incident on the first end surface, it is possible to substantially match the traveling direction of the third terahertz wave and the traveling direction of the first terahertz wave. As a result, phase matching between the third terahertz wave and the first terahertz wave can be achieved, and the efficiency of amplification or wavelength conversion of the third terahertz wave can be effectively improved by interaction between the third terahertz wave and the first terahertz wave.

The substrate may include: a first main surface facing the first clad layer; and a second main surface located on an opposite side of the first main surface. The second end surface may be inclined with respect to a second direction orthogonal to the first main surface so as to approach the first end surface from the first main surface toward the second main surface along the second direction. According to the above configuration, an incident angle of the third terahertz wave with respect to the second end surface can be small, and the reflection (loss) of the third terahertz wave on the second end surface can be suppressed.

An inclination angle of the second end surface with respect to a plane orthogonal to the first direction may substantially coincide with an angle at which an emission direction of the first terahertz wave is inclined with respect to the first direction. According to the above configuration, when the traveling direction of the third terahertz wave and the traveling direction of the first terahertz wave are substantially matched, phase matching between the third terahertz wave and the first terahertz wave can be achieved, and the efficiency of amplification or wavelength conversion of the third terahertz wave can be effectively improved by interaction between the third terahertz wave and the first terahertz wave. Further, in this case, since the incident angle of the third terahertz wave with respect to the second end surface can be brought close to 0 degrees, it is possible to effectively suppress reflection (loss) of the third terahertz wave on the second end surface.

The laser module may further include an incident lens that includes an incident surface on which the second terahertz wave is incident and a facing surface facing the first end surface. The facing surface of the incident lens may be in directly or indirectly contact with the first end surface. According to the above configuration, it is possible to suppress interface reflection of the second terahertz wave on the first end surface by causing the second terahertz wave to be incident on the first end surface via the incident lens, and it is possible to improve incidence efficiency of the second terahertz wave to the active layer by condensing the second terahertz wave.

The incident lens may be formed of Si. By forming the incident lens with Si having a very small absorption coefficient for terahertz wave, attenuation of the second terahertz wave in the incident lens can be suppressed.

The incident lens may be a meta-lens in which an uneven structure is formed on the incident surface. By configuring the incident lens with meta-lens, it is possible to reduce the size (suppress the thickness) of the incident lens.

The laser module may further include an exit lens that includes an exit surface that emits the third terahertz wave and a facing surface facing the second end surface. the facing surface of the exit lens may be in directly or indirectly contact with the second end surface. According to the above configuration, it is possible to improve extraction efficiency of the third terahertz wave by extracting the third terahertz wave from the second end surface to the outside via the exit lens.

The exit lens may be formed of Si. By forming the exit lens with Si having a very small absorption coefficient for terahertz wave, attenuation of the third terahertz wave in the exit lens can be suppressed.

The exit lens may be a meta-lens in which an uneven structure is formed on the exit surface. By configuring the exit lens with meta-lens, it is possible to reduce the size (suppress the thickness) of the exit lens.

The first end surface of the substrate may protrude to the first side more than the third end surface of the active layer in the first direction. According to the above configuration, it is possible to easily perform a process (for example, a polishing process) of processing the first end surface into an inclined surface.

The second end surface of the substrate may protrude to the second side more than the fourth end surface of the active layer in the first direction. According to the above configuration, it is possible to easily perform a process (for example, a polishing process) of processing the second end surface into an inclined surface.

The substrate may include: a first end surface located on a first side in a first direction which is a resonance direction of the quantum cascade laser element; and a second end surface located on a second side opposite to the first side in the first direction. The active layer may include: a third end surface located on the first side in the first direction; and a fourth end surface located on the second side in the first direction. The substrate may include: a first main surface facing the first clad layer; and a second main surface located on an opposite side of the first main surface. The first end surface may be inclined with respect to a second direction orthogonal to the first main surface so as to approach the second end surface from the first main surface toward the second main surface along the second direction. The first end surface of the substrate may protrude to the first side more than the third end surface of the active layer in the first direction. The second terahertz wave may be incident on the first main surface along the second direction, passes through an inside of the substrate, and is reflected by the first end surface and the second main surface to be incident on the active layer. According to the above configuration, since the incident surface of the second terahertz wave and the exit surface of the third terahertz wave can be largely separated from each other, it is possible to easily avoid interference between the light source that outputs the second terahertz wave and the member that captures the third terahertz wave, and it is possible to improve design flexibility regarding the arrangement of each member.

The substrate may include: a first end surface located on a first side in a first direction which is a resonance direction of the quantum cascade laser element; and a second end surface located on a second side opposite to the first side in the first direction. The active layer may include: a third end surface located on the first side in the first direction; and a fourth end surface located on the second side in the first direction. The substrate may include: a first main surface facing the first clad layer; and a second main surface located on an opposite side of the first main surface. The first end surface may be inclined with respect to a second direction orthogonal to the first main surface so as to approach the second end surface from the second main surface toward the first main surface along the second direction. The first end surface of the substrate may protrude to the first side more than the third end surface of the active layer in the first direction. The second terahertz wave may be incident on the second main surface along the second direction, passes through an inside of the substrate, and is reflected by the first end surface to be incident on the active layer. According to the above configuration, since the incident surface of the second terahertz wave and the exit surface of the third terahertz wave can be largely separated from each other, it is possible to easily avoid interference between the light source that outputs the second terahertz wave and the member that captures the third terahertz wave, and it is possible to improve design flexibility regarding the arrangement of each member.

According to an aspect of the present disclosure, it is possible to provide a laser module capable of improving efficiency of amplification or wavelength conversion of a terahertz wave.

DETAILED DESCRIPTION

Hereinafter, an embodiment of the present disclosure will be described in detail with reference to the drawings. In the following description, the same or corresponding elements are denoted by the same reference numerals, and redundant description is omitted. Further, terms such as “upper” and “lower” are used for convenience based on the state shown in the drawings. In the drawings, some features of the embodiments are exaggerated for easy understanding. Therefore, the dimensional ratio of each part in the drawings may be different from the actual dimensional ratio.

First Embodiment

A laser module1A according to the first embodiment will be described with reference toFIGS.1to3. As shown inFIG.1, the laser module1A includes a quantum cascade laser element2A (hereinafter referred to as a “QCL element2A”) and a light source5. The QCL element2A generates a terahertz wave T1(first terahertz wave) having a difference ω3 (= |ω1 - ω2|) between the first frequency ω1 and the second frequency ω2. The light source5emits a terahertz wave T2(second terahertz wave) that is different from the terahertz wave T1to the QCL element2A. The laser module1A is configured to amplify or wavelength-convert the terahertz wave T2emitted from the light source5by using the QCL element2A which is a terahertz quantum cascade laser of a difference frequency generation type (DFG-THz-QCL).

FIG.2shows a cross section along a plane perpendicular to a resonance direction of the QCL element2A.FIG.3shows a cross section along the line III-III inFIG.2. InFIGS.1and3, an upper contact layer44and a lower contact layer45, which will be described later, are omitted. The direction D1(first direction) is a resonance direction of the QCL element2A. The direction D2(second direction) is a stacking direction of the QCL element2A (a stacking direction of a substrate3, a lower clad layer41, an active layer42, and an upper clad layer43which will be described later). The direction D3is a direction orthogonal to the directions D1and D2.

The QCL element2A is, for example, a terahertz light source configured to be able to output a terahertz wave T1in a room temperature environment. The QCL element2A includes a substrate3, a semiconductor layer4, a first electrode6(metal electrode), and a second electrode7. The QCL element2A can be formed as a ridge-stripe laser element by a general semiconductor process. The QCL element2A is obtained, for example, by forming InGaAs/InAlAs on the InP substrate (substrate3) by epitaxial growth.

The substrate3is, for example, a rectangular plate-shaped InP single-crystal substrate (semi-insulating substrate: high-resistance semiconductor substrate not doped with impurities). A length (length in the direction D1) of the substrate3is about several hundred µm to several mm. A width (length in the direction D3) of the substrate3is about several hundred µm to several mm. A thickness (length in the direction D2) of the substrate3is and about several hundred µm. In the present embodiment, as an example, the length of the substrate3is approximately 3 mm, the width of the substrate3is approximately 1 mm, and the thickness of the substrate3is approximately 300 µm.

The substrate3includes an upper surface3a(first main surface) facing the semiconductor layer4and a lower surface3b(second main surface) located on an opposite side of the upper surface3a. The substrate3includes an end surface3c(first end surface) located on a first side S1(right side inFIG.3) in the direction D1, and an end surface3d(second end surface) located on a second side S2(left side inFIG.3) opposite to the first side S1in the direction D1.

The semiconductor layer4is provided on the upper surface3aof the substrate3. A thickness (length in the direction D2) of the semiconductor layer4is about 10 µm to 20 µm (for example, 15 µm). The semiconductor layer4includes an end surface4alocated at the first side S1in the direction D1and an end surface4blocated at the second side S2in the direction D1. The semiconductor layer4emits broadband light in the mid-infrared region (for example, 3 µm or more and 20 µm or less) from each of the end surfaces4aand4b(more specifically, the end surfaces42aand42bof the active layer42). The end surfaces4aand4bare surfaces perpendicular to the direction D1. The end surfaces4aand4bare cleavage surfaces formed by cleavage, for example. The QCL element2A may have a structure in which a plurality of active layers having center wavelengths different from each other are stacked or a structure formed of a single active layer in order to emit broadband light as described above.

The semiconductor layer4includes a lower clad layer41(first clad layer), an active layer42, an upper clad layer43(second clad layer), an upper guide layer (not shown), a lower guide layer (not shown), an upper contact layer44, a lower contact layer45, and a support layer46.

From the upper surface3aof the substrate3, the lower contact layer45, the lower clad layer41, the lower guide layer, the active layer42, the upper guide layer, the upper clad layer43, and the upper contact layer44are laminated in this order. The upper guide layer is disposed between the active layer42and the upper clad layer43. The lower guide layer is disposed between the active layer42and the lower clad layer41. The support layer46is provided between the lower clad layer41and the upper clad layer43on both sides (both sides in the direction D3) of the active layer42, the upper guide layer, and the lower guide layer formed in a ridge stripe shape. The lower contact layer45has a portion that extends outward (outward in the direction D3) from the lower clad layer41. In this embodiment, the end portion of the lower contact layer45in the direction D3coincides with the end portion of the substrate3in the direction D3.

The lower contact layer45is, for example, a high-concentration Si-doped InGaAs layer (Si: 1.0×1018/cm3) with a thickness of about 400 nm, and is provided on the upper surface3aof the substrate3.

The lower clad layer41is, for example, a Si-doped InP layer (Si: 1.5×1016/cm3) with a thickness of about 5 µm, and is provided on the lower contact layer45. That is, the lower clad layer41is provided on the upper surface3aof the substrate3via the lower contact layer45.

The lower guide layer is, for example, a Si-doped InGaAs layer (Si: 1.5×1016/cm3) with a thickness of about 250 nm, and is provided on the lower clad layer41.

The active layer42is a layer in which a quantum cascade structure is formed, and is provided on the lower guide layer. That is, the active layer42is provided on an opposite side of the lower clad layer41from the substrate3. As shown inFIG.3, the active layer42has an end surface42a(third end surface) located on the first side S1in the direction D1and an end surface42b(fourth end surface) located on the second side S2in the direction D1. The end surface42ais a part of the end surface4aof the semiconductor layer4. The end surface42bis a part of the end surface4bof the semiconductor layer4. As an example, the active layer42has a structure in which a plurality of InGaAs layers and InAlAs layers are alternately stacked along the direction D2.

The upper guide layer is, for example, a Si-doped InGaAs layer (Si: 1.5 × 1016/cm3) with a thickness of about 450 nm, and is provided on the active layer42.

The upper clad layer43is, for example, Si-doped InP layer (Si: 1.5×1016/cm3) with a thickness of about 5 µm, and is provided on the upper guide layer. That is, the upper clad layer43is provided on an opposite side of the active layer42from the lower clad layer41.

The upper contact layer44is, for example, a high-concentration Si-doped InP layer (Si: 1.5×1018/cm3) with a thickness of about 15 nm, and is provided on the upper clad layer43.

The support layer46is, for example, an Fe-doped InP layer.

As shown inFIG.2, an insulating film47is formed so as to cover an upper surface44aof the upper contact layer44, the end surface4cof the semiconductor layer4intersecting the direction D3, and a part of the lower contact layer45. The insulating film47is formed of, for example, SiN. A contact hole47ais formed in the insulating film47to expose a portion of the upper surface44aof the upper contact layer44. The contact hole47aextends along the direction D1(seeFIG.3) so as to expose a central portion of the upper surface44ain the direction D3. Further, in the direction D3, an end portion47bof the insulating film47on the lower contact layer45is located inside the end portion of the lower contact layer45. In other words, the upper surface of the lower contact layer45is exposed outside the end portion47bof the insulating film47.

The first electrode6is formed on the upper surface44aof the upper contact layer44. The first electrode6is formed of a metal such as Ti/Au. That is, the first electrode6is provided on an opposite side of the upper clad layer43from the active layer42. The first electrode6is electrically connected to a part of the upper surface44aof the upper contact layer44via the contact hole47a.

The second electrode7is formed on the lower contact layer45so as to be in contact with a portion of the lower contact layer45that is exposed outside the end portion47bof the insulating film47. The second electrode7is formed of a metal such as Ti/Au. In the present embodiment, the second electrode7is formed so as to cover the end surface and a part of the upper surface of the semiconductor layer4, but is not necessarily formed in this manner. That is, the second electrode7may be formed so as to be electrically connected to at least the lower contact layer45and separated from the first electrode6. According to the above configuration, it is possible to drive the QCL element2A by causing a current to flow from the second electrode7to the first electrode6.

As an example, in the QCL element2A, by providing two types of diffraction grating layers functioning as a distributed feedback (DFB) structure in the upper guide layer, generation of first pump light of a first frequency ω1 and second pump light of a second frequency ω2 and generation of a terahertz wave T1of a difference frequency ω3 can be realized. The diffraction grating layer may be provided inside a clad layer (for example, the upper clad layer43). Light of a first frequency ω1 (hereinafter referred to as “first light”) and light of a second frequency ω2 (hereinafter referred to as “second light”) are mid-infrared light.

The active layer42constitutes a resonator that oscillates the first light and the second light. As an example, the end surfaces42aand42bof the active layer42function as reflection surfaces that reflect the first light and the second light into the active layer42so as to confine the first light and the second light in the active layer42. However, the end surfaces42aand42bdo not necessarily have to function as the reflecting surfaces. For example, when an external resonator (for example, a diffraction grating) other than the QCL element2A is disposed and used at a position facing the end surface42a, at least one of the first light and the second light may be emitted from the end surface42ato the external resonator, and light diffracted and reflected by the external resonator may be returned to the end surface42a. The active layer42generates a terahertz wave T1of a difference frequency ω3 (= |ω1 - ω2|) between the first frequency ω1 of the first light and the second frequency ω2 of the second light by difference frequency generation due to Cherenkov phase matching.

As shown inFIG.3, the radiation direction of the terahertz wave T1generated in this manner is inclined downward (toward the substrate3) by a radiation angle θC(Cherenkov radiation angle) with respect to a direction from the first side S1toward the second side S2(leftward direction inFIG.3) along the resonance direction (direction D1). More specifically, the terahertz wave T1generated by the active layer42propagates as a plane wave (that is, in phase) in the substrate3at a radiation angle θCexpressed by the following Equation (1). In the following Equation (1), nMIRis a group index of refraction of the substrate3with respect to mid-infrared light, and nTHzis an index of refraction of the substrate3with respect to terahertz wave. The radiation angle θCdepends on the material of the substrate3(that is, the index of refraction corresponding to the material) and the frequencies of the terahertz wave T1, but is, for example, 5 degrees to 30 degrees. In the present embodiment, as an example, the radiation angle θCis 20 degrees.

As shown inFIG.3, the end surface3cof the substrate3is inclined with respect to the direction D2so as to approach the end surface3d(that is, so as to move to the second side S2) from the upper surface3atoward the lower surface3balong the direction D2. On the other hand, the end surface3dof the substrate3is inclined with respect to the direction D2so as to approach the end surface3c(that is, so as to move to the first side S1) from the upper surface3atoward the lower surface3balong the direction D2. In other words, in the present embodiment, when viewed from the direction D3, the substrate3is formed in a tapered shape that tapers from the upper surface3atoward the lower surface3b.

The inclination angle θ1of the end surface3cwith respect to the plane orthogonal to the direction D1substantially coincides with the angle at which the emission direction of the terahertz wave T1from the active layer42is inclined with respect to the direction D1(that is, the above-described radiation angle θC). Similarly, the inclination angle θ2of the end surface3dwith respect to a plane orthogonal to the direction D1also substantially coincides with the radiation angle θC.

As shown inFIG.1, in the present embodiment, the light source5is disposed at a position facing the end surface3c. The terahertz wave T2emitted by the light source5enters the active layer42via the substrate3. That is, the terahertz wave T2passes through the inside of the substrate3, transmits through the lower clad layer41, and is incident on the lower surface (surface facing the lower clad layer41) of the active layer42. The terahertz wave T2incident on the active layer42passes through the upper clad layer43and is reflected by the first electrode6. The interaction between the terahertz wave T2and the active layer42also results in amplification or wavelength-conversion of the terahertz wave T2passing through the active layer42. A terahertz wave T3(third terahertz wave) which is reflected by the first electrode6and amplified or wavelength-converted by interaction with the active layer42is emitted to the outside through the substrate3.

Here, the amplification of the terahertz wave T2means that the light amount (light intensity) of the terahertz wave T3emitted from the substrate3is larger than the light amount (light intensity) of the terahertz wave T2emitted from the light source5. Further, the wavelength-conversion of the terahertz wave T2means that the wavelength of the terahertz wave T3emitted from the substrate3is changed from the wavelength of the terahertz wave T2emitted from the light source5. The interaction between the terahertz wave T2incident on the active layer42and the active layer42may cause both the amplification and the wavelength-conversion described above or may cause only one of the amplification and the wavelength-conversion.

As illustrated inFIG.1, in the present embodiment, the terahertz wave T2emitted from the light source5is incident on the end surface3cof the substrate3along a direction inclined with respect to the direction D1so as to approach the active layer42from the first side S1toward the second side S2in the direction D1. An angle θt at which the emission direction of the terahertz wave T2is inclined with respect to the direction D1is adjusted to be substantially equal to, for example, the inclination angle θ1. In this case, since the incident angle of the terahertz wave T2with respect to the end surface3ccan be brought close to 0 degrees, the reflection of the terahertz wave T2in the end surface3ccan be suppressed. As a result, the terahertz wave T2can be efficiently introduced into the substrate3.

On the other hand, the terahertz wave T3is emitted from the end surface3dof the substrate3along a direction inclined with respect to the direction D1so as to move away from the active layer42from the first side S1toward the second side S2in the direction D1. In the present embodiment, since the inclination angle θ2 is equal to the inclination angle θ1, when the angle θt is adjusted to be substantially equal to the inclination angle θ1, the incident angle of the terahertz wave T3with respect to the end surface3dcan be brought close to 0 degrees, and thus the reflection of the terahertz wave T3in the end surface3dcan be suppressed. As a result, the terahertz wave T3can be efficiently taken out to the outside.

According to the laser module1A described above, the terahertz wave T2emitted from the light source5can be amplified or wavelength-converted by using the QCL element2A that generates the terahertz wave T1by generating a difference frequency. In the laser module1A, the terahertz wave L2emitted from the light source5is incident on the substrate3instead of the end surface42aor42bof the active layer42. That is, the terahertz wave T2passes through the inside of the substrate3and enters the active layer42(that is, a surface of the active layer42facing the substrate3). According to this method, as compared with a method in which the terahertz wave T2is incident on the end surface42aor42bof the active layer42, the terahertz wave T2can be easily incident on the active layer42, and absorption of the terahertz wave T2by the active layer42can be suppressed. That is, if the terahertz wave T2is incident into the active layer42along a direction parallel to the resonance direction (direction D1), the terahertz wave T2passes through the active layer42by a long distance, and thus the absorption amount of the terahertz wave T2in the active layer42increases. On the other hand, by causing the terahertz wave T2to enter the active layer42via the substrate3as in the present embodiment, the incident direction of the terahertz wave T2with respect to the active layer42can be inclined with respect to the resonance direction (direction D1). As a result, since the length of the terahertz wave T2passing through the active layer42can be made shorter than that in the above case, the absorption amount of the terahertz wave T2in the active layer42can be reduced. In addition, since the terahertz wave T2is reflected by the first electrode6located on the opposite side of the active layer42from the substrate3, the terahertz wave T2amplified or wavelength-converted by the active layer42(that is, the terahertz wave T3) can be easily extracted to the outside via the substrate3again. As described above, according to the laser module1A, the efficiency of amplification or wavelength-conversion of the terahertz wave T2can be improved.

The substrate3is formed of InP. According to the above configuration, since the substrate3is formed of a material having a small absorption coefficient for the terahertz wave T2, loss (attenuation) of the terahertz wave T2and T3passing through the substrate3can be suppressed, and the light amount of the terahertz wave T3taken out to the outside can be improved. From the same viewpoint, the substrate3may be formed of Si (silicon). Since Si has a smaller absorption coefficient for terahertz wave T2than InP, the above-described effect can be further enhanced.

The length L (seeFIG.3) of the active layer42in the resonance direction of the QCL element2A (direction D1) is 100 µm to 3 mm. The length L can be preferably set to 100 µm to 1 mm. More preferably, the length L is set to be substantially equal to the wave length of the terahertz wave T2(for example, 300 µm). According to the above configuration, a configuration (seeFIG.1) in which the terahertz wave T2can be taken out to the outside by reflecting the terahertz wave T3once by the first electrode6can be suitably realized. That is, it is possible to prevent the terahertz wave T2from being multiply reflected in the QCL element2A (see a second embodiment (FIG.4) described later). Accordingly, the loss of the terahertz wave T2in the QCL element2A due to the multiple reflection can be reduced, and as a result, the light amount of the terahertz wave T3can be improved.

The terahertz wave T2is incident on the end surface3calong a direction inclined with respect to the direction D1so as to approach the active layer42from the first side S1toward the second side S2in the direction D1. According to the above configuration, the terahertz wave T2can be made incident from the end surface3cof the substrate3and reliably guided to the active layer42, and can be reliably reflected by the first electrode6and extracted to the outside.

Further, the end surface3cis inclined with respect to the direction D2so as to approach the end surface3dfrom the upper surface3atoward the lower surface3balong the direction D2. According to the above configuration, the incident angle of the terahertz wave T2with respect to the end surface3ccan be small, and the reflection (loss) of the terahertz wave T2in the end surface3ccan be suppressed.

The inclination angle θ1 of the end surface3cis substantially equal to the radiation angle θCof the terahertz wave T1. According to the above configuration, by making the terahertz wave T2substantially perpendicularly incident on the end surface3c, it is possible to substantially match the traveling direction of the terahertz wave T3and the traveling direction of the terahertz wave T1. As a result, phase matching between the terahertz wave T3and the terahertz wave T1can be achieved, and efficiency of amplification or wavelength-conversion of the terahertz wave T3can be effectively improved by interaction between the terahertz wave T1and the terahertz wave T3.

Further, the terahertz wave T3is emitted from the end surface3dalong a direction inclined with respect to the direction D1so as to move away from the active layer42from the first side S1toward the second side S2in the direction D1. According to the above configuration, since it is possible to extract the output light (terahertz wave T3) from the end surface3dopposite to the end surface3cwhich is an incident surface of the terahertz wave T2, it is possible to easily avoid interference between the light source5which outputs the terahertz wave T2and a member (not illustrated) which captures the terahertz wave T3, and it is possible to improve design flexibility regarding arrangement of each member. The terahertz wave T2emitted from the light source5may enter the end surface3cvia another member such as a mirror member, and the light source5itself does not necessarily need to be disposed at a position facing the end surface3c. Even in such a case, according to the above-described configuration, there is an effect that it is possible to avoid interference between the another member disposed at a position facing the end surface3cand the member which captures the terahertz wave T3.

Further, the end surface3dis inclined with respect to the direction D2so as to approach the end surface3cfrom the upper surface3atoward the lower surface3balong the direction D2. According to the above configuration, the incident angle of the terahertz wave T3with respect to the end surface3dcan be small, and the reflection (loss) of the terahertz wave T3in the end surface3dcan be suppressed.

The inclination angle θ2 of the end surface3dis substantially equal to the radiation angle θCof the terahertz wave T1. According to the above configuration, when the traveling direction of the terahertz wave T3substantially matches the traveling direction of the terahertz wave T1, phase matching between the terahertz wave T3and the terahertz wave T1can be achieved, and efficiency of amplification or wavelength-conversion of the terahertz wave T3can be effectively improved by interaction between the terahertz wave T1and the terahertz wave T3. Further, in this case, since the incident angle of the terahertz wave T3with respect to the end surface3dcan be brought close to 0 degrees, it is possible to effectively suppress reflection (loss) of the terahertz wave T3in the end surface3d.

Second Embodiment

The laser module1B of the second embodiment will be described with reference toFIG.4. The laser module1B is different from the laser module1A in that it includes a QCL element2B that is longer in the direction D1than the QCL element2A. The substrate3of the QCL element2B is formed of Si.

In the laser module1B, after the terahertz wave T2is first reflected by the first electrode6and before the terahertz wave T2is emitted to the outside as the terahertz wave T3, the terahertz wave T2is reflected by the lower surface3bof the substrate3at least once or more (only once as an example in the present embodiment) and re-enter the active layer42. In other words, the laser module1B is configured such that the terahertz wave T2undergoes multiple reflection inside the QCL element2B. In this embodiment, the terahertz wave T3after being reflected by the first electrode6for the second time is emitted from the end surface3d. The number of reflections of the terahertz wave T2on the lower surface3bof the substrate3may be one as in the present embodiment or may be two or more. The number of reflections of the terahertz wave T2at the lower surface3bdepends on the incident angle of the terahertz wave T2(angle θt inFIG.1) and the length of the QCL element2B (in the direction D1).

According to the second embodiment, since the terahertz wave T2is multiply reflected in the QCL element2B, the terahertz wave T2can be incident on the active layer42a plurality of times (two times in the present embodiment). Therefore, the efficiency of amplification or wavelength-conversion can be effectively improved by increasing the number of times of amplification or wavelength-conversion of the terahertz wave T2. As described in the first embodiment, when the terahertz wave T2is multiply reflected in the QCL element2B, the loss of the terahertz wave T2in the QCL element2B becomes larger than that when the terahertz wave T2is not multiply reflected. However, by forming the substrate3with Si, it is possible to effectively suppress the loss of the terahertz wave T2(absorption into the substrate3). That is, according to the second embodiment, it is possible to obtain a merit of multiple reflection (improvement in efficiency of amplification or wavelength-conversion of the terahertz wave T2) while suppressing a demerit of multiple reflection (loss of the terahertz wave T2).

Third Embodiment

The laser module1C of the third embodiment will be described with reference toFIG.5. The laser module1C is different from the laser module1A in that it includes a lens8(incident lens) and a lens9(exit lens).

The lens8includes an incident surface8aon which the terahertz wave T2is incident, and a facing surface8bfacing the end surface3c. The facing surface8bof the lens8is in directly or indirectly contact with the end surface3c. In the example ofFIG.5, the facing surface8bis in directly contact with the end surface3c, but a spacer member having an index of refraction substantially equal to that of the lens8may be interposed between the facing surface8band the end surface3c. That is, the facing surface8bmay be in indirectly contact with the end surface3cvia the spacer member. By causing the terahertz wave T2to be incident on the end surface3cvia the lens8, it is possible to suppress interface reflection of the terahertz wave T2in the end surface3c, and it is possible to improve incident efficiency of the terahertz wave T2on the active layer42by condensing the terahertz wave T2.

The lens9includes an exit surface9athat emits the terahertz wave T3and a facing surface9bfacing the end surface3d. The facing surface9bof lens9is in directly or indirectly contact with the end surface3d. In the example ofFIG.5, the facing surface9bis in directly contact with the end surface3d, but a spacer member having an index of refraction substantially equal to that of the lens9may be interposed between the facing surface9band the end surface3d. That is, the facing surface9bmay be in indirectly contact with the end surface3dvia the spacer member. By extracting the terahertz wave T3from the end surface3dto the outside through the lens9, extraction efficiency of the terahertz wave T3may be improved.

The lenses8and9may be formed of Si. That is, the lenses8and9may be silicon lenses. By forming the lenses8and9with Si having a very small absorption coefficient for terahertz wave, attenuation of the terahertz waves T2and T3in the lenses8and9can be suppressed.

Alternatively, the lenses8and9may be meta-lenses in which an uneven structure is formed on the incident surface8aor the exit surface9a. As such a meta-lens, for example, an optical element for a terahertz wave disclosed in Japanese Patent Application Laid-Open No. 2021-099399 or Japanese Patent Application Laid-Open No. 2021-099400 can be used. By configuring the lenses8and9with meta-lenses, it is possible to reduce the size (suppress the thickness) of the lenses8and9.

Fourth Embodiment

The laser module1D of the fourth embodiment will be described with reference toFIG.6. The laser module1D is different from the laser module1A in that a QCL element2D is provided instead of the QCL element2A. In addition, in the laser module1D, the incident direction of the terahertz wave T2with respect to the QCL element2D is different from that of the laser module1A. The QCL element2D differs from the QCL element2A in that the substrate3D is provided in place of the substrate3.

The end surface3cof the substrate3D protrudes to the first side S1more than the end surface42aof the active layer42in the direction D1. According to the above configuration, it is possible to easily perform a process (for example, a polishing process) of processing the end surface3cinto an inclined surface. That is, it is possible to easily perform a process of forming the inclined surface (end surface3c) by polishing the end surface of the substrate3D originally formed as a surface parallel to the direction D2. More specifically, it is possible to reduce the possibility that the semiconductor layer4is damaged due to contact with the semiconductor layer4during the polishing process.

The end surface3dof the substrate3D protrudes to the second side S2more than the end surface42bof the active layer42in the direction D1. According to the above configuration, it is possible to easily perform a process (for example, a polishing process) of processing the end surface3dinto an inclined surface. That is, it is possible to easily perform a process of forming the inclined surface (end surface3d) by polishing the end surface of the substrate3D originally formed as a surface parallel to the direction D2. More specifically, it is possible to reduce the possibility that the semiconductor layer4is damaged due to contact with the semiconductor layer4during the polishing process.

In addition, in the laser module1D, the upper surface3aof the substrate3D protrudes to the first side S1more than the end surface42aof the active layer42in the direction D1, and the light source5is disposed at a position facing a portion of the upper surface3awhich protrudes to the first side S1more than the end surface42a. The terahertz wave T2emitted from the light source5is incident on the upper surface3aalong the direction D2(a direction perpendicular to the upper surface3a), passes through the inside of the substrate3D, and is reflected by the end surface3cand the lower surface3bto be incident on the active layer42.

That is, the terahertz wave T2incident on the substrate3D from the upper surface3ais reflected by the end surface3c, is further reflected by the lower surface3b, passes through the substrate3D, and reaches the active layer42. Thereafter, the terahertz wave T2is reflected by the first electrode6, becomes a terahertz wave T3, and is emitted from the end surface3dto the outside. According to the above-described configuration, the incident surface of the terahertz wave T2(a portion of the upper surface3aprotruding further to the first side S1than the end surface42a) and the exit surface of the terahertz wave T3(end surface3d) can be largely separated from each other. Accordingly, it is possible to easily avoid interference between the light source5that outputs the terahertz wave T2and a member (not illustrated) that captures the terahertz wave T3, and it is possible to improve design flexibility regarding arrangement of each member.

Fifth Embodiment

The laser module1E of the fifth embodiment will be described with reference toFIG.7. The laser module1E is different from the laser module1A in that a QCL element2E is provided instead of the QCL element2A. In addition, in the laser module1E, the incident direction of the terahertz wave T2with respect to the QCL element2E is different from that of the laser module1A. The QCL element2E differs from the QCL element2A in that the substrate3E is provided in place of the substrate3.

The end surface3cof the substrate3E is inclined with respect to the direction D2so as to approach the end surface3dfrom the lower surface3btoward the upper surface3aalong the direction D2. The end surface3cof the substrate3E protrudes to the first side S1more than the end surface42aof the active layer42in the direction S1. More specifically, in the direction D1, the end portion of the upper surface3aof the substrate3E at the first side S1is located at the same position as that of the end surface42aof the active layer42, but the end portion of the lower surface3bof the substrate3E at the first side S1is located at a position that is farther away toward the first side S1than the end surface42aof the active layer42. Note that, in the present embodiment, the end portion of the upper surface3aof the substrate3E at the second side S2is located at the same position as the end surface42bof the active layer42, and the end portion of the lower surface3bof the3E at the second side S2is located at a position that is farther away toward the second side S2than the end surface42bof the active layer42. The light source5is disposed at a position facing a portion of the lower surface3bthat protrudes from the end surface42atoward the first side S1. The terahertz wave T2emitted from the light source5is incident on the lower surface3balong the direction D2(a direction orthogonal to the lower surface3b), passes through the inside of the substrate3E, is reflected by the end surface3c, and is incident on the active layer42.

That is, the terahertz wave T2incident on the substrate3E from the lower surface3bis reflected by the end surface3cand then passes through the inside of the substrate3E to reach the active layer42. Thereafter, the terahertz wave T2is reflected by the first electrode6. In the example ofFIG.7, the terahertz wave T3reflected by the first electrode6is reflected by the lower surface3bof the substrate3E and is emitted to the outside from the end surface3d. According to the above configuration, the incident surface of the terahertz wave T2(a portion of the lower surface3bprotruding further to the first side S1than the end surface42a) and the exit surface of the terahertz wave T3(end surface3d) can be largely separated from each other. Accordingly, similarly to the fourth embodiment, it is possible to easily avoid interference between the light source5that outputs the terahertz wave T2and a member (not illustrated) that captures the terahertz wave T3, and it is possible to improve design flexibility regarding arrangement of each member.

Other Modifications

Although some embodiments (first embodiment to fifth embodiment) of the present disclosure have been described above, the present disclosure is not limited to the above embodiments. The material and shape of each component of the laser modules1A to1E are not limited to the specific material and shape described above, and various materials and shapes other than those described above may be employed.

In addition, some configurations included in each of the above-described embodiments (first embodiment to fifth embodiment) may be appropriately omitted or changed, and may be arbitrarily combined. Hereinafter, some examples of the arbitrary combinations will be described. For example, in the laser module1B of the second embodiment, both the lens8and the lens9(or one of the lens8and the lens9) in the third embodiment may be added. In the laser module1C of the third embodiment, one of the lens8and the lens9may be omitted. Also, in the fifth embodiment (FIG.7), the end surface3dmay be configured as a surface inclined in the same direction as in other embodiments, and the terahertz wave T3reflected by the first electrode6may be directly emitted from the end surface3dwithout being reflected by the lower surface3b.

In the above-described embodiment, the inclination angle θ1 of the end surface3cand the inclination angle θ2 of the end surface3dare set to substantially coincide with the Cherenkov angle (the radiation angle θCof the terahertz wave T1). However, the magnitudes of the inclination angles θ1 and θ2 are not particularly limited to the above. In addition, when the external resonator is used in the QCL element so that the wavelength of the terahertz wave T1can be swept (scanned) within a certain range, the radiation angle θCobtained by Equation (1) also changes according to the wavelength of the terahertz wave T1. In this case, the radiation angle θCmay be calculated by assuming a terahertz wave T1of arbitrary wavelength included in the sweepable range. For example, the radiation angle θCmay be calculated assuming a terahertz wave T1of the center wavelength in the sweepable range. The end surfaces3cand3ddo not necessarily have to be configured as inclined surfaces. That is, at least one of the end surfaces3cand3dmay be configured as a surface parallel to the direction D2.