A terahertz quantum cascade laser and its preparation method

Abstract

The present invention relates to a terahertz quantum cascade laser and a preparation method thereof, wherein the laser comprises a front face and a rear face; the rear face is perpendicular to the length direction of the ridge strip structure of the laser, and the front face comprises two inclined planes, and the inclined planes form an inclined angle with the vertical plane in the length direction of the ridge strip structure, so that the reflectivity and transmittance of the front face and the rear face are different. Compared with a single inclined end face, the angular inclined end face of the present invention uses two symmetrical inclined planes to couple the output laser, so that the divergence of the total output laser is reduced, and the beam quality of the output laser is effectively improved.

Description

Technical Field

[0001] The invention belongs to the field of terahertz lasers and preparation thereof, and in particular relates to a terahertz quantum cascade laser and a preparation method thereof. Background Art

[0002] Terahertz (THz) quantum cascade laser (QCL) is a very important semiconductor radiation source in the 1-5 THz frequency band, with the characteristics of compact structure, stable performance, high energy conversion efficiency, high output power, and long life. It has important application prospects in THz standard light source, optoelectronic technology, imaging system and biological effect research. Semi-insulating surface plasma waveguide is the most commonly used waveguide structure for high-power THz QCL. Since the laser mode in this structure only needs to be restricted by the surface electrode metal layer and the heavily doped lower contact layer, a large part of the laser mode can extend into the substrate. Therefore, this waveguide structure has the advantages of simple preparation process, good coupled output beam quality, and high output power.

[0003] In the ridge-shaped semi-insulating surface plasma waveguide THz QCL, a resonant cavity (Fabry–Pérot cavity) is formed by cleaving the front and rear end faces of the waveguide. Since the two end faces of the resonant cavity are symmetrical and consistent, the two end faces of the laser are in the same state during operation and output lasers of the same intensity, that is, the light output ratio of the two end faces is 50%. In order to effectively utilize the output laser of the two end faces of the laser, a high-reflection film is usually evaporated on one of the end faces of the ridge strip so that the laser of the device is output from the other end face. However, since the two end faces are parallel cleavage planes, in order to eliminate the reflection of the laser on the output end face, a layer of anti-reflection film is usually evaporated on the output end face. The thickness of the anti-reflection film depends on the wavelength of the output laser, and the optimal thickness is one-quarter of the laser wavelength. The wavelength of the mid-infrared laser is about 2μm, and the corresponding anti-reflection film thickness is about 500nm, so the above method has been well applied in mid-infrared QCL. When the operating frequency of QCL is extended to the THz band, the laser wavelength is about 100μm. At this time, the thickness of the anti-reflection film needs to reach about 25μm. However, the thickness of the dielectric film process is usually evaporated from several hundred nanometers to several microns. A thickness of 25μm is a difficult task for the dielectric film process.

[0004] Another method to effectively use the laser end face to output laser is to prepare an inclined front face, that is, to process the end face that needs to output laser so that the end face forms a certain angle with the end face that does not output laser in space, and the angle is smaller than the total reflection angle of the laser propagation process from the laser medium to the air, thereby increasing the light output ratio of the front face, and then effectively increasing the end face output power of the laser by evaporating a high-reflection film on the back face. However, due to the long wavelength of THz-band lasers (about 100 μm), which is much larger than the thickness of the laser active area (about 10 μm), the laser beam has a certain divergence when the end face emits light; at the same time, due to the construction of the inclined end face, the propagation direction of the laser after output has a certain deviation angle, the directionality of the beam propagation is greatly affected, and the overall quality of the laser output beam is poor. Therefore, how to further improve the laser beam quality while increasing the effective output power of the laser end face is a key technical problem that needs to be solved at present.

[0005] CN 110854666 A discloses a terahertz quantum cascade laser and a manufacturing method thereof. The use of a single inclined surface will cause the emitted terahertz laser beam to deflect at a large angle in a single direction. In addition, the divergence characteristics of the output beam of the terahertz quantum cascade laser make it more difficult to collect and collimate the end face output laser. In addition, the prior art directly uses grinding and polishing to realize the preparation of a single inclined surface, which takes a long time to prepare, and the inclination angle of the grinding and polishing surface is difficult to control and has a large error. Summary of the invention

[0006] The technical problem to be solved by the present invention is to provide a terahertz quantum cascade laser and a preparation method thereof, so as to overcome the large-angle deflection of the end face output laser in a single direction in the prior art. The present invention utilizes a symmetrical inclined surface structure to form a converging mode of the laser beam emitted by the laser, so that part of the divergent light beams can be effectively overlapped, which is convenient for collecting and collimating the emitted laser. At the same time, a preparation method combining graphic laser cutting and mechanical polishing is adopted, which can save the time of preparing the inclined surface to a certain extent. At the same time, according to the graphic design, it is easier to realize a symmetrical inclined surface and ensure the preparation accuracy of the inclined surface angle.

[0007] A terahertz quantum cascade laser of the present invention comprises a semi-insulating substrate, a lower contact layer, a lower electrode metal layer, an active region, an upper contact layer, an upper electrode metal layer, a front face and a rear face;

[0008] The semi-insulating substrate, the lower contact layer, the active region, the upper contact layer and the upper electrode metal layer are stacked in sequence from bottom to top, and the active region, the upper contact layer and the upper electrode metal layer form a ridge strip structure on the lower contact layer; the lower electrode metal layer is located on both sides of the ridge strip structure and maintains a spacing;

[0009] The front end face and the rear end face are two end faces of the laser along the length direction of the ridge strip structure, the front end face and the rear end face are both used to transmit the output laser, and the rear end face is perpendicular to the length direction of the ridge strip structure;

[0010] The front end surface includes a first inclined surface and a second inclined surface, and forms an angle with the upper and lower surfaces of the ridge strip structure.

[0011] The front end face includes two inclined surfaces, and the inclined surfaces form an inclined angle with a vertical surface in the length direction of the ridge strip structure, so that the reflectivity and transmittance of the front end face and the rear end face are different.

[0012] The structure of the laser is a semi-insulating surface plasma waveguide structure.

[0013] The rear end face is a cleavage face, and the first inclined face and the second inclined face of the front end face are mechanical polishing faces.

[0014] The surface of the rear end face is a plane, on which a medium and a metal layer are alternately evaporated to form a high-reflection film, thereby increasing the light output ratio of the front end face.

[0015] The first inclined plane and the second inclined plane are two symmetrical inclined planes.

[0016] The two inclined surfaces of the front end surface form an equal inclination angle with a vertical surface in the length direction of the ridge strip structure.

[0017] The included angle between the two sides of the angle is smaller than the critical angle of total reflection of the active area of ​​the laser.

[0018] Preferably, the total reflection critical angle is 15°.

[0019] A method for preparing a terahertz quantum cascade laser of the present invention comprises:

[0020] Step 1: epitaxially grow a lower contact layer, an active area and an upper contact layer in sequence on a semi-insulating substrate by using a molecular beam epitaxy method, and then evaporate an upper electrode metal layer on the upper contact layer by using a photolithography and electron beam evaporation process;

[0021] Step 2: Prepare the upper contact layer of the laser by dry etching to form a ridge stripe together with the active area, the etching depth reaches the lower contact layer, and the upper electrode metal layer is located on the ridge stripe;

[0022] Step 3: evaporating a lower electrode metal layer on the surface of the lower contact layer exposed by etching and performing annealing;

[0023] Step 4: a single ridge-shaped laser chip is cleaved out from the patterned large-area sample prepared by the process by scribing, and at this time, the front end face and the rear end face of the laser are both cleavage surfaces;

[0024] Step 5: Place the ridge laser into a precision laser cutting device, and make the front end face of the ridge laser into an angular end face including a first bevel and a second bevel according to the designed angular pattern; then, the first bevel and the second bevel of the front end face are sequentially tilted and attached to a mechanical polishing machine for polishing;

[0025] Step 6: Use high-purity indium to weld the ridge-shaped laser chip on a heat sink, and lead out the device electrodes through wire bonding to complete the device packaging.

[0026] Preferably, the included angles between the first inclined surface and the second inclined surface of the angular end surface and the end surface cleavage plane are both 12.6°.

[0027] Beneficial Effects

[0028] (1) The terahertz quantum cascade laser with an angular front facet and the method for manufacturing the same described in the present invention achieves a difference in reflectivity and transmittance between the front and rear facets by making the front facet and the rear facet non-parallel, thereby improving the light output ratio of the laser front facet.

[0029] (2) The terahertz quantum cascade laser with an angular front face and the method for manufacturing the same described in the present invention, wherein the inclination angle of the two inclined surfaces of the front face is less than the critical angle of total reflection of the laser active region, further improves the light output power ratio of the laser front face; at the same time, compared with a single inclined end face, the angular inclined end face uses two symmetrical inclined surfaces to couple the output laser, so that the total output laser divergence is reduced, and the beam quality of the output laser is effectively improved, such as Figure 3 shown.

[0030] (3) The terahertz quantum cascade laser with an angular front end face and the preparation method thereof described in the present invention, wherein the two inclined surfaces of the front end face are made by a method combining graphic design and precision laser cutting, and can be cut into the required geometric shape according to the design graphic, and has the advantages of simple preparation method, fast speed, and diverse preparation shapes.

[0031] (4) The terahertz quantum cascade laser with an angular front facet and the preparation method thereof described in the present invention can also suppress the lateral high-order mode of the laser through the angular front facet structure. Compared with the case of parallel front and rear end faces (FP cavity), when the laser is emitted, the two symmetrical inclined surfaces of the angular end facet will cause the laser to oscillate in a zigzag pattern in the resonant cavity, and the mode waveguide loss will increase, and the high-order mode waveguide loss will be greater, mainly because the lateral high-order mode is difficult to be output again through back reflection after being reflected laterally into the ridge strip by the inclined surface, and is more suppressed in the ridge strip. It is suitable for the preparation of wide ridge stripe terahertz quantum cascade lasers.

[0032] (5) Of the two end faces of the ridge laser, one is a cleavage face and the other is an angular polished face after pattern preparation. Such non-parallel end faces destroy the original symmetry of the laser light-emitting end face, which can cause differences in the transmittance and reflectivity of the front and rear end faces of the laser, thereby changing the light-emitting ratio of the front and rear end faces of the laser. When the inclination angle of the angular end face is less than the critical angle of total reflection of the laser active area, the light-emitting power ratio of the front end face of the laser is greater. At the same time, such an angular front end face structure can also effectively suppress the lateral high-order mode of the laser, and is suitable for the preparation of wide ridge terahertz quantum cascade lasers. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 A schematic side view of the terahertz quantum cascade laser of the present invention;

[0034] Figure 2 A schematic top view of the terahertz quantum cascade laser of the present invention;

[0035] Figure 3 (a) Schematic diagram of the light output path of a single tilted end face terahertz quantum cascade laser and (b) a terahertz quantum cascade laser with an angled end face according to the present invention;

[0036] In the figure: 1-semi-insulating substrate, 2-lower contact layer, 3-lower electrode metal layer, 4-active area, 5-upper contact layer, 6-upper electrode metal layer, 7-front end surface, 71-first inclined surface of the front end surface, 72-second inclined surface of the front end surface, 8-rear end surface, 9-tilt angle. DETAILED DESCRIPTION

[0037] The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms fall within the scope limited by the appended claims of the application equally.

[0038] Example 1

[0039] An embodiment of the present specification provides a terahertz quantum cascade laser, which can achieve different light output powers at the front and rear end faces.

[0040] Specifically, Figure 1 and Figure 2 As shown, the laser includes a front face 7 and a rear face 8, wherein the rear face 8 is perpendicular to the length direction of the ridge strip structure of the laser, and the two inclined surfaces 71 and 72 of the front face 7 form an equal inclination angle 9 with the vertical plane in the length direction of the ridge strip structure. Therefore, due to the non-parallelism and asymmetry of the front and rear end faces, the two have different reflectivities and transmittances, thereby forming different light output powers.

[0041] In some embodiments, the laser further comprises a stacked semi-insulating substrate 1, a lower contact layer 2, a laser active region 4, an upper contact layer 5 and an upper electrode metal layer 6, wherein the laser active region 4, the upper contact layer 5 and the upper electrode metal layer 6 form a ridge stripe structure of the laser, and the ridge stripe direction can be understood as the length direction of the ridge stripe.

[0042] In some embodiments, the upper electrode metal layer 6 is disposed on the ridge, and the width of the upper electrode metal layer 6 may be equal to the width of the ridge, that is, the upper electrode metal layer 6 completely covers the ridge. Optionally, the width of the upper electrode metal layer 5 is 400 μm.

[0043] In some embodiments, the ridge stripe is disposed on a partial area of ​​the lower contact layer 2 . Preferably, the ridge stripe is disposed in the middle of the lower contact layer 2 and partially exposes two sides of the lower contact layer 2 .

[0044] Furthermore, a lower electrode metal layer 3 is also provided on the exposed portion of the lower contact layer 2 . It should be noted that the lower electrode metal layer 3 only covers a portion of the exposed lower contact layer 2 .

[0045] Since the front end face 7 and the rear end face 8 are not parallel to each other and cannot be obtained by cleavage at the same time, one of the faces can be a cleavage face and the other can be a mechanical polishing face.

[0046] The two tilt angles 9 are smaller than the critical angle of total reflection of the laser active area. The mode reflectivity corresponding to the first bevel 71 and the second bevel 72 of the front face 7 is lower than the reflectivity of the rear face. Therefore, the light output power at the two bevels is greater than that at the rear face, and as the tilt angle 9 increases, the total light output power of the front face is also greater. When the tilt angle 9 increases to be greater than the critical angle of total reflection of the laser active area, total reflection occurs on the front face, the transmittance is almost 0, and the total light output power of the front face decreases rapidly. Therefore, the two tilt angles of the front face are generally set to be slightly smaller than the critical angle of total reflection of the laser active area, so that the front face has a larger light output power. For terahertz quantum cascade lasers, the refractive index of the active area material is about 3.8, and the corresponding critical angle of total reflection is about 15°.

[0047] Of course, in some embodiments, the rear end face 8 may also form a high-reflection film by alternately evaporating a medium and a metal layer to increase the reflectivity of the rear end face 8 and further increase the total light output power of the front end face 7 .

[0048] Example 2

[0049] This embodiment provides a method for preparing a terahertz quantum cascade laser, which is used to prepare the terahertz quantum cascade laser mentioned above.

[0050] The specific preparation method comprises the following steps:

[0051] S10: a single ridge-shaped laser chip is cleaved out from the patterned large-area sample prepared by the process by scribing, and the front and rear faces of the laser are both cleavage surfaces;

[0052] The cavity length of the laser chip obtained by cleaving may be 4 mm, and the cleavage plane may be perpendicular to the ridge direction of the laser.

[0053] S20: placing the ridge laser into a precision laser cutting device, and manufacturing the front end face of the ridge laser into an angular end face including a first inclined face and a second inclined face according to the designed angular pattern; placing the laser on a mechanical polishing machine at an angle, and mechanically polishing the first inclined face and the second inclined face formed after laser cutting, respectively, to manufacture a polished front end face with two inclined angles;

[0054] The tilt angle is smaller than the critical angle of total reflection of the active region of the laser. For a terahertz quantum cascade laser, the refractive index of the active region is about 3.8, and the corresponding critical angle of total reflection is about 15°. Therefore, in this embodiment, the tilt angle can be 12.6°. Figure 3 As shown in Figure 2, when a symmetrical tilted surface is used, the end-face emitted laser and its divergent part overlap and form a similar convergent focus, see Figure 3 (b) It is convenient to collect and collimate with a parabolic reflector or lens, while the output laser with a single inclined end face forms Figure 3 (a) shows a divergent beam with a large deflection.

[0055] S30: welding the ridge laser chip onto a heat sink using high-purity indium, and leading out device electrodes through wire bonding to complete device packaging.

[0056] In some embodiments, the waveguide structure of the terahertz quantum cascade laser may be a semi-insulating surface plasma waveguide, and thus the process of manufacturing a semi-insulating surface plasma waveguide terahertz quantum cascade laser is also included before the cleavage, specifically:

[0057] S01: On a semi-insulating substrate, a lower contact layer, a laser active region and an upper contact layer are grown epitaxially in sequence by using a molecular beam epitaxy method;

[0058] S02: using photolithography and electron beam evaporation process to evaporate an upper electrode metal layer on the upper contact layer;

[0059] The upper electrode metal layer may be a Ti / Au metal layer, the thickness of the metal layer is 10 nm for the Ti metal layer and 300 nm for the Au metal layer, and the width of the metal layer is set according to different laser specifications. Optionally, the width of the upper electrode metal layer is 400 μm.

[0060] S03: preparing the upper contact layer of the laser by dry etching to form a ridge stripe together with the active area, the etching depth reaches the lower contact layer, and the upper electrode metal layer is located on the ridge stripe;

[0061] Optionally, the ridge stripe has a width of 400 μm, and the upper electrode metal layer is located on the ridge stripe;

[0062] S04: evaporating a lower electrode metal layer on the surface of the lower contact layer portion exposed by etching and performing annealing.

[0063] A lower electrode metal layer is produced on the surface of the exposed lower contact layer by etching using photolithography and electron beam evaporation of Ge / Au / Ni / Au (13 / 33 / 30 / 300nm) and annealed, and the lower electrode metal layer does not contact the ridge. Optionally, the lower electrode metal layer may be disposed on both sides of the ridge, and the lower electrode metal layer is 50μm away from the ridge structure, wherein the width of the lower electrode metal layer is 400μm. It can be understood that the lower electrode metal layers on both sides are symmetrically disposed on both sides of the ridge, and each metal layer has a width of 400μm.

[0064] In addition, in some embodiments, the back side of the substrate may be thinned, for example, to 170 μm.

[0065] The present invention provides a terahertz quantum cascade laser and a method for manufacturing the same, wherein the rear end face of the laser is perpendicular to the ridge direction of the laser, the two inclined surfaces of the front end face have an inclination angle in the perpendicular direction to the ridge structure, one of the two end faces is a cleavage face, and the other is a mechanical polishing face. Such non-parallel end faces make the reflectivity and transmittance of the front and rear end faces different, thereby achieving the effect of changing the light output ratio of the front and rear end faces of the laser. When the inclination angle of the two inclined surfaces of the front end face is less than the critical angle of total reflection of the laser active area, the total light output power ratio of the front end face of the laser is greater. Such an angular front end face structure with two inclined surfaces is also beneficial for suppressing the lateral high-order mode of the output laser, and is suitable for the preparation of wide ridge terahertz quantum cascade lasers.

Claims

1. A terahertz quantum cascade laser, comprising a semi-insulating substrate (1), a lower contact layer (2), a lower electrode metal layer (3), an active region (4), an upper contact layer (5), an upper electrode metal layer (6), a front face (7) and a rear face (8); The semi-insulating substrate (1), the lower contact layer (2), the active region (4), the upper contact layer (5) and the upper electrode metal layer (6) are stacked in sequence from bottom to top, and the active region (4), the upper contact layer (5) and the upper electrode metal layer (6) form a ridge strip structure on the lower contact layer (2); the lower electrode metal layer (3) is located on both sides of the ridge strip structure and maintains a spacing; The front end face (7) and the rear end face (8) are two end faces of the laser along the length direction of the ridge strip structure, the front end face (7) and the rear end face (8) are both used to transmit output laser light, and the rear end face (8) is perpendicular to the length direction of the ridge strip structure; It is characterized in that The front end face (7) is composed of a first inclined surface (71) and a second inclined surface (72), and the inclined surfaces form an inclined angle (9) with a vertical surface in the length direction of the ridge strip structure; the first inclined surface (71) and the second inclined surface (72) are symmetrical inclined surfaces; the two inclined surfaces (71) and (72) of the front end face (7) form an equal inclined angle (9) with a vertical surface in the length direction of the ridge strip structure; the inclined angle (9) is smaller than a total reflection critical angle of the active region (4) of the laser; the total reflection critical angle is 15°.

2. The laser according to claim 1, characterized in that: The rear end face (8) is a cleavage face, and the first bevel (71) and the second bevel (72) of the front end face (7) are mechanical polishing faces.

3. The laser according to claim 1, characterized in that: The surface of the rear end face (8) is a plane, on which medium and metal layers are alternately evaporated.

4. A method for preparing a terahertz quantum cascade laser according to claim 1, comprising: Step 1: epitaxially growing a lower contact layer (2), an active region (4) and an upper contact layer (5) in sequence on a semi-insulating substrate, and then evaporating an upper electrode metal layer (6) on the upper contact layer (5); Step 2: using dry etching to prepare the laser upper contact layer (5) to form a ridge stripe together with the active area (4), the etching depth reaches the lower contact layer (2), and the upper electrode metal layer (5) is located on the ridge stripe; Step 3: evaporating a lower electrode metal layer (3) on the surface of the lower contact layer (2) exposed by etching and performing annealing; Step 4: a single ridge-shaped laser chip is cleaved from the patterned large-area sample prepared by the process by scribing, and at this time, the front end face (7) and the rear end face (8) of the laser are both cleavage faces; Step 5: Place the ridge laser into a precision laser cutting device, and according to the designed angular pattern, make the front end face (7) of the ridge laser into an angular end face of a first inclined surface (71) and a second inclined surface (72); then, the first inclined surface (71) and the second inclined surface (72) of the front end face (7) are tilted and attached to a mechanical polishing machine in sequence, and polished respectively; Step 6: Weld the ridge-shaped laser chip onto a heat sink, and lead out the device electrodes through wire bonding to complete the device packaging.

5. The preparation method according to claim 4, characterized in that: The included angles (9) between the first inclined surface (71) and the second inclined surface (72) of the angular end surface and the end surface cleavage plane are both 12.6°.

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