Quantum cascade laser device and method for manufacturing quantum cascade laser device

Abstract

A quantum cascade laser device (500) according to the present disclosure is provided with at least a core region (10) having formed therein a plurality of layers of stages (40) that each have an active region (38) in which barrier layers and well layers are alternately formed, and an injector region (39) which injects electrons into the active region (38). The quantum cascade laser device is characterized in that: when the gain exhibits the maximum gain value in a wavelength λqcl at the time of current injection and the width of a wavelength between two points where a gain value reduced by a preset rate with respect to the maximum gain value occurs is denoted by Δλqcl, a low reflectance film (6) having a local minimum reflectance in λcot, which is a wavelength separated by a preset wavelength from the wavelength λqcl, is provided on one end face; and when the width of a wavelength between two points where a mirror loss value is generated as a result of subtracting a preset loss value from the maximum value of a mirror loss αmir is defined as Δλmir, Δλmir is smaller than Δλqcl.

Description

Quantum cascade laser device and method for manufacturing quantum cascade laser device

[0001] The present disclosure relates to quantum cascade laser devices and methods for manufacturing quantum cascade laser devices.

[0002] Conventional terahertz nonlinear quantum cascade lasers (THz-nonlinear-QCLs, hereinafter referred to as THz nonlinear QCL devices) are designed so that two luminescence transitions occur within a single element, as described in, for example, Non-Patent Document 1. Therefore, they are capable of oscillating at two different frequencies ω1 and ω2, and output a difference frequency ω1-ω2 by utilizing the nonlinear optical effect of crystals such as InP or GaAs.

[0003] K. Fujita et. al. , “Recent progress in terahertz difference-frequency quantum cascade laser sources,” Nanophotonics, vol. 7, no. 11, pp. 1795-1817, Sept. 2018J. Kim et. al. , “Theoretical and experimental study of optical gain and linewidth enhancement factor of type-I quantum-cascade lasers,” IEEE J. Quantum. Electron. , vol. 40, no. 12, pp. 1663-1674, Dec. 2004I. Ladany et. al. , “Al2O3 half-wave films for long-life cw lasers,” Appl. Phys. Lett. , vol. 30, no. 2, pp. 87-88, 1977K. Fujita et. al. , “High-performance, homogeneous broad-gain quantum cascade lasers based on dual-upper-state,” Appl. Phys. Lett. , vol. 96, 241107, 2010K. Fujita et. al. , “Broad gain (Δλ / λ0 ~ 0.4), temperature-insensitive (T0 ~ 510K) quantum cascade lasers,” Opt. Express, vol. 19, no. 3, pp. 2694-2701, 2011

[0004] In conventional THz nonlinear QCL devices, the quantum well structure must be designed to generate two radiative transitions within a single device, and the laser output from these two radiative transitions must be roughly equal. Furthermore, to stably oscillate at two different frequencies, ω1 and ω2, the thickness and composition of the well and barrier layers in the quantum well structure must be strictly controlled during fabrication.

[0005] The present disclosure has been made to solve the above-mentioned problems, and aims to provide a quantum cascade laser device that can easily achieve terahertz oscillation and a method for manufacturing the quantum cascade laser device.

[0006] A quantum cascade laser device according to the present disclosure comprises: a semiconductor substrate; a first cladding layer of a first conductivity type formed on the semiconductor substrate; a core region formed on the first cladding layer, the core region including a plurality of stages each having an active region in which barrier layers and well layers are alternately formed, and an injector region for injecting electrons into the active region; and a second cladding layer of the first conductivity type formed on the core region, the quantum cascade laser device having a cavity length L and a wavelength λ qcl The wavelength width between two points where the gain reaches the maximum gain value and the gain value is lower by a preset percentage from the maximum gain value is defined as Δλ qcl When the wavelength λ qcl λ, which is a wavelength that is a preset wavelength away from cot A low-reflectivity film is provided at the other end surface, which minimizes the reflectivity at the other end surface. r and Δλ is the wavelength width between two points where a mirror loss value occurs when a preset loss value is subtracted from the maximum mirror loss value calculated by L. mir In this case, the Δλ mir is the above Δλ qcl is characterized by being smaller than

[0007] The method for manufacturing a quantum cascade laser device according to the present disclosure is a method for manufacturing the above-mentioned quantum cascade laser device, and includes the steps of: cleaving a wafer after wafer processing is completed to create a cleaved bar; measuring the wavelength dependence of the gain of the quantum cascade laser device by driving the cleaved bar to obtain the wavelength and gain band at which the gain is maximum; designing a low-reflectivity film whose reflectivity is minimal at a wavelength that is a predetermined wavelength away from the wavelength at which the gain is maximum; and depositing the low-reflectivity film on the front end face of the quantum cascade laser device.

[0008] According to the quantum cascade laser device and the method for manufacturing the quantum cascade laser device of the present disclosure, a low-reflectivity film optimally designed for generating terahertz light is formed on the front end face of the quantum cascade laser device, thereby providing the effect of easily obtaining a quantum cascade laser device that can easily output terahertz light, and also of easily manufacturing such a quantum cascade laser device.

[0009] 1 is a schematic view of a quantum cascade laser device according to a first embodiment; a -y b 1 is a cross-sectional view taken along a line. FIG. 1 is a schematic diagram showing a band structure of a conduction band of one stage when an electric field is applied in a quantum cascade laser device according to a first embodiment. FIG. 2 is a schematic diagram showing a quantum well structure of one stage of the quantum cascade laser device according to the first embodiment and the square of a wave function at each energy level. FIG. 3 is a diagram showing an example of wavelength dependence of gain in the quantum cascade laser device according to the first embodiment. FIG. 4 is a schematic diagram showing a case where a single-layer coating film is replaced with a three-layer coating film in the quantum cascade laser device according to the first embodiment. FIG. 5 is a schematic diagram showing a film configuration when a coating film is inserted between an end face and a first coating film of the quantum cascade laser device according to the first embodiment. FIG. 6 is a schematic diagram for explaining a method of designing a low-reflectivity film having an extremely small reflectivity for the quantum cascade laser device according to the first embodiment. FIG. 7 is a diagram showing the wavelength dependence of reflectivity and the wavelength dependence of mirror loss of a coating film provided on an end face of the quantum cascade laser device according to the first embodiment. 1A and 1B are diagrams illustrating wavelength dependence of gain and wavelength dependence of total loss in the quantum cascade laser device according to the first embodiment. a -x b1 is a cross-sectional view taken along a line; FIG. 1 is a cross-sectional view of a quantum cascade laser device according to a second embodiment; FIG. 2 is a schematic diagram showing a quantum well structure of one stage and the square of a wave function at each energy level in a quantum cascade laser device according to a third embodiment; FIG. 3 is a schematic diagram showing a quantum well structure of one stage and the square of a wave function at each energy level in a quantum cascade laser device according to a fourth embodiment; FIG. 4 is a schematic diagram of a film configuration in which a coating film is inserted between an end facet and a first coating film of a quantum cascade laser device according to a fifth embodiment, and a further coating film is provided on a third coating film; FIG. 5 is a diagram showing the wavelength dependence of the reflectance of a coating film provided on the front end facet of a quantum cascade laser device according to a fifth embodiment; FIG. 6 is a schematic diagram for explaining a method of designing a low-reflectance film having an extremely small reflectance for a quantum cascade laser device according to the fifth embodiment; and FIG. 7 is a diagram comparing the wavelength dependence of the reflectance and the wavelength dependence of the mirror loss of a coating film provided on the front end facet of a quantum cascade laser device according to the fifth embodiment with those of the first embodiment. 1 is a schematic diagram for explaining a design method for a low-reflectivity film having an extremely small reflectivity of a quantum cascade laser device according to the first and fifth embodiments; FIG. 2 is a diagram showing the wavelength dependence of the reflectivity of a low-reflectivity film designed by the design method for a low-reflectivity film having an extremely small reflectivity of a quantum cascade laser device according to the first and fifth embodiments; FIG. 3 is a schematic diagram for explaining a design method for a low-reflectivity film having an extremely small reflectivity of a quantum cascade laser device according to the first and fifth embodiments; FIG. 4 is a schematic diagram for explaining a design method for a low-reflectivity film having an extremely small reflectivity of a quantum cascade laser device according to the first and fifth embodiments; FIG. 5 is a diagram showing the wavelength dependence of the reflectivity of a low-reflectivity film designed by the design method for a low-reflectivity film having an extremely small reflectivity of a quantum cascade laser device according to the first and fifth embodiments; 10 is a schematic diagram for explaining a method for designing a low-reflectivity film having an extremely small reflectivity of the quantum cascade laser device according to the first and fifth embodiments. FIG.29 is a diagram showing the wavelength dependence of the reflectance of a low-reflectance film designed by a design method for a low-reflectance film having extremely small reflectance for the quantum cascade laser devices according to the first and fifth embodiments. 29 is a schematic view of a quantum cascade laser device according to a sixth embodiment. 29 is a diagram showing the wavelength dependence of the reflectance of a low-reflectance film designed by a design method for a low-reflectance film having extremely small reflectance for the quantum cascade laser devices according to the sixth embodiment. a -y b 31 in the quantum cascade laser device according to the seventh embodiment. a -y b FIG. 1 is a cross-sectional view taken along a line. FIG. 2 is a diagram showing one step of a method for manufacturing a quantum cascade laser device according to the first to seventh embodiments. FIG. 3 is a diagram showing one step of a method for manufacturing a quantum cascade laser device according to the first to seventh embodiments. FIG. 4 is a diagram showing one step of a method for manufacturing a quantum cascade laser device according to the first to seventh embodiments. FIG. 5 is a diagram showing one step of a method for manufacturing a quantum cascade laser device according to the first to seventh embodiments. FIG. 6 is a diagram showing one step of a method for manufacturing a quantum cascade laser device according to the first to seventh embodiments.

[0010] 1 is a schematic diagram showing a quantum cascade laser device 500 according to the first embodiment. a -y b 1 is a cross-sectional view taken along the line 500. The quantum cascade laser device 500 has a cavity length L and a ridge width W. The quantum cascade laser device 500 has a buried ridge type waveguide structure.

[0011] As shown in FIGS. 1 and 2, the quantum cascade laser device 500 includes an n-type first electrode 1, an n-type InP substrate 2, an n-type InP first cladding layer 3, an Fe-doped InP current blocking layer 4, an n-type second electrode 5, a front end facet coating film 6, a rear end facet coating film 7, a terahertz light extraction surface 8, and an n-type Ga 0.47 In0.53 The semiconductor device includes an n-type GaInAs (hereinafter abbreviated as GaInAs) first optical confinement layer (n-type GaInAs first optical confinement layer 9), a core region 10, an n-type GaInAs second optical confinement layer 11, an n-type InP second cladding layer 12, an n-type GaInAs contact layer 13, and an n-type second electrode 5. The n-type conductivity type is also referred to as the first conductivity type.

[0012] As shown in the cross-sectional view of FIG. 2 , the quantum cascade laser device 500 is composed of an n-type GaInAs first optical confinement layer 9, a core region 10 having a stacked structure (35 stages) in which 35 stages 40 each consisting of an active region 38 and an injector region 39 are stacked, an n-type GaInAs second optical confinement layer 11, an n-type InP second cladding layer 12, an n-type GaInAs contact layer 13, and an n-type second electrode 5.

[0013] 3 shows the quantum cascade laser device 500 according to the first embodiment, with a 5.0×10 6 3 is a schematic diagram showing the band structure of the conduction band of one stage 40 when an electric field of 1000 V / m is applied. Such a band structure is disclosed, for example, in Non-Patent Document 2. As shown in FIG. 3, the stage 40 is composed of an active region 38 and an injector region 39.

[0014] The active region 38 is an undoped Al layer having a thickness of 2.4 nm. 0.48 In 0.52 The AlInAs well layer 24 is made up of an undoped AlInAs barrier layer 21, an undoped GaInAs well layer 22 having a thickness of 6.5 nm, an undoped AlInAs barrier layer 23 having a thickness of 0.9 nm, an undoped GaInAs well layer 24 having a thickness of 6.6 nm, an undoped AlInAs barrier layer 25 having a thickness of 1.5 nm, an undoped GaInAs well layer 26 having a thickness of 3.2 nm, and a part of an undoped AlInAs barrier layer 27 having a thickness of 4.0 nm.

[0015] The injector region 39 is made up of the remainder of the 4.0 nm thick undoped AlInAs barrier layer 27, the 4.1 nm thick undoped GaInAs well layer 28, the 1.7 nm thick undoped AlInAs barrier layer 29, the 3.7 nm thick undoped GaInAs well layer 30, the 1.2 nm thick undoped AlInAs barrier layer 31, and the 3.4 nm thick n-type GaInAs well layer 32 doped to be n-type. The semiconductor layer 30 is composed of an InAs well layer 32, an n-type AlInAs barrier layer 33 doped to be n-type and having a thickness of 1.1 nm, an n-type GaInAs well layer 34 doped to be n-type and having a thickness of 3.4 nm, an undoped AlInAs barrier layer 35 having a thickness of 1.1 nm, an undoped GaInAs well layer 36 having a thickness of 2.9 nm, and an undoped AlInAs barrier layer 37 having a thickness of 2.4 nm.

[0016] The active region 38 is a region where electrons emit light by transitioning between subbands formed in the active region 38. The injector region 39 is a region where electrons are injected into the active region 38. In the first embodiment, the total number of well layers constituting the active region 38 is three, that is, three layers of GaInAs well layers 22, 24, and 26, is illustrated as an example. In the first embodiment, the doping concentrations of the n-type GaInAs well layer 32, n-type AlInAs barrier layer 33, and n-type GaInAs well layer 34 in the injector region 39 are, for example, 2.5×10 17 cm -3 Let's say.

[0017] 2, in the first embodiment, the total number of stages 40 is 35. Since 35 identical stages are connected in series, it is sufficient to analyze the laser characteristics using one stage 40.

[0018] 4 is a schematic diagram showing the quantum well structure of one stage of the quantum cascade laser device according to the first embodiment and the square of the wave function at each energy level, that is, FIG. 4 shows the probability of electron existence.

[0019] There are a total of 10 energy levels allowed in one stage 40, with solid lines indicating energy levels where electrons exist mainly in the active region 38 and dashed lines indicating energy levels where electrons exist mainly in the injector region 39. The five energy levels where electrons exist mainly in the active region 38 are energy level #1, energy level #2, energy level #4, energy level #7, and energy level #9, and the five energy levels where electrons exist mainly in the injector region 39 are energy level #3, energy level #5, energy level #6, energy level #8, and energy level #10.

[0020] Calculation of the electron density within the stage 40 reveals that, among the energy levels where electrons exist mainly in the active region 38, the electron density at energy level #4 is higher than the electron density at energy level #2, and that a population inversion necessary for laser oscillation is formed.

[0021] 5 is a diagram showing an example of the wavelength dependence of gain in the quantum cascade laser device 500 according to the first embodiment. FIG. 5 shows the wavelength dependence of gain when an electric field is applied from the n-type second electrode 5 to the n-type first electrode 1 and a current of 137 mA is injected in the quantum cascade laser device 500 having a cavity length L of 1.36 mm and a ridge width W of 14 μm. qcl ) At 9.466 μm, the maximum gain value g p As a result, it is 20.09 cm -1 Also, the maximum gain value g p The total width Δλ of the wavelength between the gain values ​​at two points that occurs when the gain value decreases by 10% from qcl is calculated to be 1.00 μm.

[0022] Next, the loss of the quantum cascade laser device 500 will be considered. The total loss α of the quantum cascade laser device 500 t is a loss (α ) that is almost independent of wavelength, such as waveguide loss or carrier absorption, as shown in the following equations (1) and (2). wc ) and mirror loss (α mir ) can be expressed as the sum of

[0023]

[0024] In formula (2), R f , R r and L are the front end facet reflectivity, the rear end facet reflectivity, and the cavity length, respectively. f is designed as follows assuming that it has wavelength dependency. On the other hand, the rear facet reflectance R r is assumed to have a rear end face coated with gold (Au) or the like to achieve a reflectance of nearly 100% and to be independent of wavelength.

[0025] Also, if α wc 5cm -1 Considering the asymmetry of the gain distribution, the wavelength λ cot = 9.461 μm, the total loss α t is the maximum, that is, 24.79 cm -1 If the loss band is narrower than the gain band, it is estimated that oscillation at two wavelengths is possible. cot At λ = 9.461 μm, the reflectivity is 0.459%, and the mirror loss α mir The maximum value is 19.79 cm -1 Therefore, the desired wavelength λ cot The desired minimum reflectance R0 of 0.459% is realized at the desired wavelength λ=9.461 μm. cot The desired minimum reflectivity R is the maximum gain value g of the quantum cascade laser device. p , the wavelength λ at which the maximum gain value occurs qcl The design method will be described later.

[0026] As disclosed in Non-Patent Document 3, the wavelength λ cot where the refractive index n c (3.6, GaAs) quantum cascade laser device with refractive index n f When a single layer coating made of Al2O3 with a reflectance of 1.72 is provided, the reflectance changes periodically with the thickness of the coating, and the thickness d f is λ cot / (4n f The minimum reflectance R0 in this case is expressed by the following equation (3) and is uniquely determined.

[0027] Conversely, from equation (3), the refractive index n of the coating film to realize the desired minimum reflectance R0 is fl , n fh is calculated from the following equation (4).

[0028]

[0029] The same applies to the application to quantum cascade laser devices. As is clear from equation (4), in order to obtain the desired minimum reflectance R0 using a single-layer coating film, the refractive index (n fl , n fh However, no material has been found to date that has this refractive index and is substantially free of absorption in the mid-infrared light, which is the oscillation wavelength range of quantum cascade laser devices. Therefore, the minimum reflectance R of the single-layer coating film is replaced with a multilayer coating film made of multiple materials that have no absorption in the mid-infrared light.

[0030] High refractive index fh In the first embodiment, the case of a low refractive index n fl The effective refractive index n of the quantum cascade laser device 500 is c is 3.2, and the desired minimum reflectance R by the single-layer coating film is R = 0.00459 (0.459%), the refractive index n fl is calculated as 1.67150083 from equation (4).

[0031] At least one of the single layer coating films has a refractive index n fl and at least one of the other materials has a refractive index n fl Consider a replacement with a three-layer coating of three materials with known refractive indices, each of which has a higher refractive index than the first material.

[0032] FIG. 6 shows the refractive index n fl and the film thickness d fl is λ cot / (4n fl 6 is a schematic diagram showing a case where the single-layer coating film 42 having an effective refractive index n1 and a thickness d1, a three-layer coating film having an effective refractive index n2 and a thickness d2, and a three-layer coating film having an effective refractive index n1 and a thickness d3, respectively.c The refractive index n formed on the end face of the quantum cascade laser device 41 is 3.2. fl (=1.67150083) and the film thickness d fl = λ cot / (4n fl ) is replaced with a three-layer coating film consisting of a first coating film 43 with a refractive index n1 and a thickness d1, a second coating film 44 with a refractive index n2 and a thickness d2, and a third coating film 45 with a refractive index n1 and a thickness d3. If the single-layer coating film 42 can be replaced with a three-layer coating film, the characteristic matrices of both will be equal, as expressed by the following equation (5).

[0033] The phase terms φ1, φ2, and φ3 of each coating film in equation (5) are expressed by the following equation (6).

[0034]

[0035] In equation (5), the refractive indexes n1 and n2 are known, so there are three unknowns: film thicknesses d1, d2, and d3. Each film thickness can be calculated by solving equation (5).

[0036] wavelength λ cot = 9.461 μm, and the refractive index of the first coating film 43 is n fl The second coating film 44 is made of YF3 having a refractive index n1 = 1.40, which is lower than that of the first coating film 44. fl ZnSe, which has a refractive index n = 2.41 higher than that of the third coating film 45, is used. fl Taking CeF3, which has a refractive index n1 = 1.45 lower than that of the first coating film 43, as an example, solving equation (5) gives d1 = 609.328 nm, d2 = 183.785 nm, and d3 = 697.248 nm, respectively. In order to narrow the reflectance band, a thin film having a refractive index n a and the film thickness d a is λ cot / (2n a ) may be inserted.

[0037] 7 is a schematic diagram showing the film configuration of the four-layer coating film of the first embodiment. In FIG. 7, the coating film 46 has a refractive index of n a and the film thickness d a is λcot / (2n a ) The characteristic matrix of the four-layer coating film is expressed by the following equation (7).

[0038] In equation (7), the phase term φ of each coating film a , φ1, φ2, and φ3 are expressed by the following formula (8), and the film thickness of the inserted coating film 46 is expressed by the following formula (9).

[0039] The reflectance of the coating film is expressed by the matrix component m 11 , m 12 , m 21 , m 22 Using the above, it is expressed by the following equation (10).

[0040]

[0041] An example of the material of the inserted coating film 46 is a film having a refractive index n a The thickness of the coating film 46 is set to 1.70. cot = 9.461 μm a = λ cot / (2n a ) = 2782.60 nm. In a quantum cascade laser device having a three-layer coating film with film thicknesses d1 = 609.328 nm, d2 = 183.785 nm, and d3 = 697.248 nm, when a coating film 46 is inserted between the end face of the quantum cascade laser device 41 and the first coating film 43, the wavelength dependency of the reflectance is as shown by the dashed line 47 in Figure 8. min = 9.449 μm, the minimum value R of the reflectance of the four-layer coating film min 0 In the present disclosure, R min and R min 0 The meanings of (1) R are as follows: min : The minimum reflectance of the multilayer coating film when the set reflectance R1 of the single-layer coating film is replaced with the multilayer coating film (2) R min 0 : The minimum reflectance of a multilayer coating film when the desired minimum reflectance R of a single-layer coating film is replaced with a multilayer coating film

[0042] The minimum reflectance R of the desired single-layer coating film is R = 0.459%, whereas the minimum reflectance R of the four-layer coating film is min 0 The minimum reflectance R of the four-layer coating film is 0.4576%, which is lower than the minimum reflectance R of the single-layer coating film. min In order to achieve the desired minimum reflectance R0, the set reflectance R1 is set as high as 0.460388%. The target value for the reflectance increment ΔR0 is R0 - R min 0 = 0.0014%, and adjust the increment ΔR0 until it matches the desired minimum reflectance R0. In the first embodiment, the increment ΔR0 is 0.00139%.

[0043] From the formula (11), the refractive index n of the single-layer coating film 42 to realize the set reflectance R = 0.460388% is fl 1 is calculated as 1.67132893.

[0044]

[0045] The thicknesses of the respective coating films are calculated by solving equation (12) as follows: d1 = 609.407 nm, d2 = 183.660 nm, and d3 = 697.402 nm. a is 1.70 and the film thickness d a The wavelength dependency of the reflectance when the coating film 46 having a wavelength λ of 2782.60 nm is inserted is shown by the two-dot chain line 48 in FIG. min The minimum reflectance value R of the three-layer coating film at 9.449 μm min is 0.459%, and the desired minimum reflectance R0 of 0.459% can be obtained.

[0046] Next, each film thickness is expressed as λ cot / λ min When doubled, d a The wavelength dependence of the reflectance in this case is shown by a solid line 49 in FIG. cotIt can be seen that the minimum reflectance R0 is 0.459% at λ = 9.461 μm. fl (=1.67150083) and the film thickness d fl The wavelength dependence of the reflectance of a single-layer coating film where .gtoreq. ...

[0047] The above-mentioned substitution method will be explained with reference to FIG. 9. (1) Desired wavelength (λ cot In order to replace the desired minimum reflectance (R0) of the single-layer coating film at the wavelength (wavelength dependency of reflectance is shown by the broken line 50a) with the triple-layer coating film, the thickness of each triple-layer coating film is determined so that the characteristic matrices of both are equal. Next, a and the film thickness d a is λ cot / (2n a The wavelength dependency of the reflectance of the four-layer coating film with the coating film 46 having a refractive index of n a and the film thickness is λ cot / (2n a ) the coating film 46 has a wavelength λ cot Since there is no effect on the reflectance at wavelength λ cot The minimum reflectance at the four-layer coating film is R0, but the minimum reflectance value of the four-layer coating film (minimum value R min 0 ) and the wavelength at which the minimum reflectance is reached (λ min ) deviates. The wavelength dependency of the reflectance in this case is represented by the dashed line 47a in FIG. 9. (2) The set reflectance R1 of the single-layer coating film is set to R1 = R0 + ΔR0 (wavelength dependency of the reflectance is represented by the dashed line 50b), and the thicknesses of the three-layer coating film that make the characteristic matrices equal are calculated. a and the film thickness d a is λ cot / (2n a The wavelength dependency of the reflectance of the four-layer coating film with the coating film 46 inserted is calculated. min The minimum reflectance is the desired minimum reflectance R min = minimum reflectance R0. The wavelength dependency of reflectance in this case is represented by the two-dot chain line 48a in FIG. 9. (3) Wavelength λ cot The thickness of each of the four coating layers is set to λ so that the minimum reflectance R0 is obtained at cot / λ minThe wavelength dependency of the reflectance in this case is represented by a solid line 49a in FIG.

[0048] 10 is a diagram showing the wavelength dependency of the reflectance and the wavelength dependency of the mirror loss of the coating film shown in the above-mentioned FIG. 8. In FIG. 10, the dotted line 51 is the reflectance R f and the solid line 52 represents the mirror loss α mir The other end face (rear end face) is coated with gold (Au) to give a reflectance R r The mirror loss α mir is calculated using the above-mentioned formula (2).

[0049] Mirror loss α mir is the maximum value of 19.79 cm -1 and the overall width (Δλ) of the 10% reduction in maximum mirror loss mir ) is 0.387 μm, and the full width of the wavelength between the two gain values ​​Δλ qcl As mentioned above, the loss α that is almost independent of the wavelength, such as the waveguide loss and carrier absorption, wc 5cm -1 Then, the total loss α t The maximum value is 24.79 cm -1 This becomes:

[0050] FIG. 11 shows the wavelength dependence of gain and the total loss α t 11, the dotted line 61 indicates the wavelength dependence of the gain, and the solid line 62 indicates the total loss α t The wavelength dependence of the gain and total loss α t are equal at two points, λ1 and λ2. Generally, a laser has a gain and a total loss α t When the wavelengths λ1 and λ2 are equal, the laser oscillates at two wavelengths in one quantum cascade laser device 500. The wavelength λ1 of the first laser oscillation is 8.802 μm, and the wavelength λ2 of the second laser oscillation is 10.151 μm. The angular frequency ω1 is 2.14005×10 14 Hz and ω2 are 1.85564 × 10 14 Hz.

[0051] Due to the nonlinear optical effect of the InP crystal, the difference angular frequency ω1-ω2 = 2.8441 × 10 13 Hz, difference frequency f1-f2=4.52656×10 12 Hz (4.53 THz) can be obtained. The wavelength corresponding to the difference frequency f1-f2 is 66.230 μm.

[0052] FIG. 12 shows the quantum cascade laser device 500 according to the first embodiment, where x in FIG. a -x b This is a cross-sectional view taken along the line. The terahertz waves (ω1-ω2) are emitted in the direction of the Cherenkov angle (θ) that satisfies the phase matching condition. Since the Cherenkov angle θ is usually about 20°, when the laser light reaches the end face, it satisfies the total reflection condition and cannot be extracted outside the quantum cascade laser device 500. Therefore, the end face is tilted by polishing or the like, so that the laser light can be extracted outside the quantum cascade laser device 500. Note that the total number of stages 40 in the quantum cascade laser device 500, 35, is an example and is not limited to this, and can be changed depending on the required characteristics.

[0053] The features of the quantum cascade laser device 500 according to the first embodiment are summarized below. The quantum cascade laser device 500 according to the first embodiment includes an n-type InP substrate 2, an n-type InP first cladding layer 3 of a first conductivity type (n type) formed on the n-type InP substrate 2, a core region 10 formed on the n-type InP first cladding layer 3 and including a plurality of stages 40 each having an active region 38 in which barrier layers and well layers are alternately formed and an injector region 39 for injecting electrons into the active region, and an n-type InP second cladding layer 12 of the first conductivity type formed on the core region 10, and has a cavity length L and emits a light having a wavelength λ qcl The wavelength width between the two points where the gain is the maximum gain value and the gain value is decreased by a predetermined percentage from the maximum gain value is defined as Δλ qcl When the wavelength λ qcl λ, which is a wavelength that is a preset wavelength away from cot A low-reflectivity film (front end face coating film 6) is provided at the other end face, and the reflectivity Rr and the mirror loss α calculated from the cavity length L mir The wavelength width between the two points where the mirror loss value occurs when a preset loss value is subtracted from the maximum value of mir In this case, Δλ mir is Δλ qcl is characterized by being smaller than

[0054] In the first embodiment, the mirror loss α mir The wavelength λ at which cot (9.461 mm) is the wavelength λ at which the gain is maximum. qcl (9.466 μm), but the total loss band is the mirror loss α mir If the bandwidth is wider than the wavelength λ and the total loss and gain are equal at two wavelengths, cot is λ qcl It may be the same as

[0055] In addition, a desired wavelength (λ cot The low-reflectance film having the desired minimum reflectance (R) utilizes the concept of the design method of the low-reflectance film of the first embodiment, and in manufacturing, it is not necessary for each coating film of the multilayer coating film to strictly match the values ​​shown in the first embodiment. In other words, the mirror loss α calculated from the low-reflectance film mir If the band is narrower than the gain band of the quantum cascade laser device, the function can be fully demonstrated.

[0056] <Effects of First Embodiment> As described above, according to the quantum cascade laser device of the first embodiment, when a current is injected, the wavelength λ qcl The gain reaches the maximum value at Δλ, and the wavelength width between the two points where the gain value is lower than this maximum gain value by a preset percentage is defined as qcl When the wavelength λ qcl λ, which is a wavelength that is a preset wavelength away from cot A low reflectance film is provided, which minimizes the reflectance at the mirror loss α mir The wavelength width between the two points where the mirror loss value occurs when a preset loss value is subtracted from the maximum value of mir In this case, Δλmir is Δλ qcl Since the distance is set to be smaller than the distance .theta., it is possible to obtain an effect that a quantum cascade laser device that can easily realize terahertz oscillation can be obtained.

[0057] 13 is a cross-sectional view of a quantum cascade laser apparatus 600 according to the second embodiment. The quantum cascade laser apparatus 600 according to the second embodiment includes a temperature adjustment device 71. That is, the quantum cascade laser apparatus 600 according to the second embodiment has a quantum cascade laser apparatus 74 installed on a temperature adjustment device 71 such as a Peltier element. The quantum cascade laser apparatus 74 is the same as the quantum cascade laser apparatus 500 according to the first embodiment.

[0058] 13, a quantum cascade laser device 74 according to the second embodiment includes a temperature adjustment device 71, a metal block 72 bonded onto the temperature adjustment device 71, a submount 73 bonded to the metal block 72, and a quantum cascade laser device 74 bonded onto the submount 73. In order to reduce thermal resistance, the quantum cascade laser device 74 is bonded to the submount 73 with the epitaxial crystal growth layer side facing down (epi-side down).

[0059] Mirror loss α mir , the total loss α t The distribution of the wavelength is almost independent of temperature, whereas the gain distribution is temperature dependent and shifts to longer wavelengths as the temperature increases.

[0060] When a current is injected into the quantum cascade laser device 74 to increase the gain and the temperature of the quantum cascade laser device 74 is changed by driving the temperature adjustment device 71, the gain peak wavelength and the mirror loss α mir Even if the maximum wavelength is adjusted relatively roughly in advance, the gain and total loss α t can be made equal, so that terahertz oscillation can be easily performed.

[0061] Furthermore, even if the gain distribution fluctuates over time during operation, resulting in single-wavelength oscillation and terahertz oscillation stopping, the terahertz oscillation can be easily restored to two-wavelength oscillation by adjusting the amount of current injected into the quantum cascade laser device 74 and adjusting the temperature using the temperature control device 71, making it easy to restore terahertz oscillation.

[0062] <Effects of Second Embodiment> As described above, the quantum cascade laser device according to the second embodiment has the effect of providing a quantum cascade laser device that can easily restore terahertz oscillation by adjusting the temperature of the quantum cascade laser device using a temperature adjustment device.

[0063] Third Embodiment The quantum cascade laser device of the present disclosure has a gain band that is smaller than the mirror loss α mir Therefore, in the third embodiment, an example of widening the gain bandwidth of a quantum cascade laser device is shown. Fig. 14 shows the quantum well structure of one stage 106 in the quantum cascade laser device according to the third embodiment and the square of the wave function at each energy level, which is the structure described in Non-Patent Document 4.

[0064] As shown in FIG. 14, one stage 106 is composed of an active region 104 and an injector region 105 .

[0065] The active region 104 is composed of an undoped AlInAs barrier layer 81 having a thickness of 1.8 nm, an undoped GaInAs well layer 82 having a thickness of 5.2 nm, an undoped AlInAs barrier layer 83 having a thickness of 1.5 nm, an undoped GaInAs well layer 84 having a thickness of 5.8 nm, an undoped AlInAs barrier layer 85 having a thickness of 0.7 nm, an undoped GaInAs well layer 86 having a thickness of 7.5 nm, an undoped AlInAs barrier layer 87 having a thickness of 2.7 nm, an undoped GaInAs well layer 88 having a thickness of 3.1 nm, and a part of an undoped AlInAs barrier layer 89 having a thickness of 3.7 nm.

[0066] The injector region 105 is made up of the remainder of the 3.7 nm thick undoped AlInAs barrier layer 89, the undoped GaInAs well layer 90, the 3.3 nm thick undoped AlInAs barrier layer 91, the 2.8 nm thick undoped AlInAs well layer 92, the 2.3 nm thick undoped AlInAs barrier layer 93, the 3.4 nm thick undoped GaInAs well layer 94, the 2.0 nm thick n-type AlInAs barrier layer 95 doped to be n-type, and the 3.4 nm thick undoped GaInAs well layer 96. The n-type GaInAs well layer 96 has a thickness of 1.7 nm, an n-type AlInAs barrier layer 97 doped to n-type and having a thickness of 1.7 nm, an n-type GaInAs well layer 98 doped to n-type and having a thickness of 3.5 nm, an undoped AlInAs barrier layer 99 has a thickness of 1.6 nm, an undoped GaInAs well layer 100 has a thickness of 3.8 nm, an undoped AlInAs barrier layer 101 has a thickness of 1.5 nm, an undoped GaInAs well layer 102 has a thickness of 4.1 nm, and an undoped AlInAs barrier layer 103 has a thickness of 1.8 nm.

[0067] There are two upper energy levels that contribute to light emission, energy level #3 and energy level #4, and one lower energy level, energy level #2. To quickly extract electrons from the lower energy levels, there is a ground energy level #1 below the lower energy levels. This structure is called a dual-upper-state to single-lower-state transition (DAU / SS), and has a wider gain band than those with a single transition process.

[0068] <Effects of embodiment 3> As described above, according to the quantum cascade laser device of embodiment 3, the gain band of the quantum cascade laser device is wider than the band of the mirror loss derived from the reflective film provided on the end face, and therefore, the effect is achieved that terahertz waves can be easily oscillated.

[0069] Fourth Embodiment The quantum cascade laser device of the present disclosure has a gain band that is smaller than the mirror loss α mirTherefore, in the fourth embodiment, an example in which the gain bandwidth of a quantum cascade laser device is widened will be shown.

[0070] FIG. 15 shows the quantum well structure of one stage 136 in the quantum cascade laser device according to the fourth embodiment and the square of the wave function at each energy level, which is the structure described in Non-Patent Document 5.

[0071] As shown in FIG. 15, one stage 136 is composed of an active region 134 and an injector region 135 .

[0072] The active region 134 includes an undoped AlInAs barrier layer 111 having a thickness of 1.5 nm, an undoped GaInAs well layer 112 having a thickness of 4.1 nm, an undoped AlInAs barrier layer 113 having a thickness of 1.2 nm, an undoped GaInAs well layer 114 having a thickness of 5.2 nm, an undoped AlInAs barrier layer 115 having a thickness of 1.0 nm, and an undoped GaInAs well layer 116 having a thickness of 5.8 nm. It is composed of an InAs well layer 116, an undoped AlInAs barrier layer 117 having a thickness of 0.9 nm, an undoped GaInAs well layer 118 having a thickness of 7.5 nm, an undoped AlInAs barrier layer 119 having a thickness of 2.7 nm, an undoped GaInAs well layer 120 having a thickness of 3.1 nm, and a part of an undoped AlInAs barrier layer 121 having a thickness of 3.7 nm.

[0073] The injector region 135 is made up of the remainder of the 3.7 nm thick undoped AlInAs barrier layer 121, the 3.3 nm thick undoped GaInAs well layer 122, the 2.8 nm thick undoped AlInAs barrier layer 123, the 3.4 nm thick undoped GaInAs well layer 124, the 2.3 nm thick undoped AlInAs barrier layer 125, the 3.4 nm thick undoped GaInAs well layer 126, and the 2.0 nm thick n-type AlInAs barrier layer 125 doped to be n-type. The n-type GaInAs well layer 127 is an n-type GaInAs well layer 128 doped to be n-type and having a thickness of 3.4 nm, an n-type AlInAs barrier layer 129 doped to be n-type and having a thickness of 1.7 nm, an n-type GaInAs well layer 130 doped to be n-type and having a thickness of 3.5 nm, an undoped AlInAs barrier layer 131 having a thickness of 1.6 nm, an undoped GaInAs well layer 132 having a thickness of 3.8 nm, and an undoped AlInAs barrier layer 133 having a thickness of 1.5 nm.

[0074] The upper energy levels that contribute to light emission are energy level #3 and energy level #4, and the lower energy level is a miniband #2mb consisting of four subbands. To quickly extract electrons from the lower energy levels, a ground level #1mb consisting of three subbands exists below the lower energy levels. This structure is called a dual-upper-state to multiple-lower-state transition (DAU / MS), and can widen the gain band compared to the DAU / SS of the third embodiment.

[0075] <Effects of embodiment 4> As described above, according to the quantum cascade laser device of embodiment 4, the gain band of the quantum cascade laser device is wider than the band of the mirror loss derived from the reflective film provided on the end face, and therefore, it has the effect of being able to oscillate terahertz waves more easily.

[0076] Fifth Embodiment In a quantum cascade laser device according to a fifth embodiment, the gain band of the quantum cascade laser device is mirIn the third and fourth embodiments, examples of widening the gain bandwidth of the quantum cascade laser device have been shown, but in the fifth embodiment, on the other hand, the mirror loss α mir An example of narrowing the bandwidth is shown below.

[0077] 16 is a schematic diagram showing a part of a quantum cascade laser device according to the fifth embodiment, in which a coating film 141 is added to the outside of the third coating film 45 in FIG. 7 showing the first embodiment. In FIG. 16, the coating film 141 has a refractive index of n b and the film thickness d b is λ cot / (2n b ) The characteristic matrix of the five-layer coating film is expressed by the following equation (13).

[0078] In equation (13), the phase term φ of each coating film a , φ1, φ2, φ3 and φ b is expressed by the following formula (14), and the film thicknesses of the inserted coating film 46 and the added coating film 141 are respectively expressed by the following formula (15).

[0079]

[0080] As an example, the additional coating film 141 has a refractive index n b If the thickness of the film is ZnS with a wavelength λ cot = 9.461 μm b = λ cot / (2n b ) = 2150.20 nm.

[0081] The above-mentioned film thickness d a The wavelength dependency of reflectance when a coating film 141 is provided as the final layer of each coating film having wavelengths λ = 2782.600 nm, d = 609.328 nm, d = 183.785 nm, and d = 697.248 nm (double-dot chain line 48 in FIG. 8) is shown by a dashed dotted line 142 in FIG. min = 9.458 μm, the minimum value R of the reflectance of the five-layer coating film min 0 becomes 0.4600%.

[0082] Minimum reflectance R0 of a single-layer coating a: 0.460388%, the minimum reflectance value R of the five-layer coating film min 0 The minimum reflectance R0 of the single-layer coating film is 0.4600%. a However, it is higher than the desired minimum reflectance R0. Therefore, the minimum reflectance R0 of the five-layer coating film is min In order to set the reflectance R to the desired minimum reflectance R, in equation (11), if the set reflectance R of the single-layer coating film is set to 0.459324%, the refractive index n of the single-layer coating film will be fl 1 is calculated as 1.6714607.

[0083] By solving equation (12) and calculating the thickness of each coating film, the following values ​​are obtained: d1 = 609.347 nm, d2 = 183.756 nm, and d3 = 697.284 nm. a is 1.70 and the film thickness d a The coating film 46 having a refractive index of 2782.60 nm was inserted as the final coating film. b is 2.20 and the film thickness d b The wavelength dependency of the reflectance when the coating film 141 having a reflectance of 2150.20 nm is provided is shown by the two-dot chain line 143 in FIG. min is the wavelength λ min = 9.458 μm, the desired minimum reflectivity R 0 of 0.459% can be obtained.

[0084] Next, each film thickness is expressed as λ cot / λ min When doubled, d a =2783.4826nm, d1=609.5403nm, d2=183.8143nm, d3=697.5052nm, d b The wavelength dependency of the reflectance in this case is shown by a solid line 144 in FIG. cot It can be seen that the minimum reflectance is 0.459% at wavelength = 9.461 μm.

[0085] The above-mentioned substitution method will be explained with reference to Fig. 18. (1) The dashed line 50a in Fig. 18 indicates the desired wavelength λ cotThe dashed line 48a in FIG. 18 represents the wavelength dependence of the reflectance of the single-layer coating film having the minimum reflectance R0. a 18. The reflectance of the five-layer coating film, in which a coating film 141 is provided on the final layer of the four-layer coating film, is expressed by the two-dot chain line 142a in FIG. 18. The coating film 141 has a refractive index of n b The film thickness is d b = λ cot / (2n b ) so the wavelength λ cot There is no effect on reflectance at wavelength λ cot The reflectance of the five-layer coating film at R0 is the same as the set reflectance of the four-layer coating film. a = 0.460388%. The minimum reflectance value R min 0 is 0.4600%. cot The minimum reflectance R0 a The refractive index n of the single-layer coating film is (=0.460388%) fl 1 is 1.67132893 compared to the above four-layer coating film, and the wavelength dependency of the reflectance of the single-layer coating film is shown by the dashed line 50b in FIG. cot The minimum reflectance R0 a The single layer coating film (whose reflectance is wavelength dependent 50b) is placed on the surface of the quantum cascade laser device with a refractive index of n a The film thickness is d a = λ cot / (2n a ) and the refractive index n b The film thickness is d b = λ cot / (2n b ), the wavelength dependency of the reflectance is represented by the two-dot chain line 142a in FIG. 18, and the wavelength λ min The minimum reflectance value R of the five-layer coating film min 0 This becomes:

[0086] (3) Minimum reflectance R of a single-layer coating a: 0.460388%, the minimum reflectance value R of the five-layer coating film min 0 The minimum reflectance R0 of the single-layer coating film is 0.4600%. a However, it is higher than the desired minimum reflectance R0. Therefore, the minimum reflectance R0 of the five-layer coating film is min In order to set the reflectance R to the desired minimum reflectance R, in equation (11), if the set reflectance R of the single-layer coating film is set to 0.459324%, the refractive index n of the single-layer coating film will be fl 1 The thickness of each coating film is calculated by solving equation (12), and the refractive index n a and the film thickness d a The coating film 46 having a refractive index of n b and the film thickness d b The wavelength dependency of the reflectance is calculated by providing a coating film 141 having a wavelength λ min The desired minimum reflectance R0 is obtained at wavelength λ. cot The thickness of each of the five coating layers is set to λ so that the minimum reflectance R0 is obtained at cot / λ min In this case, the wavelength dependency of the reflectance is shown by the solid line 144a in FIG.

[0087] In addition, a desired wavelength (λ cot ) a low-reflectivity film having a desired minimum reflectivity (R0) utilizes the concept of the design method of the low-reflectivity film of the fifth embodiment, and it is not necessary for each coating film of the multilayer coating film to strictly match the values ​​shown in the fifth embodiment in manufacturing. In other words, the mirror loss α calculated from the low-reflectivity film mir If the band is narrower than the gain band of the quantum cascade laser device, the function of the fifth embodiment can be fully achieved.

[0088] FIG. 19 shows the reflectance and mirror loss α mir 19 is a graph showing the wavelength dependence of the reflectance, in comparison with that of the first embodiment. In FIG. 19, the dotted line 145 indicates the wavelength dependence of the reflectance, and the solid line 146 indicates the mirror loss α mir19 shows the wavelength dependence of the reflectance distribution and the mirror loss distribution, respectively. From FIG. 19, it can be seen that the minimum reflectance and the maximum mirror loss remain unchanged, but the reflectance distribution and the mirror loss distribution become narrower. The full width (Δλ mir ) is 0.184 μm, which is about half the value achieved in the first embodiment.

[0089] <Effects of Fifth Embodiment> As described above, according to the quantum cascade laser device according to the fifth embodiment, the mirror loss α mir Since a state in which the gain band is narrower than the gain band can be easily realized, the effect is that the oscillation of terahertz waves can be performed more easily.

[0090] The present disclosure provides a method for detecting a desired wavelength (λ cot One of the components is to obtain the desired minimum reflectivity (R0) at the desired wavelength, and thus the desired maximum mirror loss at the desired wavelength. Therefore, a method for achieving the desired minimum reflectivity at the desired wavelength will be described in detail below.

[0091] The effective refractive index is n c The quantum cascade laser device 211 is cot The refractive index is n f and the film thickness is λ cot / (4n f ) is provided, the reflectance of the single-layer coating film becomes the minimum reflectance R0, which is the minimum value. The minimum reflectance R0 is expressed by the above-mentioned formula (3).

[0092] Conversely, from equation (3), the refractive index n of the coating film to realize the desired minimum reflectance R0 is fl , and the refractive index n fh is calculated using the above formula (4).

[0093] For example, the effective refractive index n c is 3.2 and the minimum reflectance R = 0.005 (0.5%), the refractive index n fl and refractive index n fhare calculated to be 1.666535 and 1.920152, respectively. However, among the materials constituting the coating film, there are currently no materials that have these refractive index values ​​and that are applicable to quantum cascade laser devices, taking into account reliability and cost. Therefore, in the present disclosure, the above-mentioned refractive index values ​​are achieved by substituting known materials.

[0094] First, the refractive index n fl The case where the refractive index of the single-layer coating film is n fl and a material with a refractive index lower than n fl Consider replacing the material with a three-layer coating of two materials, one of which has a higher refractive index than the other.

[0095] FIG. 20 shows the refractive index n fl and the film thickness d fl is λ cot / (4n fl 20 is a schematic diagram showing a case where the single-layer coating film 212 having an effective refractive index n1 and a thickness d1, a three-layer coating film having an effective refractive index n2 and a thickness d2, and a three-layer coating film having an effective refractive index n1 and a thickness d3. c and wavelength λ cot The refractive index n formed on the end face of the quantum cascade laser device 211 fl (=1.66535) and the film thickness d fl = λ cot / (4n fl ) is replaced with a three-layer coating film consisting of a first coating film 213 with a refractive index n and a thickness d, a second coating film 214 with a refractive index n and a thickness d, and a third coating film 215 with a refractive index n and a thickness d. If the single-layer coating film 212 can be replaced with a three-layer coating film, the characteristic matrices of both will be equal, as expressed by the following equation (16).

[0096] The phase terms φ1, φ2, and φ3 of each coating film in equation (16) are expressed by the following equation (17).

[0097]

[0098] In equation (16), the refractive indexes n1 and n2 are known, so there are three unknowns: film thicknesses d1, d2, and d3. Each film thickness can be calculated by solving equation (16).

[0099] wavelength λ cot = 10 μm, the first coating film 213 and the third coating film 215 have a refractive index of n fl YF3, which has a refractive index n1 = 1.40 lower than that of the second coating film 214, is used as the second coating film 214. fl Taking ZnS, which has a refractive index n2 = 2.20 higher than that of ZnS, as an example, solving equation (16) gives the thicknesses of the coating films d1 = 654.371 nm, d2 = 277.806 nm, and d3 = 654.371 nm, respectively. The wavelength dependency of the reflectance in this case is shown by the dashed-dotted line 222 in Figure 21. The reflectance is 0.5% at a wavelength of 10 μm, but this value is not the minimum value, but the minimum value R of the reflectance of the three-layer coating film. min 0 is 0.4857%.

[0100] The minimum reflectance R of the single-layer coating film is 0.5%, while the minimum reflectance R of the triple-layer coating film is min 0 The minimum reflectance R of the triple-layer coating film is 0.4857%, which is lower than the minimum reflectance R of the single-layer coating film. min In order to achieve the desired minimum reflectance R0, the set reflectance R1 is set high. The target of the reflectance increment ΔR0 is R0 - R min 0 = 0.0143%, and adjust the increment ΔR0 until it matches the desired minimum reflectance R0. In the sixth embodiment, the increment ΔR0 = 0.0146%, and the set reflectance R1 is 0.51460%. From the following equation (18), the refractive index n of the single-layer coating film to achieve the set reflectance R1 = 0.51460% is fl 1 is calculated as 1.6648189.

[0101]

[0102] That is, from the formula (18), the refractive index n of the single-layer coating film to realize R1 = 0.51460% is fl 1The thickness of each coating film is calculated by solving the following equation (19), and the calculated thickness is d1 = 655.839 nm, d2 = 276.062 nm, and d3 = 655.839 nm, respectively.

[0103]

[0104] The wavelength dependency of the reflectance in this case is shown by the two-dot chain line 223 in Figure 21. The minimum value R of the reflectance of the three-layer coating film min is the wavelength λ min = 9.876 μm, the desired minimum reflectance of 0.5% can be obtained.

[0105] Next, the wavelength λ at which the reflectance becomes minimum min the desired wavelength λ cot The thickness of each coating film is set to λ cot / λ min Multiplying these values, d1 = 664.074 nm, d2 = 279.528 nm, and d3 = 664.074 nm are calculated. The wavelength dependency of the reflectance in this case is shown by the solid line 224 in Figure 21. cot It can be seen that the reflectance reaches a minimum of 0.5% at 10 μm.

[0106] The broken line 221 in FIG. 21 indicates the refractive index n fl and the film thickness is λ cot / (4n fl 21 shows the wavelength dependence of reflectance for a single-layer coating film having a reflectance of 0.015 μm / cm. The solid line 224 and the dashed line 221 in FIG. 21 do not perfectly coincide, but they show almost the same wavelength dependence of reflectance, and the minimum reflectance and the wavelength at which the minimum reflectance occurs are completely coincident.

[0107] Minimum reflectance R = 0.5% and wavelength λ cot In the above example, YF3 is used for the first coating film 213 and the third coating film 215, and ZnS is used for the second coating film 214, but the present invention is not limited to these examples. fl and n fl 1 and one or more materials with refractive indexes higher than n fl and n fl 1The refractive index of the material can be determined by the refractive index of the material.

[0108] FIG. 22 shows the wavelength λ cot The refractive index n fl and the film thickness is λ cot / (4n fl 22 is a schematic diagram showing a method of replacing a single-layer coating film having a refractive index n fl and the film thickness λ cot / (4n fl ) represents the wavelength dependence of the reflectance of a single-layer coating film, and cot The minimum reflectance R0 is shown at

[0109] The above-mentioned substitution method will be explained below. (1) To replace the single-layer coating film (broken line 221a) with a three-layer coating film, the thicknesses of the three-layer coating film are determined so that the characteristic matrices of both are equal. In this case, the wavelength λ cot The minimum reflectance R0 is reached at the point where the triple-layer coating film has a minimum reflectance R min 0 ) and the wavelength λ at which the minimum reflectance is achieved min The wavelength dependency of the reflectance in this case is represented by the dashed line 222a in Fig. 22. (2) The set reflectance R of the single-layer coating film is set as R = R + ΔR (broken line 221b), and the thicknesses of the three-layer coating film that make the characteristic matrices equal are calculated, and the wavelength λ min The minimum reflectance value R of the three-layer coating film is desired to have a minimum reflectance. min = minimum reflectance R0. The wavelength dependency of reflectance in this case is represented by the two-dot chain line 223a in Figure 22. (3) Wavelength λ cot The thickness of each of the three coating layers is set to λ so that the minimum reflectance R0 is obtained at cot / λ min The wavelength dependency of the reflectance in this case is represented by a solid line 224a in FIG.

[0110] The method for setting the set reflectance R1 will be described in more detail below. The minimum reflectance of the three-layer coating film at the time of the initial substitution is R min0(0) If R0 is not satisfied, the difference ΔR0 from the desired minimum reflectance R0 at this time is (0) is ΔR (0) = R0 - R min 0(0) Next, the set reflectance R of the single-layer coating film for the first loop is (1) , R1 (1) = R0 + ΔR0 (0) and replaced with a three-layer coating film to achieve the minimum reflectance R min 0(1) As a result of the calculation, R min 0(1) If ΔR0 = ΔR0, the calculation ends here. (0) , R1=R1 (1) This becomes:

[0111] R min 0(1) If R0, the process moves to the second loop. At this time, the difference ΔR0 from the desired minimum reflectance R0 (1) is ΔR (1) = R0 - R min 0(1) The set reflectance R1 of the single-layer coating film (2) , R1 (2) = R0 + ΔR0 (1) and replaced with a three-layer coating film to achieve the minimum reflectance R min 0(2) Calculate.

[0112] As a result of the calculation, R min 0(2) If ΔR0 = ΔR0, the calculation ends here. (1) , R1=R1 (2) It becomes. R min 0(2) If R0, the process moves to the third loop and the calculation is repeated in the same way. min 0(k) By looping k times until the set reflectance R1 = R1 (k) The above is the method for setting the set reflectance R1.

[0113] Similarly, the refractive index n fh The case where the refractive index of the single-layer coating film is nfh and a material with a refractive index lower than n fh Consider replacing the material with a triple coating of at least two materials with a higher refractive index than the material ...

[0114] FIG. 23 shows the refractive index n fh and the film thickness d fh is λ cot / (4n fh 23 is a schematic diagram showing a case where a single-layer coating film 231 having an effective refractive index n1 and a thickness d1, a three-layer coating film having an effective refractive index n2 and a thickness d2, and a three-layer coating film having an effective refractive index n1 and a thickness d3, respectively. c and wavelength λ cot0 The refractive index n formed on the end face of the quantum cascade laser device 211 fh (=1.920152) and the film thickness d fh = λ cot / (4n fh The single-layer coating film 231 having a refractive index of n1 and a thickness of d1 is replaced with a three-layer coating film consisting of a first coating film 232 having a refractive index of n2 and a thickness of d2, a second coating film 233 having a refractive index of n1 and a thickness of d2, and a third coating film 234 having a refractive index of n1 and a thickness of d3. fl This is the same as in the case of (1), and is performed by solving the following equation (20).

[0115]

[0116] wavelength λ cot = 10 μm, the first coating film 232 and the third coating film 234 have a refractive index of n fh The second coating film 233 is made of YF3 having a refractive index n1 = 1.40, which is lower than that of the first coating film 233. fh Taking ZnS, which has a refractive index n = 2.20 higher than that of ZnS, as an example, solving equation (20) gives d = 434.257 nm, d = 547.934 nm, and d = 434.257 nm, respectively. The wavelength dependency of reflectance in this case is shown by the dashed-dotted line 236 in Figure 24. The reflectance is 0.5% at a wavelength of 10 μm, but this value is not the minimum value, but the minimum value R of the reflectance of the three-layer coating film. min 0 is 0.4715%.

[0117] The minimum reflectance R of the single-layer coating film is 0.5%, while the minimum reflectance R of the triple-layer coating film is min 0 is 0.4715%, which is lower than the minimum reflectance R of the single-layer coating film. fl As in the case of min In order to achieve the desired reflectance R0, the set reflectance R1 is set high at 0.53032%. From the following formula (21), the refractive index n of the single-layer coating film to achieve the set reflectance R1 = 0.53032% is fh 1 is calculated as 1.9243333.

[0118] The thickness of each coating film is calculated by solving the following equation (22), and the results are d1 = 430.513 nm, d2 = 552.699 nm, and d3 = 430.513 nm.

[0119]

[0120] The wavelength dependency of the reflectance in this case is shown by the two-dot chain line 237 in Figure 24. The minimum value R of the reflectance of the three-layer coating film min is the wavelength λ min = 10.174 μm, the desired minimum reflectance of 0.5% can be obtained.

[0121] Next, the wavelength λ at which the reflectance becomes minimum min the desired wavelength λ cot The thickness of each of the three coating layers is set to λ cot / λ min Multiplying by 1, the calculated values ​​are d1 = 423.150 nm, d2 = 543.247 nm, and d3 = 423.150 nm. The wavelength dependency of the reflectance in this case is shown by a solid line 238 in Figure 24. cot It can be seen that the reflectance reaches a minimum of 0.5% at 10 μm.

[0122] The broken line 235 in FIG. 24 indicates the refractive index n fh and the film thickness is λ cot / (4n fh24 shows the wavelength dependence of reflectance for a single-layer coating film having a reflectance of 0.015 μm / cm. The solid line 238 and the dashed line 235 in FIG. 24 do not perfectly coincide, but they show almost the same wavelength dependence of reflectance, and the minimum reflectance and the wavelength at which the minimum reflectance occurs are completely coincident.

[0123] Minimum reflectance R = 0.5% and wavelength λ cot In the above example, YF3 is used for the first coating film 232 and the third coating film 234, and ZnS is used for the second coating film 233, but the present invention is not limited to these examples. fh and n fh 1 and one or more materials with refractive indexes higher than n fh and n fh 1 As a result, it is possible to realize a desired minimum reflectance at a desired wavelength using a material with a known refractive index.

[0124] As described above, according to the design method of the reflectance film according to the first and fifth embodiments, by replacing the single-layer coating film with a three-layer multilayer coating film by utilizing the characteristic matrix, it is possible to design a low-reflectance film in which the reflectance is controlled to a desired minimum at a desired wavelength.

[0125] Next, we will show a case where all three layers of the multilayer coating are made of different materials. As a method for designing a low reflectivity film in this case, we will show an example of a minimum reflectivity R = 0.01 (1%). The effective refractive index n c If we set the value to 3.2, then from the above equation (4), n fl and n fh are calculated to be 1.618080 and 1.977653, respectively.

[0126] First, the refractive index n fl = 1.618080. At least one single layer coating film has a refractive index n fl and at least one of the materials has a refractive index n fl Consider replacing the film with a three-layer coating of three materials, each of which has a higher refractive index than the other.

[0127] FIG. 25 shows the refractive index n fl and the film thickness d fl is λ cot / (4n fl 25 is a schematic diagram showing a case where a single-layer coating film 241 having an effective refractive index n1 and a thickness d1, a three-layer coating film having an effective refractive index n2 and a thickness d2, and a three-layer coating film having an effective refractive index n3 and a thickness d3. c and wavelength λ cot The refractive index n formed on the end face of the quantum cascade laser device 211 fl (=1.618080) and the film thickness d fl = λ cot / (4n fl ) is replaced with a three-layer coating film consisting of a first coating film 242 with a refractive index n1 and a thickness d1, a second coating film 243 with a refractive index n2 and a thickness d2, and a third coating film 244 with a refractive index n3 and a thickness d3. If the single-layer coating film 241 can be replaced with a three-layer coating film, the characteristic matrices of both will be equal, as expressed by the following equation (23).

[0128] In equation (23), the phase terms φ1, φ2, and φ3 of each coating film are expressed as the following equation (24).

[0129]

[0130] In equation (23), the refractive indexes n1, n2, and n3 are known, so there are three unknowns: film thicknesses d1, d2, and d3. By solving equation (23), the film thickness of each coating film can be calculated.

[0131] wavelength λ cot = 10 μm, the first coating film 242 has a refractive index of n fl The second coating film 243 is YF3 having a refractive index n1 = 1.40 lower than that of the first coating film 241. fl The third coating film 244 is made of ZnS having a refractive index n = 2.20, which is higher than that of the first coating film 241. flWhen equation (23) is solved using CeF3 with n3 = 1.45, which is lower than the above, as an example, the thicknesses of the coating films are d1 = 640.642 nm, d2 = 206.894 nm, and d3 = 763.646 nm. The wavelength dependency of the reflectance in this case is shown by the dashed-dotted line 246 in Figure 26. The reflectance is 1.0% at a wavelength of 10 μm, but this value is not the minimum value, but the minimum value R of the reflectance of the three-layer coating film. min 0 is 0.9836%.

[0132] The minimum reflectance R of the single-layer coating film is 1.0%, while the minimum reflectance R of the triple-layer coating film is min 0 The reflectance of the triple-layer coating film is 0.9836%, which is lower than the minimum reflectance R of the single-layer coating film. min In order to achieve the desired minimum reflectance R0, the set reflectance R1 is set to a high value of 1.01648%. In this case, from equation (18), n fl 1 The film thickness of each coating film is calculated by solving the following equation (25), and the calculated values ​​are d1 = 641.123 nm, d2 = 205.474 nm, and d3 = 765.332 nm.

[0133]

[0134] The wavelength dependency of the reflectance in this case is shown by the two-dot chain line 247 in Figure 26. The minimum value R of the reflectance of the three-layer coating film min As the wavelength λ min = 9.864 μm, the desired minimum reflectance of 1.0% can be obtained.

[0135] Next, the wavelength λ at which the reflectance becomes minimum min the desired wavelength λ cot Each film thickness is set to λ cot / λ min Multiplying by 1, we get d1 = 649.962 nm, d2 = 208.307 nm, and d3 = 775.884 nm. The wavelength dependency of the reflectance in this case is shown by the solid line 248 in Figure 26. cot It can be seen that the minimum reflectance is 1.0% at the refractive index n fl and the film thickness is λcot / (4n fl 26 shows the wavelength dependence of reflectance for a single-layer coating film having a reflectance of 0.015 μm / cm. The solid line 248 and the dashed line 245 in FIG. 26 show almost the same wavelength dependence of reflectance, and the minimum reflectance and the wavelength at which the minimum reflectance occurs are completely the same.

[0136] Minimum reflectance R = 1.0% and wavelength λ cot The thickness of the first coating film 242 is 10 μm, and the thickness is not limited to this, and can be set to a desired value. Also, although YF3 is used for the first coating film 242, ZnS is used for the second coating film 243, and CeF3 is used for the third coating film 244, the thickness is not limited to these, and the thickness of the second coating film 243 is 10 μm. fl and n fl 1 and one or more materials with refractive indexes higher than n fl and n fl 1 The refractive index of the material can be determined by the refractive index of the material.

[0137] Similarly, n fh = 1.977653. At least one single-layer coating film has a refractive index n fh and at least one of the materials has a refractive index n fh Consider a three-layer coating of three materials, each of which has a higher refractive index than the other.

[0138] FIG. 27 shows the refractive index n fh and the film thickness d fh is λ cot / (4n fh 27 is a schematic diagram showing a case where the single-layer coating film 249 having an effective refractive index n1 and a thickness d1, a three-layer coating film having an effective refractive index n2 and a thickness d2, and a three-layer coating film having an effective refractive index n3 and a thickness d3, respectively. c and wavelength λ cot The refractive index n formed on the end face of the quantum cascade laser device 211 fh (=1.977653) and the film thickness d fh = λ cot / (4n fh) is replaced with a three-layer coating film consisting of a first coating film 250 with a refractive index n and a thickness d, a second coating film 251 with a refractive index n and a thickness d, and a third coating film 252 with a refractive index n and a thickness d. The method for replacing the single-layer coating film with the three-layer coating film is the same as in the case of the low refractive index described above, and is performed by solving the following equation (26).

[0139]

[0140] wavelength λ cot = 10 μm, the first coating film 250 has a refractive index of n fl The second coating film 251 is YF3 having a refractive index n1 = 1.40 lower than that of the first coating film 251. fl The third coating film 252 is made of ZnS, which has a refractive index n = 2.20 higher than that of the first coating film 251. fl Taking CeF3, which has n3 = 1.45, as an example, solving equation (26) gives d1 = 370.115 nm, d2 = 607.246 nm, and d3 = 395.897 nm, respectively. The wavelength dependency of reflectance in this case is shown by the dashed-dotted line 254 in Figure 28. The reflectance is 1.0% at a wavelength of 10 μm, but this value is not the minimum value, but the minimum value R of the reflectance of the three-layer coating film. min 0 is 0.94561%.

[0141] The minimum reflectance R of the single-layer coating film is 1.0%, while the minimum reflectance R of the triple-layer coating film is min 0 The reflectance of the triple-layer coating film is 0.94561%, which is lower than the minimum reflectance R of the single-layer coating film. min In order to achieve the desired minimum reflectance R0, the set reflectance R1 is set as high as 1.057485%. The refractive index n of the single-layer coating film to achieve the set reflectance R1 = 1.057485% is fh 1 is obtained from equation (21) as fh 1 When the thickness of each coating film is calculated by solving the following equation (27), the thicknesses are calculated to be d1 = 364.825 nm, d2 = 614.601 nm, and d3 = 390.009 nm, respectively.

[0142]

[0143] The wavelength dependency of the reflectance in this case is shown by the two-dot chain line 255 in Figure 28. The minimum value R of the reflectance of the three-layer coating film min is the wavelength λ min = 10.243 μm, the desired minimum reflectance of 1.0% can be obtained.

[0144] Next, the wavelength λ at which the reflectance becomes minimum min the desired wavelength λ cot The thickness of each coating film is set to λ cot / λ min Multiplying by 1, the calculations are d1 = 356.170 nm, d2 = 600.021 nm, and d3 = 380.757 nm. The wavelength dependency of the reflectance in this case is shown by the solid line 256 in Figure 28. Wavelength λ cot It can be seen that the reflectance reaches a minimum of 1.0% at this point.

[0145] The broken line 253 in FIG. fh and the film thickness is λ cot / (4n fh 28 shows the wavelength dependence of reflectance for a single-layer coating film having a reflectance of 0.01 to 0.01 mm. Although the solid line 256 and the dashed line 253 in FIG. 28 do not perfectly coincide, they show almost the same wavelength dependence of reflectance, and the minimum reflectance and the wavelength at which the minimum reflectance occurs are completely coincident.

[0146] Minimum reflectance R = 1.0% and wavelength λ cot The thickness of the first coating film 250 is 10 μm, and the thickness of the second coating film 251 is ZnS. The thickness of the third coating film 252 is 10 μm. The thickness of the first coating film 250 is 10 μm. The thickness of the second coating film 251 is ZnS. The thickness of the third coating film 252 is 10 μm ... fh and n fh 1 and one or more materials with refractive index higher than n fh and n fh 1 The refractive index of the material can be determined by the refractive index of the material.

[0147] <Effects of Fifth Embodiment> As described above, according to the method for designing a low-reflectance film according to the fifth embodiment, by replacing a single-layer coating film with a three-layer multilayer coating film by utilizing a characteristic matrix, it is possible to design a low-reflectance film in which the reflectance is controlled to a desired minimum at a desired wavelength.

[0148] In addition, as for a multilayer coating film of four or more layers made of four or more materials, it can be designed in the same way by setting the film thickness of the coating film of more than three layers in advance, as described in the first and second embodiments.

[0149] 29 is a schematic diagram showing a quantum cascade laser device 700 according to a sixth embodiment. The quantum cascade laser device 700 has a cavity length L and a ridge width W. The quantum cascade laser device 700 has a buried ridge type waveguide structure.

[0150] 29, the quantum cascade laser device 700 includes an n-type first electrode 153, a semi-insulating InP substrate 151, an n-type InP first cladding layer 3, an Fe-doped InP current blocking layer 4, an n-type GaInAs current injection layer 152, an n-type second electrode 5, a front end facet coating film 6, a rear end facet coating film 7, and a terahertz light extraction surface 8. FIG. 30 shows the quantum cascade laser device 700 in the y direction of FIG. a -y b FIG.

[0151] The quantum cascade laser apparatus 700 according to the sixth embodiment differs from the quantum cascade laser apparatus 500 according to the first embodiment in that a semi-insulating InP substrate 151 is used instead of the n-type InP substrate 2 constituting the quantum cascade laser apparatus 500 according to the first embodiment, that an n-type GaInAs current injection layer 152 is provided on the semi-insulating InP substrate 151 in order to inject current, and that an n-type first electrode 153 is provided on the n-type GaInAs current injection layer 152.

[0152] The semi-insulating InP substrate 151 has a smaller absorption of terahertz waves than the n-type InP substrate 2. Therefore, the quantum cascade laser device 700 can realize higher-power terahertz waves.

[0153] <Effects of Sixth Embodiment> As described above, according to the quantum cascade laser device of the sixth embodiment, a semi-insulating InP substrate is used as the semiconductor substrate, and therefore, an effect is achieved in that a quantum cascade laser device capable of realizing higher-output terahertz waves is obtained.

[0154] Seventh Embodiment Fig. 31 is a schematic diagram showing a quantum cascade laser device 800 according to a seventh embodiment. The quantum cascade laser device 800 has a cavity length L and a ridge width W. The quantum cascade laser device 800 has a buried ridge type waveguide structure. Fig. 32 shows the y-axis direction of the quantum cascade laser device 800 shown in Fig. 31. a -y b FIG.

[0155] The quantum cascade laser device 800 according to the seventh embodiment differs from the quantum cascade laser device 700 according to the sixth embodiment in that a Si substrate 154 is used instead of the semi-insulating InP substrate 151. The Si substrate 154 has a lower absorption of terahertz waves than an n-type InP substrate. This makes it possible to obtain high-power terahertz waves. The Si substrate 154 and the n-type GaInAs current injection layer 152 are bonded together by lamination.

[0156] <Effects of Seventh Embodiment> As described above, according to the quantum cascade laser device of the seventh embodiment, since a Si substrate is used as the semiconductor substrate, it is possible to obtain an effect of obtaining a quantum cascade laser device that can realize higher-power terahertz waves.

[0157] Eighth Embodiment. Figures 33A to 33H are diagrams showing the steps of a method for manufacturing a quantum cascade laser device according to the first to seventh embodiments. The method for manufacturing a quantum cascade laser device will be described with reference to the drawings. (1) A wafer 161 that has undergone wafer processing is prepared (Figure 33A). (2) The wafer 161 is divided into rectangular blocks, and cleaved to produce cleaved bars 162 (Figure 33B). (3) Power is supplied to each element of the quantum cascade laser device using a probe 163 or the like, and the gain distribution (wavelength dependence of gain) is measured (Figure 33C). (4) Based on the measured gain distribution, a front facet coating film 6 that provides a predetermined minimum reflectance at a desired wavelength is designed (Figure 33D). (5) The front facet is coated with the front facet coating film 6 made of the multilayer coating film designed in (4) above, and the rear facet is coated with a high-reflectivity rear facet coating film 7 (Figure 33E). (6) The front end face of the quantum cascade laser device is polished to form a terahertz light extraction surface 8 so that terahertz waves can be extracted (FIG. 33F). (7) The device is divided into individual quantum cascade laser device 74 elements (FIG. 33G). (8) After bonding the quantum cascade laser device 74, submount 73, and metal block 72, the device is mounted on a temperature adjustment device 71 such as a Peltier element (FIG. 33H).

[0158] In the eighth embodiment, polishing for extracting terahertz waves is performed after end face coating, but polishing may also be performed after cleavage.

[0159] <Effects of embodiment 8> As described above, according to the method for manufacturing a quantum cascade laser device according to embodiment 8, the gain distribution of the quantum cascade laser device is measured in advance, and the minimum reflectivity is designed based on the measured gain distribution, and then the front end face coating film (low reflectivity film) is coated, thereby achieving the effect that a quantum cascade laser device that can easily achieve terahertz oscillation can be easily manufactured with a high yield.

[0160] Although the present disclosure describes various exemplary embodiments and examples, the various features, aspects, and functions described in one or more embodiments are not limited to application to a particular embodiment, but may be applied to the embodiments alone or in various combinations.

[0161] Therefore, countless variations not illustrated are conceivable within the scope of the technology of the present disclosure, including, for example, cases where at least one component is modified, added, or omitted, and cases where at least one component is extracted and combined with a component of another embodiment.

[0162] 1, 153 n-type first electrode, 2 n-type InP substrate, 3 n-type InP first cladding layer, 4 Fe-doped InP current blocking layer, 5 n-type second electrode, 6 front end facet coating film, 7 rear end facet coating film, 8 terahertz light extraction surface, 9 n-type GaInAs first optical confinement layer, 10 core region, 40, 106, 136 stage, 38, 104, 134 active region, 39, 105, 135 injector region, 11 n-type GaInAs second optical confinement layer, 12 n-type InP second cladding layer, 13 n-type GaInAs contact layer, 21, 23, 25, 27, 29, 31, 35, 37, 81, 83, 85, 87, 89, 91, 93, 99, 101, 103, 111, 113, 115, 117, 119, 121, 123, 125, 131, 133 Undoped AlInAs barrier layer, 22, 24, 26, 28, 30, 36, 82, 84, 86, 88, 90, 92, 94, 100, 102, 112, 114, 116, 118, 120, 122, 124, 126, 132 Undoped GaInAs well layer, 32, 34, 96, 98, 128, 130 N-type GaInAs well layer, 33, 95, 97, 127, 129 N-type AlInAs barrier layer, 42, 212, 231, 241, 249 Single layer coating film, 43, 213, 232, 242, 250 First coating film, 44, 214, 233, 243, 251 Second coating film, 45, 215, 234, 244, 252 Third coating film, 46, 141 Coating film, 47, 47a, 48a, 142, 222, 222a, 236, 246, 254 Dotted chain line, 48, 142a, 143, 143a, 223, 223a, 237, 247, 255 Two-dot chain lines, 49, 49a, 52, 62, 144, 144a, 146, 224, 224a, 238, 248, 256 Solid lines, 50, 50a, 50b, 221, 221a, 221b, 235, 245, 253 Dashed lines, 51, 61, 145 Dotted lines, 41, 74, 211, 500, 600, 700, 800 Quantum cascade laser device, 71 Temperature adjustment device, 72 Metal block, 73 Submount, 151 Semi-insulating InP substrate, 152 n-type GaInAs current injection layer, 154 Si substrate, 161 Wafer, 162 Cleaved bar, 163 Probe

Claims

1. A semiconductor device comprising: a semiconductor substrate; a first cladding layer of a first conductivity type formed on the semiconductor substrate; a core region formed on the first cladding layer, the core region including a plurality of stages each including an active region in which a barrier layer and a well layer are alternately formed, and an injector region for injecting electrons into the active region; and a second cladding layer of the first conductivity type formed on the core region, the resonator length being L, and a wavelength λ qcl The gain is maximum at Δλ, and the wavelength width between two points where the gain value is lower than the maximum gain value by a preset percentage is defined as qcl In this case, the wavelength λ qcl λ is a wavelength that is a preset wavelength away from cot A low reflectance film is provided at the end surface of the other end surface, and the reflectance of the other end surface R r and Δλ is the wavelength width between two points where a mirror loss value occurs when a preset loss value is subtracted from the maximum mirror loss value calculated by L. mir In this case, the Δλ mir is the above Δλ qcl The quantum cascade laser device is characterized in that 2. The quantum cascade laser device according to claim 1, characterized in that a contact layer of a first conductivity type is provided on the second cladding layer, a first electrode of the first conductivity type is provided on the back side of the semiconductor substrate, and a second electrode of the first conductivity type is provided on the contact layer.

3. The quantum cascade laser device according to claim 1 or 2, wherein the semiconductor substrate is a first conductivity type semiconductor substrate, a semi-insulating semiconductor substrate, or a silicon substrate.

4. A quantum cascade laser device according to any one of claims 1 to 3, characterized in that the preset percentage of gain value is a gain value that is 10% of the maximum gain value.

5. A quantum cascade laser device according to any one of claims 1 to 4, characterized in that it is mounted on a temperature adjustment device.

6. A quantum cascade laser device according to any one of claims 1 to 5, characterized in that electrons transition between two upper energy levels and one lower energy level in the active region.

7. A quantum cascade laser device according to any one of claims 1 to 5, characterized in that electrons transition between two upper energy levels and a plurality of lower energy levels within said active region.

8. A method for manufacturing a quantum cascade laser device as claimed in any one of claims 1 to 7, comprising the steps of: cleaving a wafer after completion of a wafer process to create a cleavage bar; measuring the wavelength dependence of the gain of the quantum cascade laser device by driving the cleavage bar to obtain the wavelength and gain band at which the gain is maximum; designing a low-reflectance film having a minimum reflectance at a wavelength a preset wavelength away from the wavelength at which the gain is maximum; and forming the low-reflectance film on the front end face of the quantum cascade laser device.

9. The method for manufacturing a quantum cascade laser device according to claim 8, further comprising the steps of: polishing an end face of the semiconductor substrate of the quantum cascade laser device; and separating the cleavage bars into individual elements.

10. The method for manufacturing a quantum cascade laser device according to claim 9, further comprising the step of bonding the element to a temperature adjustment device via a submount and a metal block.

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