JP2008010733A - Quantum cascade laser - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To realize a quantum cascade laser oscillating in the far-infrared region or the terahertz region; particularly to realize a large gain by providing a basic structure for injecting/extracting carriers in a laser active layer with efficiency; and furthermore to realize a low threshold value, a continuous oscillating operation, and a high temperature operation by preventing thermal backfilling of carriers. <P>SOLUTION: An active layer having a laser level of a four level system is composed of a carrier injection layer, a gain layer, and a carrier extraction layer; and population inversion is improved by using layers with higher-speed relaxation than the gain layer as the carrier injection layer and the carrier extraction layer. The high-speed relaxation in the layers is mainly relaxation by making relaxation level differences in the carrier extraction layer and levels of optical phonon uniform, or relaxation by causing stimulated emission with a laser from the outside. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a semiconductor laser that oscillates in the far-infrared region or the frequency region of the terahertz region (several tens to several hundreds μm), and in particular, utilizes intersubband transition in a multiple quantum well structure fabricated on a semiconductor substrate. Quantum cascade laser.

  Conventionally, the frequency range from the mid-infrared region (5-12 μm) to the far-infrared region or terahertz region (tens to hundreds of μm) is security, environmental monitoring, biomedical diagnosis, deleterious substances / poisonous detection, wireless Although it was an important frequency band in communications and spectroscopy, its development was delayed compared to other areas. This is because a compact light source such as the interband transition semiconductor laser used in the optical communication network has not existed so far.

  Recently, however, semiconductor lasers using intersubband transitions in semiconductor quantum structures, so-called quantum cascade lasers, have been developed (Non-Patent Document 1). This is because laminating semiconductor thin films with different bandgap energies and combining the carrier injection / extraction region and the gain region in multiple stages, thereby lasing from intersubband transitions with a process energy interval corresponding to the desired emission wavelength. It is a semiconductor optical device that realizes An example of this is shown in FIG. In this device, the emission wavelength is determined by the design parameters such as the film thickness of the semiconductor thin film, regardless of the physical property parameters specific to the constituent material, and thus the emission wavelength can be freely designed (within the energy barrier height). It has the characteristics. Therefore, it is possible to realize a compact light source that covers from the mid-infrared region to the terahertz region.

  However, in particular, the conventional terahertz band quantum cascade laser is realized under a low temperature or under a pulse drive, but has not obtained good device characteristics like a near-infrared quantum cascade laser. This is because the conventional quantum cascade structure has a so-called three-level system (FIG. 4 (b)) in which the energy levels are arranged. In a high carrier injection state and at a high temperature operation, the threshold rise and carrier backfilling ( One reason is that laser oscillation itself becomes difficult due to the influence of back-filling.

As a countermeasure against high temperature operation of the terahertz band quantum cascade laser, there is an example in which high temperature operation is realized by providing a high reflection film on the laser end face and reducing the threshold value. In addition, there is an example (for example, Non-Patent Document 2) that realizes a high-temperature operation by simultaneously manifesting a high light confinement rate and efficient heat dissipation by narrowing the laser active layer with metal. However, neither example is an essential improvement in laser characteristics.
Science, vol. 264, 553 (1994) Applied Physics Letters, vol. 83, 5142 (2003).

  An object of the present invention is to realize a semiconductor laser that oscillates in the far-infrared region or the frequency region of the terahertz region (several tens to several hundreds μm). In particular, the carrier injection and extraction in the laser active layer is efficient. The goal is to provide a basic structure that is often used. This is to realize a quantum cascade laser capable of operating at a high temperature by realizing a low gain and continuous oscillation operation by realizing a large gain and further suppressing back-filling of carriers due to heat.

  According to the present invention, in the quantum cascade laser, it is possible to reduce the high optical gain and the backfilling effect by using the active layer with high-speed relaxation before and after the gain layer. Therefore, it is possible to manufacture a quantum cascade laser that operates continuously at a high temperature.

  The present invention is a kind of a multiple quantum well semiconductor laser, which is a quantum cascade laser that oscillates by transition between subbands, and an active layer having a four-level laser level includes a carrier injection layer, a gain region And a carrier extraction layer, and a carrier injection layer and a carrier extraction layer that use a layer with relaxation that is faster than the above-described gain region, so that an inversion distribution can be easily realized, thereby achieving a high optical gain. It is realized.

  In the above quantum cascade laser, high-speed relaxation is realized by configuring so that high-speed relaxation in the carrier injection layer or carrier extraction layer is mainly caused by optical phonon scattering.

  Also, in the active layer of the above quantum cascade laser, a layer in which high-speed relaxation is mainly caused by intersubband transition due to stimulated emission is used in the carrier injection layer or carrier extraction layer, thereby realizing high-speed relaxation. It is.

  In addition, in the carrier injection layer or the carrier extraction layer, the energy difference relaxed in the carrier injection layer or the carrier extraction layer is equal to or higher than the energy level of the optical phonon that causes scattering, so that the optical phonon relaxes at high speed. Is realized.

  In addition, by using a carrier injection layer and a carrier extraction layer having a subband energy level that causes stimulated radiation with light of the same wavelength, one optical system for stimulated radiation can be provided.

  In addition, since stimulated radiation does not necessarily lead to laser oscillation, it is desirable to enter laser light that causes stimulated radiation from the outside.

  Furthermore, since the laser light that induces the stimulated emission can be confined in the resonator to enhance the electric field of the light, it is desirable to install a quantum cascade laser in the resonator.

  In addition, by providing a reflection structure on the end face of the semiconductor laser, the number of resonators used can be reduced in appearance.

  In the present invention, the above problem is solved by adopting an active layer structure as shown in FIG. That is, in the active layer of the quantum cascade laser, the problem is solved by providing a carrier injection / extraction layer having a very short relaxation time at least one step before and after the gain region.

  The arrangement of the engineering energy levels corresponding to this structure is as shown in FIG. 1B, forming a four-level system. In this case, in the carrier injection part immediately before the gain region, carriers are preferentially accumulated at the laser upper level, so that the carrier density at the laser upper level can be increased. On the other hand, in the carrier extraction portion immediately after the gain region, carriers are preferentially removed from the laser level level, so that the carrier density at the lower level of the laser can always be lowered. As a result, a large difference in injected carrier density can be provided between the upper and lower laser levels, that is, a large inversion distribution can be formed, so that a large optical gain can be realized.

  Furthermore, in general, when the element temperature of a quantum scale laser rises, so-called carrier backfilling phenomenon occurs, in which carriers in the carrier injection layer are excited to the lower laser level of the previous stage by the thermal excitation of the carriers. The degree of distribution becomes small and laser oscillation becomes difficult. However, according to this method, when one unit of the active layer is cascade-connected, a large energy difference between the upper laser level and the previous laser lower level can be obtained, so that carrier backfilling can be suppressed.

  Due to the characteristics of the above two points (high optical gain, suppression of backfilling), this method allows the quantum cascade laser to have a robust structure with respect to temperature, enabling low threshold, continuous oscillation operation, and high temperature operation. To.

  Embodiments of the present invention will be described below in detail with reference to the drawings. In the following description, devices having the same function or similar functions are denoted by the same reference numerals unless there is a special reason.

  In the example given here, a quantum cascade laser that oscillates on a GaAs substrate at about 90 μm is shown, but other materials such as InP, nitride, and Si / Ge have the same energy band structure. If so, application to a laser using these materials is not precluded. This is because, as described above, the quantum cascade laser is characterized in that the emission wavelength is determined by the design parameters such as the film thickness of the semiconductor thin film, regardless of the physical property parameters inherent to the constituent materials.

  FIG. 2 shows an embodiment. This embodiment is fabricated by the MBE growth method or the like. On the semi-insulating GaAs substrate 1, a lower n-GaAs contact layer 2 (0.8 μm thick), an active layer 4 (carrier injection layer, gain region, carrier) The upper n-GaAs contact layer 5 (0.1 μm thick). The wirings 3 and 6 are connected to the lower n-GaAs contact layer 2 and the upper n-GaAs contact layer 5, respectively.

The active layer repeats the periodic structure as shown in FIG. The configuration within one cycle is
i. Al-Ga _x Ga _1-x As (3.0 nm thickness),
b. i-GaAs layer (16.5 nm thick),
c. i-Al _x Ga _1-x As (x = 0.3) layer (1.0 nm thick),
d. i-GaAs layer (9.5 nm thick),
_{e. i-Al x Ga 1} -x As (x = 0.3) layer (3.6 nm thick),
f. i-GaAs layer (8.7 nm thick),
_{g. i-Al x Ga 1} -x As (x = 0.3) layer (1.4 nm thick),
h. i-GaAs layer (7.0 nm thick),
i. i-Al _x Ga _1-x As (x = 0.3) layer (2.0 nm thick),
j. i-GaAs layer (15.2 nm thick),
_{k. i-Al x Ga 1} -x As (x = 0.3) layer (1.0 nm thick),
l. i-GaAs layer (0.6 nm thick),
m. n-GaAs layer (9.0 nm thick),
_{n. i-Al x Ga 1} -x As (x = 0.3) layer (2.0 nm thick),
o. n-GaAs layer (8.4 nm thick),
It is. The e layer is the boundary between the carrier injection layer and the gain region, and the i layer is the boundary between the gain region and the carrier extraction layer.

  Here, the current injection layer and the current extraction layer utilize fast relaxation by optical phonon, and in the case of GaAs of this time, the transition energy is about 36 meV. Here, in order to perform fast relaxation of the current injection layer or the current extraction layer mainly by scattering with the optical phonon, the transition energy and the energy level of the optical phonon are matched or the transition energy is slightly lower than the energy level of the optical phonon. Just make it bigger.

  In addition to the above, as long as the quantum cascade laser structure uses a structure in which the gain layer is sandwiched between current injection / extraction layers with fast relaxation, other structures may be used.

  FIG. 3 uses stimulated radiation to promote relaxation due to the transition from level 4 to level 3 or the transition from level 2 to level 1 in FIG. Although the quantum cascade laser element 10 is described here as having no reflecting structure at the end, the quantum cascade laser element 10 is a first mirror made up of a reflecting mirror 11 and a reflecting mirror 13 that is slightly transmissive. The laser is oscillated by being stimulated by the transition from the level 3 to the level 2 in FIG. 1B. Here, the reflecting mirror 12 has wavelength selectivity such that the laser beam generated by the laser oscillation described above is transmitted, but the other light is reflected. The reflecting mirror 14 has a wavelength selectivity for transmitting the laser light from the laser 18 but reflecting other light. At this time, the quantum cascade laser element 10 includes a second resonator made up of a reflecting mirror 15 and a reflecting mirror 15 that is slightly transmissive, and a reflecting mirror 17 that is made slightly transparent to the reflecting mirror 11. 3 is also installed in the resonator. Laser light from the laser device 16 is input to the second resonator, and the transition by the induced radiation from the level 4 to the level 3 in FIG. Similarly, the laser beam from the laser device 17 is input to the third resonator, and the transition by the induced radiation from the level 2 to the level 1 in FIG. 1B is promoted. As a matter of course, the wavelength of the laser light to be irradiated is equal to the wavelength of the light by the induced radiation. Further, it is desirable that the optical paths of the first to third resonators are made to coincide within the quantum cascade laser element 10. In addition, since the level related to the stimulated radiation has temperature dependence, it is desirable that the laser devices 16 and 18 be laser devices capable of adjusting the wavelength.

  Further, by setting the transition wavelength from the level 4 to 3 and the transition wavelength from the level 2 to 1 to be equal, either the second or third resonator and the laser associated therewith are set. The device can be omitted.

  Here, the reflecting mirror 11 can be omitted in appearance by forming a reflecting structure using a metal film or a multilayer dielectric film on the end face of the quantum cascade laser element 10.

  It is easy to apply the configuration shown in FIG. 3 to a conventional three-level quantum cascade laser. In this case, it is possible to promote the transition by the stimulated emission from the level 2 to the level 1 in FIG. 4 by omitting the reflecting mirrors 14 and 15 and the laser device 16, and as a result, the inversion distribution is made clearer. Thus, it can be expected that the laser oscillation threshold is reduced or the output is increased.

FIG. 2 is an arrangement diagram of (a) an active layer structure and (b) energy levels of the present invention. It is sectional drawing of the quantum cascade laser element of this invention. 1 is a block diagram illustrating a quantum cascade laser with improved relaxation using stimulated emission. FIG. It is an arrangement diagram of (a) active layer structure and (b) energy level of a general quantum cascade laser.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Semi-insulating GaAs substrate 2 Lower n-GaAs contact layer 3 Wiring 4 Active layer 5 Upper n-GaAs contact layer 6 Wiring 10 Quantum cascade laser element 11, 12, 13, 14, 15 Reflective mirror 16 Laser apparatus 17 Reflective mirror 18 Laser equipment

Claims (8)

  In a multiple quantum well semiconductor laser that oscillates by transition between subbands, an active layer having a four-level laser level includes a carrier injection layer, a gain region, and a carrier extraction layer. A quantum cascade laser characterized in that a high optical gain is realized by using a layer with relaxation at a higher speed than the above gain region.   2. The quantum cascade laser according to claim 1, wherein high-speed relaxation in the carrier injection layer or the carrier extraction layer is mainly caused by optical phonon scattering.   2. The quantum cascade laser according to claim 1, wherein a layer in which high-speed relaxation is mainly caused by intersubband transition by stimulated emission is used for the carrier injection layer or the carrier extraction layer.   The energy difference relaxed in the carrier injection layer or the carrier extraction layer in the carrier injection layer or the carrier extraction layer is equal to or higher than the energy level of the optical phonon that causes the scattering. Quantum cascade laser.   4. The quantum cascade laser according to claim 3, further comprising an upper carrier injection layer and a carrier extraction layer having a subband energy level that causes stimulated emission with light of the same wavelength.   6. The quantum cascade laser according to claim 3, wherein stimulated emission is caused by light irradiated from outside.   6. The quantum cascade laser according to claim 3, wherein the quantum cascade laser is installed inside a light resonator that causes stimulated emission. 8. The quantum cascade laser according to claim 7, wherein a resonator is configured by providing a reflection structure on an end face of the semiconductor laser.
NICT-05-80 1/8

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