JP6829517B2 - Infrared light element - Google Patents

Description

The present invention relates to an infrared light element. More specifically, the present invention relates to an infrared optical element that radiates or detects infrared rays having a wavelength of 10 μm or more and electromagnetic waves in the THz frequency region used for spectroscopic measurement of organic molecules such as proteins, wireless LAN, medical treatment, and the like.

Existing THz region lasers include quantum cascade lasers (QCL) and resonance tunnel diode (RTD) lasers. In QCL, it is generally necessary to lower the operating temperature as the oscillation frequency decreases. For example, it has been reported that the QCL operates at a maximum temperature of 163 K at 1.8 THz (see, for example, Non-Patent Document 1 below).

As for measures against temperature increase, there is a report that oscillation was performed at room temperature due to the generation of a difference frequency resonating with the quantum level (see Non-Patent Document 2 below).

Further, Non-Patent Document 3 below reports that the frequency of laser oscillation at room temperature by the fundamental wave is 1.46 THz in RTD.

S. Kumar et al. , Nat. Phys. 7,166 (2011) Q. Lu, Nat. Sci. Rep. 6, 23595 (2016) K. Morita et al. , Apple. Phys. Express 4,102102 (2011)

However, the technique described in Non-Patent Document 2 below has a problem that its quantum efficiency is low. The output is 2.06 μm and is only about 0.6 μW.

Further, in the technique of Non-Patent Document 3, it is necessary to take harmonics at frequencies higher than this, and there is a problem that the output drops sharply. Most of the non-linear optical crystals are generated by generating a difference frequency by an ultrashort pulse of about 1 ps or less, and an optical system for generating an ultrashort pulse laser beam is required, so that the apparatus becomes large. The continuous light is generated by using two diode lasers (LD) and a double resonator structure, but its output power is small.

Further, in the high electron mobility transistor (HEMT), the output is currently extinguished at about 200 GHz, and operation above 1 THz has not been confirmed.

On the other hand, some phonon-based electromagnetic wave radiation and lasers are based on phonon generation using the piezo effect associated with the input of an external electric field, and are lasers based on a three-level system combined with a vacuum level using two types of phonon modes. Oscillation has been confirmed. The oscillation frequency due to this structure is about 170 KHz (I. Mahboob et al. Phys. Rev. Lett., 110, 127202 (2013)), and does not reach THz. The phonon mode used in this operating principle is an acoustic phonon, not an optical phonon having a strong coupling with light, and therefore it seems difficult to obtain highly efficient oscillation.

From these situations, a new operating principle is required for the compact laser operation of room temperature operation in the range of several THz to 20 THz.

In the detector, an element capable of selectively detecting the light absorption frequency peculiar to a molecule or a crystal is required for chemical analysis and substance identification. The photodetectors in the far-infrared and THz wave regions so far have been based on the pyroelectric effect and Schottky diodes, and their detection spectrum range has been large.

Therefore, in view of the above problems, the present invention obtains a compact THz light source that operates at room temperature by a structure capable of utilizing LO phonons having a strong interaction with light, and further has a structure in which an optical gain is generated in a phonon system. The purpose is to obtain a laser beam. Another object of the THz detector is to obtain an operating principle and an element structure capable of detecting light in a narrow band according to a light absorption wavelength peculiar to a molecule or a crystal to be measured.

In the infrared optical element according to the first aspect of the present invention that solves the above problems, striped, lattice-shaped, or annular conductors are formed on a single-layer substrate of a semiconductor or an insulator, and the intervals are vertical. optical (LO) Ri der 1/2 wavelength or less of the infrared light that resonates with the phonon energy, the substrate has two or more kinds of LO phonon mode, the LO phonon mode energy conduction band or the valence band in , Or it is included in the transition energy region between the conduction band or the valence band, and can emit or detect infrared light that resonates with the mode by controlling the light absorption spectrum by electromagnetically induced transparency .

Further, in the infrared optical element according to the second aspect of the present invention, a striped, lattice-shaped or annular conductor is formed on a single-layer substrate of a semiconductor or an insulator, and the interval between them is LOphonon-. Ri der half wavelength or less of the infrared light that resonates in the binding (LOPC) mode energy of plasmon, the substrate has two or more LOPC mode, the LOPC mode energy conduction band or the valence band in , Or a transition energy region between the conduction band or the valence band, which controls the light absorption spectrum by electromagnetically induced transparency to emit or detect infrared light that resonates with the mode.

Further, in the above-described first and second aspects infrared element in, but not limited, it is preferred substrate is n-type or p-type semiconductor.

Further, the infrared optical element from the first and second viewpoints is not limited, but when the substrate is a p-type semiconductor, it has acceptability to form a deeper level in addition to the p-type impurity. When the substrate is an n-type semiconductor, it is a semiconductor having a donor-like impurity that forms a deeper level in addition to the n-type impurity, and has an energy smaller than the forbidden band width. It is preferable that the light absorption spectrum is modulated and operated by introducing a laser.

Further, in the infrared optical element from the first and second viewpoints, the light absorption spectrum can be modulated by introducing a laser that excites between bands of the valence band and the conduction band, although not limited to the above. It is preferable that it is a thing.

Further, in the infrared photon elements from the first and second viewpoints, a single infrared photon accompanied by a quantum entanglement phenomenon by inputting pulsed light having a time width of 1 ps or less is not limited. It is preferable that the light is radiated.

Further, in the infrared optical device from the first and second viewpoints, although not limited to, a semiconductor having high conductivity or other material is used instead of the metal at the interface with the semiconductor having low conductivity. It is preferable that the structure has a structure capable of shielding the electric field generated by the generated polarization charge.

Further, in the infrared optical device from the first and second viewpoints, although not limited to the above, a metal having an ohmic connection and a metal having a Schottky connection are formed as a pair. It is preferable to have.

As described above, according to the present invention, a compact THz light source that operates at room temperature is obtained by a structure that can utilize LO phonons having a strong interaction with light, and a laser beam is obtained by an element having a structure in which an optical gain is generated in a phonon system. be able to. Further, in the THz detector, it is possible to obtain an operating principle and an element structure capable of detecting light in a narrow band according to the light absorption wavelength peculiar to the molecule or crystal to be measured.

It is a figure which shows the example of the metal / semiconductor composite structure which generates radiation. The hatched part indicates metal, and the others indicate semiconductor. It is a figure which shows the high (+) low (−) energy branch of the binding (LOPC) mode of LO phonon and plasmon. It is a figure which shows the relationship of a phonon process, an electronic transition, and an optical transition in a quantum interference effect. It is a figure which shows the control example of the light absorption spectrum by Zn-doped p-type Ga _0.5 In _0.5 P quantum interference. There are doping conditions where the light absorption spectrum is almost zero at the phonon energy positions of LO ₁ and LO ₂ . The Zn densities are 2.5 × 10 ¹⁹ cm ^-3 and 3.5 × 10 ¹⁹ cm ^-3 , respectively. It is a figure which shows the example of the laser element which attached the resonator. It is a figure which shows the example of the infrared / THz detection element.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. However, the present invention can be implemented in many different embodiments, and is not limited to the specific examples of the embodiments and examples shown below.

(Embodiment 1)
FIG. 1 is a diagram showing a structure of an infrared light element according to the present embodiment.

As shown in this figure, in the infrared light element (hereinafter referred to as “the element”) according to the present embodiment, a striped, lattice-shaped or annular conductor is formed on a substrate of a semiconductor or an insulator. The interval is ½ wavelength or less of infrared light that resonates with longitudinal optical (LO) phonon energy, and infrared light that resonates with the mode can be emitted or detected. Here, the wavelength of infrared light is not particularly limited, but is preferably a wavelength of 10 μm or more and includes a wavelength of THz, and more specifically, a wavelength of 10 μm or more and 1 mm or less. The wavelength is preferably 500 μm or less. Therefore, the wavelength range of about 10 μm can be suppressed by setting the distance between the conductors to 5 μm or less. Specifically, the effect of this device can be exhibited by setting the distance between the conductors to a range of about 4 μm or more and 600 μm or less with a margin.

In this device, when laser light is introduced into this structure, LO phonons are generated by Raman scattering. In this device, a polarization charge is generated at the interface between the conductor and the semiconductor, and the radiation that resonates with the LO phonon can be generated by the polarization charge vibration.

In this device, the conductor is not limited as long as it can conduct electricity, but for example, a metal such as gold, copper, or titanium, and a high density n having an electron density of 10 ¹⁹ cm- ³ or more. Type semiconductors can be exemplified, but are not limited thereto. When this element is a light emitting element that emits light, it is not always necessary that the entire conductor is connected to the ground or the like and is conducting. The thickness of the conductor is not particularly limited, and is preferably 1 μm or more.

Further, in this element, the substrate is a semiconductor or an insulator as described above. The semiconductor is not particularly limited, and examples thereof include semiconductors such as Si, GaN, GaAs, and InP. Examples of the insulator include sapphire and SiO ₂ , and in these insulators and semiconductors, it is desirable that the photon energy of the laser beam that induces the vibration of the crystal lattice is about the band gap. Further, when the device is to be excited by heat, it is possible to heat the substrate itself at a temperature of about 200 ° C., and it is possible to superheat by a superheating means such as a heater. In the case of this material, for example, an AlGaN-based crystal is preferable because it can be used at a higher temperature.

Further, as is clear from the above description and the description described later, it is preferable that the semiconductor is doped with impurities. A p-type semiconductor makes it easier to exert a quantum interference effect, and an n-type semiconductor makes it possible to shift the wavelength.

When this device is used as a detector, two different materials must be used, and the bonding between these materials and the semiconductor must be ohmic bonding on one side and Schottky bonding on the other side. However, excitation light with photon energy larger than the forbidden bandgap of semiconductors is required. The reason why the voltage is applied by the detector is that the bias voltage is applied for the purpose of reducing the dark current. That is, when used as a detector, two different materials are arranged at the desired intervals, and a power supply device for applying a voltage is connected to each of them. On the other hand, when exciting light, it does not necessarily have to be two different materials, and it is not necessary to connect a power supply device.

The LO phonon constitutes a two-level system of the phonon excited state and the vacuum state. In principle, population inversion cannot be achieved in a two-level system. Here, by constructing a system in which two or more phonon modes and continuous levels of the electronic system exist, a coupling system of the phonon mode and the continuous levels is formed, and the emission spectrum peak wavelength and the absorption spectrum peak wavelength are shifted. This is possible, and the structure is such that this electromagnetically induced transparency is realized, an optical gain is obtained, and laser oscillation is possible. In addition, it is possible to shift the resonance energy from the LO phonon mode energy by using the high and low energy branches of the LOPC mode (FIG. 2) of LO phonon and plasmon. It is also possible to construct a three-level system by combining this high-low LOPC mode and the vacuum level, and an energy system capable of generating laser light is constructed regardless of the presence or absence of the electromagnetically induced transparency mechanism. .. In electromagnetically induced transparency, the light absorption spectrum can be controlled by controlling the addition density of p-type or n-type impurities, that is, the hole density in the valence band. Further, the change in the light absorption spectrum due to the frequency modulation in the LOPC mode and the electromagnetically induced transparency can be controlled by introducing an external laser which is an external signal.

The narrow band infrared and THz wave detectors form the striped, grid, and concentric electrode structures seen in FIG. 1, and one of the paired electrodes is ohmic connected to the semiconductor. , The other is a shotkey connection, and by irradiating light that performs interband transition of electrons, electrons are generated in the conduction band and holes are generated in the valence band, and infrared light or THz light that resonates with LOphonon or LOPC mode. The current can be increased during irradiation, and it can be operated as a light detector.

By forming a conductor such as a metal thin film into a lattice shape, a stripe shape, or an annular shape, a polarization charge having a sign opposite to that of the opposing semiconductor / metal interface is generated to form an electric dipole. A metal or a substance with high conductivity shields the electric field formed by the generated electric dipoles and reduces the cancellation of the electric fields due to the interaction between the electric dipoles. As a result, the electric dipole can be significantly present, and the electric dipole radiation is performed.

If this semiconductor is p-type, there is a valence band transition of holes. A model of this valence band transition is shown in FIG. The LO phonon mode of a semiconductor is coupled with this valence band transition, but if there are two types of LO phonon modes, the optical transition causes a destructive interference effect due to the coupling between the LO phonon mode and the valence band. An example of this result is shown in FIG. As a result, it becomes possible to control the light absorption spectrum. If the overlap between the radiation spectrum and the absorption spectrum becomes small, the radiation is emitted to the outside without being absorbed by the semiconductor.

Further, by providing the resonator structure as shown in FIG. 5, laser oscillation of LO phonon resonance becomes possible. Specifically, in the example of this figure, the device is sandwiched between laminated films such as DBR (Distributed Bag Reflector), while light is incident on the device from an excitation laser to extract THz laser light. In this case, a part of the light emitted from the resonator is fed back and re-entered into the excitation laser for re-excitation, and becomes the light from the excitation laser. Further, the present device also includes, for example, a laser for light absorption light modulation, and can perform modulation on a THz laser. As a result, surface emission is possible.

Further, when a semiconductor material doped with an impurity that forms a level having a deep acceptability is adopted, the hole density can be increased by photoexcitation to this level. This is possible because the excited electrons are energy relaxed to a state where the probability of radiative recombination with the valence band holes is small. By controlling the laser intensity of this hole excitation, the hole density can be changed to modulate the absorption spectrum. The conduction electrons due to the transition between the conduction band valence bands can generate a LOPC state and modulate the radiation / absorption peak energy position. High-speed modulation of the absorption spectrum of infrared / THz waves by controlling the input / non-input of the laser beam that generates the holes by increasing the recombination probability of electrons and valence band holes existing in the deep level. , And high speed modulation of the laser output can be performed.

When generating LO phonons with an ultrashort pulse laser, multiple LO phonon modes are coherently generated at the same time, and through continuous levels, these modes maintain their coupled state for a period of time. Photons emitted within this time will be emitted from the entangled state of multiple LO modes. Using this photon, it becomes possible to perform wireless optical communication with a high security function.

In narrow-band infrared light and THz wave detectors, light causes interband excitation to generate electrons in the conduction band and holes in the valence band. The electric field that resonates with the LO phonon incident between the metal electrodes that have made the ohmic connection and the shotkey connection as described above vibrates the generated electrons and holes with a large amplitude. By adopting the electrode structure having rectification property, this charge vibration generates a current in a certain direction. As a result, it operates as a detector (Fig. 6). As shown in this figure, in this element, the above-mentioned desired effect is obtained by arranging the conductors of the ohmic electrode and the Schottky electrode on the semiconductor or the insulator and forming an electric field between them by the power supply device. be able to.

Further, as an application, this device can be used as a waveguide by exciting polariton on the surface of a conductor. In this case, the thickness of the dielectric or the insulator is made larger than 0.1 μm, while if it is thicker than 2 μm, the speed is reduced, so it is preferable to make it thinner than this.

As described above, this element obtains a compact THz light source that operates at room temperature by a structure that can utilize LO phonons that strongly interact with light, and further obtains laser light by an element that has a structure that generates optical gain in a phonon system. be able to. Further, in the THz detector, it is possible to obtain an operating principle and an element structure capable of detecting light in a narrow band according to the light absorption wavelength peculiar to the molecule or crystal to be measured.

The present invention has industrial applicability as an infrared optical device.

Claims (8)

Striped, lattice-shaped or annular conductors are formed on a single-layer substrate of a semiconductor or insulator, and the intervals are less than 1/2 wavelength of infrared light that resonates with longitudinal optical (LO) phonon energy. Ri, the substrate has two or more LO phonon mode, the LO phonon mode energy conduction band or the valence band within or included in the transition energy region between the conduction band or the valence band, electromagnetically induced transparency An infrared light element that controls the light absorption spectrum by phononization and emits or detects infrared light that resonates with the mode. Striped, latticed or annular conductors are formed on a single layer substrate of a semiconductor or insulator, the spacing of which is 1/2 of infrared light that resonates with LOphonon-plasmon coupling (LOPC) mode energy. Ri der than the wavelength, the substrate has two or more LOPC mode, the LOPC mode energy conduction band or the valence band within or included in the transition energy region between the conduction band or the valence band, the electromagnetic An infrared light element that controls the light absorption spectrum by induced transparency to emit or detect infrared light that resonates with the mode . Infrared optical device according to claim 1 or 2 wherein the substrate is a n-type or p-type semiconductor. When the substrate is a p-type semiconductor, it is a semiconductor having an acceptor-like level that forms a deeper level in addition to the p-type impurity.
When the substrate is an n-type semiconductor, it is a semiconductor having donor impurities that form deeper levels in addition to the n-type impurities.
The infrared optical device according to claim 3 , wherein the infrared optical device operates by modulating the light absorption spectrum by introducing a laser having an energy smaller than the forbidden band width. The infrared optical element according to claim 3 , wherein a laser that performs interband excitation between the valence band and the conduction band can be introduced to modulate the light absorption spectrum. The infrared light element according to claim 3, wherein a single infrared photon accompanied by a quantum entanglement phenomenon is radiated by inputting pulsed light having a time width of 1 ps or less. The infrared optical device according to claim 1 or 2, which has a structure capable of shielding an electric field generated by a polarization charge generated at an interface with a semiconductor having a low conductivity by using a semiconductor or another material having a high conductivity instead of a metal. The infrared optical device according to claim 1 or 2, wherein a metal whose junction with a semiconductor is an ohmic connection and a metal whose junction is a shotkey connection are formed as a pair.