JP2009058875A - Optical filter - Google Patents

Abstract

An optical filter that demultiplexes into a frequency component higher than that and a frequency component lower than that at a given frequency as a boundary is realized with a simple configuration. For example, it is used to separate excitation light and terahertz light when terahertz light is generated.
SOLUTION: 1) A flat surface demultiplexing element having a plasma frequency, and 2) an incidence of a linear or planar electromagnetic wave beam having a frequency component higher and lower than the plasma frequency incident on the demultiplexing element. And 3) a first electromagnetic beam generated as a reflected electromagnetic wave of the demultiplexing element, reflected at a position away from the incident point of the electromagnetic wave beam to the demultiplexing element, and reflected from the incident point with a frequency lower than the plasma frequency. A second electromagnetic beam having a frequency higher than the plasma frequency, and a selection means for selecting one of the electromagnetic beams. Further, the carrier density at the incident point is changed by an electric field, a magnetic field, light, pressure, temperature, or the like.
[Selection] Figure 2

Description

  The present invention relates to an optical filter that demultiplexes into a frequency component higher than that and a frequency component lower than that with a predetermined frequency as a boundary. For example, when terahertz light is generated, Can be used to separate the generated terahertz light.

  Although the technical field to which the present invention can be applied is wide, an example in the field of terahertz light will be described below for the sake of clarity.

  The frequency region called far-infrared region or terahertz region (wavelength = several tens to several hundreds μm) is an important frequency band in security, environmental monitoring, biomedical diagnosis, deleterious substance / toxic substance detection, wireless communication, spectroscopy, etc . At present, for the generation of terahertz light, a laser-pumped terahertz light generation device that excites a terahertz light generation element with a femtosecond pulse laser in the near-infrared region is used. A photoconductive antenna (non-patent document 1) shown in a), an electro-optic (EO) crystal (non-patent document 2) shown in FIG. 8B, and the like are used. In particular, a terahertz light generation apparatus using an EO crystal uses a nonlinear optical effect, so that there is no fundamental limitation on the frequency band of the generated terahertz light, and generation of a terahertz light pulse having a wide band (for example, a bandwidth of 100 THz or more). It is also possible and is expected for spectroscopic applications in the terahertz band.

  However, in a terahertz light generator using an EO crystal, the near-infrared femtosecond pulse (pump pulse) for generating terahertz light and the generated terahertz light pulse are coaxial (meaning the same optical axis) or close to it. In order to obtain only the terahertz light pulse, it must be separated.

  A conventionally used method for this problem is separation using a high-resistance silicon wafer. This is an opaque medium for the pump pulse (that is, the excitation laser beam), so it does not pass through silicon, but for the terahertz light pulse, it passes through the silicon because it hits the transmission region, so silicon acts as a filter, Terahertz light can be obtained.

  However, in this method, the pump pulse is actually a very high intensity pulse, so that a part of the pump pulse passes through the silicon. Thus, complete separation is not possible, and the intensity of the transmitted light is a value that cannot be ignored. Further, when the thickness of silicon is increased, pump pulse leakage is reduced, but absorption of terahertz light by silicon cannot be ignored, and the terahertz light pulse intensity is reduced. Furthermore, in any of the above cases, a part of the terahertz light pulse is reflected by silicon, so that a reflection loss is also received.

In addition, it is well known that light having different wavelengths can be easily demultiplexed using a prism or a grating, but in this case, there are the following disadvantages.
1) There is a limit to the maximum intensity of the light pulse that can be used due to the possibility of damage to the optical element.
2) In order to demultiplex, it is necessary to block the other light wave using an imaging optical system and a slit, and the optical system becomes complicated.
3) In order to perform demultiplexing with a sufficient contrast, it is necessary to make the optical path length sufficiently large, which is easily bulky.
4) In the case of a grating, an optical element that cuts high-order light is required, and in the case of a prism, it is necessary to be transparent to light waves.

D. Grischowsky, S. Keiding, M. van Exter and Ch. Fattinger, "Far-Infrared Time-Domain Spectroscopy with Terahertz Deams of Dielectrics and Semiconductors," J. Opt. Soc. Am. Vol. 7, No. 10, pp. 2006-2013 (1990). X.-C. Zhang, Y. Jin and XF Ma, "Coherent Measurement of THz Optical Rectification from Electro-Optic Crystals," Appl. Phys. Lett. Vol.61, No.23, pp.2764-2766 (1992) .

  An object of the present invention is to realize, with a simple configuration, an optical filter that demultiplexes into a frequency component higher than that and a frequency component lower than that at some given frequency as a boundary. For example, when terahertz light is generated, it is used to separate excitation light used for generation from the generated terahertz light, thereby providing pure and high-intensity terahertz light that can be applied to a wide range of applications such as spectroscopy.

  By using the present invention, it is possible to realize an optical filter that demultiplexes into a frequency component higher than that and a frequency component lower than that with a predetermined frequency as a boundary, for example, a terahertz light with a high extinction ratio. An optical filter to be cut out can be realized.

  The operating principle of the present invention is shown in FIG. That is, incident light enters the medium 2 from the medium 1. The medium 2 is an element in which free carriers exist, terahertz light is totally reflected, and light in other frequency regions is normally reflected or refracted. Thereby, two beams (first and second electromagnetic beams) can be spatially separated. This will be described in more detail below.

When light in the medium 2 of refractive index n ₂ from a medium 1 of refractive index n ₁ is incident at an angle theta ₁ from the interface, in general, light of p-wave undergoes reflection follows in medium interfaces.

The refractive index of the above medium varies depending on the angular frequency ω of the light. However, especially when free carriers exist in the medium, the dielectric constant ε (ω), which is the square value of the refractive index, is changed as follows due to the plasma effect. receive. Here, m is the mass of the carrier, e is the elementary charge, N is the carrier density, and ε ₀ is the dielectric constant in vacuum.

ここでω_pはプラズマ周波数と呼ばれるもので、バルクの場合は以下となる。

Here, ω _p is called a plasma frequency, and in the case of a bulk, it is as follows.

It should be noted here that when the frequency of light is smaller than the plasma frequency in Equation 2, the real part of the dielectric constant of the medium becomes negative. This indicates that the refractive index becomes a complex number. For example, when light enters a medium having a complex dielectric constant ε ₂ (ε ₂ = n ₂ ² = ε _r + iε _i ) from air (n ₁ = 1). , Equation 1 is as follows.

Furthermore, since ω _p and ω such that ε _r and ε _i >> 1 can be taken, r _p can be approximated to be approximately 1. This indicates that light having a frequency smaller than the plasma frequency undergoes total reflection at the medium interface, and the phase difference before and after reflection is zero. In this case, due to the effect of Goose-Henschen shift at the medium interface, a reflection point appears at a position shifted from the incident point of light at the medium interface. On the other hand, light having a frequency higher than the plasma frequency is subjected to reflection and refraction according to the usual Snell's law at the interface, so the Goose-Henschen shift does not occur.

  For this reason, if the plasma frequency of the medium is set to the middle of the two light frequencies with respect to the two lights on the same axis, they can be spatially separated using the fact that the beam positions due to reflection are shifted from each other. . Furthermore, one of them can be completely blocked, and a high extinction ratio can be realized. Further, since the plasma frequency is determined by the free carrier density, the frequency used as the separation boundary can be tuned by controlling the carrier density.

  More specifically, the optical filter of the present invention is configured as follows.

First, an optical filter of the present invention includes a demultiplexing element having a flat surface with a plasma frequency,
Incident means for entering a linear or planar electromagnetic wave beam having a frequency component higher and lower than the plasma frequency into the demultiplexing element;
A first electromagnetic beam generated as a reflected electromagnetic wave of the branching element, reflected at a position away from an incident point of the electromagnetic wave beam to the branching element and having a frequency lower than a plasma frequency, and reflected from the incident point and plasma; A second electromagnetic beam having a frequency higher than the frequency and selection means for selecting one of the electromagnetic beams are provided.

  In addition, the above-described optical demultiplexing element can also be configured as an optical filter using a semiconductor substrate doped in N-type or P-type.

  In addition, the semiconductor substrate has a semiconductor quantum well structure at the incident portion of the electromagnetic wave beam, so that the carrier density can be controlled, and a high-performance optical (for example, a separation distance between terahertz light and excitation light is large). A filter can be configured.

Further, in addition to the above configuration, an electrode that changes the carrier concentration of the electromagnetic wave beam incident portion;
By providing means for changing the applied voltage to the electrode, the demultiplexing characteristics can be adjusted by changing the applied voltage.

Or, furthermore, a magnetic field generation source that changes the carrier concentration of the electromagnetic wave beam incident part,
By providing magnetic field control means for changing the magnetic field generated by the magnetic field generation source, the demultiplexing characteristics can be adjusted by changing the magnetic field.

Or, further, pressure applying means for changing the carrier concentration of the electromagnetic wave beam incident portion, and
By providing pressure control means for changing the pressure applied by the pressure application means, the demultiplexing characteristics can be adjusted by changing the applied pressure.

Alternatively, excitation light incident means for changing the carrier concentration of the electromagnetic wave beam incident portion,
By providing excitation light control means for changing the excitation light by the excitation light incident means, the demultiplexing characteristics can be adjusted by changing the excitation light.

Alternatively, furthermore, heating or cooling means for changing the temperature of the electromagnetic wave beam incident part,
By providing the temperature control means for controlling the heating or cooling means, the demultiplexing characteristics can be adjusted by changing the temperature of the electromagnetic wave beam incident portion.

  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.

  FIG. 2 shows a first embodiment. The example given here is an example in which an InAs (indium arsenide) substrate 1 is used as a demultiplexing element. In this embodiment, as the incident light 3, near infrared light and terahertz light are incident on the surface of the non-doped InAs substrate through the slit 11 at an angle of 45 degrees. The terahertz light is from a laser excitation type terahertz generator and is on the same optical axis or the same optical path as the excitation laser light. Here, the incident light is a linear or planar electromagnetic wave beam, and the substrate surface is flat.

Even when the InAs substrate 1 is non-doped, electrons are accumulated in the vicinity of the surface due to the pinning effect of the surface, and the plasma frequency at this time is about 1.5 THz. For example, when a near-infrared light of about 800 nm serving as a ray raising light and the generated terahertz light of about 1 THz are incident coaxially, the terahertz light 4b (first electromagnetic beam) and the near-infrared light 4a (second Electromagnetic beam) becomes reflected light. Here, since ω (terahertz light 4b) <ω _p <ω (near infrared light 4a), the terahertz light 4b is subjected to Goose-Henschen shift and is about 200 μm with respect to the near infrared light 4a. Shift.

  Here, the shielding plate 2 for the beam block is installed in the reflected optical path so that the position of the micrometer or the like can be controlled. If the shielding plate 2 is adjusted so as to block the near infrared light 4a, only the terahertz light 4b can be obtained as an output. On the contrary, it is easy to extract the near infrared light 4a by blocking the terahertz light 4b.

  FIG. 3 shows an example of a demultiplexing device using a GaAs / AlGaAs quantum well 1a on a GaAs (gallium arsenide) substrate 1c.

In this demultiplexing device, a quantum well structure 1a is formed on a GaAs substrate 1c by MBE growth or the like, and an i-Al _0.3 Ga _0.7 As layer (0.1 μm thick), an i-GaAs layer (0 μm) is formed from the substrate side. 0.17 μm thickness), n-Al _0.3 Ga _0.7 As layer (0.077 μm thickness), and i-GaAs layer (0.005 μm thickness). Furthermore, a semi-transparent electrode 1b (for example, thin NiCr) is vapor-deposited on the surface of this sample to form a gate electrode.

If the arrangement of the incident light 3 and the shielding plate 2 on the demultiplexing element is the same as that of the first embodiment, the plasma frequency ω _p determined by the carrier density of the doped quantum well layer is the same as before. Thus, terahertz light satisfying ω (terahertz light 4b) <ω _p <ω (near infrared light 4a) is separated.

In particular, in this embodiment, the carrier density can be changed by changing the voltage applied to the gate electrode 1b. As a result, ω _p becomes variable, so that the frequency that becomes the boundary when demultiplexing the terahertz light can be changed. That is, the function of a tunable filter can be provided.

FIG. 4 shows a third embodiment. Here, a layered perovskite-type manganese oxide 5b on the substrate 5a, for example, (La _0.9 Nd _0.1 ) _1.3 Sr _1.6 Mn _2O7 , is used as the demultiplexing element. When the same configuration as in Example 1 is applied to the demultiplexing element incident light 3 or the shielding plate 2 and the magnetic field 6 is applied from the outside, the carrier density can be changed by the magnetic field induced phase transition of the sample. The plasma frequency ω _p is variable. Thus, the plasma frequency at the time of extracting the terahertz light can be changed, and the function of the tunable filter can be provided. For the application of the magnetic field, either a permanent magnet or an electromagnet can be used. In the case of a permanent magnet, the magnetic field intensity can be changed by changing the distance from the incident point of incident light, and in the case of an electromagnet, it can be changed by changing the intensity of the current for the electromagnet.

FIG. 5 shows a fourth embodiment. Is used here layered perovskite type manganese oxide 7b (e.g., La _1.0 4Sr _1.96 Mn _2O7) as demultiplexing element. The incident light 3 to the demultiplexing element 3 or the shielding plate 2 has the same configuration as that of the first embodiment, and the near-infrared light and the terahertz light have pressure application anvils 8a and 8b made of a transparent material. When the pressure is applied from outside, the carrier density can be changed by the pressure-induced phase transition of the sample. Because of this phenomenon, the plasma frequency ω _p is variable. That is, the plasma frequency at the time of cutting out the terahertz light can be changed, and the function of the tunable filter can be provided.

FIG. 6 shows a fifth embodiment. Here, the quantum well 1a is grown on the GaAs substrate 1 by MBE (divided line epitaxy) growth method or the like, and an i-Al _0.3 Ga _0.7 As layer (0.1 μm thickness), an i-GaAs layer (from the substrate side) 0.017 μm thickness), i-Al _0.3 Ga _0.7 As layer (0.077 μm thickness), and i-GaAs layer (0.005 μm thickness). The incident light 3 or the shielding plate 2 to the demultiplexing element has the same configuration as that of the first embodiment, and is irradiated with the carrier excitation laser light 9 from the outside at the same position as the condensing point of the incident light, When this is interrupted, the carrier density in the quantum well 1a changes. Using this phenomenon, the plasma frequency ω _p can be controlled by changing the intensity of the laser beam 9 for carrier excitation. Thereby, the plasma frequency which cuts out terahertz light can be made variable, and the function of a tunable filter can be given.

FIG. 7 shows a sixth embodiment. Here, a non-doped InAs substrate 1 is used for the branching element. If the incident light 3 or the shielding plate 2 or the like to the demultiplexing element has the same configuration as that of the first embodiment, and the demultiplexing element temperature is changed by the variable temperature control mechanism 10, the carrier density changes due to the temperature change. The plasma frequency ω _p can be made variable. Thereby, the frequency of the terahertz light to be cut out can be controlled, and the function of a tunable filter can be provided.

  In addition to the above, it is obvious that any element having a flat surface and having a plasma frequency can be used as the demultiplexing element of the present invention.

  In the above description, the terahertz light (first electromagnetic beam) and the near-infrared light (second electromagnetic beam) are separated using a shielding plate. However, when the distance between these lights is not sufficiently large, a convex mirror is used. It is desirable to use a shielding plate after using a or concave lens as a beam splitter and sufficiently increasing the distance between them.

  In the above description, an example in which the present invention is applied to generation of terahertz light has been shown. However, the present invention is generally applied to a case in which a group is divided into a higher frequency group and a lower frequency group. Obviously, it can be used in a wider area.

It is a figure which shows the principle of operation of this invention. It is a figure which shows the basic composition of this invention. It is a schematic diagram which shows the example which controls a demultiplexing characteristic by voltage application. It is a schematic diagram which shows the example which controls a demultiplexing characteristic by a magnetic field application. It is a schematic diagram which shows the example which controls a demultiplexing characteristic by pressure application. It is a schematic diagram which shows the example which controls a demultiplexing characteristic by carrier excitation light irradiation. It is a schematic diagram which shows the example which controls a demultiplexing characteristic by temperature control. It is a schematic diagram which shows the principle of the conventional terahertz wave generation.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 InAs (indium arsenide) board | substrate 1a Quantum well 2 Shielding board 3 Incident light 4a Near-infrared light 4b Terahertz light 5b Layered perovskite-type manganese oxide 6 Magnetic field 7b Layered perovskite-type manganese oxide 8a, 8b Anvil 9 Laser light for carrier excitation 10 Variable temperature control mechanism 11 Slit

Claims (8)

A demultiplexing element having a flat surface with a plasma frequency;
Incident means for entering a linear or planar electromagnetic wave beam having a frequency component higher and lower than the plasma frequency into the demultiplexing element;
A first electromagnetic beam generated as a reflected electromagnetic wave of the branching element, reflected at a position away from an incident point of the electromagnetic wave beam to the branching element and having a frequency lower than a plasma frequency, and reflected from the incident point and plasma; A second electromagnetic beam having a frequency higher than the frequency, and a selection means for selecting one of the electromagnetic beams,
An optical filter comprising: 2. The optical filter according to claim 1, wherein the branching element uses a semiconductor substrate doped in N-type or P-type. The optical filter according to claim 2, wherein the semiconductor substrate has a semiconductor quantum well structure in an electromagnetic wave beam incident portion. further,
An electrode for changing the carrier concentration of the electromagnetic wave beam incident portion;
Means for changing the voltage applied to the electrode;
The optical filter according to claim 2, wherein the demultiplexing characteristic is adjusted by changing the applied voltage. further,
A magnetic field source that changes the carrier concentration of the electromagnetic wave beam incident portion;
Magnetic field control means for changing the magnetic field generated by the magnetic field source;
The optical filter according to claim 1, wherein the demultiplexing characteristic is adjusted by changing the magnetic field. further,
Pressure applying means for changing the carrier concentration of the electromagnetic wave beam incident portion;
Pressure control means for changing the pressure applied by the pressure application means,
The optical filter according to claim 1, wherein the demultiplexing characteristic is adjusted by changing the applied pressure. further,
Excitation light incident means for changing the carrier concentration of the electromagnetic wave beam incident portion;
Comprising excitation light control means for changing excitation light by the excitation light incident means,
The optical filter according to claim 1, wherein the demultiplexing characteristic is adjusted by changing the excitation light. further,
Heating or cooling means for changing the temperature of the electromagnetic wave beam incident portion;
Temperature control means for controlling the heating or cooling means,
4. The optical filter according to claim 1, wherein the demultiplexing characteristic is adjusted by changing a temperature of the electromagnetic wave beam incident portion. 5.

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