JP2019152501A - Method for detecting electromagnetic wave and electromagnetic wave detector - Google Patents

Abstract

To provide an electromagnetic wave detector which can increase the sensitivity of the detection with an easy configuration.SOLUTION: The present invention relates to a method for detecting electromagnetic waves used for detecting detection lights with two different polarization components, using an EO sampling method. According to the method, the component part of a polarization state expression which shows the polarization state of the detection lights in which the intensity of the detected lights is 1/β(β<1) times is obtained by inserting the coefficient β into the polarization state expression. After that, polarization control is performed so that the polarization component for the component part is converted into β times.SELECTED DRAWING: Figure 2

Description

  The present invention relates to an electromagnetic wave detection method and an electromagnetic wave detection apparatus used for a nondestructive inspection apparatus using electromagnetic waves.

Devices and methods for performing nondestructive inspection using electromagnetic waves are known. In recent years, as a safe inspection method that replaces an X-ray apparatus, a technique for non-destructive inspection using an electro-optic (EO) sampling method using electromagnetic waves in a terahertz (THz) frequency band has been developed (for example, patents). References 1 and 2).
As an EO sampling method, for example, a method is known in which the birefringence induced in the electro-optic crystal by the electromagnetic field in the frequency band is sampled and detected by probe light using a femtosecond laser as a light source.

  FIG. 7 is a schematic diagram illustrating an example of an electromagnetic wave detection device that performs EO sampling detection. A sufficiently short laser pulse (referred to as probe light, particularly referred to as detection light after being modulated by the electromagnetic wave) with a delay time and an electromagnetic wave to be detected is converted into the same linearly polarized light and irradiated to the electro-optic crystal 11 collinearly. . The elliptically polarized detection light emitted from the electro-optic crystal 11 is incident on the λ / 4 wave plate 12 and the phase thereof is shifted by 90 degrees (phase bias). Then, the detection light in the polarization state emitted from the λ / 4 wavelength plate 12 is incident on a polarizer (for example, Wollaston Prism) 13 so that the polarized light is converted into a longitudinal polarization component on the major axis side and a lateral polarization on the minor axis side. The polarization components of the direction are separated, and the respective intensities are detected by the photodetector 14.

JP 2013-174513 A Japanese Patent No. 3388319

By the way, due to demands for strengthening countermeasures against terrorism, high precision and accurate fluoroscopic inspection is required for baggage inspection and whole body inspection at facilities where people gather, such as airports, event venues, and conference halls. X-ray fluoroscopy is limited in accuracy, accuracy, and image clarity, and the rapid development and popularization of a new technique to replace X-ray fluoroscopy is desired.
Although the EO sampling method is attracting attention as a new electromagnetic wave detection technique that replaces the fluoroscopic examination, the conventional electromagnetic wave detection method and the electromagnetic wave detection device described above are difficult to scan and map at high speed, and are still There is a problem that the range of applications is limited.

  The present invention was made for the purpose of rapidly expanding the use of the EO sampling method, and can greatly increase the sensitivity of electromagnetic wave detection by the conventional EO sampling method at a simple and inexpensive cost, and can also be used for high-speed mapping. An object of the present invention is to provide a possible electromagnetic wave detection method and an electromagnetic wave detection device.

In the electromagnetic wave detection apparatus using the conventional EO sampling method described above, the polarization state of the detection light is changed by providing a phase difference (optical path difference) between two orthogonal polarization components by the λ / 4 wavelength plate 12. The detection light emitted from the λ / 4 wavelength plate 12 is converted to a state close to circularly polarized light, and is proportional to the electric field of the electromagnetic wave by detecting the difference between the longitudinal polarization component and the lateral polarization component separated by the polarizer. Getting the signal.
The signal intensity (polarization modulation degree of detection light) σ in the EO sampling according to this conventional method is expressed as follows: ΔI is an intensity difference due to deviation from the circularly polarized light in the polarization components in the two directions, and I ₀ is the total intensity of the detection light. Although it can be expressed as σ = ΔI / I ₀ , this equation corresponds to the phase change ΔΓ = ΔI / I ₀ induced by the electro-optic effect.

Specifically, the signal intensity σ is (Iv−Ih) / (Iv + Ih) where Iv is the light intensity of the polarization component in the vertical direction detected by the photodetector and Ih is the light intensity of the polarization component in the horizontal direction. ) = ΔI / I ₀ = ΔΓ. The longitudinal polarization component of the detection light immediately after passing through the electro-optic crystal is considerably larger than the lateral polarization component, but the signal due to the electro-optic effect is mainly contained in the lateral polarization component. Therefore, it can be expected that the signal intensity σ can be increased if the ratio between the lateral polarization component and the longitudinal polarization component immediately after passing through the electro-optic crystal can be controlled. An expression representing the signal intensity in EO sampling when the change rate of the EO signal intensity σ by the polarization control of the detection light is α is shown below.

  From this equation, it can be seen that if the polarization control is performed so that the coefficient α is larger than 1, the signal intensity σ can be increased α times.

  Specifically, the electromagnetic wave detection method according to the present invention is an electromagnetic wave detection method for detecting detection light of two different polarization components using an EO sampling method as described in claim 1, wherein the polarization state of the detection light is changed. By inserting a coefficient β into a part of the polarization state expression to be expressed, a component part of the polarization state expression in which the signal intensity of the detection light is α (= 1 / β, β <1) times is obtained, and the component part The polarization control is performed so that the polarization component corresponding to is converted to β times in amplitude.

  The electromagnetic wave incident on the electro-optic crystal is emitted as elliptically-polarized detection light, and the longitudinal polarization component of such elliptically-polarized detection light is considerably larger than the lateral polarization component. In such elliptically polarized light, the signal intensity can be enhanced to 1 / β times by converting (attenuating) the polarization component on the long axis side to β times as described in claim 2.

The present invention spatially separates a sum frequency generation light component and a difference frequency generation light component contained in detection light, and performs heterodyne-type EO sampling that detects electromagnetic waves using an interference effect with the original detection light component. The same applies to the detection method. That is, as described in claim 3, after making the detection light into two orthogonal polarization components, the major axis polarization component is attenuated to generate a sum frequency generation (SFG) light component and a difference frequency generation (DFG). ) For the polarization component on the short axis side including the light component, the signal intensity of the detection light is 1 / β (β = sin θ / cos θ: where θ is the incident light incident on the electro-optic crystal in the embodiment described later. (The tilt angle of the detection light (probe light) from the longitudinal polarization.) The polarization may be controlled so as to be doubled.
The polarization control can be performed using an optical filter as described in claim 4, for example.
The signal intensity can be increased as the value of the coefficient β is decreased. However, if the value of the coefficient β is too small, noise will increase and the detection accuracy will be lowered. Therefore, as shown in claim 5, the intensity of the detection light when the electric field amplitude of the detection light is E _0. The value of the coefficient β may be determined within a range where (| E ₀ | ² β ² ) is larger than the noise intensity.

  The electromagnetic wave detection apparatus using the electromagnetic wave detection method of the present invention described above is an electromagnetic wave detection apparatus that detects detection light of two different polarization components using an EO sampling method, as described in claim 6, Of the polarization components, the polarization control means for attenuating the polarization component on the long axis side is arranged after the electro-optic crystal. The polarization control unit may attenuate a polarization component on the long axis side of the elliptically polarized light of the detection light emitted from the electro-optic crystal. As the polarization control means, an optical filter that attenuates the polarization component on the long axis side can be used.

The electromagnetic wave detection apparatus of the present invention is a heterodyne that detects an electromagnetic wave by spatially separating a sum frequency generated light component and a difference frequency generated light component and utilizing an interference effect with the original detected light (probe light) component. In this case, the polarization control means attenuates the polarization component on the long axis side among the polarization components of the detection light emitted from the electro-optic crystal, and produces a sum frequency. The polarization component on the short axis side including the generated (SFG) light component and the difference frequency generated (DFG) light component may be transmitted.
In the present invention, the term “attenuation” includes a case where the polarization component on the long axis side is completely removed (cut).

  Since the present invention is configured as described above, the sensitivity of one of the two orthogonal polarization components of the detection light can be increased by simply performing control such as relatively attenuating the polarization component. . Moreover, an optical filter can be used as the polarization control means, and the sensitivity of electromagnetic wave detection can be increased by simply inserting a commercially available optical filter into an existing electromagnetic wave detection device. Furthermore, although there is a certain limit on the attenuation rate in relation to the signal-to-noise ratio, the sensitivity can be dramatically increased, which enables high-speed mapping of electromagnetic wave detection signals, enabling people at airports, event venues, conference halls, etc. It will be possible to broaden the scope of application, such as baggage at facilities where people gather and fluoroscopy of the whole body of visitors.

Hereinafter, preferred embodiments of the electromagnetic wave measurement method of the present invention will be described in detail.
[Principle of the present invention]
When electromagnetic waves (THz waves) and probe light (visible, near-infrared pulsed light having a width of about 100 fs) are incident on the electro-optic crystal 11 (see FIG. 7), the electric field due to the electromagnetic waves is unequal in the electro-optic crystal. Induced refractive index change (birefringence) Δn. At this time, when the propagation distance in the electro-optic crystal 11 is L and the wavelength of the detection light emitted from the electro-optic crystal 11 is λ, the phase difference ΔΓ is expressed by the following equation.

This phase difference ΔΓ is usually very small, and the detection light emitted from the electro-optic crystal 11 is very slightly elliptically polarized (long axis is the vertical direction), but in a polarization state close to linearly polarized light. When the detection light in this state is transmitted through the ¼λ plate, the detection light is substantially circularly polarized.
Here, assuming that the intensity of the polarization component in the vertical direction is Iv, the intensity of the polarization component in the horizontal direction is Ih, and the intensity of the detection light before separating the polarization is I ₀ , each is given by the following equations.

  In Equations 3 and 4, ΔΓ / 2 in the final term approximates sin ΔΓ / 2 because ΔΓ is sufficiently small. The deviation ΔI from the circularly polarized light is given by the difference Iv−Ih between the longitudinally polarized light and the horizontally polarized light intensity Iv and Ih.

That is,

Get. Thus, the signal intensity of EO sampling is proportional to the phase difference ΔΓ between orthogonal (in this case, +/− 45 degrees) polarization components generated as a result of induction of birefringence by electromagnetic waves, and π / 2 of the detection light. It can be seen that this corresponds to a deviation from circularly polarized light after the phase bias.
In order to increase the signal strength of EO sampling, I ₀ in the denominator of Equation 6 may be attenuated. However, if I ₀ is attenuated, ΔI is also attenuated. Therefore, it is necessary to efficiently increase the signal intensity ΔI / I ₀ of EO sampling by making the attenuation rate of ΔI smaller than the attenuation rate of I ₀ .

Therefore, the inventor of the present invention decided that the detected light intensity after attenuation of the detected light immediately after being emitted from the electro-optic crystal is I ₀ ′ = β ² I ₀ . In other words, the amplitude of the detection light is attenuated by β times (β ² times the detection light intensity). In this way, ΔI also attenuates, but when the attenuation rate is s and ΔI after attenuation is ΔI ′, ΔI ′ = sΔI, and Equation 6 representing the signal strength of EO sampling at this time is

Can be rewritten. Here, if α = s / β ² , the signal intensity is enhanced if α> 1. In order to satisfy this condition, s> β ² (0 ≦ β <1) may be satisfied. At this time, even if the denominator I ₀ is attenuated, the numerator ΔI is not attenuated as much as the denominator I ₀ .
By controlling the polarization state of the detection light in this way, if the above Equation 7 can be derived, the signal intensity of EO sampling can be increased.

Based on this prediction, how to control the polarization state of the detection light immediately after passing through the electro-optic crystal, that is, if the coefficient β is inserted into any part of the polarization state equation, the result shown in Equation 7 is obtained. Ask if you can.
The expression of the polarization state of the detection light immediately after passing through the electro-optic crystal is such that the major axis and minor axis directions of the birefringence are +45 degrees and −45 degrees, respectively, and the respective phase differences are + ΔΓ / 2, −ΔΓ / Therefore, the amplitude can be expressed by the following equation.

  In Equation 8, the subscript indicates the direction of the polarization component, the subscript 0 indicates the vertical direction, and the subscript 90 indicates the horizontal direction. If the time factor (exp (−iωt)) is omitted from Equation 8,

It becomes. In Equation 9, the vertical amplitude is E ₀ cos (ΔΓ / 2), the horizontal amplitude is E ₉₀ sin (ΔΓ / 2), and the latter phase is elliptically polarized light that oscillates with a delay of 90 degrees. Show.
Now, the coefficient β is inserted into one component of the elliptically polarized light component of Equation 9 (longitudinal polarized light component (polarized light component with subscript 0) in the following example). Equation 9 can be rewritten as follows by inserting the coefficient β.

Here, e ₊₄₅ and e ₋₄₅ are unit vectors in directions inclined by +45 degrees and −45 degrees respectively from the vertical (vertical) direction, and there is the following relationship between e ₀ and e ₉₀ .

  When a phase bias of π / 2 is added to Equation 10 and the phase of the −45 degree polarization component is shifted by 90 degrees,

  Therefore,

となるので、ＥＯサンプリング信号は、 It becomes. From this, it can be seen that ΔI has an attenuation factor s = β. The intensity of the detection light is from I ₀ = E ₀ ²

Therefore, the EO sampling signal is

It becomes.
This equation 16 is found to be α = 1 / β when compared with equation 7 for increasing the signal strength of EO sampling. Therefore, the intensity of the EO sampling signal can be increased by inserting a coefficient β into the component part representing one polarization component in the polarization state expression representing the polarization state of the elliptically-polarized detection light emitted from the electro-optic crystal. I understand that I can do it.
Here, in order to increase the intensity of the EO sampling signal, the coefficient β needs to be smaller than 1 (β <1). That is, according to the example of Equation 10, the component that inserts the coefficient β corresponds to the longitudinal polarization component on the long axis side. Therefore, by attenuating the polarization component in the longitudinal direction, the EO sampling signal Strength can be increased. The magnitude that can be increased is 1 / β times from Equation 7 and Equation 15.

Note that the smaller the coefficient β, the larger the value of ΔΓ / β in Equation 15, but in practice, if the coefficient β is reduced below a certain value, the light intensity becomes too small and the noise component becomes relatively large. . When the intensity | E ₀ ² β ² | of the detection light attenuated by an optical filter or the like becomes smaller than the noise intensity, the SN ratio is deteriorated. Therefore, the intensity, frequency, and electricity of the detection light immediately after being emitted from the electromagnetic wave irradiation unit It is necessary to appropriately select the value of the coefficient β after comprehensively considering the characteristics of the optical crystal, the performance of the λ / 4 wavelength plate, the optical filter, and the like.

Hereinafter, a preferred embodiment of the present invention based on this principle will be described in detail with reference to the drawings.
FIG. 1 is a schematic diagram illustrating a specific device configuration of an electromagnetic wave detection device according to the present invention, and FIG. 2 is a schematic diagram illustrating details of a main part thereof.
As shown in FIG. 1, the electromagnetic wave detection apparatus of this embodiment includes a probe light irradiation unit including a femtosecond laser device 24 that emits probe light and an optical system 23 that controls the polarization state of the probe light, and probe light. An electromagnetic wave irradiation unit that emits an electromagnetic wave having a frequency larger than 30 GHz and smaller than 30 THz, and includes an emitter 22. The probe light irradiated from the probe light irradiation unit and the electromagnetic wave (for example, THz light) irradiated from the electromagnetic wave irradiation unit are incident on the electro-optic crystal 11 which is a light modulation unit through the reflecting mirror 25, the concave mirror 27a, and the concave mirror 27b. The
The electromagnetic wave incident on the electro-optic crystal 11 is a combination of the probe light irradiated from the probe light irradiation unit and the electromagnetic wave such as a THz wave irradiated from the electromagnetic wave irradiation unit. The probe light emitted from the electro-optic crystal 11 after being modulated by the electro-optic effect in the electro-optic crystal is referred to as “detection light” to distinguish it from the light before modulation.

Based on the above equation 10, as shown in FIG. Among them, an optical filter 15 is disposed as a polarization control means for attenuating the polarization component on the long axis side. This optical filter 15 attenuates the polarization component on the long axis side (vertical polarization component in this embodiment) by a coefficient β, and transmits the polarization component on the short axis side orthogonal to this as it is.
When the amplitude of the polarization component on the long axis side is attenuated to, for example, 1/10 (coefficient β = 1/10) by using this optical filter 15, the EO sampling detected by the photodetector 14 according to Equation 16 is performed. The intensity of the signal will be enhanced about 10 times.
However, if the coefficient β is too small, the intensity of the probe light becomes too small and the noise component becomes relatively large. That is, if the intensity | E ₀ | ² β ² of the electromagnetic wave after attenuation of the polarization component in the vertical direction becomes smaller than the noise intensity, the SN ratio deteriorates, so that | E ₀ | ² β ² does not become less than the noise intensity. The coefficient β is preferably selected. The coefficient β can be 1/100 to 50/100.

[Experiment 1]
In order to verify the effect of the present invention, the inventor of the present invention uses a (001) plane-cut ZnTe crystal having a thickness of 1 mm as the electro-optic crystal 11, a frequency band of 30 GHz to 4 THz, and an intensity attenuation rate of detection light β ² = 5 % (Β = 0.22) was used for the experiment in the apparatus of FIG. The result is shown in the graph of FIG.
FIG. 3A is a graph showing a displacement when the delay time of the probe light of the signal intensity is changed, that is, a time waveform of the THz wave, and FIG. 3B is a relationship between the signal intensity and the frequency obtained by Fourier transforming it, that is, It is a graph which shows a power spectrum.
In the graphs of FIGS. 3A and 3B, the optical filter 15 before being provided is indicated by a broken line, and after the optical filter 15 is provided by a solid line. From the graph of FIG. 3A, it can be seen that the signal intensity is enhanced about 2.5 times before and after the optical filter 15 is provided (the enhancement rate expected from the theory is about 4.5 times). ). Further, from the graph of FIG. 3B, signal enhancement could be confirmed over the entire frequency band in which the experiment was performed.
The frequency band of the electromagnetic wave used in the experiment is in the range of 30 GHz to 4 THz, but the present invention can be suitably applied up to a frequency of about 30 THz.

[Other Embodiments]
The present invention can be similarly applied to a heterodyne EO sampling detection system.
In the heterodyne EO sampling method, the sum frequency generation (SFG) light and the difference frequency generation (DFG) light generated by the nonlinear interaction in the electro-optic crystal 11 of the electromagnetic wave such as THz wave and the probe light are converted into the original probe light. In this method, the intensity modulation of the detection light is directly detected by the light wave interference.

In the heterodyne EO sampling, the electromagnetic wave irradiated from the electromagnetic wave irradiation unit is incident on the electro-optic crystal 11 non-coaxially with respect to the probe light irradiated from the probe light irradiation unit. Frequency generation (SFG) light and difference frequency generation (DFG) light are detected with respect to the central axis of detection light (an electromagnetic wave emitted from the electro-optic crystal 11 and detected by the photodetector 14 is referred to as “detection light”). The light is emitted from the electro-optic crystal 11 in opposite directions. In addition, the phases of the SFG light and the DFG light are reversed, and the polarity of the intensity modulation due to the interference with the original probe light is reversed. Using this property, the detector 14 detects the modulation of the detection light by the heterodyne EO sampling of the SFG light and the DFG light.
The probe light incident on the electro-optic crystal 11 is slightly tilted in the polarization direction from the vertically polarized light (the tilt angle θ is about 0.5 to 5 ° as a guide), and the vertically polarized component E ₀ cos θ (main polarized light) Component) and a very small transverse polarization component E ₀ sin θ.
Here, the amplitude of incident detection light is E ₀ , its intensity is I ₀ = | E ₀ | ² , and the amplitudes of SFG light and DFG light generated by the interaction of electromagnetic waves (THz light) and probe light are E _{SFG / DFG} Then, the heterodyne EO sampling signal is expressed by the following equation.

In Equation 17, n _opt is the refractive index of the detection light of the EO crystal, r _ij is the EO coefficient, ω _opt is the angular frequency of the electromagnetic wave, L is the interaction length (the thickness of the EO crystal), E _THz is the amplitude of the electromagnetic wave, Im [g (ΔkL)] is a phase mismatch factor.
In this embodiment, sin θ is inserted into the denominator on the right side of Equation 17 and cos θ is inserted into the numerator. This is because the probe light incident on the electro-optic crystal is tilted by θ from the longitudinal polarization, so that the transverse polarization amplitude of the probe light that interferes with the denominator SFG light and DFG light is cos θ times, and the amplitude of the SFG light and DFG light is sin θ. Corresponds to being doubled. As a result, Equation 17 becomes

  When the right side of Equation 18 is rewritten as β = sin θ / cos θ,

It becomes. This equation 19 has the same form as equation 16 in the previous embodiment, and the intensity of the EO sampling signal is 1 / β (β <1) times before and after inserting the coefficient β corresponding to the polarization control of the probe light. Only will be strengthened.
FIG. 4 is a schematic diagram illustrating a specific device configuration of the electromagnetic wave detection device according to the second embodiment of the present invention, and FIG. 5 is a schematic diagram illustrating details of a main part of the device of FIG.
In the heterodyne EO sampling detection system of this embodiment, an optical filter (for example, a λ / 2 wavelength plate) 16 that tilts the probe light by a slight angle θ is disposed in front of the electro-optic crystal 11. Then, an optical filter 15 that cuts the longitudinal polarization component of the polarization components of the detection light emitted from the electro-optic crystal 11 is disposed immediately after the electro-optic crystal 11. Thus, the remaining lateral polarization component is modulated by interference with the SFG light and the DFG light.

In heterodyne EO sampling, the value of 1 / β increases and the signal multiplication factor increases as the inclination θ from the longitudinal polarization of the detection light is reduced by polarization control.
However, if θ is made too small, the intensity of the probe light after the longitudinally polarized light component is cut becomes too small, and the noise component becomes relatively large. Therefore, the intensity of the detected light after cutting the polarization component in the vertical direction | E ₀ | ² β ² (where β ² ≈sin θ ² when cos θ≈1, this equation is expressed as | E ₀ | ² sinθ ² If (equivalent) becomes smaller than the noise intensity, the SN ratio deteriorates. Therefore, it is preferable not to reduce θ below the value at which the intensity | E ₀ | ² sinθ ² of the polarization component in the horizontal direction becomes comparable to the noise intensity. .

[Experiment 2]
About this 2nd embodiment, it experimented similarly to the experiment 1. FIG. Experiments were performed using the apparatus shown in FIG. 4 with θ of 2.8 ° and θ = 0.87 °, and a frequency band of 30 GHz to 4 THz. The result is shown in the graph of FIG.
FIG. 6A is a graph showing the displacement when the delay time of the probe light of the signal intensity is changed, that is, the time waveform of the THz wave, and FIG. 6B is the relationship between the signal intensity and the frequency obtained by Fourier transforming it, that is, It is a graph which shows a power spectrum.
In the graphs of FIGS. 6A and 6B, the optical filter 15 is provided with a broken line before being provided, and the solid line (θ = 2.8 °) and the dashed line (θ = 0.87 °) after being provided. .
From the graph of FIG. 6A, it can be seen that the signal intensity is enhanced by about 20 times (θ = 2.8 °) and 66 times (θ = 0.87 °) before and after the optical filter 15 is provided. . Further, from the graph of FIG. 6 (b), it was confirmed that the signal was enhanced over the entire frequency band in which the experiment was performed.
However, in the case of θ = 0.87 ° (one-dot chain line), the SN ratio deteriorated, and practical results could not be obtained.

Although a preferred embodiment of the present invention has been described, the present invention is not limited to the above description.
For example, the optical filter 15 has been described as an example of the polarization control means in the above description, but other means may be used as long as the same effect is obtained.
Further, in the polarization state equation (for example, Equation 10) indicating the polarization state of the detection light, it is interpreted that the coefficient β is inserted into one component and the coefficient “1” is inserted into the other component. Can do. That is, the coefficient β is relative in relation to the coefficients of other constituent parts, and it is possible to insert coefficients (τ ₁ , τ ₂ ...) Other than 1 into other constituent parts. For example, according to the above-described embodiment, if the lateral polarization component is amplified by inserting the coefficient τ ₁ (> 1) into the constituent part on the short axis side in the polarization state formula of Expression 12, the relatively long axis The polarization component in the vertical direction on the side is attenuated, and the same result as Expression 17 can be obtained. As described above, when other coefficients τ ₁ , τ _2, ... Are inserted into other components, a coefficient τ that satisfies β <1 in relation to these coefficients τ ₁ , τ ₂ ,. ₁ , τ ₂ ... And coefficient β may be selected.

The electromagnetic wave detection device and electromagnetic wave detection method of the present invention can be applied even in the frequency band of millimeter wave band or gigahertz wave band, but is particularly preferably applied in the terahertz wave band of frequency 30 GHz to 30 THz (wavelength 10 mm to 10 μm). In principle, it can also be applied to electromagnetic wave detection in a frequency band exceeding 30 THz.
In addition, the electromagnetic wave detection device and the electromagnetic wave detection method of the present invention can be applied to various sensing devices and imaging devices.

It is the schematic which shows the specific apparatus structure of the electromagnetic wave detection apparatus of this invention. It is the schematic explaining the detail of the principal part of the apparatus of FIG. FIG. 3A is a graph for explaining the effect of the electromagnetic wave detection device of the first embodiment, and FIG. 3A is a graph showing a displacement when the delay time of the probe light of the signal intensity is changed, that is, a time waveform of a THz wave. b) is a graph showing the relationship between the signal intensity and frequency obtained by Fourier transform, that is, the power spectrum. It is the schematic which shows the specific apparatus structure of the electromagnetic wave detection apparatus concerning 2nd embodiment of this invention. It is the schematic explaining the detail of the principal part of the apparatus of FIG. FIG. 6A is a graph for explaining the effect of the electromagnetic wave detection device according to the second embodiment. FIG. 6A is a graph showing a displacement when the delay time of the probe light of the signal intensity is changed, that is, a time waveform of a THz wave. b) is a graph showing the relationship between the signal intensity and frequency obtained by Fourier transform, that is, the power spectrum. It is the schematic which shows an example of the electromagnetic wave detection apparatus concerning the prior art example of this invention which performs EO sampling detection.

11 Electro-optic crystal 12 λ / 4 wavelength plate 13 Polarizer 14 Photo detector 15 Optical filter 16 Optical filter (λ / 2 wavelength plate)
21 Femtosecond laser device 22 Emitter 23 Lens 24 Femtosecond laser device 25 Reflector

Claims (6)

In an electromagnetic wave detection method for detecting detection light of two different polarization components using an EO sampling method,
By inserting a coefficient β into a part of the polarization state expression representing the polarization state of the detection light, a component part of the polarization state expression in which the signal intensity of the detection light is 1 / β (β <1) times is obtained. ,
Performing polarization control so as to convert the polarization component corresponding to the component to β times,
An electromagnetic wave detection method characterized by the above. The electromagnetic wave detection method according to claim 1, wherein after the detection light is elliptically polarized, the polarization component on the long axis side of the elliptically polarized light is controlled to be polarized by a factor of β.
An electromagnetic wave detection method characterized by the above. 2. The electromagnetic wave detection method according to claim 1, wherein after the detection light is made into two orthogonal polarization components, the polarization component on the long axis side is attenuated to generate a sum frequency generation (SFG) component and a difference frequency generation (DFG). A method of detecting an electromagnetic wave, wherein polarization of a short-axis side polarization component including a component is controlled so that a signal intensity of the detection light is 1 / β (β = sin θ / cos θ) times. The electromagnetic wave detection method according to claim 1, wherein the polarization control is performed by transmitting through an optical filter. The value of the β is such that the intensity (| E ₀ | ² β ² ) of the detection light when the electric field amplitude of the detection light is E ₀ is larger than the noise intensity. The electromagnetic wave detection method in any one. An electromagnetic wave detection apparatus using the electromagnetic wave detection method according to any one of claims 1 to 5, wherein detection light of two different polarization components is detected using an EO sampling method,
Among the polarization components, the polarization control means for attenuating the polarization component on the long axis side is disposed after the electro-optic crystal,
An electromagnetic wave detection device characterized by the above.

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