JP5963080B2 - Electromagnetic wave detection method and electromagnetic wave detection device - Google Patents

Description

  The present invention relates to an electromagnetic wave detection method and an apparatus therefor, and more particularly, to an electromagnetic wave detection method and an electromagnetic wave detection apparatus suitable for detecting an electromagnetic wave in a frequency region with a frequency of 30 GHz to 30 THz and a light length of 10 mm to 10 μm.

  In this type of frequency region, technology for non-destructive inspection, etc., has recently been developed, particularly by detecting the electric field strength in the terahertz light band, and constitutes a safe fluoroscopic inspection device that replaces, for example, an X-ray device. The use for imaging technology is progressing. Also, spectroscopic techniques for examining physical properties such as molecular binding states by obtaining absorption spectra and complex permittivity inside substances, measurement techniques for examining physical properties such as carrier concentration, mobility, and conductivity, and biomolecule analysis techniques. The use is also proceeding (see, for example, Patent Documents 1, 2, and 3).

JP 2011-33700 A JP 2010-204488 A JP 2011-203718 A

IQEC / CLEO Pacific Rim 2011 “Efficient electro-optic sampling detection and generation of intenseTHz radiation via Cherenkov-type phase matching in a LiNbO3 crystalcoupled to a Si prism” S. P. Kovalev and G. Kh. Kitaeva, JETP Lett.Vol.94, No.2, pp.91-96 (2011).

  By the way, as a method for detecting the electric field strength of an electromagnetic wave such as terahertz light, an EO sampling method that detects an electric field using an electro-optic (EO) effect can be used. In this EO sampling method, an electro-optic crystal is used. When using a nonlinear optical crystal with a large nonlinear optical coefficient, the phase velocity of the terahertz light sampled for high efficiency of detection coincides with the velocity of the group velocity of the femtosecond laser pulse used as the sampling light (phase It is a necessary condition for crystals having birefringence characteristics to compensate for the phase delay due to the birefringence.

The former has proposed a technique by which the phase matching condition can be obtained with any crystal and sampling light length by using the Cherenkov phase matching method (see Non-Patent Document 1). However, regarding the latter problem of intrinsic birefringence, a method of rotating the polarizing plate by 90 degrees and propagating the same crystal optical path twice has been proposed by the patent applicant (see Non-Patent Document 1). Has the disadvantage of becoming complicated.
Further, Non-Patent Document 2 proposes an EO sampling detection method for THz light by a method called “Energy sensitive electro optical detection”. This detection method has a sensitivity higher than that of the conventional method by ω _THz / ω ₀ (ω _THz : frequency of THz light, and ω ₀ : frequency of sampling light).

  The present invention has been made in view of the above problems, and can detect electromagnetic waves without phase delay compensation even for crystals having intrinsic birefringence, and can detect differences using a wave plate, a polarizing element, and a photodetector. An object of the present invention is to provide an electromagnetic wave detection method and an electromagnetic wave detection device that can be simplified and can be simplified in configuration, and can detect electromagnetic waves with high sensitivity and high efficiency.

In order to achieve the above object, the invention according to claim 1 is an electromagnetic wave detection method for detecting an electromagnetic wave by an EO sampling method using an electro-optic effect, wherein the sampling light irradiated from the sampling light irradiation means is converted into a nonlinear optical crystal. Increasing and focusing the electromagnetic wave to be detected irradiated from the electromagnetic wave irradiation means, and making the enhanced electromagnetic wave incident on the nonlinear optical crystal at a Cherenkov phase matching angle with respect to the optical axis of the sampling light, and being phase-matched The wave vector matched output including the sum frequency component and the difference frequency component output by combining the electromagnetic wave and the sampling light is divided into two symmetrically at the center of the output, and each of the divided outputs Are input in advance to two detection units arranged at positions where the signal polarity is reversed, and the other one of the detection units detects the sum frequency component. Wherein by detecting the difference frequency component at the detecting portion of, it is a method for determining the difference between the two detected signals by heterodyne detection method or homodyne detection method at each of the detecting portion.

As means for enhancing the electromagnetic waves, those that generate surface plasmons can be used. For example, as described in claim 2, at least the surface of the electromagnetic waves irradiated from the electromagnetic wave irradiation means is formed of a metal. After being introduced into the V-groove, it is recommended to propagate it to the tip of the V-groove and strengthen it.
The nonlinear optical crystal may be a single crystal, but as described in claim 3, it may be a waveguide formed from the nonlinear optical crystal.

The apparatus of the present invention for carrying out the invention of the above method is the sampling light for irradiating the sampling light in the electromagnetic wave detecting apparatus for detecting the electromagnetic wave by the EO sampling method using the electro-optic effect as described in claim 4. Irradiation means, electromagnetic wave irradiation means for irradiating an electromagnetic wave to be detected, electromagnetic wave enhancement means for focusing and enhancing the electromagnetic wave, and the enhanced electromagnetic wave incident on the optical axis of the sampling light at a Cherenkov phase matching angle a phase matching means for, is formed by the non-linear optical crystal, and sum and difference frequency generation means for generating an output comprising a phase matched the electromagnetic wave and the sampling light and the by coupling sum frequency components and difference frequency components, wherein the sum A dividing means for dividing the wave vector matched output including the frequency component and the difference frequency component symmetrically into two at the center of the output; Each of the outputs divided by the dividing means is inputted, and on the one hand, the sum frequency component is detected, and on the other hand, the difference frequency component is detected, and two detections arranged at positions where the polarity of the detected signal is reversed. And detecting means for obtaining a difference between two signals detected by the heterodyne detection method or the homodyne detection method in each of the detection units .

As the electromagnetic wave enhancing means, one that generates surface plasmon can be used. For example, as described in claim 5 , the electromagnetic wave enhancing means is formed of a V-shaped groove having at least a surface formed of a metal. Can be used. As described in claim 6 , the angle of the top of the V groove can be determined based on the focusing angle of the electromagnetic wave incident on the V groove. According to a seventh aspect of the present invention, the phase matching unit and the sum / difference frequency generating unit are formed so that the electromagnetic wave incident from one inclined surface is output to the other inclined surface at a Cherenkov phase matching angle. The prism may be composed of a combined body of a non-linear optical crystal formed on the other inclined surface, and the prism may be disposed at the tip of the V groove. According to an eighth aspect of the present invention, a parallel portion may be provided at the tip of the V groove, and the prism may be disposed in the parallel portion.

Since the present invention is configured as described above, it is possible to perform EO sampling of the electromagnetic wave to be detected without phase delay compensation even for a crystal having intrinsic birefringence, and a wave plate, a polarizing element, and two light beams. It is not necessary to use a difference detection method by a detector, and EO sampling detection can be performed with a simple configuration, high sensitivity and high efficiency.
In particular, in the present invention, the detection sensitivity of EO sampling can be dramatically increased by enhancing the electric field strength of the terahertz wave detected by EO sampling by the electromagnetic wave enhancing means.
Further, the output including the sum frequency component and the difference frequency component is divided into two, and two detectors are arranged so that the signal polarity is inverted, and a difference between signals detected by the two detectors is obtained. Thus, the detection sensitivity by the detector can be increased and the noise component can also be reduced.

DESCRIPTION OF EMBODIMENTS Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings.
[Device configuration]
FIG. 1 is a block diagram showing the apparatus configuration of the present invention.
The apparatus of FIG. 1 includes a sampling light irradiation unit 1 that irradiates sampling light, a THz light irradiation unit 2 that emits terahertz (hereinafter referred to as “THz”) waves that are electromagnetic waves to be detected, and a phase of THz light. The phase matching unit 3 that performs adjustment, the enhancement unit 9 that combines and enhances the THz light emitted from the THz light irradiation unit 2 with a metal surface plasmon, and the phase-adjusted and enhanced THz light and sampling light. It has a sum / difference frequency generator 4 that generates a sum frequency by combining, and a detector 5 that detects a sum frequency component from the output from the sum / difference frequency generator 4.

[Sampling light irradiation unit 1 and THz light irradiation unit 2]
As the sampling light irradiation unit 1 and the THz light irradiation unit 2, a femtosecond laser device can be used. In addition, you may convert into the laser beam of a half wavelength by letting the laser beam irradiated from these laser apparatuses pass through a well-known 2nd harmonic generator. The timing at which the sampling light irradiated from the sampling light irradiation unit 1 enters the phase matching unit 3 is adjusted by a known delay means so that the phase matching unit 3 described below can be optically coupled with the THz light. The

[Enhancer 9]
The enhancement unit 9 of this embodiment is to enhance the THz light by combining the THz light with the metal surface plasmon in the V-shaped groove (V-groove) and focusing it on the tip of the V-groove. The principle of such an enhancement unit 9 is 9 April.
“Terahertz field enhancement to the MV / cm regime in K. Iwaszczuk et al. Published in 2012 / Vol.20, No.8 / OPTICS EXPRESS 8355
a tapered parallel plate waveguide ”can be used.
The type of metal is not particularly limited as long as it causes a bond with surface plasmon, but gold (Au) or silver (Ag) can be suitably used. A V-groove may be formed in a metal block made of aluminum or iron, and a metal such as gold (Au) or silver (Ag) may be plated or deposited on the inner surface of the V-groove, or a resin or ceramic A V-groove may be formed in a block of a non-metallic material such as gold, and a metal layer such as gold (Au) or silver (Ag) may be formed on the inner surface of the V-groove.
The angle of the top of the V groove (indicated by α in FIG. 4) should be as small as possible. However, by adjusting the angle of the V groove to the beam focusing angle of the incident THz light, Even if the angle α is large, the THz light can be focused efficiently. The optimum angle α of the top of the V-groove can be obtained by an experiment or the like with reference to the beam focusing angle of the THz light. For example, when the beam focusing angle of the THz light is 18 °, The apex angle α can be about 20 °.
By providing a narrow parallel groove at the tip of the V groove and propagating the focused THz light in the parallel groove, the enhanced THz light can be guided to the phase matching unit 3 described below. The narrower the width of the parallel grooves, the higher the enhancement effect, and it is preferable that the width of the parallel grooves be narrower than the wavelength of THz light.

[Phase matching unit 3]
When optically coupling two light beams of sampling light and THz light, it is necessary to phase match the two light beams to be combined, and the condition is that the phase velocities in the medium of the two light beams are the same. . The phase matching unit 3 adjusts the phase velocity of the incident THz light so as to be the same as the group velocity of the sampling light. In the present invention, a method using a Cherenkov phase matching method in which THz light is incident obliquely with respect to the optical axis of the sampling light is used.
As materials for adjusting the phase velocity of THz light to be the same as the group velocity of sampling light, lithium niobate (LiNbO ₃ : abbreviation LN) or lithium tetraborate (Li ₂ B ₄ O ₇ : abbreviation) A crystal having a second-order nonlinear optical effect such as LBO) or β-barium borate (abbreviated as BBO) can be used, but an LN crystal having a high electro-optic coefficient is preferably used.

In order to make THz light incident at a Cherenkov phase matching angle (θ _c ) on sampling light incident on an LN crystal or a waveguide, for example, a Si prism having an inclination angle (90 ° −θ _c ) may be used. At this time, the Cherenkov phase matching angle θ _c is determined by the ratio of the phase refractive index n _THz of the THz light in the Si prism and the group refractive index _ng of the sampling pulse light in the LN crystal. That is, the Cerenkov phase matching angle θ _c is determined by cos θ _c = n _g / n _THz . As the LN crystal, it is preferable to use a crystal having a waveguide structure rather than using a bulk crystal in order to avoid absorption in the LN crystal and widen the detection band.

[Sum / Frequency Generator 4]
The sum / difference frequency generator 4 generates a sum frequency or a difference frequency of the two lights whose phases are adjusted.
A single crystal of the material having the nonlinear optical effect described above or a waveguide formed of this material can be used as the sum / difference frequency generator 4.

Here, if the sampling light frequency is ω ₀ and the THz light frequency is ω _THz , the sum frequency ω _SFG (= ω ₀ + ω _THz ) of these two waves is generated in the nonlinear optical crystal constituting the waveguide. To do. Sum frequency generation (Sum Frequency) where d _eff is the effective nonlinear optical coefficient (determined depending on the polarization direction of sampling light and terahertz light and the crystal orientation of the nonlinear optical crystal)
The nonlinear polarization by Generation (abbreviated as SFG) is given by the following equation. The sum frequency P ⁽²⁾ is

ε_０は真空の誘電率である。この非線形分極により和周波が発生する。サンプリング光が十分強く，その非線形相互作用による減衰を無視できるとし、ＳＦＧにより発生した光波の電界振幅をＥ_SFGとすると

ε ₀ is the vacuum dielectric constant. A sum frequency is generated by this nonlinear polarization. If the sampling light is strong enough and the attenuation due to the nonlinear interaction can be ignored, and the electric field amplitude of the light wave generated by _SFG is E _SFG

である。ここでμ_０は真空の透磁率，cは真空中の光速度，Ｌはサンプリング光が電気光学結晶中を伝搬する距離，ｎ_ω0＋ωTHzは周波数ω₀＋ω_THzにおける屈折率，
ｇ（ΔkL）は次で定義される位相不整合因子である。

It is. Where μ ₀ is the permeability of vacuum, c is the speed of light in vacuum, L is the distance that sampling light propagates through the electro-optic crystal, n _{ω0 + ωTHz} is the refractive index at frequency ω ₀ + ω _THz ,
g (ΔkL) is a phase mismatch factor defined as follows.

ここで波数不整合Δkは次で与えられる。

Here, the wave number mismatch Δk is given as follows.

k₀はサンプリング光の波数，ｋ_SFGはＳＦＧによる光波の波数，k_THzはＴＨｚ光の波数である。光波の周波数はＴＨｚ光の周波数に比べて２桁以上大きい（ω₀>>ω_THz）ので、ω₀＋ω_THzはω₀，ｎ_ω0＋ωTHzはｎ_ω0で近似してよい。すると（２）式は

k ₀ is the wave number of sampling light, k _SFG is the wave number of light wave by _SFG , and k _THz is the wave number of THz light. Since the frequency of the light wave is at least two orders of magnitude higher than the frequency of the THz light (ω ₀ >> ω _THz ), ω ₀ + ω _THz may be approximated by ω ₀ and n _{ω0 + ωTHz} may be approximated by n _ω0 . Then equation (2) becomes

と書き換えられる。

It can be rewritten as

The original sampling light and the light generated by the sum frequency generation are superposed and detected. Assuming that the efficiency of sum frequency generation is small and that the original sampling light is hardly attenuated, the change rate of the light intensity (total light intensity normalized by the original sampling light intensity I ₀ ) is given by the following equation.

最後の式では発生した和周波発生の電界振幅Ｅ_SFGは、サンプリング光の電界振幅E₀よりずっと小さいとしているので、Ｅ_SFGの自乗に対応する項は無視した。

In the last equation, the electric field amplitude E _{SFG of the} generated sum frequency is assumed to be much smaller than the electric field amplitude E ₀ of the sampling light, so the term corresponding to the square of E _SFG was ignored.

[Detector 5]
The detector 5 detects the sum frequency component or the difference frequency component from the output from the sum difference frequency generator 4.
When only the change amount of the sampling light intensity is detected by the lock-in detection method, the signal intensity is given by the following equation. In the equation (6), the electric field amplitude E _THz of the light wave generated by the sum frequency generation is almost the same as the wavelength of the original sampling light. Therefore, due to interference, E _THz is a product of E ₀ ((6) It is shown that it is detected as the middle term of equation 2). This is the same as the heterodyne detection, the signal is enhanced by E _0.
(6) If we rewrite the formula,

となり、これに（５）式を用いると、（７）式は次式で与えられる。

Then, using equation (5) for this, equation (7) is given by the following equation.

ＬＮ結晶の場合，非線形感受率テンソル成分のなかでｄ₃₃が一番大きい。
また

In the case of the LN crystal, d ₃₃ is the largest among the nonlinear susceptibility tensor components.
Also

の関係を用いて（８）式を書き換えると

Rewriting equation (8) using the relationship

を得る。ここでｎ_eはＬＮ結晶の異常光の屈折率で、ｒ₃₃はｄ₃₃に対応した電気光学係数である。
さらにＩｍ［g（ΔkL）］はΔkL=2.216 のとき最大値ｇ_max=0.723をとる。チェレンコフ位相整合では波数不整合Δkを結晶の角度で任意に調整できるので、ΔkL =2.216となるように調整することで容易にｇ_max=0.723を得る（結晶の伝搬長Ｌは一定とする）ことができる。
すなわち、波数不整合Δkを最適化することでヘテロダイン方式による信号の最大値を得ることができ、このときのヘテロダイン方式による信号強度は

Get. Where n _e is the extraordinary refractive index of the LN crystal, r ₃₃ is the electro-optic coefficient corresponding to the d _33.
Further, Im [g (ΔkL)] takes the maximum value g _max = 0.723 when ΔkL = 2.216. In Cherenkov phase matching, wave number mismatch Δk can be adjusted arbitrarily by the crystal angle, so g _max = 0.723 can be easily obtained by adjusting ΔkL = 2.216 (the crystal propagation length L is constant). Can do.
That is, by optimizing the wave number mismatch Δk, the maximum value of the signal by the heterodyne method can be obtained, and the signal strength by the heterodyne method at this time is

で与えられる。

Given in.

[Comparison]
The method of the present invention is compared with a conventional EO sampling method using an LN crystal.
The signal modulation degree in the conventional EO sampling method is given by the following equation (assuming perfect phase matching condition).

ここで、DGはＴＨｚ光によって引き起こされた位相遅延である。ｎ_oはＬＮ結晶の正常光の屈折率である。
例えば、ｒ₁₃ =9.6 [pm/V], ｒ₃₃ =30.9 [pm/V], ｎ_o= 2.255 and ｎ_e=2.176 （800
nm）を用いてそれぞれの性能指数を計算すると従来のＥＯサンプリング法では、

Where DG is the phase delay caused by THz light. n _o is the refractive index of normal light of the LN crystal.
For _{example, r 13 = 9.6 [pm /} V], r 33 = 30.9 [pm / V], n o = 2.255 and n e = 2.176 (800
nm) to calculate each figure of merit, the conventional EO sampling method

となり、本発明のＥＯサンプリング法では、

In the EO sampling method of the present invention,

となる。
以上から、本発明のＥＯサンプリング法は従来のＥＯサンプリング法よりも高効率（ＬＮ結晶の場合で約２倍）であることがわかる。

It becomes.
From the above, it can be seen that the EO sampling method of the present invention is more efficient than the conventional EO sampling method (about twice in the case of LN crystal).

[Other Embodiments]
In the above description, the sum frequency is detected. However, in the present invention, the difference frequency may be detected. In the crystal, not only the sum frequency but also the difference frequency is generated simultaneously. In this case, the sign of the light heterodyne signal generated by the difference frequency is inverted compared to the sum frequency. Which signal is dominant depends on the phase of the THz light with respect to the sampling light wave.
In the above description, the sampling light and the THz light have a single frequency. However, since each is actually a pulse wave, it has a finite frequency spectrum distribution. The expression corresponding to the expression (10) considering the difference frequency generation and the finite frequency spectrum is given by the following expression.

ここでＣ（ω_THｚ）は次で定義されるサンプリング光の相互相関関数である。

Here, C (ω _THz ) is a cross-correlation function of sampling light defined as follows.

ω_THz =0のとき

When ω _THz = 0

はサンプリング光の自己相関関数であり、サンプリング光強度に比例した量である。サンプリング光のスペクトル分布がＴＨｚ光のそれよりも十分広い（サンプリング光のパルス幅がＴＨｚ光に対して十分狭い場合に相当）とすると

Is an autocorrelation function of the sampling light, which is an amount proportional to the sampling light intensity. If the spectral distribution of sampling light is sufficiently wider than that of THz light (corresponding to the case where the pulse width of sampling light is sufficiently narrower than that of THz light)

と近似でき、（１５）式は

(15) can be approximated as

とあらわされる。（１９）式の導出においてはΔkはω_THzに依存しないと仮定した。この仮定はチェレンコフ位相整合では妥当な仮定である。
（１９）式は（１０）式と類似しているが，周波数ω_THzに対するＴＨｚ光の振幅Ｅ_THz（ω_THz）の代わりに、ＴＨｚ光の時間波形Ｅ_THz（τ）が用いられている。ここでτはＴＨｚ光とサンプリング光の相対遅延時間である。したがって（１９）式はＴＨｚ光の時間波形に依存したＥＯ信号が検出されることを示している。

It is expressed. In the derivation of the equation (19), it is assumed that Δk does not depend on ω _THz . This assumption is a reasonable assumption for Cherenkov phase matching.
The equation (19) is similar to the equation (10), but the THz light time waveform E _THz (τ) is used instead of the THz light amplitude E _THz (ω _THz ) with respect to the frequency ω _THz . Here, τ is a relative delay time between the THz light and the sampling light. Therefore, equation (19) indicates that an EO signal depending on the time waveform of THz light is detected.

[Specific equipment configuration]
An example of a specific apparatus configuration of the present invention is shown in FIG.
As shown in FIG. 2, the terahertz light detecting device includes a sampling light irradiation unit 1 including a femtosecond laser device 11 and a lens 12, and a THz light irradiation unit including a femtosecond laser device 21 and a THz emitter 22. 2 and an intensifying unit 9 that combines and enhances the THz light emitted from the THz light irradiating unit 2 with a metal surface plasmon.
In the example of FIG. 2, the phase matching unit 3 and the sum / frequency generator 4 are configured to include a prism 31 and a waveguide 41 formed on one surface of the prism 31.
As the prism 31, it is preferable to use a silicon (Si) prism having small absorption and dispersion in the terahertz band.
FIG. 3 shows an example of the phase matching unit 3 and the sum / frequency generator 4 used in the apparatus of FIG.

The prism 31 has a triangular shape in which one inclined surface is formed as an incident surface for THz light, and a waveguide 41 made of a thin film of LN crystal is formed on the surface on which the THz light incident from the inclined surface is incident obliquely. Is formed.
The inclined angle of the inclined surface is formed so that the THz light incident from the inclined surface becomes an angle θ suitable for Cherenkov phase matching. The Cherenkov phase matching angle is determined by the ratio of the phase refractive index n _THz of the THz light in the prism 31 to the group refractive index _ng of the sampling pulse light in the LN crystal. That is, the Cerenkov phase matching angle θ is determined by cos θ _c = n _g / n _THz .

The prism 31 having the above-described configuration is attached to the augmenting section 9 as shown in FIG.
The reinforcing portion 9 has a main body 90 formed of a metal block, a V-groove 91 formed in the main body 90, and a narrow parallel portion 92 formed at the tip of the V-groove 91. When the main body 90 is formed of a metal material such as aluminum or iron, or when formed of a non-metal material such as resin or ceramic, gold (Au) or silver (Ag) is formed on the inner surfaces of the V groove 91 and the parallel portion 92. It is preferable to form a metal layer.
A wide portion of the V groove is opened at one end surface of the main body 90, and THz light taken from the opening is propagated to the narrow tip portion while being enhanced by being combined with the metal surface plasmon of the V groove.

  Since the enhancement effect increases as the width dimension t1 of the parallel portion 92 decreases, the width dimension t1 is preferably as small as possible. More preferably, it is smaller than the wavelength of THz light. For example, when the wavelength of THz light is 1 THz (wavelength 0.3 mm), the width dimension t1 is preferably set to 0.3 mm or less.

  Two prism holding portions 93, 93 are provided opposite to the other end surface of the main body 90 where the tip of the parallel portion 92 is open. The prism holding portions 93 and 93 may be formed integrally with the main body 90 or may be formed of a block-like member that is separate from the main body 90 as shown in the figure. The two prism holding portions 93 and 93 are spaced apart from each other in parallel so as to have a gap communicating with the parallel portion 92. And the prism 31 in which the THz light which propagated the parallel part 92 injects is inserted and hold | maintained in the said clearance gap.

The opposing surfaces of the two prism holding portions 93 and 93 that come into contact with the prism 31 are preferably finished in the same manner as the inner surface of the parallel portion 92 so as to form a part of the parallel portion 92.
The prism 31 directs the slope on which THz light is incident to the parallel portion 92 side, and adjusts the orientation so that the THz light enters perpendicularly to the slope, and is held by the prism holders 93 and 93. .

The width t2 of the prism 31 is preferably the same as the width t1 of the parallel portion 92. However, when it is difficult to form the thin prism 31 or the waveguide 41, the width t2 of the parallel portion 92 is larger than the width t1 of the parallel portion 92. It may be large.
Although the support substrate 6 shown in FIG. 3 is omitted in the example of FIG. 4, the prism holding members 93 and 93 correspond to the support substrate 6 of FIG. 3 in the example of FIG. In the example of FIG. 4, a covering material that covers the waveguide 41 exposed from the gap between the prism holding members 93 and 93 may be provided as necessary.

[Light guide path]
As shown in FIG. 2, the light guide path for guiding the sampling light emitted from the sampling light irradiation unit 1 and the THz light emitted from the THz light irradiation unit 2 to the prism 31 and the waveguide 41 through the enhancement unit 9 is as shown in FIG. It is composed of a reflecting mirror 8 installed facing the sampling light irradiation unit 1, a concave mirror 7a installed facing the THz light irradiation unit 2, and a concave mirror 7b installed facing the concave mirror 7a. .

The reflecting mirror 8 is arranged so that the sampling light enters the waveguide 41, and the concave mirrors 7 a and 7 b are arranged so that the THz light enters the inclined surface of the prism 31 from the V groove 91 of the enhancement unit 9.
With the above configuration, the sampling light irradiated from the sampling light irradiation unit 1 is changed in its traveling direction by the reflecting mirror 8 and is incident from the side surface of the waveguide 41.
Further, the THz light emitted from the THz light irradiating unit 2 is converted in its traveling direction by the concave mirrors 7a and 7b, introduced into the V-groove 91 of the intensifying unit 9, and focused at the tip. The THz light enhanced in this way propagates through the parallel portion 92 and enters the inclined surface of the prism 31.
The enhanced THz light and sampling light are combined in the waveguide 41 and output from the waveguide 41 as a sum frequency output or a difference frequency output. The outputted sum frequency output or difference frequency output is detected by the heterodyne detector 5 whose position is adjusted so that the signal to be detected becomes maximum.

[Example 1]
The effect of the present invention was tested under the following conditions.
Laser light source: wavelength 780nm, pulse width <100fs, repetition rate 50MHz
THz light generator: Bowtie photoconductive antenna, bias +/- 50 Vp-p @ 95kHz (Sin wave), pump power approx. 6mW
EO element: Si prism-coupled LiNbO ₃ crystal (thickness 2mm, 1% Mol. MgO-doped stoichiometric LN), probe power about 1mW
Angle α at the top of the V-groove of the enhancement part: 20 ° (THz light focusing angle 18 °)
Width dimension t1: 0.3 mm of the parallel part 92 of the reinforcing part
Prism width t2: 0.3 mm
Waveguide: A waveguide having a thickness of 2 mm was attached to the surfaces of the prism holding portions 93 and 93 so as to be in contact with the prism.
[Example 2]
Example 1 is the same as Example 1 except that the thickness of the waveguide is 40 μm.
[Comparative example]
The present embodiment is the same as the first embodiment except that the enhancement unit 9 is not provided and THz light is directly incident on the prism 31.

In FIG. 5, the detection result of the THz light by a comparative example and a conventional method is shown. FIG. 5 shows a time domain waveform of a THz pulse wave detected by the EO sampling method in the comparative example, together with a waveform when detected by the conventional EO sampling. It can be seen that, even in the comparative example that does not have the enhancement unit 9, THz light can be detected in the same manner as in the past without using a wave plate or a polarizing element.
FIG. 6 is a graph similar to FIG. 5 showing a comparison result between the comparative example not using the enhancement unit 9 and Examples 1 and 2 using the enhancement unit 9.
It can be seen that the signal intensity is enhanced by using the enhancement unit 9 as in the present invention. When the thickness of the waveguide is the same as in Example 1 and the comparative example (thickness 2 mm), Example 1 is about 5.5 times the comparative example. Further, the degree of enhancement becomes higher as the waveguide thickness is smaller, and is about 17 times that of the comparative example when the thickness of the waveguide is 40 μm as in the second embodiment.

[Other Embodiments]
FIG. 7A shows another embodiment of the detector 5 in the detection method and detection apparatus of the present invention. As described above, not only the sum frequency but also the difference frequency is generated simultaneously in the crystal, and the sign of the light heterodyne signal generated by the difference frequency is inverted with respect to the sum frequency. Further, the sum frequency output and the difference frequency output can be divided into two symmetrically about the center of the output by wave vector matching. Therefore, the detector 5 of this embodiment includes a pair of left and right photodiodes PDA and PDB, and the sum frequency output and the difference frequency output output from the waveguide 41 through the lens 52 are three reflecting mirrors. It is divided by 51a, 51b, 51c. That is, the output pointed to the reflecting mirror 51b by the reflecting mirror 51a is changed in direction from the output II that goes straight to the detector 5 by the reflecting mirror 51b as shown in FIG. The output I is directed to the reflecting mirror 51c. The two divided outputs are input to the respective photodiodes PDA and PDB. In this embodiment, the detector 5 is aligned so that the polarities of the signals detected by the photodiodes PDA and PDB are inverted as shown in FIGS. 7 (a) (i) (ii).

FIG. 8 shows a graph in which the respective signals detected by the photodiodes PDA and PDB are superimposed and a graph in which the difference between the two signals (Balance detection) is obtained.
As can be seen from the graph of FIG. 8, in the detector 5 of this embodiment, the detection sensitivity can be almost doubled by obtaining the difference between the signals detected by the photodiodes PDA and PDB. Further, since the noise signal has the same polarity in the left and right photodiodes PDA and PDB, the noise component can be reduced by obtaining the difference.

Although a preferred embodiment of the present invention has been described, the present invention is not limited to the above description.
For example, in the above description, the parallel portion 92 is provided at the tip of the V groove 91, and THz light enhanced by the V groove 91 is guided to the prism 31 through the parallel portion 92. It is also possible to dispose the prism 31 so that the parallel portion 92 is not provided. In this case, the prism 31 may be disposed outside the tip of the V groove 91 or may be disposed inside the tip of the V groove 91.
Furthermore, the parallel part 92 which propagates THz light is not limited to a straight line shape, and a part or all of the parallel part 92 may be curved.

The present invention can be suitably applied to detection of electromagnetic waves in a region (frequency 30 MHz to 30 THz: 10 m to 10 μm) including a millimeter wave band, a microwave band, and a terahertz light band.
In addition, the present invention can be applied to various sensing devices and imaging devices.

It is a block diagram explaining the apparatus structure of this invention. It is the schematic which shows a specific apparatus structure. It is the schematic which shows an example of the phase matching part and sum-and-frequency generator used for the apparatus of FIG. It is a schematic perspective view explaining the structure concerning one Embodiment of the reinforcement part of this invention. It is a graph which shows the detection result of the THz light by a comparative example and a conventional method. It is a graph which shows the detection result of THz light by Examples 1 and 2 and a comparative example. It is the schematic explaining the structure of the detector concerning other embodiment of this invention which detects the output divided into 2 with two detection parts. It is a graph explaining the effect of the detector in other embodiments.

DESCRIPTION OF SYMBOLS 1 Sampling light irradiation part 2 Terahertz light irradiation part 3 Phase matching part 4 Sum frequency generator 5 Detector 9 Enhancing part 90 Main body 91 V groove | channel 92 Parallel part 93 Prism holding | maintenance part

Claims (8)

In an electromagnetic wave detection method for detecting an electromagnetic wave by an EO sampling method using an electro-optic effect,
The sampling light irradiated from the sampling light irradiation means is incident on the nonlinear optical crystal,
Focusing and enhancing the electromagnetic wave of the detection target irradiated from the electromagnetic wave irradiation means,
Increasing the electromagnetic wave incident on the nonlinear optical crystal at a Cherenkov phase matching angle with respect to the optical axis of the sampling light,
A wave vector matched output including a sum frequency component and a difference frequency component output by combining the phase-matched electromagnetic wave and the sampling light is divided into two symmetrically at the center of the output,
Each of the divided outputs is input to two detectors arranged in advance at positions where the signal polarity is inverted,
One of the detection units detects the sum frequency component and the other detection unit detects the difference frequency component,
Obtaining a difference between two signals detected by heterodyne detection method or homodyne detection method in each of the detection units;
An electromagnetic wave detection method characterized by the above. 2. The electromagnetic wave according to claim 1, wherein the electromagnetic wave irradiated from the electromagnetic wave irradiation means is introduced into a V groove having at least a surface formed of a metal, and then propagated to a tip portion of the V groove to enhance the electromagnetic wave. Detection method. The electromagnetic wave detection method according to claim 1, wherein a waveguide is formed by the nonlinear optical crystal, and the sampling light is incident on the waveguide. In an electromagnetic wave detection device for detecting an electromagnetic wave by an EO sampling method using an electro-optic effect,
Sampling light irradiation means for irradiating sampling light;
An electromagnetic wave irradiation means for irradiating an electromagnetic wave to be detected;
Electromagnetic wave enhancing means for focusing and enhancing the electromagnetic wave;
Phase matching means for causing the enhanced electromagnetic wave to be incident on the optical axis of the sampling light at a Cherenkov phase matching angle;
A sum / difference frequency generating means for generating an output including a sum frequency component and a difference frequency component by combining the sampling light and the electromagnetic wave formed of a nonlinear optical crystal and phase-matched;
A dividing unit that divides the wave vector matched output including the sum frequency component and the difference frequency component into two symmetrically at the center of the output, and each of the outputs divided by the dividing unit is input. And detecting the difference frequency component on the other side, and two detection units arranged at positions where the polarity of the detected signal is reversed, and each of the detection units includes a heterodyne detection method or a homodyne Detection means for obtaining a difference between two signals detected by the detection method;
An electromagnetic wave detection device comprising: 5. The electromagnetic wave detecting device according to claim 4 , wherein the electromagnetic wave enhancing means is formed of a V-groove having at least a surface made of metal. The electromagnetic wave detection device according to claim 5 , wherein an angle of a top portion of the V groove is determined based on a convergence angle of an electromagnetic wave incident on the V groove. The phase matching means and the sum-and-difference frequency generating means have a prism formed to output the electromagnetic wave incident from one inclined surface to the other inclined surface at a Cherenkov phase matching angle, and the other inclined surface. It is a conjugate with the formed nonlinear optical crystal,
Arranging the prism at the tip of the V-groove;
The electromagnetic wave detection device according to claim 5 or 6 . The electromagnetic wave detection device according to claim 7 , wherein a parallel portion is provided at a tip portion of the V groove, and the prism is disposed in the parallel portion.

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