JP6306363B2 - Time domain spectrometer - Google Patents

Description

  The present invention relates to a time-domain spectroscopic device using a photoconductive electromagnetic wave generator and an electromagnetic wave detector.

  Conventionally, as this type of time domain spectroscopic device, a laser irradiation device that generates a femtosecond laser (excitation light), a beam splitter that divides the femtosecond laser into pump light and probe light, and terahertz electromagnetic waves are generated by the pump light. Together with an electromagnetic wave generator for irradiating the measurement sample with the terahertz electromagnetic wave, an electromagnetic wave detection device for detecting the terahertz electromagnetic wave transmitted through the measurement sample by the probe light, a delay optical system for delaying the femtosecond laser incident on the electromagnetic wave detection device, An apparatus including various optical systems that reflect and collect a femtosecond laser and a signal processing device that processes an input signal is known (see Patent Document 1). In this time domain spectroscopic device, the same electromagnetic wave generation detection device is used as the electromagnetic wave generation device and the electromagnetic wave detection device. The electromagnetic wave generation detection device includes a substrate, a buffer layer grown on the substrate, a semiconductor layer grown on the buffer layer, and an antenna formed on the semiconductor layer.

JP 2013-2995 A

By the way, in this type of time domain spectroscopic device, it is conceivable to use InGaAs having a predetermined In ratio (for example, 35%) as the semiconductor layer of the electromagnetic wave generation detecting device.
However, as a result of applying this electromagnetic wave generation detection device to both the electromagnetic wave generation device and the electromagnetic wave detection device, there is a problem that noise (dark current) of the electromagnetic wave detection device increases and the detection sensitivity of the terahertz wave is lowered. It was.

  An object of the present invention is to provide a time domain spectroscopic device capable of improving the detection sensitivity of terahertz waves by using an optimum photoconductive layer in each of the electromagnetic wave generation unit and the electromagnetic wave detection unit.

The time domain spectroscopic device of the present invention includes an electromagnetic wave generation unit that generates electromagnetic waves by pump light and irradiates the generated electromagnetic waves to the target, an electromagnetic wave detection unit that detects electromagnetic waves transmitted or reflected by the target by probe light, comprising a wave generation unit and the electromagnetic wave detection unit, the _{in X Ga 1-X As (} 0 ≦ X ≦ 0.53) photoconductive layer made of a compound semiconductor containing quantum dots, each containing Mutotomoni photoconductive layer a pair of antennas disposed opposite and in presence of gaps formed thereon, respectively have, in ratio X of the photoconductive layer of the electromagnetic wave detection unit, rather less than the in ratio X of the photoconductive layer of the electromagnetic wave generator , the gap of the electromagnetic wave detection unit is characterized smaller than the gap of the electromagnetic wave generator Ikoto.

  In this case, the In ratio X of the photoconductive layer of the electromagnetic wave generation unit is preferably 30% or more and 40% or less, and the In ratio X of the photoconductive layer of the electromagnetic wave detection unit is preferably 0% or more and 10% or less.

Further, a part or all of the photoconductive layer of the electromagnetic wave generating part is added with Be in a ratio of 1 × 10 ¹⁷ cm ⁻³ or more and 1 × 10 ¹⁸ cm ⁻³ or less, and part of the photoconductive layer of the electromagnetic wave detecting part. Alternatively, all of Be is preferably added at a ratio of 1 × 10 ¹⁷ cm ⁻³ or more and 1 × 10 ¹⁸ cm ⁻³ or less.

  On the other hand, an excitation light source that outputs the excitation light and a light splitting unit that divides the excitation light output from the excitation light source into pump light and probe light are provided. It is preferable to irradiate excitation light of 45 μm or more and 1.65 μm or less.

  Each of the photoconductive layers of the electromagnetic wave generation unit and the electromagnetic wave detection unit preferably includes a plurality of individual photoconductive layers and a plurality of quantum dots formed between the individual photoconductive layers. .

  In this case, it is preferable that the quantum dots of each layer are formed based on the strain caused by the difference in lattice constant with respect to the individual photoconductive layer of each layer located in the lower layer.

  The quantum dot is preferably any of InAs, InGaAs, InAsSb, and InGaSb.

It is a block diagram of the time-domain spectroscopy apparatus which concerns on embodiment. (A) is the perspective view which represented typically the generation side photoconductive antenna element, (b) is the perspective view which represented the detection side photoconductive antenna element typically. (A) is sectional drawing which represented the generation | occurrence | production side photoconductive antenna element typically, (b) is sectional drawing which represented the detection side photoconductive antenna element typically. (A) is the figure (graph) showing the relationship between In ratio of the photoconductive layer of a detection side photoconductive antenna element, and a dynamic range, (b) is the photoconductive layer of a detection side photoconductive antenna element. It is a figure (graph) showing the relationship between In ratio and S / N ratio. (A) is the top view showing the generation side photoconductive antenna element in a modification, (b) is the top view showing the detection side photoconductive antenna element in a modification.

  Hereinafter, a time domain spectroscopic device according to an embodiment of the present invention will be described with reference to the accompanying drawings. This time domain spectrometer is an application of time domain spectroscopy, which generates terahertz waves using pulsed laser light, detects the terahertz waves after passing through the sample, and detects the terahertz of the sample from the detection result. Spectral analysis such as material analysis by waves is performed. In addition, the terahertz wave in this case includes not only electromagnetic waves of 0.1 THz to 10 THz but also electromagnetic waves of several tens GHz to several hundreds THz.

  FIG. 1 is a configuration diagram of a time domain spectroscopic device 1. As shown in the figure, the time domain spectroscopic device 1 includes a pumping light source 2 that generates and outputs pumping light that is pulsed laser light, and a beam that splits the output pumping light into pump light LP1 and probe light LP2. A photoconductive terahertz wave generating unit that generates a terahertz wave Th (electromagnetic wave) by the splitter 3 (light splitting unit) and the pump light LP1 from the beam splitter 3 and irradiates the sample S (target) with the terahertz wave Th. 4 (electromagnetic wave generation unit) and a photoconductive terahertz wave detection unit 5 (electromagnetic wave detection unit) that detects the terahertz wave Th transmitted through the sample S by the probe light LP2 from the beam splitter 3.

  In addition, the time domain spectroscopic device 1 delays the probe light LP2 incident on the terahertz wave detection unit 5 and the first reflection that guides the probe light LP2 output from the beam splitter 3 to the delay optical system 11. A mirror 12, and a second reflecting mirror 13 and a third reflecting mirror 14 that appropriately reflect the probe light LP 2 output from the delay optical system 11 and guide it to the terahertz wave detection unit 5 are provided.

  Further, the time domain spectroscopic device 1 supplies the first parabolic mirror 15 that guides the terahertz wave Th irradiated from the terahertz wave generation unit 4 to the sample S, and the terahertz wave Th that has passed through the sample S to the terahertz wave detection unit 5. And a second parabolic mirror 16 for guiding.

  Furthermore, the time domain spectroscopic device 1 includes a bias power supply circuit 17 that applies a predetermined bias voltage to the generation-side photoconductive antenna element 21A (described later) of the terahertz wave generation unit 4, and the detection-side light of the terahertz wave detection unit 5. And a signal processing circuit 18 for detecting and processing a current generated in the conductive antenna element 21B (described later). Although details will be described later, the signal processing circuit 18 performs spectral analysis processing such as imaging processing and material analysis.

  The excitation light source 2 irradiates excitation light such as a femtosecond pulse laser. Specifically, the excitation light source 2 emits pulsed laser light having a wavelength of 1.45 μm or more and 1.65 μm or less as excitation light.

  The beam splitter 3 divides the excitation light output from the excitation light source 2, outputs one of the divided excitation lights as pump light LP1, and outputs the other divided excitation light as probe light LP2.

  The terahertz wave generation unit 4 includes a generation side photoconductive antenna element 21A irradiated with the pump light LP1, and a hemispherical lens 22a attached to the sample S side of the generation side photoconductive antenna element 21A. The generation side photoconductive antenna element 21A is arranged so that the pump light LP1 is irradiated on the surface side (parallel transmission line 32 side described later).

  The terahertz wave detection unit 5 includes a detection-side photoconductive antenna element 21B irradiated with the probe light LP2, and a hemispherical lens 22b attached to the sample S side of the detection-side photoconductive antenna element 21B. The detection-side photoconductive antenna element 21B is arranged so that the probe light LP2 is irradiated on the front surface side (parallel transmission line 32 side described later) and the terahertz wave Th transmitted through the sample S is irradiated on the back surface side. Has been.

  In such a time domain spectroscopic device 1, the excitation light output from the excitation light source 2 is divided into pump light LP 1 and probe light LP 2 by the beam splitter 3. The pump light LP1 divided by the beam splitter 3 is incident on the generation side photoconductive antenna element 21A. At this time, by applying a predetermined bias voltage to the generation side photoconductive antenna element 21A by the bias power supply circuit 17, the terahertz wave Th is generated from the generation side photoconductive antenna element 21A. The terahertz wave Th passes through the hemispherical lens 22a and is irradiated to the sample S through the first parabolic mirror 15. Then, the terahertz wave Th that has passed through the sample S enters the detection-side photoconductive antenna element 21B via the second parabolic mirror 16 and the hemispherical lens 22b.

  On the other hand, the probe light LP <b> 2 divided by the beam splitter 3 is reflected by the first reflecting mirror 12 and irradiated on the delay optical system 11. Thereafter, the probe light LP2 to which the time delay is given by the delay optical system 11 enters the detection-side photoconductive antenna element 21B via the second reflecting mirror 13 and the third reflecting mirror 14. A signal detected by the detection side photoconductive antenna element 21B by the incident terahertz wave Th and the probe light LP2 is input to the signal processing circuit 18. The signal processing circuit 18 stores the time waveform of the terahertz electromagnetic wave that has passed through the sample S and the time waveform of the terahertz electromagnetic wave without the sample S as time-series data, respectively, and performs Fourier transform processing to convert them into a frequency space. . Thus, a spectrum of intensity amplitude and phase of the terahertz electromagnetic wave from the sample S is obtained. Thereby, analysis information (spectral analysis information) for searching for partial physical and scientific properties of the sample S and imaging information of the terahertz wave Th can be obtained.

  Here, the generation side photoconductive antenna element 21A and the detection side photoconductive antenna element 21B will be described with reference to FIGS. 2A and 2B are perspective views of the generation side photoconductive antenna element 21A and the detection side photoconductive antenna element 21B. 3A and 3B are cross-sectional views of the generation side photoconductive antenna element 21A and the detection side photoconductive antenna element 21B. The generation side photoconductive antenna element 21A and the detection side photoconductive antenna element 21B are the same except for the elemental composition ratio of the photoconductive layers 53a and 53b.

  As shown in the figure, the two photoconductive antenna elements 21A and 21B are composed of a photoconductive substrate 31 which is a main body and a parallel transmission line 32 formed on the photoconductive substrate 31.

  The parallel transmission line 32 includes a pair of dipole antennas 41 and 41 formed on a photoconductive substrate 31 (photoconductive layers 53a and 53b described later). Each antenna 41 has a line portion 42 extending linearly and an electrode portion 43 extending inward from the center of the line portion 42, and at least one end of the line portion 42 serves as an electrode pad 44. Function. The pair of antennas 41, 41 are arranged such that the line portions 42, 42 are parallel to each other, and the mutual electrode portions 43, 43 are opposed to each other with a predetermined gap. That is, a gap 45 having a width of several μm (in the embodiment, about 5 μm) is formed between the opposing ends of the electrode portions 43 and 43.

  Each antenna 41 of the embodiment is made of Au (gold), but may be a conductive material such as Al, Ti, Cr, Pd, Pt, Au—Ge alloy, Al—Ti alloy. The form of the parallel transmission line 32 (antenna 41) may be a bow tie type, a strip line type, a spiral type, or the like. Moreover, the structure which forms multiple gap parts 45 (electrode part 43) may be sufficient.

  The photoconductive substrate 31 includes a substrate 51, a buffer layer 52 formed on the substrate 51, and photoconductive layers 53 a and 53 b formed on the buffer layer 52. Although details will be described later, in this embodiment, the elemental composition ratio of the photoconductive layers 53a and 53b is made different between the generation-side photoconductive antenna element 21A and the detection-side photoconductive antenna element 21B, thereby increasing the dynamic range. And the S / N ratio is improved.

  The substrate 51 is a single crystal substrate of a compound semiconductor and is made of GaAs (gallium arsenide). As a material of the substrate 51, a single crystal semiconductor such as Si, Ge, InP or the like can be used according to the material (lattice constant) of the buffer layer 52 and the photoconductive layers 53a and 53b in addition to GaAs.

  The buffer layer 52 is formed on the substrate 51 using a material having a lattice constant not less than the lattice constant of the substrate 51, not more than the lattice constant of the photoconductive layers 53a and 53b or not more than the lattice constant of the substrate 51, and not less than the lattice constant of the photoconductive layers 53a and 53b. In the embodiment, the thin film is formed of GaAs. The buffer layer 52 is provided to control the crystallinity of the photoconductive layers 53a and 53b laminated thereon. Therefore, the semiconductor material constituting the buffer layer 52 can be arbitrarily selected and used according to the material (lattice constant) of the substrate 51 and the photoconductive layers 53a and 53b. The buffer layer 52 may be a plurality of layers.

The photoconductive layers 53 a and 53 b are made of an In _X Ga _1-X As (0 ≦ X ≦ 0.53) compound semiconductor containing the quantum dots 62. Specifically, the photoconductive layers 53a and 53b include a plurality of individual photoconductive layers 61a and 61b, and a plurality of quantum dots 62 formed between the plurality of individual photoconductive layers 61a and 61b. Has been.

The individual photoconductive layers 61a and 61b are formed by growing In _x Ga _1-x As (0 ≦ X ≦ 0.53) on the substrate 51 through the buffer layer 52 at a low temperature (so-called LT-GaAs or LT -InGaAs), and the quantum dots 62 are obtained by growing InAs (indium arsenide) on the individual photoconductive layers 61a and 61b. The quantum dot 62 of the present embodiment uses a lattice constant mismatch (difference) between InAs constituting the quantum dot 62 and GaAs (or InGaAs: indium gallium arsenide) in the lower layer (or upper layer). The island-like structure (quantum dots 62) is realized by the strain force generated thereby. Then, the photoconductive layers 53a and 53b are formed by alternately forming the individual photoconductive layers 61a and 61b and forming the quantum dots 62 so as to contain the quantum dots 62 in the photoconductive layers 53a and 53b. ing.

  The quantum dots 62 are preferably made of InAs, InGaAs, InAsSb, InGaSb, or the like in consideration of the difference in lattice constant from the photoconductive layers 53a and 53b (individual photoconductive layers 61a and 61b). Furthermore, as a method for forming the quantum dots 62, the quantum dots 62 may be formed during crystal growth, such as selective growth using a fine mask or a method using a surfactant.

  In the generation side photoconductive antenna element 21A configured as described above, excitation light (pump light LP1) is applied to the gap portion 45 in a state where an electric field is generated between the electrode portions 43 and 43 by applying a bias voltage. When irradiated, the photoconductive layer 53a of the gap 45 is excited by the excitation light. Thereby, carriers (electrons and holes) are generated in the photoconductive layer 53a, and the carriers are accelerated by the voltage (electric field) between the electrode portions 43 and 43, so that an instantaneous current flows. A terahertz wave Th (strictly, a terahertz pulse wave) is generated by the time fluctuation (ultra-high speed current modulation) of the pulse current, and is strongly radiated toward the substrate 51 having a large dielectric constant. Further, in the detection-side photoconductive antenna element 21B configured as described above, since a current flows between the pair of electrode portions 43 and 43 when receiving the terahertz wave Th, the terahertz wave Th can be detected.

Next, the difference between the generation side photoconductive antenna element 21A and the detection side photoconductive antenna element 21B in the time domain spectroscopic device 1 of the present embodiment will be described. In the time domain spectroscopic device 1 of the present embodiment, as described above, the element composition ratios in the photoconductive layers 53a and 53b of both the photoconductive antenna elements 21A and 21B are made different. Specifically, both the photoconductive layers 53a and 53b are made of In _x Ga _1-x As (0 ≦ X ≦ 0.53) containing the quantum dots 62, but the detection side photoconductive antenna element The In ratio X (indium ratio) in the photoconductive layer 53b (each individual photoconductive layer 61b) of 21B is the In ratio X in the photoconductive layer 53a (each individual photoconductive layer 61a) of the generation side photoconductive antenna element 21A. It is getting smaller. For example, the In ratio X of the photoconductive layer 53a of the generation side photoconductive antenna element 21A is 30% or more and 40% or less, and the In ratio X of the photoconductive layer 53b of the detection side photoconductive antenna element 21B is 0% or more and 10%. % Or less. Thereby, the dynamic range or S / N ratio of the spectrum (terahertz wave spectrum) obtained by Fourier transforming the terahertz wave time waveform is improved. Note that the In ratio X of the photoconductive layer 53a of the generation-side photoconductive antenna element 21A is ideally 35%, and more preferably around 35% (34% or more and 36% or less).

Here, the effectiveness of the difference in the In ratio X will be described with reference to the experimental results of FIG. FIG. 4A is a graph (graph) showing the relationship between the In ratio X of the photoconductive layer 53b of the detection-side photoconductive antenna element 21B and the dynamic range. On the other hand, FIG. 4B is a diagram (graph) showing the relationship between the In ratio X of the photoconductive layer 53b of the detection-side photoconductive antenna element 21B and the S / N ratio. The experimental results in both figures show that without the sample S, the photoconductive layers 53a of In _x Ga _1-x As (0 ≦ X ≦ 0.53) containing the quantum dots 62 in the photoconductive antenna elements 21A and 21B, 53b, and the In ratio X of the photoconductive layer 53a of the generation side photoconductive antenna element 21A was obtained as 35% (In ₃₅ Ga ₆₅ As).

  As shown in the figure, when the In ratio X in the photoconductive layer 53b of the detection side photoconductive antenna element 21B is the same as the In ratio X in the photoconductive layer 53a of the generation side photoconductive antenna element 21A (In ratio 35) %) Has a dynamic range of about 36 dB and an S / N ratio of about 3800. On the other hand, when the In ratio X in the photoconductive layer 53b of the detection-side photoconductive antenna element 21B is smaller than the In ratio X in the photoconductive layer 53a of the generation-side photoconductive antenna element 21A (In ratio is less than 35%), dynamic Both the range and the S / N ratio rise, the dynamic range exceeds 36 dB, and the S / N ratio exceeds 3800. As described above, the In ratio X in the photoconductive layer 53b of the detection-side photoconductive antenna element 21B is made smaller than the In ratio X in the photoconductive layer 53a of the generation-side photoconductive antenna element 21A. The ratio is improved. This is considered to be because by reducing the In ratio X, the resistance in the photoconductive layer 53b of the detection-side photoconductive antenna element 21B increases, and noise (dark current) is reduced. In other words, in the experimental results shown in the figure, the photoconductive layer 53b of the detection-side photoconductive antenna element 21B reduces the noise by increasing the In ratio X and increasing the resistance, rather than increasing the In ratio X and increasing the excitation efficiency. This suggests that it is relatively effective.

  According to the above configuration, the In ratio X in the photoconductive layers 53a and 53b of both photoconductive antenna elements 21A and 21B is made different, and the In ratio X in the photoconductive layer 53b of the detection side photoconductive antenna element 21B is set as follows. By making the size smaller than the photoconductive layer 53a of the generation side photoconductive antenna element 21A, the optimum photoconductive layers 53a and 53b can be used in each of the generation side photoconductive antenna element 21A and the detection side photoconductive antenna element 21B. The dynamic range and S / N ratio can be improved. That is, the detection sensitivity of the terahertz wave Th in the time domain spectroscopic device 1 can be improved.

  Further, the In ratio X of the photoconductive layer 53a of the generation side photoconductive antenna element 21A is set to an In ratio (30% to 40%) in a predetermined range in which the excitation efficiency is prioritized over the resistance increase (noise reduction), By setting the In ratio X in the photoconductive layer 53b of the detection side photoconductive antenna element 21B to an In ratio (0% or more and 10% or less) in a predetermined range in which resistance increase (noise reduction) is prioritized over excitation efficiency. The range and the S / N ratio can be further improved. This is because when the inventor of the present application increases the In ratio X in the generation-side photoconductive antenna element 21A, the noise does not become so large. In the detection-side photoconductive antenna element 21B, when the In ratio X is increased, the excitation efficiency exceeds In addition, it is confirmed that the noise becomes large, and this is an effect obtained by using this. That is, in the generation side photoconductive antenna element 21A, when the In ratio X is increased, the effect of the excitation light rate increase accompanying this is stronger than the effect of the resistance decrease (noise increase) associated therewith. In the side photoconductive antenna element 21B, when the In ratio X is increased, the effect of the resistance decrease (noise increase) is stronger than the effect of the excitation light rate increase associated therewith. Therefore, the dynamic range and the S / N ratio can be further improved by setting the In ratio X of each of the photoconductive layers 53a and 53b to the above-described predetermined ranges taking this into consideration.

  In the above embodiment, only the In ratio X of the photoconductive layers 53a and 53b is different between the photoconductive antenna elements 21A and 21B. In addition to this, as shown in FIG. The gap may be different between the parallel transmission lines 32 (the pair of antennas 41 and 41) of both the photoconductive antenna elements 21A and 21B. Specifically, the gap between the electrode parts 43 and 43 in the detection side photoconductive antenna element 21B is made smaller than the gap between the electrode parts 43 and 43 in the generation side photoconductive antenna element 21A. According to this configuration, by reducing the gap of the detection-side photoconductive antenna element 21B, it is possible to reduce the gap portion 45 that is the probe light irradiation region. Thereby, the spot size of the probe light condensed on the gap portion 45 can be reduced, and the laser light power density of the probe light irradiated on the gap portion 45 can be increased. Thereby, the detection sensitivity of the detection side photoconductive antenna element 21B is improved, and the detection sensitivity of the terahertz wave Th in the time domain spectroscopic device 1 can be further improved.

In the above embodiment, the photoconductive layers 53a and 53b of both photoconductive antenna elements 21A and 21B are made of undoped In _X Ga _1-X As (0 ≦ X ≦ 0.53). However, In _X Ga _1-X As (0) in which Be (beryllium) is added to the photoconductive layers 53a and 53b at a predetermined ratio (1 × 10 ¹⁷ cm ⁻³ or more and 1 × 10 ¹⁸ cm ⁻³ or less). ≦ X ≦ 0.53). According to this configuration, noise (dark current) in the photoconductive layers 53a and 53b is reduced, and the detection sensitivity of the terahertz wave Th in the time domain spectroscopic device 1 can be further improved.

Further, only part of the photoconductive layers 53a and 53b (for example, the surface side), In _X Ga to which Be is added at a predetermined ratio (1 × 10 ¹⁷ cm ⁻³ or more and 1 × 10 ¹⁸ cm ⁻³ or less). _It may be composed of _1-X As (0 ≦ X ≦ 0.53).

  In the above embodiment, the generated terahertz wave Th is transmitted through the sample S and detected, but it may be reflected by the sample S and detected. It is also possible to detect (imaging) the sample S in a plane using a plurality of generation side photoconductive antenna elements 21A and a plurality of detection side photoconductive antenna elements 21B arranged in a matrix. Of course, the sample S can be finely moved in the X and Y directions orthogonal to the optical path, and this can be detected (imaging) in a plane.

  1: time domain spectroscopic device, 2: excitation light source, 3: beam splitter, 4: terahertz wave generation unit, 5: terahertz wave detection unit, 41: antenna, 53a, 53b: photoconductive layer, 61a, 61b: individual light Conductive layer, 62: quantum dot, LP1: pump light, LP2: probe light, S: sample, Th: terahertz wave

Claims (7)

An electromagnetic wave generating unit that generates electromagnetic waves by pump light and irradiates the generated electromagnetic waves to a target;
An electromagnetic wave detection unit for detecting the electromagnetic wave transmitted or reflected by the target by probe light, and
The electromagnetic wave generating unit and the electromagnetic wave detection unit, the _{In X Ga 1-X As (} 0 ≦ X ≦ 0.53) photoconductive layer made of a compound semiconductor containing quantum dots, each containing Mutotomoni, the photoconductive layer Each having a pair of antennas formed on and facing each other with a gap between them,
In ratio X of the photoconductive layer of the electromagnetic wave detection unit is rather smaller than the In ratio X of the photoconductive layer of the electromagnetic wave generator,
The electromagnetic wave the gap of the detector, a time domain spectroscopic apparatus according to claim smaller Ikoto than the gap of the electromagnetic wave generator. In ratio X of the photoconductive layer of the electromagnetic wave generator is 30% or more and 40% or less,
2. The time domain spectroscopic device according to claim 1, wherein an In ratio X of the photoconductive layer of the electromagnetic wave detection unit is 0% or more and 10% or less . Part or all of the photoconductive layer of the electromagnetic wave generating part is added at a ratio of Be of 1 × 10 ¹⁷ cm ⁻³ or more and 1 × 10 ¹⁸ cm ⁻³ or less,
The electromagnetic wave detection unit of part or all of the photoconductive layer, claim 1, characterized in that Be is added in a proportion of 1 × ¹⁰ 17 ^{cm -3} to 1 × ¹⁰ 18 ^{cm -3} or less, or 2. The time domain spectroscopic apparatus according to 2. An excitation light source that outputs excitation light;
A light splitting unit that splits the pumping light output from the pumping light source into the pump light and the probe light; and
4. The time domain spectroscopic device according to claim 1 , wherein the excitation light source emits the excitation light having a wavelength of 1.45 μm or more and 1.65 μm or less. 5. Each of the photoconductive layers of the electromagnetic wave generation unit and the electromagnetic wave detection unit includes a plurality of individual photoconductive layers,
5. The time domain spectroscopic device according to claim 1 , comprising: a plurality of quantum dots formed between layers of the plurality of individual photoconductive layers. 6. 6. The time-domain spectroscopy according to claim 5, wherein the quantum dots of each layer are formed based on a strain caused by a difference in lattice constant with respect to the individual photoconductive layer of each layer positioned below the quantum dot. apparatus. 7. The time domain spectroscopic device according to claim 1 , wherein the quantum dot is any one of InAs, InGaAs, InAsSb, and InGaSb.

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