Electromagenetic wave detection device and electromagnetic wave detection method

Abstract

This electromagnetic wave detection device 10 comprises an electromagnetic wave transmitter 11, an electromagnetic wave detector 12, a waveform deformation unit 13, a control device 15, a reference signal generation unit 14a, a waveform generation unit 16, and a synchronous detection unit 14b. The electromagnetic wave transmitter 11 transmits electromagnetic waves to a sample 1, and the electromagnetic wave detector 12 detects electromagnetic waves that have interacted with the sample 1 and outputs an electric signal. The reference signal generation unit 14a generates a reference signal with a frequency that is a natural number multiple of a standard frequency, the natural number multiple being at least two. The waveform deformation unit 13, 23 deforms the waveforms of either one or both of the electric signal and the reference signal. After this waveform deformation, the synchronous detection unit 14b synchronously detects the electrical signal with the reference signal and outputs a detection signal.

Description

Electromagnetic wave detection device and electromagnetic wave detection method

[0001] The present invention relates to a technique for transmitting electromagnetic waves (for example, terahertz waves) to a sample and detecting the electromagnetic waves that have interacted with the sample (for example, acquiring the waveform of the electromagnetic waves).

[0002] Terahertz time domain spectroscopy (THz-TDS) is a method for acquiring and analyzing the waveform of electromagnetic waves that have interacted with a sample.

[0003] Conventionally, the THz-TDS method is performed as follows: Terahertz waves as electromagnetic waves are transmitted to a sample at a reference frequency (e.g., several tens of MHz), and the intensity of the terahertz waves that have interacted with the sample is detected at the reference frequency, and an electrical signal is output. This electrical signal is synchronously detected with a reference signal in a lock-in amplifier, thereby generating a detection signal. The reference signal is generated by an external modulator such as a chopper. Such a reference signal is, for example, a signal with a frequency obtained by dividing the reference frequency by a natural number greater than or equal to 2.

[0004] In this case, for example, by generating the detection signal while changing the time difference between the transmission and detection of the terahertz wave, waveform data representing the magnitude of the detection signal for each time difference can be obtained as the waveform of the terahertz wave that has interacted with the sample. The obtained waveform data is converted into spectral data by Fourier transform. This spectral data can be used to inspect and analyze the sample.

[0005] Such a THz-TDS method is described, for example, in the following Patent Document 1. Furthermore, regarding the THz-TDS method, a technology using an electro-optic modulator (EOM) as an external modulator for generating a reference signal is described in the following Non-Patent Document 1.

[0006] JP 2016-057138 A

[0007] MH Balgos, et al. "Megahertz repetition rate-based lock-in detection scheme for rapid data acquisition in terahertz time domain spectroscopy", Review of Scientific Instruments, 94, 043002 (2023); https: / / doi.org / 10.1063 / 5.0138938

[0008] In the THz-TDS method described above, the detection signal obtained by synchronously detecting the electrical signal generated by detecting the terahertz wave with a reference signal is susceptible to disturbances, such as noise generated around the sample.

[0009] This is because the electrical signal generated by detecting terahertz waves contains disturbances (noise) at the reference frequency. Therefore, even if a reference signal having a frequency obtained by dividing the reference frequency by a natural number equal to or greater than 2 (e.g., 1 / 2, 1 / 3, ..., 1 / 10 of the reference frequency) is generated using an external modulator such as a chopper or EOM, and the electrical signal is synchronously detected using this reference signal, it is not possible to remove the noise present in the electrical signal at a frequency that is a natural number multiple of the frequency of the reference signal.

[0010] The same applies to the case where other electromagnetic waves are used instead of the above-mentioned terahertz waves and the electromagnetic waves that have interacted with the sample as described above are detected (for example, the waveform of the electromagnetic waves is acquired). That is, the same applies to the case where a pump-probe method other than the THz-TDS method is implemented.

[0011] The inventors of the present application have found the above problem and have made the present invention based on this problem. That is, an object of the present invention is to suppress noise and obtain a detection signal having the intensity required for inspecting and analyzing a sample when, in an environment where disturbances (noise) are present, an electromagnetic wave is transmitted to a sample at a reference frequency, the electromagnetic wave that has interacted with the sample is detected at the reference frequency to output an electrical signal, and the electrical signal is synchronously detected using a reference signal.

[0012] An electromagnetic wave detection device according to one aspect of the present invention comprises: an electromagnetic wave emitter that emits electromagnetic waves to a sample; an electromagnetic wave detector that detects the electromagnetic waves that have interacted with the sample and outputs an electrical signal; a control device that controls the repeated emission and detection of the electromagnetic waves at a reference frequency; a reference signal generation unit that generates a reference signal having a frequency that is a natural number multiple of at least twice the reference frequency; a waveform modification unit that modifies the waveforms of one or both of the electrical signal and the reference signal; and a synchronous detection unit that receives the one or both of the signals that have passed through the waveform modification unit, synchronously detects the electrical signal with the reference signal, and outputs a detection signal.

[0013] In one aspect of the electromagnetic wave detection method of the present invention, (A) an electromagnetic wave is sent to a sample, and an electrical signal is output by detecting the electromagnetic wave that has interacted with the sample; (B) control is performed to repeat the sending and detection of the electromagnetic wave at a reference frequency; (C) a reference signal having a frequency that is a natural number multiple of at least twice the reference frequency is generated; (D) the waveforms of one or both of the electrical signal and the reference signal are deformed; and (E) after (D), the electrical signal is synchronously detected with the reference signal to output a detected signal.

[0014] According to one aspect of the present invention, an electromagnetic wave is transmitted to a sample at a reference frequency, and the electromagnetic wave that has interacted with the sample is detected at the reference frequency to output an electrical signal. When the transmission and detection of the electromagnetic wave are repeated at the reference frequency, the electrical signal is synchronously detected with a reference signal having a frequency that is a natural number multiple of at least twice the reference frequency. This makes it possible to obtain a detection signal in which noise is suppressed (e.g., a detection signal in which the noise has been removed) from an electrical signal having noise at the reference frequency.

[0015] On the other hand, if the above-mentioned electrical signal is synchronously detected using a reference signal whose frequency is a natural number multiple of the reference frequency (2 or more), the detected signal will be weak. Therefore, before synchronous detection, the waveforms of one or both of the electrical signal and the reference signal are modified. This allows for a detection signal with the strength required for sample inspection or analysis.

[0016] 7 shows an electromagnetic wave detection device according to an embodiment of the present invention. It shows experimental data showing the relationship between the electrical signal input to the synchronous detection unit (left side of FIG. 2) and the magnitude of the detection signal at each frequency of the reference signal (right side of FIG. 2). It is a flowchart of an electromagnetic wave detection method according to an embodiment of the present invention. It shows waveform data actually generated by the electromagnetic wave detection device in a state where noise is generated around the sample. It shows a waveform generated by the waveform generation unit of the embodiment using a reference signal with a frequency twice the reference frequency in the case of the THz-TDS method. It shows an electromagnetic wave detection device according to a modified example. It shows an electromagnetic wave detection device according to another modified example. It is a flowchart of an electromagnetic wave detection method using the electromagnetic wave detection device of FIG.

[0017] An embodiment of the present invention will be described with reference to the drawings. In addition, common parts in the drawings are given the same reference numerals, and duplicated explanations will be omitted.

[0018] 1 shows an electromagnetic wave detection device 10 according to an embodiment of the present invention. In this embodiment, the electromagnetic wave detection device 10 transmits an electromagnetic wave to a sample 1 and generates a time waveform of the electromagnetic wave that has interacted with the sample 1 by passing through or being reflected by the sample 1.

[0019] The electromagnetic wave detection device 10 includes an electromagnetic wave transmitter 11, an electromagnetic wave detector 12, a waveform transforming section 13, a synchronous detection section 14b, a control device 15, and a reference signal generating section 14a.

[0020] The electromagnetic wave emitter 11 generates an electromagnetic wave and emits it to the sample 1. This electromagnetic wave may be a pulsed electromagnetic wave. The emission time of the electromagnetic wave emitter 11 is repeated at a predetermined reference frequency (hereinafter also simply referred to as the reference frequency) under the control of a control device 15, which will be described later. The electromagnetic wave emitted from the electromagnetic wave emitter 11 to the sample 1 may interact with the sample 1 by passing through the sample 1 or by being reflected by the sample 1. The pulsed electromagnetic wave that has interacted with the sample 1 in this way is incident on the electromagnetic wave detector 12.

[0021] The electromagnetic wave detector 12 detects the electromagnetic wave emitted from the electromagnetic wave emitter 11 and interacting with the sample 1. The electromagnetic wave detector 12 outputs an electric signal corresponding to the intensity (displacement of the electric field or magnetic field, i.e., amplitude) of the detected electromagnetic wave. This electric signal has a pulse shape. In other words, the electric signal is a pulse signal that occurs in a pulse shape over time.

[0022] The detection time points at which the electromagnetic wave detector 12 detects the electromagnetic wave are repeated at a reference frequency under the control of the control device 15, which will be described later. Each detection time point may be a single time point that is sufficiently shorter than the duration (pulse width) of the pulse electromagnetic wave.

[0023] The waveform modification unit 13 modifies the waveform of the electrical signal output by the electromagnetic wave detector 12. The magnitude of the electrical signal output by the electromagnetic wave detector 12 indicates the intensity of the electromagnetic wave at the corresponding detection point, and the pulse shape of the electromagnetic signal has no physical meaning. Therefore, the waveform modification unit 13 modifies the waveform of the electrical signal from the electromagnetic wave detector 12 so as to obtain a detection signal (described below) having the intensity required for inspecting or analyzing the sample 1.

[0024] The synchronous detection unit 14b synchronously detects the electrical signal that has passed through the waveform modification unit 13 using the reference signal from the reference signal generation unit 14a, and outputs the detected signal. Here, the electrical signal that has passed through the waveform modification unit 13 is an electrical signal whose waveform has been modified by passing through the waveform modification unit 13. The synchronous detection unit 14b constitutes the lock-in amplifier 14 as shown in FIG. 1.

[0025] The control device 15 controls the electromagnetic wave transmitter 11 and the electromagnetic wave detector 12 so that the transmission of electromagnetic waves by the electromagnetic wave transmitter 11 and the detection of electromagnetic waves by the electromagnetic wave detector 12 are repeated at the reference frequency.

[0026] The reference signal generator 14a generates a reference signal having a frequency that is a natural number multiple of the reference frequency, which is equal to or greater than twice the reference frequency, and inputs the reference signal to the synchronous detector 14b. In parallel with this, as will be described later, the reference signal generator 14a may also generate a reference signal of the reference frequency and input the reference signal to the synchronous detector 14b. The reference signal generator 14a may be incorporated into the lock-in amplifier 14 as shown in FIG. 1, but is not limited to this, as will be described later.

[0027] Therefore, the synchronous detection unit 14b synchronously detects the above-mentioned electrical signal using a reference signal having a frequency that is a natural number multiple of the reference frequency, which is equal to or greater than twice the reference frequency. This makes it possible to obtain a detected signal in which noise is suppressed (e.g., a detected signal from which the noise has been removed) from an electrical signal having noise at the reference frequency. In parallel with this synchronous detection, as will be described later, the synchronous detection unit 14b can also synchronously detect the above-mentioned electrical signal using a reference signal at the reference frequency.

[0028] On the other hand, if the above-mentioned electrical signal is synchronously detected using a reference signal having a frequency that is a natural number multiple of the reference frequency (2 or more), the detected signal will be weak. Therefore, before synchronous detection as described above, the waveform of the electrical signal from the electromagnetic wave detector 12 is modified by the waveform modification unit 13. This allows a detected signal to be obtained that has the strength required for inspecting or analyzing the sample 1. As a result, it becomes possible to generate, for example, waveform data as described below.

[0029] The electromagnetic wave detection device 10 may further include a time difference changer 21 and a waveform generator 16. However, according to the present invention, the electromagnetic wave detection device 10 does not necessarily have to include the time difference changer 21 and the waveform generator 16, as in a third modification example described below.

[0030] The time difference change device 21 changes the time difference (hereinafter simply referred to as the time difference) between the transmission of the electromagnetic wave by the electromagnetic wave sender 11 and the detection of the electromagnetic wave by the electromagnetic wave detector 12 during the repeated transmission and detection of the electromagnetic wave.

[0031] The waveform generating unit 16 generates a waveform representing the magnitude of the detection signal for each time difference based on the detection signal synchronously detected for each time difference. This waveform is a time waveform of the electromagnetic wave that has interacted with the sample 1. In other words, this waveform is waveform data representing the displacement (electric field amplitude) of the electromagnetic wave with respect to the elapsed time (time difference). The sample 1 can be inspected or analyzed based on this waveform data. For example, spectral data representing each frequency component of the waveform data can be obtained, and the sample 1 can be inspected or analyzed based on this spectral data.

[0032] (Detailed Description of Electromagnetic Wave Detector) The electromagnetic wave detector 10 according to this embodiment will be described in more detail below.

[0033] <Type of Electromagnetic Wave> In the embodiment, the electromagnetic wave emitted by the electromagnetic wave emitter 11 to the sample 1 may be a terahertz wave. That is, the electromagnetic wave detection device 10 may be a device that performs a THz-TDS method. The frequency of the terahertz wave may be, for example, in the range of 0.1 THz or more and approximately 10 THz or less, or in the range of 0.05 THz or more and 20 THz or less, but is not limited to these ranges.

[0034] In this embodiment (hereinafter also simply referred to as an embodiment) in which the THz-TDS method is performed, the electromagnetic wave transmitter 11 may be a photoconductive antenna (PCA) that transmits terahertz waves when a pump pulse (pulse laser light) described below is input by the control device 15. In this case, the above-mentioned transmission time point is the time point when the pump pulse (pulse laser light) described below is input (incident) to the electromagnetic wave transmitter 11.

[0035] Furthermore, in an embodiment in which the THz-TDS method is performed, the electromagnetic wave detector 12 may be a photoconductive antenna (PCA) or an electro-optical sampling (EOS) element that detects the terahertz waves that are transmitted from the electromagnetic wave transmitter 11 and have interacted with the sample 1 at the timing when a probe pulse (pulse laser light) described below is input (incident), and outputs an electric signal (e.g., a current signal) according to the intensity of the detected terahertz waves. In this case, the above-mentioned detection time point is the timing when the probe pulse is incident on the electromagnetic wave detector 12.

[0036] The electromagnetic waves emitted by the electromagnetic wave emitter 11 to the sample 1 are not limited to terahertz waves, but may be, for example, ultraviolet light, visible light, infrared light, microwaves (e.g., electromagnetic waves with a frequency ranging from about 300 MHz to about 30 GHz), RF waves (e.g., electromagnetic waves with a frequency ranging from about 30 kHz to about 300 MHz), X-rays, or gamma rays. The ultraviolet light may be, for example, an electromagnetic wave having a wavelength ranging from about 10 nm to about 400 nm. The visible light may be, for example, an electromagnetic wave having a wavelength ranging from about 380 nm to about 780 nm. The infrared light may be, for example, an electromagnetic wave having a wavelength ranging from about 780 nm to about 1 mm. The electromagnetic wave detector 12 is not limited to a PCA or EOS, and may be a detector that detects the electromagnetic waves emitted from the electromagnetic wave emitter 11.

[0037] <Waveformation section>

[0038] The waveform modification section 13 may be configured to perform any one of the following waveform modifications (1) to (7). However, the waveform modification section 13 is not limited to a configuration that performs any one of the following processes (1) to (7).

[0039] (1) The waveform modification unit 13 modifies the waveform of the electrical signal from the electromagnetic wave detector 12 into a waveform different from a sine wave. Note that the electrical signal output by the electromagnetic wave detector 12 may have a sine wave waveform or another pulse shape.

[0040] (2) The waveform modification unit 13 modifies the waveform of the electrical signal while increasing or maintaining the pulse width of the electrical signal from the electromagnetic wave detector 12. Here, the pulse width may mean the length of time from when the magnitude of the electrical signal becomes equal to or exceeds a threshold value (e.g., 50% of the maximum magnitude of the electrical signal) to when the magnitude becomes smaller than the threshold value again.

[0041] (3) The waveform modification unit 13 modifies the waveform of the electrical signal from the electromagnetic wave detector 12 so that the electrical signal rises exponentially and, in addition to this, or instead of this, the electrical signal falls exponentially.

[0042] (4) The waveform modification unit 13 modifies the waveform of the electrical signal from the electromagnetic wave detector 12 into a Gaussian waveform.

[0043] (5) The waveform modification unit 13 modifies the waveform of the electrical signal from the electromagnetic wave detector 12 into a rectangular shape (rectangular pulse).

[0044] (6) The waveform modifying unit 13 may modify the waveform of the electrical signal from the electromagnetic wave detector 12 while increasing or maintaining at least one of the rise time and fall time of the electrical signal from the electromagnetic wave emitter 11. Here, the rise time may be the time required for the magnitude of the electrical signal to reach a predetermined upper limit value from a predetermined lower limit value, and the fall time may be the time required for the magnitude of the electrical signal to reach a predetermined lower limit value from a predetermined upper limit value. In one example, the lower limit value may be 10% of the maximum value (peak value) of the electrical signal, and the upper limit value may be 90% of the maximum value (peak value) of the electrical signal.

[0045] (7) The waveform modification unit 13 modifies the waveform of the electrical signal from the electromagnetic wave detector 12 into an arbitrary waveform.

[0046] In each of the above (1) to (7), the reference signal may have a sine wave waveform, but the reference signal is not limited to such a sine wave. If the reference signal in each of the above (1) to (7) is not a sine wave, under the assumptions of each of the above (1) to (7), the waveform modification unit 13 may modify the waveform of the electrical signal from the electromagnetic wave detector 12 into a waveform that is arbitrarily adjusted according to the waveform of the reference signal.

[0047] 2 shows experimental data of the electrical signal input to the synchronous detection unit 14b (left side of FIG. 2) and the magnitude of the detection signal synchronously detected by the synchronous detection unit 14b using a reference signal having a sine wave shape and a frequency n times that of the reference signal (right side of FIG. 2). In this experiment, the electrical signal (voltage signal) input to the synchronous detection unit 14b was a simulation of the electrical signal waveform-modified by the waveform modification unit 13 (hereinafter referred to as the simulated electrical signal), and was generated by a function generator.

[0048] In FIG. 2 as a whole, the first row from the top shows the case where the simulated electrical signal rises and falls exponentially, the second row from the top shows the case where the simulated electrical signal has a Gaussian shape, the third row from the top shows the case where the simulated electrical signal has a rectangular shape, and the fourth row from the top shows the case where the simulated electrical signal has a sine wave shape.

[0049] 2, the horizontal axis represents the elapsed time, and the vertical axis represents the displacement (amplitude) of the electrical signal input to the synchronous detection unit 14b. In the graph on the right side of Fig. 2, the numerical value on the horizontal axis represents how many times (n times) the reference signal is as compared to the reference frequency, and the vertical axis represents the magnitude of the detected signal.

[0050] 2, when a simulated electrical signal having a sine wave shape is synchronously detected with a sine wave reference signal, the magnitude of the detected signal becomes zero if the frequency of the reference signal is twice or more the reference frequency. In contrast, when a simulated electrical signal having a non-sine wave shape is synchronously detected with a sine wave reference signal, the magnitude of the detected signal does not become zero even if the frequency is twice or more the reference frequency, and the detected signal has the strength required for inspecting or analyzing the sample 1. This makes it possible to generate waveform data, as described below, for example.

[0051] The waveform modification section 13 may be configured to perform waveform modification by arbitrarily combining two or more of the above (1) to (7) within a range where no contradiction occurs.

[0052] The waveform modification unit 13 may amplify the signal strength of the electrical signal from the electromagnetic wave detector 12 in addition to modifying the waveform as described above. That is, the waveform modification unit 13 modifies the waveform of the electrical signal from the electromagnetic wave detector 12, amplifies the waveform of the electrical signal, and outputs the amplified electrical signal (voltage signal) to the synchronous detection unit 14b.

[0053] Such a waveform modification unit 13 may be, for example, an IV amplifier (current-voltage conversion amplifier). In this case, the waveform modification unit 13 may modify the waveform of the electric signal, which is the current signal from the electromagnetic wave detector 12, and output a voltage signal obtained by amplifying the signal strength as the electric signal.

[0054] Alternatively, the waveform modification unit 13 may be configured with a pulse generator. In this case, the pulse generator may output an electrical signal having a waveform obtained by modifying the waveform of the electrical signal each time a current signal is input from the electromagnetic wave detector 12. Even in this case, the waveform modification unit 13 having the pulse generator may amplify the signal intensity as described above in addition to modifying the waveform of the electrical signal from the electromagnetic wave detector 12 as described above.

[0055] The ratio of the magnitude (maximum value) of the electrical signal output from the waveform modifying section 13 to the magnitude (maximum value) of the electrical signal input to the waveform modifying section 13 may be constant.

[0056] The synchronous detector 14b includes a multiplier 14b1 and a low-pass filter 14b2. The synchronous detector 14b may constitute the lock-in amplifier 14.

[0057] The multiplier 14b1 multiplies a reference signal input as an electrical signal (for example, a voltage signal) by an electrical signal (for example, a voltage signal) input from the waveform transformer 13, to generate and output a multiplied signal.

[0058] The low-pass filter 14b2 removes components of frequencies higher than a predetermined cutoff frequency (e.g., all frequencies higher than the cutoff frequency) from the multiplied signal output by the multiplier 14b1. As a result, the low-pass filter 14b2 outputs a detection signal (electrical signal) obtained by synchronously detecting the electrical signal from the waveform transformer 13 using the reference signal. The detection signal output from the low-pass filter 14b2 may be a DC signal. In other words, the detection signal may be a signal indicating the magnitude of components of the electrical signal from the waveform transformer 13 that have the same frequency as the frequency of the reference signal.

[0059] The timing of synchronously detecting the electrical signal input from the waveform modifying unit 13 at the reference frequency using a reference signal repeatedly input at a frequency that is a natural number multiple of at least twice the reference frequency may be adjusted in advance and set in the synchronous detecting unit 14b, thereby allowing the synchronous detecting unit 14b to function appropriately as described above.

[0060] <Control Device> The control device 15 inputs a pump pulse at a reference frequency to the electromagnetic wave emitter 11 and inputs a probe pulse at a reference frequency to the electromagnetic wave detector 12. As a result, the electromagnetic wave emitter 11 emits an electromagnetic wave to the sample 1 at each time point (hereinafter simply referred to as a transmission time point) when a pump pulse is input, and the electromagnetic wave detector 12 detects the electromagnetic wave that has interacted with the sample 1 at a detection time point (hereinafter simply referred to as a detection time point) corresponding to each transmission time point and outputs an electrical signal. In this way, the electromagnetic wave emitter 11 emits an electromagnetic wave at the reference frequency, and the electromagnetic wave detector detects the electromagnetic wave at the reference frequency and outputs an electrical signal.

[0061] In addition, the control device 15 includes, for example, a laser 15a and a beam splitter 15b as shown in FIG.

[0062] The laser 15a may output pulsed laser light at a reference frequency. In principle, the reference frequency has no upper or lower limit, and may be a frequency on the order of GHz, for example, but may also be a value of 10 MHz or more and 100 MHz or less (e.g., 80 MHz), or a value of 1 kHz or more and 1 MHz or less (e.g., about 1 kHz or about 200 kHz). In this embodiment, the pulsed laser light output by the laser 15a is a femtosecond (1×10 －１５ The laser beam may be, but is not limited to, femtosecond pulsed laser light having a pulse width on the order of 1 / 2 second.

[0063] The beam splitter 15b splits the pulsed laser light from the laser 15a into a pulsed laser light serving as a pump pulse and a pulsed laser light serving as a probe pulse. In the example of Fig. 1, the pump pulse is input from the beam splitter 15b to the electromagnetic wave emitter 11 via the time difference changer 21, and the probe pulse is input from the beam splitter 15b to the electromagnetic wave detector 12 without via the time difference changer 21. However, the pump pulse may also be input from the beam splitter 15b to the electromagnetic wave emitter 11 without via the time difference changer 21, and the probe pulse may also be input from the beam splitter 15b to the electromagnetic wave detector 12 via the time difference changer 21.

[0064] The control device 15 may include a reference signal generator 15c. In one example, as shown in FIG. 1 , the reference signal generator 15c generates a reference signal, which is a pulse signal, at a reference frequency and inputs it to the laser 15a and the reference signal generator 14a. The laser 15a may output pulsed laser light at the reference frequency as described above, based on the reference signal input at the reference frequency from the reference signal generator 15c. In another example, the reference signal generator 15c may receive a signal (electrical signal or pulsed laser light) at the reference frequency from the laser 15a, and based on the signal, generate a reference signal, which is a pulse signal, at the reference frequency and input it to the reference signal generator 14a.

[0065] <Reference Signal Generator> The reference signal generator 14a may be incorporated into the lock-in amplifier 14. That is, the lock-in amplifier 14 may be composed of a synchronous detector 14b and a reference signal generator 14a. However, the present invention is not limited to this, and the reference signal generator 14a may be provided outside the lock-in amplifier 14. That is, the reference signal generator 14a and the lock-in amplifier 14 may be separate devices.

[0066] Based on the reference signal input at the reference frequency from the reference signal generation unit 15c, the reference signal generation unit 14a generates a reference signal at a frequency that is a natural number multiple of the reference frequency (hereinafter also referred to as the reference frequency) that is at least twice the reference frequency, and inputs the reference signal to the synchronous detection unit 14b.

[0067] In the following, the reference frequency may be represented as ω, and the reference frequencies may be represented as 2ω, 3ω, 4ω, etc. The reference signal generator 14a may generate a reference signal of a reference frequency of one specific value (for example, 2ω) based on the input ω reference signal, and input the generated reference signal to the synchronous detector 14b.

[0068] Alternatively, the reference signal generator 14a may generate multiple types of reference signals (e.g., 2ω, 3ω, 4ω, and 5ω reference signals) based on the input ω reference signal, as shown in FIG. 1 , with different reference frequencies, and input these signals to the synchronous detector 14b independently and in parallel. In this case, the synchronous detector 14b synchronously detects the electrical signal from the waveform transformer 13 for each type of input reference signal using the reference signal of that type and outputs a detected signal. More specifically, for each type of input reference signal, the multiplier 14b1 multiplies the reference signal of that type by the electrical signal input from the waveform transformer 13 to generate and output a multiplied signal, and the low-pass filter 14b2 removes frequency components higher than a predetermined cutoff frequency from the multiplied signal, thereby outputting the detected signal as described above. In this way, the synchronous detector 14b outputs multiple types of detection signals corresponding to the multiple types of reference signals independently and in parallel to the waveform generator 16. In this case, for each type of detection signal input from the synchronous detection unit 14 b, the waveform generation unit 16 generates a waveform representing the magnitude of the detection signal of that type for each time difference based on the detection signal of that type synchronously detected for each time difference. In this way, the waveform generation unit 16 generates multiple types of waveforms (waveform data) corresponding to multiple types of detection signals.

[0069] 1, four types of reference signals (e.g., 2ω, 3ω, 4ω, and 5ω reference signals) are input to the synchronous detection unit 14b, but this is not limiting, and multiple types of reference signals other than four may be input to the synchronous detection unit 14b. In this case, as described above, the synchronous detection unit 14b outputs multiple types of detection signals corresponding to the multiple types of reference signals in parallel and independently of each other.

[0070] Although not shown in FIG. 1 , the reference signal generator 14a may generate a reference signal having a reference frequency ω in addition to a reference signal having a frequency twice or more the reference frequency and input the reference signal to the synchronous detector 14b. In this case, the synchronous detector 14b synchronously detects the electrical signal from the waveform transformer 13 using the reference signal having the reference frequency ω and outputs a detected signal, in parallel with the above-described synchronous detection using the reference signal having a frequency twice or more the reference frequency. That is, the multiplier 14b1 multiplies the reference signal having the reference frequency by the electrical signal input from the waveform transformer 13 to generate and output a multiplied signal, and the low-pass filter 14b2 outputs a detected signal obtained by removing frequency components higher than a predetermined cutoff frequency from the multiplied signal. In this case, the waveform generator 16 may generate a waveform representing the magnitude of the detected signal for each time difference based on the detected signal synchronously detected using the reference signal having the reference frequency for each time difference.

[0071] The time difference changing device 21 includes, for example, a movable body 21 b on which two reflecting mirrors 21 a that reflect electromagnetic waves are installed, and a drive control device 21 c that moves the movable body 21 b. The drive control device 21 c moves the movable body 21 b (for example, in the left-right direction in FIG. 1 ) to change the length of the propagation path of the pulsed laser light (for example, in FIG. 1 , the path from the beam splitter 15 b to the electromagnetic wave emitter 11 via the reflecting mirrors 21 a), thereby changing the time difference between the transmission time and the detection time.

[0072] The time difference changing device 21 (drive control device 21c) outputs a time difference signal indicating the time difference to the waveform generating unit 16. For example, the time difference changing device 21 (drive control device 21c) outputs a time difference signal indicating the initial value of the time difference to the waveform generating unit 16, and every time the time difference is changed, outputs a time difference signal indicating the changed time difference to the waveform generating unit 16. The waveform generating unit 16 generates waveform data as described above based on each time difference signal from the time difference changing device 21.

[0073] (Electromagnetic Wave Detection Method) An electromagnetic wave detection method according to this embodiment will now be described. This electromagnetic wave detection method is performed using the electromagnetic wave detection device 10 described above. Fig. 3 is a flowchart showing the electromagnetic wave detection method according to this embodiment. This electromagnetic wave detection method has steps S1 to S9.

[0074] In step S1, a sample 1 is placed on an electromagnetic wave detection device 10, as shown in FIG.

[0075] In step S2, the electromagnetic wave is transmitted to the sample 1 by the electromagnetic wave transmitter 11. More specifically, as described above, the control device 15 inputs a pump pulse to the electromagnetic wave transmitter 11, causing the electromagnetic wave transmitter 11 to transmit the electromagnetic wave to the sample 1.

[0076] In step S3, the electromagnetic wave that was transmitted in step S2 and interacted with the sample 1 is detected by the electromagnetic wave detector 12, and the electromagnetic wave detector 12 outputs an electric signal having a magnitude corresponding to the intensity (amplitude of the electric field) of the detected electromagnetic wave. More specifically, as described above, the control device 15 inputs a probe pulse corresponding to the pump pulse in step S2 to the electromagnetic wave detector 12, and the electromagnetic wave detector 12 outputs an electric signal having a magnitude corresponding to the intensity of the electromagnetic wave that was incident on the electromagnetic wave detector 12 at the timing when the probe pulse was input to the electromagnetic wave detector 12.

[0077] When steps S2 and S3 are performed for the first time, the time difference between the time when the electromagnetic wave is transmitted in step S2 and the time when the electromagnetic wave is detected in step S3 (hereinafter simply referred to as the time difference) is set to a predetermined initial value as the current time difference. This initial value may be, for example, a time difference close to the time difference at which the detection signal obtained in step S5 described below is expected to output a maximum value.

[0078] In step S4, the waveform of the electrical signal output from the electromagnetic wave detector 12 in step S3 is modified by the waveform modification unit 13. At this time, the waveform modification unit 13 also amplifies the signal strength of the electrical signal from the electromagnetic wave detector 12. In step S4, the electrical signal whose waveform has been modified and whose signal strength has been amplified in this manner is input from the waveform modification unit 13 to the synchronous detection unit 14b.

[0079] In step S5, the synchronous detection unit 14b synchronously detects the electrical signal after waveform modification in step S4 using the reference signal and outputs the detected signal. More specifically, as described above, the reference signal generation unit 14a inputs the reference signal to the synchronous detection unit 14b, and the synchronous detection unit 14b synchronously detects the electrical signal using the reference signal. In step S5, the signal thus synchronously detected is output from the synchronous detection unit 14b.

[0080] The above steps S2 to S5 are continuously repeated at the above-mentioned reference frequency for one time difference. During this repetition, a reference signal having a frequency that is a natural number multiple of the reference frequency (i.e., twice or more), is also continuously input to the synchronous detection unit 14b. Therefore, during the repetition of the above steps S2 to S5, the synchronous detection unit 14b repeatedly outputs the above-mentioned detection signal in step S5.

[0081] Regarding the repetition of steps S2 to S5, if the detection signal generated in step S5 is stable (YES in step S6), proceed to step S7; if not (NO in step S6), continue repeating steps S2 to S5.

[0082] For example, in step S6, the control device 15 (e.g., a device controlling the laser 15a) may determine whether the above steps S2 to S5 have been repeated a predetermined number of times. If the above steps S2 to S5 have been repeated a predetermined number of times for one time difference (YES in step S6), the process proceeds to step S7. If not (NO in step S6), the process continues to repeat steps S2 to S5. This predetermined number of times may be, for example, 10 or more and 15 or less, or 20 or less, but is not limited thereto. The above-mentioned repetition of steps S2 to S5 a predetermined number of times is performed while the above-mentioned time difference is maintained constant by the time difference change device 21. Furthermore, this repetition may be performed while adjusting the entire optical system constituting the electromagnetic wave detection device 10 and the device environment. Furthermore, if the detection signal is weak, the time difference may be finely adjusted by the time difference change device 21 and steps S2 to S5 may be repeated.

[0083] In step S7, the time difference change device 21 (e.g., the drive control device 21c) may determine whether the time difference between the time point at which the electromagnetic waves are transmitted in step S2 and the time point at which they are detected in step S3 has been changed to a value within a set range (values ​​spaced apart by a set interval). If the time difference has been changed to a value within the set range in step S7, the process proceeds to step S9; otherwise, the process proceeds to step S8.

[0084] In the case of the THz-TDS method, for example, the setting range of the time difference may be from -several tens of picoseconds (-30 picoseconds in one example) to +several tens of picoseconds (+30 picoseconds in one example), and the setting interval may be 1 picosecond or less (0.1 picoseconds in one example), but the setting range and setting interval are not limited to these.

[0085] In step S8, the time difference is changed by the time difference change device 21. After the time difference is changed in step S8, the changed time difference is set as the current time difference, and the process returns to step S2, where steps S2 to S7 are performed again as described above.

[0086] In step S9, the waveform generation unit 16 generates waveform data representing the magnitude of the detection signal relative to the time difference based on each time difference changed as described above and the magnitude of the detection signal (for example) generated in step S5 while the time difference is maintained.

[0087] This waveform data represents, for each time difference (i.e., elapsed time), the waveform of the electromagnetic wave that has interacted with the sample 1. In this waveform data, the magnitude of the signal synchronously detected in step S5 for each time difference may be the magnitude of the detection signal immediately before the time difference is changed, or may be the time average value or time integral value of the magnitude of the detection signal over a certain period of time (the period over which the above-mentioned steps S2 to S5 are repeated the above-mentioned predetermined number of times), but is not limited to these, and may be based on the detection signal in step S5.

[0088] In step S9, the waveform generating unit 16 may input the waveform data generated as described above to the waveform processing unit 17 (see FIG. 1 ). The waveform processing unit 17 converts the input waveform data into spectrum data. This spectrum data indicates each frequency component of the waveform represented by the waveform data. The waveform data generated by the waveform generating unit 16 and the spectrum data generated by the waveform processing unit 17 may be stored in the memory unit 18. The waveform data and spectrum data may be displayed on the screen of a display device 19 (display). The waveform processing unit 17, the memory unit 18, and the display device 19 may be components of the electromagnetic wave detection device 10. The memory unit 18 may be a storage area in a storage device such as a memory or a hard disk. The waveform generating unit 16, the waveform processing unit 17, the memory unit 18, and the display device 19 may be configured by computer hardware and software, but are not limited to this.

[0089] Note that, if multiple types of reference signals with multiple different reference frequencies (e.g., 2ω, 3ω, 4ω, and 5ω) are used in step S5, steps S2 to S9 are performed simultaneously for each type of reference signal, as described above. Therefore, in step S9, multiple types of waveform data and spectrum data corresponding to the multiple types of reference signals are generated. The multiple types of waveform data and spectrum data may be displayed on display device 19. If a reference signal with a reference frequency ω is also used in step S5, steps S2 to S9 are also performed for the reference signal, as described above. Therefore, in step S9, waveform data and spectrum data corresponding to the reference signal with the reference frequency are further generated. The waveform data and spectrum data may also be displayed on display device 19. This makes it possible to compare the waveform data and spectrum data with waveform data and spectrum data generated by a reference signal with a frequency that is a natural number multiple of twice or greater than the reference frequency.

[0090] (Effects of this embodiment) According to the above-described embodiment, the following effects (i) to (iv) can be obtained.

[0091] (i) Regardless of whether or not the waveform modifying unit 13 is present, when an electrical signal of a reference frequency from the electromagnetic wave detector 12 is synchronously detected using a reference signal having a frequency obtained by dividing the reference frequency by a natural number multiple (e.g., the reference frequency), the level of a noise signal in the detected signal is generated in synchronization with the reference frequency. Therefore, in this embodiment, the synchronous detection unit 14b synchronously detects the electrical signal generated by electromagnetic wave detection using a reference signal having a frequency obtained by dividing the reference frequency by a natural number multiple that is equal to or greater than twice the reference frequency. This makes it possible to obtain a detected signal in which noise at the reference frequency due to disturbances around the sample 1 (e.g., noise or vibration caused by human work) is suppressed (e.g., a detected signal from which the noise has been removed) from the electrical signal.

[0092] On the other hand, if the above-mentioned electrical signal is synchronously detected using a reference signal having a frequency that is a natural number multiple of the reference frequency (2 or more), the detected signal will be weak (for example, zero intensity). Therefore, a waveform modification unit 13 is provided before synchronous detection to modify the waveform of the electrical signal from the electromagnetic wave detector 12. This allows a detection signal having the intensity required for inspection or analysis of the sample 1 to be obtained.

[0093] For example, Figures 4A to 4D show waveform data actually generated by the waveform generating unit 16 when the electromagnetic wave detecting device 10 performs the THz-TDS method in a state where noise and vibration are present around the sample 1. Figures 4A to 4D illustrate cases where the frequency of the reference signal is n times the reference frequency in the electromagnetic wave detecting device 10 described above. That is, Figure 4A illustrates a reference example where n = 1, and Figures 4B to 4D illustrate cases where n = 2 to 4, respectively. In Figures 4A to 4D, the horizontal axis represents the delay time as the time difference, and the vertical axis represents the strength of the detection signal (current value in this example). In the reference example of Figure 4A, the waveform data is affected by noise (especially in the delay time range of -30 to +30 ps). However, when the frequency of the reference signal is twice or more the reference frequency (when n = 2 to 4), the noise in the waveform data is significantly suppressed, and the waveform is clearly defined near the delay time of 0 ps.

[0094] In this embodiment, the magnitude of the electrical signal from the electromagnetic wave detector 12 indicates the intensity of the electromagnetic wave, but the shape of the signal has no physical meaning, and is therefore modified by the waveform modification unit 13. This prevents the waveform of the electrical signal input to the synchronous detection unit 14b from being weaker than the waveform of the reference signal. For example, if the waveforms of the electrical signal input to the synchronous detection unit 14b and the reference signal are both sine-shaped, and the reference signal has a frequency equal to or greater than twice the reference frequency, the magnitude of the output signal (detection signal) from the synchronous detection unit 14b will be zero (see, for example, the fourth row from the top of Figure 2). Therefore, the waveform of the electrical signal is modified by the waveform modification unit 13 as described above. Furthermore, the effects of this embodiment can be achieved not only when a sine wave is used as the reference signal, but also when a rectangular or other waveform reference signal is used, by performing waveform modification by the waveform modification unit 13 accordingly.

[0095] (ii) In principle, there is no upper limit to the reference frequency, and no external modulator such as a chopper or EOM is provided. Therefore, the reference frequency and the reference frequency (frequency of the reference signal) are not affected by the characteristics of the external modulator, and can be determined only by the repetition frequency of the laser 15 a. Therefore, by using a laser 15 a with a higher repetition frequency, it is possible to repeatedly generate the detection signal used for inspecting and analyzing the sample 1 at a higher speed (at a higher frequency).

[0096] FIG. 5 shows waveforms generated by the waveform generator 16 using a reference signal with a frequency twice the reference frequency in the case of the THz-TDS method. In FIG. 5, the horizontal axis represents the delay time, which is the time difference described above, and the vertical axis represents the intensity of each of the multiple waveforms. These waveforms represent the detection signals output from the synchronous detector 14b for each time difference integrated over the respective integration times. That is, the six waveforms in FIG. 5, from top to bottom, represent waveforms obtained when the integration times are 100 μs, 10 μs, 1 μs, 500 ns, 250 ns, and 100 ns, respectively. As can be seen from FIG. 5, the integration time can be set to the order of nanoseconds, and a stable detection signal (time average or time integral value of the detection signal) can be obtained for one time difference in a short time on the order of nanoseconds. This makes it possible to track ultrafast phenomena on the order of nanoseconds in the sample 1.

[0097] Furthermore, in the case of the THz-TDS method, the electromagnetic wave detection device 10 of this embodiment can be applied to fields of application such as the medical field, drug testing, security field, agriculture field, materials field, biotechnology field, etc. In this case, the benefit of the above-mentioned high speed is that the above-mentioned spectral data can be obtained at high speed at all measurement points during imaging, making real-time hyperspectral imaging possible.

[0098] (iii) Furthermore, in this embodiment, an external modulator such as a chopper is not required, and therefore the configuration of the electromagnetic wave detection device 10 is simplified.

[0099] (iv) The strength and shape of the electrical signal can be controlled by controlling the output waveform of the waveform modification unit 13 (e.g., a current-voltage amplifier), and furthermore, the device configuration is simpler as described above in (iii). Therefore, the electromagnetic wave detection device 10 of this embodiment can be realized by easily replacing part of a device of the prior art (e.g., an external modulator).

[0100] The present invention is not limited to the above-described embodiment, and various modifications may be made within the scope of the technical concept of the present invention. For example, the electromagnetic wave detection device 10 according to the embodiment of the present invention may not have all of the above-described features, but may have only some of the above-described features. Furthermore, the electromagnetic wave detection device 10 according to the embodiment of the present invention may not achieve all of the above-described effects, but may achieve only some of the above-described effects. For example, while the absence of an external modulator is a major advantage of the above-described embodiment, this is not limiting. An external modulator may be provided outside the control device 15, and the external modulator may modulate an input signal (e.g., pulsed laser light from the laser 15a or another input signal) to generate a modulation signal with a modulation frequency that is a reference frequency. The modulation signal may then be input to the reference signal generator 14a as a reference signal. Even in this case, the same effects as those of the above-described embodiment can be achieved, except that an external modulator is not required. Furthermore, in this case, the reference signal generated by the reference signal generator 15c is not input to the reference signal generator 14a. Such an external modulator may or may not be provided in Modifications 1 to 3 described below. The frequency of the input signal to the external modulator may be a frequency other than the reference frequency. Furthermore, the modulated signal generated by the external modulator may be input to the electromagnetic wave emitter 11 or the electromagnetic wave detector 12, thereby controlling the electromagnetic wave emitter 11 and the electromagnetic wave detector 12 so that the electromagnetic wave emitter 11 repeatedly emits electromagnetic waves or the electromagnetic wave detector 12 repeatedly detects electromagnetic waves at the reference frequency. In this case, the device combining the control device 15 and the external modulator functions as a device (control device) that controls the repeated emission and detection of electromagnetic waves at the reference frequency.The external modulator may be a device (e.g., a chopper) that physically turns on and off the input signal (e.g., pulsed laser light from laser 15a), and in this case, the electromagnetic wave sender 11 or the electromagnetic wave detector 12 may be controlled by a combination of the control device 15 and the external modulator (e.g., by physically turning on and off the input of pulsed laser light from laser 15a to the electromagnetic wave sender 11 or the electromagnetic wave detector 12 using the chopper as the external modulator) so that the transmission of electromagnetic waves by the electromagnetic wave sender 11 or the detection of electromagnetic waves by the electromagnetic wave detector 12 is repeated at a reference frequency. Points not described here regarding the case where the above external modulator is provided may be the same as those described above.

[0101] In addition, any one of the following modified examples 1 to 3 may be adopted, or two or all of modified examples 1 to 3 may be combined and adopted. In this case, points not described below may be the same as those described above.

[0102] (Modification 1) The control device 15 is not limited to the configuration described above. For example, the laser 15a in the control device 15 may be a single laser as shown in FIG. 1 , or may include a pump laser that inputs pump pulses at a reference frequency to the electromagnetic wave emitter 11 and a probe laser that inputs probe pulses at a reference frequency to the electromagnetic wave detector 12. In this case, the time difference changer 21 described above may be provided in the propagation path of the pump pulse or the probe pulse, or the time difference changer 21 may be incorporated into the control device 15 and change the time difference by controlling the output timing of the pump pulse and the probe pulse from the pump laser and the probe laser.

[0103] (Modification 2) Fig. 6 shows a configuration example of an electromagnetic wave detection device 10 according to modification 2. In the above description, the electromagnetic wave detection device 10 includes the waveform modification unit 13 that modifies the waveform of the electrical signal from the electromagnetic wave detector 12. However, instead of or in addition to this, the electromagnetic wave detection device 10 may include a waveform modification unit 23 that modifies the waveform of the reference signal. In the example of Fig. 6, the electromagnetic wave detection device 10 includes both the waveform modification unit 13 and the waveform modification unit 23.

[0104] The waveform modification unit 23 modifies the waveform of the reference signal from the reference signal generation unit 14 a so that the magnitude of the detected signal does not become 0. Furthermore, the waveform modification unit 23 may modify the waveform of the reference signal from the reference signal generation unit 14 a so that the intensity of the detected signal from the synchronous detection unit 14 b is increased compared to when the waveform modification unit 23 is not provided.

[0105] For example, the waveform modification unit 23 may be configured to perform any one of the waveform modifications (1) to (7) described above for the waveform modification unit 13, or may be configured to perform any combination of two or more of the waveform modifications (1) to (7) described above. In this case, the above explanations (1) to (7) apply to the waveform modification performed by the waveform modification unit 23. In this application, in the above explanations (1) to (7), the waveform modification unit 13 is replaced with the waveform modification unit 23, the electrical signal from the electromagnetic wave detector 12 is replaced with the reference signal from the reference signal generation unit 14a, and the reference signal is replaced with the electrical signal from the electromagnetic wave detector 12.

[0106] 6 , when the reference signal generating unit 14a generates multiple types of reference signals having different reference frequencies as described above, a waveform modifying unit 23 may be provided for each of the multiple types. That is, multiple waveform modifying units 23 may be provided corresponding to the multiple types. In this case, as shown in FIG. 6 , for each type, the waveform modifying unit 23 corresponding to that type modifies the waveform of the reference signal of that type and inputs the reference signal after the waveform modification to the corresponding multiplier 14b1. The multiplier 14b1 multiplies the reference signal after the waveform modification by the electrical signal input from the waveform modifying unit 13 to generate and output a multiplied signal. The corresponding low-pass filter 14b2 removes frequency components higher than a predetermined cutoff frequency from the multiplied signal, thereby outputting the detection signal as described above.

[0107] In addition, when only the waveform transformation unit 23 is provided out of the waveform transformation unit 13 and the waveform transformation unit 23, an amplifier may be provided instead of the waveform transformation unit 13. This amplifier may or may not transform the waveform of the electrical signal from the electromagnetic wave detector 12, but may amplify the electrical signal and output it to the synchronous detection unit 14b (multiplication unit 14b1). Here, the ratio of the magnitude (maximum value) of the electrical signal output from the amplifier to the magnitude (maximum value) of the electrical signal input to the amplifier may be constant.

[0108] 7 shows a configuration example of an electromagnetic wave detection device 10 according to Modification 3. This electromagnetic wave detection device 10 includes a time-series data generation unit 22 instead of the waveform generation unit 16 and the waveform processing unit 17 described above (or in addition to the waveform generation unit 16 and the waveform processing unit 17).

[0109] The time series data generating unit 22 generates time series data of the time average value or time integral value of the magnitude of the detection signal. Alternatively, the time series data generating unit 22 may generate time series data of the magnitude of the detection signal.

[0110] Fig. 8 is a flowchart showing an electromagnetic wave detection method according to Modification 3. This electromagnetic wave detection method may be performed using the electromagnetic wave detection device 10 of Fig. 7. In Modification 3, as shown in Fig. 8, steps S17 and S18 are performed instead of steps S7 to S9 described above.

[0111] In this case, as described above, in step S6, when steps S2 to S5 are repeated and the detection signal generated in step S5 becomes stable (for example, when steps S2 to S5 described above have been repeated a predetermined number of times), the process proceeds to step S17.

[0112] In step S17, a time average value or a time integral value of the magnitude of the detection signal over a certain period of time (for example, the period of time over which steps S2 to S5 are repeated the predetermined number of times) is generated. This time average value or time integral value may be generated by the time-series data generator 22.

[0113] After performing step S17, the process returns from step S17 to step S2, and steps S2 to S6 are repeated (for example, a predetermined number of times) as described above with the same time difference as the previous time difference without changing the time difference, and the time average value or time integral value of the magnitude of the detection signal is generated again in step S17 as described above.

[0114] By repeating this process, the time-averaged or time-integrated value of the magnitude of the detection signal is output at each step S17. In step S18, the time-series data generator 22 generates time-series data of the time-averaged or time-integrated values ​​generated at multiple steps S17. Alternatively, the time-series data is updated each time step S17 is performed. The time-series data may be data indicating the time-averaged or time-integrated value of the magnitude of the detection signal generated at step S17 relative to the time point at which each step S17 is performed (e.g., the time point at which steps S2 to S5 have been repeated a predetermined number of times). For example, the time-series data may be data obtained by plotting the time-averaged or time-integrated value at each step S17 in a two-dimensional coordinate system having a horizontal axis indicating the time point at which each step S17 was performed and a vertical axis indicating the time-averaged or time-integrated value of the magnitude of the detection signal generated at each step S17.

[0115] Furthermore, the above-described steps S6 and S17 may be omitted. In this case, in repeating steps S2 to S5, the time-series data generator 22 may generate time-series data of the detection signal generated in step S5 each time. In this case, the time-series data may be data indicating the magnitude of the detection signal relative to the time point at which each detection signal is generated.

[0116] Note that, after performing step S17 a predetermined number of times with one time difference to generate time series data in step S18, the time difference may be changed and steps S1 to S6 and steps S17 and S18 shown in the flowchart of Figure 8 may be performed again in the same manner as described above to generate the above-mentioned time series data for the changed time difference. However, the time difference does not have to be changed, and in this case, the time difference change device 21 may be omitted.

[0117] In the third modified example, the time series data generated by the time series data generating unit 22 may be stored in the storage unit 18 and displayed on the display device 19 .

[0118] In this third modification, when the reference signal generator 14a generates multiple types of reference signals having different reference frequencies as described above, the time-series data generator 22 generates the above-described time-series data for each of the multiple types. The reference signal generator 14a may also generate a reference signal of a reference frequency and input it to the synchronous detector 14b. In this case, the synchronous detector 14b may synchronously detect the electrical signal using the reference signal and output the detected signal, and the time-series data generator 22 may generate the above-described time-series data of the magnitude of the repeatedly output detected signal.

[0119] The present invention is applicable not only to the THz-TDS method but also to other pump-probe methods in general.

[0120] REFERENCE SIGNS LIST 1 Sample 10 Electromagnetic wave detection device 11 Electromagnetic wave emitter 12 Electromagnetic wave detector 13 Waveform modification section 14 Lock-in amplifier 14a Reference signal generation section 14b Synchronous detection section 14b1 Multiplication section 14b2 Low-pass filter 15 Control device 15a Laser 15b Beam splitter 15c Reference signal generation section 16 Waveform generation section 17 Waveform processing section 18 Memory section 19 Display device 21 Time difference change device 21a Reflection mirror 21b Movable body 21c Drive control device 22 Time series data generation section 23 Waveform modification section

Claims

1. An electromagnetic wave detection device comprising: an electromagnetic wave emitter that emits electromagnetic waves to a sample; an electromagnetic wave detector that detects the electromagnetic waves that have interacted with the sample and outputs an electrical signal; a control device that controls the repeated emission and detection of the electromagnetic waves at a reference frequency; a reference signal generation unit that generates a reference signal having a frequency that is a natural number multiple of at least twice the reference frequency; a waveform modification unit that modifies the waveforms of one or both of the electrical signal and the reference signal; and a synchronous detection unit that receives one or both of the signals that have passed through the waveform modification unit, synchronously detects the electrical signal with the reference signal, and outputs a detection signal.

2. The electromagnetic wave detection device according to claim 1, wherein the waveform modification section modifies one or both of the waveforms so that the magnitude of the detection signal does not become zero.

3. The electromagnetic wave detection device according to claim 1 or 2, wherein the waveform modification section modifies one or both of the waveforms so as to increase the intensity of the detection signal.

4. An electromagnetic wave detection device according to any one of claims 1 to 3, wherein the waveform modification section modifies the waveform of one or both of the pulses while increasing or maintaining the pulse width of one or both of the pulses.

5. An electromagnetic wave detection device according to any one of claims 1 to 4, wherein the reference signal has a sine wave waveform, and the waveform modification section modifies the waveform of the electrical signal into a waveform different from the sine wave waveform.

6. An electromagnetic wave detection device according to any one of claims 1 to 5, comprising: a time difference changing device that changes the time difference between the transmission and detection of the electromagnetic wave; and a waveform generating unit that generates a waveform that represents the magnitude of the detection signal for each of the time differences.

7. An electromagnetic wave detection device according to any one of claims 1 to 6, comprising a time series data generation unit that generates time series data of the time average value or time integral value of the magnitude of the detection signal, or time series data of the magnitude of the detection signal.

8. An electromagnetic wave detection device according to any one of claims 1 to 7, wherein the synchronous detection unit comprises: a multiplication unit that generates a multiplied signal by multiplying the electrical signal by the reference signal; and a low-pass filter that outputs the detection signal by removing frequency components higher than a predetermined cutoff frequency from the multiplied signal.

9. The electromagnetic wave detection device according to any one of claims 1 to 8, wherein the electromagnetic waves are terahertz waves, ultraviolet light, visible light, infrared light, microwaves, RF waves, X-rays, or gamma rays.

10. An electromagnetic wave detection method comprising: (A) transmitting electromagnetic waves to a sample and detecting the electromagnetic waves that have interacted with the sample to output an electrical signal; (B) controlling the repeated transmission and detection of the electromagnetic waves at a reference frequency; (C) generating a reference signal having a frequency that is a natural number multiple of at least twice the reference frequency; (D) modifying the waveforms of one or both of the electrical signal and the reference signal; and (E) after (D), synchronously detecting the electrical signal with the reference signal to output a detected signal.

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