JP2006266908A - Instrument and method for measuring terahertz pulse light - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a plurality of time-serial waveforms for reducing a noise component by one time delay operation. <P>SOLUTION: A sample is irradiated with terahertz pulse light L6 generated in a terahertz light generator 5, so as to detect a time-serial change with time of intensity in an electric field of a terahertz pulse light L8 transmitted through the sample S by a terahertz light detector 8. The intensity in the electric field of the terahertz pulse light L8 transmitted through the sample S is detected finely with a prescribed delay time interval, during the one time delay operation for moving a turning mirror 11 by a distance d along an A direction from an original position. A storage circuit 21 stores an electric field intensity data together with positional information of the turning mirror 11 corresponding to the time delay amount in the detection. A computing circuit 22 extracts the plurality of electric field intensity data in every of the prescribed delay time amounts from the storage circuit 21 to obtain the plurality of time-serial waveforms, and the respective time-serial waveforms are Fourier-transformed respectively to obtain a plurality of spectral waveforms. The plurality of spectral waveforms include respectively a plurality of amplitude information pieces and a plurality of phase information pieces, and the computing circuit 22 averages further the plurality of amplitude information pieces or phase information pieces. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a measurement apparatus using terahertz pulse light and a measurement method using terahertz pulse light.

In various measuring apparatuses using terahertz light, the sample is irradiated with terahertz pulse light in a frequency range of approximately 0.01 × 10 ^{12 to} 100 × 10 ¹² hertz (0.01 to 100 THz), and transmitted through the sample. By detecting light or reflected light reflected from the sample, the electrical characteristics, component concentration, etc. of the sample are measured. Conventionally, a time series waveform of terahertz pulse light is measured by delaying the time that probe pulse light that operates a detector that detects terahertz pulse light reaches the terahertz light detector and measuring the electric field intensity according to the time delay amount. There is known a terahertz light measuring device that acquires a physical property value and a component amount based on this time-series waveform. The time delay device includes a mirror and a moving stage for continuously changing the optical path length of the probe pulse light by moving the mirror, and one time series waveform is obtained by one movement operation (time delay operation). Is obtained. (For example, refer to Patent Document 1).

JP 2002-277393 (pages 10, 11 and 1, 4)

  The conventional terahertz pulse light measurement apparatus can obtain only one time-series waveform by one time delay operation, and needs to obtain a plurality of time-series waveforms by repeating the time delay operation. There was a problem to do.

(1) A terahertz pulse light measuring device according to the invention of claim 1 includes a terahertz light generator that generates terahertz pulse light by irradiation of pump pulse light, and an optical that irradiates a sample with terahertz pulse light generated by the terahertz light generator System, a terahertz light detector that detects terahertz pulse light from a sample irradiated with terahertz pulse light through an optical system as an electric field intensity signal by irradiation with probe pulse light, and the terahertz pulse light reaches the terahertz light detector A time delay device that continuously changes the time difference between the time when the probe pulse light reaches the terahertz light detector, and an electric field acquired at a predetermined sampling interval in one time delay operation by the time delay device Memory circuit for storing intensity signal data and time delay device position information stored in the memory circuit Extracting a plurality of arithmetic field strength data based, characterized in that an arithmetic circuit for calculating a measurement result by processing the plurality of operation for the electric field intensity data.
(2) The invention according to claim 2 is the terahertz pulse light measurement device according to claim 1, wherein each of the plurality of operation electric field intensity data extracted is time-series waveform data, A plurality of pieces of amplitude information obtained by Fourier transforming each of the above are averaged to calculate the amplitude.
(3) The invention of claim 3 is the terahertz pulse light measurement device according to claim 1 or 2, wherein each of the plurality of operation electric field intensity data extracted is time-series waveform data, and the operation circuit is time-series A plurality of pieces of phase information obtained by Fourier transforming each of the waveform data are averaged to calculate the phase.
(4) The invention according to claim 4 is the terahertz pulse light measuring device according to claim 3, wherein the arithmetic circuit calculates the phase by averaging the plurality of pieces of phase information and sets the sampling interval of the electric field intensity signal. Accordingly, the phase shift included in the time-series waveform data is corrected.
(5) In the terahertz pulse light measurement method according to the invention of claim 5, the terahertz pulse light is generated by irradiating the pump pulse light, the generated terahertz pulse light is irradiated to the sample, and the terahertz pulse light generated by the pump pulse light is generated. While continuously changing the timing to reach the terahertz light detector and the timing to detect the terahertz pulse light as the electric field intensity signal by the probe pulse light, the terahertz pulse light from the sample is detected and the timing is continuously changed. In one time delay operation, the detected electric field strength signal data is acquired and stored at a predetermined sampling interval, and a plurality of calculation electric field strength data is extracted based on the stored electric field strength signal data. A plurality of calculation electric field intensity data are arithmetically processed to obtain measurement results.

  According to the present invention, a plurality of sets of time-series waveform data are obtained based on electric field strength signal data obtained in one time delay operation, and desired electric field strength data are obtained from the plurality of calculation field strength data. Since the measurement result is processed, the measurement time can be shortened.

Hereinafter, an embodiment of a terahertz pulse light measuring apparatus according to the present invention will be described with reference to FIGS.
FIG. 1 is an overall configuration diagram schematically showing a configuration of a terahertz pulse light measuring apparatus according to an embodiment of the present invention. The terahertz pulsed light measurement apparatus 100 irradiates the sample S with terahertz pulsed light, detects temporal changes in the electric field intensity of the terahertz pulsed light that has passed through the sample S, and performs Fourier transform on the detected data to thereby change the amplitude at each frequency. Information and phase information are obtained.

  The terahertz pulse light measuring apparatus 100 includes a laser light source 1 that emits pulsed light L1, a beam splitter 2 that divides the pulsed light L1 into pulsed light L2 and L9, mirrors 3 and 4, and terahertz that generates terahertz pulsed light L5. A light generator 5, curved mirrors 6 and 7, and a terahertz light detector 8 that detects the terahertz pulsed light L 8 after passing through the sample S are provided. Further, the terahertz pulse light measuring device 100 is configured to change the arrival time of the pulsed light L11 that is guided to the terahertz light detector 8 for the purpose of detecting the terahertz pulse light by a known time-series terahertz light detection method, And a mirror 14.

  Further, the terahertz pulse light measuring device 100 includes an A / D conversion circuit 16 connected to the terahertz light detector 8, a synchronization circuit 17, a stage controller 18, a control device 20, and a display connected to the time delay device 10. 23. The control device 20 includes a storage circuit 21 and an arithmetic circuit 22 and is connected to the A / D conversion circuit 16, the synchronization circuit 17, the stage controller 18 and the display 23, respectively.

  For example, a femtosecond pulse laser is used as the laser light source 1 that emits the pulsed light L1. The pulsed light L1 is linearly polarized pulsed light having a center wavelength of about 780 to 830 nm in the near infrared region, a repetition period on the order of several kHz to 100 MHz, and a pulse width of about 10 to 150 fs.

  The pump pulse light L2 divided by the beam splitter 2 sequentially passes through the mirrors 3 and 4 and enters the terahertz light generator 5 as pump pulse light L4. The terahertz light generator 5 includes an optical switch element 5a and a bias circuit 5b. When the pump pulse light L4 is applied to the optical switch element 5a in a state where a DC voltage is applied to the optical switch element 5a by the bias circuit 5b, terahertz pulse light L5 is generated. The terahertz pulse light L5 is collected by the curved mirror 6 and passes through the sample S. The terahertz pulsed light L7 that has passed through the sample S is light including physical property information of the transmission point, is collected by the curved mirror 7, and becomes the terahertz pulsed light L8 and enters the terahertz photodetector 8.

  The probe pulse light L9 divided by the beam splitter 2 sequentially passes through the time delay device 10 and the mirror 14 and becomes probe pulse light L11 and enters the terahertz light detector 8. The terahertz light detector 8 includes an optical switch element 8a and an I / V conversion circuit 8b similar to the optical switch element 5a. When the optical switch element 8a is irradiated with the probe pulse light L11 in the state where the terahertz pulse light L8 transmitted through the sample S is incident on the optical switch element 8a, the optical switch element 8a has the terahertz pulse light for the instantaneous irradiation time. A photocurrent generated by the electric field of L8 flows. That is, the electric field intensity of the terahertz pulsed light L8 that has passed through the sample S and entered the terahertz light detector 8 is detected by the I / V conversion circuit 8b.

  The terahertz photodetector 8 converts the detected current signal into a voltage signal by the I / V conversion circuit 8 b and sends it to the A / D conversion circuit 16. The A / D conversion circuit 16 converts the voltage signal into a digital quantity and sends it to the control device 20.

  The time delay device 10 is a device for changing the arrival time of probe pulse light from the beam splitter 2 to the terahertz light detector 8, and includes a folding mirror 11, a movable stage 12, and a linear encoder 13. The folding mirror 11 is composed of two or three reflecting mirrors, is placed on the movable stage 12 and can move in the A direction indicated by the arrow in the figure, and the moving distance d is precisely measured by the linear encoder 13. The

  The movable stage 12 is driven and controlled by the stage controller 18, and the total optical path length of the probe pulse lights L9 and L10 changes continuously. The stage controller 18 controls a motor (not shown) that moves the movable stage 12 based on a command signal from the control device 20. If the folding mirror 11 is moved by a distance d in the A direction from the origin position, the amount of change in the optical path length is 2 × d. Accordingly, the time for the probe pulse light to reach the terahertz light detector 8 is continuously delayed in proportion to the change amount of the optical path length. The moving distance d from the origin position, that is, the position information of the folding mirror 11 is sequentially sent from the linear encoder 13 to the synchronization circuit 17.

  As described above, the electric field strength of the terahertz pulsed light L8 incident on the optical switch element 8a is detected at the moment when the pulsed light L11 as the probe light irradiates the optical switch element 8a. This means that the probe pulse light gates the terahertz light detector 8. The electric field intensity at each delay time of the terahertz pulsed light L8 that repeatedly arrives at the terahertz light detector 8 is detected while moving the movable stage 12 continuously and shifting the timing for applying the gate.

The electric field strength detected at each delay time is sent as a voltage signal from the I / V conversion circuit 8b to the A / D conversion circuit 16, and is converted into a digital quantity by the A / D conversion circuit 16. The conversion operation is performed as follows. That is, the synchronization circuit 17 performs A / D conversion for the A / D conversion circuit 16 each time the position information of the folding mirror 11 sent from the linear encoder 13 matches a plurality of position information set in advance. Outputs a trigger. Specifically, the synchronization circuit 17 counts the number of pulses from the linear encoder 13 and outputs a trigger each time the count number (the moving distance of the folding mirror 11) reaches a preset count number. This count number corresponds to a minimum time interval r described later. These digital data of the electric field strength are sent to the control device 20 and stored in the storage circuit 21. Further, position data at the time of trigger output is also sent from the synchronization circuit 17 to the control device 20 and stored in the storage circuit 21. In the control device 20, the digital data relating to the electric field intensity is spliced based on the position data at the time of detecting the electric field intensity, and the time series waveform E _(t) of the electric field intensity of the terahertz pulse light L8 is acquired.

FIG. 2 is a graph of the time-series waveform E _(t) measured by the terahertz pulse light measurement device of the present embodiment. This time series waveform E _(t) is displayed on the screen of the display 23 (see FIG. 1). The horizontal axis represents the delay time calculated from the position data output from the synchronization circuit 17, and the vertical axis represents the electric field strength output from the A / D conversion circuit 16, which is detected by the I / V conversion circuit 8b. The current signal or the converted voltage signal.

This time-series waveform E _(t) is a graph obtained by one time delay operation of moving the movable stage 12 by a distance d from the origin position. That is, the delay time t when the movable stage 12 is at the origin position is t = 0, and the delay time t when the distance is d from the origin position is t = p.

In the graph of FIG. 2, the curve of the time series waveform E _{(t) in} the time period from time t ₁ to t ₂ is enlarged and shown in FIG. Now, in FIG. 3 (a), and divided into three equal parts the time from time t ₁ to t _2, represents one of which time interval q, further time interval q and n equal parts, one of which Expressed as time interval r. Then, it is assumed that data constituting the time series waveform E _(t) is stored in the storage circuit 21 of the control device 20 for each minimum time interval r.

FIGS. 3B to 3E show time series obtained by extracting data (calculation electric field intensity data) at predetermined time intervals from the time series waveform E _{(t) of} FIG. Waveform is shown. FIG. 3B shows a time-series waveform E _{(t + 0r)} configured by extracting data from the storage circuit 21 every time interval q from time t _1. This waveform is stored in the first storage area of the storage circuit 21. Stored. FIG. 3C shows a time-series waveform E _{(t + 1r)} configured by extracting data from the storage circuit 21 at time intervals q with reference to the time t ₁ + r. This waveform is the second waveform of the storage circuit 21. Stored in the storage area. FIG. 3D shows a time-series waveform E _{(t + 2r)} configured by extracting data from the storage circuit 21 for each time interval q with reference to the time t ₁ + 2r. This waveform is the third waveform of the storage circuit 21. Stored in the storage area. In this way, time-series waveforms are acquired with a time interval r shifted, and q / r (n) waveforms are stored in the storage area. FIG. 3E shows a time-series waveform E _{(t + (n−1) r)} configured by extracting data from the storage circuit 21 at time intervals q with reference to the time t ₁ + (n−1) r. Yes, this waveform is stored in the nth storage area of the storage circuit 21. Finally, q / r waveforms are stored in each storage area over the entire area (delay time p) of the time-series waveform E _(t) in FIG.

The arithmetic circuit 22 of the control device 20 performs Fourier transform on each of these q / r time series waveforms.
FIG. 4 is a graph of a spectrum waveform obtained by subjecting the time series waveform E _(t) of FIG. The horizontal axis represents frequency and the vertical axis represents spectrum intensity. The spectrum waveforms obtained by subjecting the time series waveforms of FIGS. 3B to 3E to Fourier transform are also similar curves. These spectrum waveforms are displayed on the screen of the display 23.

Q / r spectrum waveforms are obtained by Fourier transform, and amplitude information and phase information are included in each spectrum waveform. The amplitude information obtained as a result of the Fourier transform for the _n- th time series waveform E _{(t + (n−1) r )} is represented as | E _{n (ω)} |, and the phase information is represented as θ _{n (ω)} . Q / r pieces of amplitude information | E _{n (ω)} | and phase information θ _{n (ω)} are obtained.

The q / r pieces of amplitude information | E _{n (ω)} | can be integrated and averaged to reduce white noise to 1 / √ (q / r). White noise is noise that overlaps almost uniformly in the entire frequency region of the spectrum waveform of FIG. The integration and averaging operations are executed by the arithmetic circuit 22, and the amplitude information | E _(ω) | in which the white noise is reduced is expressed by Equation 1.

Also, q / r pieces of phase information θ _{n (ω)} can be integrated and averaged to reduce white noise. However, in the case of phase information, time series waveforms are acquired with a time interval r shifted, and therefore, when performing integration and averaging processing after Fourier transform, phase correction corresponding to each frequency is required. . Phase correction, integration, and averaging operations are executed by the arithmetic circuit 22, and phase information θ _(ω) in which white noise is reduced is expressed by Equation 2. In Equation 2, the phase correction term is rω (n−1).

Through the above calculation, amplitude information | E _(ω) | and phase information θ _(ω) with reduced white noise are obtained. Using both the amplitude information | E _(ω) | and the phase information θ _(ω) , for example, the refractive index and dielectric constant of the semiconductor material can be calculated, but only using the amplitude information | E _(ω) | In the case of the physical property value, the calculation related to the phase information θ _(ω) can be omitted. In the present embodiment, the time series waveform of FIG. 2 is obtained by one time delay operation. However, m time series waveforms are obtained by performing m time delay operations, and each time series waveform is obtained. If the above-described calculation and averaging are performed, the number of data becomes m times, so that the noise reduction effect is further increased. It should be noted that the present invention is not intended only for noise reduction but also for the purpose of greatly reducing the measurement time.

The present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the present invention. For example, instead of inserting the time delay device 10 into the optical path of the probe pulse light, it may be inserted into the optical path of the pump pulse light (pump pulse light L2, L3, L4) to delay the generation of the terahertz pulse light L5. . The terahertz pulse light measurement apparatus 100 may be provided with a scanning mechanism for moving the sample S two-dimensionally to form a scanning imaging apparatus. In this case, since the number of times of the time delay operation is large, the effect of shortening the measurement time is great.
The terahertz pulse light measuring apparatus 100 is a transmissive apparatus, but may be a reflective apparatus that detects reflected light from a sample.

1 is an overall configuration diagram schematically showing a configuration of a terahertz pulse light measurement apparatus according to an embodiment of the present invention. It is a graph of time series waveform E _(t) measured by the terahertz pulse light measuring device concerning an embodiment of the invention. It is a figure which expands and shows the time series waveform E _(t) curve of FIG. FIG. 3A shows a time series waveform E _(t) in a time period from time t ₁ to t ₂ , and FIGS. 3B to 3E show the time series waveforms of FIG. E _(t) shows a time-series waveform obtained by extracting data by shifting a predetermined time and reconstructing each data. It is a graph of the spectrum waveform obtained by performing a Fourier transform on the time series waveform E _(t) of FIG.

Explanation of symbols

1: laser light source 2: beam splitter 5: terahertz light generator 8: terahertz light detector 10: time delay device 11: folding mirror 12: movable stage 13: linear encoder 16: A / D conversion circuit 17: synchronization circuit 18: Stage controller 20: Control device 21: Memory circuit 22: Arithmetic circuit 23: Display 100: Terahertz pulse light measurement device L1: Laser pulse light L2-L4: Pump pulse light L5-L8: Terahertz pulse light L9-L11: Probe pulse light S: Sample

Claims (5)

A terahertz light generator that generates terahertz pulse light by irradiation with pump pulse light; and
An optical system for irradiating the sample with terahertz pulse light generated by the terahertz light generator;
A terahertz light detector for detecting terahertz pulse light from a sample irradiated with terahertz pulse light through the optical system as an electric field intensity signal by irradiation with probe pulse light; and
A time delay device for continuously changing a time difference between a time for the terahertz pulse light to reach the terahertz light detector and a time for the probe pulse light to reach the terahertz light detector;
A storage circuit that stores data of electric field strength signals acquired at a predetermined sampling interval in one time delay operation by the time delay device;
An arithmetic circuit that extracts a plurality of calculation electric field intensity data based on the position information of the time delay device stored in the storage circuit, and calculates the measurement result by calculating the plurality of calculation electric field intensity data. A terahertz pulsed light measurement apparatus comprising the terahertz pulse light measurement apparatus. In the terahertz pulsed light measurement device according to claim 1,
Each of the extracted plurality of electric field strength data for calculation is time-series waveform data,
The terahertz pulse light measuring apparatus, wherein the arithmetic circuit averages a plurality of pieces of amplitude information obtained by Fourier transforming each of the time series waveform data and calculates the amplitude. In the terahertz pulse light measuring device according to claim 1 or 2,
Each of the extracted plurality of electric field strength data for calculation is time-series waveform data,
The terahertz pulse light measuring device, wherein the arithmetic circuit calculates a phase by averaging a plurality of pieces of phase information obtained by Fourier transforming each of the time-series waveform data. In the terahertz pulse light measuring device according to claim 3,
The arithmetic circuit corrects a phase shift included in the time-series waveform data according to a sampling interval of the electric field intensity signal when calculating a phase by averaging the plurality of phase information. Terahertz pulse light measurement device. Terahertz pulse light is generated by irradiation with pump pulse light,
The sample is irradiated with the generated terahertz pulse light,
While continuously changing the timing at which the terahertz pulse light generated by the pump pulse light reaches the terahertz photodetector and the timing at which the terahertz pulse light is detected as an electric field intensity signal by the probe pulse light, the terahertz pulse from the sample is changed. Detect light,
In one time delay operation that continuously changes the timing, the data of the detected electric field strength signal is acquired and stored at a predetermined sampling interval,
Terahertz pulse light measurement characterized in that a plurality of calculation electric field strength data are extracted based on the stored electric field strength signal data, and a plurality of calculation electric field strength data are processed to obtain a measurement result. Method.

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