JP2007101370A - Terahertz spectral device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a terahertz spectral device capable of obtaining exact spectrum regardless of positioning accuracy of a delay stage. <P>SOLUTION: At every movement of the delay stage 8, a detection signal corresponding to the electric intensity of terahertz pulse wave is gained from a terahertz detector 7. Simultaneously, a pulse corresponding to the position of the delay stage 8 output from a stage position meter 9 is gained and a time wave corresponding to the position of the delay stage 8 in gaining the electric field intensity of the terahertz wave is obtained. After correcting the time wave to the data of equivalent time internals, it is Fourier-converted to obtain an amplitude spectrum and a phase spectrum. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a terahertz spectrometer.

  Conventionally, a terahertz spectrometer using a terahertz wave (an electromagnetic wave having a frequency ranging from about 0.01 THz to 100 THz) is known.

  This terahertz spectrometer includes a beam splitter, a delay stage, a terahertz generator, and a terahertz detector.

  The pulsed light from the laser light source is split into two by a beam splitter, one is guided to the terahertz generator as pump light, and the other is guided to the terahertz detector as probe light. The terahertz wave generated by the terahertz generator is focused on the sample. The terahertz wave transmitted or reflected from the sample is incident on the terahertz detector. When a terahertz wave is incident on the terahertz detector, an electric field is generated. At this time, when probe light enters the terahertz detector, a photocurrent according to the electric field strength flows. By measuring this, it is possible to know the electrical characteristics and impurity concentration of the sample.

When a terahertz wave is incident on the terahertz detector, for example, when the movable mirror of the delay stage is moved, the optical path length of the probe light changes, and the time for the probe light to reach the terahertz detector changes. Therefore, time series terahertz spectroscopy can be performed by measuring the photocurrent detected by the terahertz detector while changing the time.
JP 2004-191302 A

  By the way, positioning accuracy of 0.1 μm or less is required for a delay stage whose terahertz wave measurement frequency band is 5 to 10 THz.

  However, since it is very difficult to make a delay stage that satisfies this requirement, there is a problem that an accurate spectrum cannot be obtained.

  The present invention has been made in view of such circumstances, and is to provide a terahertz spectrometer capable of obtaining an accurate spectrum regardless of the positioning accuracy of the delay stage.

  In order to solve the above problem, a terahertz spectrometer according to the present invention includes a delay stage that delays one time of pulse light divided into two by a beam splitter, a terahertz generator that generates a terahertz wave by irradiation with the pulse light, A terahertz detector that detects a terahertz wave reflected from or transmitted through the sample, a position measuring unit that measures the position of the delay stage, and a detection performed by the terahertz detector based on a signal output from the position measuring unit And a control means for correcting a detection signal representing a time change in electric field intensity corresponding to the terahertz wave.

  According to a second aspect of the present invention, in the terahertz spectrometer according to the first aspect, the control means performs a Fourier transform after correcting the time change of the electric field intensity to obtain a spectrum.

  According to a third aspect of the present invention, in the terahertz spectrometer according to the first aspect, the control means performs the correction process simultaneously with performing spectral analysis of the detection signal using a predetermined algorithm.

  According to the present invention, an accurate spectrum can be obtained regardless of the positioning accuracy of the delay stage.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 is a block diagram showing a part of a terahertz spectrometer according to an embodiment of the present invention, FIG. 2 is a conceptual diagram showing the entire terahertz spectrometer according to an embodiment of the present invention, and FIG. 3 shows an amplitude intensity spectrum. FIG. In FIG. 3, the vertical axis and the horizontal axis represent the electric field strength and frequency (THz), respectively.

  The terahertz spectrometer includes a beam splitter 2, a delay stage 8, a terahertz generator 3, a terahertz detector 7, a first terahertz optical system 4, a second terahertz optical system 6, and a stage position measuring device. (Position measuring means) 9 and a computer (control means) 10 are provided.

  The beam splitter 2 splits the pulsed light P1 from the laser light source 1 into pump light P2 and probe light P3. As the laser light source 1, for example, a femtosecond pulse laser is used.

  The delay stage 8 performs time delay of the probe light P3 divided by the beam splitter 2. The delay stage 8 is a combination of a movable movable mirror 8a and fixed planar reflecting mirrors 8b and 8c. The movable mirror 8a moves as indicated by the arrow in FIG. 2 to change the optical path length of the probe light P3.

  The terahertz generator 3 generates a terahertz wave by irradiation with the pump light P2. As the terahertz generator 3, a so-called photoconductive antenna in which two electrodes having an antenna shape are attached on a semiconductor substrate formed of semi-insulating GaAs or the like and a bias voltage is applied between these electrodes, a ZnTe or the like is used. Nonlinear optical crystals are used.

  The terahertz detector 7 detects the terahertz wave that has passed through the sample 5. As the terahertz detector 7, a photoconductive antenna similar to the terahertz generator 3 or an electro-optic crystal such as ZnTe is used.

  The first terahertz optical system 4 converges the terahertz wave and guides it to the sample 5. The first terahertz optical system 4 is, for example, an ellipsoidal mirror.

  The second terahertz optical system 6 guides the terahertz wave transmitted through the sample 5 to the terahertz detector 7. The second terahertz optical system 6 is, for example, an ellipsoidal mirror.

  The stage position measuring device 9 measures the position of the delay stage 8. As the stage position measuring device 9, for example, a linear encoder (resolution: several nm to several tens of nm) is used.

  The computer 10 processes the detection signal 7 a detected by the terahertz detector 7 based on the pulse signal 9 a from the stage position measuring device 9. Further, the computer 10 receives the pulse signal 9 a output from the stage position measuring device 9 and outputs a signal 10 a for controlling the movement of the delay stage 8 to execute closed loop control for controlling the position of the delay stage 8.

  The pulsed light P1 emitted from the laser light source 1 is split into two pulsed lights by the beam splitter 2 through the plane reflecting mirror M1. The pulsed light P1 is linearly polarized pulsed light having a center wavelength of about 780 to 800 nm in the near infrared region, a repetition period of about several kHz to 100 MHz, and a pulse width of about 10 to 150 fs.

  One pulsed light split by the beam splitter 2 becomes pump light P2 for exciting the terahertz generator 3 to generate a terahertz wave. The pump light P2 is guided to the terahertz generator 3 through the plane reflecting mirror M2. When the terahertz generator 3 is irradiated with pulsed light of about 100 femtoseconds, a terahertz wave P4 having a frequency in the terahertz region is emitted from the terahertz generator 3 with the same repetition period as the pulsed light P1 emitted from the laser light source 1. The

The terahertz wave P4 is an electromagnetic wave in a frequency range from about 0.01 × 10 ¹² to 100 × 10 ¹² hertz. The terahertz wave P4 is condensed on the sample 5 through the first terahertz optical system 4.

  The terahertz wave P <b> 5 that has passed through the sample 5 enters the terahertz detector 7 through the second terahertz optical system 6. When the terahertz wave P5 is incident on the terahertz detector 7, an electric field is generated. At this time, when the terahertz detector 7 is irradiated with the probe light P3, a detection signal 7a corresponding to the electric field strength is output. The probe light P3 enters the terahertz detector 7 as follows.

  The other pulse light split by the beam splitter 2 becomes probe light P3 for detecting the terahertz wave P5. The probe light P3 enters the terahertz detector 7 through the delay stage 8, the plane reflecting mirror M3, and the plane reflecting mirror M4. When the movable mirror 8a of the delay stage 8 moves as indicated by the arrow in FIG. 2, the optical path length of the probe light P3 changes. The delay stage 8 moves in a predetermined direction by 12 μm, for example, in response to a signal 10a from the computer 10. As a result, the time for the probe light P3 to reach the terahertz detector 7 changes. For example, changing the optical path length of the probe light P3 by 3 mm corresponds to changing the delay time by 10 picoseconds.

  Each time the delay stage 8 moves, the computer 10 inputs a detection signal 7a corresponding to the electric field strength of the terahertz wave incident at the moment when the probe light P3 is applied to the terahertz detector 7, and the stage position measuring device 9 The pulse signal 9a corresponding to the position of the delay stage 8 output from is input. The computer 10 repeats the output of the signal instructing the movement of the delay stage 8 and the input of the pulse signal 9a, for example, 128 times.

  When the pulse signal 9a indicating that the delay stage 8 has reached the target position is input from the stage position measuring device 9 to the computer 10, the computer 10 outputs a signal for stopping the movement of the delay stage 8. However, the delay stage 8 may actually stop before and after the target position, and there is an error in the positioning accuracy of the delay stage 8.

  As described above, since the stage position measuring instrument 9 has a resolution of several nanometers to several tens of nanometers, the amount of time delay when the stage position measuring instrument 9 actually acquires the electric field strength of the terahertz wave is detected, and is accurate. It is possible to obtain a time waveform (time change data) of the electric field intensity of a terahertz wave.

  The computer 10 converts (corrects) the accurately obtained time waveform into a time waveform at equal time intervals using an interpolation method or the like, and Fourier-transforms the result to obtain a spectrum (amplitude intensity, phase difference with respect to frequency (terahertz)). )) Get.

  When the stop position of the delay stage 8 detected by the stage position measuring device 9 is P1, P2, P3... Pm... Pn, ideally, the electric field intensity E1, with respect to each position P1, P2, P3. E2, E3 ... Em ... En should be acquired by the terahertz detector 7. However, the stop positions P1, P2, P3... Pm... Pn of the delay stage 8 cannot be detected due to insufficient positioning accuracy of the delay stage 8. Actually, the stop positions P1 ', P2', P3 '. “... Pn” is detected by the stage position measuring device 9, and the electric field strengths E 1 ′, E 2 ′, E 3 ′. Em ′. In order to correct the time waveform, Em is obtained from Em ′ using Equation 1 or Equation 2.

Ｐｍ＞Ｐｍ’の場合

時間波形の補正なしにフーリエ変換を行なうと、振幅強度スペクトルにおいて、ディレイステージ８の位置決め精度不足は高周波側に疑似的なシグナル、すなわちノイズとなって現れる（図３の波形ａ参照）。 When Pm <Pm '

When Pm> Pm ′

When Fourier transform is performed without correcting the time waveform, inadequate positioning accuracy of the delay stage 8 appears as a pseudo signal, that is, noise on the high frequency side in the amplitude intensity spectrum (see waveform a in FIG. 3).

  When Fourier transform is performed after time correction by the above formula or the like, noise is reduced and an amplitude intensity spectrum having a high S / N (signal / noise) ratio is obtained (see waveform b in FIG. 3).

  Note that the transmittance of the sample 5 is known from the amplitude intensity, and the refractive index of the sample 5 is known from the phase difference. The obtained time waveform and spectrum are displayed on a display unit such as a liquid crystal display or CRT (not shown).

  As another correction method, a spectrum may be obtained from a time waveform that is not at equal time intervals by a predetermined algorithm without performing Fourier transform. At this time, conversion from the time domain to the frequency domain and correction processing are performed simultaneously.

  According to this embodiment, since the position of the delay stage can be accurately obtained, an accurate spectrum can be obtained regardless of the positioning accuracy of the delay stage. As a result, the reliability of the spectrum can be improved.

  In addition, although the case where the transmission type measurement optical system is used has been described in the above embodiment, a reflection type measurement optical system may be used instead of the transmission type measurement optical system.

FIG. 1 is a block diagram showing a part of a terahertz spectrometer according to an embodiment of the present invention. FIG. 2 is a conceptual diagram showing the entire terahertz spectrometer according to an embodiment of the present invention. FIG. 3 is a diagram showing an amplitude intensity spectrum.

Explanation of symbols

  1: laser light source, 2: beam splitter, 3: terahertz generator, 5: sample, 7: terahertz detector, 8: delay stage, 9: stage position measuring device (position measuring means), 10: computer (control means) .

Claims (3)

A delay stage for delaying one time of the pulsed light divided into two by the beam splitter;
A terahertz generator that generates a terahertz wave by irradiation with the pulsed light;
A terahertz detector that detects a terahertz wave reflected by the sample or transmitted through the sample;
Position measuring means for measuring the position of the delay stage;
Control means for correcting a detection signal representing a temporal change in electric field intensity corresponding to the terahertz wave detected by the terahertz detector based on a signal output from the position measuring means. Terahertz spectrometer.   2. The terahertz spectrometer according to claim 1, wherein the control means performs a Fourier transform after correcting the time variation of the electric field intensity to obtain a spectrum.   2. The terahertz spectrometer according to claim 1, wherein the control means performs the correction process simultaneously with performing a spectrum analysis of the detection signal using a predetermined algorithm.

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