JP2007327897A - Terahertz spectroscopic device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a terahertz spectroscopic device capable of shortening a measuring time without lowering reliability of measurement. <P>SOLUTION: In this terahertz spectroscopic device equipped with a beam splitter for splitting a light pulse from a laser light source into pump light and probe light P3, and an optical path length changing optical system for changing the optical path length of the pump light or the probe light P3 split by the beam splitter, the optical path length changing optical system is constituted of a plurality of pairs of mirrors 9a, 10a, 9b, 10b, and one pair of mirrors 9a, 10a among the plurality of pairs of mirrors is set to be movable. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a terahertz spectrometer.

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

In recent years, research on generating and detecting pulsed terahertz waves using a laser light source that emits an optical pulse of about 100 femtoseconds has been conducted, and a new spectroscopic method called terahertz time domain spectroscopy has been developed. A time domain spectroscopic device has appeared for implementation. The principle of terahertz time domain spectroscopy is disclosed in the following patent document.
JP 2000-212110 A

  In this time domain spectroscope, if the moving speed of the time delay stage that optically delays the optical pulse is increased, the time required for measurement is shortened, but the vibration of the time delay stage is increased, and the surrounding optical elements are increased. Since the element also vibrates, there is a problem that the reliability of measurement is lowered.

  The present invention has been made in view of such circumstances, and an object thereof is to provide a terahertz spectrometer capable of shortening the measurement time without reducing the reliability of measurement.

  In order to solve the above problems, the present invention provides a beam splitter that divides a light pulse from a laser light source into pump light and probe light, and a pair of a fixed mirror and a movable mirror, or a pair of a movable mirror and a movable mirror, respectively. An optical path length changing optical system for changing the optical path length of the pump light or the probe light divided into a plurality of pairs and split by the beam splitter, and the plurality of pairs of movable mirrors among the plurality of pairs of mirrors. And a movable member movable in the opposite direction.

  According to a second aspect of the present invention, in the terahertz spectrometer according to the first aspect, the movable mirror is provided with driving means for driving the movable mirrors in opposite directions at the same speed.

  According to the present invention, the measurement time can be shortened without reducing the reliability of measurement.

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

  1 is a conceptual diagram showing a terahertz spectrometer according to a first embodiment of the present invention, FIG. 2 is a conceptual diagram showing a time delay stage according to a comparative example of the terahertz spectrometer of FIG. 1, and FIG. FIG. 3B is a side view of the stage, and FIG.

  This terahertz spectrometer includes a beam splitter 2, a time delay stage 8, a terahertz generator 3, a terahertz detector 7, a first terahertz optical system 4, and a second terahertz optical system 6. .

  The time delay stage 8 includes a plurality of pairs of mirror units including a pair of a movable mirror 9a and a fixed mirror 9b and a pair of a movable mirror 10a and a fixed mirror 10b.

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

  The time delay stage 8 performs time delay of the probe light P3 divided by the beam splitter 2. The time delay stage 8 includes movable mirrors (mirrors) 9a and 10a, fixed mirrors (mirrors) 9b and 10b, a motor 15, and a crank mechanism 16 (see FIG. 3). The movable mirrors 9a and 10a and the fixed mirrors 9b and 10b constitute an optical path length changing optical system. By the motor (driving means) 15 and the crank mechanism (movable member) 16, the movable mirror 9a and the movable mirror 10a are continuously moved at the same speed and the same distance in opposite directions.

  The movable mirrors 9 a and 10 a and the motor 15 are connected via a crank mechanism 16. The crank mechanism 16 changes the rotational movement of the motor 15 to the linear movement of the movable mirrors 9a and 10a as indicated by arrows. The optical path length of the probe light P3 is changed by the reciprocating motion of the movable mirrors 9a and 10a.

  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 antenna-shaped electrodes are formed on a semiconductor substrate made of semi-insulating GaAs or the like and a bias voltage is applied between these electrodes, ZnTe, or the like is used. Nonlinear optical crystals are used.

  The terahertz detector 7 detects a terahertz wave. 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 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 several tens of 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.

The terahertz wave P4 is an electromagnetic wave in a frequency range from approximately 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, if the terahertz detector 7 is irradiated with the probe light P3, a photocurrent according to the electric field strength flows, and by measuring this, the electrical characteristics, impurity concentration, etc. of the sample 5 can be detected. . 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 time delay stage 8, the plane reflecting mirror M3, and the plane reflecting mirror M4. By moving the movable mirrors 9a, 10a of the time delay stage 8 as shown by arrows, the optical path length of the probe light P3 is changed, and the electric field of the terahertz wave is changed while changing the time for the probe light P3 to reach the terahertz detector 7. Measure the time waveform.

  A pump-probe method is used to measure the time waveform of the terahertz wave. That is, the movable mirrors 9a and 10a are placed on the optical path that guides the probe light P3, the optical path length is changed by moving the movable mirrors 9a and 10a, and the timing at which the probe light P3 reaches the terahertz wave detector 7 is shifted. However, the time waveform of the electric field of the terahertz wave is measured. When measuring a time waveform of 40 picoseconds of a terahertz pulse wave, it is necessary to extend the time delay of 40 picoseconds, that is, the optical path length of the probe light P3 by 12 mm.

  As a comparative example, when one movable mirror 11 is used as shown in FIG. 2, it is necessary to move the movable mirror 11 relative to the fixed mirror 12 in order to extend the optical path length by 12 mm. On the other hand, when two opposing movable mirrors 9a and 10a are used, the opposing movable mirrors 9a and 10a may be moved by 3 mm in opposite directions (the moving distance of the movable mirrors 9a and 10a is a necessary optical path). It is 1/4 times the length of 12 mm).

  In addition, by making the moving distance of the movable mirrors 9a and 10a ½ times, the time for obtaining a predetermined optical path length can be halved if the moving speed is made the same. Moreover, if the time for obtaining a predetermined optical path length is made the same, the moving speed can be halved and vibration can be reduced. Furthermore, since the opposing movable mirrors 9a and 10a move in the opposite directions, the mutual vibrations cancel each other.

  According to this embodiment, since the moving distance of the movable mirrors 9a and 10a is halved, it is possible to change the optical path length in a short time and the opposing movable mirrors 9a and 10a move in the opposite direction. Therefore, the mutual vibrations cancel each other and the vibrations can be reduced.

  FIG. 4 is a conceptual diagram showing a modification of the time delay stage.

  This time delay stage includes movable mirrors 19a and 20a, fixed mirrors 19b and 20b, and driving means (not shown) for driving the movable mirrors 19a and 20a. Movable mirrors 19a and 20a, fixed mirrors 19b and 20b, and an optical path length changing optical system are configured.

  The movable mirrors 19a and 20a and the fixed mirrors 19b and 20b have the same shape.

  The movable mirror 19a has reflecting surfaces R1 and R2 and reflecting surfaces R3 and R4 that are substantially perpendicular to each other.

  The fixed mirror 19b has reflecting surfaces R5 and R6 and reflecting surfaces R7 and R8 that are substantially perpendicular to each other.

  The movable mirror 19a and the fixed mirror 19b are arranged as follows.

  The reflecting surface R6 and the reflecting surface R1 face each other, the reflecting surface R2 and the reflecting surface R7 face each other, and the reflecting surface R8 and the reflecting surface R3 face each other.

  The movable mirror 20a has reflecting surfaces R11 and R12 and reflecting surfaces R13 and R14 that are substantially perpendicular to each other.

  The fixed mirror 20b has reflecting surfaces R15 and R16 and reflecting surfaces R17 and R18 that are substantially perpendicular to each other.

  The movable mirror 20a and the fixed mirror 20b are arranged as follows.

  The reflecting surface R16 and the reflecting surface R11 face each other, the reflecting surface R12 and the reflecting surface R17 face each other, and the reflecting surface R18 and the reflecting surface R13 face each other.

  The reflecting surface R5 and the reflecting surface R15 are in a relationship in which the other receives the reflected light.

  In the case of this modification, as can be seen from the figure, the probe light P3 is repeatedly reflected 16 times on the time delay stage, so that the moving distance of the movable mirrors 19a and 20a is 1/8 times the optical path length necessary for the time delay. .

  According to this modification, the optical path length can be changed in a shorter time than in the first embodiment, and vibration can be reduced.

  FIG. 5 is a conceptual diagram showing another modification of the time delay stage.

  The time delay stage includes movable mirrors 21a, 22a, 23a, 24a and movable members (not shown) and driving means for driving the movable mirrors 21a, 22a, 23a, 24a. An optical path length changing optical system is composed of a pair of movable mirrors 21a and 22a and a plurality of pairs of mirror units forming a pair of movable mirrors 23a and 24a.

  The movable mirrors 22a and 23a move in opposite directions so as to cancel vibrations, and the movable mirrors 21a and 24a move in opposite directions so as to cancel vibrations. The movable mirrors 21a and 23a move in the same direction, and the movable mirrors 22a and 24a move in the same direction.

  Specifically, for example, when the movable mirrors 21a and 22a move in a direction approaching each other, the movable mirrors 23a and 24a continuously move at the same speed and the same distance in the direction approaching each other.

  In the case of this modification, the moving distance of the movable mirrors 21a, 22a, 23a, 24a is 1/8 times the required optical path length.

  According to this modification, the optical path length can be changed in a shorter time than in the first embodiment, and vibration can be reduced.

FIG. 1 is a conceptual diagram showing a terahertz spectrometer according to a first embodiment of the present invention. FIG. 2 is a conceptual diagram showing a time delay stage according to a comparative example of the terahertz spectrometer shown in FIG. 3A is a side view of the time delay stage, and FIG. 3B is a plan view thereof. FIG. 4 is a conceptual diagram showing a modification of the time delay stage. FIG. 5 is a conceptual diagram showing another modification of the time delay stage.

Explanation of symbols

  1: laser light source, 2: beam splitter, 3: terahertz wave generator, 8: time delay stage, 9a, 10a, 19a, 20a, 21a, 22a, 23a, 24a: movable mirror (mirror), 9b, 10b, 19b , 20b: fixed mirror (mirror), 15: motor (drive means), 16: crank mechanism (movable member).

Claims (2)

A beam splitter that splits a light pulse from a laser light source into pump light and probe light;
Optical path length change for changing the optical path length of the pump light or the probe light split by the beam splitter, each of which includes a pair of a fixed mirror and a movable mirror, or a pair of a movable mirror and a movable mirror. Optical system,
A terahertz spectrometer comprising: a movable member configured to move the plurality of pairs of movable mirrors in directions opposite to each other among the plurality of pairs of mirrors. The terahertz spectrometer according to claim 1, further comprising driving means for driving the movable mirrors in opposite directions at the same speed.

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