# Patent

## Title
Terahertz electromagnetic wave generator, terahertz spectrometer and method of generating terahertz electromagnetic wave

## Abstract

A terahertz electromagnetic wave generator according to the present disclosure includes: a thermoelectric material layer; a metal layer which partially covers the surface of the thermoelectric material layer; and a light source system which is configured to irradiate both a surface region of the thermoelectric material layer that is not covered with the metal layer and an edge of the metal layer with pulsed light, thereby generating a terahertz wave from the thermoelectric material layer.

## Background
This is a continuation of International Application No. PCT/JP2014/000928 with an international filing date of Feb. 21, 2014, which claims priority of Japanese Patent Application No. 2013-054780, filed on Mar. 18, 2013, the contents of which are hereby incorporated by reference.


BACKGROUND
1. Technical Field
The present application relates to a terahertz electromagnetic wave generator, a terahertz spectrometer, and a method of generating a terahertz electromagnetic wave.
2. Description of the Related Art
In this specification, the “terahertz electromagnetic wave” will refer herein to an electromagnetic wave, of which the frequency falls within the range of 0.1 THz to 100 THz. 1 THz (terahertz) is 1×1012 (=the twelfth power of 10) Hz. Terahertz electromagnetic waves are now used in various fields including security, medical treatments, and nondestructive tests on electronic parts. Since there are excitation, vibration and rotation modes of various electronic materials, organic molecules and gas molecules in the terahertz electromagnetic wave frequency range, people have proposed that a terahertz electromagnetic wave be used as a sort of “fingerprint” to recognize a given material. On top of that, since a terahertz electromagnetic wave is safer than an X ray or any of various other electromagnetic waves, the terahertz electromagnetic wave can be used to make a medical diagnosis without doing harm on the body of a human subject.
As disclosed in Nature Mater. 1, 26, (2002), a photoconductor or a nonlinear optical crystal is used as a conventional terahertz electromagnetic wave generator. In any of those elements, a terahertz electromagnetic wave is generated by irradiating the element with a laser beam, of which the pulse width falls within the range of a few femtoseconds to several hundred femtoseconds (and which will be hereinafter referred to as a “femtosecond laser beam”). 1 femtosecond is 1×10−15 (=the minus fifteenth power of 10) seconds. In a vacuum, an electromagnetic wave travels approximately 300 nm in one femtosecond.
Such a terahertz electromagnetic wave is generated by taking advantage of a so-called “dipole radiation” phenomenon in classical electromagnetism. That is to say, a variation in electric polarization or current in accelerated motion with time generates an electromagnetic wave at a frequency corresponding to the rate of that variation. Since a variation in polarization or current is induced in a few femtoseconds to several hundred femtoseconds (which depends on the pulse width of the laser beam) by being irradiated with a femtosecond laser beam, the electromagnetic wave generated by dipole radiation has a frequency falling within the terahertz range.
SUMMARY
According to a method of generating a terahertz electromagnetic wave using a photoconductor, a bias voltage needs to be applied to the photoconductor. That is why to generate a terahertz electromagnetic wave, not only a femtosecond laser diode but also an external voltage supply are needed. However, to use the terahertz electromagnetic wave technologies in a broader range of fields in practice, a method of generating a terahertz electromagnetic wave without using such an external voltage supply should be provided so that the technologies work in various operating environments.
According to a method of generating a terahertz electromagnetic wave using a nonlinear optical crystal, on the other hand, no external voltage supply is needed. However, since the second-order nonlinear optical effect is used, a femtosecond laser beam needs to be radiated precisely toward a predetermined crystal orientation of a nonlinear optical crystal. In addition, the phase matching condition needs to be satisfied, and therefore, the nonlinear optical crystal should be designed, shaped and controlled precisely enough.
The present disclosure provides a technique for generating a terahertz electromagnetic wave using a simpler configuration.
In one general aspect, a terahertz electromagnetic wave generator disclosed herein includes: a thermoelectric material layer; a metal layer which partially covers the surface of the thermoelectric material layer; and a light source system which is configured to irradiate both a surface region of the thermoelectric material layer that is not covered with the metal layer and an edge of the metal layer with pulsed light, thereby generating a terahertz wave from the thermoelectric material layer.
In another aspect, a terahertz spectrometer disclosed herein includes: the terahertz electromagnetic wave generator described above; an optical system which irradiates an object with a terahertz electromagnetic wave that has been generated by the terahertz electromagnetic wave generator; and a detector which detects the terahertz electromagnetic wave that has been transmitted through, or reflected from, the object.
In another aspect, a method of generating a terahertz electromagnetic wave disclosed herein includes the steps of: (A) providing a thermoelectric material body which is partially covered with a metal layer; and (B) locally heating the thermoelectric material body by irradiating both a surface region of the thermoelectric material body which is not covered with the metal layer and an edge of the metal layer with pulsed light. The step (B) includes the steps of: locally heating the thermoelectric material body so that an asymmetric heat distribution is formed in the thermoelectric material body; and producing thermal diffusion current in the portion of the thermoelectric material body that has been heated locally, thereby generating a terahertz electromagnetic wave.
According to the present disclosure, by irradiating a thermoelectric material layer with a femtosecond laser beam, a macroscopic current can be induced, and a terahertz electromagnetic wave can be generated from this current.
Additional benefits and advantages of the disclosed embodiments will be apparent from the specification and Figures. The benefits and/or advantages may be individually provided by the various embodiments and features of the specification and drawings disclosure, and need not all be provided in order to obtain one or more of the same.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A illustrates how the Seebeck effect is produced.
FIG. 1B is a perspective view illustrating a terahertz electromagnetic wave generator according to the present disclosure.
FIG. 1C is a schematic representation illustrating a terahertz electromagnetic wave generator according to the present disclosure.
FIG. 2A is a cross-sectional view schematically illustrating how the terahertz electromagnetic wave generator 4 operates when a femtosecond laser beam 5 is incident on the generator 4.
FIG. 2B is a top view schematically illustrating how the terahertz electromagnetic wave generator 4 operates when a femtosecond laser beam 5 is incident on the generator 4.
FIG. 2C is a top view schematically illustrating how the terahertz electromagnetic wave generator 4 operates when a center portion of the surface 20 of the thermoelectric material layer is irradiated with the femtosecond laser beam 5.
FIG. 2D illustrates an exemplary configuration for a terahertz spectrometer according to an embodiment of the present disclosure.
FIG. 2E is a flowchart showing the procedure of generating a terahertz electromagnetic wave.
FIG. 2F is a cross-sectional view schematically illustrating a terahertz electromagnetic wave generator 4 with a thermoelectric material layer 2 which has been patterned into a linear shape.
FIG. 2G is a top view schematically illustrating a terahertz electromagnetic wave generator 4 with a thermoelectric material layer 2 which has been patterned into a linear shape.
FIG. 2H is a cross-sectional view schematically illustrating a terahertz electromagnetic wave generator 4 including a thermoelectric material layer 2 which is partially covered with a metal layer 13 with a hole.
FIG. 2I is a top view schematically illustrating a terahertz electromagnetic wave generator 4 including a thermoelectric material layer 2 which is partially covered with a metal layer 13 with a hole.
FIG. 3A is a perspective view illustrating how a terahertz electromagnetic wave generator including Bi and Au layers operates when irradiated with a femtosecond laser beam so that the beam spot will cover both a left edge portion of the Au layer and a portion of the Bi layer.
FIG. 3B is a graph showing the time domain waveform of a terahertz electromagnetic wave that was generated when the terahertz electromagnetic wave generator including Bi and Au layers was irradiated with a femtosecond laser beam so that the beam spot would cover both a left edge portion of the Au layer and a portion of the Bi layer.
FIG. 4 is a graph showing the power spectrum of the terahertz electromagnetic waveform shown in FIG. 3B.
FIG. 5A is a perspective view illustrating how a terahertz electromagnetic wave generator including Bi and Au layers operates when irradiated with a femtosecond laser beam so that the beam spot will cover only the Au layer.
FIG. 5B is a graph showing the time domain waveform of a terahertz electromagnetic wave that was generated when the terahertz electromagnetic wave generator including Bi and Au layers was irradiated with a femtosecond laser beam so that the beam spot would cover only the Au layer.
FIG. 6A is a perspective view illustrating how a terahertz electromagnetic wave generator including Bi and Au layers operates when irradiated with a femtosecond laser beam so that the beam spot will cover only the Bi layer.
FIG. 6B is a graph showing the time domain waveform of a terahertz electromagnetic wave that was generated when the terahertz electromagnetic wave generator including Bi and Au layers was irradiated with a femtosecond laser beam so that the beam spot would cover only the Bi layer.
FIG. 7A is a perspective view illustrating how a terahertz electromagnetic wave generator including Bi2Te3 and Au layers operates when irradiated with a femtosecond laser beam so that the beam spot will cover both a left edge portion of the Au layer and a portion of the Bi2Te3 layer.
FIG. 7B is a graph showing the time domain waveform of a terahertz electromagnetic wave that was generated when the terahertz electromagnetic wave generator including Bi2Te3 and Au layers was irradiated with a femtosecond laser beam so that the beam spot would cover both a left edge portion of the Au layer and a portion of the Bi2Te3 layer.
FIG. 8 is a graph showing the power spectrum of the terahertz electromagnetic waveform shown in FIG. 7B.
FIG. 9A is a perspective view illustrating how a terahertz electromagnetic wave generator including Bi2Te3 and Au layers operates when irradiated with a femtosecond laser beam so that the beam spot will cover only the Au layer.
FIG. 9B is a graph showing the time domain waveform of a terahertz electromagnetic wave that was generated when the terahertz electromagnetic wave generator including Bi2Te3 and Au layers was irradiated with a femtosecond laser beam so that the beam spot would cover only the Au layer.
FIG. 10A is a perspective view illustrating how a terahertz electromagnetic wave generator including Bi2Te3 and Au layers operates when irradiated with a femtosecond laser beam so that the beam spot will cover only the Bi2Te3 layer.
FIG. 10B is a graph showing the time domain waveform of a terahertz electromagnetic wave that was generated when the terahertz electromagnetic wave generator including Bi2Te3 and Au layers was irradiated with a femtosecond laser beam so that the beam spot would cover only the Bi2Te3 layer.
FIG. 11A is a perspective view illustrating how a terahertz electromagnetic wave generator including CaxCoO2 and Au layers operates when irradiated with a femtosecond laser beam so that the beam spot will cover both a left edge portion of the Au layer and a portion of the CaxCoO2 layer.
FIG. 11B is a graph showing the time domain waveform of a terahertz electromagnetic wave that was generated when the terahertz electromagnetic wave generator including CaxCoO2 and Au layers was irradiated with a femtosecond laser beam so that the beam spot would cover both a left edge portion of the Au layer and a portion of the CaxCoO2 layer.
FIG. 12 is a graph showing the power spectrum of the terahertz electromagnetic waveform shown in FIG. 11B.
FIG. 13A is a perspective view illustrating how a terahertz electromagnetic wave generator including CaxCoO2 and Au layers operates when irradiated with a femtosecond laser beam so that the beam spot will cover only the Au layer.
FIG. 13B is a graph showing the time domain waveform of a terahertz electromagnetic wave that was generated when the terahertz electromagnetic wave generator including CaxCoO2 and Au layers was irradiated with a femtosecond laser beam so that the beam spot would cover only the Au layer.
FIG. 14A is a perspective view illustrating how a terahertz electromagnetic wave generator including CaxCoO2 and Au layers operates when irradiated with a femtosecond laser beam so that the beam spot will cover only the CaxCoO2 layer.
FIG. 14B is a graph showing the time domain waveform of a terahertz electromagnetic wave that was generated when the terahertz electromagnetic wave generator including CaxCoO2 and Au layers was irradiated with a femtosecond laser beam so that the beam spot would cover only the CaxCoO2 layer.
FIG. 15A is a perspective view illustrating how a terahertz electromagnetic wave generator including Bi and Al layers operates when irradiated with a femtosecond laser beam so that the beam spot will cover both a left edge portion of the Al layer and a portion of the Bi layer.
FIG. 15B is a graph showing the time domain waveform of a terahertz electromagnetic wave that was generated when the terahertz electromagnetic wave generator including Bi and Al layers was irradiated with a femtosecond laser beam so that the beam spot would cover both a left edge portion of the Al layer and a portion of the Bi layer.
FIG. 16 is a graph showing the power spectrum of the terahertz electromagnetic waveform shown in FIG. 15B.
FIG. 17A is a perspective view illustrating how a terahertz electromagnetic wave generator including Bi and Al layers operates when irradiated with a femtosecond laser beam so that the beam spot will cover only the Al layer.
FIG. 17B is a graph showing the time domain waveform of a terahertz electromagnetic wave that was generated when the terahertz electromagnetic wave generator including Bi and Al layers was irradiated with a femtosecond laser beam so that the beam spot would cover only the Al layer.
FIG. 18A is a perspective view illustrating how a terahertz electromagnetic wave generator including Bi2Te3 and Al layers operates when irradiated with a femtosecond laser beam so that the beam spot will cover both a left edge portion of the Al layer and a portion of the Bi2Te3 layer.
FIG. 18B is a graph showing the time domain waveform of a terahertz electromagnetic wave that was generated when the terahertz electromagnetic wave generator including Bi2Te3 and Al layers was irradiated with a femtosecond laser beam so that the beam spot would cover both a left edge portion of the Al layer and a portion of the Bi2Te3 layer.
FIG. 19 is a graph showing the power spectrum of the terahertz electromagnetic waveform shown in FIG. 18B.
FIG. 20A is a perspective view illustrating how a terahertz electromagnetic wave generator including Bi2Te3 and Al layers operates when irradiated with a femtosecond laser beam so that the beam spot will cover only the Al layer.
FIG. 20B is a graph showing the time domain waveform of a terahertz electromagnetic wave that was generated when the terahertz electromagnetic wave generator including Bi2Te3 and Al layers was irradiated with a femtosecond laser beam so that the beam spot would cover only the Al layer.
FIG. 21A is a perspective view illustrating how a terahertz electromagnetic wave generator including CaxCoO2 and Al layers operates when irradiated with a femtosecond laser beam so that the beam spot will cover both a left edge portion of the Al layer and a portion of the CaxCoO2 layer.
FIG. 21B is a graph showing the time domain waveform of a terahertz electromagnetic wave that was generated when the terahertz electromagnetic wave generator including CaxCoO2 and Al layers was irradiated with a femtosecond laser beam so that the beam spot would cover both a left edge portion of the Al layer and a portion of the CaxCoO2 layer.
FIG. 22 is a graph showing the power spectrum of the terahertz electromagnetic waveform shown in FIG. 21B.
FIG. 23A is a perspective view illustrating how a terahertz electromagnetic wave generator including CaxCoO2 and Al layers operates when irradiated with a femtosecond laser beam so that the beam spot will cover only the Al layer.
FIG. 23B is a graph showing the time domain waveform of a terahertz electromagnetic wave that was generated when the terahertz electromagnetic wave generator including CaxCoO2 and Al layers was irradiated with a femtosecond laser beam so that the beam spot would cover only the Al layer.
FIG. 24A is a perspective view illustrating how a terahertz electromagnetic wave generator including Bi and Ti layers operates when irradiated with a femtosecond laser beam so that the beam spot will cover both a left edge portion of the Ti layer and a portion of the Bi layer.
FIG. 24B is a graph showing the time domain waveform of a terahertz electromagnetic wave that was generated when the terahertz electromagnetic wave generator including Bi and Ti layers was irradiated with a femtosecond laser beam so that the beam spot would cover both a left edge portion of the Ti layer and a portion of the Bi layer.
FIG. 25 is a graph showing the power spectrum of the terahertz electromagnetic waveform shown in FIG. 24B.
FIG. 26A is a perspective view illustrating how a terahertz electromagnetic wave generator including Bi and Ti layers operates when irradiated with a femtosecond laser beam so that the beam spot will cover only the Ti layer.
FIG. 26B is a graph showing the time domain waveform of a terahertz electromagnetic wave that was generated when the terahertz electromagnetic wave generator including Bi and Ti layers was irradiated with a femtosecond laser beam so that the beam spot would cover only the Ti layer.
FIG. 27A is a perspective view illustrating how a terahertz electromagnetic wave generator including Bi2Te3 and Ti layers operates when irradiated with a femtosecond laser beam so that the beam spot will cover both a left edge portion of the Ti layer and a portion of the Bi2Te3 layer.
FIG. 27B is a graph showing the time domain waveform of a terahertz electromagnetic wave that was generated when the terahertz electromagnetic wave generator including Bi2Te3 and Ti layers was irradiated with a femtosecond laser beam so that the beam spot would cover both a left edge portion of the Ti layer and a portion of the Bi2Te3 layer.
FIG. 28 is a graph showing the power spectrum of the terahertz electromagnetic waveform shown in FIG. 27B.
FIG. 29A is a perspective view illustrating how a terahertz electromagnetic wave generator including Bi2Te3 and Ti layers operates when irradiated with a femtosecond laser beam so that the beam spot will cover only the Ti layer.
FIG. 29B is a graph showing the time domain waveform of a terahertz electromagnetic wave that was generated when the terahertz electromagnetic wave generator including Bi2Te3 and Ti layers was irradiated with a femtosecond laser beam so that the beam spot would cover only the Ti layer.
FIG. 30A is a perspective view illustrating how a terahertz electromagnetic wave generator including CaxCoO2 and Ti layers operates when irradiated with a femtosecond laser beam so that the beam spot will cover both a left edge portion of the Ti layer and a portion of the CaxCoO2 layer.
FIG. 30B is a graph showing the time domain waveform of a terahertz electromagnetic wave that was generated when the terahertz electromagnetic wave generator including CaxCoO2 and Ti layers was irradiated with a femtosecond laser beam so that the beam spot would cover both a left edge portion of the Ti layer and a portion of the CaxCoO2 layer.
FIG. 31 is a graph showing the power spectrum of the terahertz electromagnetic waveform shown in FIG. 30B.
FIG. 32A is a perspective view illustrating how a terahertz electromagnetic wave generator including CaxCoO2 and Ti layers operates when irradiated with a femtosecond laser beam so that the beam spot will cover only the Ti layer.
FIG. 32B is a graph showing the time domain waveform of a terahertz electromagnetic wave that was generated when the terahertz electromagnetic wave generator including CaxCoO2 and Ti layers was irradiated with a femtosecond laser beam so that the beam spot would cover only the Ti layer.



DETAILED DESCRIPTION
A terahertz electromagnetic wave generator according to the present disclosure uses the Seebeck effect to be expressed by a thermoelectric material. The Seebeck effect is a phenomenon that a difference in temperature in an object is directly transformed into a voltage and is a kind of a thermoelectric effect. FIG. 1A schematically illustrates how the Seebeck effect is produced. In FIG. 1A, illustrated are an n-type thermoelectric material and a p-type thermoelectric material. In each of these materials, the temperature is higher at its left end than at its right end. In this case, in the n-type thermoelectric material, electrons that are majority carriers move (i.e., diffuse thermally) from the left at a relatively high temperature to the right at a relatively low temperature, thus generating a voltage. On the other hand, in the p-type thermoelectric material, holes that are majority carriers move (i.e., diffuse thermally) from the left at a relatively high temperature to the right at a relatively low temperature, thus generating a voltage. In both of these two materials, the majority carriers move in the same direction from a portion at the higher temperature toward a portion at the lower temperature. However, the polarity of the majority carriers of the n-type thermoelectric material is opposite from that of the majority carriers of the p-type thermoelectric material, and therefore, currents flow through the materials in mutually opposite directions.
In general, a thermoelectric material is a material which generates a voltage and current by producing a temperature gradient within the substance. According to the present disclosure, a temperature gradient is introduced into a thermoelectric material with a femtosecond laser beam, thereby producing current through the thermal diffusion. And by using that current, a terahertz electromagnetic wave is generated by dipole radiation. Nevertheless, if current has been induced symmetrically in a space, it can be said that no current has been produced macroscopically, and therefore, no terahertz electromagnetic wave is generated, either. On the other hand, if asymmetric current can be induced, then the current will flow in one direction macroscopically, and a terahertz electromagnetic wave can be generated by dipole radiation.
A terahertz electromagnetic wave generator according to the present disclosure includes: a thermoelectric material layer; a metal layer which partially covers the surface of the thermoelectric material layer; and a light source system which is configured to irradiate both a surface region of the thermoelectric material layer that is not covered with the metal layer and an edge of the metal layer with pulsed light, thereby generating a terahertz wave from the thermoelectric material layer.
Embodiments of the present disclosure will now be described.
EMBODIMENTS
FIG. 1B is a perspective view illustrating a terahertz electromagnetic wave generator 4 as an embodiment of the present disclosure. As shown in FIG. 1B, the terahertz electromagnetic wave generator 4 for use in this embodiment includes a substrate 1, a thermoelectric material layer 2 which is supported by the substrate 1, and a metal layer 13 which covers the surface of the thermoelectric material layer 2 partially. For your reference, XYZ coordinates represented by X, Y and Z axes that intersect with each other at right angles are shown in FIG. 1B. The substrate 1 illustrated in FIG. 1B has a flat plate shape. The principal surface of the substrate 1 is parallel to an XY plane and intersects with the Z axis at right angles.
FIG. 1C is a perspective view schematically illustrating an exemplary configuration for a terahertz electromagnetic wave generator according to an embodiment of the present disclosure. As shown in FIG. 1C, the terahertz electromagnetic wave generator of this embodiment includes a femtosecond laser light source 3 and a terahertz electromagnetic wave generator 4, which includes the thermoelectric material layer 2 supported by the substrate 1 and the metal layer 13 stacked on the thermoelectric material layer 2 as described above. The femtosecond laser light source 3 of this embodiment irradiates an edge of the metal layer 13 of the terahertz electromagnetic wave generator 4 and a surface region of the thermoelectric material layer 2 which is not covered with the metal layer 13 with a pulsed femtosecond laser beam 5 simultaneously. The femtosecond laser beam may have a pulse width of 1 femtosecond to 1 nanosecond, and typically has a pulse width falling within the range of 10 femtoseconds to 100 femtoseconds. Such a pulsed femtosecond laser beam can be radiated 1 to 108 times per second. In the thermoelectric material layer 2 irradiated with such a pulsed femtosecond laser beam, the temperature rises in a short time (which is approximately as long as a laser beam radiation time). As a result, in the thermoelectric material layer 2, carriers diffuse toward the non-heated peripheral area of the layer 2 due to thermal diffusion. In the meantime, current flows from the edge of the metal layer 13 toward the heated area of the thermoelectric material layer 2 so as to compensate for the loss of those carriers. Consequently, a terahertz electromagnetic wave 6 is generated from such a current that flows in one direction from the edge of the metal layer 13.
FIG. 2A is a cross-sectional view schematically illustrating how the terahertz electromagnetic wave generator 4 operates when a femtosecond laser beam 5 is incident on the generator 4, and FIG. 2B is a schematic top view of the generator 4. In the terahertz electromagnetic wave generator 4 shown in FIGS. 2A and 2B, the surface of the thermoelectric material layer 2 supported by the substrate 1 is irradiated with a femtosecond laser beam 5 at the boundary between its area covered with the metal layer 13 and its area not covered with the metal layer 13. More specifically, the radiation position of the femtosecond laser beam 5 is adjusted so that a spot of the femtosecond laser beam 5 crosses an edge 20e of the metal layer 13. In this manner, part of the femtosecond laser beam 5 irradiates the thermoelectric material layer 2 and the rest of the femtosecond laser beam 5 irradiates the edge of the metal layer 13. In the thermoelectric material layer 2, the temperature rises steeply in a short time in its portion irradiated with the femtosecond laser beam 5, and a temperature gradient is produced generally in the direction indicated by the open arrow 30 in FIG. 2A. The temperature gradient has not only a component which is parallel to the X-axis direction shown in FIG. 2B but also a component in the Y-axis direction as well. However, in the temperature gradient, components in the +Y-axis and −Y-axis directions may be symmetric to each other, but components in the +X-axis and −X-axis directions are not symmetric to each other.
FIG. 2C is a top view schematically illustrating how the terahertz electromagnetic wave generator 4 operates when a center portion of the surface 20 of the thermoelectric material layer 2 is irradiated with the femtosecond laser beam 5. In this case, the thermoelectric material layer 2 is irradiated with the femtosecond laser beam 5 fully. Consequently, as schematically indicated by the arrows 30 in FIG. 2C, the temperature gradient produced in this case is isotropic within the XY plane.
The present inventors discovered via experiments that when the temperature gradient was isotropic within the XY plane, no terahertz electromagnetic wave was generated (as will be described later). In an embodiment of the present disclosure, on the other hand, a terahertz electromagnetic wave is generated. This is probably because thermally diffused current is produced asymmetrically within the XY plane as shown in FIG. 28, for example.
According to this embodiment, if the spot radius of the femtosecond laser beam 5 is r, the radiation position of the femtosecond laser beam is adjusted so that the edge of the metal layer 13 is located within the distance r from the center of the laser beam, and a portion of the surface of the thermoelectric material layer 2 is selectively heated by being irradiated with a pulsed laser beam. The spot radius r of the femtosecond laser beam 5 is defined to be the radius of an area where the beam intensity becomes equal to or greater than 1/e of the beam center intensity, where e is the base of natural logarithms and is represented by an approximate value of 2.7.
Also, the femtosecond laser beam raises the temperature of the thermoelectric material layer 2 with pulses, and therefore, the wavelength of the laser beam is set to be a value falling within the range in which the laser beam is absorbed into the thermoelectric material layer 2. The wavelength range in which the laser beam is absorbed into the thermoelectric material layer 2 may vary according to the type of the thermoelectric material that forms the thermoelectric material layer 2.
In the terahertz electromagnetic wave generator of the present disclosure, the thermally diffused current produced by the Seebeck effect becomes the source of a terahertz electromagnetic wave, and therefore, it does not matter whether the material of the thermoelectric material layer 2 is an n-type material or a p-type one. The thermoelectric material layer 2 may be made of a material with a large Seebeck coefficient and a high degree of electrical conductivity. Examples of thermoelectric materials which may be used to make the thermoelectric material layer 2 include single-element thermoelectric materials such as Bi and Sb, alloy-based thermoelectric materials such as BiTe, PbTe and SiGe based materials, and oxide-based thermoelectric materials such as CaxCoO2, NaxCoO2, and SrTiO3. In this description, the “thermoelectric material” refers herein to a material with a Seebeck coefficient, of which the absolute value is equal to or greater than 30 μV/K, and an electrical resistivity of 10 m Ωcm or less. Such a thermoelectric material may be either crystalline or amorphous.
The substrate 1 of this embodiment is made of a material which can transmit the terahertz electromagnetic wave generated and may be made of a dielectric material, for example. Examples of dielectric materials which may be used to make the substrate 1 include SiO2, Al2O3, MgO, Si and LSAT. Not the entire substrate 1 has to be made of the same material, and the substrate 1 does not have to have a uniform thickness, either. The principal surface of the substrate 1 is typically flat but may have some unevenness, too. The function to be performed by the substrate 1 is to support the thermoelectric material layer 2. As long as the substrate 1 can perform this function, the substrate 1 may have any of various forms.
In the terahertz electromagnetic wave generator 4 of the present disclosure, the thermoelectric material layer 2 may have a thickness at which 50% or more of the terahertz electromagnetic wave generated can be transmitted. In the area where the terahertz electromagnetic wave is generated, the thickness of the thermoelectric material layer 2 may be set to fall within the range of 10 nm to 1000 nm (=1 μm), for example. The thermoelectric material layer 2 may be formed by any method, which may be a sputtering process, an evaporation process, a laser ablation process, a vapor deposition process such as chemical vapor deposition, a liquid phase deposition process, or any of various other methods. The thermoelectric material layer 2 does not have to be deposited directly on the principal surface of the substrate 1. Alternatively, the thermoelectric material layer 2 may be deposited on another substrate and then transferred onto the principal surface of the substrate 1.
The metal layer 13 to be stacked on the thermoelectric material layer 13 may be, but does not have to be, made of a material with a high degree of electric conductivity such as Au, Al or Ti. The thickness of the metal layer 13 may be set to fall within the range of 10 nm to 100 μm, for example.
FIG. 2D illustrates an exemplary configuration for a terahertz spectrometer according to an embodiment of the present disclosure. This spectrometer includes a light source system 100 which emits the femtosecond laser beam 5, the terahertz electromagnetic wave generator 4 of the present disclosure, an optical system (including mirrors 200a and 200b) which irradiates an object (sample 300) with the terahertz electromagnetic wave 6 generated by the terahertz electromagnetic wave generator 4, and a detector 400 which detects the terahertz electromagnetic wave 16 that has been transmitted through the sample 300. Optionally, this terahertz spectrometer may further include a processing apparatus 500 which generates an image representing a terahertz electromagnetic wave with a particular wavelength based on the output of the detector 400.
As shown in FIG. 2E, a method of generating a terahertz electromagnetic wave according to the present disclosure includes the step S100 of providing a thermoelectric material body, of which the surface is partially covered with a metal layer 13 and the step S200 of irradiating an edge of the metal layer of the thermoelectric material body with pulsed light (e.g., the femtosecond laser beam) so that the beam spot crosses that edge. This step S200 includes the step S210 of locally heating the thermoelectric material body so that an asymmetric heat distribution is formed in the thermoelectric material body, and the step S220 of producing thermal diffusion current in the portion of the thermoelectric material body that has been heated locally, thereby generating a terahertz electromagnetic wave.
The thermoelectric material layer 2 does not have to have the shape shown in FIGS. 2A and 2B. Alternatively, the thermoelectric material layer 2 may also be patterned by a known method to have any other shape. For example, the thermoelectric material layer 2 may have a linear pattern, a bent curved pattern or a curvilinear pattern or may also have a single or a plurality of holes. The thermoelectric material layer 2 may be divided into a plurality of portions on the single substrate 1 or may cover the principal surface of the substrate 1 entirely. Optionally, the thermoelectric material layer 2 may be partially extended out of the principal surface of the substrate 1. The thermoelectric material layer 2 may also be a nanowire layer. The surface of the thermoelectric material layer 2 does not have to be flat and its thickness does not have to be uniform within the plane, either.
FIG. 2F is a cross-sectional view schematically illustrating a terahertz electromagnetic wave generator 4 with a thermoelectric material layer 2 which has been patterned into a linear shape, and FIG. 2G is a schematic top view thereof. In the terahertz electromagnetic wave generator 4 illustrated in FIGS. 2F and 2G, the surface of the thermoelectric material layer 2 that is supported on the substrate 1 is also irradiated with a femtosecond laser beam 5 so that the beam spot crosses an edge 20e of the metal layer 13. In the example illustrated in FIGS. 2F and 2G, one of the three linear portions into which the thermoelectric material layer 2 has been patterned is irradiated with the femtosecond laser beam 5.
FIG. 2H is a cross-sectional view schematically illustrating a terahertz electromagnetic wave generator 4 including a thermoelectric material layer 2, of which the surface is partially covered with a metal layer 13 with a hole, and FIG. 2I is a schematic top view thereof. In the terahertz electromagnetic wave generator 4 illustrated in FIGS. 2H and 2I, a surface region of the thermoelectric material layer 2 which is not covered with the metal layer 13 is also irradiated with a femtosecond laser beam 5 so that the beam spot crosses an edge of the metal layer 13.
The following are some specific examples of the present disclosure.
Example 1
An element which used n-type Bi as a thermoelectric material and SiO2 as a substrate material, respectively, was fabricated by the following method. A Bi layer was deposited by evaporation process to a thickness of 50 nm on an SiO2 substrate (10 mm×10 mm×0.5 mm). The evaporation process was carried out without heating the SiO2 substrate after the film deposition chamber had been evacuated to a pressure of 1.0×10−3 Pa or less. Bi had a Seebeck coefficient of −75 pV/K and an electrical resistivity of 0.1 mQcm. Next, an Au layer was selectively deposited by evaporation process to a thickness of 150 nm on the surface of the Bi layer. The Au layer had an area of approximately 1 mm×1 mm. In this manner, a terahertz electromagnetic wave generator including a Bi layer and an Au layer was fabricated.
As the femtosecond laser light source, a Ti: Sapphire laser diode with a wavelength of 800 nm, a pulse width of 100 fs and a pulse rate of 80 MHz was used.
The terahertz electromagnetic wave generator thus fabricated was irradiated with a femtosecond laser beam that had been condensed to 100 μm (=2r) so that the beam spot would cover both a left edge portion of the Au layer and a portion of the Bi layer. The time domain waveform of the electromagnetic wave measured in such a situation is shown in FIG. 3B. On the other hand, the relative positions of the Bi and Au layers and the laser beam are generally illustrated in FIG. 3A. As can be seen from the time domain waveform of the electromagnetic wave, a pulse wave with a positive peak intensity was generated at around 20 ps (picoseconds). FIG. 4 shows a power spectrum obtained by subjecting this time domain waveform to a Fourier transform. The electromagnetic wave generated had a frequency range of about 0.1 THz to about 1 THz. Thus, it was confirmed that a terahertz electromagnetic wave had been actually generated.
Next, the terahertz electromagnetic wave generator was irradiated with a femtosecond laser beam that had been condensed to 100 μm (=2r) so that the beam spot would cover only the Au layer. The time domain waveform of the electromagnetic wave measured in such a situation is shown in FIG. 5B. On the other hand, the relative positions of the Bi and Au layers and the laser beam are generally illustrated in FIG. 5A. As can be seen from the time domain waveform, there was nothing but noise with no definite peaks observed, and no terahertz electromagnetic wave had been generated.
Next, the terahertz electromagnetic wave generator was irradiated with a femtosecond laser beam that had been condensed to 100 μm (=2r) so that the beam spot would cover only the Bi layer. The time domain waveform of the electromagnetic wave measured in such a situation is shown in FIG. 6B. On the other hand, the relative positions of the Bi and Au layers and the laser beam are generally illustrated in FIG. 6A. As can be seen from the time domain waveform, there was nothing but noise with no definite peaks observed, and no terahertz electromagnetic wave had been generated.
Thus, the present inventors discovered that in the method of generating a terahertz electromagnetic wave according to the present disclosure, the element should be irradiated with a femtosecond laser beam so that the beam spot covers both an edge of the metal layer and a portion of the thermoelectric material layer.
Example 2 
An element which used p-type Bi2Te3 as a thermoelectric material and MgO as a substrate material, respectively, was fabricated by the following method.
A Bi2Te3 layer was deposited by evaporation process to a thickness of 50 nm on an MgO substrate (10 mm×10 mm×0.5 mm). The evaporation process was carried out without heating the MgO substrate after the film deposition chamber had been evacuated to a pressure of 1.0×10−3 Pa or less. Bi2Te3 had a Seebeck coefficient of +210 pV/K and an electrical resistivity of 1 m Ωcm. Next, an Au layer was selectively deposited by evaporation process to a thickness of 150 nm on the surface of the Bi2Te3 layer. The Au layer had an area of approximately 1 mm×1 mm. In this manner, a terahertz electromagnetic wave generator including a Bi2Te3 layer and an Au layer was fabricated.
As a femtosecond laser diode, a Ti: Sapphire laser diode with a wavelength of 800 nm, a pulse width of 100 fs and a pulse rate of 80 MHz was used.
The terahertz electromagnetic wave generator thus fabricated was irradiated with a femtosecond laser beam that had been condensed to 100 μm (=2r) so that the beam spot would cover both a left edge portion of the Au layer and a portion of the Bi2Te3 layer. The time domain waveform of the electromagnetic wave measured in such a situation is shown in FIG. 7B. On the other hand, the relative positions of the Bi2Te3 and Au layers and the laser beam are generally illustrated in FIG. 7A. As can be seen from the time domain waveform of the electromagnetic wave, a pulse wave with a negative peak intensity was generated at around 20 ps. FIG. 8 shows a power spectrum obtained by subjecting this time domain waveform to a Fourier transform. The electromagnetic wave generated had a frequency range of about 0.1 THz to about 1 THz. Thus, it was confirmed that a terahertz electromagnetic wave had been actually generated.
Next, the terahertz electromagnetic wave generator was irradiated with a femtosecond laser beam that had been condensed to 100 μm (=2r) so that the beam spot would cover only the Au layer. The time domain waveform of the electromagnetic wave measured in such a situation is shown in FIG. 9B. On the other hand, the relative positions of the Bi2Te3 and Au layers and the laser beam are generally illustrated in FIG. 9A. As can be seen from the time domain waveform, there was nothing but noise with no definite peaks observed, and no terahertz electromagnetic wave had been generated.
Next, the terahertz electromagnetic wave generator was irradiated with a femtosecond laser beam that had been condensed to 100 μm (=2r) so that the beam spot would cover only the Bi2Te3 layer. The time domain waveform of the electromagnetic wave measured in such a situation is shown in FIG. 10B. On the other hand, the relative positions of the Bi2Te3 and Au layers and the laser beam are generally illustrated in FIG. 10A. As can be seen from the time domain waveform, there was nothing but noise with no definite peaks observed, and no terahertz electromagnetic wave had been generated.
Comparing the results obtained in these Examples 1 and 2 to each other, it can be seen that although the element was irradiated with a laser beam with the same experimental arrangement adopted in both of those examples, the polarity at the peak of the terahertz electromagnetic wave (i.e., whether the peak is positive or negative) inverted depending on whether the thermoelectric material used was n-type Bi or p-type Bi2Te3. These results reveal that the phase of the terahertz electromagnetic wave generated by the terahertz electromagnetic wave generator of the present disclosure reflects the type of the carriers (i.e., whether they are electrons or holes) in the thermoelectric material layer.
The terahertz electromagnetic wave radiation properties of the Bi and Bi2Te3 layer based elements that have been described for Examples 1 and 2 exhibited no dependence on the polarization of the femtosecond laser beam. This indicates that the terahertz electromagnetic wave generating mechanism is not a secondary nonlinear effect. As for semiconductors and insulators, on the other hand, a so-called “photo-Dember effect” that is a terahertz electromagnetic wave generating mechanism which has something to do with diffusion of photo-excited carriers has also been reported. However, the polarity of a terahertz electromagnetic wave generated under the photo-Dember effect does not depend on the type of the majority carriers (i.e., whether the majority carriers are electrons or holes) (see Physical Review B73, 155330, (2006)), which is different from the terahertz electromagnetic wave radiation properties of Examples 1 and 2. Consequently, the terahertz electromagnetic wave generating mechanism of the present disclosure does not result from the photo-Dember effect.
As can be seen from the foregoing description, a method of generating a terahertz electromagnetic wave according to the present disclosure is based on a novel mechanism, and the present disclosure provides a simple terahertz electromagnetic wave source which needs no external voltage supply.
Example 3
A terahertz electromagnetic wave generator according to the present disclosure was fabricated with p-type CaxCoO2 used as a thermoelectric material and Au used as a metal layer.
A CaxCoO2 layer was deposited to a thickness of 150 nm on an Al2O3 substrate (10 mm×10 mm×0.5 mm). The CaxCoO2 layer was formed by radio frequency (RF) magnetron sputtering process. As a sputter target, a target in which Ca and Co had been mixed together to have a molar ratio of one to one was used. After the film deposition chamber had been evacuated to a pressure of 1.0×10−3 Pa or less, a mixed gas of argon (96%) and oxygen (4%) was introduced thereto to fix the gas pressure inside the chamber at 5 Pa. In such a state, the Al2O3 substrate was heated to 450° C. with a resistance heating heater and then a CaxCoO2 layer was deposited by sputtering on the Al2O3 substrate at an RF power of 100 W. After the layer had been deposited, the layer was cooled to room temperature in 60 minutes with the argon-oxygen mixed gas still introduced at 5 Pa. The cation composition ratio of the CaxCoO2 layer thus formed was estimated with an energy dispersive X-ray spectrometer to be one to two (which corresponds to x=0.5). CaxCoO2 had a Seebeck coefficient of +60 pV/K and an electrical resistivity of 0.1 mQcm. Next, an Au layer was selectively deposited by evaporation process to a thickness of 150 nm on the surface of the CaxCoO2 layer. The Au layer had an area of approximately 1 mm×1 mm. In this manner, a terahertz electromagnetic wave generator including a CaxCoO2 layer and an Au layer was fabricated.
As a femtosecond laser diode, a Ti: Sapphire laser diode with a wavelength of 800 nm, a pulse width of 100 fs and a pulse rate of 80 MHz was used.
The terahertz electromagnetic wave generator thus fabricated was irradiated with a femtosecond laser beam that had been condensed to 100 μm (=2r) so that the beam spot would cover both a left edge portion of the Au layer and a portion of the CaxCoO2 layer. The time domain waveform of the electromagnetic wave measured in such a situation is shown in FIG. 11B. On the other hand, the relative positions of the CaxCoO2 and Au layers and the laser beam are generally illustrated in FIG. 11A. As can be seen from the time domain waveform of the electromagnetic wave, a pulse wave with a negative peak intensity was generated at around 20 ps. FIG. 12 shows a power spectrum obtained by subjecting this time domain waveform to a Fourier transform. The electromagnetic wave generated had a frequency range of about 0.1 THz to about 1 THz. Thus, it was confirmed that a terahertz electromagnetic wave had been actually generated.
Next, the terahertz electromagnetic wave generator was irradiated with a femtosecond laser beam that had been condensed to 100 μm (=2r) so that the beam spot would cover only the Au layer. The time domain waveform of the electromagnetic wave measured in such a situation is shown in FIG. 13B. On the other hand, the relative positions of the CaxCoO2 and Au layers and the laser beam are generally illustrated in FIG. 13A. As can be seen from the time domain waveform, there was nothing but noise with no definite peaks observed, and no terahertz electromagnetic wave had been generated.
Next, the terahertz electromagnetic wave generator was irradiated with a femtosecond laser beam that had been condensed to 100 μm (=2r) so that the beam spot would cover only the CaxCoO2 layer. The time domain waveform of the electromagnetic wave measured in such a situation is shown in FIG. 14B. On the other hand, the relative positions of the CaxCoO2 and Au layers and the laser beam are generally illustrated in FIG. 14A. As can be seen from the time domain waveform, there was nothing but noise with no definite peaks observed, and no terahertz electromagnetic wave had been generated.
Comparing the result obtained in this Example 3 to the ones obtained in Examples 1 and 2, it can be seen that although the element was irradiated with a laser beam with the same experimental arrangement adopted in all of these examples, the polarity at the peak of the terahertz electromagnetic wave (i.e., whether the peak is positive or negative) inverted depending on whether the thermoelectric material used was n-type Bi or CaxCoO2 but the polarity did not invert no matter whether the thermoelectric material used was p-type Bi2Te3 or CaxCoO2. These results reveal that the phase of the terahertz electromagnetic wave generated by the terahertz electromagnetic wave generator of the present disclosure reflects the type of the carriers (i.e., whether they are electrons or holes) in the thermoelectric material layer.
Example 4
A Bi layer was deposited in the same way as in Example 1 to a thickness of 50 nm on an SiO2 substrate (10 mm×10 mm×0.5 mm). Next, an Al layer was selectively deposited by evaporation process to a thickness of 150 nm on the surface of the Bi layer. The Al layer had an area of approximately 1 mm×1 mm. In this manner, a terahertz electromagnetic wave generator including a Bi layer and an Al layer was fabricated.
As a femtosecond laser diode, a Ti: Sapphire laser diode with a wavelength of 800 nm, a pulse width of 100 fs and a pulse rate of 80 MHz was used.
The terahertz electromagnetic wave generator thus fabricated was irradiated with a femtosecond laser beam that had been condensed to 100 μm (=2r) so that the beam spot would cover both a left edge portion of the Al layer and a portion of the Bi layer. The time domain waveform of the electromagnetic wave measured in such a situation is shown in FIG. 15B. On the other hand, the relative positions of the Bi and Al layers and the laser beam are generally illustrated in FIG. 15A. As can be seen from the time domain waveform of the electromagnetic wave, a pulse wave with a positive peak intensity was generated at around 20 ps. FIG. 16 shows a power spectrum obtained by subjecting this time domain waveform to a Fourier transform. The electromagnetic wave generated had a frequency range of about 0.1 THz to about 1 THz. Thus, it was confirmed that a terahertz electromagnetic wave had been actually generated.
Next, the terahertz electromagnetic wave generator was irradiated with a femtosecond laser beam that had been condensed to 100 μm (=2r) so that the beam spot would cover only the Al layer. The time domain waveform of the electromagnetic wave measured in such a situation is shown in FIG. 17B. On the other hand, the relative positions of the Bi and Al layers and the laser beam are generally illustrated in FIG. 17A. As can be seen from the time domain waveform, there was nothing but noise with no definite peaks observed, and no terahertz electromagnetic wave had been generated.
Example 5 
A Bi2Te3 layer was deposited in the same way as in Example 2 to a thickness of 50 nm on an MgO substrate (10 mm×10 mm×0.5 mm). Next, an Al layer was selectively deposited by evaporation process to a thickness of 150 nm on the surface of the Bi2Te3 layer. The Al layer had an area of approximately 1 mm×1 mm. In this manner, a terahertz electromagnetic wave generator including a Bi2Te3 layer and an Al layer was fabricated.
As a femtosecond laser diode, a Ti: Sapphire laser diode with a wavelength of 800 nm, a pulse width of 100 fs and a pulse rate of 80 MHz was used.
The terahertz electromagnetic wave generator thus fabricated was irradiated with a femtosecond laser beam that had been condensed to 100 μm (=2r) so that the beam spot would cover both a left edge portion of the Al layer and a portion of the Bi2Te3 layer. The time domain waveform of the electromagnetic wave measured in such a situation is shown in FIG. 18B. On the other hand, the relative positions of the Bi2Te3 and Al layers and the laser beam are generally illustrated in FIG. 18A. As can be seen from the time domain waveform of the electromagnetic wave, a pulse wave with a negative peak intensity was generated at around 20 ps. FIG. 19 shows a power spectrum obtained by subjecting this time domain waveform to a Fourier transform. The electromagnetic wave generated had a frequency range of about 0.1 THz to about 1 THz. Thus, it was confirmed that a terahertz electromagnetic wave had been actually generated.
Next, the terahertz electromagnetic wave generator was irradiated with a femtosecond laser beam that had been condensed to 100 μm (=2r) so that the beam spot would cover only the Al layer. The time domain waveform of the electromagnetic wave measured in such a situation is shown in FIG. 20B. On the other hand, the relative positions of the Bi2Te3 and Al layers and the laser beam are generally illustrated in FIG. 20A. As can be seen from the time domain waveform, there was nothing but noise with no definite peaks observed, and no terahertz electromagnetic wave had been generated.
Example 6 
A CaxCoO2 layer was deposited in the same way as in Example 3 to a thickness of 150 nm on an Al2O3 substrate (10 mm×10 mm×0.5 mm). Next, an Al layer was selectively deposited by evaporation process to a thickness of 150 nm on the surface of the CaxCoO2 layer. The Al layer had an area of approximately 1 mm×1 mm. In this manner, a terahertz electromagnetic wave generator including a CaxCoO2 layer and an Al layer was fabricated.
As a femtosecond laser diode, a Ti: Sapphire laser diode with a wavelength of 800 nm, a pulse width of 100 fs and a pulse rate of 80 MHz was used.
The terahertz electromagnetic wave generator thus fabricated was irradiated with a femtosecond laser beam that had been condensed to 100 μm (=2r) so that the beam spot would cover both a left edge portion of the Al layer and a portion of the CaxCoO2 layer. The time domain waveform of the electromagnetic wave measured in such a situation is shown in FIG. 21B. On the other hand, the relative positions of the CaxCoO2 and Al layers and the laser beam are generally illustrated in FIG. 21A. As can be seen from the time domain waveform of the electromagnetic wave, a pulse wave with a negative peak intensity was generated at around 20 ps. FIG. 22 shows a power spectrum obtained by subjecting this time domain waveform to a Fourier transform. The electromagnetic wave generated had a frequency range of about 0.1 THz to about 1 THz. Thus, it was confirmed that a terahertz electromagnetic wave had been actually generated.
Next, the terahertz electromagnetic wave generator was irradiated with a femtosecond laser beam that had been condensed to 100 μm (=2r) so that the beam spot would cover only the Al layer. The time domain waveform of the electromagnetic wave measured in such a situation is shown in FIG. 23B. On the other hand, the relative positions of the CaxCoO2 and Al layers and the laser beam are generally illustrated in FIG. 23A. As can be seen from the time domain waveform, there was nothing but noise with no definite peaks observed, and no terahertz electromagnetic wave had been generated.
As can be seen from the results of Examples 4, 5 and 6, even if the metal layer is replaced with an Al layer, a terahertz electromagnetic wave can also be generated in the same mechanism.
Example 7
A Bi layer was deposited in the same way as in Example 1 to a thickness of 50 nm on an SiO2 substrate (10 mm×10 mm×0.5 mm). Next, a Ti layer was selectively deposited by evaporation process to a thickness of 150 nm on the surface of the Bi layer. The Ti layer had an area of approximately 1 mm×1 mm. In this manner, a terahertz electromagnetic wave generator including a Bi layer and a Ti layer was fabricated.
As a femtosecond laser diode, a Ti: Sapphire laser diode with a wavelength of 800 nm, a pulse width of 100 fs and a pulse rate of 80 MHz was used.
The terahertz electromagnetic wave generator thus fabricated was irradiated with a femtosecond laser beam that had been condensed to 100 μm (=2r) so that the beam spot would cover both a left edge portion of the Ti layer and a portion of the Bi layer. The time domain waveform of the electromagnetic wave measured in such a situation is shown in FIG. 24B. On the other hand, the relative positions of the Bi and Ti layers and the laser beam are generally illustrated in FIG. 24A. As can be seen from the time domain waveform of the electromagnetic wave, a pulse wave with a positive peak intensity was generated at around 20 ps. FIG. 25 shows a power spectrum obtained by subjecting this time domain waveform to a Fourier transform. The electromagnetic wave generated had a frequency range of about 0.1 THz to about 1 THz. Thus, it was confirmed that a terahertz electromagnetic wave had been actually generated.
Next, the terahertz electromagnetic wave generator was irradiated with a femtosecond laser beam that had been condensed to 100 μm (=2r) so that the beam spot would cover only the Ti layer. The time domain waveform of the electromagnetic wave measured in such a situation is shown in FIG. 26B. On the other hand, the relative positions of the Bi and Ti layers and the laser beam are generally illustrated in FIG. 26A. As can be seen from the time domain waveform, there was nothing but noise with no definite peaks observed, and no terahertz electromagnetic wave had been generated.
Example 8
A Bi2Te3 layer was deposited in the same way as in Example 2 to a thickness of 50 nm on an MgO substrate (10 mm×10 mm×0.5 mm). Next, a Ti layer was selectively deposited by evaporation process to a thickness of 150 nm on the surface of the Bi2Te3 layer. The Ti layer had an area of approximately 1 mm×1 mm. In this manner, a terahertz electromagnetic wave generator including a Bi2Te3 layer and a Ti layer was fabricated.
As a femtosecond laser diode, a Ti: Sapphire laser diode with a wavelength of 800 nm, a pulse width of 100 fs and a pulse rate of 80 MHz was used.
The terahertz electromagnetic wave generator thus fabricated was irradiated with a femtosecond laser beam that had been condensed to 100 μm (=2r) so that the beam spot would cover both a left edge portion of the Ti layer and a portion of the Bi2Te3 layer. The time domain waveform of the electromagnetic wave measured in such a situation is shown in FIG. 27B. On the other hand, the relative positions of the Bi2Te3 and Ti layers and the laser beam are generally illustrated in FIG. 27A. As can be seen from the time domain waveform of the electromagnetic wave, a pulse wave with a negative peak intensity was generated at around 20 ps. FIG. 28 shows a power spectrum obtained by subjecting this time domain waveform to a Fourier transform. The electromagnetic wave generated had a frequency range of about 0.1 THz to about 1 THz. Thus, it was confirmed that a terahertz electromagnetic wave had been actually generated.
Next, the terahertz electromagnetic wave generator was irradiated with a femtosecond laser beam that had been condensed to 100 μm (=2r) so that the beam spot would cover only the Ti layer. The time domain waveform of the electromagnetic wave measured in such a situation is shown in FIG. 29B. On the other hand, the relative positions of the Bi2Te3 and Ti layers and the laser beam are generally illustrated in FIG. 29A. As can be seen from the time domain waveform, there was nothing but noise with no definite peaks observed, and no terahertz electromagnetic wave had been generated.
Example 9
A CaxCoO2 layer was deposited in the same way as in Example 3 to a thickness of 150 nm on an Al2O3 substrate (10 mm×10 mm×0.5 mm). Next, a Ti layer was selectively deposited by evaporation process to a thickness of 150 nm on the surface of the CaxCoO2 layer. The Ti layer had an area of approximately 1 mm×1 mm. In this manner, a terahertz electromagnetic wave generator including a CaxCoO2 layer and a Ti layer was fabricated.
As a femtosecond laser diode, a Ti: Sapphire laser diode with a wavelength of 800 nm, a pulse width of 100 fs and a pulse rate of 80 MHz was used.
The terahertz electromagnetic wave generator thus fabricated was irradiated with a femtosecond laser beam that had been condensed to 100 μm (=2r) so that the beam spot would cover both a left edge portion of the Ti layer and a portion of the CaxCoO2 layer. The time domain waveform of the electromagnetic wave measured in such a situation is shown in FIG. 30B. On the other hand, the relative positions of the CaxCoO2 and Ti layers and the laser beam are generally illustrated in FIG. 30A. As can be seen from the time domain waveform of the electromagnetic wave, a pulse wave with a negative peak intensity was generated at around 20 ps. FIG. 31 shows a power spectrum obtained by subjecting this time domain waveform to a Fourier transform. The electromagnetic wave generated had a frequency range of about 0.1 THz to about 1 THz. Thus, it was confirmed that a terahertz electromagnetic wave had been actually generated.
Next, the terahertz electromagnetic wave generator was irradiated with a femtosecond laser beam that had been condensed to 100 μm (=2r) so that the beam spot would cover only the Ti layer. The time domain waveform of the electromagnetic wave measured in such a situation is shown in FIG. 32B. On the other hand, the relative positions of the CaxCoO2 and Ti layers and the laser beam are generally illustrated in FIG. 32A. As can be seen from the time domain waveform, there was nothing but noise with no definite peaks observed, and no terahertz electromagnetic wave had been generated.
As can be seen from the results of Examples 7, 8 and 9, even if the metal layer is replaced with a Ti layer, a terahertz electromagnetic wave can also be generated in the same mechanism.
In the examples described above, the thermoelectric material layer is supposed to be irradiated with the femtosecond laser beam so that its beam spot crosses an edge of the surface of the thermoelectric material layer on the right- or left-hand side (i.e., an edge X which runs parallel to the Y axis of an XY plane). However, this is just an example and the laser beam spot does not always have to be formed there. Alternatively, the laser beam spot may also be formed across an upper or lower edge of the surface of the thermoelectric material layer (i.e., an edge which runs parallel to the X axis of the XY plane), for example. Also, since the thermoelectric material layer may be patterned into any arbitrary shape, the location and direction of the edge do not have to be the exemplary ones of the examples described above, either.
Also, in the examples described above, the thickness of the thermoelectric material layer is set to be 50 nm or 150 nm to make the comparison easily. However, it would be obvious to those skilled in the art from the entire disclosure of the present application that even if the thickness of the thermoelectric material layer is not 50 nm or 150 nm but falls within the range of 10 nm to 1000 nm (=1 μm), for example, similar effects will also be achieved. Likewise, the thickness of the metal layer does not have to be the exemplary value adopted in the examples described above, either.
Furthermore, as can be seen easily from the principle of generating a terahertz electromagnetic wave that has already been described in the foregoing description of the present disclosure, respective materials for the thermoelectric material layer, metal layer and substrate according to the present disclosure do not have to be the ones used in the examples described above, but any of a wide variety of thermoelectric materials, metallic materials and substrate materials may be used in an arbitrary combination.
With the technique of generating a terahertz electromagnetic wave according to the present disclosure, a terahertz electromagnetic wave can be generated by using a simpler configuration than conventional ones without providing any external voltage supply. Thus, the present disclosure is applicable to not only evaluation of the properties of various kinds of materials, but also security, healthcare and numerous other fields, using a terahertz electromagnetic wave.
While the present invention has been described with respect to preferred embodiments thereof, it will be apparent to those skilled in the art that the disclosed invention may be modified in numerous ways and may assume many embodiments other than those specifically described above. Accordingly, it is intended by the appended claims to cover all modifications of the invention that fall within the true spirit and scope of the invention.

## Claims

1. A terahertz electromagnetic wave generator comprising:
a thermoelectric material layer;
a metal layer that partially covers a surface of the thermoelectric material layer; and
a light source system configured to irradiate both a surface region of the thermoelectric material layer that is not covered with the metal layer and an edge of the metal layer with pulsed light, thereby generating a terahertz wave from the thermoelectric material layer.

2. The terahertz electromagnetic wave generator of claim 1, wherein the terahertz electromagnetic wave has a frequency falling within the range of 0.1 THz to 100 THz.

3. The terahertz electromagnetic wave generator of claim 2, wherein the pulsed laser light source system includes:
a light source which emits the pulsed light, of which the pulse width falls within the range of 1 femtosecond to 1 nanosecond; and
an optical system which guides the pulsed light that has been emitted from the light source toward the edge of the surface of the metal layer.

4. The terahertz electromagnetic wave generator of claim 3, wherein the light source is a femtosecond laser light source.

5. The terahertz electromagnetic wave generator of claim 1, wherein the thermoelectric material layer is made of a material selected from the group consisting of a single-element thermoelectric material, an alloy-based thermoelectric material, and an oxide-based thermoelectric material.

6. The terahertz electromagnetic wave generator of claim 5, wherein the thermoelectric material layer is made of at least one material selected from the group consisting of Bi, Sb, a BiTe-based alloy, a PbTe-based alloy, an SiGe-based alloy, CaxCoO2, NaxCoO2, and SrTiO3.

7. The terahertz electromagnetic wave generator of claim 1, wherein the thermoelectric material layer has a thickness of 10 nm to 1 μm.

8. The terahertz electromagnetic wave generator of claim 1, wherein the metal layer has a thickness of 10 nm to 100 μm.

9. The terahertz electromagnetic wave generator of claim 8, wherein the substrate is configured to transmit the terahertz wave.

10. A terahertz spectrometer comprising:
the terahertz electromagnetic wave generator of claim 1;
an optical system which irradiates an object with a terahertz electromagnetic wave that has been generated by the terahertz electromagnetic wave generator; and
a detector which detects the terahertz electromagnetic wave that has been transmitted through, or reflected from, the object.

11. The terahertz spectrometer of claim 10, further comprising a processing apparatus which generates an image representing a terahertz electromagnetic wave with a particular wavelength based on the output of the detector.

12. A method of generating a terahertz electromagnetic wave, the method comprising the steps of:
(A) providing a thermoelectric material body which is partially covered with a metal layer; and
(B) locally heating the thermoelectric material body by irradiating both a surface region of the thermoelectric material body which is not covered with the metal layer and an edge of the metal layer with pulsed light,
the step (B) comprising:
locally heating the thermoelectric material body so that an asymmetric heat distribution is formed in the thermoelectric material body; and
producing thermal diffusion current in the portion of the thermoelectric material body that has been heated locally, thereby generating a terahertz electromagnetic wave.