JP2013171954A - Terahertz wave generating element, application device using the same and device manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a terahertz generating element which can improve a ratio of exciting light contributing to the generation of a terahertz wave pulse.SOLUTION: A terahertz generating element for generating a terahertz wave by irradiation of exiting light 8 includes an exciting light absorption part 2 made of a semiconductor having a smaller band gap than energy corresponding to a wavelength range that the exciting light has, and having a reference surface which is determined considering an irradiation direction of the exciting light. The exciting light absorption part 2 has a stair shape part 3 in which a plurality of stair-steps 4, 5, 6 and 7, which have different distances in step-wise in a vertical direction with respect to the reference surface and have respective planar surfaces parallel to the reference surface, are constructed ranging via a side face connecting between planar surfaces having different distances.

Description

The present invention relates to a terahertz wave generating element that generates a terahertz wave including an electromagnetic wave component in a frequency region from a millimeter wave band to a terahertz wave band (30 GHz to 30 THz), a terahertz wave generating device using the terahertz wave generating device, and terahertz time domain spectroscopy (THz). -TDS) apparatus, terahertz imaging apparatus, terahertz wave generator manufacturing method, and the like.

In recent years, nondestructive sensing technology using terahertz waves has been developed. As an application field of electromagnetic waves in this frequency band, imaging techniques, spectroscopic techniques for examining physical properties such as molecular binding states by obtaining absorption spectra and complex dielectric constants of substances, measurements for examining physical properties such as carrier concentration, mobility, and conductivity Technology, biomolecule analysis technology, etc. have been developed. In order to put these technologies into practical use, progress in terahertz wave generation technology is one of the important factors. As a technique for generating such a terahertz wave, a terahertz wave generating element using a photodenver effect has been proposed (see Patent Document 1). Here, the photodenver effect means that a terahertz wave pulse is generated by an instantaneous change in surface current due to a difference in diffusion speed between two photoexcited carriers (electrons and holes) generated when an excitation light pulse is irradiated on a semiconductor surface. This is the effect that occurs. At the moment when the excitation light pulse is applied to the semiconductor surface, the excitation carriers are dense in the vicinity of the surface, so they try to diffuse into the semiconductor. Since the diffusion rate of electrons is generally greater than the diffusion rate of holes, electrons diffuse faster and farther. This causes a difference in the number density distribution of electrons and holes. This can be regarded as an instantaneous change in current, and a terahertz wave pulse having a frequency corresponding to the Fourier component of the time scale of this current change (typically about several picoseconds) is generated.

In the above description, the case where the direction of current change due to the photodenver effect is perpendicular to the semiconductor surface has been described. However, in the generating element described in Patent Document 1, the direction of current change is parallel to the semiconductor surface. It has a simple structure. This is a structure considering that the terahertz wave pulse is radiated most strongly in the direction perpendicular to the direction of the current change. When the direction of the current change is parallel to the semiconductor surface, the direction in which the terahertz wave pulse is radiated strongly is perpendicular to the semiconductor surface. As a method of adjusting the direction of current change, the direction of current change is adjusted so that the distribution of excited carriers has a steep gradient in the semiconductor surface parallel direction by covering a part of the semiconductor surface with a light shielding film, etc. An example is disclosed. With such an element configuration, by making the radiation direction of the terahertz wave pulse perpendicular to the semiconductor surface, it is easy to extract the terahertz wave pulse to the outside of the generating element, and the power of the terahertz wave pulse radiated to the space is reduced. It is said that there is an effect to improve.

International Publication WO2010142313

However, the generating element described in Patent Document 1 does not contribute to generation of a terahertz wave pulse even when an excitation light pulse is irradiated in a structure in which the distribution of excitation carriers has a steep gradient parallel to the semiconductor surface. There were places (for example, places on the light shielding film). For this reason, there is a concern that the utilization efficiency of the excitation light pulse is lowered.

In view of the above problems, the terahertz wave generating element of the present invention is an element that generates a terahertz wave by irradiation of excitation light, and is made of a semiconductor having a band gap smaller than energy corresponding to the wavelength range of the excitation light. And an excitation light absorbing portion having a reference plane determined in consideration of the irradiation direction of the excitation light. The excitation light absorption unit includes a plurality of stepped steps each having a stepwise difference in a vertical direction with respect to the reference plane and each having a plane parallel to the reference plane. At least one staircase-shaped portion configured in series.

According to the terahertz wave generating element of the present invention, the excitation light applied to each step is largely used to generate a terahertz wave pulse in a direction substantially perpendicular to the reference plane (a plane parallel to the plane of each step). Can be used. Therefore, there is an effect that the ratio of the excitation light contributing to the generation of the terahertz wave pulse can be improved.

FIG. 3 illustrates a configuration example of a terahertz wave generating element according to the first embodiment. FIG. 4 is a diagram for explaining a carrier number distribution in the terahertz wave generating element according to the first embodiment. FIG. 6 is a diagram illustrating a modification of the terahertz wave generating element according to the first embodiment. FIG. 3 is a view for explaining an excitation light pulse incidence method to the terahertz wave generating element according to the first embodiment. FIG. 6 is a diagram illustrating a modification of the terahertz wave generating element according to the first embodiment. FIG. 6 is a diagram illustrating a modification of the terahertz wave generating element according to the first embodiment. FIG. 6 is a diagram illustrating a modification of the terahertz wave generating element according to the first embodiment. FIG. 3 is a diagram for explaining an example of a method for manufacturing the terahertz wave generating element according to the first embodiment. FIG. 3 is a diagram for explaining an example of a method for manufacturing the terahertz wave generating element according to the first embodiment. FIG. 3 is a diagram for explaining an example of a method for manufacturing the terahertz wave generating element according to the first embodiment. FIG. 6 is a diagram illustrating a modification of the terahertz wave generating element according to the first embodiment. FIG. 6 is a diagram illustrating a configuration example of an apparatus according to a second embodiment. FIG. 6 illustrates an example of a measurement method according to a second embodiment. The figure explaining the inclination of the side of a stair step.

The terahertz wave generating element of the present invention is characterized in that a terahertz wave can be generated in a direction substantially perpendicular to the plane at each step by forming a stepped portion in the excitation light absorbing portion. . Thereby, it is intended to improve the utilization efficiency of the excitation light pulse. As described above, the staircase shape portion is a side surface in which a plurality of staircase steps each having a plane that is stepwise different in a direction perpendicular to the reference plane and each parallel to the reference plane connects between the planes having the different distances. Consecutive via. Here, in the staircase step of the staircase shape part, the angle of the side surface connecting between adjacent stairsteps is necessarily formed at a right angle (that is, perpendicular to the reference plane or the plane of the stairstep parallel to it). There is no limitation as long as a substantially steep excitation carrier distribution can be realized in the direction parallel to the reference plane at the level of the plane parallel to the reference plane of each step of the staircase shape portion.

Hereinafter, embodiments will be described with reference to the drawings.
(Embodiment 1)
Embodiment 1 of the present invention will be described with reference to FIG. FIG. 1A is a cross-sectional view of a terahertz wave generating element, and FIG. 1B is a top view. In FIG. 1, an excitation light absorption unit 2 is disposed in contact with the substrate 1. The excitation light absorption unit 2 has a stepped portion 3. Here, the substrate 1 has a parallel plate shape having upper and lower surfaces parallel to the reference surface, and the substrate 1 and the excitation light absorbing portion 2 are in contact with each other on a surface parallel to the reference surface. First, the direction in this specification is defined. In FIG. 1, a plane parallel to the plane of the stair step of the staircase shape portion 3 is set as a reference plane. A direction perpendicular to the direction of the reference plane is defined as a reference plane vertical direction. Since the excitation light is irradiated at a certain angle (typically perpendicular) to the plane or reference plane of the stair step, and the terahertz wave is generated in a direction substantially perpendicular to this plane, the reference plane is irradiated with the excitation light. The direction is determined in consideration of the direction, and the plane of the stair step of the excitation light absorber is determined accordingly. Further, when the substrate is provided in contact with the excitation light absorption unit, the side where the excitation light absorption unit 2 is present when viewed from the substrate 1 is the element upper surface side. The opposite side is the element lower surface side.

In the present embodiment, the excitation light absorber 2 absorbs the irradiated excitation light pulse 8 to generate photoexcited carriers, and thus has a band gap smaller than the energy corresponding to the main wavelength range of the excitation light pulse 8. It consists of a semiconductor. The main wavelength range is, for example, the peak wavelength of the excitation light pulse 8 or the long wavelength end or center wavelength of the minus 3 dB band. The pumping light absorption unit 2 is configured according to the main wavelength range of the pumping light pulse 8, for example, GaAs, InAs, InSb, GaSb, InGaAs, InP, GaN, GaAsN, GaInAsN, Si, C (diamond), InN, ZnO, and the like. Can be used. Other material selection criteria include high carrier mobility and a large difference in mobility between electrons and holes in order to generate stronger terahertz waves.

The staircase shape of the staircase shape portion 3 of the excitation light absorber 2 refers to a shape in which the thickness in the direction perpendicular to the reference surface (distance in the direction perpendicular to the reference surface) is gradually changing in the direction parallel to the reference surface. In the present specification, each step where the thickness changes stepwise (steps) is referred to as a step. The number of stair steps is two or more and is arbitrary. In FIG. 1, the staircase shape portion 3 of the excitation light absorbing portion 2 includes four step steps 4, 5, 6, and 7.

The substrate 1 is preferably made of a highly insulating material with few free carriers in order to reduce terahertz wave absorption. For example, semi-insulating gallium arsenide (SI (Semi-Insulating) -GaAs) or semi-insulating indium phosphide (SI-InP) can be used. The substrate 1 and the excitation light absorber 2 may be made of the same material. Further, the substrate 1 may be omitted. When the excitation light absorber 2 is grown on the substrate 1 by MBE (Molecular Beam Epitaxy) or the like, the material of the substrate 1 is selected from the viewpoint of lattice matching or the like according to the type of the excitation light absorber 2. Another layer may be provided between the substrate 1 and the excitation light absorber 2. Further, as the substrate 1, a resin such as high resistance silicon (Si) or cycloolefin with little absorption in the terahertz wave region may be used. Quartz or ceramics can also be used. In this case, the excitation light absorption unit 2 grown on another substrate can be manufactured by applying a method of transferring and bonding the substrate 1 to the substrate 1. There is also a method of growing the excitation light absorbing portion 2 on the Si substrate by an MBE method or the like through a buffer layer such as Al _x Ga _1-x As (0 ≦ x ≦ 1) / Ge.

For generation of terahertz waves, a steep carrier distribution generated in the direction parallel to the reference plane when the pumping light absorption unit 2 irradiates the pumping light pulse 8 is used. With such carrier distribution, a photodenver effect occurs in the direction parallel to the reference plane. That is, a terahertz wave pulse is generated due to an instantaneous change in surface current due to a difference in diffusion speed between two photoexcited carriers (electrons and holes) generated when the excitation light absorption unit 2 is irradiated with the excitation light pulse 8. An effect is produced. At this time, increasing the carrier mobility and increasing the difference between the electron and hole mobility are effective in increasing the intensity of the terahertz wave pulse. This is because the current increases as the mobility increases, and the current increases as the mobility difference between electrons and holes increases. Here, a current change substantially occurs in the direction parallel to the reference plane. When the direction of the current change is the direction parallel to the reference plane, the direction in which the terahertz wave pulse is radiated strongly becomes the direction perpendicular to the reference plane, that is, the direction perpendicular to the plane of the step. In the configuration shown in FIG. 1, since the dielectric constant of the substrate 1 is generally larger than the dielectric constant of air (for example, the dielectric constant of GaAs is about 13), a large terahertz radiation intensity can be obtained particularly on the lower surface side of the element. Here, since the surface where the excitation light absorber 2 and the substrate 1 are in contact with each other and the bottom surface of the substrate 1 are parallel to the reference surface, that is, the plane of the step, the direction in which the terahertz wave pulse is radiated strongly is almost perpendicular to the bottom surface of the substrate 1. Become a direction. On the other hand, when the surface where the excitation light absorbing unit 2 and the substrate 1 are in contact and / or the bottom surface of the substrate 1 is not parallel to the reference surface, the terahertz wave pulse is refracted on the contact surface or / and the bottom surface of the substrate 1. Will be emitted.

A configuration that causes the steep carrier distribution in the reference plane parallel direction as described above will be described. In order to simplify the explanation, consider a case where the excitation light pulse 8 is applied to the generating element in the direction perpendicular to the reference plane from the upper surface side of the element (see FIG. 2A). Here, focus on step 4. The excitation light pulse 8 reaching the step 4 is divided into two types. One is an excitation light pulse 8 that propagates in the air and reaches the stair step 4 directly, and the other is partially absorbed or reflected by the stair step 5 or the like and then reaches the level of the plane of the stair step 4. This is the excitation light pulse 8. The power of the latter pumping light pulse 8 is greatly reduced. As a result, there is a difference in the number of carriers excited by both at the level of the plane of the step 4. In the excitation light absorbing portion 2, the region 22 having a large number of photoexcited carriers is distributed as shown in FIG. Therefore, as shown in FIG. 2B, a steep carrier distribution is generated in the region parallel to the reference plane in the region 21 near the step between the stair step 4 and the stair step 5 (the side surface between the planes of both stair steps).

Here, FIG. 2A is an enlarged view of the vicinity of the step 4 in FIG. 1, and FIG. 2B is a diagram showing the carrier distribution at the level of the step 4 immediately after the excitation light pulse 8 irradiation. is there. Such a carrier distribution is also generated in the other step steps 5 and 6. In the configuration of FIG. 1, there is no configuration in which the intensity of the excitation light pulse 8 irradiated to the uppermost stair step 7 is steeply changed in the reference plane parallel direction. Does not occur.

In the present specification, the “pulse” of the excitation light pulse 8 is excitation light having a pulse width (full width at half maximum) of a level that strongly generates electromagnetic waves in the terahertz frequency region. Since the frequency of 1 THz corresponds to 1 ps in the time domain, the pulse width of the excitation light pulse 8 is preferably 10 ps or less at full width at half maximum. 1 ps or less is more desirable.

The size of each stair step will be described. The thickness d (see FIG. 2A) in the direction perpendicular to the reference plane of each step is preferably equal to or less than the light absorption length for the main wavelength range of the excitation light pulse. This is to reduce the steepness of the carrier distribution that occurs in the direction perpendicular to the reference plane, thereby suppressing the current change in the direction perpendicular to the reference plane and preventing the terahertz wave from being strongly emitted in the direction parallel to the reference plane. The light absorption length is a material thickness at which the amount of transmitted light is 1 / e when light is transmitted through the material. On the other hand, if d is too small, the amount of light absorption decreases, so the value of d is preferably 0.5 to 2 times the light absorption length. That is, the thickness in the direction perpendicular to the reference plane of each stair step layer constituting the staircase shape portion is 0.5 times the light absorption length in the wavelength range of the excitation light of the material constituting the excitation light absorption portion. It is preferably 2 times or less. In addition, the width W (see FIG. 2A) of the plane of each stair step in the direction parallel to the reference plane is preferably less than or equal to the spatial scale of carrier diffusion of the photodenver effect. This is because the excited carriers generated in each step are effectively used for the generation of terahertz waves. Excited carriers generated at a location far from the spatial scale of carrier diffusion from a location where a steep carrier distribution occurs (for example, region 21 in FIG. 2B) cannot reach a location where a steep carrier distribution occurs. Accordingly, since the contribution of these excited carriers to the generation of the terahertz wave becomes small, the width W is preferably less than or equal to the spatial scale of carrier diffusion of the photodenver effect. The spatial scale of carrier diffusion of the photodenver effect is about 1 μm in the case of GaAs. Considering such a case, it is preferable that the width between the side portions of the plane of each stair step constituting the stair shape portion is 1 μm or less. Of course, the thickness d in the direction perpendicular to the reference plane and the width W in the direction parallel to the reference plane may be different from each other.

A light shielding film 9 that shields the excitation light pulse 8 may be formed on a part of the upper surface of the uppermost stair step 7 so as to form a steep carrier distribution in the direction parallel to the reference plane (FIG. 3). The light shielding film 9 may be a metal or a dielectric as long as it reflects or absorbs the excitation light pulse 8. For example, a titanium / gold (Ti / Au) film having a thickness of about 200 nm can be used. The light shielding film 9 is disposed at a position within the beam diameter of the excitation light pulse 8 on the plane of the uppermost step of the staircase shape portion 3. This is because terahertz waves are efficiently generated both in the staircase shape portion 3 and in the vicinity of the light shielding film 9. Here, the beam diameter of the excitation light pulse 8 is the diameter of a circle connecting points that are 1 / e ² of the peak power in the power distribution in the plane perpendicular to the optical axis of the excitation light pulse 8. In this manner, the light shielding portion in contact with the staircase shape portion can be disposed at a position within the beam diameter of the excitation light in the staircase shape portion.

As such a generating element, for example, SI-GaAs can be used for the substrate 1 and U / D (Undope) -InAs can be used for the excitation light absorber 2. For the pumping light pulse, a 1.5 μm band femtosecond laser capable of absorbing InAs which is a material of the pumping light absorbing unit 2 can be used. When there is a possibility that the crystallinity of the excitation light absorption unit 2 is deteriorated due to a lattice constant difference between the substrate 1 and the excitation light absorption unit 2, in order to improve the crystallinity between the substrate 1 and the excitation light absorption unit 2. A buffer layer may be formed. As the buffer layer, a layer having a single composition of AlInAsSb, a graded layer, or the like can be used. The thicknesses of the substrate 1 are 500 μm, the buffer layer is 1 μm, the excitation light absorption unit 2 is 2.4 μm, and the thickness of each step 4, 5, 6, 7 of the excitation light absorption unit 2 (the height of the side surface). ) Is 0.6 μm. The thickness of each step is set equal to about 0.6 μm, which is the light absorption length of InAs in the wavelength band of the excitation light pulse of 1.5 μm.

In the above configuration, the terahertz waves generated from the respective step steps are combined and radiated to the outside of the terahertz wave generating element. As described above, the angle of the side surface of the stair step with respect to the reference surface does not need to be formed at a right angle. This will be described in detail with reference to FIG. Even if the inclination of the side surface 111 of the stepped portion with respect to the reference plane is not vertical (α = 90 °), it is only necessary that the excitation light absorption unit 2 can realize a substantially steep excitation carrier distribution. Since the number of pumping carriers is considered to be proportional to the power density of the pumping light pulse 8, the allowable range of the inclination of the step-shaped side surface 111 can be calculated from the viewpoint of the power density of the pumping light pulse 8. As described above, when InAs (refractive index of 3.5) is used as a material, the inclination α of the side surface 111 of the stepped portion is desirably 75 degrees or more. This is because the power density on the plane parallel to the reference plane of the excitation light pulse 8 which is refracted by the side surface 111 of the staircase shape portion and is incident on the excitation light absorbing portion 2 is about 75 degrees of the power density on the light vertical plane. This is because the angle corresponds to half. That is, as is generally known, when the incident angle of the light ray on the evaluation surface is θ, the power density on the evaluation surface is reduced by cos θ times compared to the power density on the light vertical surface (cos (60 ° ) = 1/2). However, the above calculation is a simplified example. The allowable range of the inclination of the side surface 111 of the staircase shape portion is other parameters (for example, the incident direction and polarization direction of the excitation light pulse 8, the absorption coefficient and thickness of the excitation light absorption portion 2, the photodenver length, the staircase shape portion Of course, it also depends on the surface state (roughness, etc.) of the side surface 111. Therefore, the allowable range of the inclination of the side surface of the stair step with respect to the reference surface capable of realizing a substantially steep excitation carrier distribution should be determined in consideration of various parameters in each configuration. Typically, from the viewpoint of significantly improving the ratio of the excitation light pulse contributing to the generation of the terahertz wave pulse as compared with the conventional example, the power density of the excitation light pulse 8 on the plane parallel to the reference plane is It is desirable that the angle range be approximately half or more of the power density (for example, a range of about 75 degrees to 90 degrees).

A method of irradiating the generating element with the excitation light pulse will be described with reference to FIG. 4B and 4C show a case where the excitation light pulse 8 is irradiated from an oblique direction on the upper surface side of the substrate. In FIG. 4B, the excitation light pulse 8 is irradiated while being inclined toward the thicker side of the staircase shape portion than the direction perpendicular to the reference plane. On the other hand, in FIG. 4C, the excitation light pulse 8 inclined to the opposite side is irradiated. In the configuration as shown in FIG. 4B, the convex portion of a certain stair step (near the boundary between the plane and the side surface) steeply distributes the excitation light pulse 8 irradiated to the stair step on the lower surface side of the element. Can be cut. Therefore, it is possible to generate a steep carrier distribution in the reference plane parallel direction. On the other hand, in FIG. 4C, the distribution of photoexcited carriers becomes gentle compared to FIGS. 4A and 4B. Therefore, in order to generate a higher-intensity terahertz wave, an irradiation method as shown in FIGS. 4A and 4B is desirable. Further, although the excitation light pulse 8 is irradiated from the upper surface side of the element here, it may be irradiated from the lower surface side of the element as described above. In this case, a stair step is formed on the lower surface side (substrate side) of the excitation light absorbing portion. For this, a transfer adhesion process or the like may be used. In order to avoid frequency dispersion, absorption, and reflection of the excitation light pulse 8 due to passing through the substrate 1, a configuration where the excitation light pulse 8 passes through the substrate 1 may be hollowed out. In the pumping light absorption unit 2, when the stepped portion is formed symmetrically with respect to the vertical plane of the substrate and irradiated with the pumping light pulse 8 symmetrically (see FIG. 5A), it is generated on both sides of the plane of symmetry. Terahertz waves have electric field strengths whose phases are inverted by 180 degrees. Since these two terahertz waves cancel each other, the arrangement as shown in FIG. 5A is not preferable. When the stepped portion of the excitation light absorbing portion 2 is formed symmetrically with respect to the substrate vertical plane, the excitation light pulse 8 may be irradiated asymmetrically (see FIG. 5B).

As the shape of the excitation light absorbing portion 2, a concentric circular shape as shown in FIG. 6 can be used. In this case, the excitation light pulse 8 irradiates the excitation light absorption unit 2 asymmetrically as shown in FIG. The electric field direction of the terahertz wave coincides with the direction of the current change generated in the excitation light absorbing unit 2. Therefore, it is possible to control the polarization of the terahertz wave according to the position at which the excitation light pulse 8 is irradiated to the excitation light absorption unit 2 in FIG.

A carrier barrier layer or an excitation light reflecting layer may be provided between the layers of each step (see FIG. 7). In FIG. 7, 10 constituent elements are a carrier barrier layer or an excitation light reflecting layer. When the carrier barrier layer 10 is used, the probability that photoexcited carriers generated at each step will move to another step is reduced. Therefore, it is possible to reduce the influence of carriers on each other between the respective stair steps, and to generate a terahertz wave with higher intensity. For example, when the excitation light absorber 2 is InAs, AlInAsSb or the like can be used as the carrier barrier layer 10. Further, when the excitation light reflecting layer 10 is used, it is possible to reduce the penetration of the excitation light pulse through a certain stepped step layer to a layer below it. Therefore, a steeper photoexcited carrier distribution can be formed, and a higher-intensity terahertz wave can be generated. In particular, the excitation light reflecting layer 10 is effective when the thickness d in the direction perpendicular to the reference plane of each step is less than or equal to the light absorption length for the main wavelength range of the excitation light pulse 8. This is because the excitation light reflecting layer 10 makes the power distribution in the direction parallel to the reference plane of the excitation light pulse 8 reaching the stair step closer to the substrate side steep.

An example of a method for manufacturing the generating element will be described with reference to FIG. FIG. 8A shows a state in which the excitation light absorber 2 is formed on the substrate 1. A photoresist film 73 is formed by patterning on the surface on the upper surface side of the element that is to be irradiated with the excitation light of the excitation light absorbing portion by photolithography (FIG. 8B). Next, using the patterned photoresist film 73 as a mask, the excitation light absorbing portion 2 is etched by dry etching (FIG. 8C). That is, the excitation light absorbing portion other than the region covered with the photoresist film is etched. Here, in order to form a stepped shape portion later, the excitation light absorbing portion 2 is etched halfway through the film. As shown in FIG. 8C, in order to stop the etching in the middle of the film of the excitation light absorption unit 2, the etching amount is calculated from the etching rate or an etching stop layer is provided inside the excitation light absorption unit 2. Good. Etching may use a wet etching method. When the etching is completed, the photoresist film 73 is peeled off. By repeating such a process for each step, a structure as shown in FIG. 8D can be finally formed.

Alternatively, a method of applying a stepped resist 73 may be used. First, as shown in FIG. 9A, a stepped resist 73 is formed on the excitation light absorbing portion 2. The step-like resist 73 can be formed using a spray coater. Further, it may be formed by lithography by adjusting the exposure amount using a multi-tone photomask or a laser drawing apparatus. Next, the excitation light absorbing portion 2 is etched by a dry etching method. At that time, the resist 73 is also etched simultaneously. When the thinnest region 73 of the resist 73 is not etched, the excitation light absorbing portion 2 below (on the substrate side) begins to be etched. By repeating such steps, the excitation light absorbing portion 2 having a stepped portion as shown in FIG. 9B can be formed.

In addition, there is a method of etching the excitation light absorbing portion 2 while gradually retracting the resist 73 applied first. First, a resist 73 is formed so as to have an end face on the excitation light absorbing portion 2 (FIG. 10A). The excitation light absorbing portion 2 is etched using the resist 73 as a mask (FIG. 10B). The resist 73 is moved backward by the length in the width direction of the stair step (FIG. 10C). For this purpose, the resist 73 is exposed and removed by the laser drawing apparatus as much as it is retracted, or is removed by laser ablation. By repeating such a process, the excitation light absorbing portion 2 having a stepped portion as shown in FIG. 10D can be formed.

A plurality of generating elements as described above may be arranged in an array shape in parallel with the reference plane (see FIG. 11). That is, the excitation light absorbing portion can have a plurality of stepped portions. At this time, it is necessary to consider the electric field direction of the terahertz wave pulses (and thus the generation direction of the carrier distribution) so that the terahertz wave pulses generated in the array do not cancel each other. For example, as described in the case of a single element, the structure of the generating element in the array is asymmetrical (that is, the stepped portion includes a region formed asymmetrically with respect to the plane including the excitation light irradiation direction). The excitation light pulse irradiation position is made asymmetric. The excitation light pulse may be applied to the entire array or a part thereof. FIG. 11A is a cross-sectional view of a terahertz wave generating element arranged in an array shape, and has a plurality of excitation light absorbing portions 2 as shown in FIG. The direction of the stepped shape portions of the respective excitation light absorbing portions 2 is the same in the reference plane parallel direction as shown in FIG. 11A so that generated terahertz waves do not cancel each other. Is desirable. FIG. 11B is a top view of a terahertz wave generating element in which nine excitation light absorbing units 2 are arranged. The excitation light pulse 8 may be applied to the entire array as shown in FIG. 9, or may be applied only to the area of each excitation light absorption unit 2 using a lens array or the like. By controlling the phase of the excitation light pulse 8 applied to each excitation light absorption unit 2, the radiation direction of the generated terahertz wave can be changed (phased array).

With the generating element as described above, it is possible to improve the efficiency with which the excitation light pulse is used for generating the terahertz wave. That is, the terahertz wave can be efficiently generated by irradiating the terahertz wave generating element with excitation light.

(Embodiment 2)
Embodiment 2 relates to a Terahertz Time Domain Spectroscopy (THz-TDS) apparatus using the generating element as described in Embodiment 1. FIG. 12 shows a configuration example of the terahertz time domain spectroscopic device in the present embodiment. This terahertz time domain spectroscopic device is a device that uses a terahertz wave including an electromagnetic wave component in a frequency region of about 30 GHz to 30 THz. That is, the present embodiment is a terahertz wave generation device including the terahertz wave generation element of the present invention, an excitation light generation unit that generates excitation light that irradiates the generation element, and an irradiation unit that irradiates the terahertz wave generation element with the excitation light. Equipment. And a terahertz wave detecting device that detects the electric field strength of the terahertz wave, a terahertz wave optical system that irradiates the sample with the terahertz wave and transmits or reflects the terahertz wave transmitted to or reflected from the sample to the terahertz wave detecting device. And a processing unit that calculates a time waveform of the terahertz wave from the electric field intensity of the terahertz wave detected by the terahertz wave detection device.

This will be specifically described. In FIG. 12, a pumping light pulse generator 80 emits a pumping light pulse 81. As the excitation light pulse generator 80, a fiber laser or the like can be used. Here, the excitation light pulse 81 is a pulse laser having a wavelength of 1.5 μm and a pulse time width (full width at half maximum in power display) of about 30 fs. The excitation light pulse 81 is divided into two by the beam splitter 82. One excitation light pulse 81 is incident on the terahertz wave pulse generation unit 83, and the other excitation light pulse 81 is incident on the second harmonic generation unit 84.

The terahertz wave pulse generator 83 includes the terahertz wave generating element as described in the above embodiment. The excitation light pulse 81 is collected by a lens and irradiated to the light absorption part of the terahertz wave generating element with a beam diameter of about 10 μm. When the terahertz wave pulse 85 is radiated strongly toward the back surface of the substrate on which the generating element is formed, a silicon hemispherical lens may be disposed in contact with the back surface of the substrate to increase the radiation power to the space. Here, since the wavelength of the excitation light pulse 81 is in the 1.5 μm band, InAs that can absorb the excitation light of this wavelength and generate photoexcited carriers can be used as the light absorbing portion.

With the configuration described above, it is possible to emit a terahertz wave pulse 85 having a pulse time width (full width at half maximum) of several hundreds fs to several ps. The terahertz wave pulse 85 radiated to the space is condensed onto the sample 86 by an optical element such as a lens or a mirror. The terahertz wave pulse 85 reflected from the sample 86 is incident on the terahertz wave pulse detecting element 87 by the optical element.

The other excitation light pulse 81 divided by the beam splitter 82 and incident on the second harmonic generation unit 84 becomes a pulse laser having a wavelength of 0.8 μm band by the second harmonic conversion process. As the second harmonic conversion element, a PPLN crystal (Periodically Poled Lithium Niobate) or the like can be used. Lasers generated in other nonlinear processes or emitted in a wavelength of 1.5 μm without being converted are excluded from the excitation light pulse 81 by a dichroic mirror or the like (not shown). The excitation light pulse 81 converted to the wavelength of 0.8 μm band passes through the excitation light delay system 88 and enters the terahertz wave pulse detection element 87.

A photoconductive element can be used as the terahertz wave pulse detecting element 87. On the detection side, low-temperature grown GaAs is preferably used for the photoconductive layer of the photoconductive element in order to absorb the excitation light pulse 81 having a wavelength of 0.8 μm generated by the second harmonic generation unit 88. The photoexcited carriers generated in the photoconductive layer are accelerated by the electric field of the terahertz wave pulse 85 to generate a current between the electrodes. This current value reflects the electric field strength of the terahertz wave pulse 84 within the time during which the photocurrent flows. The current may be converted into a voltage by a current-voltage conversion device. The terahertz wave pulse detecting element 87 is not limited to a photoconductive element, and other methods generally used in the terahertz region, for example, a detecting method using birefringence of an electro-optic crystal may be used.

The pumping light delay system 88 includes a movable retro-reflector and the like, and is controlled by the processing unit 89 to sweep the delay time of the pumping light pulse 81 reaching the terahertz wave pulse detection element 87. Thereby, the electric field intensity of the terahertz wave pulse 87 can be measured while changing the relative time difference between the terahertz wave pulse 85 and the excitation light pulse 81 reaching the terahertz wave pulse detecting element 87. Therefore, the time waveform of the electric field strength of the terahertz wave pulse 85 can be reconstructed. The processing unit 89 controls the delay time of the excitation light pulse 81 by the excitation light delay system 88. Further, information (complex refractive index, shape, etc.) of the sample 86 is acquired from the time waveform of the terahertz wave pulse 85 and its frequency component, and displayed on the display unit 90.

By obtaining the time interval of the terahertz wave pulse 85 reflected from the surface of the sample 86 or the internal interface, it is also possible to evaluate the surface interval (Time of Flight method). FIG. 13 is an explanatory diagram thereof. The incident terahertz pulse 100 is reflected by the sample 86. If the sample 86 has three interfaces that reflect terahertz pulses, the reflected terahertz pulse 101 is composed of three pulses. Furthermore, it is possible to perform tomographic imaging by scanning the measurement location in the sample 86. Although the terahertz wave pulse 85 reflected from the sample 86 is detected in FIG. 12, the terahertz wave pulse 85 that passes through the sample 86 may be detected. Thus, for example, the terahertz imaging apparatus can be configured by acquiring the refractive index interface structure of the sample or the physical properties of the constituent object from the time waveform of the terahertz wave calculated by the processing unit.

By using the terahertz wave generating element of the present invention, a high-intensity terahertz wave pulse can be used. Therefore, in an application apparatus such as the above terahertz time domain spectroscope, object identification and imaging can be performed with high accuracy. Is possible. Taking advantage of these features, it can be used in the fields of medicine, beauty, industrial product inspection, and the like.

DESCRIPTION OF SYMBOLS 1 ... Board | substrate, 2 ... Excitation light absorption part, 3 ... Stair-shaped part 4, 5, 6, 7 ... Stair step, 8 ... Excitation light

Claims (12)

An element that generates terahertz waves when irradiated with excitation light,
An excitation light absorbing portion having a reference surface determined in consideration of the irradiation direction of the excitation light, made of a semiconductor having a band gap smaller than the energy corresponding to the wavelength range of the excitation light,
The excitation light absorption unit has a plurality of stair steps each having a plane that is stepwise different in a direction perpendicular to the reference plane and parallel to the reference plane, via side surfaces that connect the planes having different distances. A terahertz wave generating element having at least one step-shaped portion configured in a row. 2. The terahertz wave generating element according to claim 1, wherein the staircase shape portion includes a region formed asymmetrically with respect to a plane including an irradiation direction of the excitation light. The thickness in the direction perpendicular to the reference plane of each stair step layer constituting the staircase portion is 0.5 of the light absorption length in the wavelength range of the excitation light of the material constituting the excitation light absorber. The terahertz wave generating element according to claim 1, wherein the terahertz wave generating element is not less than twice and not more than twice. 4. The terahertz wave generating element according to claim 1, wherein a width between the side portions of the flat surface of each stair step constituting the stair shape portion is 1 μm or less. 5. 5. The apparatus according to claim 1, further comprising a carrier barrier layer or an excitation light reflection layer that reflects the excitation light pulse between layers of the respective step steps constituting the step shape portion. Terahertz wave generator. 6. The light-shielding portion disposed in contact with the staircase-shaped portion, wherein the light-shielding portion is disposed at a position within the beam diameter of the excitation light in the staircase-shaped portion. The terahertz wave generating element according to claim 1. 7. The terahertz wave generating element according to claim 1, wherein the excitation light that irradiates the stepped portion is a pulse having a full width at half maximum of 10 ps or less. The terahertz wave generating element according to claim 1, further comprising a substrate in contact with the excitation light absorbing unit. The terahertz wave generating element according to any one of claims 1 to 8, an excitation light generating unit that generates the excitation light that irradiates the terahertz wave generating element, and irradiating the terahertz wave generating element with the excitation light. The terahertz wave generator characterized by having the irradiation means to do. A terahertz time domain spectroscopic device using the terahertz wave generator according to claim 9,
A terahertz wave detecting device for detecting electric field intensity of the terahertz wave;
A terahertz wave optical system that irradiates the terahertz wave to the sample and guides the terahertz wave transmitted or reflected by the sample to the terahertz wave detection device;
A processing unit that calculates a time waveform of the terahertz wave from an electric field intensity of the terahertz wave detected by the terahertz wave detection device;
A terahertz time domain spectroscopic device characterized by comprising: A terahertz imaging device that images the sample using the terahertz time domain spectroscopy device according to claim 10,
A terahertz imaging apparatus that acquires the refractive index interface structure of the sample or the physical properties of a constituent object from the time waveform of the terahertz wave calculated by the processing unit. A manufacturing method for a terahertz wave generating device according to any one of claims 1 to 8,
A patterning step of patterning and forming a photoresist film on the upper surface side of the device that is to be irradiated with the excitation light of the excitation light absorbing portion;
An etching step of removing at least a part of the region of the excitation light absorbing portion other than the region covered with the photoresist film;
A peeling step of peeling the photoresist film from the excitation light absorbing portion;
Including
A method of manufacturing a terahertz wave generating element, wherein the patterning step, the etching step, and the peeling step are repeated so that the excitation light absorbing portion forms a stepped portion.

Images