JP2012023134A - Terahertz electromagnetic wave generation source and terahertz electromagnetic wave generation device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a terahertz electromagnetic wave generation source capable of generating a terahertz electromagnetic wave with a simple element configuration while the generation is maintained for a long period of time.SOLUTION: A terahertz electromagnetic wave generation source 101 includes an nipi laminate 110 having a semiconductor layered structure comprising at least three layers of an N-type semiconductor layer and a P-type semiconductor layer which are laminated alternately. A terahertz electromagnetic wave is generated by irradiating the nipi laminate 110 with an ultrashort pulse laser beam. Thereby, the terahertz electromagnetic wave can be generated by a simple configuration while the generation is maintained for a long period of time.

Description

  The present invention relates to a terahertz electromagnetic wave generation source and a terahertz electromagnetic wave generation device that generate a terahertz electromagnetic wave by irradiation with a pulse laser.

  In recent years, as one of the electromagnetic waves attracting attention, there is an electromagnetic wave having a wavelength of 30 μm to 3 mm (hereinafter referred to as a terahertz electromagnetic wave) that is an intermediate wavelength between light and radio waves.

  Although terahertz electromagnetic waves do not pass through water, they cannot be used as a substitute for human X-ray inspection, but are attracting attention in other nondestructive inspection fields.

  As generation methods of terahertz electromagnetic waves, two types of generation methods are known.

  For example, as disclosed in Patent Documents 1 to 3, Non-Patent Document 1, etc., by irradiating the surface of the semiconductor layer with a pulse laser, electrons and holes are diffused on the surface of the semiconductor layer. This is a method for generating a terahertz electromagnetic wave (hereinafter referred to as “pulse laser irradiation method”).

  The other is, for example, as disclosed in Patent Documents 4 and 5, Non-Patent Document 2 and the like, in a junction layer constituting a semiconductor, a cascade is formed from a layer having a high electron potential to a layer having a low electron potential. In which electrons are caused to flow to generate terahertz electromagnetic waves (hereinafter referred to as “cascade method”).

JP 2005-019472 A (published January 20, 2005) JP 2005-519458 A (published on June 30, 2005) JP-A-2005-332954 (published on December 02, 2005) JP 2006-310784 A (published on November 09, 2006) JP 2001-320136 A (published November 16, 2001)

"Generation of femtosecond electromagnetic pulses from semiconductor surfaces" X. -C. Zhang et. Al., APPL, p.1011-1013, published March 12, 1990 "Terahertz semiconductorheterostructure laser" Rudeger Kohler et. Al., NATURE VOL 417, p.156-157, published March 9, 2002

  However, in the conventional pulsed laser irradiation method, the semiconductor surface is irradiated with a pulsed laser, and electrons or holes are diffused to generate terahertz electromagnetic waves. There is a problem that terahertz electromagnetic waves can be generated only for a second (about ps).

  In addition, in the conventional cascade method, in order to allow electrons to move from a layer with a high electron potential to a layer with a low electron potential by the tunnel effect, the layer with a high electron potential is made thin or an electron potential gradient is formed in the stacking direction. There is a problem that the degree of difficulty in element configuration, such as, is extremely high.

  The present invention has been made in view of the above-described problems, and an object of the present invention is to provide a terahertz electromagnetic wave generation source and a terahertz electromagnetic wave generator that can generate terahertz electromagnetic waves for a long time with a simple element configuration. Is to provide.

  As a result of intensive investigations to solve the above problems, the inventors of the present application, as a plasma wave having a uniform phase, for a stacked body in which P-type semiconductors and N-type semiconductors doped with impurities are alternately stacked. By irradiating an ultrashort pulse laser with a pulse width of 100 femtoseconds (fs) or less, coherent plasma oscillation (coherent plasmon) of carriers such as electrons or holes is generated for a long time inside the semiconductor. I observed that. As a result, it is considered that an electromagnetic wave (terahertz electromagnetic wave) having the same period as the period of the coherent plasmon is generated while the coherent plasmon continues.

  Accordingly, a terahertz electromagnetic wave generation source of the present invention is a terahertz electromagnetic wave generation source that generates a terahertz electromagnetic wave by irradiating an ultrashort pulse laser beam in order to solve the above-described problem, and includes a P-type semiconductor layer and an N-type semiconductor It has a semiconductor stacked structure in which at least three layers are alternately stacked.

  In the semiconductor stacked structure in which at least three P-type semiconductor layers and N-type semiconductor layers are alternately stacked as in the above configuration, at least one semiconductor layer has a polarity different from that of the semiconductor layer. The structure is sandwiched between the remaining semiconductor layers. When this semiconductor laminated structure is irradiated with an ultrashort pulse laser, electrons or holes doped as impurities in all the semiconductor layers are subjected to coherent plasma oscillation (coherent plasmon) at a frequency represented by the following formula according to the carrier concentration. )

In the above equation, ω _p is the plasma frequency, e is the elementary charge, n is the electron or hole concentration, ε is the dielectric constant of the semiconductor, and m ^* is the effective mass of the electron or hole.

  In the semiconductor stacked structure, of the stacked semiconductor layers, a semiconductor layer sandwiched between the semiconductor layers, that is, a semiconductor layer other than the two semiconductor layers on the outermost surface has a polarity different from that of the semiconductor layer. Since the electric field of an arbitrary magnitude is formed by being surrounded by the layers, the coherent plasmon of electrons or holes in the semiconductor layer is stabilized by this electric field.

  Here, the ultrashort pulse laser is a laser having a very short pulse whose width (time width) of one pulse is several picoseconds (ps) to several femtoseconds (fs). Therefore, when this ultrashort pulse laser is irradiated, the period of the coherent plasmon of electrons or holes in the semiconductor layer becomes very short terahertz. That is, the electromagnetic wave generated from this semiconductor laminated structure becomes a terahertz electromagnetic wave. Therefore, while coherent plasmons are sustained in the semiconductor multilayer structure, terahertz electromagnetic waves are continuously generated from the semiconductor multilayer structure.

  As described above, the terahertz electromagnetic wave generation source configured as described above can generate terahertz electromagnetic waves continuously for a long time with a simple element configuration in which P-type semiconductor layers and N-type semiconductor layers are alternately stacked. Yes.

  Here, the long time means a time longer than 2 picoseconds (ps) to 3 picoseconds (ps), which is a duration of generation of terahertz electromagnetic waves in the conventional pulsed laser irradiation method. According to the terahertz electromagnetic wave generation source configured as described above, it is possible to continuously generate the terahertz electromagnetic wave, for example, for 40 picoseconds (ps) to 100 picoseconds (ps).

  In addition, the period of coherent plasmon oscillation generated in the semiconductor layer varies depending on the concentration (doping concentration) of the impurity doped in the semiconductor layer. That is, if the concentration of impurities contained in the semiconductor layer increases, the number of electrons or holes that vibrate in the semiconductor layer increases, so the period of coherent plasmons is shortened, and terahertz electromagnetic waves on the higher frequency side are generated, On the contrary, if the impurity concentration is lowered, the number of electrons or holes that vibrate in the semiconductor layer is reduced, so that the period of coherent plasmons becomes longer, and a terahertz electromagnetic wave on the lower frequency side is generated.

  Therefore, in the semiconductor stacked structure as described above, the frequency of the terahertz electromagnetic wave can be easily adjusted only by adjusting the impurity doping concentration of the semiconductor layer.

  Further, in the semiconductor multilayer structure, terahertz electromagnetic waves are generated from the respective semiconductor layers. Therefore, if the number of stacked semiconductor layers is increased, the signal intensity of the terahertz electromagnetic waves is increased accordingly. Conversely, if the number of stacked semiconductor layers is small, the signal intensity of the terahertz electromagnetic wave is reduced accordingly.

  In particular, by making the impurity doping concentration in each semiconductor layer the same, the signal intensity of the terahertz electromagnetic wave having a frequency corresponding to the impurity doping concentration at this time can be adjusted only by the number of stacked semiconductor layers. That is, if the doping concentration of impurities in each semiconductor layer is adjusted so as to generate a terahertz electromagnetic wave having a desired frequency, the signal intensity of the terahertz electromagnetic wave having the desired frequency adjusts the number of stacked semiconductor layers. It becomes possible to carry out by the simple method.

  It is preferable that an I layer not doped with impurities is formed between the N-type semiconductor layer and the P-type semiconductor layer.

  As described above, an I layer that is not doped with impurities is formed between the N type semiconductor layer and the P type semiconductor layer. By adjusting the thickness of the I layer, the N type semiconductor layer is adjusted. The intensity of the electric field formed between the layer and the P-type semiconductor layer can be adjusted. That is, by increasing the thickness of the I layer, the strength of the electric field can be reduced, and by reducing the thickness of the I layer, the strength of the electric field can be increased.

  As a result, if the thickness of the I layer is increased, the intensity of the above-described electric field is weakened, so that it is difficult to stably maintain the vibration of electrons or holes, which occurs in the N-type semiconductor layer or the P-type semiconductor layer. The duration of the coherent plasmon is shortened. On the other hand, if the thickness of the I layer is reduced, the intensity of the electric field is increased, so that the vibration of electrons or holes can be stably maintained. As a result, it occurs in the N-type semiconductor layer or the P-type semiconductor layer. The duration of the coherent plasmons you are doing is longer. As described above, it is possible to easily adjust the duration of coherent plasmon only by adjusting the thickness of the I layer formed between the N-type semiconductor layer and the P-type semiconductor layer.

  Therefore, it is possible to easily adjust the generation duration of the terahertz electromagnetic wave simply by adjusting the thickness of the I layer.

  Each of the N-type semiconductor layer, the I-layer, and the P-type semiconductor layer is preferably formed of the same semiconductor material.

  As described above, each of the N-type semiconductor layer, the I-layer, and the P-type semiconductor layer is formed of the same semiconductor material, so that there is no need to switch the semiconductor material when the semiconductor layers are stacked. In addition, since the N-type semiconductor layer and the P-type semiconductor layer are formed using the same semiconductor material, impurities can be easily doped into the semiconductor layer.

  The terahertz electromagnetic wave generation device of the present invention is characterized by including the terahertz electromagnetic wave generation source having the above-described configuration and a laser irradiation device that irradiates the terahertz electromagnetic wave source with an ultrashort pulse laser.

  Thereby, the terahertz electromagnetic wave generator which can generate a terahertz electromagnetic wave for a long time with a simple configuration can be realized.

  The wavelength of the ultrashort pulse laser is preferably shorter than the band gap energy of the semiconductor layer constituting the terahertz electromagnetic wave generation source.

  Since the wavelength of the ultrashort pulse laser is shorter than the band gap energy of the semiconductor layer constituting the terahertz electromagnetic wave generation source, carriers can be generated by optical transition, and the terahertz electromagnetic wave generation source can reliably Terahertz electromagnetic waves can be generated.

  In the laser irradiation apparatus, the pulse width of the ultrashort pulse laser is equal to or less than one cycle on the low frequency side of the frequency indicating two types of peaks of the terahertz electromagnetic wave generated from the terahertz electromagnetic wave generation source, and 1 on the high frequency side. It is preferable to set a value longer than the period.

  In this case, the pulse width of the ultrashort pulse laser is equal to or less than one cycle on the low frequency side of the frequency showing two types of peaks of the terahertz electromagnetic wave generated from the terahertz electromagnetic wave generation source 101 and from one cycle on the high frequency side. By setting the value to a long value, the peak on the high frequency side of the terahertz electromagnetic wave is not detected, and only the peak on the low frequency side is detected.

  Therefore, if necessary, the detection of the high-frequency peak in the terahertz electromagnetic wave generated from the terahertz electromagnetic wave source is suppressed by a simple method of adjusting the pulse width of the ultrashort pulse laser, and only the low-frequency peak is detected. Can be detected.

  The terahertz electromagnetic wave generation source is preferably cooled to 130 k (Kelvin) or less.

  As described above, the terahertz electromagnetic wave generation source can cool the coherent plasmon for a long time by being cooled to 130 k (Kelvin) or less.

  In addition, the lower the cooling temperature of the terahertz electromagnetic wave generation source, the more stable the coherent plasmon is generated and the longer the duration.

  The terahertz electromagnetic wave generation source of the present invention is a terahertz electromagnetic wave generation source that generates a terahertz electromagnetic wave by irradiating an ultrashort pulse laser, and at least three layers of P-type semiconductor layers and N-type semiconductor layers are alternately stacked. This is a configuration having a semiconductor stacked structure. This produces an effect that the terahertz electromagnetic wave can be generated for a long time with a simple configuration.

It is a schematic block diagram of a terahertz electromagnetic wave generation source according to an embodiment of the present invention. It is a schematic sectional drawing of the nip laminated body in the terahertz electromagnetic wave generation source shown in FIG. It is a graph which shows the relationship between reflectance change and time obtained by using the nipi laminated body shown in FIG. 2 as a semiconductor sample. 3 is a graph showing the relationship between signal intensity and frequency obtained using the nip laminate shown in FIG. 2 as a semiconductor sample. It is a graph which shows the relationship between reflectance change and time obtained by using the nipi laminated body shown in FIG. 2 as a semiconductor sample. It is a schematic block diagram of the nondestructive inspection apparatus provided with the terahertz electromagnetic wave generator which has the terahertz electromagnetic wave generation source shown in FIG.

  An embodiment according to the present invention will be described as follows.

  The terahertz electromagnetic wave generation source according to the present embodiment will be described below with reference to FIGS.

<Terahertz electromagnetic wave source structure>
As shown in FIG. 1, the terahertz electromagnetic wave generation source 101 has a structure in which a nip stack 110 in which three or more semiconductor layers made of GaAs are stacked is stacked on a Si substrate 130 via a buffer layer 120. .

  In the terahertz electromagnetic wave generation source 101, the surface opposite to the contact surface with the buffer layer 120 in the nip laminate 110 is a pulse laser incident surface 101 a, and the contact surface with the buffer layer 120 in the Si substrate 130. The surface on the opposite side is a terahertz electromagnetic wave emitting surface 101b.

  In the above configuration, the essential structure for generation of the terahertz electromagnetic wave is the nip laminate 110. Moreover, the nipi laminate 110 can generate the terahertz electromagnetic wave continuously for a long time compared to the conventional terahertz electromagnetic wave generation source (the semiconductor layer has a single layer structure). This mechanism will be described below.

<Nipi laminate>
The nip stack 110 is formed by epitaxially growing GaAs, which is a semiconductor material, on the buffer layer 120. In the present embodiment, the nip laminate 110 is
As shown in FIG. 2, 20 sets are formed by laminating four layers of an N-type semiconductor layer 1, an I layer 3, a P-type semiconductor layer 2, and an I layer 3.

  The N-type semiconductor layer 1 is formed by doping Be as an impurity into GaAs epitaxially grown to a layer thickness of 10 nm. Thereby, in the N-type semiconductor layer 1, when an ultrashort pulse laser is irradiated from the outside, electrons are diffused inside to generate coherent plasma oscillation (coherent plasmon).

  The P-type semiconductor layer 2 is formed by doping Si as an impurity into GaAs epitaxially grown to a layer thickness of 10 nm. Thereby, in the P-type semiconductor layer 2, when an ultrashort pulse laser is irradiated from the outside, holes are diffused inside to generate coherent plasma oscillation (coherent plasmon).

  The I layer 3 is a GaAs layer epitaxially grown to a thickness of 10 nm on the N-type semiconductor layer 1 or the P-type semiconductor layer 2, but is not doped with impurities. The I layer 3 is formed when switching from the N-type semiconductor layer 1 to the P-type semiconductor layer 2 or when switching from the P-type semiconductor layer 2 to the N-type semiconductor layer 1. That is, the I layer 3 is formed while switching the type of impurities doped in the semiconductor layer. Therefore, the thickness of the I layer 3 can be adjusted or eliminated by adjusting the switching time.

  In the nip stacked body 110 having the above-described configuration, 20 sets are stacked with the four layers of the N-type semiconductor layer 1, the I-layer 3, the P-type semiconductor layer 2, and the I-layer 3 as one set. The N-type semiconductor layer 1 or the P-type semiconductor layer 2 other than the semiconductor layer 1 or the P-type semiconductor layer 2 has a structure sandwiched between semiconductor layers having different polarities.

  For example, the N-type semiconductor layer 1 is sandwiched between the P-type semiconductor layers 2, and conversely, the P-type semiconductor layer 2 is sandwiched between the N-type semiconductor layers 1. For this reason, the electron layer inside the N-type semiconductor layer 1 is sandwiched between the hole layers of the two P-type semiconductor layers 2, and a certain electric field strength is formed between the P-type semiconductor layers 2. An electric field having the following characteristic is formed, and electrons can stably oscillate in the electric field. That is, the coherent plasmon duration due to the diffusion of electrons inside the N-type semiconductor layer 1 can be increased. Similarly, the hole layer inside the P-type semiconductor layer 2 is sandwiched between two electron layers of the N-type semiconductor layer 1, and a certain electric field strength is formed between the N-type semiconductor layers 1. Thus, holes can be stably vibrated in the electric field. That is, the coherent plasmon duration due to the diffusion of holes inside the P-type semiconductor layer 2 can be increased.

  By irradiating the nipi stack 110 with an ultrashort pulse laser having a pulse width of about 100 femtoseconds or less as a plasma wave having the same phase, the period of the coherent plasmon is in the terahertz band (30 μm to 3 mm wavelength). Is equal to the period. Therefore, it means that terahertz electromagnetic waves are generated from the nip laminate 110.

  Here, the period of coherent plasmon changes according to the impurity concentration (doping concentration) doped in the N-type semiconductor layer 1 and the P-type semiconductor layer 2. If the impurity concentration is high, the movement of electrons or holes in the semiconductor layer becomes active, the period of coherent plasmons is shortened, and if the impurity concentration is low, the movement of electrons or holes in the semiconductor layer is reduced. Since the period of coherent plasmon becomes long, the frequency of the terahertz electromagnetic wave increases when the impurity concentration is increased, and the frequency of the terahertz electromagnetic wave decreases when the impurity concentration is decreased. Therefore, the frequency of the terahertz electromagnetic wave can be easily adjusted by changing the impurity concentration of the semiconductor layer.

  Here, in the nip laminated body 110 having the above configuration, coherent plasma oscillation (coherent plasmon) is performed at a frequency represented by the following formula.

In the above equation, ω _p is the plasma frequency (frequency of the terahertz electromagnetic wave), e is the elementary charge, n is the electron or hole concentration, ε is the dielectric constant of the semiconductor, and m ^* is the effective mass of the electron or hole. is there.

  Therefore, it can be seen from the above formula that the frequency of the terahertz electromagnetic wave increases as the value of n indicating the electron or hole concentration in the N-type semiconductor layer 1 and the P-type semiconductor layer 2 increases.

  In addition, since the terahertz electromagnetic wave is generated from the semiconductor layer sandwiched between the semiconductor layers in the nip multilayer body 110 as described above, the signal intensity of the terahertz electromagnetic wave is increased as much as the number of stacked semiconductor layers is increased. Can do.

  Therefore, in order to set the signal intensity of the terahertz electromagnetic wave to a necessary value, it is only necessary to adjust the number of stacked semiconductor layers in the nip stacked body 110.

  Further, according to the terahertz electromagnetic wave generation source 101 having the above-described configuration, the coherent plasmon vibration is stably maintained in the semiconductor layer, so that the generated terahertz electromagnetic wave has high monochromaticity.

<Each condition for generation of terahertz electromagnetic waves>
In order to generate a terahertz electromagnetic wave, an ultrashort pulse laser that irradiates the terahertz electromagnetic wave generation source 101 will be discussed.

  Here, as the semiconductor sample, the above-described nip stack 110 is used, and the thickness of the buffer layer 120 is 500 nm. Incidentally, the layer thickness of the nip multilayer body 110 is (10 + 10 + 10 + 10) × 20 = 800 nm as 20 sets of four layers of the N-type semiconductor layer 1, the I-layer 3, the P-type semiconductor layer 2, and the I-layer 3.

The doping concentration in the N-type semiconductor layer 1 and the P-type semiconductor layer 2 in the nip stack 110 is set to 3.0 × 10 ¹⁷ cm ⁻³ .

  Furthermore, the environmental temperature for measuring coherent plasmon vibration in the semiconductor sample is set to 8 k (Kelvin).

The excitation photon energy is 1.520 ev and the excitation intensity is 0.028 to 0.552 uJ / cm ² .

  The pulse laser to be irradiated is an ultrashort pulse laser having a pulse width of 100 femtoseconds or less, and the semiconductor sample is irradiated by changing the output value to three types of 1 mW, 10 mW, and 20 mW.

  The relationship between the reflectance of the pulse laser by the semiconductor sample and the time obtained as a result is the graph shown in FIG.

  From the results shown in FIG. 3, it can be seen that when a pulse laser with output values of 10 mW and 20 mW is irradiated, the reflectance change from the semiconductor sample continues for 40 psec or more. However, the degree of amplitude of the change in reflectance increases as the output value of the pulse laser increases. That is, the degree of amplitude of the change in reflectance when a pulse laser with an output value of 20 mW is used is greater than the degree of amplitude of the change in reflectance when a pulse laser with an output value of 10 mW is used. . This is because electrons or holes in the semiconductor layer vibrate more actively as the output value of the pulse laser increases.

  From the results shown in FIG. 3, the degree of amplitude of the change in reflectivity when a pulse laser with an output value of 1 mW is used is hardly seen. That is, it can be seen that with a pulse laser with an output value of 1 mW, coherent plasmons that can generate terahertz electromagnetic waves cannot be obtained, or even if they are generated, they cannot be detected by a detector. This is clear from the results shown in FIG.

  FIG. 4 is a graph showing the relationship between signal intensity and frequency between 1.0 and 100 psec. In FIG. 4, the result of examining the signal intensity of each frequency by Fourier transforming the reflectance in the graph shown in FIG. 3 is shown as a graph. FIG. 4 shows only the peak (low frequency side) of the terahertz electromagnetic wave caused by the P-type semiconductor layer.

  From the results shown in FIG. 4, when the output value of the pulse laser is 10 mW and 20 mW, a signal intensity peak is observed between 1.0 and 2.0 THz, but when the output value of the pulse laser is 1 mW, the signal intensity peak is obtained. It can be seen that is hardly seen.

  As described above, from FIG. 3 and FIG. 4, in order to generate a terahertz electromagnetic wave, an output value of 1 mW is insufficient as an ultrashort pulse laser to be irradiated to a terahertz electromagnetic wave generation source, and a value larger than 1 mW is required. In particular, it was found that a value of 10 mW or more is necessary.

  When the semiconductor sample at room temperature is irradiated with an ultrashort pulse laser, coherent plasmon can be generated, but the duration is extremely short. On the other hand, when the ultrashort pulse laser is irradiated in a state where the semiconductor sample is cooled to a temperature lower than room temperature, the duration of the coherent plasmon is extended. This is because the lower the temperature of the semiconductor, the more the carrier scattering due to lattice vibration is suppressed, and the carrier scattering due to impurities and lattice defects that are not ionized can be suppressed. Therefore, in order to maintain the coherent plasmon for a long time, it is necessary to irradiate the ultrashort pulse laser with the semiconductor sample cooled as much as possible.

  FIG. 5 is a graph showing the relationship between the reflectance of the pulse laser by the semiconductor sample and time due to the difference in temperature of the semiconductor sample.

  From the result shown in FIG. 5, it can be seen that when the temperature of the terahertz electromagnetic wave generation source is lower than 130 k (Kelvin), the coherent plasmon lasts for a long time of 40 psec or more. As described above, in the terahertz electromagnetic wave generation source (semiconductor sample) having the above configuration, in order to generate the terahertz electromagnetic wave for a long time, the temperature of the terahertz electromagnetic wave generation source (semiconductor sample) needs to be considerably lower than room temperature. It turns out that it is.

The following three types of cooling methods for the semiconductor sample are conceivable. The semiconductor sample can be cooled to 200 k (Kelvin) or less by any of the following cooling methods (1) to (3).
(1) Use a fan. In this case, the wind generated by the fan is applied to the semiconductor sample. In this method, it can cool to about 200 k (Kelvin).
(2) Use liquid nitrogen. In this case, the semiconductor sample is covered with liquid nitrogen. In this method, it can be cooled to about 77 k (Kelvin).
(3) A Peltier element is used. In this case, the Peltier device is brought into contact with the semiconductor sample and cooled. In this method, it is possible to cool to 150 k (Kelvin).

  Although any of the above cooling methods can generate carrier coherent plasmons in the terahertz electromagnetic wave generation source 101, the temperature of the terahertz electromagnetic wave generation source 101 is 130 k (Kelvin) or less in order to maintain the coherent plasmons for a long time. It is preferable to cool it down. Therefore, the method (2) is preferable as the cooling method.

<Description of each effect by the terahertz electromagnetic wave generation source configured as described above>
In the terahertz electromagnetic wave generation source 101 having the above-described configuration, the case where GaAs (gallium arsenide) is used as a semiconductor material for both the N-type semiconductor layer 1 and the P-type semiconductor layer 2 constituting the nip stack 110 has been described. In addition to GaAs, InAs, GaSb, and InSb can be used.

  Here, regardless of which semiconductor material is used, if the nipi stack 110 is irradiated with an ultrashort pulse laser having a wavelength shorter than the band gap energy of the semiconductor material, the coherent plasmon vibrates for a long time. As a result, terahertz electromagnetic waves can be generated for a long time.

  At this time, if the band gap energy of the semiconductor material is the same as the wavelength of the ultrashort pulse laser, the loss of carriers can be suppressed by suppressing the in-band relaxation due to lattice vibration. As a result, the coherent plasmon The vibration intensity increases. That is, it is preferable to use an ultrashort pulse laser having a wavelength as close as possible to the band gap energy of the semiconductor material.

  Therefore, in this embodiment using GaAs as the semiconductor material, titanium sapphire having an ultrashort pulse laser wavelength of 800 nm is used as the laser source. As the semiconductor material, InAs, GaSb, and InSb, which have a lighter effective mass than GaAs, have a lower band gap energy than GaAs. Therefore, using an ultrashort pulse laser having a wavelength longer than 800 nm can reduce the vibration intensity of coherent plasmons. Necessary to secure.

  In the terahertz electromagnetic wave generation source 101, the N-type semiconductor layer 1, the P-type semiconductor layer 2, and the I layer 3 all use the same GaAs. That is, the same semiconductor material is used. Thus, when the same semiconductor material is used, it is not necessary to change the semiconductor material when stacking semiconductor layers in the manufacturing process. Further, if the N-type semiconductor layer 1 and the P-type semiconductor layer 2 are the same semiconductor material, there is also an advantage that the impurity with a required concentration can be easily doped.

  Further, in the terahertz electromagnetic wave generation source 101 having the above-described configuration, the I layer 3 is provided between the N-type semiconductor layer 1 and the P-type semiconductor layer 2 in the nip laminate 110. Although the I layer 3 is not an essential component as described above, the duration of the coherent plasmon can be easily adjusted by adjusting the thickness of the I layer 3.

  For example, if the thickness of the I layer 3 is reduced, the electric field formed between the N-type semiconductor layer 1 and the P-type semiconductor layer 2 becomes stronger, so that the vibration of electrons or holes can be stably maintained. it can. That is, as the thickness of the I layer 3 becomes thinner, the electric field becomes stronger, and the vibration of electrons or holes is stably maintained, so that the duration of the coherent plasmon becomes longer.

  On the other hand, if the thickness of the I layer 3 is increased, the electric field formed between the N-type semiconductor layer 1 and the P-type semiconductor layer 2 is weakened, so that the vibration of electrons or holes is stably maintained. It becomes difficult to let you. That is, as the thickness of the I layer 3 is increased, the electric field is weakened, and the oscillation of electrons or holes is not stably maintained, so that the coherent plasmon duration is shortened.

  For example, in the nip laminate 110, the thickness of the I layer 3 is 10 nm. However, when the thickness is 30 nm, the electric field formed between the N-type semiconductor layer 1 and the P-type semiconductor layer 2 is weakened. The Coulomb force acting on the carriers of the N-type semiconductor layer 1 and the P-type semiconductor layer 2 is about 1/10 of the Coulomb force when the thickness of the I layer 3 is 10 nm. As a result, the vibration of electrons or holes is stabilized. The duration of coherent plasmons is shortened because they no longer last.

  Furthermore, if the thickness of the I layer 3 is increased to 300 nm, the Coulomb force acting on the carriers of the N-type semiconductor layer 1 and the P-type semiconductor layer 2 becomes about 1/100 of the Coulomb force when the thickness of the I layer 3 is 10 nm. The vibration of electrons or holes becomes unstable, and the duration of coherent plasmons becomes extremely short.

  Therefore, if the thickness of the I layer 3 in the nip laminate 110 is made as thick as 300 nm, the effect of sustaining coherent plasmons may not be obtained, so the thickness of the I layer 3 is made thinner than 300 nm. Is preferred.

<Non-destructive inspection equipment>
A nondestructive inspection apparatus using the terahertz electromagnetic wave generation source 101 having the above configuration will be described below with reference to FIG.

  As shown in FIG. 6, the nondestructive inspection apparatus includes a terahertz electromagnetic wave generator 100 and a detector 200 arranged in the direction in which the terahertz electromagnetic wave is emitted from the terahertz electromagnetic wave generator 100. This is an apparatus that performs a destructive inspection by irradiating a test object 300 disposed between the generator 100 and the detector 200 with a terahertz electromagnetic wave.

  The terahertz electromagnetic wave generation apparatus 100 includes the above-described terahertz electromagnetic wave generation source 101 and a laser apparatus 102 that generates a pulse laser that irradiates the terahertz electromagnetic wave generation source 101.

  The laser device 102 generates an ultrashort pulse laser having a pulse width of 100 femtoseconds or less.

  Since the terahertz electromagnetic wave generation device 100 having the above-described configuration includes the nip stack 110 in the terahertz electromagnetic wave generation source 101, the terahertz electromagnetic wave is continuously generated for 40 picoseconds (ps) to 100 picoseconds (ps). be able to.

  Here, in order to avoid interference of terahertz electromagnetic waves, it is necessary to set the irradiation interval of the ultrashort pulse laser to 12.5 nanoseconds (nsec). For this reason, since the duration of generation of terahertz electromagnetic waves is as short as 2 picoseconds (ps) to 3 picoseconds (ps) by one pulse of an ultrashort pulse laser as in the conventional case, the laser irradiation interval is 12.5 nanometers. During the second (ns), the terahertz electromagnetic wave is generated only for 2 to 3 periods.

  However, according to the terahertz electromagnetic wave generating apparatus 100 having the above-described configuration, the generation duration of the terahertz electromagnetic wave is as long as 40 picoseconds (ps) to 100 picoseconds (ps) by one pulse of the ultrashort pulse laser. Terahertz electromagnetic waves are generated for 40 to 100 cycles or more during 12.5 nanoseconds.

  Therefore, it takes a very long time to obtain a necessary amount of terahertz electromagnetic waves. In other words, as in the present invention, when the terahertz electromagnetic wave is generated for 40 to 100 periods in 12.5 nanoseconds (ns), the time required for the inspection and observation using the terahertz electromagnetic wave becomes longer. Arise. For example, the observation time by the terahertz electromagnetic wave can be significantly shortened as compared with the case where the terahertz electromagnetic wave is generated only for 2 to 3 periods during the laser irradiation interval of 12.5 nanoseconds (ns).

  Thereby, according to said nondestructive inspection apparatus, since it becomes possible to continuously irradiate the to-be-inspected object 300 with the terahertz electromagnetic wave for a long time, the inspection time with respect to the to-be-inspected object can be shortened.

  In the nondestructive inspection apparatus, the frequency of the terahertz electromagnetic wave according to the type of the object to be inspected 300 can be easily changed. For example, a terahertz electromagnetic wave generation source 101 that generates a terahertz electromagnetic wave having a frequency corresponding to the type of the object to be inspected is prepared, and the terahertz electromagnetic wave generation device 100 uses the terahertz electromagnetic wave generation source 101 corresponding to the type of the object 300 to be inspected. Replace it.

<Terahertz electromagnetic wave peak detection suppression on the high frequency side>
In the terahertz electromagnetic wave generation device 100, the terahertz electromagnetic wave generated from the terahertz electromagnetic wave generation source 101 has peaks detected on the low frequency side and the high frequency side. This is due to the difference in carriers in the N-type semiconductor layer 1 and the P-type semiconductor layer 2. In other words, since the effective mass of electrons that are carriers of the N-type semiconductor layer 1 is smaller than that of holes that are carriers of the P-type semiconductor layer 2, the peak of the terahertz electromagnetic wave generated from the N-type semiconductor layer 1 is detected on the high frequency side. On the contrary, the peak of the terahertz electromagnetic wave generated from the P-type semiconductor layer 2 is detected on the low frequency side.

  Here, if the pulse width of the ultrashort pulse laser irradiated to the terahertz electromagnetic wave generation source 101 is narrower than one period at the peak frequency on the high frequency side, two peaks on the low frequency side and the high frequency side can be detected. . On the other hand, if the pulse width of the ultrashort pulse laser applied to the terahertz electromagnetic wave generation source 101 is wider than one period at the peak frequency on the high frequency side, the peak on the high frequency side cannot be detected. Only the peak on the low frequency side is detected.

  Therefore, when detecting only the low frequency side peak among the terahertz electromagnetic waves generated from the terahertz electromagnetic wave generation source 101, the pulse width of the ultrashort pulse laser is set to be the same as that of the terahertz electromagnetic wave generated from the terahertz electromagnetic wave generation source 101. It is set to a value not longer than one cycle on the low frequency side of the frequency showing two types of peaks and longer than one cycle on the high frequency side. This setting is performed in the laser device 102.

  As described above, by adjusting the pulse width of the ultrashort pulse laser as necessary, the peak on the high frequency side of the terahertz electromagnetic wave generated from the terahertz electromagnetic wave generation source 101 may be detected or may not be detected. it can.

  The present invention is not limited to the above-described embodiments, and various modifications can be made within the scope shown in the claims, and embodiments obtained by appropriately combining technical means disclosed in different embodiments. Is also included in the technical scope of the present invention.

  INDUSTRIAL APPLICABILITY The present invention is suitable for fields and devices that use terahertz electromagnetic waves, and is particularly useful as an alternative light beam for X-rays, so that it can be used for the field of observing the internal structure of substances that do not contain water. .

DESCRIPTION OF SYMBOLS 1 N type semiconductor layer 2 P type semiconductor layer 3 I layer 100 Terahertz electromagnetic wave generator 101 Terahertz electromagnetic wave source 102 Laser apparatus 110 nipi laminated body (semiconductor laminated structure)
120 Buffer layer 130 Si substrate 200 Detector 300 Inspected object

Claims (7)

A terahertz electromagnetic wave source that generates a terahertz electromagnetic wave by irradiating an ultrashort pulse laser,
A terahertz electromagnetic wave generation source having a semiconductor stacked structure in which at least three N-type semiconductor layers and P-type semiconductor layers doped with impurities are alternately stacked.   2. The terahertz electromagnetic wave generation source according to claim 1, wherein an I layer not doped with impurities is formed between the N-type semiconductor layer and the P-type semiconductor layer.   The terahertz electromagnetic wave generation source according to claim 2, wherein each of the N-type semiconductor layer, the I-layer, and the P-type semiconductor layer is formed of the same semiconductor material. The terahertz electromagnetic wave generation source according to any one of claims 1 to 3,
A terahertz electromagnetic wave generation apparatus comprising: a laser irradiation apparatus that irradiates the terahertz electromagnetic wave source with an ultrashort pulse laser.   5. The terahertz electromagnetic wave generation device according to claim 4, wherein a wavelength of the ultrashort pulse laser is shorter than a band gap energy of a semiconductor layer constituting the terahertz electromagnetic wave generation source. The laser irradiation device is
The pulse width of the ultrashort pulse laser is set to a value not longer than one cycle on the low frequency side and longer than one cycle on the high frequency side, showing the two types of peaks of the terahertz electromagnetic wave generated from the terahertz electromagnetic wave generation source. The terahertz electromagnetic wave generator according to claim 4, wherein the terahertz electromagnetic wave generator is set.   The terahertz electromagnetic wave generation device according to any one of claims 4 to 6, wherein the terahertz electromagnetic wave generation source is cooled to 130 k (Kelvin) or less.

Images