JP6554641B2 - Terahertz oscillator - Google Patents

Description

  The present invention relates to electromagnetic waves in the terahertz region.

  The application of electromagnetic waves in the terahertz region is attracting attention as an evaluation diffraction technique.

  The application of terahertz waves has been studied because of the merit of identifying the organic matter (explosives, poisons, etc.) and identifying the morphology of the structure without destruction.

  On the other hand, research on devices suitable for terahertz wave oscillation itself is also being conducted in parallel (Non-Patent Document 1).

  The terahertz wave oscillation includes a superconducting material (Patent Document 1) and a plurality of antenna structures (Patent Document 2).

JP-A-2005-251863 JP2012-044688

“Current Status and Prospects of Terahertz Wave Technology”, Masayoshi Tonouchi, Applied Physics Vol. 75, No. 2, page 160 (2006) Kyoko Nakada and Mitsutaka Fujita, Gene Dresselhaus and Mildred S. Dresselhaus, Phys. Rev. B54, 17954 (1996) Young-Woo Son, Marvin L. Cohen, and Steven G. Louie, Phys. Rev. Lett. Vol. 97, ref. 216803 (2003) Hong Zhang and Yoshiyuki Miyamoto, Appl. Phys. Lett. 95, 053109 (2009) Susumu Okada, Phys. Rev. B Vol.77, 041408 (R) (2008) Xiaolin Li, Xinran Wang, Li Zhiang, Sangwon Lee, and Hongjie Dai, Science Vol.319, 1229 (2008).

  A material using a superconducting material is required to be precisely processed in the element structure and bonding with an electrode.

  On the other hand, an element having a plurality of antenna structures has a complex structure that realizes terahertz oscillation through the oscillation and feeding of electromagnetic waves between antennas, which are also highly difficult to process accurately and remove internal resistance. Processing is required.

  Terahertz waves oscillate when an ultraviolet laser is irradiated through a graphene nanoribbon film provided on a semiconductor exhibiting photoconductivity, and the intensity of the ultraviolet light is modulated by the film at a frequency of terahertz, so that a current that is strongly modulated is passed through the semiconductor. .

(1)
An oscillation element comprising a photoconductive semiconductor capable of producing electrical conduction characteristics when irradiated with light, and a light intensity modulation material capable of periodically modulating the intensity of emitted light when incident on the light,
The oscillation element characterized in that the photoconductive semiconductor has a length of micron order in the longitudinal direction.
(2)
A voltage is applied in the longitudinal direction of the photoconductive semiconductor,
When light is irradiated on the surface of the light intensity modulating substance,
The light whose intensity is periodically modulated from the back surface of the light intensity modulating material is applied to the photoconductive semiconductor, the modulated current flows, and the photoconductive semiconductor oscillates a terahertz wave. The oscillation element according to (1).

(3)
The light intensity modulation material is an armchair graphene nanoribbon in which carbon atoms at the width ends are terminated with hydrogen atoms,
The oscillation element according to (2), wherein the light is ultraviolet light.
(4)
The oscillation element according to (3), wherein the ultraviolet light is linearly polarized, and is irradiated with being polarized vertically to the longitudinal direction of the graphene nanoribbon and perpendicular to the width direction of the graphene nanoribbon .

  The present invention can provide a terahertz antenna that realizes oscillation of a terahertz radiation field by making a semiconductor exhibiting photoconductivity into an antenna shape and passing a current modulated at a terahertz frequency through the antenna.

(5)
A terahertz antenna including the oscillation element described in (4),
The photoconductive semiconductor has a rectangular or circular cross section, a rod shape, a ring shape, or an U shape, and has an antenna shape with a part of a straight line,
The terahertz antenna, wherein the graphene nanoribbon is disposed in proximity to a surface of the partially linear portion of the photoconductive semiconductor.
(6)
The graphene nanoribbon is characterized in that a graphite piece solution is applied to the surface of the partially linear portion of the photoconductive semiconductor and dried, or pasted or transferred to be adjacently disposed (5 ) Terahertz antenna.
(7)
The terahertz antenna according to (6), wherein the graphene nanoribbons are mechanically rubbed in the partial linear direction before being dried after the application, and are arranged close to each other in a strip shape.

  Using an armchair graphene nanoribbon, a terahertz oscillation device with a simple structure and easy manufacture was successfully fabricated.

The conceptual diagram of irradiating a circuit containing a photoconductive semiconductor with an ultraviolet-ray. FIG. 4 is a conceptual diagram showing time dependence of ultraviolet intensity in the upper stage and time dependence of photovoltaic current in the lower stage. The conceptual diagram of a mode that a ultraviolet-ray is irradiated to a photoconductive semiconductor via a light intensity modulation substance. The light whose intensity is modulated at a certain period and the time change of the current flowing in the circuit of FIG. 3 when the light is irradiated are shown, the upper part shows the light intensity and the lower part shows the current value. Time is an arbitrary unit, but it is common to the upper and lower stages. FIG. 4 is a conceptual diagram showing a state in which a terahertz wave is transmitted from an antenna when the photoconductive semiconductor is an antenna of several microns in length and the photocurrent modulation period is terahertz in the configuration of FIG. 3. Example of armchair graphene nanoribbon structure (N = 7). A thick bidirectional arrow indicates the polarization direction of the electric field of the irradiated ultraviolet light. The graph which shows the modulation | alteration of the electric field strength of the graphene nanoribbon vicinity generated by ultraviolet irradiation of light energy 6.20eV (upper stage) and 6.53eV (lower stage). The conceptual diagram of a structure of a nanoribbon graphene film. Each rectangle with different gray values represents an armchair nanoribbon with a different width. FIG. 6 is a mounting diagram of the circuit configuration of FIG. 5. In the figure, the length of the linear antenna including the light intensity modulating material is on the order of μ. FIG. 6 is another mounting diagram of the circuit configuration of FIG. 5. The length of the straight portion including the light intensity modulation material of the U-shaped antenna in the figure is μ order. FIG. 6 is still another mounting diagram of the circuit configuration of FIG. 5. The diameter of the U-shaped curved antenna in the figure is μ order.

The principle of obtaining THz oscillation from an ultraviolet laser will be described below.
First, a semiconductor exhibiting photoconduction is formed.

A semiconductor itself is not a material having electrical conductivity, but is a material that exhibits conductivity characteristics by doping with impurities.
Even in the case where impurities are not doped, a semiconductor that exhibits conduction characteristics by generating electrons and holes in the semiconductor by irradiating light is called a photoconductive semiconductor in the present invention.

Consider a structure in which ultraviolet rays are applied to a circuit configuration of a photoconductive semiconductor as shown in FIG.
In the circuit configuration of FIG. 1, on / off of the amount of current flowing through the circuit is determined by on / off of ultraviolet irradiation.
FIG. 2 shows this as time dependence of ultraviolet irradiation.

Next, consider a case where the photoconductive semiconductor is irradiated with ultraviolet rays via a material having photoelectric field modulation characteristics.
FIG. 3 shows a circuit configuration at that time.

Here, the light intensity modulation substance will be described.
When a material is irradiated with light, the light passes through the material and usually only decreases its intensity.

  However, when light passes through the material, the electron trajectory inside the material is modulated by the energy of the light, and when the modulation occurs in many electron orbits, the modulated electric field is generated inside the material and in the vicinity of the material surface. Occur.

  When superposed on the electric field of the incident light, the amplitude intensity of the electric field near the surface of the material may be modulated.

  In the present invention, a material having a strong property of modulating incident light by such a mechanism is called a light intensity modulating substance.

This mechanism can be simulated by first-principles calculations considering quantum mechanical electron behavior.
Using the time-dependent Schrödinger equation, the motion of electrons in a material can be calculated numerically with a photoelectric field, allowing the calculation of electric field modulation near the surface of the material by the mechanism described above. It is possible to examine the photoelectric field modulation characteristics.

In the present invention, a material having a property that intensity modulation is periodic is selected as the light intensity modulating substance.
Therefore, the current intensity flowing through the circuit is also modulated with the same period. This is shown in FIG.
When the modulation frequency in FIG. 4 is terahertz, if the photoconductive semiconductor portion is set to a length of several μm equivalent to the terahertz wavelength, a terahertz wave having a frequency due to current modulation is oscillated.
This is shown in FIG.

The photoconductive semiconductor functions as an antenna.
The definition of an antenna is as follows. The current flowing through the photoconductive semiconductor is preferably an alternating current, but it is sufficient for the periodic modulation of the current in one direction to oscillate the terahertz radiation field.

In the present invention, graphene nanoribbons are used as a material exhibiting light modulation characteristics. The reason why the graphene nanoribbon is used is that the effect by the principle described above is remarkable, and since it is a film-like substance, it can be easily applied to a photoconductive semiconductor.
In addition to application, a method of pasting or transferring may be used. The graphene nanoribbon will be described below.

  The graphene nanoribbon is a ribbon-like material obtained by cutting graphene with a finite width, and the width of the ribbon is on the order of nanometers. Here, graphene refers to a two-dimensional material composed of one atomic layer of graphite, which is a layered material.

Graphene nanoribbon structures are roughly divided into two types, called armchair type and zigzag type, respectively.
Although the structure is shown in FIG. 1 of Non-Patent Document 2, the structure is shown in FIG. 6 because only the graphene nanoribbon of the armchair is used in the present invention.

  The feature of the armchair graphene nanoribbon in FIG. 6 is that the carbon atoms at the ends are terminated with hydrogen atoms, and three chemical bond arms come out from all the carbon atoms constituting the nanoribbon. The direction of the CC bond consisting of carbon atoms is parallel to the longitudinal direction of the ribbon, and it has a structure like an armrest of a chair.

Also, the width of the armchair nanoribbon is defined. In the case of FIG. 6, there are a total of seven CC bonds parallel to the longitudinal direction of the ribbon per ribbon width, so N = 7 armchair nanoribbon. Call it.
This designation is based on Non-Patent Document 3.

  In the structure of FIG. 6, when ultraviolet light polarized perpendicular to the longitudinal direction of the ribbon is irradiated, the intensity of the ultraviolet light is modulated in the vicinity of the nanoribbon film.

The relationship between the irradiation electric field and the electric field generated near the nanotube is shown in FIG.
The result of FIG. 7 is the first knowledge obtained by executing the simulation by the first principle calculation described above.

  The photoelectric field enhancement effect of such graphene nanoribbons has not been known so far, but for the first time, the above-mentioned understanding of the mechanism by which the amplitude intensity of the electric field near the material surface is modulated and the simulation method based on first-principles calculations It became clear.

However, as described in the explanation of FIG. 6, the ultraviolet rays are linearly polarized , and the fact that they are perpendicularly polarized in the longitudinal direction of the ribbon and in the width direction of the ribbon obtains photoelectric field modulation as shown in FIG. 7. It becomes a condition.

  Furthermore, the light energy of the irradiated ultraviolet light is in the range of 5 eV to 7 eV, which is the condition for light enhancement.

  Similar to graphene nanoribbons, the photoelectric field enhancement effect has also been reported for carbon nanotubes (Non-Patent Document 4), but in the case of carbon nanotubes, the photoelectric field enhancement occurs in the hollow of the nanotubes, so it is adjacent to the photoconductive semiconductor. However, the effect of increasing the electric field does not reach and is ineffective.

  The first-principles calculation simulation revealed that such a photoelectric field modulation did not change even when the width of graphene increased slightly to N = 9 and N = 11.

  Therefore, in order to cause the photoelectric field modulation as shown in FIG. 7, it is only necessary to irradiate the film made of armchair graphene nanoribbons of various widths with ultraviolet rays and use the modulation electric field in the vicinity of the film.

  Finally, a method for applying such an armchair nanoribbon to a semiconductor exhibiting photoconductivity will be described.

  It has been proved that when a nanoribbon is thermodynamically produced from graphene, an armchair type is stably produced (Non-patent Document 5).

  As a coating method, a graphene nanoribbon film is formed by coating a cleaved graphite piece by ultrasonic treatment in an organic solvent and centrifuging, and then drying the solution (Non-patent Document 6).

  At this time, the width of the graphene nanoribbon may vary, but if possible, the film may be formed by an operation in which the longitudinal direction of the graphene nanoribbon is oriented.

As a method, before the solution is dried, the coating film is mechanically rubbed with a soft material in one direction.
Thereby, as shown in FIG. 8, nanoribbons having various widths are spread in the film formation.

  The above is the explanation of the principle of terahertz wave oscillation.

  Examples will now be described.

FIG. 9 is an image of mounting a terahertz wave oscillation device in the circuit described in FIG. 5, and the antenna has a linear structure.
The application location of the graphene film is not particularly limited, and is most efficient when it is equivalent to the diameter of the ultraviolet laser spot to be irradiated.
The same applies to the following examples.

  FIG. 10 is a mounting image of the terahertz wave oscillation device in the circuit described in FIG. 5, and the antenna has a U-shaped structure.

  FIG. 11 is an image of mounting a terahertz wave oscillation device in the circuit described in FIG. 5, and the antenna has a ring-like structure.

  The structure as shown in FIGS. 9 to 11 can be arbitrarily determined according to the application such as the wavelength of the generated terahertz wave and the oscillation direction.

  As shown in FIGS. 9 to 11, in order to irradiate the photoconductive semiconductor with ultraviolet rays via a substance exhibiting light modulation characteristics, a substance exhibiting light modulation characteristics is applied to a part of the surface of the photoconductive semiconductor, What is necessary is just to irradiate an ultraviolet-ray to the applied part.

1 Photoconductive semiconductor 2 Light intensity modulation material (light intensity modulation material)
3 Light 4 Terahertz wave 5 Voltage source
6 Intensity modulated light 7 Ultraviolet light 8 Current
10 length

Claims (4)

An oscillating device in which a photoconductive semiconductor capable of producing electrical conduction characteristics when irradiated with light and a graphene nanoribbon are arranged close to each other,
The photoconductive semiconductor have a length of micron order in the longitudinal direction,
When a voltage is applied in the longitudinal direction of the photoconductive semiconductor and the graphene nanoribbon is irradiated with light, transmitted light whose intensity is periodically modulated is irradiated to the photoconductive semiconductor, and a current corresponding to the modulation flows. An oscillation element, wherein a photoconductive semiconductor oscillates a terahertz wave . The graphene nanoribbon is an armchair graphene nanoribbon in which carbon atoms at the width ends are terminated with hydrogen atoms,
2. The oscillation element according to claim 1 , wherein the light is ultraviolet light. 3. The oscillation device according to claim 2 , wherein the ultraviolet light is linearly polarized, and is irradiated while being vertically polarized in the longitudinal direction of the graphene nanoribbon and perpendicularly polarized in the width direction of the graphene nanoribbon . A terahertz antenna comprising the oscillation element according to claim 3 ,
The photoconductive semiconductor has a rectangular or circular cross section, a rod shape, a ring shape, or an U shape, and has an antenna shape with a part of a straight line,
Terahertz antenna, wherein the graphene nanoribbons are arranged close to the surface of the part straight portions of the photoconductive semiconductor.

Images