CN112531348A - All-dielectric super-surface terahertz photoconductive antenna based on embedded metal nano structure - Google Patents
All-dielectric super-surface terahertz photoconductive antenna based on embedded metal nano structure Download PDFInfo
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- 229910000530 Gallium indium arsenide Inorganic materials 0.000 claims description 2
- KXNLCSXBJCPWGL-UHFFFAOYSA-N [Ga].[As].[In] Chemical compound [Ga].[As].[In] KXNLCSXBJCPWGL-UHFFFAOYSA-N 0.000 claims description 2
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Abstract
The invention provides an all-dielectric super-surface terahertz photoconductive antenna based on an embedded metal nano structure, and solves the problems that in the prior art, the terahertz photoconductive antenna generates terahertz waves and is low in radiation power and conversion efficiency. The device comprises a semiconductor substrate, an all-dielectric antireflection super-surface array, a metal nano-structure array, a metal anode and a metal cathode; the all-dielectric antireflection super-surface array, the metal anode and the metal cathode are arranged on the upper surface of the semiconductor substrate, and the all-dielectric antireflection super-surface array is positioned between the metal anode and the metal cathode; the metal nano-structure array is arranged in the semiconductor substrate and has a distance with the upper surface of the semiconductor substrate.
Description
Technical Field
The invention belongs to terahertz photoconductive antennas, and particularly relates to an all-dielectric super-surface terahertz photoconductive antenna based on an embedded metal nano structure.
Background
Terahertz (THz) waves are electromagnetic radiation having a frequency between the microwave and infrared bands, and are generally defined as electromagnetic waves having a frequency of 100GHz-10THz[1]。
The terahertz wave has the advantages of broadband property, transmission property, low energy property and the like due to the unique frequency spectrum position of the terahertz wave, and has important application value in the fields of broadband communication, medical imaging, nondestructive testing, security inspection and the like[2]。
The terahertz photoconductive antenna (THz-PCA) is used as an important artificial terahertz source and can emit broadband terahertz radiation (the whole terahertz frequency band can be covered), and a terahertz time-domain spectroscopy technology formed by the terahertz photoconductive antenna has important application in the aspects of biomolecule and material analysis and the like.
The photoconductive antenna is generally composed of a metal antenna working in a terahertz frequency band and a semiconductor material serving as a substrate, when a femtosecond laser pulse is incident to the semiconductor substrate, electrons in the substrate material are transited from a valence band to a conduction band to form a photogenerated carrier, and the photogenerated carrier accelerates to move under the action of a bias electric field to form an ultrafast photocurrent, so that terahertz waves are radiated outwards. The generated ultrafast photocurrent is a narrow-band pulse with sub-picosecond magnitude in time domain, namely, the terahertz wave corresponding to wider frequency band[3]. Albeit terahertz photoelectricityThe conductive antenna has the advantages of low cost, small volume, room-temperature operation and the like, but the traditional terahertz photoconductive antenna still faces the limitation of lower output power.
In recent years, with the development of micro-nano technology, terahertz photoconductive antennas based on surface metal nano-structures become an important research direction. Due to the action of surface plasma resonance, the metal nano structure can remarkably enhance the absorption of the substrate to incident femtosecond laser and reduce the transmission distance of current carriers, thereby greatly improving the radiation power and the conversion efficiency of the terahertz photoconductive antenna[4-5]. However, the use of metallic nanostructures also inevitably introduces two major problems: 1) the larger ohmic loss limits the conversion efficiency of the terahertz photoconductive antenna and reduces the damage threshold of the device; 2) due to the good electrical conductivity of metals, the "switching" process (i.e. the ultrafast process of current generation to extinction) of the photoconductive antenna will be prolonged, thereby reducing its frequency range of generating terahertz radiation. These two main problems greatly limit the further improvement of the radiation power and the conversion efficiency of the terahertz photoconductive antenna. In order to solve the defects of introducing metal nano-structures, in recent years, terahertz photoconductive antennas based on all-dielectric materials have attracted attention[6-8]But the local field enhancement effect is far weaker than that of the metal nano structure[9]Therefore, the radiation power generated by the terahertz photoconductive antenna still cannot meet the requirement of practical application.
Therefore, it is highly desirable to design a terahertz photoconductive antenna to improve the radiation power and conversion efficiency of the terahertz wave generated by the photoconductive antenna.
The references are as follows:
[1]Y.S.Lee.“Principle of terahertz science and technology”,New York:Springer Science&Business Media,2009.
[2]D.M.Mittleman.“Perspective:Terahertz science and technology”,Journal of Applied Physics,Vol.122(23):230901(2017).
[3]N.T.Yardimci,M.Jarrahi.“Nanostructure-Enhanced Photoconductive Terahertz Emission and Detection”,Small,Vol.14:1802437(2018).
[4]S-G.Park,Y.Choi,Y-J.Oh,et al.“Terahertz photoconductive antenna with metal nano-islands”,Optics Express,Vol.20(23):25530-25535(2012).
[5]C.W.Berry,N.Wang,M.R.Hashemi,et al.“Significant performance enhancement in photoconductive terahertz optoelectronics by incorporating plasmonic contact electrodes”,Nature Communications,Vol.4(3):1622(2013).
[6]S.Jahani,Z.Jacob.“All-dielectric metamaterials”,Nature Nanotechnology,Vol.11(1):23-36(2016).
[7]T.Siday,P.P.Vabishchevich,L.Hale,et al.“Terahertz detection with perfectly-absorbing photoconductive metasurface”,Nano Letter,Vol.19:2888-2896(2019).
[8]K.E.Wang,J.Q.Gu,W.Q.Shi,et al.“All-dielectric nanograting for increasing terahertz radiation power of photoconductive antennas”,Optics Express,Vol.28(13):19144-19151(2020).
[9]M.R.Shcherbakow,D.N.Neshev,B.Hopkins,et al.“Enhanced third-harmonic generation in silicon nanoparticles driven by magnetic response”,Nano Letter,Vol.14:6488-6492(2014).
disclosure of Invention
The invention aims to overcome the defects of low radiation power and conversion efficiency of a terahertz photoconductive antenna in the prior art, and provides an all-dielectric super-surface terahertz photoconductive antenna based on an embedded metal nano structure.
The conception of the invention is as follows:
the all-dielectric antireflection super-surface array is arranged on the surface of the semiconductor substrate, high light transmittance and diffraction efficiency are guaranteed by using the low-loss characteristic of an all-dielectric material, reflection of incident femtosecond laser is reduced, more photon-generated carriers are generated, and the photocurrent of the semiconductor substrate is improved; the metal nano array is arranged in the semiconductor substrate, the drift speed of the photo-generated carriers is improved and the photo-generated carriers are limited between the surface of the antenna and the metal nano array by utilizing the stronger local field enhancement effect of the metal nano, the transmission distance of the carriers is reduced, more photo-generated carriers reach the electrode before being compounded, and therefore the radiation power and the conversion efficiency of the photoconductive antenna are improved.
In order to achieve the purpose, the technical solution provided by the invention is as follows:
the all-dielectric super-surface terahertz photoconductive antenna based on the embedded metal nano structure is characterized by comprising a semiconductor substrate, an all-dielectric antireflection super-surface array, a metal nano structure array, a metal anode and a metal cathode;
the all-dielectric antireflection super-surface array, the metal anode and the metal cathode are arranged on the upper surface of the semiconductor substrate, and the all-dielectric antireflection super-surface array is positioned between the metal anode and the metal cathode;
the metal nano-structure array is arranged inside the semiconductor substrate and has a distance with the upper surface of the semiconductor substrate.
Further, the all-dielectric antireflection super-surface array comprises a plurality of all-dielectric antireflection super-surface units which are periodically arranged;
the metal nanostructure array comprises a plurality of metal nanostructure units which are periodically arranged;
the arrangement period of the all-dielectric antireflection super-surface unit is the same as that of the metal nano-structure unit.
Further, in order to improve the conversion efficiency, the plurality of all-dielectric antireflection super-surface units and the plurality of metal nano-structure units are arranged in a one-to-one correspondence manner.
Furthermore, the material of the all-dielectric antireflection super-surface unit is the same as that of the semiconductor substrate, so that the lattice matching degree of the all-dielectric antireflection super-surface unit and the semiconductor substrate is higher, and the antireflection effect is favorably improved.
Further, in order to improve the local field enhancement effect of the metal nano array, the metal nano structure unit is made of gold.
Further, the thickness of the semiconductor substrate is preferably selected based on a calculation result of a finite difference time domain method.
Further, the selection of the arrangement period is based on the working wavelength, and the size is in the sub-wavelength order.
Furthermore, the shapes and the sizes of the all-dielectric antireflection super-surface unit and the metal nano-structure unit are preferably selected based on respective arrangement periods and materials and based on a calculation result of a time domain finite difference method;
the distance between the metal nanostructure unit and the upper surface of the semiconductor substrate is selected based on the position where the incident light can be transmitted into the substrate in the calculation result of the finite difference time domain method.
Furthermore, the working wavelength is 800nm, the material of the semiconductor substrate is low-temperature grown gallium arsenide, and the thickness is 2 microns; the all-dielectric antireflection super-surface unit is in a nano-cube column shape, is made of intrinsic potassium arsenide, and is 240nm in side length, 150nm in height and 400nm in arrangement period; the terahertz photoconductive antenna manufactured according to the parameters has the advantages that the reflectivity is greatly reduced, the terahertz radiation intensity is multiplied, and the radiation power and the conversion efficiency are obviously improved.
Furthermore, the working wavelength is 1550nm, the material of the semiconductor substrate is indium gallium arsenide, and the materials and the sizes of the all-dielectric antireflection super-surface unit and the metal nano-structure unit are selected preferentially according to the actual processing condition and the calculation result.
The invention has the advantages that:
1. the invention creatively combines the all-dielectric antireflection super-surface array and the metal nano-array structure with the terahertz conductive antenna, reduces the reflection of incident light, improves the drift velocity of carriers and reduces the transmission distance of the carriers, so that more photon-generated carriers reach an electrode before being compounded, the radiation power and the conversion efficiency of the terahertz conductive antenna are improved, the terahertz conductive antenna can be used as a radiation source for a terahertz time-domain spectroscopy system, the terahertz time-domain spectroscopy system is used for measuring and checking the working performance of the terahertz conductive antenna, and the invention lays a foundation for the large-scale practical application of terahertz waves.
2. The all-dielectric antireflection super-surface array and the metal nano-structure array are both composed of unit structures which are periodically arranged at fixed intervals. Through accurate design and simulation, the columnar all-dielectric super-surface structure has obvious antireflection effect on incident femtosecond laser, so that more photon-generated carriers are generated, and the photocurrent of a semiconductor substrate is improved. The strong local field enhancement effect of the gold nanometer disc structure (cylindrical shape) improves the drift velocity of carriers and limits photon-generated carriers between the surface of the antenna and the gold nanometer disc array, thereby reducing the transmission distance of the carriers, leading more photon-generated carriers to reach an electrode before being compounded, and further improving the radiation power and the conversion efficiency of the photoconductive antenna.
3. The semiconductor substrate absorbs the incident femtosecond laser, reduces the electromagnetic wave energy interacted with the metal nano structure, and improves the damage threshold of the device.
Drawings
FIG. 1 is a schematic diagram of an all-dielectric super-surface terahertz photoconductive antenna based on an embedded metal nanostructure according to the present invention;
FIG. 2 is a top view of an all-dielectric super-surface terahertz photoconductive antenna based on an embedded metal nanostructure according to the present invention;
FIG. 3 is a side view of an all-dielectric super-surface terahertz photoconductive antenna based on an embedded metal nanostructure according to the present invention;
FIG. 4 is a schematic diagram of a unit structure of an all-dielectric antireflection super-surface array;
FIG. 5 is a schematic diagram of a cell structure of a metal nanostructure array;
FIG. 6 is a time domain finite difference method-based simulated all-dielectric antireflection super-surface reflectivity;
fig. 7 is a simulated electric field distribution of a semiconductor substrate based on a time-domain finite difference method, wherein (a) is the electric field distribution of the semiconductor substrate in a conventional terahertz photoconductive antenna, (b) is the electric field distribution of the semiconductor substrate in the terahertz photoconductive antenna when an all-dielectric antireflection super surface is coupled, and (c) is the electric field distribution of the semiconductor substrate in the all-dielectric antireflection super surface terahertz photoconductive antenna based on an embedded metal nanostructure;
fig. 8 is a photocurrent generated by a terahertz photoconductive antenna simulated based on a finite element method with time change, a lower curve is the photocurrent generated by a conventional terahertz photoconductive antenna, a middle curve is the photocurrent generated by the terahertz photoconductive antenna when an all-dielectric antireflection super surface is coupled, an upper curve is the photocurrent generated by the all-dielectric antireflection super surface terahertz photoconductive antenna based on an embedded metal nanostructure, and d1, d2 and d3 are respectively half-height widths of a lower curve, a middle curve and an upper curve;
fig. 9 is terahertz radiation intensity generated by the terahertz photoconductive antenna simulated based on the finite element method with time change, a lower curve is terahertz radiation intensity generated by the conventional terahertz photoconductive antenna, a middle curve is terahertz radiation intensity generated by the terahertz photoconductive antenna when the all-dielectric antireflection super surface is coupled, and an upper curve is terahertz radiation intensity generated by the all-dielectric antireflection super surface terahertz photoconductive antenna based on the embedded metal nanostructure.
The reference numbers are as follows:
1-full-medium antireflection super-surface array, 2-metal nano-structure array, 3-semiconductor substrate, 4-metal anode, 5-full-medium antireflection super-surface unit, 6-metal nano-structure unit and 7-metal cathode.
Detailed Description
The invention is described in further detail below with reference to the following figures and specific examples:
the all-dielectric super-surface terahertz photoconductive antenna based on the embedded metal nano structure comprises a semiconductor substrate 3, an all-dielectric antireflection super-surface array 1, a metal nano structure array 2, a metal anode 4 and a metal cathode 7. The all-dielectric antireflection super-surface array 1, the metal anode 4 and the metal cathode 7 are all arranged on the upper surface of the semiconductor substrate 3 (when the all-dielectric antireflection super-surface array is used, voltage is applied to the metal anode, and the metal cathode is grounded), the all-dielectric antireflection super-surface array 1 is located between the metal anode 4 and the metal cathode 7, the metal anode 4 is connected with the voltage, and the metal cathode 7 is grounded. The metal nanostructure array 2 is disposed inside the semiconductor substrate 3 and spaced apart from the upper surface of the semiconductor substrate 3. The all-dielectric antireflection super-surface array 1 comprises a plurality of all-dielectric antireflection super-surface units 5 which are periodically arranged; the metal nanostructure array 2 includes a plurality of metal nanostructure units 6 arranged periodically.
In the embodiment, the working wavelength is 785 nm-820 nm (the actual standard of the existing titanium sapphire femtosecond laser generates femtosecond laser with the wavelength of 800nm, the femtosecond laser is influenced by objective factors such as temperature, the wavelength is changed, the change range is 785 nm-820 nm, and the period of the full-medium antireflection super-surface array and the metal nano-structure array is 400nm according to the selection standard of the arrangement period; the material of the semiconductor substrate is low-temperature grown gallium arsenide, the material of the all-dielectric antireflection surface array is intrinsic potassium arsenide, and the material of the metal nano-structure array is gold.
Based on the arrangement period and the material, the optimal parameters are selected based on the calculation result of the finite difference time domain method, as follows:
the thickness of the semiconductor substrate is 2 microns, the short carrier service life of the semiconductor substrate accelerates the switching process of the antenna, and terahertz radiation with a wide frequency spectrum can be obtained.
The shape of the all-dielectric antireflection super-surface unit is a nano-scale cube column, the side length is 240nm, and the height is 150 nm; the semiconductor substrate provides an adhesion layer for the nano-scale columnar structure, and is a key structure for reducing reflection of incident femtosecond laser.
The metal nanostructure unit is cylindrical, the diameter is 240nm, the height is 150nm, the distance between the position where the incident light can be transmitted into the substrate and the upper surface of the semiconductor is 300nm according to the calculation result of the time domain finite difference method, and the metal nanostructure unit is called a gold nanometer disc structure; the semiconductor substrate provides an adhesion layer for the gold nanometer disk structure, and is a key structure for providing a local field enhancement effect.
The all-dielectric antireflection super-surface terahertz photoconductive antenna based on the embedded gold nano disk can replace the traditional terahertz photoconductive antenna in a terahertz time-domain spectroscopy system, and the actual working performance of the photoconductive antenna is tested through experiments as follows:
the rate of photogenerated carriers generated in the semiconductor substrate under the irradiation of femtosecond laser pulses is as follows:
G(x,y,z,t)＝(4πk/hc)Ix(x,y,z)exp(-4In(2)×((t-t0)2/Dt 2)
where k is the semiconductor extinction coefficient, h is the Planck constant, π is the circumference ratio, Ix(x, y, z) is the laser intensity, t0Is the peak time of the laser pulse, DtIs the laser pulse width, x, y, z denote the direction, t denotes time;
the current between the metal electrodes of the photoconductive antenna is defined as:
Js(t)＝-ne(t)qvs(t)，
wherein n ise(t) is the electron concentration, q is the unit charge amount, vs(t) is the electron mean drift velocity;
the intensity of the generated far-field terahertz radiation is related to the density of the generated photocurrent in the semiconductor substrate:
ETHz(t)＝-(A/4zπε0c0 2)(dJ(t)/dt)，
wherein epsilon0Is the dielectric constant in vacuum, c0Is the vacuum light velocity, a is the illuminated area of the antenna electrode gap, and z is the distance from the radiation center to the observation point. From the equation, the terahertz radiation intensity is proportional to the first derivative of the current density with respect to time, i.e., the higher the photocurrent and the faster the "switching" process, the stronger the terahertz radiation intensity. Therefore, the design of the invention can significantly improve the performance of the terahertz photoconductive antenna.
The reflectivity of a pure substrate to an incident femtosecond laser is defined as:
R＝(n1(v)-n2(v))2/(n1(v)+n2(v))2，
where v is the electromagnetic frequency, n 11 is the refractive index of air, n23.5 is the refractive index of low temperature grown gallium arsenide, and for a wavelength of 800nm, it is calculated that R is 30%; the pure substrate material has lower utilization rate of the energy of the incident femtosecond laser.
As shown in FIG. 6, the reflectivity of the designed all-dielectric antireflection super-surface to incident femtosecond laser with the wavelength of 800nm is reduced from 30% to 1.4%, and the reflectivity in the wavelength range of 785nm to 820nm is less than 2%, so that the absorption of the semiconductor substrate to the incident femtosecond laser is effectively improved.
FIG. 7 is an electric field distribution of a semiconductor substrate in a terahertz photoconductive antenna for three different cases simulated based on a time-domain finite difference method; as shown in fig. 7 (a), incident light is substantially transmitted to a distance of 300nm from the upper surface of the base material, and thus the metal nanostructure is placed there. As shown in fig. 7 (c), the peak electric field intensity in the embedded gold nano disc semiconductor substrate is increased by 11.2 times compared with that of the conventional antenna structure, and compared with 4.95 times (as shown in fig. 7 (b)) of the peak electric field intensity in the terahertz photoconductive antenna substrate when the full-dielectric antireflection super-surface is coupled, the local field enhancement effect is significantly improved.
Fig. 8 is a photocurrent generated by the time-varying terahertz photoconductive antenna simulated based on the finite element method. As shown in the figure, the full-dielectric antireflection super-surface terahertz photoconductive antenna based on the embedded gold nano disk is improved by 4.9 times compared with the photocurrent of the traditional antenna structure, is improved by 2.2 times compared with the photocurrent of the antenna coupled with the full-dielectric antireflection super-surface structure, and has half-height width d30.61ps, less than half-width of photocurrent (d) of the other two antennas2＝0.82ps，d11.21ps), i.e., corresponding to a spectrally wider terahertz wave.
Fig. 9 is terahertz radiation intensity generated by a terahertz photoconductive antenna simulated based on a finite element method as a function of time. As shown in the figure, compared with the other two types of antennas, the terahertz radiation intensity of the all-dielectric antireflection super-surface terahertz photoconductive antenna based on the embedded gold nano disk is respectively improved by 2.5 times and 7.8 times.
According to the relationship that the radiation power of the antenna is in direct proportion to the radiation intensity:
PTHz∝E2 THz
therefore, the radiation power of the antenna structure is improved by 60.8 times compared with the traditional structure. The design of the present invention thus significantly improves the performance of the antenna.
Similarly, if the operating wavelength is around 1550nm (actual target time wavelength is 1550nm of femtosecond laser), the material of the semiconductor substrate is indium gallium arsenic, and the optimal structure parameters can be designed by the same method as in the above embodiment, and the result is verified.
Therefore, the radiation power and the conversion efficiency of the terahertz photoconductive antenna adopting the structure are greatly improved compared with the prior art, the purpose of improving the performance of the terahertz photoconductive antenna is met, and the terahertz photoconductive antenna has a good development prospect.
While the invention has been described with reference to specific embodiments, the invention is not limited thereto, and various equivalent modifications or substitutions can be easily made by those skilled in the art within the technical scope of the present disclosure.
Claims (10)
1. An all-dielectric super-surface terahertz photoconductive antenna based on an embedded metal nano structure is characterized in that: the device comprises a semiconductor substrate (3), an all-dielectric antireflection super-surface array (1), a metal nano-structure array (2), a metal anode (4) and a metal cathode (7);
the all-dielectric antireflection super-surface array (1), the metal anode (4) and the metal cathode (7) are all arranged on the upper surface of the semiconductor substrate (3), and the all-dielectric antireflection super-surface array (1) is positioned between the metal anode (4) and the metal cathode (7);
the metal nano structure array (2) is arranged inside the semiconductor substrate (3) and has a distance with the upper surface of the semiconductor substrate (3).
2. The all-dielectric super-surface terahertz photoconductive antenna based on the embedded metal nano structure as claimed in claim 1, wherein:
the all-dielectric antireflection super-surface array (1) comprises a plurality of all-dielectric antireflection super-surface units (5);
the metallic nanostructure array (2) comprises a plurality of metallic nanostructure units (6);
the arrangement period of the plurality of all-dielectric antireflection super-surface units (5) is the same as that of the plurality of metal nano-structure units (6).
3. The all-dielectric super-surface terahertz photoconductive antenna based on the embedded metal nano structure as claimed in claim 2, wherein:
the all-dielectric antireflection super-surface units (5) and the metal nano-structure units (6) are arranged in a one-to-one correspondence manner.
4. The all-dielectric super-surface terahertz photoconductive antenna based on the embedded metal nano structure as claimed in claim 2 or 3, wherein:
the material of the all-dielectric antireflection super-surface unit (5) is the same as that of the semiconductor substrate (3).
5. The all-dielectric super-surface terahertz photoconductive antenna based on the embedded metal nano structure as claimed in claim 4, wherein:
the metal nano-structure unit (6) is made of gold.
6. The all-dielectric super-surface terahertz photoconductive antenna based on the embedded metal nano structure as claimed in claim 5, wherein:
the thickness of the semiconductor substrate (3) is selected based on the calculation result of the finite difference time domain method.
7. The all-dielectric super-surface terahertz photoconductive antenna based on the embedded metal nano structure as claimed in claim 6, wherein:
the selection of the arrangement period takes the working wavelength as a standard, and the size is in the sub-wavelength order.
8. The all-dielectric super-surface terahertz photoconductive antenna based on the embedded metal nano structure as claimed in claim 7, wherein:
the shapes and the sizes of the all-dielectric antireflection super-surface unit (5) and the metal nano-structure unit (6) are selected based on respective arrangement periods and materials and based on a calculation result of a finite difference time domain method;
the distance between the metal nanostructure unit (6) and the upper surface of the semiconductor substrate (3) is selected based on the position where the incident light can be transmitted into the substrate in the calculation result of the finite difference time domain method.
9. The all-dielectric super-surface terahertz photoconductive antenna based on the embedded metal nano structure as claimed in claim 8, wherein:
the working wavelength is 800nm, the material of the semiconductor substrate (3) is low-temperature grown gallium arsenide, and the thickness is 2 microns; the all-dielectric antireflection super-surface unit (5) is in a cubic column shape, is made of intrinsic gallium arsenide, and has the side length of 240nm, the height of 150nm and the arrangement period of 400 nm; the metal nano-structure unit (6) is cylindrical, the diameter is 240nm, the height is 150nm, the distance between the metal nano-structure unit and the upper surface of the semiconductor is 300nm, and the arrangement period is 400 nm.
10. The all-dielectric super-surface terahertz photoconductive antenna based on the embedded metal nano structure as claimed in claim 8, wherein:
the working wavelength is 1550nm, and the material of the semiconductor substrate (3) is indium gallium arsenide.
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