RU2622093C9 - Source of terahertz radiation - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: proposed source of terahertz radiation consists of a body located inside the body of an electromagnetic radiator, an irradiated substrate with gold objects located on it and a resonance filter whose plane is parallel to the plane of the substrate. Also, the source of terahertz radiation is equipped with a metal chamber installed in the housing with a gap with an inlet and an outlet on adjacent walls. The substrate with gold objects is placed inside the metal chamber so that the axis of the inlet is in the plane with the gold objects. The body also has a hole that coincides in shape with the outlet of the chamber and coaxial to it. A resonance filter is mounted in the hole in the housing. The electromagnetic radiator is made in the form of a magnetron with a waveguide installed coaxially with the inlet of the metal chamber so that the end of the waveguide is inside the chamber. Gold objects have an arbitrary shape and consist of the number of atoms N_a, satisfying the inequality: (4/3)⋅(E_F/E_m)≤N_a<(4/3)⋅(E_F/hν), where E_F - Fermi energy of gold, E_m - the energy of the peak of the energy distribution of the density of states of longitudinal phonons in gold, ν - working frequency of the magnetron, h - Planck's constant.

EFFECT: increase in the cross-section of the terahertz radiation flux and increase the power of the terahertz radiation source.

2 cl, 1 tbl, 6 dwg

Description

The invention relates to sources of terahertz (THz) radiation and can be used in medical research for the visualization of malignant tumors, for non-destructive quality control of drugs in pharmaceuticals and for scanning people at airports, train stations and places of public events in order to detect items hidden under clothes.

A known sub-terahertz and terahertz radiation generator based on an optical transistor (patent RU 2536327, H01L 31/112, G02F 1/00, published in 2014), including a laser radiation source, an electrical circuit with voltage sources and an impedance load, and an optically active element. The optically active element is equipped with an additional field-effect transistor, which has a semiconductor layer with a short lifetime of photoexcited charge carriers, a gate made of transparent or translucent material in the gate region, and the electric bias is applied to the drain and source of the conducting channel of the field-effect transistor.

Also known is a multi-element terahertz radiation generator (patent RU 2523746, G02F 1/00, published in 2014), containing a test sample, a laser emitting femtosecond laser radiation, a multi-element emitter consisting of at least one elementary emitter, which is a layer of a crystalline semiconductor with sprayed metal mask, forming a sharp gradient of illumination of the crystalline semiconductor layer by femtosecond laser radiation, while at the border of the illuminated and unlit parts of the crista layer -crystal semiconductor formed sharp concentration gradient photoexcited charge carriers parallel to the surface of the crystalline semiconductor layer. It also contains an elliptical mirror made by forming a focused beam of terahertz radiation and containing a hole for transmitting femtosecond laser radiation, and a multi-element emitter is made containing a raster of cylindrical microlenses that distributes femtosecond laser radiation between elementary emitters and generates illumination of only T regions on the crystal semiconductor layer that are involved in the generation of Hz radiation, in addition, the metal mask is made in the form of a flat metal bands.

A common disadvantage of the described devices is the use of an expensive source of laser radiation, a small cross section of the THz radiation stream and low THz radiation power.

For the prototype, a metamaterial-based terahertz radiation device has been selected (L. Luo, I. Chatzakis, J. Wang, FBP Niesler, M. Wegener, T. Koschny, CM Soukoulis, Broadband terahertz generation from metamaterials. Nature Communications, vol. 5 , Article number: 3055 doi: 10.1038 / ncomms4055 (2014)), containing an electromagnetic emitter in the form of a near-infrared femtosecond laser with a wavelength of 1500 nm, irradiating pulses of 140 fs suprasil substrate with a thickness of 1 mm with U-shaped gold objects in the form of split ring resonators with overall dimensions measuring 212 nm × 220 nm × 40 nm and a period of 382 nm, a polarizer in the form of a metal wire mesh and a focusing system in the form of a parabolic mirror, while a titanium-sapphire amplifier with a wavelength of 800 nm and a pulse duration of 35 fs is used to pump a femtosecond laser and a repetition rate of 1 kHz.

The disadvantages of a device for broadband generation of terahertz radiation based on the metamaterial selected for the prototype include: high cost, since it requires an expensive emitter in the form of a pulsed femtosecond near-infrared laser; limited possibilities for increasing the THz radiation power due to the small number of irradiated split ring resonators due to the small irradiated substrate area (limited by a laser beam diameter of ≈4 mm) with large resonator sizes (212 nm × 220 nm) and their placement period (382 nm); and also the small cross section of the THz radiation flux due to the small diameter of the laser beam (≈4 mm).

An object of the invention is to reduce the cost of a THz radiation source while increasing its power by increasing the THz radiation flux cross section.

The problem is solved due to the fact that the source of terahertz radiation, including a housing in which an electromagnetic emitter is located, an irradiated substrate with gold objects located on it and a resonant filter, the plane of which is parallel to the plane of the substrate, is equipped with a metal camera with inlet and outlet openings on adjacent walls installed in the housing with a gap, the substrate with gold objects is placed inside the metal chamber so that the axis of the inlet lies in the plane with the gold object and, moreover, a hole is made in the housing that matches the shape and is coaxial with the outlet of the chamber, in which the resonant filter is installed, while the electromagnetic emitter is made in the form of a magnetron with a waveguide mounted coaxially with the inlet of the metal chamber so that the end of the waveguide is inside the chamber , and gold objects are used of arbitrary shape, the number of atoms N _a in which satisfies the inequality:

where E _F is the Fermi energy of gold, E _m is the energy of the peak of the energy distribution of the density of states of longitudinal phonons in gold, ν is the working frequency of the magnetron, and ah is the Planck constant.

The magnetron can be made with inverter power control.

The implementation of an electromagnetic emitter in the form of a magnetron with a waveguide mounted coaxially with the inlet of the metal chamber and irradiating a substrate with gold objects mounted in a metal chamber of sufficiently large sizes allows simultaneously to realize in a large number of gold objects the condition for the absorption of microwave radiation, and then generation THz quanta. This condition requires that the energy of an integer m _{el of} quantized energy gaps ΔE _{el of} electrons in the interval from the Fermi level to the "excited" electronic level is equal to the sum of the energies of an integer n _{vm of} quantized energy gaps ΔE _{vm of} longitudinal vibrational modes of gold atoms (i.e., longitudinal phonons) and microwave photon hν:

Given the fact that:

1) in the microwave range hν << n _vm ⋅ΔE _vm (for example, at a magnetron frequency ν = 2.45 GHz, the energy of the microwave photon is hν = 0.01 meV, which is much less than n _vm ⋅ΔE _vm = 15.6 meV , energy of the most common longitudinal phonon in a gold object);

2) the highest absorption efficiency of the microwave photon (and the highest THz power of the radiation source) occurs when m _el = n _vm ;

3) ΔE _el ≈ (4/3) ⋅ (E _F / N _a );

relation (2) implies the requirement for the required number of atoms N _a in gold objects:

N _a ≈ (4/3) ⋅ (E _F / ΔE _vm ),

and from it - inequality (1). The left side of inequality (1) reflects the limiting case when the “excited” electron level is the level closest to the Fermi level, and the energy gap ΔE _vm is equal to Е _m , the peak energy of the energy distribution of the density of states of longitudinal phonons in gold (Е _m = 15 , 6 meV). The right-hand side of inequality (1) reflects another limiting case when there are many electronic energy levels between the “excited” electron level and the Fermi level, and the gap ΔE _vm = ΔE _el = hν. This limiting case is due to the undesirability of the possible heating of gold objects due to the fulfillment of the laws of conservation of energy and momentum due to the Heisenberg uncertainty relation.

Ensuring the number of atoms N _a in gold objects of arbitrary shape according to inequality (1) creates the possibility of increasing the power of THz radiation due to:

(a) the simultaneous absorption by the Fermi electron of the object of both a microwave photon and the longitudinal phonon most widespread in the object, which numerically dominates the energy distribution of longitudinal phonons in gold (E _m = 15.6 meV);

(b) an increase in the number of irradiated gold objects due to, firstly, the smallness of their overall dimensions in comparison with the overall dimensions of the split ring resonators in the prototype device and, secondly, due to the larger area of the substrate irradiated by microwave photons;

(c) scattering of an excited electron at the boundary of the object (and not inside it, where the absorbed energy could be released in the form of heat instead of the radiation of the THz quantum), which, when relaxed, will emit a THz quantum.

For example, at the working frequency of a magnetron with a waveguide ν = 2.45 GHz, E _F = 5.53 eV, E _m = 15.6 meV, h = 4.14⋅10 ^-15 eV⋅s, inequality (1) takes the form:

473≤N _a <7.3⋅10 ⁵ .

In order to ensure the maximum number of THz quanta passing through the resonance filter per unit time, the sum of the energies of the microwave photon and the longitudinal phonon that dominates the energy distribution of the longitudinal phonons in gold is equal to the transmission energy of the resonant filter. This will also increase the THz radiation power of the source.

The use of microwave photons to obtain THz radiation reduces the cost of a THz radiation source, since the cost of microwave emitters is three orders of magnitude lower than the cost of a femtosecond laser. And since the power of the microwave emitters is easily provided at a level of 1-2 kW, which is much higher than the average power of existing femtosecond near-infrared lasers, limited to 0.01-15 W, this creates the possibility of increasing the power of the THz source. In addition, the use of microwave radiation allows you to irradiate a large substrate: it can have a width and height of the chamber. Due to this, firstly, the THz radiation flux cross section increases, and secondly, the substrate area can significantly exceed the substrate area with split ring resonators in the prototype device, therefore, it is direct (directly from the magnetron with the waveguide) and reflected from the walls of the metal chamber Microwave radiation provides simultaneous and comprehensive irradiation of a much larger number of objects than split ring resonators in the prototype device. Therefore, by collecting THz radiation from a larger number of objects by focusing, it is also possible to increase the power of THz radiation at the object under study.

The implementation of a magnetron with inverter control of its power will allow you to smoothly change the output power of a THz radiation source, which is important in medical research.

The source of THz radiation is illustrated in the drawings, where in FIG. 1 shows a General view in section; in FIG. 2 - the same, top view. In FIG. 3 shows a top view of a gold nanostrip; in FIG. 4 - same, side view in section. In FIG. 5 - shows a top view of a gold nanowire; in FIG. 6 is the same sectional side view.

The terahertz radiation source consists of a housing 1, in which with a gap 2 there is a metal chamber 3 with an inlet 4 and an outlet 5 holes on adjacent walls, an irradiated substrate 6 with deposited gold objects 7, an electromagnetic emitter in the form of a magnetron 8 with a waveguide 9 and a resonant filter 10 , power, control and cooling systems (not shown in Fig.). The substrate 6 is placed in the chamber 3 so that the axis of the inlet 4 is in the plane with the gold objects 7, while the plane of the substrate 6 is parallel to the plane of the outlet 5. The waveguide 9 is installed coaxially with the inlet 4. The end face of the waveguide 9 is inside the chamber 3. B the housing 1 has a hole 11, matching in shape with the outlet 5 and coaxial with it. In the gap 2, a resonance filter 10 is tightly installed in the holes 5 and 11. At the output of the resonance filter 10, a focusing system 12 can be placed that collects the radiation emitted by the THz objects and focuses it on the object under study 13. When using a THz radiation source to scan people for detection objects hidden under clothing, the focusing system 12 is not installed, since in this case an unfocused wide stream of THz radiation is required, providing a spot with dimensions of ≈0.5 m × 0.5 m on the human body.

The source of THz radiation works as follows. Magnetron 8 (Fig. 1, 2) generates microwave photons, which through a waveguide 9 enter the cavity of a metal chamber 3, in which a substrate 6 with a plurality of gold objects 7 is mounted on it. Objects 7 are irradiated by a magnetron 8 with a waveguide 9 and reflected from the walls metal chamber 3 by a stream of microwave photons. In objects 7, Fermi electrons simultaneously absorb microwave photons and longitudinal phonons existing in objects 7. The energy of longitudinal phonons is 15.6 meV and corresponds to the peak of the energy distribution of longitudinal phonons in gold. The dominant number of absorbed longitudinal phonons also causes an increased flux of THz photons with an energy of 15.6 meV emitted by excited electrons as a result of their scattering at the boundaries of objects 7. THz photons with an energy of 15.6 meV emitted in the direction of the resonant filter 10 exit the metal camera 3 through a resonant filter 10 having a transmission energy of 15.6 meV (transmission frequency of 3.77 THz), and the background of the microwave photons is absorbed by the resonant filter 10 and does not go outside. In the case of using a THz radiation source to study the object 13, the THz photon flux transmitted by the resonance filter 10 is collected by the focusing system 12 and focuses on the sample being studied 13. In the case of using a THz radiation source to scan people to detect objects covered under clothes, the flow The THz photons transmitted by the resonance filter 10 should be wide, so it does not focus, and the focusing system 12 is not used.

Table 1 shows examples of the implementation of a THz radiation source for various combinations of the substrate, such as objects, a resonant filter and a focusing system, the working frequency of the magnetron with the waveguide is 2.45 GHz, and the microwave power of the magnetron is 1 kW.

Thus, due to two geometrical factors contributing to an increase in the number of gold objects-sources of THz radiation (increased substrate area for objects and reduced sizes of the latter), and also due to the fact that in gold objects Fermi electrons simultaneously absorb microwave photons and longitudinal phonons, which dominate in objects, the emitted THz power of the THz radiation source increases. And the use of a microwave emitter can significantly reduce the cost of a THz radiation source. Thus, the technical task is solved.

The calculations showed that in gold objects, the angle γ (see Fig. 3 and Fig. 5) between the direction of the momentum s of the excited electron (absorbing the microwave photon and the longitudinal phonon) and the longitudinal phonon momentum n _vm ⋅q is ≈72.3 °. Therefore, an excited electron will inevitably be scattered at the boundary of the object. In this case, due to the significant work function of the electron from gold (≈4.3 eV), the scattered electron will not leave the object, but will relax, going to the Fermi level with the emission of a THz photon with an energy of 15.6 meV (frequency 3.77 THz) to which gold objects are “tuned” by choosing the number of atoms N _and in them according to inequality (1) (the size of the objects must also be less than the depth of the skin layer in gold in the microwave range, for example, at a magnetron frequency of 2.45 GHz - less than 1 5 μm). Thus, objects become sources of THz radiation, the bandwidth of which is determined by the full width at half the height of the energy distribution of longitudinal phonons in gold, that is, ≈3.74 meV (≈0.9 THz).

The calculations for the 2.45 GHz magnetron frequency showed the following sizes of nanobands: d = 3 nm, L = 100 nm, H = 3.1 nm, while the size d / 2 is less than the mean free path of electrons in gold nanoparticles (≈1, 7 nm), and all sizes of the nanoband are much smaller than the depth of the skin layer of gold at a frequency of 2.45 GHz (1.5 μm). Similarly, calculations showed that for nanorings, the sizes should be as follows: R = 21.5 nm, r = 18.5 nm, H = 3.1 nm, while the size (Rr) / 2 is less than the mean free path of electrons in the nanoparticles gold, and all sizes of the nanorings are much smaller than the depth of the skin layer of gold at a frequency of 2.45 GHz.

Using a THz radiation source of the proposed design will satisfy the urgent need for equipment for cancer research and safety.

Claims (4)

1. A terahertz radiation source, including an electromagnetic emitter placed in the housing, an irradiated substrate with gold objects and a resonant filter, the plane of which is parallel to the substrate plane, characterized in that it is equipped with a metal camera with inlet and outlet openings on adjacent walls mounted in the housing with a gap, the substrate with gold objects is placed inside the metal chamber so that the axis of the inlet lies in the plane with the gold objects, a hole is made in the housing that matches in shape and coaxial with the outlet of the chamber in which the resonant filter is installed, the electromagnetic emitter is made in the form of a magnetron with a waveguide mounted coaxially with the inlet of the metal chamber so that the end of the waveguide is inside the chamber, and gold objects are used in an arbitrary shape, the number of atoms N _a in which it satisfies the inequality: (4/3) ⋅ (E _F / E _m ) ≤N _a <(4/3) ⋅ (E _F / hν), where E _F is the Fermi energy of gold, E _m is the energy of the peak of the energy distribution of the density of states of longitudinal phonons in gold, ν is the working frequency of the magnetron, and ah is the Planck constant. 2. The terahertz radiation source according to claim 1, characterized in that the magnetron is made with inverter power control.

Images