A tunable lens assembly based on the terahertz jet effect for terahertz super-resolution imaging

Abstract

The present invention discloses a tunable lens assembly based on the terahertz jet effect for terahertz super-resolution imaging. The lens assembly comprises three terahertz optical elements: a negative axicon, a positive axicon, and a dielectric sphere. Each terahertz optical element is coaxially mounted in an optical path, with its axis aligned with the center of an incident light beam. The negative axicon, the positive axicon, and the dielectric sphere are mounted in this order according to the direction of the incident light beam. The distance between the positive axicon and the dielectric sphere is 1-5λ, and the distance between the negative axicon and the positive axicon is between L = 0 and L = L(max). The incident light beam is a plane beam or a Gaussian beam. When coupled to a terahertz imaging system, the present invention can achieve tunable working distance, depth of field, and resolution, with the optimal resolution exceeding the diffraction limit.

Description

Technical Field

[0001] The present invention belongs to the technical field of tunable lens groups, and in particular relates to a tunable lens group based on the terahertz jet effect for terahertz super-resolution imaging. Background Art

[0002] Terahertz (THz) waves represent a distinct region of the electromagnetic spectrum, with frequencies between 0.1 and 10 THz, or wavelengths between 3000 and 30 microns. Positioned between infrared and microwaves, THz waves possess unique properties not found in microwave and optical bands. These include high penetrability, with THz waves penetrating non-polar materials such as plastics, paper, and fabric, facilitating X-ray inspections. Low energy, with THz photon energies in the millielectronvolt range (1 THz photons are 4.1 meV), far below the peptide bond energy of biological tissue, resulting in no radiation damage. Existing THz pulse sources typically cover the GHz to several THz range, enabling spectral detection. Transient, with THz pulses typically limited to a few wavelengths, and typical pulse durations in the picosecond range, allows for precise time-resolved studies. These unique advantages have led to THz waves being widely used in security inspections, biological testing, medicine, aerospace, and other fields.

[0003] However, when terahertz waves are used for imaging, there is an unavoidable problem, namely the diffraction limit. According to the Rayleigh criterion, the resolution under the diffraction limit is δ = 0.61λ / NA (where λ is the wavelength of the incident light and NA is the numerical aperture of the objective lens). Since the wavelength of terahertz is very large compared to visible light, close to millimeters, its resolution limit is also on the order of millimeters. With the rapid development of biology, medicine, materials and other fields, the requirements for imaging are also getting higher and higher. The resolution of conventional THz imaging technology is low and it is difficult to meet the current increasing application needs. Therefore, improving imaging resolution and breaking through the diffraction limit of THz imaging technology has been unremittingly pursued.

[0004] Existing solutions: So far, a variety of THz super-resolution imaging technologies have been proposed, such as scanning probe THz imaging, subwavelength aperture or subwavelength source THz imaging, THz image restoration processing, etc., which have improved the resolution accordingly.

[0005] Among them, scanning probe imaging technology breaks the diffraction limit by scanning the sample surface to detect evanescent waves containing ultra-fine information, thus achieving very high resolution. However, it is a near-field detection method, and the detection probe can only be close to the sample surface and cannot be imaged in the far field, so its application scenarios are very limited. Subwavelength aperture or subwavelength source THz imaging can achieve super-resolution imaging, but the subwavelength aperture method requires a metal aperture to limit the terahertz wave irradiation range on the sample, which limits the spectral bandwidth and energy. Subwavelength source THz imaging often requires ultra-high peak power femtosecond lasers to stimulate the generation of terahertz radiation. The generation conditions are high, the equipment is expensive, and the operation is difficult, which is not conducive to large-scale application. THz image restoration processing uses digital image processing technology to improve image contrast, resolution, remove noise and other aspects to improve image quality. It is a post-processing technology for the imaged optical system. It cannot improve the resolution from the source and can only be used as an auxiliary means to improve image quality.

[0006] Furthermore, super-resolution imaging based on the terrestrial jet method, which directly and simply utilizes structures such as microspheres, hemispheres, and cubes, has overcome the shortcomings of spectral bandwidth loss, energy loss, and the need to be close to the surface, but there are still some shortcomings that need to be further improved. For example, the terrestrial jets generated by these structures are relatively close to the structure. Although the detection distance is greater than that of the near-field probe method, most are still 1-2 times λ, which requires a small distance between the sample and the structure, increasing the risk of sample contamination. In addition, the half-width at half maximum of the terrestrial jets generated by traditional structures is usually around 0.5λ, which can only be considered to barely break the diffraction limit. In practical applications, the resolution still needs to be further improved, which requires an even smaller half-width. In addition, once the shape of these existing structures is determined, the terrestrial jets they generate cannot be adjusted. They do not have the tuning function for resolution, focal length, working distance, depth of field, etc., which seriously limits the application needs in different scenarios. Summary of the Invention

[0007] In order to overcome the shortcomings of the above-mentioned prior art, the purpose of the present invention is to provide a tunable lens group based on the terahertz effect for terahertz super-resolution imaging. The lens group is coupled to a terahertz imaging system, which can achieve tunable working distance, depth of field and resolution, and the optimal resolution can reach beyond the diffraction limit.

[0008] In order to achieve the above object, the technical solution adopted by the present invention is:

[0009] A tunable lens assembly based on the terahertz jet effect for terahertz super-resolution imaging, comprising three terahertz optical elements: a negative axicon, a positive axicon, and a dielectric sphere;

[0010] Each terahertz optical element is coaxially installed in the optical path, keeping the central axis aligned with the center of the incident light beam, and is installed in the order of negative axicon, positive axicon and dielectric sphere according to the direction of the incident light beam.

[0011] The distance between the positive axicon and the dielectric sphere is 1-5λ (λ refers to the wavelength of the incident terahertz wave), and the distance between the negative axicon and the positive axicon is between L=0 and L=L(max).

[0012] The incident light beam is a plane light beam or a Gaussian light beam.

[0013] The negative axicon and the positive axicon have the same base angle and are a pair of complementary pyramids.

[0014] The negative axicon is a body shape with a circular bottom surface and a conical concave top surface, which is rotationally symmetrical along the central axis. When in use, the incident light beam is incident from the bottom surface (i.e., the plane side).

[0015] The axicon is a body shape with a circular bottom surface and a conical protrusion on the top surface, which is rotationally symmetrical along the central axis. When in use, the incident light beam is incident from the top surface (i.e., the conical surface).

[0016] The incident light beam is converged into a small local area through the negative axicon, the positive axicon and the dielectric sphere.

[0017] To meet the aforementioned convergence requirements, the dimensional parameters of the negative axicon, positive axicon, and dielectric sphere must meet the following requirements: the dielectric sphere radius must be between 5λ and 30λ, and the radial dimensions of both the negative and positive axicons must be slightly larger than the dielectric sphere by 1-5λ. The base angles of the negative and positive axicons must be between 10° and 60°. The refractive indices of the negative axicon, positive axicon, and dielectric sphere must all be between 1.2 and 1.9.

[0018] A method for using a tunable lens assembly based on the terahertz jet effect for terahertz super-resolution imaging, comprising the following steps:

[0019] First, after the terahertz wave passes through the negative axicon, the terahertz beam is refracted on the rear surface of the negative axicon and becomes a divergent annular beam.

[0020] Then, the light is irradiated on the front cone surface of the axicon, and after being refracted again on the front cone surface, it becomes a parallel propagating annular light beam and passes through the axicon.

[0021] Then, the parallel propagating annular beam illuminates a part of the dielectric sphere and generates a jet after passing through the dielectric sphere.

[0022] When the jet is tuned, the tuning relationship is:

[0023]

[0024] Among them, the parameters in the tuning relationship of the lens group for the annular beam are: n0 represents the refractive index of the terahertz wave in air; n1 represents the refractive index of the terahertz wave in the lens medium; θ1 represents the base angle of the pyramid pair; θ0 represents the exit angle of the terahertz wave passing through the negative prism; θ2 represents the angle between the terahertz wave passing through the negative prism and the central axis of the lens group; r in represents the inner radius of the annular beam; h represents the contact position of the innermost light of the annular beam with the cone surface of the positive axicon, and the axial distance from the apex of the cone; L represents the axial distance between the positive and negative axicons.

[0025] Adjust the axial distance L between the negative axis pyramid and the positive axis pyramid. The adjustment range is between L = 0 and L = L (max). L (max) is the axial distance between the positive and negative axis pyramids when the annular beam is at the maximum aperture. in (max) determines, where r in (max) is equal to the radius of the dielectric sphere minus the waist radius of the incident beam.

[0026] Furthermore, the lens group tunes the annular beam to produce annular beams with different apertures to illuminate the dielectric sphere. The aperture range of the annular beam is r in =0 to r in =r in (max), thereby generating jets with different characteristics, and realizing flexible tuning of the jet intensity, focal length, jet length, half-height full width, etc.

[0027] Specifically, when L = 0, the positive and negative axicons are close to each other in the axial direction, and the beams passing through the positive and negative axicons are not expanded into annular beams, and their aperture is r in = 0, the light beam is irradiated in the middle area of the dielectric sphere. At this time, the jet is in the most divergent state, with the smallest maximum intensity, the largest focal length, the largest jet length, the largest full width at half maximum, and the smallest maximum light intensity.

[0028] When L=L(max), the axial distance between the positive and negative axis pyramids is the largest, and the annular beam aperture after passing through the positive and negative axis pyramids is r in =r in (max), the annular beam illuminates the edge area of the dielectric sphere. At this time, the jet is in the most concentrated state, with the smallest focal length, the smallest jet length, the smallest full width at half maximum, and the largest maximum light intensity;

[0029] When 0<L<L(max), the positive and negative axicons have appropriate spacing in the axial direction, and the light beam passing through the positive and negative axicons is an annular light beam with an aperture of 0<r in <r in(max), the annular beam illuminates the area between the edge and the middle of the dielectric sphere. At this point, the jet is in a state between the most divergent and the most convergent, with its maximum intensity, focal length, jet length, full width at half maximum, and maximum light intensity all between the minimum and maximum values.

[0030] The lens group is combined with a terahertz scanning imaging system and applied to terahertz imaging to achieve imaging with tunable working distance, depth of field, and resolution.

[0031] Beneficial effects of the present invention:

[0032] This invention can be used in terahertz imaging applications. This lens assembly, coupled to a terahertz imaging system, enables tunable working distance, depth of field, and resolution, with optimal resolution exceeding the diffraction limit. This method can significantly improve the overall performance of terahertz imaging systems and greatly expand their application scenarios.

[0033] This invention is based on the terahertz effect (a strong focusing effect of terahertz waves characterized by high local intensity and a small beam waist) that can be generated by a mesoscopic dielectric sphere under terahertz wave irradiation. A pair of positive and negative axicons is first used to generate an annular beam with tunable aperture, which is then used to illuminate different areas of the dielectric sphere. By simply adjusting the aperture of the annular beam, the terahertz jet can be flexibly tuned from "tight focus" to "loose focus," thereby achieving tunable focal length, resolution, depth of field, and working distance during imaging. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 Schematic diagram of the three-dimensional structure and combination of lens group elements.

[0035] Figure 2 Schematic diagram of the lens group tuning relationship.

[0036] Figure 3 Schematic diagram of the optical path of a terahertz scanning imaging system coupled with a tunable lens group.

[0037] Figure 4 This is a simulation analysis diagram of the jet characteristics under different tuning states. DETAILED DESCRIPTION

[0038] The present invention will be described in further detail below with reference to the accompanying drawings.

[0039] The present invention proposes a tunable lens group based on the terahertz jet effect for terahertz super-resolution imaging. The lens group consists of three terahertz optical elements: a negative axicon, a positive axicon and a dielectric sphere.

[0040] When the lens group is assembled, the three elements should be coaxially installed in the optical path, keeping the axis aligned with the center of the incident light beam, and installed in the order of negative axicon, positive axicon and dielectric sphere according to the direction of the incident light beam. The three-dimensional structure and assembly diagram of the lens group elements are shown in the attached figure. Figure 1 shown.

[0041] When working, the terahertz wave is incident from the left side of the lens group.

[0042] After passing through the negative axicon, the terahertz beam is refracted on the rear surface of the negative axicon and becomes a divergent annular beam.

[0043] Then, the light is irradiated on the front cone surface of the axicon, and after being refracted again on the front cone surface, it becomes a parallel propagating annular light beam and passes through the axicon.

[0044] Then, the parallel propagating annular beam illuminates a part of the dielectric sphere and generates a jet after passing through the dielectric sphere.

[0045] When the jet is tuned, the tuning relationship is as shown in the attached Figure 2 As shown, after sorting, the tuning relationship can be expressed as: X is denoted as Wherein: the adjustment range of L is between L=0 and L=L(max); in The adjustment range is r in =0 to r in =r in (max). r in (max) is the maximum aperture of the annular beam, which is equal to the radius of the dielectric sphere minus the waist radius of the incident beam; L(max) is the axial distance between the positive and negative axicons when the annular beam is at maximum aperture.

[0046] Specifically, when L = 0, the positive and negative axicons are close to each other in the axial direction, and the beams passing through the positive and negative axicons are not expanded into annular beams, and their aperture is r in = 0, the light beam is irradiated in the middle area of the dielectric sphere. At this time, the jet is in the most divergent state, with the smallest maximum intensity, the largest focal length, the largest jet length, the largest full width at half maximum, and the smallest maximum light intensity.

[0047] When L=L(max), the axial distance between the positive and negative axis pyramids is the largest, and the annular beam aperture after passing through the positive and negative axis pyramids is r in =r in (max), the annular beam illuminates the edge of the dielectric sphere. At this time, the jet is in the most concentrated state, with the smallest focal length, the smallest jet length, the smallest full width at half maximum, and the largest maximum light intensity.

[0048] When 0<L<L(max), the positive and negative axicons have appropriate spacing in the axial direction, and the light beam passing through the positive and negative axicons is an annular light beam with an aperture of 0<r in <r in (max), the annular beam illuminates the area between the edge and the middle of the dielectric sphere. At this point, the jet is in a state between the most divergent and the most convergent, with its maximum intensity, focal length, jet length, full width at half maximum, and maximum light intensity all between the minimum and maximum values.

[0049] Thus, by simply adjusting L, annular beams of different apertures are achieved, and after irradiating the dielectric sphere, terahertz jets with different characteristics such as focal length, jet length, and half-maximum full width are generated. Since the focal length, jet length, and half-maximum full width of the terahertz jet characteristics correspond to the working distance, depth of field, and resolution of the imaging performance when applied to imaging, the working distance, depth of field, and resolution can all be tuned when applied to imaging. In addition, the other parameters in the tuning relationship of the lens group to the annular beam are: n0 represents the refractive index of the terahertz wave in air; n1 represents the refractive index of the terahertz wave in the lens medium; θ1 represents the base angle of the pyramid pair; θ0 represents the exit angle of the terahertz wave passing through the negative prism; θ2 represents the angle between the terahertz wave passing through the negative prism and the central axis of the lens group; r in represents the inner radius of the annular beam; h represents the contact position of the innermost light of the annular beam with the cone surface of the positive axicon, and the axial distance from the apex of the cone; L represents the axial distance between the positive and negative axicons.

[0050] Application examples:

[0051] The tunable lens assembly proposed in this invention is a key component of tunable terahertz high-resolution imaging. In actual use, it must be coupled with a conventional terahertz scanning imaging system to form an improved and complete scanning imaging device. To complete the tunable terahertz high-resolution imaging process proposed in this invention, the actual operation should include the following complete steps:

[0052] Step 1: Build a terahertz scanning imaging system;

[0053] In order to demonstrate or realize the complete process of terahertz high-resolution imaging, a set of equipment is needed as a basis. This step is the first step in building the equipment.

[0054] The terahertz scanning imaging system should at least include:

[0055] 1. Terahertz source. Common terahertz sources include terahertz photoconductive antenna, terahertz Gunn diode, terahertz quantum cascade laser, etc.

[0056] 2. Terahertz detectors. Common terahertz detectors include photoconductive antennas, pyroelectric detectors, terahertz balanced detectors, etc.

[0057] 3. Scanning stage, which is generally an electrically controlled high-precision stage that can automatically scan under programmed control;

[0058] 4. Signal acquisition module, generally an acquisition card, etc. This module is responsible for collecting the terahertz signal detected by the terahertz detector, converting it into a digital signal and outputting it to the signal recording and processing module.

[0059] 5. Signal recording and processing module, generally a computer, is responsible for recording and storing the terahertz detection signal during the point-by-point scanning process, and combining it with digital image processing to form the final terahertz imaging image.

[0060] 6. Several terahertz optical components, such as terahertz lenses, parabolic mirrors, terahertz polarizers, etc.

[0061] The above components need to be connected in a suitable relationship to form a terahertz scanning imaging system. The basic connection relationship can be described as follows:

[0062] First, a terahertz source emits a terahertz wave. A terahertz lens is then used to collimate the beam so that it propagates in parallel. The terahertz lens is then used to focus the beam onto the imaging sample, which is fixed to a scanning stage. The terahertz wave that passes through the imaging sample is then focused and collected by two lenses and directed to a terahertz detector. The detector detects the terahertz wave and transmits the signal to a signal acquisition module. The acquisition module collects the signal, converts it into a digital signal, and outputs it to a signal recording and processing module. After processing, a terahertz image of the sample is finally generated. Step 2: Couple a tunable lens group to the terahertz scanning imaging system;

[0063] A tunable lens group is added between the terahertz source and the terahertz detector, and the sample to be scanned and imaged is placed in the shadow measurement area illuminated by the terahertz wave of the dielectric sphere in the lens group;

[0064] The main purpose of this step is to couple the tunable lens group proposed in the present invention to the terahertz scanning imaging system described in step 1 to form a complete scanning imaging device. The optical path diagram of the terahertz scanning imaging system coupled with the tunable lens group is shown in the attached figure. Figure 3 As shown,

[0065] A terahertz source emits a terahertz wave under a modulated voltage. After being collimated by L1, it propagates in parallel and then strikes the flat side of the negative axicon in the lens assembly. After passing through the negative axicon, it refracts on the conical side, becoming a diverging annular beam. After passing through the positive axicon, it becomes a parallel annular beam and strikes a dielectric sphere, generating a terahertz jet on the shadowed side of the sphere. A sample, mounted on a scanning stage, is placed within the terahertz jet. After irradiation, the terahertz wave passes through the sample and is collimated by L2 and L3 before being directed to a terahertz detector. The terahertz detector detects the terahertz wave and transmits the signal to an acquisition module, which collects and converts the signal into a digital signal. This signal is then output to a signal recording and processing module, where it is processed to generate a terahertz image of the sample. L1, L2, and L3 represent terahertz focusing lenses. Furthermore, the radius of the dielectric sphere is preferably between 5λ and 30λ. The radial dimensions of the negative and positive axicons can be slightly larger than the sphere. Step 3: Adjust the lens group to achieve a tunable solar jet effect;

[0066] This step demonstrates the use of the tunable lens assembly proposed in the present invention.

[0067] This lens assembly is tunable, and its tuning method is simple. By simply adjusting the axial distance between the positive and negative axicons in the lens assembly, annular beams of different apertures can be generated to illuminate the dielectric sphere, thereby generating terahertz jets with different characteristics. The intensity, focal length, jet length, and full width at half maximum of the terahertz jet can be flexibly tuned. This allows for tunable imaging with working distance, depth of field, and resolution when applied to terahertz imaging.

[0068] The tuning relationship is as follows Figure 2 As shown, it can be expressed as: X is denoted as Wherein: the adjustment range of L is between L=0 and L=L(max); in The adjustment range is r in =0 to r in =r in (max). r in (max) is the maximum aperture of the annular beam, which is equal to the radius of the dielectric sphere minus the waist radius of the incident beam; L(max) is the axial distance between the positive and negative axicons when the annular beam is at maximum aperture.

[0069] The specific characteristics of the jet under various tuning states are as follows:

[0070] When L = 0, the positive and negative axicons are close to each other in the axial direction, and the light beams passing through the positive and negative axicons are not expanded into annular beams, and their aperture is r in= 0, the light beam is irradiated in the middle area of the dielectric sphere. At this time, the jet is in the most divergent state, with the smallest peak intensity, the largest focal length, the largest jet length, the largest full width at half maximum, and the smallest peak intensity.

[0071] When L=L(max), the axial distance between the positive and negative axis pyramids is the largest, and the annular beam aperture after passing through the positive and negative axis pyramids is r in =r in (max), the annular beam illuminates the edge of the dielectric sphere. At this time, the jet is in the most convergent state, with the smallest focal length, the smallest jet length, the smallest full width at half maximum, and the largest peak intensity.

[0072] When 0<L<L(max), the positive and negative axicons have appropriate spacing in the axial direction, and the light beam passing through the positive and negative axicons is an annular light beam with an aperture of 0<r in <r in (max), the annular beam illuminates the area between the edge and the middle of the dielectric sphere. At this point, the jet exhibits a state between the most divergent and the most convergent, with its peak intensity, focal length, jet length, full width at half maximum, and maximum light intensity all between the minimum and maximum values.

[0073] Step 4: Perform scanning imaging under different tuning states of the lens group;

[0074] This step is an experimental demonstration of the key functions of the tunable lens assembly proposed in this invention.

[0075] When the axial distance between the positive and negative axicons is adjusted, the lens group can generate jets with different characteristics, and the imaging system will have imaging capabilities with different characteristics;

[0076] When the axial distance between the positive and negative axicons is large, a large-aperture annular beam is obtained to illuminate the dielectric sphere, resulting in a more convergent jet with a larger peak intensity, smaller focal length, smaller jet length, and smaller full width at half maximum. This results in a lower resolution when used for imaging.

[0077] When the axial distance between the positive and negative axicons is small, a small-aperture annular beam illuminates the dielectric sphere, resulting in a more divergent jet with a smaller peak intensity, a larger focal length, a longer jet length, and a larger full width at half maximum. This results in a larger working distance and depth of field when used for imaging.

[0078] The four steps mentioned above will ensure that the tunable terahertz imaging experiment can proceed smoothly and realize terahertz super-resolution imaging with tunable working distance, depth of field and resolution.

[0079] As a further alternative to this lens group,

[0080] The lens group and lens group components may be made of materials other than polypropylene, such as polyethylene, polytetrafluoroethylene, high-resistance silicon, quartz, etc.

[0081] The specific dimensions of the lens group and lens group components can be adjusted to achieve similar functions, such as changing the base angles and radial dimensions of the positive and negative axis pyramids, and the diameter of the dielectric sphere.

[0082] The terahertz waves irradiating the lens group, in addition to plane waves with specific wavelengths and beam diameters, can also achieve similar functions with terahertz waves of other different parameters, such as different terahertz wave wavelengths, beam diameters, polarization states, and structured light under various parameters.

[0083] The imaging system that can be coupled with the lens group can be a variety of terahertz scanning imaging systems, such as pulsed time-domain spectroscopy scanning imaging system, continuous wave terahertz scanning imaging system, terahertz microscopy imaging system, etc.

[0084] In addition to being used individually, this lens group can also be arranged in an array. For example, it can be arranged in a grid or ring configuration to create an array of tunable lens groups, expanding its application scenarios. Tuning can be performed synchronously for all lens groups in the array, or individually for each lens group.

[0085] In addition, in order to further describe the specific characteristics of the tunable solar jet effect mentioned in step 3 under different tuning states, the following is further analyzed and demonstrated by means of geometric optics and numerical simulation, as shown in the attached figure. Figure 4 shown.

[0086] As attached Figure 4 As shown in the figure, taking the incident beam diameter of 10λ, the dielectric sphere radius of 15λ, the refractive index of the lens group material of 1.49, and the positive and negative axicon base angle θ=33.7° as an example, the jet characteristics under different tuning states are analyzed.

[0087] When the distance between the positive and negative axicons is minimal, that is, when they are close to each other, the light beam passes through the positive and negative axicons and illuminates the central area of the dielectric sphere. At this time, the jet generated by the dielectric sphere has the longest jet distance of 19λ and the maximum focal length of 30λ, corresponding to a maximum depth of field of 19λ and a working distance of 15λ during imaging (working distance = focal length - R). However, the light beam is more divergent at this time, and the full width at half maximum of the light beam is larger. The full width at half maximum of the light beam at the focus is 2.5λ, corresponding to a maximum resolution of 2.5λ during imaging.

[0088] When the distance between the positive and negative axis pyramids is adjusted to the maximum, that is, a ring-shaped beam with an outer aperture equal to the outer diameter of the dielectric sphere is obtained, and when it irradiates the edge area of the dielectric sphere, the jet generated by the dielectric sphere has a minimum jet distance of 3.2λ and a minimum focal length of 1.3λ, corresponding to a minimum depth of field of 3.2λ and a working distance of 1.3λ during imaging (working distance = focal length - R). However, at this time, the beam is most concentrated and has the smallest half-maximum full width, where the half-maximum full width at the focus can reach 0.44λ, corresponding to a minimum resolution of 0.44λ during imaging.

[0089] When the distance between the positive and negative axis pyramids is adjusted between the minimum and maximum, the focal length, injection length, and resolution can be obtained to have values between the maximum and minimum values. Therefore, in actual use, the most suitable injection state required can be adjusted according to different usage scenarios, thereby obtaining the most desired imaging effect. The tuning of this injection state is to tune the distance between the positive and negative axis pyramids to produce a ring-shaped light beam with adjustable aperture, so it can be infinitely tuned and easy to use. In addition, this example is only an example. In actual applications, its experimental parameters such as the incident light beam diameter, the dielectric sphere radius, the refractive index of the lens group material, the bottom angle of the positive and negative axis pyramid, etc. can be adjusted according to actual conditions.

[0090] The above simulation results fully demonstrate the excellent performance and practical application feasibility of the tunable lens group based on the terahertz jet effect for terahertz super-resolution imaging proposed in the present invention.

[0091] This paper proposes a novel method for generating tunable terrestrial jets by illuminating a dielectric sphere with an annular light beam with an adjustable aperture. Adjusting the annular light aperture can produce terrestrial jets with varying characteristics, such as intensity, focal length, jet length, and full width at half maximum, enabling diverse application scenarios.

[0092] The present invention proposes a new method for generating and tuning annular beams using a positive and negative axicon pair. The generation of annular beams and aperture tuning can be achieved by adjusting the axial distance between the positive and negative axicon pairs.

[0093] The present invention proposes a design method for a tunable lens group, which can be combined with a traditional terahertz scanning imaging system to achieve tunable terahertz super-resolution imaging. The construction method of the terahertz imaging system coupled with the lens group is the key to realizing the practical application of tunable imaging.

[0094] The present invention proposes such a tunable lens group, which requires appropriate parameters and parameter combinations to successfully achieve an ideal tunable jet effect and thus achieve good tunable imaging.

[0095] The lens assembly in this invention has excellent performance: the tunable lens assembly proposed in this invention can achieve an optimal depth of field of approximately 19λ, a working distance of 15λ, and a minimum super-diffraction resolution of 0.44λ when applied to terahertz imaging. It also has low bandwidth and energy loss.

[0096] This lens assembly is tunable: This invention proposes a tunable lens assembly that can achieve tunable depth of field, working distance, and resolution when applied to terahertz imaging. Tuning is flexible and simple, requiring only the axial distance between the positive and negative axicons to achieve this.

[0097] The lens assembly is easy to use: it can be easily integrated into existing conventional terahertz imaging systems. Simply coupling the lens assembly into the conventional terahertz imaging optical path can significantly improve imaging performance and achieve tunable imaging.

[0098] The lens group is easy to prepare. The elements of the lens group are positive and negative axis pyramids and dielectric spheres. They have simple shapes and large sizes. The materials are generally dielectric materials such as polypropylene (PP). They are easy to process and manufacture and have low costs.

[0099] The tunable lens assembly based on the terahertz jet effect for terahertz super-resolution imaging proposed in the present invention can solve multiple shortcomings of the prior art.

[0100] First, the imaging of this lens system is based on the highly focused terrestrial jet spot produced by the terrestrial jet effect of the dielectric sphere. This spot has high intensity, low bandwidth, and minimal energy loss. Furthermore, this lens system achieves high, super-diffraction-limited resolution. It generates an annular beam with adjustable aperture. When illuminating the edge of the dielectric sphere with a larger aperture annular beam, it produces an extremely fine terrestrial jet spot with a full-width at half-maximum of 0.44λ, enabling imaging with a super-diffraction-limited resolution of up to 0.44λ. Furthermore, this lens system has a large depth of field. When illuminating the center of the dielectric sphere with the minimum aperture annular beam, it produces an extremely long terrestrial jet spot with a jet length of up to 19λ, significantly extending the imaging depth of field. Furthermore, this lens system has a large working distance. When tuned to the appropriate distance, the resulting terrestrial jet spot has a large focal length and a large distance from the dielectric sphere, preventing contact and friction between the sample and the dielectric sphere during imaging. Finally, the lens assembly is tunable, allowing for flexible adjustment of the terahertz jet's intensity, focal length, jet length, and full width at half maximum, thereby enabling flexible tuning of the working distance, depth of field, and resolution during imaging. Tuning is simple, requiring only the axial distance between the positive and negative axicons to adjust the annular beam aperture, which in turn adjusts the area illuminated by the dielectric sphere. This allows the sphere to produce terahertz jet spots of varying shapes, from tightly focused to loosely focused. This enables the terahertz imaging system to achieve high-resolution terahertz imaging with tunable parameters across multiple imaging parameters.

Claims

1. A tunable lens assembly based on the terahertz jet effect for terahertz super-resolution imaging, characterized in that: It includes three terahertz optical elements: negative axicon, positive axicon and dielectric sphere; Each terahertz optical element is coaxially installed in the optical path, keeping the axis aligned with the center of the incident beam, and is installed in the order of negative axicon, positive axicon and dielectric sphere according to the direction of the incident beam; The distance between the positive axicon and the dielectric sphere is 1-5λ, and the distance between the negative axicon and the positive axicon is between L=0 and L=L(max); L(max) is the axial distance between the positive and negative axicons when the annular beam is at maximum aperture; The incident light beam is a plane light beam or a Gaussian light beam; The negative axicon and the positive axicon have the same base angle and are a pair of complementary pyramids; The negative axis pyramid is a body shape with a circular plane at the bottom and a conical concave top, which is rotationally symmetrical along the central axis; when in use, the incident light beam is incident from the bottom surface; The axicon is a body shape with a circular bottom surface and a conical protrusion on the top surface, which is rotationally symmetrical along the central axis. When in use, the incident light beam is incident from the top surface. The incident light beam is converged to a small local area through the negative axicon, the positive axicon and the dielectric sphere; The dimensional parameters of the negative axis pyramid, positive axis pyramid, and dielectric sphere should meet the following requirements: the radius of the dielectric sphere is between 5λ and 30λ, the radial dimensions of the negative axis pyramid and the positive axis pyramid are both 1-5λ larger than those of the dielectric sphere, the base angles of the negative axis pyramid and the positive axis pyramid are between 10° and 60°, and the refractive indices of the negative axis pyramid, the positive axis pyramid, and the dielectric sphere are all between 1.2 and 1.

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2. The tunable lens assembly based on the terahertz jet effect for terahertz super-resolution imaging according to claim 1, characterized in that: The tunable lens group is combined with a terahertz scanning imaging system and applied to terahertz imaging to achieve imaging with tunable working distance, depth of field, and resolution.

3. The method for using the tunable lens assembly based on the terahertz jet effect for terahertz super-resolution imaging according to claim 1, characterized in that: The following steps are included: First, after the terahertz wave passes through the negative axicon, the terahertz beam is refracted on the rear surface of the negative axicon and becomes a divergent annular beam. Then, the light is irradiated on the front cone surface of the axicon, and after being refracted again on the front cone surface, it becomes a parallel propagating annular light beam and passes through the axicon. Then, the parallel propagating annular beam illuminates a part of the dielectric sphere and generates a jet after passing through the dielectric sphere.

4. The method for using the tunable lens assembly based on the terahertz jet effect for terahertz super-resolution imaging according to claim 3, characterized in that: When the jet is tuned, the tuning relationship is: Among them, the parameters in the tuning relationship of the lens group to the annular beam are: n0 represents the refractive index of the terahertz wave in air; n1 represents the refractive index of the terahertz wave in the lens medium; θ1 represents the base angle of the pyramid pair; r in represents the inner radius of the annular beam; L represents the axial distance between the positive and negative axicons.

5. The method for using the tunable lens assembly based on the terahertz jet effect for terahertz super-resolution imaging according to claim 4, characterized in that: Adjust the axial distance L between the negative axis pyramid and the positive axis pyramid. The adjustment range is between L = 0 and L = L (max). L (max) is determined by r in (max) determines, where r in (max) is equal to the radius of the dielectric sphere minus the waist radius of the incident beam.

6. The method for using the tunable lens assembly based on the terahertz jet effect for terahertz super-resolution imaging according to claim 5, characterized in that: The tuning relationship of the lens group to the annular beam produces annular beams with different apertures to illuminate the dielectric sphere. The aperture range of the annular beam is r in =0 to r in =r in (max), thereby generating space jets with different characteristics, and realizing flexible tuning of the intensity, focal length, jet length, and half-height full width of the space jet.

7. The method for using the tunable lens assembly based on the terahertz jet effect for terahertz super-resolution imaging according to claim 6, characterized in that: When L = 0, the positive and negative axicons are close to each other in the axial direction, and the light beams passing through the positive and negative axicons are not expanded into annular beams, and their aperture is r in = 0, the light beam is irradiated in the middle area of the dielectric sphere. At this time, the jet is in the most divergent state, with the smallest maximum intensity, the largest focal length, the largest jet length, the largest full width at half maximum, and the smallest maximum light intensity.

8. The method for using the tunable lens assembly based on the terahertz jet effect for terahertz super-resolution imaging according to claim 6, characterized in that: When L=L(max), the axial distance between the positive and negative axis pyramids is the largest, and the annular beam aperture after passing through the positive and negative axis pyramids is r in =r in (max), the annular beam illuminates the edge area of the dielectric sphere. At this time, the jet is in the most concentrated state, with the smallest focal length, the smallest jet length, the smallest full width at half maximum, and the largest maximum light intensity.

9. The method for using the tunable lens assembly based on the terahertz jet effect for terahertz super-resolution imaging according to claim 6, characterized in that: When 0<L<L(max), the positive and negative axicons have appropriate spacing in the axial direction, and the light beam passing through the positive and negative axicons is an annular light beam with an aperture of 0<r in <r in (max), the annular beam is irradiated between the edge and the middle area of the dielectric sphere. At this time, the jet shows a state between the most divergent and the most convergent, and its maximum intensity, focal length, jet length, full width at half maximum and maximum light intensity are all between the minimum and maximum values.

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