An immersion tunable hemispherical lens for terahertz high-resolution imaging

Abstract

The application discloses an immersion type tunable hemispherical lens for terahertz high-resolution imaging, which comprises a lens shell, a medium hemispherical shell arranged in the lens shell and a liquid injected in the lens shell; the immersion type tunable hemispherical lens is coupled to a terahertz imaging system during use, the injection amount of the liquid in the lens shell is adjusted to realize the tuning of the focusing characteristics of the lens, and then the comprehensive tuning of the focal length, the depth of field and the resolution in imaging is realized; the immersion type tunable hemispherical lens can realize the adjustment of the focal length, the depth of field and the resolution, and has the advantages of high resolution, large depth of field and large working distance.

Description

Technical Field

[0001] This invention relates to the field of terahertz high-resolution imaging technology, and more specifically to a liquid-immersed tunable hemispherical lens for terahertz high-resolution imaging. Background Technology

[0002] Traditional terahertz (THz) high-resolution imaging includes scanning probe THz imaging, subwavelength aperture or subwavelength source THz imaging, and THz image restoration processing. Among these, scanning probe imaging 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, where the probe must be in close contact with the sample surface and cannot image in the far field, limiting its application scenarios. Subwavelength aperture or subwavelength source THz imaging can improve resolution to the diffraction limit, but subwavelength apertures typically use metallic apertures to limit the terahertz wave's illumination range on the sample, which limits spectral bandwidth and energy. When the aperture size is as small as the wavelength, significant energy and bandwidth are lost. Subwavelength source THz imaging often requires ultra-high peak power femtosecond lasers to excite terahertz radiation, which has high generation conditions, expensive equipment, and high operational difficulty, hindering its widespread application. THz image restoration processing is a method to improve image quality by using digital image processing techniques to enhance image contrast and resolution, and remove noise. It is a post-processing technique for images already formed by an optical system and cannot fundamentally improve resolution; it can only serve as an auxiliary means to improve image quality. Therefore, there is a real need to develop terahertz imaging systems with high resolution, strong overall performance, low cost, convenient maintenance, and wide applicability. In recent years, the nanophoton jet effect has been introduced into the terahertz band, generating a "terahertz jet" effect when terahertz waves irradiate structures such as cubic and sphere media (Appl. Phys. Lett. 105, 084102). Terahertz super-resolution imaging has been achieved based on the terahertz jet effect (Appl. Phys. Lett. 113, 031105).

[0003] The terahertz jet effect refers to the highly focused light spot produced on the shadow side of a medium structure of a certain size and refractive index when terahertz waves irradiate it, due to the special diffraction and scattering effects that occur during light propagation. This light spot has a jet-like intensity cross-section and is characterized by high brightness, a small beam waist, and a long jetting distance.

[0004] Super-resolution imaging based on the terahertz jet effect generally involves placing a medium structure with appropriate parameters in front of the sample. A highly focused terahertz jet spot generated by the medium structure illuminates a small local area of ​​the sample, obtaining imaging information for that area. Then, the entire sample is acquired through point-by-point scanning. Finally, the overall terahertz image of the sample is spatially stitched together. The principle behind achieving super-resolution is that in the point-by-point scanning imaging process, the terahertz jet spot illuminates only a small local area in a single imaging session. The transmitted or reflected beam carries only the sample information of the illuminated area, which corresponds to only one pixel in the image. Therefore, the size of the illuminated area directly determines the imaging resolution. Since the terahertz jet spot has a super-diffraction-limited beam waist diameter, super-diffraction-limited resolution can be achieved in the imaging. This method has advantages such as low energy loss, no loss of spectral bandwidth, far-field detection capability, and low sample contamination. However, despite these advantages, this method still has some shortcomings that need to be addressed. First, the solar jets generated by structures such as dielectric cubes and dielectric spheres are relatively close to the structure. Although the detection distance is greater than that of near-field probes, it is mostly still 1-2 times λ. This requires a small distance between the sample and the structure, increasing the risk of sample contamination. In addition, the full width at half maximum (FWHM) of solar jets generated by traditional structures is usually around 0.5λ, which can only be considered as barely breaking the diffraction limit. In practical applications, the resolution still needs to be further improved, and a smaller FWHM is required. More importantly, once the shape of traditional structures is determined, the solar jets they generate cannot be adjusted. They do not have the tuning function of resolution, focal length, working distance, depth of field, etc., which seriously limits the application requirements in different scenarios. Summary of the Invention

[0005] In order to overcome the shortcomings of the prior art, the present invention aims to provide a liquid-immersed tunable hemispherical lens for terahertz high-resolution imaging, which can fully adjust the focal length, depth of field and resolution, and has the advantages of high resolution, large depth of field and large working distance.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A liquid-immersed tunable hemispherical lens for terahertz high-resolution imaging includes a lens housing 1, a dielectric hemisphere 2 inside the lens housing 1, and a liquid 3 injected into the lens housing 1. In use, the liquid-immersed tunable hemispherical lens is coupled to a terahertz imaging system. By adjusting the amount of liquid 3 injected into the lens housing 1, the focusing characteristics of the lens are tuned, thereby achieving comprehensive tuning of focal length, depth of field, and resolution in imaging.

[0008] The bottom material of the lens housing 1 must have high transmittance in the terahertz band, including polyethylene, polytetrafluoroethylene, high-resistivity silicon, and quartz; the side wall material can be any solid material; the dielectric hemisphere 2 is made of a solid material with high transmittance to terahertz waves, including polyethylene, polytetrafluoroethylene, high-resistivity silicon, and quartz; the liquid 3 is made of a non-polar liquid material with high transmittance to terahertz waves, including white oil, vegetable oil, mineral oil, and electrical insulating oil.

[0009] The lens housing 1 has a regular or irregular shape and can accommodate the medium hemisphere 2 and the liquid 3; the regular shape includes cuboid, cube, cylinder and polygonal prism.

[0010] The medium hemisphere 2 is replaced by polyhedra such as spheres, ellipsoids, cubes, cuboids, pyramids, and irregular shapes.

[0011] Terahertz imaging systems with coupled immersion tunable hemispherical lenses include pulsed time-domain spectral scanning imaging systems, continuous wave terahertz scanning imaging systems, and terahertz microscopic imaging systems.

[0012] The liquid-immersed tunable hemispherical lenses are arranged in arrays in different spatial arrangements. The array arrangement can be in the form of straight lines, curves, grids, concentric rings, or other arbitrary arrays. When performing the tuning function, each liquid-immersed tunable hemispherical lens in the array can be tuned individually, or all liquid-immersed tunable hemispherical lenses can be tuned synchronously.

[0013] A method for fabricating a liquid-immersion tunable hemispherical lens for terahertz high-resolution imaging includes the following steps:

[0014] Step 1: Construct a lens housing 1 capable of containing the medium hemisphere 2 and the liquid 3;

[0015] Step 2: Install the medium hemisphere 2 inside the bottom surface of the lens housing 1, keeping the flat side facing down;

[0016] Step 3: Inject liquid 3 into the lens housing 1 to partially immerse the medium hemisphere 2, forming a liquid-immersed hemispherical lens; adjust the focusing characteristics of the lens by adjusting the amount of liquid 3 injected, thus forming a liquid-immersed tunable hemispherical lens.

[0017] A method for adjusting a liquid-immersion tunable hemispherical lens for terahertz high-resolution imaging, comprising:

[0018] When the liquid 3 is reduced, the immersion depth of the medium hemisphere 2 decreases. This is accompanied by a decrease in the focal length of the lens, the length of the jet spot, and the size of the beam waist. When combined into a terahertz imaging system, this results in a reduction in the focal length, working distance, depth of field, and resolution of the imaging system.

[0019] When liquid 3 is added, the immersion depth of the medium hemisphere 2 increases. Accompanied by the increase of the focal length of the lens, the length of the jet spot, and the size of the beam waist, when combined into a terahertz imaging system, the focal length, working distance, depth of field, and resolution of the imaging system are increased.

[0020] Infinitely variable adjustment is achieved by slowly increasing or decreasing the volume of liquid 3.

[0021] A terahertz imaging system with a coupled liquid-immersed tunable hemispherical lens includes a terahertz source controlled by a modulation module. The terahertz wave emitted by the terahertz source passes downward through a first terahertz lens and illuminates the liquid-immersed tunable hemispherical lens. The terahertz beam emitted by the liquid-immersed tunable hemispherical lens is focused downward onto the imaging sample located on the scanning stage. The imaging terahertz signal passes downward through a second terahertz lens and a third terahertz lens and is transmitted to a terahertz detector. The terahertz signal received by the terahertz detector is transmitted to a computer via a signal acquisition module. The computer records and processes the signal output from the signal acquisition module to ultimately form a complete terahertz image, and controls the scanning stage and modulation module.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0023] The imaging of the liquid-immersion tunable hemispherical lens of this invention is based on a highly focused light spot generated by the "too jet" effect, which has high light spot intensity, small beam waist size, long longitudinal length, and no bandwidth or energy loss.

[0024] The immersion-type tunable hemispherical lens of this invention has high resolution and subwavelength resolution over a wide adjustment range; when the immersion state of the medium hemispherical is adjusted to a suitable state, the optimal super-diffraction limit resolution of 0.5λ can be achieved.

[0025] The immersion-type tunable hemispherical lens of this invention has a large depth of field. When the medium hemisphere is at the minimum immersion height, its light spot jet length is 2.9λ. When the immersion height is increased by injecting liquid, the light spot jet length will gradually increase, and its maximum jet length can reach 20λ. When applied to imaging, it can achieve a great expansion of the depth of field.

[0026] The immersion-type tunable hemispherical lens of this invention has a large working distance. When the incident light is incident from the spherical surface of the dielectric hemisphere, the distance between the focal point of the focused spot and one side of the lens plane is approximately the radius of the hemisphere. Taking a hemisphere with a diameter of 10λ as an example, the working distance is 5λ. Compared with the light spot of the dielectric spherical lens that is close to the surface, its working distance is greatly increased, which can avoid the lens from contacting and rubbing against the sample during imaging.

[0027] The immersion-type tunable hemispherical lens of this invention is tunable. By simply increasing or decreasing the amount of liquid, the immersion height of the medium hemisphere can be adjusted, thereby regulating the intensity, focal length, jet length, and full width at half maximum (FWHM) characteristics of the focused spot. This allows for comprehensive tuning of the working distance, depth of field, and resolution during terahertz imaging.

[0028] In summary, the present invention has the following advantages:

[0029] 1. Excellent performance: When the liquid-immersion tunable hemispherical lens of this invention is applied to terahertz imaging, it has the superior performance of low bandwidth and energy loss, high resolution, large working distance and wide adjustable depth of field. It can achieve a working distance of more than 4λ, a depth of field adjustment range of 0.8λ to more than 14λ, and a minimum high resolution of 0.59λ.

[0030] 2. Tunable focusing characteristics: The liquid-immersion tunable hemispherical lens of this invention can achieve flexible adjustment of focusing characteristics such as focal length, focusing depth, and beam waist by simply adjusting the immersion depth of the medium hemisphere in the liquid, thereby achieving comprehensive adjustment of working distance, depth of field, and resolution in imaging.

[0031] 3. High practicality: The liquid-immersion tunable hemispherical lens of this invention can be easily combined with existing terahertz imaging systems, thereby being applied to terahertz imaging to achieve high-quality tunable imaging; in addition, the liquid-immersion tunable hemispherical lens of this invention is simple to operate, and only requires simply increasing or decreasing the injected liquid to adjust the immersion depth of the medium hemisphere in the liquid, so as to achieve tunable terahertz imaging.

[0032] 4. Easy to manufacture and low cost: The liquid-immersion tunable hemispherical lens of this invention consists of only three parts: lens shell, medium hemisphere and liquid. It has a simple structure, regular shape, cheap materials, simple manufacturing process and low cost. Attached Figure Description

[0033] Figure 1 This is a schematic diagram of the structure of the liquid-immersion tunable hemispherical lens of the present invention.

[0034] Figure 2 This is a schematic diagram illustrating the fabrication process of the liquid-immersion tunable hemispherical lens of the present invention.

[0035] Figure 3 This is a schematic diagram of the tuning method of the immersion-type tunable hemispherical lens of the present invention.

[0036] Figure 4 This is a schematic diagram of the terahertz imaging optical path of the present invention coupled with a liquid-immersed tunable hemispherical lens.

[0037] Figure 5 This is a simulation analysis diagram of the focusing characteristics of the immersion-type tunable hemispherical lens under different immersion conditions according to the present invention. Detailed Implementation

[0038] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0039] like Figure 1 As shown, a liquid-immersed tunable hemispherical lens for terahertz high-resolution imaging includes a lens housing 1, a dielectric hemisphere 2 inside the lens housing 1, and a liquid 3 injected into the lens housing 1. In use, the liquid-immersed tunable hemispherical lens is coupled to a terahertz imaging system. By adjusting the amount of liquid 3 injected into the lens housing 1, the focusing characteristics of the lens are tuned, thereby achieving comprehensive tuning of focal length, depth of field, and resolution in imaging.

[0040] The bottom material of the lens housing 1 must have high transmittance in the terahertz band, including polyethylene, polytetrafluoroethylene, high-resistivity silicon, quartz, etc.; the sidewall material can be any solid material. The dielectric hemisphere 2 is made of a solid material with high transmittance to terahertz waves, including polyethylene, polytetrafluoroethylene, high-resistivity silicon, quartz, etc. The liquid 3 is made of a non-polar liquid material with high transmittance to terahertz waves, including white oil, vegetable oil, mineral oil, electrical insulating oil, etc.

[0041] The lens housing 1 has a regular or irregular shape and can accommodate the medium hemisphere 2 and the liquid 3; the regular shape includes cuboid, cube, cylinder and polygonal prism.

[0042] The liquid-immersed tunable hemispherical lenses are arranged in arrays in different spatial arrangements. The array arrangement can be in the form of straight lines, curves, grids, concentric rings, or other arbitrary arrays. When performing the tuning function, each liquid-immersed tunable hemispherical lens in the array can be tuned individually, or all liquid-immersed tunable hemispherical lenses can be tuned synchronously.

[0043] like Figure 2 As shown, a method for fabricating a liquid-immersion tunable hemispherical lens for terahertz high-resolution imaging includes the following steps:

[0044] Step 1: Construct a lens housing 1 capable of containing the medium hemisphere 2 and the liquid 3;

[0045] Step 2: Install the medium hemisphere 2 inside the bottom surface of the lens housing 1, keeping the flat side facing down;

[0046] Step 3: Inject liquid 3 into the lens housing 1 to partially immerse the medium hemisphere 2, forming a liquid-immersed hemispherical lens; adjust the focusing characteristics of the lens by adjusting the amount of liquid 3 injected, thus forming a liquid-immersed tunable hemispherical lens.

[0047] like Figure 3As shown, a method for adjusting a liquid-immersion tunable hemispherical lens for terahertz high-resolution imaging includes:

[0048] Since the key to achieving tunable focusing characteristics of the liquid-immersed tunable hemispherical lens lies in the immersion depth of the liquid 3 in the medium hemisphere 2, the tunability of the liquid-immersed tunable hemispherical lens can be achieved by adjusting the amount of liquid 3 injected into the lens housing 1. When the liquid 3 is reduced, the immersion depth of the medium hemisphere 2 decreases, which, along with the reduction of the lens's focal length, jet spot length, and beam waist size, results in a reduction of the focal length, working distance, depth of field, and resolution when coupled to a terahertz imaging system.

[0049] When liquid 3 is added, the immersion depth of the medium hemisphere 2 increases, which is accompanied by an increase in the focal length of the lens, the length of the jet spot, and the size of the beam waist. When coupled to the terahertz imaging system, this results in an increase in the focal length, working distance, depth of field, and resolution of the imaging system.

[0050] Since the tuning of the liquid-immersion adjustable hemispherical lens is achieved by increasing and decreasing the amount of liquid 3 in the immersion medium hemisphere 2, stepless adjustment can be achieved by slowly increasing or decreasing the amount of liquid.

[0051] In addition, in practical applications, parameters such as the incident beam diameter, the radius of the medium hemisphere, and the refractive index of the lens group material can be adjusted according to the actual situation.

[0052] like Figure 4 As shown, a terahertz imaging system with a coupled liquid-immersed tunable hemispherical lens is generally vertically mounted. It includes a terahertz source controlled by a modulation module, which emits terahertz waves. The modulation module modulates the terahertz source to emit terahertz waves with specific radiation characteristics. The terahertz waves emitted by the source pass downwards through a first terahertz lens and illuminate the liquid-immersed tunable hemispherical lens, used to tune the focusing characteristics of the terahertz beam, achieving comprehensive tuning of the imaging system's focal length, working distance, depth of field, and resolution. The terahertz beam, after passing through the liquid-immersed tunable hemispherical lens, focuses downwards onto the image at the location where the scanning movement... On the imaging sample on the stage, the scanning displacement stage is used to move the imaging sample to achieve imaging; the imaging terahertz signal is transmitted downward through the second terahertz lens and the third terahertz lens to the terahertz detector, which is used to receive the terahertz signal; the terahertz signal received by the terahertz detector is transmitted to the computer through the signal acquisition module, which is used to acquire the terahertz signal received by the terahertz detector, convert it into a digital signal and output it to the computer for recording, and the computer is used to record and process the signal output by the signal acquisition module to finally form a complete terahertz image, as well as control the scanning displacement stage and the modulation module.

[0053] The first terahertz lens, the second terahertz lens, and the third terahertz lens are used to adjust and transmit terahertz beams.

[0054] like Figure 5 As shown, with the incident light as a plane wave, the radius of the medium hemisphere 2 being 5λ, the refractive index of the medium hemisphere 2 being 1.49, and the refractive index of the liquid 3 being 1.46, simulation analysis was conducted on four different immersion depths: no immersion, immersion depth 2λ, immersion depth 3λ, and immersion depth 4λ. This is to specifically demonstrate the characteristics of the liquid-immersed tunable hemispherical lens in achieving comprehensive tuning of focusing properties by adjusting the immersion depth. When the medium hemisphere 2 is not immersed in liquid 3, its focal length is 4.1λ, the jet length is 2.9λ, and the full width at half maximum (FWHM) at the focal point is 0.59λ. When the immersion depth of the medium hemisphere 2 is 2λ, 3λ, and 4λ, its focal length is 4.7λ, 4.8λ, and 6.3λ, respectively; the jet length is 5.5λ, 8.6λ, and 14.2λ, respectively; and the FWHM at the focal point is 0.82λ, 1.0λ, and 1.36λ, respectively. Therefore, by adjusting the immersion depth of the medium hemisphere 2 in the liquid-immersed tunable hemisphere lens, the focusing characteristics of the lens, such as focal length, jet length, and beam waist, can be comprehensively tuned. These results fully demonstrate the excellent performance and feasibility of the tunable liquid-immersed hemisphere lens for terahertz high-resolution imaging proposed in this invention.

Claims

1. A liquid-immersed tunable hemispherical lens for terahertz high-resolution imaging, comprising a lens housing (1), characterized in that: The lens shell (1) is provided with a medium hemisphere (2), and the lens shell (1) is filled with liquid (3); in use, the liquid-immersed tunable hemispherical lens is coupled into a terahertz imaging system, the injection amount of the liquid (3) in the lens shell (1) is adjusted, the tuning of the focusing characteristics of the lens is realized, and then the comprehensive tuning of the focal length, the depth of field and the resolution in imaging is realized; The liquid-immersed tunable hemispherical lens is combined into an array through different spatial arrangements, and the array arrangement shape is a straight line, a curved arrangement, a grid, a concentric circular ring surface arrangement and other arbitrary array modes; when the tuning function is performed, each liquid-immersed tunable hemispherical lens in the array is tuned individually, or all the liquid-immersed tunable hemispherical lenses are synchronously tuned. When the liquid (3) is reduced, the immersion depth of the medium hemisphere (2) is reduced, the focal length, the ejection spot length and the beam waist size of the lens are reduced, and when the lens is coupled into the terahertz imaging system, the focal length, the working distance, the depth of field and the resolution of the imaging system are reduced. When the liquid (3) is increased, the immersion depth of the medium hemisphere (2) is increased, the focal length, the ejection spot length and the beam waist size of the lens are increased, and when the lens is coupled into the terahertz imaging system, the focal length, the working distance, the depth of field and the resolution of the imaging system are increased.

2. The immersion tunable hemispherical lens of claim 1, wherein: The bottom surface material of the lens shell (1) needs to have high transmissivity in the terahertz wave band, including polyethylene, polytetrafluoroethylene, high-resistance silicon or quartz; the sidewall material is any solid material; the medium hemisphere (2) adopts a solid material with high transmissivity to terahertz waves, including polyethylene, polytetrafluoroethylene, high-resistance silicon or quartz; the liquid (3) adopts a nonpolar liquid material with high transmissivity to terahertz waves, including white oil, vegetable oil, mineral oil or electrical insulating oil.

3. The immersion tunable hemispherical lens of claim 1, wherein: The lens shell (1) is in a regular shape or an irregular shape, and can accommodate the medium hemisphere (2) and the liquid (3); the regular shape includes a cuboid, a cube, a cylinder or a multi-prism.

4. The immersion tunable hemispherical lens of claim 1, wherein: The medium hemisphere (2) is replaced by a spherical ball, an elliptical ball, a cube, a cuboid, a pyramid or an irregular shape.

5. The immersion tunable hemispherical lens of claim 1, wherein: The terahertz imaging system coupled with the liquid-immersed tunable hemispherical lens includes a pulsed time-domain spectral scanning imaging system, a continuous wave terahertz scanning imaging system or a terahertz microscopic imaging system.

6. The method of claim 1-5 for making an immersion tunable hemispherical lens for terahertz high-resolution imaging, characterized in that, The method comprises the following steps: First step: manufacturing a lens shell (1) capable of accommodating a medium hemisphere (2) and a liquid (3); Second step: installing the medium hemisphere (2) on the inner bottom surface of the lens shell (1) and keeping the plane downward; Third step: injecting the liquid (3) into the lens shell (1) to immerse part of the medium hemisphere (2) and form a liquid-immersed hemispherical lens; the focusing characteristics of the lens are adjusted by adjusting the injection amount of the liquid (3), and thus the liquid-immersed tunable hemispherical lens is formed.

7. The immersion tunable hemispherical lens of claim 1, wherein: The non-polar liquid material is slowly increased or reduced to realize stepless adjustment.

8. The terahertz imaging system of claim 1, wherein: The application comprises a terahertz source controlled by a modulation module, terahertz waves emitted by the terahertz source pass through a first terahertz lens and irradiate on an immersion tunable hemispherical lens, a terahertz light spot passing through the immersion tunable hemispherical lens is focused on an imaging sample on a scanning moving stage, an imaged terahertz signal passes through a second terahertz lens and a third terahertz lens and is transmitted to a terahertz detector, a terahertz signal received by the terahertz detector is transmitted to a computer through a signal acquisition module, the computer is used for recording and processing signals output by the signal acquisition module and finally forms a complete terahertz image, and the computer is used for controlling the scanning displacement stage and the modulation module.

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