Femtosecond time-resolved imaging device for weak-phase objects based on vortex filtering

Abstract

The present invention relates to a device for realizing femtosecond time-resolved imaging of weak-phase objects based on vortex filtering, comprising a detection unit and a pump unit; after the emitted light of a femtosecond laser light source passes through a beam splitter, the transmitted light enters the detection unit as detection light, and the reflected light enters the pump unit as pump light; a vortex filter is added to the detection unit to introduce a geometric phase, and a weak-phase object is introduced on the incident surface of the detection light to realize high-contrast imaging of the weak-phase object in a dark background; an ultrafast time-resolved optical delay device is introduced into the pump unit to control and adjust the optical path between the pump optical path and the detection optical path to realize time resolution of the weak-phase object and realize imaging detection of the ultrafast dynamic process of the weak-phase object. The device is applied to detecting weak-phase objects in the biological field, such as biological cells, transparent organisms, etc.; high-contrast imaging of weak-phase objects in a dark background and imaging detection of ultrafast dynamic processes are realized.

Description

Technical Field

[0001] The invention relates to an imaging technology, and in particular to a device for realizing femtosecond time-resolved imaging of weak-phase objects based on vortex filtering. Background Art

[0002] Vortex light is a special light beam whose equal phase surface presents a spiral shape during transmission, with a spiral phase factor e imθ , where θ is the azimuth angle; m is the topological charge, also known as the quantum number of orbital angular momentum; and i is the imaginary unit. Due to the presence of a phase singularity in the center, the vortex light presents a dark hollow ring distribution, that is, the central field amplitude is zero, and a dark area with zero brightness appears on the beam cross section, which is called an optical vortex. Compared with ordinary Gaussian beams, vortex light has the advantages of good confidentiality, large transmission capacity, high efficiency, and easy encoding and decoding. It has broad application prospects in micro-nano processing, quantum communication, optical tweezers, etc.

[0003] In Fourier optics, placing a spatial filter on the spatial spectrum of an optical system and changing the amplitude and phase of the spatial frequency can change the image of the object field. Vortex filtering refers to a spatial filtering method that introduces a spatial filter with a spiral phase factor on the spectrum to achieve image change. In 1992, SNKhonina et al. introduced the transmittance function H(ρ,θ)=e on the spectrum of the 4f system. iθ , that is, a spatial filter with a topological charge of 1, which achieves uniform image edge enhancement effects through radial Hilbert transform. In 2005, Grover A swartzlander Jr proposed the optical vortex coronagraph technology [prior technology: G. Foo, DM Palacios, and GA Swartzlander, Optical vortex coronagraph, Optics letters (A). 2005: 30, 3308-3310]. This method eliminates the bright light from the propagation axis by introducing a spiral phase plate with a topological charge of 2 on the spectrum of the 4f system, but the signal of the dim planet or star outside the axis is not weakened. This method has greater advantages than other similar technologies because of its high-contrast imaging characteristics. At the same time, theory and experiments have proved that compared with the vortex filter with m = 1, the central dark core area obtained by using the vortex filter with m = 2 is larger, which means that a wider range of high contrast can be achieved.

[0004] Although image edge enhancement through radial Hilbert transform can play a certain role in the imaging field, its low contrast characteristic obviously limits its application in weak phase or weak signal imaging. Summary of the invention

[0005] Aiming at the limitation of the existing vortex filter imaging, a femtosecond time-resolved imaging device based on vortex filter is proposed, which can be applied to the optical vortex coronagraph for imaging weak phase objects and realize the femtosecond (1fs=10 -15 s) precision optical imaging with ultrafast time resolution capability.

[0006] The technical solution of the present invention is: a device for realizing femtosecond time-resolved imaging of weak-phase objects based on vortex filtering, comprising a detection unit and a pump unit; after the emitted light of a femtosecond laser light source passes through a beam splitter, the transmitted light enters the detection unit as the detection light, and the reflected light enters the pump unit as the pump light; a vortex filter is added to the detection unit to introduce a geometric phase, and a weak-phase object is introduced on the incident surface of the detection light to realize high-contrast imaging of the weak-phase object in a dark background; a femtosecond time-resolved optical delay device is introduced into the pump unit to control and adjust the optical path between the pump optical path and the detection optical path to realize time resolution of the weak-phase object and realize imaging detection of the ultrafast dynamic process of the weak-phase object.

[0007] Preferably, the detection unit includes a beam expansion system, a pinhole, a 4f system, a vortex filter and a CCD detector; the detection light passes through the beam expansion system to obtain an expanded beam with uniform light intensity distribution, then passes through the pinhole and enters the 4f system, a vortex filter is placed on the focal plane of the 4f system, a vortex phase factor is introduced to modulate the spectrum information, and the 4f system finally reaches the CCD detector after emitting, and the weak phase object is placed in the expanded beam of the incident pinhole.

[0008] Preferably, the beam expansion system is composed of a concave lens and a convex lens, the beam expansion ratio of the light spot is determined by the focal lengths of the two lenses, and the light passes through the beam expansion system to obtain a light spot with uniform energy distribution.

[0009] Preferably, the 4f system is composed of two imaging convex lenses, and the vortex filter is placed on the confocal plane of the two imaging lenses.

[0010] Preferably, the vortex filter is a spiral phase plate, a vortex half-wave plate or a spatial light modulator with a topological charge equal to ±2.

[0011] Preferably, when the vortex filter is a vortex half-wave plate, the light incident on the vortex filter is required to be circularly polarized light.

[0012] Preferably, after the vortex filter with a topological charge equal to ±2, the output light field expression on the final dark background imaging plane is:

[0013]

[0014] Where R is the radius of the pinhole, E2(r,θ) is the emitted light field expressed in polar coordinates, the light field inside the pinhole is zero, and the light field outside the pinhole is 1 / r2 distributed.

[0015] Preferably, the pump unit includes a delay device and a focusing lens. The delay device is composed of an electric translation stage with fs-level accuracy and two reflectors placed thereon. The pump light is adjusted in optical path by the delay device and then enters the focusing lens for focusing.

[0016] The beneficial effects of the present invention are as follows: the present invention realizes a femtosecond time-resolved imaging device of a weak-phase object based on vortex filtering, and realizes ultrafast time-resolved imaging of a weak-phase object based on vortex filtering, which is a brand-new application of spectral filtering and is aimed at weak-phase objects, especially weak-phase objects, and is expected to be applied to the detection of weak-phase objects in the biological field, such as biological cells, transparent organisms, etc.; the present invention introduces an ultrafast time-resolved system, which can realize time-resolved imaging of ultrafast physical processes, and expands the application field of the present invention; the pump-detection technology is used to accurately control the relative delay of the two beams of light by adjusting the relative optical path of the two beams of light, and the time resolution accuracy of imaging can reach the femtosecond level. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a schematic diagram of the structure of a device for realizing femtosecond-level time-resolved imaging of weak-phase objects based on vortex filtering according to the present invention;

[0018] Figure 2 This is a pinhole imaging diagram when the vortex filter modulation is not added in the present invention;

[0019] Figure 3 After adding the vortex filter to the present invention, the corona effect image with dark background is obtained;

[0020] Figure 4 This is an imaging diagram of a weak phase object under different time delays of the pump light and the detection light of the present invention. DETAILED DESCRIPTION

[0021] The present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process, but the protection scope of the present invention is not limited to the following embodiments.

[0022] The device for realizing femtosecond time-resolved imaging of weak-phase objects based on vortex filtering includes two parts: a detection unit and a pump unit; the detection unit part is used to image weak-phase objects on the incident surface; the pump unit part provides ultrafast time resolution capability for the optical system of the present invention. By adding a vortex filter to the detection unit and introducing a weak-phase object on the incident surface, high-contrast imaging of the weak-phase object in a dark background can be achieved; at the same time, an ultrafast time-resolved optical delay device is introduced into the pump unit part to realize imaging detection of ultrafast dynamic processes of weak-phase objects.

[0023] Figure 1 The present invention is a schematic diagram of the structure of a device for realizing femtosecond time-resolved imaging of weak phase objects based on vortex filtering. The device includes a femtosecond laser light source 1, a beam splitter 2, a reflector 3, a reflector 4, a reflector 5, a focusing lens 6, a reflector 7, a reflector 8, a reflector 9, a concave lens 10, a convex lens 11, a quarter wave plate 12, a pinhole 14, an imaging lens 15, a vortex filter 16, an imaging lens 17 and an imaging CCD 18. In the figure, A is the pump optical path, which includes a time delay device and a focusing lens 6. The time delay device is composed of a precision electric translation stage and two reflectors 3 and 4 placed thereon. The pump light enters the focusing lens 6 for focusing after the optical path is adjusted by the time delay device: it is composed of reflectors 3 and 4; B is the detection optical path, which includes a beam expansion system composed of reflectors 5, 7, 8, 9, a concave lens 10 and a convex lens 11, a quarter wave plate 12, a pinhole 14, a 4f system composed of two imaging lenses 15 and 17, a vortex filter 16 and a CCD 18. The linearly polarized beam of the femtosecond laser source 1 is divided into reflected light and transmitted light at different energy ratios after passing through the beam splitter 2. The transmitted light is the detection light. After the detection light is compensated by multiple reflectors to achieve the optical path difference with the pump optical path, it is expanded by the beam expansion system composed of a concave lens 10 and a convex lens 11 in sequence. The beam expansion ratio of the light spot is determined by the focal length of the two lenses, and the purpose is to obtain a light spot with uniform energy distribution; the expanded linear polarized light passes through the quarter wave plate 12 and becomes circularly polarized light (Note: when the vortex filter is a spiral phase plate, it is not necessary to convert the linear polarized light into circularly polarized light), and then passes through the small hole 14 at the entrance pupil, and then passes through the imaging lens 15 of the 4f system, the vortex filter 16 and the imaging lens 17 of the 4f system in sequence, and finally forms an image on the CCD 18. Without the introduction of the weak phase object 13 and the absence of the pump optical path, the CCD detects a dark background. After the weak phase object 13 is introduced between the quarter wave plate 12 and the small hole 14, the weak phase object can be detected in the aforementioned dark background. The reflected light of the beam splitter 2 is the pump light. After passing through the delayed optical path, the pump light is focused by the focusing lens 6. At the focus, the pump light interacts with the weak phase object 13. By controlling the optical path between the pump optical path and the detection optical path, the delay between the two can be controlled, so that the change of the weak phase object with time after the interaction can be detected in the aforementioned dark background, that is, the time resolution of the phase-type object is realized, and then its ultrafast dynamic physical mechanism is studied.

[0024] The following are the parameters of the embodiment. This embodiment uses a titanium sapphire femtosecond laser source with a central wavelength of 796nm. The weak phase object of this embodiment uses a plasma filament formed by focusing a femtosecond laser on the air as the research object. Plasma filaments are generated when a femtosecond strong laser is transmitted in the air under the combined effects of the self-focusing effect, the plasma defocusing effect generated by ionization, and many other physical effects. At this time, the energy of the femtosecond laser reaches a dynamic balance and forms a slender plasma filament. Plasma filaments can cause a slight change in the refractive index and are ideal weak phase objects.

[0025] The central wavelength of the femtosecond laser source 1 is 796nm, the pulse width is 35fs, the repetition frequency is 1kHz, and the horizontally polarized pulsed light is incident on the beam splitter 2 with a reflectivity of 75% and a transmittance of 25% to be divided into reflected light and transmitted light. The transmitted light passes through multiple reflectors to compensate for the optical path difference with the pump optical path A, and then passes through a concave lens 10 with a focal length of -10cm and a convex lens 11 with a focal length of 40cm to obtain an expanded light beam, where the spot diameter is expanded by 4 times. This embodiment uses a vortex half-wave plate as a filter, so it is necessary to convert linear polarization into circular polarization. The expanded linear polarization enters the quarter-wave plate to become circularly polarized light, and then passes through a small hole 14 with a diameter of 2mm. After that, the circularly polarized light enters the 4f system, and passes through the imaging lens 15 with a focal length of 30cm, the vortex half-wave plate (vortex filter 16) with a vortex topological charge number m=2, the imaging objective lens 17 with a focal length of 30cm, and the imaging CCD18 with a pixel size of 3.69μm×3.69μm. Among them, the weak phase object 13 and the pinhole 14 should be placed in the same object plane as much as possible to obtain a better imaging effect; the vortex half-wave plate is placed on the confocal plane of the imaging lenses 15 and 17 to modulate the spectrum information (it is not required that the focal lengths of the two imaging lenses are the same). When no weak phase object is introduced and there is no vortex half-wave plate, CCD18 detects ordinary pinhole imaging, such as Figure 2 As shown; without introducing a weak phase object and adding a vortex half-wave plate, the focal plane spectrum information of the pinhole in the 4f system is modulated, and at this time, CCD18 detects a corona-like image on the imaging surface, as shown Figure 3 As shown in the figure, the light intensity inside the pinhole is 0, and the detection light is diffracted outside the pinhole. When a weak phase object, i.e., a plasma filament, is introduced, a light filament pattern is detected on the imaging surface CCD18, as shown in Figure 4 As shown in the figure, it is a diagram of light filaments under different delays. The optical path for the formation of plasma filaments is: the femtosecond laser is reflected by the beam splitter 2, then passes through the delay device of the pump optical path A and the focusing lens 6 in sequence, and finally forms a plasma filament at the focus of the lens 6. This embodiment uses an electric translation stage delay device with an accuracy of 33fs (the minimum step length of the translation stage is 0.005mm. Since the incident and return pass through the delay line twice, the minimum step length provides an optical path difference of 0.005mm*2 between the two beams, divided by the speed of light in air 3*108 m / s, and the accuracy of the delay between the pump and detection beams is 33fs. ), the step size of the translation stage can be set in the software to control the optical path difference between the pump light path A and the detection light path B, thereby changing the delay between them; the focal length of the focusing lens 6 is 10cm. When a weak-phase object plasma filament is introduced and the optical path lengths of the two light paths A and B are made equal at the same time, a bright line will appear in the dark background originally detected by CCD18. The bright line is due to the introduction of the refractive index change by the plasma, which changes the phase of the incident surface of the detection light. At this time, it can be regarded as the detection light path imaging the plasma, a weak-phase object. Furthermore, by adjusting the optical path difference between the pump light path and the detection light path, the dynamic formation process of the plasma and the decay process after formation can be detected. Figure 4 These are images of plasma filaments when the time delays of the detection light and the pump light are 1.163ps, 1.578ps, 2.076ps, and 2.574ps, respectively, demonstrating the formation and evolution process of plasma filaments.

[0026] The vortex half-wave plate is used as the vortex filter 16 because the geometric phase can be introduced after the circularly polarized light is incident on the vortex half-wave plate, so that the outgoing light carries the spiral phase factor. The vortex light obtained by the circularly polarized light passing through the vortex half-wave plate can be expressed by the Jones matrix of the light field:

[0027] represents the incident circularly polarized light, is the Jones matrix expression of the vortex half-wave plate, where m is the vortex topological charge and α is the radial angle of the fast axis of the vortex half-wave plate. In this embodiment, a vortex half-wave plate with m=2 is used, and the output light field after the circularly polarized light is incident on the vortex half-wave plate 16 is:

[0028]

[0029] From equation (1), it can be concluded that the output light field after the circularly polarized light is incident on the vortex half-wave plate 16 carries the spiral phase factor e i2α , becomes vortex light, and the polarization state becomes reverse circular polarization. Introducing the spiral phase factor e i2α After modulating the spectrum information of the 4f system, the output light field of the pinhole with a diameter of 2 mm on the imaging surface is:

[0030]

[0031] Where R is the radius of the pinhole, and E2(r,θ) is the emitted light field expressed in polar coordinates. From this expression, we can see that the light field inside the pinhole is zero, and the light field outside the pinhole is 1 / r. 2 The light intensity is diffracted outside the small hole and the Figure 3 dark background.

[0032] The vortex topological charge number m=+2 or -2, which respectively indicates whether the vortex light beam phase rotates counterclockwise or clockwise around the central axis. Therefore, the final output light field on the dark background imaging plane is:

[0033]

[0034] Figure 1 In the embodiment, if plasma filaments are used as weak phase objects, the focusing lens 6 can be of other focal lengths to form plasma filaments; if other weak phase objects are used, the focusing lens 6 may not be needed when the laser energy is sufficient to interact with the weak phase objects. The imaging lens 15 and the imaging lens 17 of the 4f system can be replaced by two objective lenses, and the imaging resolution of the system is higher at this time. The vortex filter is a spiral phase plate, a vortex half-wave plate, or a spatial light modulator with a topological charge equal to 2. The vortex filter needs to be placed at the confocal plane of the 4f system, that is, the spectrum plane position, wherein, if a vortex half-wave plate is used, the light incident on the vortex filter is required to be circularly polarized light.

[0035] The above-mentioned embodiments only express several implementation methods of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A device for realizing femtosecond time-resolved imaging of weak-phase objects based on vortex filtering, characterized in that: It includes a detection unit and a pump unit; after the light emitted by the femtosecond laser light source passes through a beam splitter, the transmitted light enters the detection unit as the detection light, and the reflected light enters the pump unit as the pump light; a vortex filter is added to the detection unit to introduce a geometric phase, and a weak-phase object is introduced on the incident surface of the detection light to achieve high-contrast imaging of the weak-phase object in a dark background; The pump unit introduces a femtosecond time-resolved optical delay device to control and adjust the optical path between the pump optical path and the detection optical path, thereby achieving time resolution of weak-phase objects and imaging detection of ultrafast dynamic processes of weak-phase objects. The detection unit comprises a beam expansion system, a pinhole, a 4f system, a vortex filter and a CCD detector; the detection light passes through the beam expansion system to obtain a beam expansion light with uniform light intensity distribution, then passes through the pinhole and enters the 4f system, a vortex filter is placed on the focal plane of the 4f system, and a vortex phase factor is introduced to modulate the spectrum information. After the 4f system is emitted, it finally reaches the CCD detector, and the weak phase object is placed in the beam expansion light incident on the pinhole; The 4f system consists of two imaging convex lenses, and a vortex filter is placed on the confocal plane of the two imaging lenses; the vortex filter is a spiral phase plate, a vortex half-wave plate or a spatial light modulator with a topological charge equal to ±2.

2. According to claim 1, the device for realizing femtosecond time-resolved imaging of weak-phase objects based on vortex filtering is characterized in that: The beam expansion system is composed of a concave lens and a convex lens. The beam expansion ratio of the light spot is determined by the focal lengths of the two lenses. The light passes through the beam expansion system to obtain a light spot with uniform energy distribution.

3. The device for realizing femtosecond time-resolved imaging of weak-phase objects based on vortex filtering according to claim 1, characterized in that: When the vortex filter is a vortex half-wave plate, it is required that the light incident on the vortex filter is circularly polarized light.

4. The device for realizing femtosecond time-resolved imaging of weak-phase objects based on vortex filtering according to claim 1, characterized in that: After the vortex filter with a topological charge equal to ±2, the output light field expression on the dark background imaging surface is: Where R is the radius of the pinhole, E2(r,θ) is the emitted light field expressed in polar coordinates, the light field inside the pinhole is zero, and the light field outside the pinhole is 1 / r 2 distributed.

5. The device for realizing femtosecond time-resolved imaging of weak-phase objects based on vortex filtering according to claim 1, characterized in that: The pump unit comprises a time delay device and a focusing lens. The time delay device is composed of an electric translation stage with fs-level accuracy and two reflectors placed thereon. The pump light is adjusted in optical path by the time delay device and then enters the focusing lens for focusing.

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