CN114322863A - Method for far-field illumination and detection of diffraction-limit-breaching objects - Google Patents
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Abstract
The embodiment of the application provides a method for far-field illumination and detection of an object breaking through diffraction limit, which comprises the following steps: the method comprises the steps that a wave source emits waves to an object, the waves are scattered by the object to generate an evanescent field, wave signals capable of being transmitted to a far field are generated through excitation of the evanescent field, and the characteristic size of the object and the wave intensity of the wave signals changing along with time have a corresponding relation; the wave intensity detector detects the wave intensity changing along with time; the calculation unit acquires the wave intensity and the correspondence relation that vary with time and determines the characteristic size of the object based on both. The technical scheme of the embodiment of the application breaks through the limitation of diffraction effect, and can detect the object with any size based on the corresponding relation.
Description
Technical Field
The invention relates to the field of object detection, in particular to a method for carrying out far-field super-resolution detection on a microscopic object breaking through a diffraction limit.
Background
When an object is detected, high spatial frequency information (also referred to as information beyond the diffraction limit) of less than one-half wavelength of the detected object is carried by an evanescent field, which is, however, confined near the surface of the detected object and cannot propagate to the far field. Thus, typical far-field detection techniques can detect spatial imaging with a resolution limit of about one-half the wavelength of the incident wave.
The existing technologies for detecting the super-diffraction limit information of an object mainly comprise a near-field scanning microscopy technology, a super-lens detection technology based on an artificial microstructure material, a fluorescence microscopy detection based on a fluorescence label and the like.
In the near-field scanning microscopy technology, a wave source or a detector is placed in the near field of a detected object, and the evanescent field of the detected object is scanned point by point in the near field, so that the imaging speed is low, the information of the detected object to be detected is difficult to destroy, and a false image is easy to generate.
In the detection technology based on the artificial microstructure material super lens, the lens is placed in the near field of a detected object, so that the detected object needs to be placed close to the lens; also, since such lenses are constructed of artificial microstructured materials, the spatial resolution that can be achieved is severely limited by the fine microstructure, complexity (limited to micro-machining techniques), absorption losses, etc. of the lens.
In fluorescence microscopy based on fluorescent labeling, the nonlinear effect of materials is utilized, and high-power laser and nonlinear response materials (fluorescent labels) are needed; and hundreds of original images need to be scanned point-to-point or taken for image reconstruction, resulting in a complex and slow imaging process.
Disclosure of Invention
The invention solves the technical problems that the existing technology for detecting the super-diffraction limit information of an object has the defects of low detection speed, easy generation of false images, limited space resolution by a fine microstructure of a lens and the like, and needs to use a nonlinear material and a high-power laser.
To solve the above technical problem, an embodiment of the present invention provides a method for far-field illumination and detection of an object that breaks through a diffraction limit, including: the method comprises the steps that a wave source emits waves to an object, the waves are scattered by the object to generate an evanescent field, wave signals capable of being transmitted to a far field are generated through excitation of the evanescent field, and the characteristic size of the object and the wave intensity of the wave signals changing along with time have a corresponding relation; the wave intensity detector detects the wave intensity changing along with time; the calculation unit acquires the wave intensity and the correspondence relation that vary with time and determines the characteristic size of the object based on both.
Optionally, the wave is an electromagnetic wave, an elastic wave or an acoustic wave of any frequency band.
Optionally, the wave source and the wave intensity detector are both located in the far field, which is the region at a distance from the surface of the object greater than the wavelength of the wave.
Optionally, the wave has a single wavelength, the wavelength of the wave being a single wavelength; alternatively, the wave includes a plurality of discrete wavelengths, and the wavelength of the wave is any one of the plurality of discrete wavelengths.
Optionally, the computing unit is adapted to perform fourier transform on the wave intensity varying with time to obtain a characteristic vibration frequency, and obtain a characteristic size of the object based on the characteristic vibration frequency and a corresponding relationship, where the corresponding relationship is:
Ps＝λ/(1+fs/f0)，
wherein ,Psλ is the wavelength of the wave, f, being the characteristic dimensionsIs a characteristic vibration frequency, f0The center frequency of the wave.
Optionally, the method comprises: selecting a corresponding wave source based on the estimated size of the object; judging whether the characteristic vibration frequency can be obtained or not; if so, instructing the calculation unit to calculate the feature size based on the feature vibration frequency and the corresponding relation, if not, selecting wave sources with different wavelengths, repeating the judging steps until the feature vibration frequency is obtained, and instructing the calculation unit to calculate the feature size based on the feature vibration frequency and the corresponding relation.
Optionally, the characteristic dimension is a distance between portions adjacent to each other at the surface of the object, or an actual dimension of the object.
Optionally, the characteristic dimension is a distance between two locations inside the object, and the two locations scatter the waves emitted by the wave source to obtain a corresponding evanescent field, which excites a corresponding wave signal propagating to the far field, the time-varying wave intensity of the wave signal having a characteristic vibration frequency in the frequency domain after fourier transformation.
Alternatively, the generation of a wave signal that can be transmitted to the far field via excitation by the evanescent field is calculated by the formula for the complex amplitude of the wave signal:
wherein ,ψs(z, t) is the complex amplitude of the wave signal, i is the imaginary unit, t is the time of transmission of the wave signal, z is the propagation distance of the wave signal, ωsIs the characteristic angular frequency of the wave signal, ω is the angular frequency of the wave source, c is the velocity of the wave traveling in free space, k⊥For the wavenumber of the evanescent field, theta (t-z/c) is a switching function which takes a value of 0 when t ≦ z/c and takes a value of t>And z/c is 1.
Compared with the prior art, the technical scheme of the embodiment of the invention has at least the following beneficial effects.
For example, in the prior art, a wave emitted by a wave source toward an object is scattered by the object to generate an evanescent field, which is located in the near field and therefore cannot be detected in the far field. In the technical scheme of the embodiment of the invention, the wave emitted to the object by the wave source is scattered by the object to generate an evanescent field, and the evanescent field is excited to generate a wave signal which can be transmitted to a far field; since there is a correspondence between the characteristic dimension of the object and the wave intensity of the wave signal that varies with time, the characteristic dimension of the object can be determined based on the correspondence and the wave intensity detected by the wave intensity detector. The technical scheme of the embodiment of the invention can detect the wave signal excited by the evanescent field, which not only breaks through the limit of diffraction effect (super diffraction limit), but also can detect the object with any size based on the corresponding relation, for example, the characteristic size of the object.
For another example, in the existing near-field scanning microscopy, a wave source or a detector needs to be placed in the near field of a detected object to directly detect an evanescent field, and in the superlens detection technology based on an artificial microstructure material, a lens needs to be placed in the near field of the detected object, and the evanescent field needs to be converted into a propagation field to be detected; this makes it difficult to set the wave source, the detector or the lens, and the requirements for the fine microstructure, the absorption loss, etc. of the lens are strict, and the detection operation is complicated. In the technical scheme of the embodiment of the invention, the wave signal which is generated by being excited by the evanescent field and can be transmitted to the far field is detected, and the wave source and the detector can be placed in the far field in the detection process, so that the arrangement of the wave source and the detector is easy, and the detection operation is simple.
For another example, in the existing fluorescence microscopy, not only the nonlinear effect of the material is utilized, but also high-power laser and nonlinear response material, such as fluorescent marker, are needed, and a physical mechanism for detecting a super-resolution object is not provided in principle; but still require point-to-point scanning or taking hundreds or thousands of original images for image reconstruction, resulting in a complex and slow imaging process. In the technical scheme of the embodiment of the invention, a linear method is directly provided for detecting the super-resolution object in principle without depending on additional marking materials and nonlinear effects.
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FIG. 1 is a schematic diagram of one configuration of an apparatus for far field illumination and detection of diffraction limited breached objects in an embodiment of the present invention;
FIG. 2 is a schematic diagram of another configuration of an apparatus for far field illumination and detection of diffraction limited breached objects in an embodiment of the present invention;
FIG. 3 is a schematic illustration of the wave intensity and its corresponding frequency spectrum over time in an embodiment of the present invention;
FIG. 4 is a schematic illustration of the detection of a characteristic dimension of an object that breaches the diffraction limit in an embodiment of the present invention, where the characteristic dimension represents the variation of relief of the surface of the object;
FIG. 5 is a schematic illustration of the detection of a characteristic dimension of an object that breaches the diffraction limit in an embodiment of the present invention, where the characteristic dimension represents the actual size of the object;
FIG. 6 is a schematic illustration of the detection of a characteristic dimension of an object that breaches the diffraction limit in an embodiment of the present invention, wherein the characteristic dimension represents the internal structure of the object;
FIG. 7 is a flow chart of a method of far field illumination and detection of objects that breach the diffraction limit in an embodiment of the present invention.
Detailed Description
In order to make the aforementioned objects, features and advantages of the present invention comprehensible, embodiments accompanied with figures are described in detail below.
Embodiments of the present invention provide an apparatus for far field illumination and detection of objects that breach diffraction limits.
As shown in fig. 1, 2, 4 to 6, the apparatus 100, 200, 300, 400, 500 is used for far-field illumination and far-field detection of an object. The apparatus 100, 200, 300, 400, 500 may comprise a wave source 110, a wave intensity detector 120 and a calculation unit 130.
The wave source 110 may emit waves toward the object.
The wave may have a single wavelength (i.e., a wave having only one frequency), in which case the wavelength λ of the wave is the single wavelength. The wave may include a plurality of discrete wavelengths (i.e., a wave having a plurality of frequencies), and in this case, the wavelength λ of the wave may be any one of the plurality of discrete wavelengths.
In a specific implementation, the wave source 110 may be an electromagnetic field source that emits waves that are electromagnetic waves, such as radio waves, microwaves, infrared rays, visible light, ultraviolet rays, X-rays, and gamma rays; the wave emitted by the wave source 110 may also be an elastic wave source, and the emitted wave is an elastic wave, such as a bulk wave and a boundary wave; the wave source 110 may also be a sonic wave source that emits waves that are sonic waves, such as infrasonic, audible, ultrasonic, and hypersonic waves.
The object scatters the waves emitted by the wave source 110, thereby generating evanescent and propagating fields.
In the embodiment of the present invention, when the object scatters the wave emitted from the wave source 110, evanescent fields of different frequencies are generated, and the evanescent fields of different frequencies excite and generate a wave signal that can be transmitted to a far field, and the principle of generating the wave signal can be expressed by the following formula of complex amplitude of the wave signal:
wherein ,ψs(z, t) is the complex amplitude of the wave signal, i is the imaginary unit, t is the time of transmission of the wave signal, z is the propagation distance of the wave signal, ωsIs the characteristic angular frequency of the wave signal, ω is the angular frequency of the wave source, c is the velocity of the wave traveling in free space, k⊥For the wavenumber of the evanescent field, theta (t-z/c) is a switching function which takes a value of 0 when t ≦ z/c and takes a value of t>And z/c is 1.
The above formula applies to evanescent fields (e.g. its wavenumber k)⊥) And wave signals generated by its excitation and transmissible to far field [ e.g. its complex amplitude psis(z,t)]An association is established between them.
In the above formula, the complex amplitude ψ of the wave signal is such that the propagation distance z of the wave signal gradually increases with the other parameters being knowns(z, t) is gradually decreased.
In embodiments of the invention, scattering of waves by an object includes both transmission type scattering and reflection type scattering. The transmission type scattering means that incident waves are scattered after being irradiated on the surface, edge or internal structure of an object, and the waves generated after scattering can penetrate through the object and reach the wave intensity detector 120 located at the rear side of the object; reflection scattering means that an incident wave is scattered when it is irradiated on a surface, an edge, or an internal structure of an object, and a wave generated by scattering is reflected to reach a wave intensity detector 120 located on the front side of the object.
As shown in fig. 1, the apparatus 100 performs far-field illumination and far-field detection of an object 11. The object 11 may be transmission-type scattered, and the scattered wave passes through the object 11 to reach the intensity detector 120 located at the rear side of the object 11.
As shown in fig. 2, device 200 performs far-field illumination and far-field detection of object 12. Object 12 may reflectively scatter the waves, and the waves generated after scattering are reflected through object 12 to a wave intensity detector 120 located on the front side of object 12.
The wave intensity detector 120 can detect evanescent field excitation to produce a wave signal that is time varying wave intensity transmitted into the far field.
In some embodiments, the wave intensity detector 120 may be a known illuminometer, a wave intensity meter, a real part of amplitude detecting device, or the like, which converts the wave intensity of the wave signal into a current of a corresponding intensity, and determines the magnitude of a corresponding detected physical quantity based on the measured current.
The intensity detector 120 may also measure the intensity of waves at different wavelengths (or different frequencies) separately.
The wave intensity may vary with time, and the wave intensity detector 120 may detect the wave intensity in real time, thereby obtaining the distribution of the wave intensity in the time domain.
In the embodiment of the present invention, the wave emitted from the wave source has a wavelength λ, and a region having a distance to the surface of the detected object smaller than λ may be referred to as a near field, and a region having a distance to the surface of the detected object larger than λ may be referred to as a far field.
When the apparatus 100, 200, 300, 400, 500 is used for far-field illumination and far-field detection of an object, the wave source 110 and the wave intensity detector 120 are both located in the far-field.
In one embodiment, as shown in sub-diagram 3a of FIG. 3, the wave intensity detector 120 detects the distribution of the wave intensity over time in real time.
The time-varying wave intensity distribution can be fourier transformed to obtain a frequency spectrum, whose abscissa is the frequency domain corresponding to the wave intensity distribution, as shown in sub-diagram 3b of fig. 3.
In the frequency spectrum, has a characteristic vibration frequency fs。
The object may be a solid or a liquid, which may have a characteristic dimension PsWhich is related to the characteristic vibration frequency f in the above frequency spectrumsAnd (7) corresponding.
Characteristic dimension P of objectsLess than the diffraction limit, e.g. its length, width, height, diameter or the distance between the points whose surfaces are furthest apart may be less than half the wavelength λ of the wave.
In some embodiments, the feature size PsCan represent the great fluctuation of the surface of the objectIs small.
As shown in fig. 4, the apparatus 300 performs far-field illumination and far-field detection of the object 13. The surface of the object 13 has portions adjacent to each other, such as a crest a and a trough B, which are adjacent to each other, wherein the crest a and the trough B can be compared with respect to each other so as to have the crest and the trough opposite to each other, respectively. The peaks a and valleys B adjacent to each other are similar to a grating structure, and scatter the waves emitted by the wave source 110 to obtain corresponding evanescent fields, which excite corresponding wave signals propagating to the far field, the wave signals having a time-varying wave intensity, which, after fourier transformation, have a frequency (characteristic vibration frequency) f with a high frequency amplitude in the frequency domainsRefer to fig. 3. Characteristic dimension PsThe distance between the peak a and the valley B, i.e., the distance between the peak of the peak a and the valley bottom of the valley B, is indicated.
In other embodiments, the feature size PsThe actual size of the object may be represented.
As shown in fig. 5, the apparatus 400 provides far field illumination and far field detection of the object 14. The actual size of the object 14 may be represented by the edge positions C and D at which the surfaces are further or farthest apart. The object 14 is located between the edge positions C and D, which are similar to a grating structure, which scatters the waves emitted by the wave source 110 to obtain a corresponding evanescent field, which excites a corresponding wave signal propagating to the far field, which has a time-varying wave intensity, which, after fourier transformation, has a frequency with a high frequency amplitude in the frequency domain (characteristic vibration frequency) fsRefer to fig. 3. Characteristic dimension PsRepresenting the distance between edge positions C and D, i.e., the actual size of object 14.
In still other embodiments, the feature size PsMay represent the internal structure of the object.
As shown in fig. 6, the apparatus 500 provides far field illumination and far field detection of the object 15. The two locations E, F inside the object 15 have different materials, densities, configurations, concentrations, or other physical properties. The portion of the object 15 between the two locations E, F resembles a grating structure, scattering the waves emitted by the wave source 110 to obtain corresponding evanescent fields that excite corresponding wave signals propagating to the far fieldHaving a time-varying wave intensity which, after Fourier transformation, has a frequency (characteristic vibration frequency) f of high frequency amplitude in the frequency domainsRefer to fig. 3. Characteristic dimension PsIndicating the distance between the two locations E, F.
The above description with respect to the feature size P is described in connection with fig. 4 to 6sAlthough fig. 4 to 6 illustrate transmission type scattering, it should be understood that the above description is with respect to the characteristic dimension PsThe embodiments of (1) can also be applied to reflective scattering scenes.
The calculation unit 130 may acquire the wave intensity and the correspondence over time and determine the characteristic size of the object based on both.
In an embodiment of the invention, there is a correspondence between the size of the object and the time-varying wave intensity of the wave signal.
Specifically, the calculation unit 130 may receive the time-varying wave intensity output by the wave intensity detector 120, and may perform fourier transform on the time-varying wave intensity distribution to obtain a corresponding frequency spectrum. Thus, the correspondence between the characteristic dimension of the object and the time-varying wave intensity of the wave signal can be expressed as a correspondence between the characteristic dimension of the object and the corresponding frequency spectrum, which is expressed by the following formula:
Ps＝λ/(1+fs/f0)，
wherein ,Psλ is the wavelength of the wave, f, being the characteristic dimensionsIs a characteristic vibration frequency, f0The center frequency of the wave.
When the wave has a single wavelength, the wavelength λ of the wave in the above formula takes the single wavelength. When the wave comprises a discrete plurality of wavelengths, e.g. comprising λ1、λ2In this case, the wavelength λ of the medium wave in the above formula may be any one of a plurality of discrete wavelengths, for example, λ1Or λ2。
The calculation unit 130 may obtain the correspondence between the size of the object and the corresponding frequency spectrum or the characteristic size P of the objectsAnd characteristic vibration frequency fsThe corresponding relation of (2), which may be stored in advance in a memoryIn this case, the memory may be provided inside or outside the calculation unit 130. When the calculation unit 130 performs the calculation, the correspondence relationship may be read from the memory.
Theoretically, by the characteristic dimension P of the objectsAnd characteristic vibration frequency fsBased on the wavelength λ of the wave emitted from the wave source 110 and the characteristic vibration frequency fsCenter frequency f of sum wave0The calculating unit 130 can calculate the characteristic dimension P of the object with any sizes。
In the detection of practical experiments, the characteristic dimension P of the detectable objectsIs determined by the detection accuracy of the intensity detector 120. The present embodiment is based on the detection accuracy of the autonomous detector, and the characteristic dimension P of the detectable objectsOne sixth of the wavelength λ of the wave can be reached, breaking the diffraction limit (which is about one half of the wavelength).
In numerical simulation experiments, the characteristic dimension P of the object is passedsAnd characteristic vibration frequency fsIs calculated, the characteristic dimension P of the object which can be detectedsOne-thirtieth of the wavelength λ of the wave can be reached, which far breaks the diffraction limit.
In the actual detection, the device for far-field illumination and far-field detection of an object may comprise a control unit, and the wave source may be selectively set. For example, several wave sources with different wavelengths can be controlled by the control unit to selectively emit waves to the object, and the control unit can select one of the wave sources to emit waves to the object.
Specifically, first, the size of the detected object may be estimated, and then the control unit selects the corresponding wave source based on the estimated size. For example, based on the input estimated size a of the detected object, the control unit selects a wave source of the emitted wave having a wavelength of 2 a.
Then, the control unit judges whether or not the characteristic vibration frequency f can be determined in obtaining the corresponding frequency spectrum by performing Fourier transform on the wave intensity distributions. If the characteristic vibration frequency f can be determinedsThe control unit instructs the calculation unit to calculate the characteristic vibration frequency f based on the characteristic vibration frequencysCalculating the characteristic dimension P of the object by the corresponding relations. If the characteristic vibration frequency f cannot be determinedsThen the control unit can select wave sources with different wavelengths; the control unit repeats this step until a characteristic vibration frequency f can be determined in the frequency spectrumsAnd based on the characteristic vibration frequency fsCalculating a characteristic dimension P of an objects。
In an embodiment of the present invention, the computing Unit 130 and the control Unit may be processors, such as a Central Processing Unit (CPU), a general-purpose Processor, a microprocessor, a Digital Signal Processor (DSP), an Application Specific Integrated Circuit (ASIC), an off-the-shelf Programmable Gate Array (FPGA), other Programmable logic devices, a discrete Gate or transistor logic device, or a discrete hardware component.
Embodiments of the present invention also provide a method for far field illumination and detection of objects that breach diffraction limits.
As shown in fig. 7, a method 600 of far field illumination and detection of a diffraction-limited object includes: step S610, a wave source emits waves to an object, the waves are scattered by the object to generate an evanescent field, wave signals which can be transmitted to a far field are generated through excitation of the evanescent field, and the characteristic size of the object and the wave intensity of the wave signals, which changes along with time, have a corresponding relation; step S620, detecting the wave intensity changing along with time by a wave intensity detector; in step S630, the calculation unit acquires the wave intensity and the correspondence relation that vary with time and determines the characteristic size of the object based on both.
In a specific implementation, the method 600 may be performed based on any of the apparatuses 100, 200, 300, 400, 500 described above.
In a specific implementation, the calculating unit performs fourier transform on the wave intensity changing with time to obtain the characteristic vibration frequency, and the corresponding relation is as follows:
Ps＝λ/(1+fs/f0)，
wherein ,Psλ is the wavelength of the wave, f, being the characteristic dimensionsIs a characteristic vibration frequency, f0The center frequency of the wave.
In a specific implementation, the control unit may perform the following steps: selecting a corresponding wave source based on the estimated size of the object; judging whether the characteristic vibration frequency can be obtained or not; if so, instructing the calculation unit to calculate the feature size based on the feature vibration frequency and the corresponding relation, if not, selecting wave sources with different wavelengths, repeating the judging steps until the feature vibration frequency is obtained, and instructing the calculation unit to calculate the feature size based on the feature vibration frequency and the corresponding relation.
In specific implementation, the wave is an electromagnetic wave, an elastic wave or an acoustic wave in any frequency band.
In a specific implementation, both the wave source and the wave intensity detector are located in the far field, which is the region at a distance from the surface of the object greater than the wavelength of the wave.
In a specific implementation, the wave has a single wavelength, the wavelength of the wave being a single wavelength; alternatively, the wave includes a plurality of discrete wavelengths, and the wavelength of the wave is any one of the plurality of discrete wavelengths.
In a specific implementation, the characteristic dimension is a distance between portions adjacent to each other at a surface of the object, or an actual dimension of the object.
In a specific implementation, the characteristic dimension is the distance between two positions in the object, the two positions scatter the waves emitted by the wave source to obtain corresponding evanescent fields, the evanescent fields excite corresponding wave signals which propagate to a far field, and the time-varying wave intensity of the wave signals has characteristic vibration frequency in a frequency domain after Fourier transformation.
In a specific implementation, the generation of a wave signal that can be transmitted to the far field via evanescent field excitation is calculated by the following formula for the complex amplitude of the wave signal:
wherein ,ψs(z, t) is the complex amplitude of the wave signal, i is the imaginary unit, t is the time of transmission of the wave signal, z is the propagation distance of the wave signal, ωsIs a characteristic of a wave signalThe characteristic angular frequency, ω the angular frequency of the wave source, c the velocity of the wave traveling in free space, k⊥For the wavenumber of the evanescent field, theta (t-z/c) is a switching function which takes a value of 0 when t ≦ z/c and takes a value of t>And z/c is 1.
For specific principles, embodiments, and the like of the method 600 for far-field illumination and far-field detection of an object, reference may be made to the above-mentioned description of the apparatus for far-field illumination and far-field detection of an object in conjunction with fig. 1 to 6, and no further description is provided here.
Although the present invention is disclosed above, the present invention is not limited thereto. Various changes and modifications may be effected therein by one skilled in the art without departing from the spirit and scope of the invention as defined in the appended claims.
Claims (9)
1. A method for far field illumination and detection of a diffraction-limited object, comprising:
a wave source emits waves to the object, the waves are scattered by the object to generate an evanescent field, wave signals which can be transmitted to a far field are generated through excitation of the evanescent field, and the characteristic size of the object and the wave intensity of the wave signals change along with time have a corresponding relation;
a wave intensity detector detects the time-varying wave intensity;
a calculation unit acquires the time-varying wave intensity and the correspondence and determines a characteristic dimension of the object based on both.
2. The method according to claim 1, wherein the wave is an electromagnetic wave, an elastic wave, or an acoustic wave of an arbitrary frequency band.
3. The method of claim 1, wherein the wave source and the wave intensity detector are both located in a far field, the far field being a region of greater distance from the surface of the object than the wavelength of the waves.
4. The method of claim 1, wherein the wave has a single wavelength, the wavelength of the wave being the single wavelength; alternatively, the wave comprises a discrete plurality of wavelengths, and the wavelength of the wave is any one of the discrete plurality of wavelengths.
5. The method according to claim 1, wherein the computing unit is adapted to perform a fourier transform on the time-varying wave intensity to obtain a characteristic vibration frequency, and to obtain a characteristic dimension of the object based on the characteristic vibration frequency and the correspondence relationship:
Ps＝λ/(1+fs/f0)，
wherein ,Psλ is the wavelength of the wave, f, for the characteristic dimensionsIs the characteristic vibration frequency, f0Is the center frequency of the wave.
6. The method of claim 5, comprising:
selecting a corresponding wave source based on the estimated size of the object;
judging whether the characteristic vibration frequency can be obtained or not;
if so, instructing the calculating unit to calculate the characteristic dimension based on the characteristic vibration frequency and the corresponding relation, if not, selecting wave sources with different wavelengths, repeating the judging steps until the characteristic vibration frequency is obtained, and instructing the calculating unit to calculate the characteristic dimension based on the characteristic vibration frequency and the corresponding relation.
7. The method of claim 5, wherein the characteristic dimension is a distance between portions adjacent to each other at the surface of the object or an actual dimension of the object.
8. The method according to claim 5, wherein the characteristic dimension is a distance between two locations inside the object, said two locations scattering the waves emitted by the wave source to obtain respective evanescent fields which excite respective wave signals propagating to the far field, the time-varying wave intensities of the wave signals having the characteristic vibration frequency in the frequency domain after Fourier transformation.
9. The method of claim 1, wherein the generation of a wave signal transmissible to the far field via excitation of the evanescent field is calculated by the formula for the complex amplitude of the wave signal as follows:
wherein ,ψs(z, t) is the complex amplitude of the wave signal, i is the imaginary unit, t is the time of transmission of the wave signal, z is the propagation distance of the wave signal, ωsIs the characteristic angular frequency of the wave signal, ω is the angular frequency of the wave source, c is the velocity of the wave traveling in free space, k⊥Theta (t-z/c) is a switching function for the wavenumber of the evanescent field, the switching function having a value of 0 when t ≦ z/c and t ≦ z/c>And z/c is 1.
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