CN117170089A - Terahertz optical system and mode conversion method - Google Patents
Terahertz optical system and mode conversion method Download PDFInfo
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Abstract
The application discloses a terahertz optical system and a mode conversion method, which solve the problem that a plurality of divergence suppression devices are needed for long distance of conversion of THz working modes. A terahertz optical system includes a divergence suppression module and a mode conversion module. The divergence suppression module is used for receiving and sending the incident terahertz light beam to the mode conversion module. The mode conversion module is used for changing different THz working modes by changing the positions of the optical elements, and the optical paths of the different THz working modes are the same and are used for receiving terahertz light and sending the terahertz light to the STM. According to the application, by adopting the magnetic attraction structure, the switching of multiple THz-STM working modes can be realized on the premise of not changing the optical path, and the THz strength electric control device is added, so that corresponding THz working mode support can be provided for various application scenes of the THz-STM, the experiment is more convenient, the operability is strong, and the cost is reduced.
Description
Technical Field
The application relates to the technical field of microscopes, in particular to a terahertz optical system and a mode conversion method.
Background
In 2013, frank subject group of university of Alberta in Canada proposes an experimental framework of a terahertz scanning tunnel microscope system (THz-STM) for the first time, and verifies the advantage of the THz-STM in super-high space-time resolution characteristic material surface characteristics in a rough environment of normal temperature atmosphere, wherein the space-time resolution can reach 2nm and 500fs. In the later experiments, the record is advanced to 0.1nm spatial resolution and 100fs time resolution, technical advantages are presented in multiple directions of THz high-resolution imaging, molecular dynamics detection, carrier diffusion process detection and the like, and the record has important technical value for researching the ultra-fast dynamics physical process of the material surface and developing a novel semiconductor device.
In these THz-STM-related experiments, phase stabilization is required, intensity-stabilized adjustable THz light is coupled to the STM tip region, and verification of multiple experimental functions by THz-STM needs to be done in different modes of operation, e.g., intensity and polarity adjustable THz pulses are required to measure the THz-STM intensity current curve. For this reason, the THz-STM layout involves multiple THz operation modes designed to fulfill the functional experimental requirements, where the designs include a michelson interferometer structure to generate coherent THz pulse pairs, a method of performing polarity inversion by a method of a coriolis phase shift, half-wave loss, and the like, and a rotatable Jin Xianshan THz polarizer combination to perform THz intensity control, and the like. However, the current optical system design cannot cover the design of multiple functions at the same time, the optical path change is not introduced when the working mode is switched, the strong divergence problem of long-distance transmission THz cannot be solved, and the divergence problem is usually solved by combining a plurality of off-axis parabolic mirrors, so that the experimental cost is increased.
Disclosure of Invention
The embodiment of the application provides a terahertz optical system and a mode conversion method, which solve the problem that a plurality of divergence suppression devices are needed for long distance of converting THz working modes.
The embodiment of the application provides a terahertz optical system, which comprises a divergence suppression module and a mode conversion module. The divergence suppression module is used for receiving and sending the incident terahertz light beam to the mode conversion module. The mode conversion module is used for changing different THz working modes by changing the positions of the optical elements, and the optical paths of the different THz working modes are the same and are used for receiving terahertz light and sending the terahertz light to the STM.
Further, the divergence-suppressing module includes an incident parabolic mirror, an exit parabolic mirror, and an enhanced-suppression parabolic mirror. The incident parabolic mirror is used for receiving terahertz light, and the incident parabolic mirror is used for enhancing and inhibiting the parabolic mirror after focusing. The enhancement suppression parabolic mirror is used for receiving the focused terahertz light and enhancing the focused terahertz light to be emitted into the emergent parabolic mirror. The emergent parabolic mirror is used for receiving the focused terahertz light and converting the focused terahertz light into terahertz light beams with the divergence degree smaller than a set threshold value. The parameters of the parabolic mirror are calculated through a Gaussian beam parameter transformation formula. The minimum beam waist position of the terahertz light which enhances focusing is at the focal position of the outgoing parabolic mirror.
Further, the mode conversion module includes: a plurality of reflectors, polarizers and an incidence module. The reflecting mirror is used for reflecting terahertz light. The polaroid is used for adjusting the terahertz. Terahertz light enters the incidence module after changing the light path and adjusting through the reflector and the polaroid. The incidence module is used for receiving terahertz light and sending the terahertz light to the STM after passing through.
Further, the device also comprises a beam splitter and a displacement table. The beam splitter is arranged on an incident terahertz light path and used for reflecting terahertz light to the first reflector and projecting the terahertz light to the second reflector. The first mirror or the second mirror is disposed on the displacement stage. The displacement table moves back and forth close to or far from the beam splitter and is used for adjusting the distance between the first reflecting mirror or the second reflecting mirror and the beam splitter.
Preferably, the parabolic mirror is an off-axis parabolic mirror.
Further, a gold mirror is also included. The gold mirror is used for reflecting THz full power to achieve the purpose of full reflection.
Further, the incidence module includes a terahertz lens, an off-axis parabolic mirror, and a third mirror. The terahertz lens is used for making terahertz light incident to the off-axis parabolic mirror and ensuring that the minimum beam waist position of the terahertz light is positioned at the focal position of the off-axis parabolic mirror. The reflecting mirror is used for changing the light path of the terahertz light and enabling the terahertz light to be incident to the STM.
Further, a fourth mirror is also included. And the fourth reflector is used for realizing a pi shift structure with the third reflector.
Preferably, the components in the mode conversion module adopt a magnetic attraction structure.
The embodiment of the application also provides a mode conversion method of the terahertz optical system, which uses the terahertz optical system in any one of the embodiments, and comprises the following steps:
selecting a THz working mode, and transforming the position of a component in the module according to the THz working mode;
performing divergence suppression on the terahertz light source;
and the terahertz light source after divergence inhibition is incident into an STM after passing through a mode conversion module.
The above at least one technical scheme adopted by the embodiment of the application can achieve the following beneficial effects:
according to the application, by adopting the magnetic attraction structure, the switching of multiple THz-STM working modes can be realized on the premise of not changing the optical path, and the THz strength electric control device is added, so that corresponding THz working mode support can be provided for various application scenes of the THz-STM, the experiment is more convenient, the operability is strong, and the cost is reduced.
Drawings
The accompanying drawings, which are included to provide a further understanding of the application and are incorporated in and constitute a part of this specification, illustrate embodiments of the application and together with the description serve to explain the application and do not constitute a limitation on the application. In the drawings:
FIG. 1 is a block diagram of a prior art THz transmission to address spot divergence;
FIG. 2 is a block diagram of a terahertz optical system according to an embodiment of the present application;
FIG. 3 is a block diagram of a divergence suppression module in accordance with an embodiment of the present application;
FIG. 4 is a graph of the variation of object distance with different focal lengths for the same incident Rayleigh length in accordance with an embodiment of the present application;
FIG. 5 is a graph of the same focal length, different incident Rayleigh lengths affecting the change in object distance according to an embodiment of the present application;
FIG. 6 is a schematic diagram of an autocorrelation mode of a mode conversion module according to an embodiment of the present application;
FIG. 7 is a diagram illustrating a total reflection mode structure of a mode conversion module according to an embodiment of the present application;
FIG. 8 is a schematic diagram illustrating a polarity switching configuration of a mode conversion module according to an embodiment of the present application;
Detailed Description
In order to make the objects, technical solutions and advantages of the present application more apparent, the technical solutions of the present application will be clearly and completely described below with reference to specific embodiments of the present application and corresponding drawings. It will be apparent that the described embodiments are only some, but not all, embodiments of the application. All other embodiments, which can be made by those skilled in the art based on the embodiments of the application without making any inventive effort, are intended to be within the scope of the application.
The following describes in detail the technical solutions provided by the embodiments of the present application with reference to the accompanying drawings.
Fig. 1 is a block diagram of a prior art THz transmission to address spot divergence.
The THz transmission has strong divergence effect, the problem of divergence of light spots needs to be solved before THz conversion is carried out, the problem of divergence of THz is solved in the prior art in a mode of combining a plurality of off-axis parabolic mirrors, the cost is increased, aberration can be generated due to the action of the off-axis parabolic mirrors on the THz, and the accumulation of the aberration also causes the problems of THz distortion at the needle point and the like. Compared with the prior art, the application only uses half of the off-axis parabolic mirrors, so that on one hand, the aberration of THz is reduced, and on the other hand, the cost is saved.
The application successfully realizes the design of the THz optical system applied to the terahertz scanning tunnel microscope system (THz-STM), outputs the THz light with controllable polarity and controllable intensity, can generate coherent THz light beam pairs, can also work in a single THz pulse mode, has high energy transmission efficiency and stable space transmission, can complete the optical path requirements of multiple working modes of the THz-STM under the condition of not changing the relative optical path of the THz, and can give consideration to the functions of the THz-TDS (terahertz time domain spectroscopy system). Because THz-STM has strict requirements on incident light, how to couple THz with stable phase and adjustable intensity to an STM needle point area is a key problem for researching the physical process of THz induced tunneling current on the surface of a material.
Thus, the optical properties of incident THz are of great significance to the development of various experiments on THz-STM, for example, verifying the time resolution capability of THz-STM requires a pair of phase-stable coherent THz pulses. This places various demands on the THz optical path, including THz polarity, THz stability, THz intensity adjustability, and THz optical path separability, which requires various optical path function and mode switching designs for THz. The function switching of the THz part layout of the THz-STM experiment in the prior art can change the optical path, so that the position of an aplanatic point is changed drastically when the THz-TDS is detected or the optical pumping-THz detection experiment is carried out, the inconvenience of the experiment is increased, and the problem of realizing the function under the condition of not changing the optical path size is a great difficulty of the THz-STM layout.
On the other hand, THz-STM requires long distance spatial transmission of THz, but the specific wavelength range of THz determines strong divergence in the spatial light transmission process, and a distance of 10cm is enough to enable THz light spots to diverge by several times, and larger mirror surfaces are needed for reflection, so that the cost is increased. Some THz-TDS experiments in existence do not need to consider the serious problem of light spot divergence due to the short THz transmission distance, but the light spot divergence caused by long-distance transmission becomes a major problem for THz light paths which need to be switched in multiple working modes and coupled to the STM tip.
Aiming at the requirements of multiple working modes of THz-STM on the optical characteristics of THz and the divergence problem caused by long-distance transmission of THz, the application provides the design of a related optical system through theoretical and experimental verification, effectively solves two technical problems, and provides a reference template for the construction of a domestic THz-STM system.
Fig. 2 is a block diagram of a terahertz optical system according to an embodiment of the application.
The embodiment of the application provides a terahertz optical system, which comprises a divergence suppression module 1 and a mode conversion module 2.
The divergence suppression module is used for receiving and sending the incident terahertz light beam to the mode conversion module.
Fig. 3 is a block diagram of a divergence suppression module in accordance with an embodiment of the present application.
Further, the divergence-suppressing module includes an incident parabolic mirror 11, an exit parabolic mirror 12, and an enhanced suppression parabolic mirror 13.
The incident parabolic mirror is used for receiving terahertz light, and the incident parabolic mirror is used for enhancing and inhibiting the parabolic mirror after focusing.
The enhancement suppression parabolic mirror is used for receiving the focused terahertz light and enhancing the focused terahertz light to be emitted into the emergent parabolic mirror.
The emergent parabolic mirror is used for receiving the focused terahertz light and converting the focused terahertz light into terahertz light beams with the divergence degree smaller than a set threshold value.
The emergent parabolic mirror converts focused terahertz light into near-uniform terahertz light, and a threshold value is set as the terahertz light beam with the divergence degree smaller than the threshold value is regarded as the uniform terahertz light beam because absolute uniformity is difficult to realize.
The parameters of the parabolic mirror are calculated through a Gaussian beam parameter transformation formula.
Terahertz beams generated by exciting an electro-optic crystal with a gaussian beam emitted from a 1030nm laser are also gaussian beams, which are represented as longitudinal sections (z=z 0 ) Optical power density in gaussian distribution.
The following wavefront features:
wherein, the wave front refers to z=z in the light wave transmission direction 0 Plane of constitution, A 0 Is a constant describing the electric field intensity mode, W (z) is the beam waist width at the gaussian beam cross section at a point in the transmission direction, x, y is the coordinates of two dimensions in a plane perpendicular to the transmission direction, k is the wave number of the gaussian beam, R (z) is the true radius of curvature of the equiphase plane at a point in the transmission direction, and phi (z) is the phase at that point.
The change of the curvature of the equiphase surface and the beam waist width along with z is as follows:
wherein W is 0 Is the minimum beam waist width and λ is the spatial light center wavelength.
The key parameters that typically describe the transmission of gaussian beams in space are the rayleigh length and the minimum beam waist radius. Rayleigh length is defined as the expansion of the beam waist radius to the minimum beam waist radiusA multiple of the transmission length, which implies whether the optical transmission is more linear or wave-like.
For example, for a 1THz, the minimum beam waist width is 3mm, which is only 20cm, i.e. light outside the range of 10cm from the focal point has diverged very much, while for a 1030nm laser this value is 55m, so it is apparent that the THz spot becomes very easily large and difficult to transmit in space.
For efficient transmission of THz, it is necessary to design a transmission path in space with an off-axis parabolic mirror, and for this purpose, it is necessary to study the variation of gaussian beam under the combination of parabolic mirror and reflecting mirror, and it is necessary to study the transmission of gaussian beam after passing through an imaging system.
W′ 0 ＝MW 0 Equation 5
(z′-f)＝M 2 (z-f) equation 6
2L′ 0 ＝M 2 (2L 0 ) Equation 7
Wherein W is 0 Is the minimum beam waist radius, L 0 Is the Rayleigh length, θ 0 Is the divergence angle, f is the focal length of the lens, and z is the object distance, which is the distance from the center of the paraboloid at the location of the smallest beam waist of the incident THz. Skimmed physical quantity (e.g., L' 0 ) Representing the physical quantity corresponding to the outgoing gaussian beam after off-axis parabolic mirror action.
The minimum beam waist position of the terahertz light which enhances focusing is at the focal position of the outgoing parabolic mirror.
In order to achieve divergence suppression of the THz spot, for any incident THz beam, an off-axis parabolic mirror combination of three specific parameters are required to achieve divergence suppression of the exiting THz, the specific parameters of which can be given in connection with simulation analysis.
For example, in order to obtain as uniform a beam as possible, i.e. as large an exit rayleigh length L 'as possible' 0 According to equations 7 and 9, it is desirable to have the minimum beam waist position of the incident THz at the focal position of the lens or off-axis parabolic mirror, where the rayleigh length of the exiting THz is maximum, but the minimum beam waist width is also enlarged to a maximum, which determines the position of the third parabolic mirror OPM3 relative to OPM2, as shown in fig. 3.
On the basis, the OPM3 emergent Rayleigh length L is further improved 0 ' on the one hand a shorter incident rayleigh length L can be achieved by a combination of OPM2 and OPM1 0 It is also possible to use a longer focal length OPM3, but this also increases the spot minimum beam waist radius W 0 ' i.e. the exit spotIs of a size of (a) and (b). For the optical mirror surface with the same section, a larger W 0 ' resulting divergent effect with shorter L 0 ' As such, strong divergence of the spot is easily blocked by the edge of the lens, which is detrimental to chopper modulation, thus W 0 ' also is a factor that requires optimization by OPM1 and OPM2, the actual optimization being required to optimize L 0 ' mainly, to optimize W 0 ' is auxiliary, both of which have an effect on the divergence of the THz spot.
For example, first the effect of off-axis parabolic mirror OPM2 focal length is analyzed for a fixed incident Rayleigh length L 0 Variable object distance z of =40 mm 0 Analyzing focal length of a common off-axis parabolic mirror by-20 mm: 50.8mm,76.2mm,101.2mm effect on the exit beam waist width and rayleigh length.
The off-axis parabolic mirror can significantly reduce aberrations and chromatic aberration, which is not possible with conventional parabolic mirrors. The off-axis parabolic mirror is functionally equivalent to a lens with a certain focal length, so that three lenses with a specific focal length are arranged on a straight line to achieve suppression, but the lens loss of the THz band is particularly serious, the lens loss of the high-density polyethylene material is as high as 30%, the use of a plurality of THz lenses can significantly reduce the intensity of THz, and the lens has great aberration and chromatic aberration for THz with a large light spot, so that the parabolic mirror is preferably an off-axis parabolic mirror.
Fig. 4 is a graph of the variation of object distance with different focal lengths for the same incident rayleigh length in accordance with an embodiment of the present application.
For example, for the same incident rayleigh length of 40mm, the effects of different focal lengths are analyzed (a) changing object distance, the ratio of the outgoing minimum beam waist radius to the incoming minimum beam waist radius changes with the focal length, the smaller the ratio, the smaller the focused light spot (b) changing object distance, and the outgoing rayleigh length changes with the focal length.
As shown in fig. 4, it can be seen from fig. 4 (a) (b), in order to satisfy the minimum incident rayleigh length condition proposed by equation 10, OPM2 should focus with an off-axis parabolic mirror with f=50.8 mm to achieve the minimum rayleigh length and beam waist radius, so that the longest exiting rayleigh length and minimum beam waist radius, that is, the divergence suppression effect, can be obtained after thz passes through OPM3 according to equation 10, and therefore OPM2 should select f=50.8 mm.
Fig. 5 is a graph of the same focal length, different incident rayleigh lengths affecting the varying object distance according to an embodiment of the present application.
For example, for the same focal length f=50.8 mm, the effects of different incident rayleigh lengths are analyzed (a) the change in object distance, the ratio of the minimum beam waist radius at exit to the minimum beam waist radius at different rayleigh lengths is analyzed, the smaller the ratio, the smaller the focused spot (b) the change in object distance, the exit rayleigh lengths at different rayleigh lengths are analyzed
When f=50.8mm is taken, when the influence of the incident rayleigh length of the OPM2 is analyzed, as can be seen from fig. 5 (a) and (b), the longer the incident rayleigh length is, the smaller the minimum beam waist radius and the emergent rayleigh length are in a large object distance range, and the stronger focusing effect can be realized by the OPM2, so that the divergence of the emergent beam of the OPM3 is inhibited. Furthermore, in order to achieve the effect of negative object distance to reduce the OPM2 exit rayleigh length and spot size, OPM1 should also select a longer focal length, in combination with the analysis results of fig. 4 (a) (b), so OPM1 should select the longest focal length f=101.2 mm to achieve the longest incident rayleigh length of OPM2
The focal length f of OPM3 is selected to directly influence the final emergent Rayleigh length and the size of the light spot, and a larger f can realize a longer Rayleigh length and can lead to the light spot to be enlarged, and a smaller f is opposite, so that moderate f=76.2 mm is selected
By semi-quantitative analysis of gaussian beam space transmission, the parameters of three off-axis parabolic mirrors can be determined to be selected as:
| focal length/parabolic mirror | OPM1 | OPM2 | OPM3 |
| f(mm) | 101.2 | 50.8 | 76.2 |
The process of combining the three parabolic mirrors is as follows:
THz generated by lithium niobate crystal source (LiNbO 3) emits THz beam with strong divergence and longer rayleigh length after passing through OPM1 (l 1); the THz beam with longer Rayleigh length realizes strong focusing through the OPM2 (l 3), wherein the OPM1 (l 1) is arranged so that the minimum beam waist position of the emergent THz beam is positioned behind the center of the OPM2 (l 3) to realize negative object distance; the strong focused THz beam emitted by the OPM2 (l 3) emits a weak divergent THz beam l with a longer rayleigh length after passing through the OPM3 (l 2), wherein the OPM2 (l 3) should be placed such that the focal point of the OPM2 (l 3) is located at the focal point of the OPM3 (l 1).
The ideal divergence suppression effect can be finally realized, which is the basis for carrying out THz working mode switching structural design.
The mode conversion module is used for changing different THz working modes by changing the positions of the optical elements, and the optical paths of the different THz working modes are the same and are used for receiving terahertz light and sending the terahertz light to the STM.
Further, the mode conversion module includes: a number of mirrors 3, a polarizer 4 and an entrance module.
The reflecting mirror is used for reflecting terahertz light.
The polaroid is used for adjusting the terahertz. Terahertz light enters the incidence module after changing the light path and adjusting through the reflector and the polaroid.
The incidence module is used for receiving terahertz light and sending the terahertz light to the STM after passing through.
Further, the incidence module includes a terahertz lens 5, an off-axis parabolic mirror 6, and a third mirror 33.
The terahertz lens is used for making terahertz light incident to the off-axis parabolic mirror and ensuring that the minimum beam waist position of the terahertz light is positioned at the focal position of the off-axis parabolic mirror.
The reflecting mirror is used for changing the light path of the terahertz light and enabling the terahertz light to be incident to the STM.
After the divergence of the THz wave beam transmitted in a long distance is restrained, a mode conversion module compatible with 4 THz-STM working modes can be designed, the mode conversion module can switch the working modes under the condition of not changing the optical path, the mode conversion module is very important for experiments such as optical pumping detection and far-field THz-TDS detection, and the THz-STM can conveniently realize multiple functions in the same layout.
The THz operation modes include an autocorrelation mode, a total reflection mode, a positive polarity mode, and a negative polarity mode.
The autocorrelation mode, the working application scene comprises: and (3) characterizing the THz-STM time resolution capability, a THz-pump-THz-probe pumping detection experiment, and a field emission acquisition THz near-field waveform experiment.
The self-correlation mode needs to prepare two coherent THz beams, and after convergence, the self-correlation scanning is carried out through an electric control displacement table, so that self-correlation peaks with a certain time width can be observed in THz-STM tunneling current signals, and the width of the self-correlation peaks is an important parameter for representing the time resolution capability of the THz-STM, and is an important experiment in the THz-STM.
Fig. 6 is a schematic diagram of an autocorrelation mode structure of a mode conversion module according to an embodiment of the present application.
Further, a beam splitter 7 and a displacement table 8 are also included.
The beam splitter is disposed on an incident terahertz light path for reflecting terahertz light to the first mirror 31 and projecting the terahertz light to the second mirror 32.
The first mirror or the second mirror is disposed on the displacement stage.
The displacement table moves back and forth close to or far from the beam splitter and is used for adjusting the distance between the first reflecting mirror or the second reflecting mirror and the beam splitter.
For example, as shown in FIG. 6, WG1-3 is a THz linear polarizer that can be used to adjust THz size; M1-M4 are gold-plated mirrors, where M1 with motorized displacement stages can be used for scanning.
BS is THz 1:1 beam splitter, F1 is high density polyethylene material THz lens.
OPM is an off-axis parabolic mirror. The BS is removable with the magnetic attraction device, and the M2 is switchable between 0 ° (shown) and 45 °.
The autocorrelation mode can conveniently modulate one path to carry out a pumping detection experiment or adjust the whole THz to carry out an autocorrelation tunneling current adoption experiment.
Fig. 7 is a diagram illustrating a total reflection mode structure of a mode conversion module according to an embodiment of the present application.
The total reflection mode, the working application scene comprises: THz-STM imaging experiment, THz-STM optical pumping detection experiment, THz-STM field fluorescence experiment, THz-STM IETS experiment, and light emission acquisition THz near field waveform experiment.
The autocorrelation mode is a Michelson interferometer structure, so 50% of total energy is lost, and in the face of some optical pumping detection or THz scanning experiments, no wave division is needed, but THz with full power is needed, and the total reflection mode needs to be switched.
Further, a gold mirror is also included. The gold mirror is used for reflecting THz total power from M1 to M3 and then transmitting the THz total power to the STM through the incidence module.
T1 is shown in FIG. 7 as the core position of the total reflection mode, with T1 absent in the autocorrelation mode; the replacement of the T1 position with a 50% reflectivity BS mirror may form a pair of polarity inversion modes with the case in the autocorrelation mode.
For example, WG1-3 is a THz linear polarizer that can be used to adjust THz size; M1-M4 are gold-plated mirrors; t1 is a golden mirror; f1 is a high density polyethylene material THz lens; OPM is an off-axis parabolic mirror. T1 with magnetic attraction device can be removed or replaced by other reflecting mirrors, and M1 with magnetic attraction device can be switched between 0 degree and 45 degrees (shown in the figure).
The working application scene comprises: and (3) researching THz-STM imaging experiments under positive and negative bias, and IETS experiments to obtain THz tunneling current-THz incident peak electric field intensity curve experiments.
Positive polarity mode: because THz acts on STM tunneling junction in bias form, the carrier phase of THz is a very important parameter, for THz of 0 degree and 180 degree carrier phase, the tunneling current direction induced in tunneling junction is opposite, and for studying THz effect of certain polarity it is necessary to switch THz moment to positive or negative polarity
For example, the positive polarity mode corresponds to the autocorrelation mode of fig. 6, and THz is output at the autocorrelation zero point by controlling the position of the motor mirror M1, at which time THz is embodied as a positive bias with 50% energy.
Negative polarity mode: the negative polarity mode is similar to the positive polarity mode, and is the case of replacing the magnetic gold attraction mirror with the magnetic gold attraction 1:1THz-BS at the T1 in FIG. 7, at this time, as the number of mirrors through which THz passes in the structure of FIG. 7 is increased by one, as known from half-wave loss of electromagnetic theory, this introduces 180 degrees of carrier phase difference, so as to realize polarity inversion, so that both the negative polarity mode and the positive polarity mode act on the STM tunneling junction in the form of negative bias and positive bias respectively with 50% of total energy, and switching of the two polarities is realized.
Fig. 8 is a schematic diagram of a polarity switching module according to an embodiment of the present application.
Polarity switching
The polarity switching can lose 50% of energy, and in the total reflection mode, the following pi shift structure can be designed to realize the polarity switching in the total reflection mode.
It should be noted that, the polarity switching may be implemented by using this structure in the autocorrelation mode, but this polarity switching structure may cause optical path change, and if this switching structure is adopted in the autocorrelation mode, the optical path change may be caused by both 50% energy loss and the optical path loss; the above-mentioned auto-correlation polarity switching without changing the optical path loses 50% of the energy, and the total reflection polarity switching referred to herein is polarity switching at 100% of the energy, which has the disadvantage of optical path change.
Further, a fourth mirror 34 is also included. And the fourth reflector is used for realizing a pi shift structure with the third reflector.
Pishift refers to a 180 degree (Pi) change in carrier phase, which can be achieved by adding one or fewer mirrors and using half-wave loss of electromagnetic theory; it can also be realized by adding a reflecting mirror at the focusing point and utilizing the method of the Gu's phase shift. Here we take the approach of half wave loss, so the purpose of the switching structure in fig. 8 is to add or subtract a mirror.
It should be noted that this configuration can effectively switch the polarity of THz at 100% power, but similar to the existing polarity inversion scheme, switching of two polarities introduces optical path differences, resulting in a change in the electric field strength at the STM tip attachment, introducing measurement errors, and polarity inversion can only be performed without changing the optical path in the embodiments of positive polarity mode and negative polarity mode described above.
Switching between these modes of operation does not change the optical path, otherwise the zero optical path point position in the pump detection experiment is changed, resulting in limited scanning range.
Because the components in the mode conversion module of the application need to be moved to realize the switching of different THz working modes, the components are required to be frequently disassembled, the components in the mode conversion module can be arranged in any disassembly mode, and the magnetic structure is more convenient to assemble and disassemble and is not easy to damage, so that the components in the mode conversion module are preferably in the magnetic structure.
The embodiment of the application also provides a mode conversion method of the terahertz optical system, which uses the terahertz optical system in any one of the embodiments, and comprises the following steps:
step 110, selecting a THz working mode, and changing the positions of components in the conversion module according to the THz working mode;
the THz operation mode to be performed is selected from an autocorrelation mode, a total reflection mode, a positive polarity mode, and a negative polarity mode according to the operation scene to be performed.
It should be noted that, the auto-correlation mode and the total reflection mode are one set of corresponding modes, and the positive polarity mode and the negative polarity mode are the other set of corresponding modes. Different sets of modes may be combined.
The optical path is not changed, and the terahertz light path is modified by disassembling and assembling the components, so that the requirements of corresponding THz working modes are met.
Step 120, performing divergence suppression on the terahertz light source;
the terahertz light enters the divergence suppression module and then exits as suppressed terahertz light.
And 130, enabling the terahertz light source after divergence suppression to enter an STM after passing through a mode conversion module.
The suppressed terahertz light enters the mode conversion module, exits from the mode conversion module, and enters the STM.
The application designs a THz optical system applied to a THz-STM system, and can realize the switching of various THz-STM working modes on the premise of not changing optical paths by adopting a magnetic attraction structure, wherein the THz optical system comprises a THz autocorrelation mode, a THz total reflection mode, a working mode with positive polarity and a working mode with negative polarity. And the THz intensity electric control device is added, and the THz intensity electric control device is applied to an electric control displacement table device for THz scanning, so that a generating device for generating long-distance uniform transmission THz can provide corresponding THz working mode support for various application scenes of the THz-STM, the experiment is more convenient, the operability is strong, and the cost is reduced.
The foregoing is merely exemplary of the present application and is not intended to limit the present application. Various modifications and variations of the present application will be apparent to those skilled in the art. Any modification, equivalent replacement, improvement, etc. which come within the spirit and principles of the application are to be included in the scope of the claims of the present application.
Claims (10)
1. A terahertz optical system, characterized by comprising a divergence suppression module and a mode conversion module;
the divergence suppression module is used for receiving and sending the incident terahertz light beam to the mode conversion module;
the mode conversion module is used for changing different THz working modes by changing the positions of the optical elements, and the optical paths of the different THz working modes are the same and are used for receiving terahertz light and sending the terahertz light to the STM.
2. The terahertz optical system according to claim 1, wherein the divergence-suppressing module includes an incident parabolic mirror, an exit parabolic mirror, and an enhanced-suppression parabolic mirror;
the incident parabolic mirror is used for receiving terahertz light, and after focusing, the incident parabolic mirror is shot into the enhanced inhibition parabolic mirror;
the enhancement suppression parabolic mirror is used for receiving the focused terahertz light, enhancing the focused terahertz light and then injecting the focused terahertz light into the emergent parabolic mirror;
the emergent parabolic mirror is used for receiving the focused terahertz light and converting the focused terahertz light into terahertz light beams with the divergence degree smaller than a set threshold value;
the parameters of the parabolic mirror are calculated through a Gaussian beam parameter transformation formula;
the minimum beam waist position of the terahertz light which enhances focusing is at the focal position of the outgoing parabolic mirror.
3. The terahertz optical system according to claim 2, wherein the parabolic mirror is an off-axis parabolic mirror.
4. The terahertz optical system according to claim 1, wherein the mode conversion module comprises: a plurality of reflectors, polarizers and an incidence module;
the reflecting mirror is used for reflecting terahertz light;
the polaroid is used for adjusting the terahertz;
terahertz light enters an incidence module after changing the light path and adjusting through a reflector and a polaroid;
the incidence module is used for receiving terahertz light and sending the terahertz light to the STM after passing through.
5. The terahertz optical system according to claim 4, further comprising a beam splitter and a displacement stage;
the beam splitter is arranged on an incident terahertz light path and used for reflecting terahertz light to the first reflector and projecting the terahertz light to the second reflector;
the first reflecting mirror or the second reflecting mirror is arranged on the displacement table;
the displacement table moves back and forth close to or far from the beam splitter and is used for adjusting the distance between the first reflecting mirror or the second reflecting mirror and the beam splitter.
6. The terahertz optical system according to claim 4, further comprising a gold mirror;
the gold mirror is used for reflecting THz full power to achieve the purpose of full reflection.
7. The terahertz optical system of claim 4, wherein the incidence module comprises a terahertz lens, an off-axis parabolic mirror, and a third mirror;
the terahertz lens is used for making terahertz light incident to the off-axis parabolic mirror and ensuring that the minimum beam waist position of the terahertz light is positioned at the focal position of the off-axis parabolic mirror;
the reflecting mirror is used for changing the light path of the terahertz light and enabling the terahertz light to be incident to the STM.
8. The terahertz optical system of claim 7, further comprising a fourth mirror;
and the fourth reflector is used for realizing a pi shift structure with the third reflector.
9. The terahertz optical system according to any one of claims 4-8, wherein components in the mode conversion module employ a magnetic attraction structure.
10. A terahertz optical system mode conversion method using the terahertz optical system according to any one of claims 1 to 9, comprising the steps of:
selecting a THz working mode, and changing the position of a component in the conversion module according to the THz working mode;
performing divergence suppression on the terahertz light source;
and the terahertz light source after divergence inhibition is incident into an STM after passing through a mode conversion module.
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| CN117706216A (en) * | 2024-02-06 | 2024-03-15 | 广东大湾区空天信息研究院 | Terahertz near-field waveform sampling method and device based on scanning tunnel microscope, storage medium and computer equipment |
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| CN117706216A (en) * | 2024-02-06 | 2024-03-15 | 广东大湾区空天信息研究院 | Terahertz near-field waveform sampling method and device based on scanning tunnel microscope, storage medium and computer equipment |
| CN117706216B (en) * | 2024-02-06 | 2024-04-12 | 广东大湾区空天信息研究院 | Terahertz near-field waveform sampling method and device based on scanning tunnel microscope, storage medium and computer equipment |
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