CN117170089A - Terahertz optical system and mode conversion method - Google Patents

Abstract

This application discloses a terahertz optical system and a mode conversion method, which solves the problem that multiple divergence suppression devices are needed to convert the THz operating mode over a long distance. Terahertz optical system, including divergence suppression module and mode conversion module. The divergence suppression module is used to receive and divergence suppress the incident terahertz beam, and sends it to the mode conversion module. The mode conversion module changes the position of the optical element to change different THz working modes. The optical path lengths of different THz working modes are the same, and is used to receive terahertz light and send it to the STM. By adopting a magnetic structure, this application can realize switching of various THz‑STM working modes without changing the optical path. It also adds a THz intensity electronic control device, which can provide corresponding solutions for various application scenarios of THz‑STM. THz working mode support, experiments are more convenient, easy to operate and cost is reduced.

Description

A terahertz optical system and mode conversion method

Technical field

The present application relates to the field of microscope technology, and in particular to a terahertz optical system and a mode conversion method.

Background technique

In 2013, Frank's research group at the University of Alberta in Canada proposed the experimental architecture of the Terahertz Scanning Tunneling Microscope (THz-STM) for the first time, and verified the THz-STM's ability to characterize material surfaces with ultra-high spatial and temporal resolution in the rough environment of the normal temperature atmosphere. The advantage of the characteristics is that the spatio-temporal resolution can reach 2nm and 500fs. In subsequent experiments, this record was advanced to 0.1nm spatial resolution and 100fs time resolution, and demonstrated technical advantages in multiple directions such as THz high-resolution imaging, molecular dynamics detection, and carrier diffusion process detection. It is of great technical value to study ultrafast dynamic physical processes on material surfaces and develop new semiconductor devices.

In these THz-STM related experiments, THz light with stable phase and stable and adjustable intensity is required to be coupled to the STM tip area, and the verification of multiple experimental functions of THz-STM needs to be completed in different working modes, such as requiring intensity and polarity. Adjustable THz pulse to measure THz-STM intensity current curve. To this end, the THz-STM layout involves the design of multiple THz working modes to complete functional experimental requirements. These designs include using a Michelson interferometer structure to generate coherent THz pulse pairs, using Guy phase shift, half-wave Loss and other methods are used to achieve polarity reversal, and a rotatable gold wire grid THz polarizer combination is used to achieve THz intensity control. However, the current optical system design cannot cover multiple functions at the same time. Switching working modes does not introduce changes in the optical path, and it cannot solve the strong divergence problem of long-distance transmission of THz. This is usually solved by a combination of multiple off-axis parabolic mirrors. Divergence problems increase experimental costs.

Contents of the invention

Embodiments of the present application provide a terahertz optical system and a mode conversion method, which solve the problem of requiring multiple divergence suppression devices for long-distance conversion of THz operating modes.

Embodiments of the present application provide a terahertz optical system, including a divergence suppression module and a mode conversion module. The divergence suppression module is used to receive and divergence suppress the incident terahertz beam, and sends it to the mode conversion module. The mode conversion module changes different THz working modes by changing the position of the optical element. The optical path lengths of different THz working modes are the same, and is used to receive terahertz light and send it to the STM.

Further, the divergence suppression module includes an incident parabolic mirror, an exit parabolic mirror and an enhanced suppression parabolic mirror. The incident parabolic mirror is used to receive terahertz light, focus it and then inject it into the enhanced suppression parabolic mirror. The enhanced suppression parabolic mirror is used to receive the focused terahertz light, and then inject the focused terahertz light into and out of the parabolic mirror. The exit parabolic mirror is used to receive focused terahertz light and convert it into a terahertz beam with a divergence less than a set threshold. The parameters of the parabolic mirror are calculated through the Gaussian beam parameter transformation formula. The minimum beam waist position of the enhanced focused terahertz light is at the focus position of the exit parabolic mirror.

Further, the mode conversion module includes: several reflecting mirrors, polarizing plates and incident modules. The reflector is used to reflect terahertz light. The polarizing plate is used to adjust the size of terahertz. The terahertz light enters the incident module after changing the optical path and adjusting the reflector and polarizer. The incident module is used to receive terahertz light and then send it to the STM.

Furthermore, it also includes a beam splitter and a displacement stage. The beam splitter is disposed on the incident terahertz light path for reflecting the 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 stage. The displacement stage moves back and forth toward or away from the beam splitter to adjust the distance between the first reflector or the second reflector and the beam splitter.

Preferably, the parabolic mirror is an off-axis parabolic mirror.

Furthermore, gold mirrors are also included. The gold mirror is used to reflect the full power of THz to achieve the purpose of total reflection.

Further, the incident module includes a terahertz lens, an off-axis parabolic mirror and a third reflecting mirror. The terahertz lens is used to incident terahertz light into the off-axis parabolic mirror and ensure that the minimum beam waist position of the terahertz light is at the focal position of the off-axis parabolic mirror. The reflecting mirror is used to change the optical path of the terahertz light so that the terahertz light is incident on the STM.

Further, a fourth reflector is also included. The fourth reflector is used to realize the pi shift structure with the third reflector.

Preferably, the components in the mode conversion module adopt a magnetic attraction structure.

Embodiments of the present application also provide a terahertz optical system mode conversion method, using the terahertz optical system described in any of the above embodiments, including the steps:

Select the THz working mode and change the position of the components in the module according to the THz working mode;

Divergence suppression of terahertz light sources;

The divergence-suppressed terahertz light source passes through the mode conversion module and then enters the STM.

At least one of the above technical solutions adopted in the embodiments of the present application can achieve the following beneficial effects:

By adopting a magnetic structure, this application can realize switching of various THz-STM working modes without changing the optical path, and adds a THz intensity electronic control device, which can provide corresponding solutions for various application scenarios of THz-STM. THz working mode support, experiments are more convenient, easy to operate and cost is reduced.

Description of drawings

The drawings described here are used to provide a further understanding of the present application and constitute a part of the present application. The illustrative embodiments of the present application and their descriptions are used to explain the present application and do not constitute an improper limitation of the present application. In the attached picture:

Figure 1 is a structural diagram of the existing technology THz transmission to solve the problem of light spot divergence;

Figure 2 is a structural diagram of a terahertz optical system according to an embodiment of the present application;

Figure 3 is a structural diagram of a divergence suppression module according to an embodiment of the present application;

Figure 4 is a graph showing the change in object distance affected by the same incident Rayleigh length and different focal lengths according to the embodiment of the present application;

Figure 5 is a graph showing the change in object distance affected by different incident Rayleigh lengths at the same focal length according to the embodiment of the present application;

Figure 6 is a structural diagram of an autocorrelation mode of a mode conversion module according to an embodiment of the present application;

Figure 7 is a total reflection mode structural diagram of a mode conversion module according to an embodiment of the present application;

Figure 8 is a structural diagram of polarity switching of a mode conversion module according to an embodiment of the present application;

Detailed ways

In order to make the purpose, technical solutions and advantages of the present application clearer, the technical solutions of the present application will be clearly and completely described below in conjunction with specific embodiments of the present application and corresponding drawings. Obviously, the described embodiments are only some of the embodiments of the present application, but not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of this application.

The technical solutions provided by each embodiment of the present application will be described in detail below with reference to the accompanying drawings.

Figure 1 is a structural diagram of the prior art THz transmission to solve the problem of light spot divergence.

THz transmission has a strong divergence effect. Before performing THz conversion, the problem of light spot divergence needs to be solved first. The existing technology uses a combination of multiple off-axis parabolic mirrors to solve the THz divergence problem, which not only increases the cost but also increases the distance. The effect of the axial parabolic mirror on THz will produce aberration, and the accumulation of aberration also leads to problems such as THz distortion at the tip. Compared with the existing technology, this application only uses half the number of off-axis parabolic mirrors, which on the one hand reduces THz aberration and on the other hand saves costs.

This application successfully realized the design of a THz optical system applied to the terahertz scanning tunneling microscope system (THz-STM). The polarity and intensity of the output THz light are controllable. It can produce coherent THz beam pairs and work In the single THz pulse mode, the energy transmission efficiency is high and the spatial transmission is stable. Without changing the relative optical path of THz, it can complete the optical path requirements of multiple working modes of THz-STM, and can take into account THz-TDS (terahertz time). domain spectroscopy system) function. Since THz-STM has strict requirements on incident light, how to couple THz with stable phase and adjustable intensity to the STM tip area is a key issue in studying the physical process of THz-induced tunneling current on the material surface.

Therefore, the optical properties of incident THz are of great significance to the development of various THz-STM experiments. For example, verifying the time resolution capability of THz-STM requires a pair of phase-stable coherent THz pulse pairs. This puts forward a variety of requirements for the THz optical path, including THz polarity, THz stability, adjustable THz intensity and divisible THz optical path, which requires the design of various optical path functions and working mode switching for THz. In the existing technology, the function switching of the THz part layout of the THz-STM experiment will change the optical path, causing the position of the optical path point to change drastically when detecting THz-TDS or performing optical pump-THz detection experiments, increasing the inconvenience of the experiment. How to achieve the above functions without changing the optical path size is a major problem in THz-STM layout.

On the other hand, THz-STM requires long-distance spatial transmission of THz, but the specific wavelength range of THz determines its strong divergence during spatial light transmission. A distance of 10cm is enough to make the THz spot diverge several times in size, which requires Larger mirrors for reflection increase cost. Some existing THz-TDS experiments do not need to consider serious light spot divergence due to the short THz transmission distance. However, for THz optical paths that require multiple working modes to switch and be coupled to the STM tip, the light spot divergence caused by long-distance transmission will become the main problem.

In view of the requirements for THz optical properties in various operating modes of THz-STM and the divergence problems caused by long-distance transmission of THz, the present invention has proposed a relevant optical system design through theoretical and experimental verification, effectively overcoming two technical problems and providing domestic THz solutions. -The construction of the STM system provides a reference template.

Figure 2 is a structural diagram of a terahertz optical system according to an embodiment of the present application.

The embodiment of the present application provides a terahertz optical system, including a divergence suppression module 1 and a mode conversion module 2 .

The divergence suppression module is used to receive and divergence suppress the incident terahertz beam, and sends it to the mode conversion module.

Figure 3 is a structural diagram of a divergence suppression module according to an embodiment of the present application.

Further, the divergence suppression 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 to receive terahertz light, focus it and then inject it into the enhanced suppression parabolic mirror.

The enhanced suppression parabolic mirror is used to receive the focused terahertz light, and then inject the focused terahertz light into and out of the parabolic mirror.

The exit parabolic mirror is used to receive focused terahertz light and convert it into a terahertz beam with a divergence less than a set threshold.

The exit parabolic mirror converts the focused terahertz light into nearly uniform terahertz light. Since it is difficult to achieve absolute uniformity, a threshold is set. The terahertz beam with a divergence less than the threshold is regarded as a uniform terahertz beam. .

The parameters of the parabolic mirror are calculated through the Gaussian beam parameter transformation formula.

The terahertz beam generated by the electro-optical crystal excited by the Gaussian beam emitted by the 1030nm laser is also a Gaussian beam, showing an optical power density with a Gaussian distribution in the longitudinal section (z= _z0 ).

And the following wavefront characteristics:

Among them, the wavefront refers to the plane formed by z = z ₀ in the direction of light wave transmission, A ₀ is a constant describing the electric field intensity mode, W (z) is the beam waist width of the Gaussian beam cross-section at a certain point in the transmission direction, x, y are the two-dimensional coordinates on the plane perpendicular to the transmission direction, k is the transmission wave number of the Gaussian beam, R(z) is the real isophase surface curvature radius at a certain point in the transmission direction, φ(z) is It's the phase at that point.

The changes in the curvature of the equal phase surface and the width of the beam waist with z are:

Among them, W ₀ is the minimum beam waist width, and λ is the center wavelength of spatial light.

The key parameters that usually describe the propagation of Gaussian beams in space are the Rayleigh length and the minimum beam waist radius. The Rayleigh length is defined as the extension of the beam waist radius to the minimum beam waist radius times the transmission length, it implies whether the light transmission is more like a straight line or more like a wave.

For example, for 1THz, when the minimum beam waist width is 3mm, this value is only 20cm, which means that the light outside the range of 10cm from the focus has diverged very much, and this value is 55m for a 1030nm laser. , so it is obvious that the THz light spot can easily become very large and difficult to transmit in space.

In order to effectively transmit THz, it is necessary to use off-axis parabolic mirrors to design the transmission path in space. To this end, it is necessary to study the changes of Gaussian beams under the combination of parabolic mirrors and reflectors, and to study the transmission of Gaussian beams after passing through the imaging system.

W′ ₀ =MW ₀ formula 5

(z′-f)=M ² (zf) Formula 6

2L′ ₀ =M ² (2L ₀ ) Formula 7

Among them, W ₀ is the minimum beam waist radius, L ₀ is the Rayleigh length, θ ₀ is the divergence angle, f is the lens focal length, and z is the object distance. The object distance refers to the distance between the incident THz minimum beam waist position and the center of the paraboloid. . The primed physical quantity (for example, L′ ₀ ) represents the physical quantity corresponding to the outgoing Gaussian beam after the action of the off-axis parabolic mirror.

The minimum beam waist position of the enhanced focused terahertz light is at the focus position of the exit parabolic mirror.

In order to achieve divergence suppression of the THz spot, for any incident THz beam, a combination of three off-axis parabolic mirrors with specific parameters is required to achieve divergence suppression of the outgoing THz. The specific parameters can be given in combination with simulation analysis.

For example, in order to obtain a beam as uniform as possible, that is, the exit Rayleigh length L′ ₀ as large as possible, according to Formula 7 and Formula 9, the minimum beam waist position of the incident THz needs to be at the focus position of the lens or off-axis parabolic mirror , at this time, the Rayleigh length of the outgoing THz is the largest, but the minimum beam waist width will also be enlarged to the maximum, which determines the position of the third parabolic mirror OPM3 relative to OPM2, as shown in Figure 3.

On this basis, to further improve the incident Rayleigh length L ₀ ′ of OPM3, on the one hand, a shorter incident Rayleigh length L ₀ can be achieved by combining OPM2 and OPM1, or by using OPM3 with a longer focal length, but this will also increase The minimum beam waist radius of the light spot is W ₀ ′, which is the size of the exit light spot. For optical mirrors of the same cross-section, the divergence effect caused by a larger W ₀ ' is the same as that of a shorter L ₀ '. The strong divergence of the light spot is easily blocked by the edge of the lens, which is not conducive to chopper modulation, so W ₀ ' is also Factors that need to be optimized through OPM1 and OPM2. The actual optimization needs to be based on optimizing L ₀ ' and optimizing W ₀ ' as a supplement. Both of them have an impact on the divergence of the THz spot.

For example, first analyze the influence of the focal length of the off-axis parabolic mirror OPM2. For a fixed incident Rayleigh length L ₀ =40mm and varying object distance z ₀ :-20mm～20mm, analyze the focal lengths of commonly used off-axis parabolic mirrors: 50.8mm, 76.2mm, 101.2 The effect of mm on the exit beam waist width and Rayleigh length.

The off-axis parabolic mirror can significantly reduce aberrations and chromatic aberrations, which cannot be achieved with ordinary parabolic mirrors. The off-axis parabolic mirror is functionally equivalent to a lens with a certain focal length. Therefore, three lenses with a specific focal length selected and arranged in a straight line can also achieve suppression. However, the lens loss in the THz band is particularly serious. Lenses made of high-density polyethylene The loss is as high as 30%. The use of multiple THz lenses will significantly reduce the intensity of THz. Moreover, the lens has large aberration and chromatic aberration for large spot THz. Therefore, preferably, the parabolic mirror is an off-axis parabolic mirror.

Figure 4 is a graph showing the change in object distance affected by the same incident Rayleigh length and different focal lengths according to the embodiment of the present application.

For example, for the same incident Rayleigh length of 40mm, analyze the effects of different focal lengths (a) changing the object distance, the ratio of the exit minimum beam waist radius to the incident minimum beam waist radius changes with the focal length, the smaller the ratio, the smaller the focus spot The smaller (b) changes the object distance, the exit Rayleigh length changes with the focal length.

As shown in Figure 4, it can be seen from Figure 4(a)(b) that in order to meet the minimum incident Rayleigh length condition proposed by Formula 10, OPM2 should use an off-axis parabolic mirror of f=50.8mm for focusing to achieve the minimum Rayleigh length. Rayleigh length and beam waist radius, so according to formula 10, THz can obtain the longest exit Rayleigh length and minimum beam waist radius after passing through OPM3, which is the effect of divergence suppression, so OPM2 should choose f=50.8mm.

Figure 5 is a graph showing the influence of different incident Rayleigh lengths on object distance changes at the same focal length according to the embodiment of the present application.

For example, for the same focal length f = 50.8mm, analyze the influence of different incident Rayleigh lengths (a) changing the object distance, and analyze the ratio of the exit minimum beam waist radius to the incident minimum beam waist radius under different Rayleigh lengths. The smaller the ratio, The smaller the focused spot (b) changes the object distance and analyzes the exit Rayleigh length under different Rayleigh lengths

When f=50.8mm is determined, when analyzing the influence of the incident Rayleigh length of OPM2, it can be seen from Figure 5(a)(b) that the longer the incident Rayleigh length, the minimum beam waist radius will be within a large object distance range. and the exit Rayleigh length are very small. At this time, OPM2 can achieve a stronger focusing effect, thus suppressing the divergence of the exit beam of OPM3. In addition, in order to achieve the effect of negative object distance and reduce the exit Rayleigh length and spot size of OPM2, OPM1 should also choose a longer focal length. Combined with the analysis results in Figure 4(a)(b), OPM1 should choose the longest focal length f =101.2mm to achieve the longest incident Rayleigh length of OPM2

The choice of focal length f for OPM3 will directly affect the final exit Rayleigh length and spot size. A larger f can achieve a longer Rayleigh length, and will also cause the spot to become larger. A smaller f will do the opposite, so choose Moderate f=76.2mm

Through semi-quantitative analysis of the spatial transmission of Gaussian beams, it can be determined that the parameter selection of the three off-axis parabolic mirrors is:

Focal length/parabolic mirror OPM1 OPM2 OPM3 f(mm) 101.2 50.8 76.2

The process of combining three parabolic mirrors is:

The THz generated by the lithium niobate crystal source (LiNbO3) passes through OPM1(l1) and emits a THz beam with a strong divergence and a longer Rayleigh length; the THz beam with a longer Rayleigh length achieves strong focusing through OPM2(l3), where, OPM1(l1) should be placed so that the minimum beam waist position of the outgoing THz beam is behind the center of OPM2(l3) to achieve a negative object distance; the strongly focused THz beam outgoing from OPM2(l3) exits weakly divergent after passing through OPM3(l2) , a longer Rayleigh length THz beam l, in which OPM2(l3) should be placed so that the focus of OPM2(l3) is at the focus position of OPM3(l1).

Ultimately, the ideal divergence suppression effect can be achieved, which is the basis for THz working mode switching structure design.

The mode conversion module changes different THz working modes by changing the position of the optical element. The optical path lengths of different THz working modes are the same, and is used to receive terahertz light and send it to the STM.

Further, the mode conversion module includes: several reflecting mirrors 3, polarizing plates 4 and incident modules.

The reflector is used to reflect terahertz light.

The polarizing plate is used to adjust the size of terahertz. The terahertz light enters the incident module after changing the optical path and adjusting the reflector and polarizer.

The incident module is used to receive terahertz light and then send it to the STM.

Further, the incident module includes a terahertz lens 5 , an off-axis parabolic mirror 6 and a third reflecting mirror 33 .

The terahertz lens is used to incident terahertz light into the off-axis parabolic mirror and ensure that the minimum beam waist position of the terahertz light is at the focal position of the off-axis parabolic mirror.

The reflecting mirror is used to change the optical path of the terahertz light so that the terahertz light is incident on the STM.

After suppressing the divergence of long-distance transmission THz beams, a mode conversion module compatible with four THz-STM operating modes can be designed. This module can switch operating modes without changing the optical path. For optical pump detection It is very important for experiments such as far-field THz-TDS detection, allowing THz-STM to easily implement multiple functions in the same layout.

The THz working mode includes autocorrelation mode, total reflection mode, positive polarity mode and negative polarity mode.

The working application scenarios of the autocorrelation mode include: characterizing the THz-STM time resolution capability, THz-pump-THz-probe pump detection experiments, and field emission acquisition THz near-field waveform experiments.

The autocorrelation mode requires the preparation of two coherent THz beams. After converging, the electronically controlled displacement stage is used to perform autocorrelation scanning. An autocorrelation peak with a certain time width can be observed in the THz-STM tunneling current signal. This autocorrelation peak Width is an important parameter characterizing the time resolution capability of THz-STM and is a very important type of experiment in THz-STM.

Figure 6 is a structural diagram of an autocorrelation mode of a mode conversion module according to an embodiment of the present application.

Furthermore, it also includes a beam splitter 7 and a displacement stage 8 .

The beam splitter is disposed on the incident terahertz light path for reflecting the terahertz light to the first reflecting mirror 31 and projecting the terahertz light to the second reflecting mirror 32 .

The first reflecting mirror or the second reflecting mirror is arranged on the displacement stage.

The displacement stage moves back and forth toward or away from the beam splitter to adjust the distance between the first reflector or the second reflector and the beam splitter.

For example, as shown in Figure 6, WG1-3 are THz linear polarizers that can be used to adjust the THz size; M1-M4 are gold-plated mirrors, of which M1 has an electric displacement stage that can be used for scanning.

BS is a THz 1:1 beam splitter, and F1 is a high-density polyethylene material THz lens.

OPM is an off-axis parabolic mirror. BS has a magnetic device that can be removed, and M2 has a magnetic device that can be switched between 0° (pictured) and 45°.

This autocorrelation mode can easily modulate one of the channels for pump detection experiments or adjust the entire THz for autocorrelation tunneling current experiments.

Figure 7 is a total reflection mode structure diagram of a mode conversion module according to an embodiment of the present application.

The working application scenarios of the total reflection mode include: THz-STM imaging experiments, THz-STM optical pump detection experiments, THz-STM field fluorescence experiments, THz-STM IETS experiments, and optical emission collection THz near-field waveform experiments.

The autocorrelation mode uses a Michelson interferometer structure, so 50% of the total energy is lost. When facing some optical pump detection or THz scanning experiments, wavelength splitting is often not needed, but full-power THz is required. This It is necessary to switch to total reflection mode.

Furthermore, gold mirrors are also included. The gold mirror is used to reflect the THz full power from M1 to M3, and then transmit it to the STM through the incident module.

T1 shown in Figure 7 is the core position of the total reflection mode. T1 does not exist in the autocorrelation mode; replacing the T1 position with a 50% reflectivity BS mirror can form a pair of polarity inversions with the situation in the autocorrelation mode. switch mode.

For example, WG1-3 are THz linear polarizers that can be used to adjust the THz size; M1-M4 are gold-plated reflectors; T1 is a gold mirror; F1 is a high-density polyethylene material THz lens; OPM is an off-axis parabolic mirror. T1 has a magnetic device that can be removed or replaced with other reflectors. M1 has a magnetic device that can be switched between 0° and 45° (as shown in the picture).

The working application scenarios of the positive polarity mode and negative polarity mode include: studying THz-STM imaging experiments under positive and negative bias voltages, IETS experiments, and obtaining THz tunneling current-THz incident peak electric field intensity curve experiments.

Positive polarity mode: Since THz acts on the STM tunnel junction in the form of bias, the carrier phase of THz is a very important parameter. For THz with carrier phases of 0 degrees and 180 degrees, the direction of the tunneling current caused by the tunnel junction On the contrary, to study the THz influence of a certain polarity, it is necessary to switch the THz to positive or negative polarity at all times.

For example, the positive polarity mode corresponds to the autocorrelation mode in Figure 6. By controlling the position of the electric mirror M1, THz is output at the autocorrelation zero point. At this time, THz is reflected as a positive bias voltage with 50% of the energy.

Negative polarity mode: The negative polarity mode is similar to the positive polarity mode. It is the case where the magnetic gold mirror is replaced by the magnetic 1:1THz-BS at T1 in Figure 7. At this time, the number of THz mirrors passing through in the structure of Figure 7 increases. One, from the half-wave loss in electromagnetic theory, this will introduce a 180-degree carrier phase difference, thereby achieving polarity flipping, so that both the negative polarity mode and the positive polarity mode are negatively biased and positively biased with 50% of the total energy, respectively. The form acts on the STM tunnel junction to achieve switching of the two polarities.

Figure 8 is a structural diagram of polarity switching of a mode conversion module according to an embodiment of the present application.

Polarity switching

The above polarity switching will lose 50% of the energy. In total reflection mode, the following pi shift structure can also be designed to achieve polarity switching in total reflection mode.

It should be noted that polarity switching can also be achieved using this structure in the autocorrelation mode, but this polarity switching structure will cause changes in the optical path. If this switching structure is adopted for the autocorrelation mode, it will result in 50% energy loss. , will also cause the optical path to change; the above-mentioned autocorrelation polarity switching that does not change the optical path loses 50% of the energy. The total reflection polarity switching mentioned here is polarity switching at 100% energy, and the disadvantage is that the optical path changes.

Furthermore, a fourth reflecting mirror 34 is also included. The fourth reflector is used to implement a pishift structure with the third reflector.

The Pi shift refers to a 180-degree (pi) change in the carrier phase. This change can be achieved by adding or subtracting a mirror and using the half-wave loss of electromagnetic theory; it can also be achieved by adding or subtracting a reflector at the focus point. Add a reflector and use the Guy phase shift method to achieve it. Here we adopt the method of half-wave loss, so the purpose of switching the structure in Figure 8 is to add or remove a reflector.

It should be noted that this structure can effectively switch the polarity of THz at 100% power, but similar to the existing polarity reversal scheme, the switching of two polarities will introduce an optical path difference, resulting in an optical path difference near the STM tip. Changes in electric field strength introduce measurement errors. Only the above-mentioned positive polarity mode and negative polarity mode embodiments can perform polarity reversal without changing the optical path.

Switching between these working modes cannot change the optical path, otherwise it will change the position of the zero optical path point in the pump-detection experiment, resulting in a limited scanning range.

Since the components in the mode conversion module of this application need to be moved to achieve switching between different THz working modes, the components need to be frequently disassembled. You can choose any disassembly method to arrange the components in the mode conversion module. The magnetic structure makes installation and disassembly more convenient, and It is not easy to be damaged, so it is preferred that the components in the mode conversion module adopt a magnetic structure.

Embodiments of the present application also provide a terahertz optical system mode conversion method, using the terahertz optical system described in any of the above embodiments, including the steps:

Step 110. Select the THz working mode and change the position of the components in the module according to the THz working mode;

According to the working scenario to be carried out, select the THz working mode to be carried out from autocorrelation mode, total reflection mode, positive polarity mode and negative polarity mode.

It should be noted that the autocorrelation mode and the total reflection mode are one set of corresponding modes, and the positive polarity mode and the negative polarity mode are another set of corresponding modes. Patterns from different groups can be combined.

Based on the principle that the optical path does not change, the path of terahertz light can be modified by disassembling and assembling components to meet the requirements of the corresponding THz working mode.

Step 120: Suppress divergence of the terahertz light source;

The terahertz light enters the divergence suppression module and emerges as suppressed terahertz light.

Step 130: Pass the divergence-suppressed terahertz light source through the mode conversion module and then enter the STM.

The suppressed terahertz light enters the mode conversion module, emerges from the mode conversion module, and then enters the STM.

This application designs a set of THz optical systems for THz-STM systems. By adopting a magnetic structure, it can switch between multiple THz-STM working modes without changing the optical path, including THz autocorrelation mode. THz total reflection mode, positive polarity working mode, negative polarity working mode. In addition, a THz intensity electronic control device, an electronically controlled displacement stage device used for THz scanning, and a generating device for uniformly transmitting THz over long distances are added, which can provide corresponding THz working mode support for various application scenarios of THz-STM, making the experiment more Convenient, easy to operate and reduce costs.

The above descriptions are only examples of the present application and are not intended to limit the present application. To those skilled in the art, various modifications and variations may be made to this application. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of this application shall be included in the scope of the claims of this 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 to receive and divergence suppress the incident terahertz beam, and sends it to the mode conversion module; The mode conversion module changes the position of the optical element to change different THz working modes. The optical path lengths of different THz working modes are the same, and is used to receive terahertz light and send it to the STM. 2. The terahertz optical system according to claim 1, wherein the divergence suppression module includes an incident parabolic mirror, an exit parabolic mirror and an enhanced suppression parabolic mirror; The incident parabolic mirror is used to receive terahertz light, focus it and then inject it into the enhanced suppression parabolic mirror; The enhanced suppression parabolic mirror is used to receive the focused terahertz light, and enhance the focusing before entering and exiting the parabolic mirror; The exit parabolic mirror is used to receive focused terahertz light and convert it into a terahertz beam with a divergence less than a set threshold; The parameters of the parabolic mirror are calculated through the Gaussian beam parameter transformation formula; The minimum beam waist position of the enhanced focused terahertz light is at the focus position of the exit 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, characterized in that the mode conversion module includes: a number of reflectors, polarizers and incident modules; The reflector is used to reflect terahertz light; The polarizing plate is used to adjust the terahertz size; The terahertz light enters the incident module after changing the optical path and adjusting the reflector and polarizer; The incident module is used to receive terahertz light and then send it to the STM. 5. The terahertz optical system according to claim 4, further comprising a beam splitter and a displacement stage; The beam splitter is arranged on the incident terahertz light path for reflecting the 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 stage; The displacement stage moves back and forth toward or away from the beam splitter to adjust the distance between the first reflector or the second reflector and the beam splitter. 6. The terahertz optical system according to claim 4, further comprising a gold mirror; The gold mirror is used to reflect the full power of THz to achieve the purpose of total reflection. 7. The terahertz optical system according to claim 4, wherein the incident module includes a terahertz lens, an off-axis parabolic mirror and a third reflector; The terahertz lens is used to incident terahertz light into the off-axis parabolic mirror and ensure that the minimum beam waist position of the terahertz light is at the focal position of the off-axis parabolic mirror; The reflecting mirror is used to change the optical path of the terahertz light so that the terahertz light is incident on the STM. 8. The terahertz optical system according to claim 7, further comprising a fourth reflector; The fourth reflector is used to realize the pi shift structure with the third reflector. 9. The terahertz optical system according to any one of claims 4 to 8, characterized in that the components in the mode conversion module adopt a magnetic structure. 10. A terahertz optical system mode conversion method, characterized in that, using the terahertz optical system according to any one of claims 1 to 9, it includes the steps: Select the THz working mode and change the position of the components in the module according to the THz working mode; Divergence suppression of terahertz light sources; The divergence-suppressed terahertz light source passes through the mode conversion module and then enters the STM.