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Abstract
Dental caries is listed as one of the three major non-communicable diseases by the World Health Organization, and its main treatment method is to restore it by filling it with resin. At present, the visible light-cure method has the problems of non-uniform curing and low penetration efficiency, which makes the bonding area easy to develop marginal leakages, thus leading to secondary caries and requiring repeated treatment. In this work, through the strong terahertz (THz) irradiation-weak THz detection technique, it is found that the strong THz electromagnetic pulses can accelerate the curing process of the resin, and the weak-field THz spectroscopy can be used to monitor this dynamic change in real time, which will greatly promote the potential application of THz technology in dentistry.
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1. Introduction
Teeth are the hardest organ of the human body, undertaking the function of cutting and chewing food, which can fully grind food to ensure the absorption of nutrients and promote the healthy development of the human body [1]. However, with the development of economy and society, people's living standard has improved. Food refinement has increased significantly, resulting in a significant increase in human oral diseases in all age groups [2], making oral diseases the world's most harmful problem after cardiovascular diseases, and increasingly concerned by the medical profession [3–7]. The structure of the tooth consists of the crown and the root. The enamel layer of the crown contains minerals and is hard enough to protect the pulp and nerve tissue [8,9]. Human teeth have certain self-protective functions, but with the accumulation of chewing wear, unhealthy eating habits, poor oral cleaning habits and other problems, it can cause many oral diseases, such as dental caries [10], cracked tooth [11] and periodontal disease [12]. The caries is caused by the dominant action of bacteria [13], which leads to the continuous erosion and breakage of the tooth, and the external stimulus connects to the pulp through the nerve tissue, causing great pain and even complication of inflammation in the human body [14]. Caries is the most common and easily neglected, and is listed as one of the top three non-communicable diseases by the World Health Organization [15].
Relevant research indicates that caries may also cause a variety of diseases, such as endocarditis, kidney inflammation and sepsis [16]. Timely and effective treatment of caries can effectively prevent further damage to teeth leading to serious secondary diseases [17]. After the treatment of caries, the artificially prepared cavities need to be filled with restorative materials in order to restore the shape of the tooth defect and to ensure that the pulp is not irritated by the environment [18]. Currently, the commonly used material for restorative dentistry is a polymer resin (e.g. composite resin), which has the advantages of a well-insulated seal, stable physicochemical properties and simple restorative operation. Composite resin is a new type of restorative material developed on the basis of acrylate, mainly composed of resin and inorganic filler, and is the most clinically used dental restorative material [19]. Light-cure composite resins occupy an important place in modern aesthetic dentistry and are widely used for the restoration of dental defects and discolored teeth [20].
The commonly used light-cure lamp is an LED lamp with a wavelength of ∼470 nm, and the curing time is determined by the doctor experience, usually not less than 30 s. During light curing, patient saliva may contaminate the curing interface, resulting in microleakage or even filling failure [21]. In clinical practice, the probability of light-cured resin fillings failure is <5% based on the dentist's experience. In addition, the coefficient of thermal expansion between the tooth and the restorative resin does not match [22,23], and the light-cure lamp has a short wavelength and shallow penetration depth. When filling a tooth, it is often necessary to fill the tooth in several layers. If the curing process was too fast or non-uniform, it could affect the bonds between the different layers of resin, and the bond was easily damaged when the person chewed hard food, inducing the resin to fall off resulting in marginal leakage. The bacteria caused by food residue will enter the tooth tissue through the marginal leakage and cause secondary caries that needs to be treated again, which is a problem that needs to be solved urgently [24]. In addition, because blue light is a short wavelength visible light with low penetration, prolonged curing light irradiation still only increases the degree of curing of the resin material on the surface, but not on the inside. Clinical treatment of cavity filling usually requires thin layers, multiple fillings, and repeated irradiation, which further increases the operation time and the risk of contamination. More importantly, the curing process is not monitored in real time by other means, and the degree of curing is judged solely by the dentist operating experience. For this reason, it is necessary not only to improve the curing depth of resin, but also to develop more advanced technologies in the real-time monitoring of the curing process.
Terahertz (THz) is an electromagnetic wave between millimeter waves and near-infrared light that has not yet been fully exploited, with a recognized frequency range of 0.1-10 THz and a wavelength of 3 mm to 30 µm. Compared with visible light, THz electromagnetic wave has many advantages such as longer wavelength, stronger penetrating ability, and has shown important applications in early screening of dental caries and dental imaging. For example, the anisotropy due to the significant THz birefringence effect in tooth enamel, combined with the strong attenuation of THz caries, can be used as a new diagnostic tool for dental caries[25]. THz pulse and X-ray are used to image dental caries in vitro, which proved that THz pulse imaging can be used to detect dental caries and has no significant difference with the existing dental caries imaging methods [26]. In an addition, THz pulse imaging can also be used to examine and evaluate the extent of remineralization of artificial caries lesions [27]. THz waves generated by using an 80 MHz oscillator is usually defined as a weak-field THz with its focused peak electric field much lower than V/cm, while strong-field THz implies its peak field strength larger than kV/cm. The essential difference between the two types of THz waves is the electric field intensity and their potential applications. Strong-field THz has been employed to induce phase transition in matters, accelerate electrons for compact accelerators, and observe some biological effects. Weak-field THz spectroscopy and imaging techniques are more appropriate for contactless detection, substance identification and cancer section imaging. However, almost all the above-mentioned reported work has used weak-field THz spectroscopy or imaging for early screening of dental diseases, and there are no reports on the application of strong-field THz technology in the diagnosis and treatment of dental diseases, let alone the application of THz technology in the curing and dynamic monitoring of dental fillings.
In this work, we employ for the first time the curing of the tooth filling material-composite resin by strong-field THz electromagnetic radiation and monitored the curing process dynamically in real time by weak-field THz time-domain spectroscopy. By constructing a strong-field THz irradiation-weak-field THz spectroscopy detection system, the curing effects of strong-field THz pulses, near-infrared femtosecond laser pulses, and near-infrared continuous waves on composite resin materials were systematically studied. It is found that all three electromagnetic radiation sources could make the composite resin material cured, and the composite resin material is functionally stable after the radiation source is removed. However, strong-field THz pulses have the best curing effect, requiring the lowest power density and the most uniform curing results. This work demonstrates for the first time the potential of strong-field THz electromagnetic radiation in dental restorative applications, and is expected to greatly promote the application of THz technology in the treatment of dental diseases.
2. Methods and theories
2.1 Sample preparation
Composite resin in dentistry is an adhesive restorative material composed of resin matrix, surface treated inorganic filler and initiator system. The samples used in this experiment are Valux, Z250, Z350 and P60 light-curing composite resin produced by 3 M company. These four light-curing composite resins are all visible light-activated, radiation-blocking filling composite resins. To ensure that the thickness of the composite resin material is uniform and indicates flatness, a sample holder structure is designed specifically for the THz spectroscopy study of the filling material. The sample holder is a three-layer sandwich structure, as shown in Fig. 1. It is covered with 1-mm thick round quartz glass on both sides and 3-mm thick Teflon O-ring in the middle. Place a piece of quartz glass at the bottom, then place the Teflon O-ring on the quartz glass, turn the sample tube syringe clockwise for 6 turns, extrude the composite resin material to the center of the O-ring, then place another piece of quartz glass on top of the O-ring, press evenly until the composite resin material fills the inside of the O-ring evenly, and complete the sandwich structure of the composite resin sample. The O-ring is made of Teflon material, which is not easily deformed, and the quartz glass and O-ring fit tightly to ensure the sample thickness so that the thickness of each sample is approximately 3 mm.
2.2 Experimental setup
In this experiment, a strong-field THz/near-infrared femtosecond pulse irradiation-weak-field THz detection system (ST/NIP-WTDS), and a near-infrared continuous-wave irradiation-weak-field THz time-domain spectroscopy system (NICW-WTDS) are used to systematically investigate the composite resin material. The schematic diagram of our home-built ST/NIP-WTDS is shown in Fig. 2(A). It consists of a femtosecond laser amplifier (center wavelength of 800 nm, pulse width of 35 fs, repetition frequency of 1 kHz, and maximum output pump energy of ∼7 mJ) pumping a lithium niobate nonlinear crystal to generate strong-field THz pulses via tilted pulse front technique. Through systematically optimize the lithium niobate generation crystal at room temperature, it is possible to generate strong-field THz pulses with a single pulse energy of several microjoules (average power of milliwatts). To match the study of the nonlinear response characteristics of multiple materials under strong-field THz pumping, the system is also configured with time-resolved multispectral detection capabilities such as weak-field THz detection and optical detection, laying a solid experimental foundation for subsequent dental section imaging, early screening studies of dental caries, and curing of dental filling materials. The strong THz pulse irradiation with various powers is achieved by adding attenuators to the optical path. Not only that, the versatile system also has the capability of near-infrared 800 nm femtosecond pulse irradiation-weak field THz detection function. In Fig. 2(A), the thicker red-light path indicates that most of the laser energy is used to generate THz radiation, the thinner red-light path is the detection beam, the green light path is the 800 nm near-infrared femtosecond pulsed irradiation, and the orange light path is used to generate weak-field THz pulses for real-time monitoring of the curing process of the composite resin material. Figure 2(B) shows the strong-field THz time-domain waveform and its corresponding spectrum.
Fig. 2. Schematic diagram of the experimental systems. A. Strong-field THz/near-infrared femtosecond pulse irradiation-weak-field THz detection system (ST/NIP-WTDS). B. A typical strong-field THz temporal waveform and its corresponding spectrum; C. Near-infrared continuous wave irradiation-weak-field THz time-domain spectroscopy system (NICW-WTDS).
Meanwhile, another NICW-WTDS is also used for comparison experiments. In this system, weak-field THz pulses are generated from a commercial low-temperature-grown InGaAs photoconductive antenna, and the experimental setup is shown in Fig. 2(C). THz pulses are generated by excitation of a fiber laser (model: Femtoferb FD 6.5, Toptica) with a central wavelength of 1560 nm, a repetition rate of 80 MHz and a pulse width of 60 fs. The horizontally polarized THz wave is first collimated by TPX lens L1 (focal length =50 mm), then focused onto the sample by TPX lens L2 (focal length =100 mm), with a focused THz spot diameter ∼4 mm, and then focused onto the detection antenna by a TPX lens L3 (focal length =100 mm) and a TPX lens L4 (focal length =50 mm). A high-sensitivity InGaAs photoelectric antenna with electro-optical sampling method is used to record THz time-domain waveform signals. For subsequent studies on the curing of dental fillings, the system also integrates an 808 nm near-infrared continuous wave and power tunable laser. Irradiation is applied to the sample surface at a fixed angle of 30° to excite the phase transition of the filling material and to characterize its THz transmission spectrum.
2.3 Data processing
The experimental data are not collected on the same day, resulting in slightly different laser power, and the samples are not perfectly uniform in thickness during preparation. Therefore, a normalized preprocess is required to compare the temporal waveforms of different samples using Eq. (1).
where, ${E_{sam}}$ is the THz time-domain signal of different samples, ${E_{\max \_ref}}$ denotes the reference signal of the corresponding sample.The refractive index of the sample is obtained from Eq. (2).
where, c is the speed of light in free space, $\phi (\omega )$ is the phase difference between the reference signal and the sample signal, d is the sample thickness.The degree of curing of the composite resin material is characterized by the peak-to-peak shift $\Delta t$.
where, ${t_{sam}}$ and ${t_{ref}}$ are the peak positions of the sample signal and the reference signal, respectively.2.4 Possible mechanism for the composite resin cured by THz radiation
The composite resin is cured by strong-field THz pulses as shown in Fig. 3. Microwave curing technology has been widely used in resins and their composites. THz frequency is as close to the rotational vibration frequency of certain chemical groups as microwave, which can change the conformation of molecules, selectively activate certain reactive groups and accelerate the rate of chemical reactions [28]. Under the effect of THz electromagnetic field, the polar molecules in the resin material change from random distribution to orientation arrangement according to the linearly polarized THz electric field. The material molecules move, collide and rub at high speed in a short time, thus heating up the material through dielectric loss, reducing the activation energy and molecular bonding strength, greatly accelerating the reaction speed and shortening the reaction cycle [29,30]. Materials with molecular dipole response can be described using Debye's theory and Lambert's law [31,32]. The energy absorbed by a dielectric material is expressed by the given Lamberts law as shown in Eq. (4)
where, Q is the THz energy, $\sigma$ denotes the effective electrical conductivity, $\bar{E}$ is the field intensity, f represents the THz frequency, ${\varepsilon _0}$ is the permittivity of free space, $\varepsilon$ is the dielectric constant that quantifies the stored and transmitted energy from material, $\tan \sigma$ is the loss tangent coefficient.Therefore, it can be concluded that the energy absorbed by the resin material is proportional to the square of the electric field strength of the radiation, and proportional to the electromagnetic wave frequency. The strong-field THz used in this experiment is pulsed and has a higher frequency and stronger electric field compared to microwaves of the same power magnitude, which can obtain the same degree of curing with lower power. At the same time, having a longer wavelength compared to visible light allows for a more uniform curing effect.
3. Experimental results and discussion
3.1 Characterization of static weak-field THz transmission properties of resins materials
Before the irradiation experiments, to obtain the static THz refractive indices of the samples, we first measured the THz transmission spectra of the different resin composites using a NICW-WTDS system and obtained the dielectric response spectra of the materials in the THz band with reference to the equivalent refractive index method described above. The types of samples measured include Valux, Z250, Z350, and P60, and a reference sample without any filling in the middle (only two pieces of quartz glass with O-rings) is made. Figure 4(A) shows the THz transmission temporal waveforms of the resin sample as well as the reference sample. Compared to the reference signal, all four types of resins show strong absorption of THz pulses, with a delay in peak time. With the same thickness, the Value absorbs the least THz wave with a peak time delay of 4.67 ps, the Z350 absorbs the most THz wave with a peak time delay of 5.07 ps.
Fig. 4. A. THz transmission temporal waveforms, and B. Corresponding equivalent refractive indices for different resin composites.
With approximately the same thickness, the peak time delays of the transmitted signals of the Valux, Z250, P60, and Z350 resins differ, indicating significant differences in THz refractive index. To clarify the refractive index differences of the four composite resin, we calculate their refractive indices at 0.2 THz-1.6 THz, as shown in Fig. 4(B). The refractive indices of the four composite resins are significantly different, in order from large to small, it is Z350 (∼1.51), P60 (∼1.47), Z250 (∼1.45), Valux (∼1.38).
3.2 Curing of resin materials by strong-field THz pulses
Valux resin is the most used cost-effective composite resin after 15 years of proven clinical performance. It is also less absorbing than the other three resins in the THz band, so it is selected for the experimental investigation of strong-field THz irradiation. Since the composite resin is a light-cured material, to exclude the influence of ambient light on the experiment, the weak-field THz transmission spectra of the Valux resin samples are collected at 20-minute intervals during 60-minutes. It is found that there is no significant change in the amplitude and peak position of the transmitted THz waveform, indicating that the ambient light had basically no effect on the curing of the Valux resin and the material performance is stable. THz waves have a longer wavelength and deeper penetration than commonly used curing lamps, so if THz waves can cause a phase shift in resin composites, it may make the composite resin material cure more firmly. For this purpose, we used strong-field THz pulsed irradiation of Value resin with power of 0.6 mW, 0.9 mW, and 1.2 mW, peak frequency at ∼0.45 THz, and focused spot diameter of ∼1.6 mm, respectively. The samples were continuously irradiated with strong-field THz pulses during 0-30 min, and weak-field THz transmission spectra were collected at 10 min intervals (Fig. 5(A)). When the 30-minute set of data measurements was completed, the strong-field THz pulses were immediately masked and weak-field THz transmission spectra were recorded at 10-minute intervals during the 30–60 min time period (Fig. 5(B)). From Fig. 5(A), the temporal waveform obtained by irradiating the Valux composite resin using THz pulses tends to shift to the left within 30 min, and the tendency becomes more and more obvious with the increase of laser power. As can be seen from Fig. 5(B), after the THz pulse of the strong-field is blocked, the temporal waveform basically does not change significantly and does not shift to the left, staying in the state at the end of the irradiation. The results of this experiment shows that strong-field THz pulses on the milliwatt scale had a significant curing effect on the filling material.
Fig. 5. Strong-field THz pulses on the curing of Value resin. A. Continuous irradiation for 30 min; B. 30 min after removal of irradiation; C. Plot of irradiation time versus peak-to-peak displacement.
To compare more visually the curing effect of strong-field THz pulses of different powers on the filling material. We plotted pictures of the irradiation time and the peak-to-peak displacement of the temporal waveform in the strong-field THz. It is clearly observed from Fig. 5(C) that the higher the strong-field THz irradiation power, the faster the peak displacement velocity. THz pulses of 0.6 mW, 0.9 mW, and 1.2 mW shifted the peak-to-peak values to the left by 0.067 ps, 0.2 ps, and 0.2 ps, respectively, at 10 min of irradiation. THz pulses of 0.9 mW and 1.2 mW had the same curing effect in the first 10 min and reached the same degree of curing at 30 min. When irradiated with the THz pulse of 1.2 mW for 20 min, the Value composite resin finished curing and the THz temporal waveform did not continue to shift to the left as the irradiation time increased. It is further indicated that the THz pulse of 0.9 mW enables the curing of Value resin, and further increase in THz pulse power will only shorten the curing time without increasing the degree of curing. The unchanged peak positions at 30-60 min indicate that the Value resin irradiated with THz pulses is stable and does not lose its filling properties due to the THz irradiation.
3.3 Curing effect of a near-infrared pulsed laser and continuous laser on resin materials
The THz band wavelength range is from 3000 to 30 µm, while the commonly used LED light curing lamp wavelength range is between 400-500 nm. Having established that the THz band can accelerate the curing of dental fillings, we wanted to investigate whether infrared light in both wavelength ranges could also accelerate the curing of resin materials. Firstly, a femtosecond pulsed laser source with a wavelength of 800 nm is selected, and the composite resin is irradiated with laser powers of 6 mW, 18 mW and 36 mW, respectively, the experimental method is the same as above. From Fig. 6(A) we can still observe the leftward shift of the THz temporal spectrum, and the peak leftward shift increases with increasing laser irradiation power. The peak position is basically not displaced after the laser is blocked, which proves that it is indeed the 800 nm pulsed laser that can induce the change of properties of the composite resin, and the stability of the composite resin after the laser is removed does not result in the change of resin material properties. Figure 6(C) plots the irradiation time of the 800 nm pulsed laser versus peak displacement, and it can be seen that the curing ability of the laser with 6 mW and 18 mW power is the same for the composite resin when irradiated for 10 min. Moreover, after 10 min of irradiation, the pulsed laser with a power of 6 mW was unable to induce further curing of the composite resin and had reached the upper limit of curing capacity. With the increase of irradiation time, both the 18 mW and 36 mW pulsed lasers could induce further curing of the composite resin material with peak displacements of 0.27 ps and 0.33 ps at the end of 30 min irradiation, respectively. We can find that the peak position always shifts to the left as the irradiation time increases before the laser is removed, indicating that perhaps the complete curing of the composite resin material is not achieved at the time of laser removal.
Fig. 6. Curing of resin material by NIR laser. Weak-field THz temporal waveforms monitoring for 60 minutes under the radiation of A. 800 nm femtosecond laser, B. 808 nm continuous laser; Corresponding THz peak value displacement with respect to that of for C. 800 nm femtosecond laser, and D. 808 nm continuous laser radiation.
Both above-mentioned laser sources that caused the curing of the composite resin are pulsed, and we also carried out similar experiments using a continuous source. The experiment was completed using the NICW-WTDS system. The power of continuous laser is set to 2 W, 4 W and 6 W respectively, and the curing effect of resin material is evaluated by the same method, as shown in Fig. 6(B). The same experimental trend as for the previous pulsed source is that the peak of the THz temporal spectrum is similarly shifted to the left when irradiating with a continuous source. The difference is that the peak amplitude increases after the laser source is turned off, although the peak position does not continue to move. As the power of the laser source increases, the peak amplitude increase also increases. This may be caused by the thermal effect accumulated under continuous source irradiation, but the thermal effect of the laser source only affects the amplitude properties of the resin material without changing its refractive index. Similarly, we also summarize the peak displacement law at different power levels, as shown in Fig. 6(D). The increase in irradiation power accelerated the rate of peak displacement, and the 2 W power laser source did not shift the peak position to the left after 10 min of irradiation, while it took 20 min for the peak position to remain unchanged using the 4 W power laser source. The peak position of the laser source with 6 W power was always shifted to the left during the irradiation time of 30 min. None of the peak positions of the composite resin changed after removal of the laser source. It is worth noting that the laser source power is higher in the W scale than the mW scale in the above experiment, but the peak shift distance is smaller than the above experiment. The highest power continuous laser also shifts only 0.2 ps, which is lower than the 0.27 ps for the strong-field THz pulse and the 0.33 ps for the 800 nm pulsed laser.
3.4 Comparison of curing resins with different irradiation sources
In the investigations of sections 3.2 and 3.3, it is found that the degree of curing of the composite resin material is not only related to the wavelength of the irradiation source, but also to the type of irradiation source. To compare the effects of different types of radiation sources on the composite resin horizontally, we plot the power density versus peak displacement (Fig. 7) and found that the degree of curing using pulsed sources is significantly higher than that of continuous sources. Calculating the power density of different radiation sources, we conclude that the power density of strong-field THz pulses is between 0.6-1.2 mW/mm2, and the power density of 800 nm near-infrared pulses is between 2.5-15.5 mW/mm2, which is ∼4.2-12.9 times that of THz pulses. And the power density of 808 nm continuous laser is as high as 33.9-101.9 mW/mm2, which is ∼56.5-84.9 times that of THz pulses. Based on these results, we can infer that the main factor affecting the curing of resin materials is the wavelength. THz waves have a longer wavelength than near-infrared light, so it has stronger penetration than near-infrared light. Therefore, only a smaller power density is needed to obtain the same degree of curing. Besides, we can also deduce that due to the deeper penetration depth, the using of THz waves to cure the resin material can make them cured better in the depth, rather than just on the surface as in the case of light-curing lamps. The 800 nm femtosecond laser pulses cure the resin material better, although the power density is much lower than that of the 808 nm continuous laser. We infer that the instantaneous power affects the curing effect of the resin material more than the average power.
We define the incident side of the light source as the front side and the exit side of the light source as the back side. After irradiating the resin material with different light sources and placing the resin material for 24 hours to make the resin material stable in nature, the hardness of the cured resin was measured using a Shore durometer, as shown in Fig. 8. In this case, the control samples were without light source irradiation and the resin material achieved a significantly lower hardness than the resin irradiated by the light source. Both strong THz pulses and near-infrared light sources were used to irradiate the resin to enhance the hardness of the resin cure. The commercial visible light method of curing the resin resulted in a higher degree of cure. However, it is important to note that the hardness of the front and back was almost the same after curing the resin using strong field THz pulses, which confirms from the side that the penetration depth of THz is higher than that of visible light.
However, it is obvious that the use of strong-field THz pulses for resin curing still has a major shortcoming compared to conventional methods. When the thickness of the resin material is increased, the hardness of the resin material cured by strong-field THz pulses is reduced. This is due to the fact that the power density of our strong-field THz is less than 20 times the power density of visible light. At the same time, the visible light sensitive substances mentioned in the resin material can efficiently absorb visible light energy and cure rapidly. Therefore, curing resin using strong-field THz requires a longer irradiation time than that provided by visible light, and the curing hardness is lower than that of visible light. However, if materials sensitive to the THz band could be added to the resin material, we believe that it would be possible to significantly reduce the time and improve the hardness of the resin material using strong-field THz curing.
4. Conclusion
In this work, we initially demonstrated the ability of strong-field THz curing resin, but have not yet investigated whether strong-field THz pulses affect the adhesive properties and the bond strength between the composite resin and the tooth. We have systematically investigated the factors affecting the curing of composite resin using the ST/NIP-WTDS and NICW-WTDS experimental systems, and found that both strong-field THz pulses and near-infrared light can induce the curing of composite resin, and the properties of composite resin do not change when the irradiation source is blocked. However, comparing the THz band and NIR band, we find that the THz frequency band requires the least amount of power density to achieve the same curing effect. Comparing a pulsed light source with a continuous light source, we find that the instantaneous power is more important than the average power, even if the instantaneous power is an order of magnitude lower to obtain better curing results. At the same time, the THz band is invisible and not harmful to the human eye, and the high penetration can better cure the resin material at depth. Our results show that the curing of composite resins can be achieved using strong-field THz pulses, and the degree of curing of composite resins can be characterized by real-time monitoring during the curing of resin materials using weak THz spectroscopy. Although the use of strong-field THz pulses for resin curing purposes is a proof-of-principle experiment and the cost of this system is currently higher than that of the light-curing method and is currently in the laboratory stage unsuitable for clinical use, the system has not only strong-field THz waves but also a high-energy laser. In the following work, we will investigate the techniques of developing cost-effective and compact strong-field THz generation methods, such as nanostructure enhanced local strong-field method. Also, we may explore other frequencies such as 100-GHz with 80-mW output average power and 4.3-THz with 1-mW. Both high power THz sources hold the advantage of low cost and small size, and may be more appropriate for applications in dentistry. In our next work, we will also explore the effect of strong-field THz pulses on adhesive and the possibility of using high-energy laser to treat caries. At the same time, we will combine weak-field THz to achieve early detection of caries and realize a commercial prototype that integrates detection, diagnosis and treatment.
Funding
Shenzhen Fundamental Research Program (2021Szvup080).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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