A system and method for structured undiffracted optical chirped femtosecond laser radiation supercontinuum terahertz wave

Abstract

This invention proposes a system and method for generating ultra-intense broadband terahertz waves using a structured, diffraction-free, chirped femtosecond laser. In the system, a first femtosecond laser emitted by a femtosecond laser is split by a beam splitter to obtain a pump beam and a probe beam. An optical parametric amplifier modulates the pump beam into a second femtosecond laser, which, after passing through a laser pulse shaper, outputs a chirped-modulated third femtosecond laser. A reflective spatial light modulator converts and reflects the incident third femtosecond laser, and the reflected fourth femtosecond laser is reflected by a second mirror to a first off-axis parabolic mirror. After focusing, it is incident on a BBO crystal, converting part of the light into a second harmonic wave. The remaining light from the fourth femtosecond laser, along with the second harmonic wave, is focused into air, generating air plasma. The air plasma radiates terahertz waves. These terahertz waves are incident on a second off-axis parabolic mirror and reflected as collimated terahertz waves. This invention generates ultra-intense broadband terahertz waves with flexible spectral polarization control.

Description

Technical Field

[0001] This invention belongs to the field of terahertz wave radiation enhancement technology, and particularly relates to a system and method for structured, diffraction-free, chirped femtosecond laser radiation of ultra-intense broadband terahertz waves. Background Technology

[0002] In recent years, terahertz waves have made breakthrough progress in applications such as high-speed broadband communication, precision biomedicine, non-destructive testing and imaging, and spectral material identification. Developing high-power, wide-bandwidth, stable and flexibly adjustable terahertz wave sources has become a core research topic to support the implementation of technologies in this field.

[0003] Among the many methods for generating terahertz waves, the technique of using air as a medium to focus ultrashort femtosecond laser pulses to excite air plasma and generate terahertz waves has become a research hotspot in the terahertz field due to its significant advantages such as short pulse width, high electric field intensity, and wide spectral coverage. The energy conversion efficiency between laser and terahertz waves is a key factor determining the output performance of a terahertz wave source.

[0004] Currently, the technology of generating terahertz waves by exciting air plasma with traditional Gaussian lasers has made significant progress. Researchers have effectively enhanced the intensity of terahertz waves by controlling the wavelength, energy, and initial phase of two-color and multi-color laser fields, and have gained a full understanding of the modulation laws of terahertz wave spectra and polarization. However, due to the inherent characteristics of Gaussian lasers, they are prone to diffraction and divergence during propagation, resulting in low laser energy utilization and making it difficult to further increase the radiation power and transmission distance of terahertz waves.

[0005] Meanwhile, existing technologies for controlling femtosecond lasers suffer from limitations due to their singular approach. The femtosecond laser filamentation and terahertz supercontinuum radiation processes are highly dependent on the spatiotemporal intensity and phase distribution of the initial laser pulse. While temporal shaping and chirped modulation techniques using devices such as liquid crystal spatial light modulators and phase plates can optimize the control of these nonlinear processes, these approaches primarily focus on controlling the temporal parameters of the laser pulse and fail to combine this with optimization of the spatial configuration of the light field to achieve a synergistic control effect.

[0006] The structured diffraction-free light described in this invention refers to a beam with diffraction-free or self-healing propagation characteristics that differs from Gaussian lasers in spatial intensity and phase distribution. Specifically, it includes types such as Bessel beams, Airy beams, and vortex beams. These beams can maintain their cross-sectional shape and energy concentration after long-distance propagation, extending the interaction length between the laser and the medium. Furthermore, the orbital angular momentum carried by vortex beams can optimize the spatial morphology of plasmas, providing a new technical path for improving terahertz wave conversion efficiency. However, current research has not organically combined the spatial manipulation of structured diffraction-free beams with the temporal manipulation of femtosecond laser chirps, failing to fully leverage their synergistic effect and making it difficult to overcome the performance bottlenecks of existing terahertz wave sources in terms of power, bandwidth, and stability. Summary of the Invention

[0007] To address the aforementioned technical problems, this invention proposes a system and method for structured, diffraction-free, chirped femtosecond laser radiation of ultra-intense broadband terahertz waves, aiming to overcome existing technical issues.

[0008] The first aspect of this invention proposes a system for emitting ultra-intense broadband terahertz waves using a structured, diffraction-free, chirped femtosecond laser, the system comprising: The system includes a femtosecond laser, a beam splitter, an optical parametric amplifier, a first reflector, a laser pulse shaper, a reflective spatial light modulator, a second reflector, a first off-axis parabolic mirror, a BBO crystal, and a second off-axis parabolic mirror, which are sequentially arranged on the optical path. The femtosecond laser emits a first femtosecond laser of the first wavelength. After the first femtosecond laser is split by a beam splitter, a pump beam and a probe beam with mutually perpendicular propagation directions are obtained. The probe beam is emitted to a terahertz time-domain spectroscopy detection device. The optical parametric amplifier modulates the pump beam into a second femtosecond laser with a second wavelength. The second femtosecond laser is reflected by the first mirror to the laser pulse shaper, which outputs a chirped modulated third femtosecond laser. A reflective spatial light modulator converts and reflects the incident third femtosecond laser, transforming its Gaussian optical field distribution into a structured, diffraction-free optical field distribution. The reflected structured, diffraction-free femtosecond laser is the fourth femtosecond laser. The fourth femtosecond laser is reflected by a second mirror to a first off-axis parabolic mirror, which focuses the incident fourth femtosecond laser onto a BBO crystal. The BBO crystal converts a portion of the incident fourth femtosecond laser light into a second harmonic of the third wavelength. The remaining light from the incident fourth femtosecond laser light and the second harmonic are focused together into the air to generate air plasma. The air plasma radiates terahertz waves with an electric field strength of not less than 100 MV / cm and a spectral coverage of (0.1~50) THz. These terahertz waves are incident on the second off-axis parabolic mirror and reflected as collimated terahertz waves. Among them, the structured non-diffraction light field distribution includes Bessel beam distribution, ring Airy beam distribution, and vortex beam distribution; the angle between the third femtosecond laser incident on the reflective spatial light modulator (6) and the reflected beam is less than 10 degrees.

[0009] Preferably, the femtosecond laser is a titanium-doped sapphire femtosecond laser amplifier with a first wavelength of 800 nm, a second wavelength of 1550 nm, and a third wavelength of 775 nm.

[0010] Preferably, the laser pulse shaper uses a multiphoton pulse intra-pulse interference phase scanning method to modulate the amplitude and phase, thereby achieving chirp compression, compensation, and modulation of the second femtosecond laser.

[0011] Preferably, the reflective spatial light modulator is a reflective spatial light modulator suitable for 1550 nm wavelength lasers.

[0012] Preferably, the terahertz time-domain spectroscopy detection device is independent of the system of structured, diffraction-free, chirped femtosecond laser radiating ultra-intense broadband terahertz waves. The terahertz time-domain spectroscopy detection device includes, in the optical path, a chopper, a terahertz filter, a perforated off-axis parabolic mirror, an electric translation device, a third mirror, a fourth mirror, a first focusing lens, a zinc telluride crystal, a quarter-wave plate, a second focusing lens, a Wollaston prism, and a dual-eye photodiode balanced detector, wherein: The collimated terahertz wave reflected by the second off-axis parabolic mirror of the structured, diffraction-free, chirped femtosecond laser radiating ultra-intense broadband terahertz waves is incident on the perforated off-axis parabolic mirror via a chopper and a terahertz filter; wherein the perforated off-axis parabolic mirror has an opening on its back side; The probe beam obtained by the beam splitter of the structured diffraction-free chirped femtosecond laser emitting ultra-intense broadband terahertz waves is sequentially incident on the first focusing lens through an electric translation device, a third reflecting mirror, and a fourth reflecting mirror. The first focusing lens focuses the incident probe beam. The focused probe beam is then incident from the back of a perforated off-axis parabolic mirror through an opening. The incident direction of the focused beam is perpendicular to the incident direction of the collimated terahertz wave filtered by the terahertz filter. The perforated off-axis parabolic mirror concentrically focuses the focused probe beam and the filtered collimated terahertz wave, so that the focused probe beam and the filtered collimated terahertz wave are incident at the same focal point onto the zinc telluride crystal. The probe beam from the zinc telluride crystal is focused by passing through a quarter-wave plate and a second focusing lens in sequence, and the resulting focused probe beam is incident on the Wollaston prism. The Wollaston prism splits the incident focusing probe beam into a first beam and a second beam with mutually perpendicular polarization directions. The first beam and the second beam are focused onto the first probe and the second probe of the binocular photodiode balanced detector, respectively. The first probe and the second probe are used to acquire the differential signal of the first beam and the second beam, thereby obtaining the intensity signal of the terahertz wave.

[0013] A second aspect of this invention proposes a method for radiating ultra-intense broadband terahertz waves using a structured, chirp-free femtosecond laser, based on the aforementioned system for radiating ultra-intense broadband terahertz waves. The method includes: Step S1: The femtosecond laser of the structured, diffraction-free, chirped femtosecond laser radiation ultra-intense broadband terahertz wave system emits a first femtosecond laser of the first wavelength; after the first femtosecond laser is split by a beam splitter, a pump beam and a probe beam with mutually perpendicular propagation directions are obtained, and the probe beam is emitted to the terahertz time-domain spectroscopy detection device. Step S2: The optical parametric amplifier modulates the pump beam into a second femtosecond laser of the second wavelength. The second femtosecond laser is reflected by the first mirror to the laser pulse shaper. The laser pulse shaper performs pulse compression, compensation and chirp modulation on the second femtosecond laser to obtain a third femtosecond laser. Step S3: The reflective spatial light modulator converts the third femtosecond laser from a Gaussian beam into a structured, diffraction-free light field energy distribution beam, which becomes the fourth femtosecond laser. Step S4: The first off-axis parabolic mirror focuses the incident fourth femtosecond laser onto the BBO crystal; the BBO crystal converts part of the incident fourth femtosecond laser into a second harmonic of the third wavelength, and the remaining light in the incident fourth femtosecond laser and the second harmonic are focused together into the air to generate air plasma; the air plasma radiates terahertz waves with an electric field strength of not less than 100 MV / cm and a spectrum coverage of (0.1~50) THz.

[0014] Compared with the prior art, the present invention has the following beneficial effects: This invention relates to a system and method for enhancing the intensity of terahertz wave radiation and controlling its spectral and polarization characteristics by using structured, diffraction-free optical field distribution shaping and modulation of chirped femtosecond lasers.

[0015] This invention employs a laser pulse shaper to chirp-modulate a femtosecond laser, and utilizes a reflective spatial light modulator to shape the femtosecond Gaussian beam into structured, diffraction-free beams such as Bessel, ring Airy, and vortex beams. The chirped and field-controlled femtosecond laser is then focused into air to generate air plasma, radiating strong-field broadband terahertz waves. Compared to other terahertz radiation techniques, this invention can generate ultra-strong broadband terahertz waves, and the spectral polarization can be flexibly controlled, making it of significant value for terahertz wave imaging and spectroscopic applications. Attached Figure Description

[0016] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0017] Figure 1 This is a schematic diagram of the structure of a structured, diffraction-free, chirped femtosecond laser system emitting ultra-intense broadband terahertz waves, according to the present invention.

[0018] Explanation of reference numerals in the attached figures: 1- Femtosecond laser; 2- Beam splitter; 3- Optical parametric amplifier; 4- First reflecting mirror; 5- Laser pulse shaper; 6- Reflective spatial light modulator; 7- Second reflecting mirror; 8- First off-axis parabolic mirror; 9- BBO crystal; 10- Second off-axis parabolic mirror; 11- Chopper; 12- Terahertz filter; 13- Drilled off-axis parabolic mirror; 14- Motorized translation device; 15- Third reflecting mirror; 16- Fourth reflecting mirror; 17- First focusing lens; 18- Zinc telluride crystal; 19- Quarter-wave plate; 20- Second focusing lens; 21- Wollaston prism; 22- Binocular photodiode balanced detector; A- Terahertz time-domain spectroscopy detection device; P- Air plasma Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] The first aspect of this invention proposes a structured, diffraction-free, chirped femtosecond laser system for radiating ultra-intense broadband terahertz waves. The system comprises, in sequence, a femtosecond laser 1, a beam splitter 2, an optical parametric amplifier 3, a first reflector 4, a laser pulse shaper 5, a reflective spatial light modulator 6, a second reflector 7, a first off-axis parabolic mirror 8, a BBO crystal 9, and a second off-axis parabolic mirror 10 arranged in the optical path. Femtosecond laser 1 emits a first femtosecond laser of the first wavelength. After the first femtosecond laser is split by beam splitter 2, a pump beam and a probe beam with mutually perpendicular propagation directions are obtained. The probe beam is emitted to the terahertz time-domain spectroscopy detection device. The optical parametric amplifier 3 modulates the pump beam into a second femtosecond laser of the second wavelength. The second femtosecond laser is reflected by the first reflector 4 to the laser pulse shaper 5, which outputs a chirped modulated third femtosecond laser. The reflective spatial light modulator 6 converts and reflects the incident third femtosecond laser, transforming the Gaussian light field distribution of the third femtosecond laser into a structured, diffraction-free light field distribution. The reflected structured, diffraction-free light field distribution femtosecond laser is the fourth femtosecond laser. The fourth femtosecond laser is reflected by the second mirror 7 to the first off-axis parabolic mirror 8, which focuses the incident fourth femtosecond laser onto the BBO crystal 9. BBO crystal 9 converts part of the incident fourth femtosecond laser light into a second harmonic of the third wavelength. The remaining light in the incident fourth femtosecond laser light and the second harmonic are focused together into the air to generate air plasma. The air plasma radiates terahertz waves with an electric field strength of not less than 100 MV / cm and a spectral coverage of (0.1~50) THz. The terahertz waves are incident on the second off-axis parabolic mirror 10 and reflected as collimated terahertz waves. Among them, the structured diffraction-free light field distribution includes Bessel beam distribution, ring Airy beam distribution, and vortex beam distribution; the angle between the incident third femtosecond laser and the reflected beam of the reflective spatial light modulator 6 is less than 10 degrees.

[0021] Furthermore, the femtosecond laser 1 is a titanium-doped sapphire femtosecond laser amplifier with a first wavelength of 800 nm, a second wavelength of 1550 nm, and a third wavelength of 775 nm.

[0022] The laser pulse shaper 5 utilizes a multiphoton pulse intra-pulse interference phase scanning method to modulate amplitude and phase, achieving chirp compression, compensation, and modulation of the second femtosecond laser. The reflective spatial light modulator 6 is a reflective spatial light modulator suitable for 1550 nm wavelength lasers.

[0023] In this invention, the reflective spatial light modulator 6 modulates its phase modulation spectrum through software control, thereby changing the beam distribution from Gaussian to structured, diffraction-free beams such as Bessel, ring Airy, and vortex beams. Specifically, the energy distribution of the beam is modulated by controlling the phase modulation spectrum parameters of each type of beam, including the central spot diameter, central peak intensity, and number of concentric rings of the Bessel beam; the main ring radius, main ring width, number of concentric rings, and main ring peak intensity of the ring Airy beam; and the phase singularity position, topological charge, and pulse intensity of the vortex beam.

[0024] This invention sequentially passes a femtosecond laser through a pulse chirp modulation device and a spatial light modulator optical field distribution shaping device. It uses a long-wavelength infrared 1550 nm femtosecond laser and a second harmonic 775 nm laser generated by a BBO crystal to confocal ionize air and form air plasma, thereby radiating strong field terahertz waves outward to obtain a high-power, wide-spectrum terahertz radiation source, achieving the purpose of enhancing terahertz wave intensity and broadening the spectrum.

[0025] Furthermore, the terahertz time-domain spectroscopy detection device is independent of the system of structured diffraction-free chirped femtosecond laser radiating ultra-intense broadband terahertz waves. The terahertz time-domain spectroscopy detection device includes, in its optical path, a chopper 11, a terahertz filter 12, a perforated off-axis parabolic mirror 13, an electric translation device 14, a third mirror 15, a fourth mirror 16, a first focusing lens 17, a zinc telluride crystal 18, a quarter-wave plate 19, a second focusing lens 20, a Wollaston prism 21, and a binocular photodiode balanced detector 22, wherein: The collimated terahertz wave reflected by the second off-axis parabolic mirror 10 of the structured, diffraction-free, chirped femtosecond laser radiating ultra-intense broadband terahertz waves is incident on the perforated off-axis parabolic mirror 13 via the chopper 11 and the terahertz filter 12; wherein, the perforated off-axis parabolic mirror 13 has an opening on its back side. The probe beam obtained by the beam splitter 2 of the structured diffraction-free chirped femtosecond laser radiating ultra-intense broadband terahertz waves passes sequentially through the electric translation device 14, the third reflecting mirror 15, and the fourth reflecting mirror 16 and is incident on the first focusing lens 17. The first focusing lens 17 focuses the incident probe beam. The focused probe beam is incident from the back of the perforated off-axis parabolic mirror 13 through an opening. The incident direction of the focused beam is perpendicular to the incident direction of the collimated terahertz wave filtered by the terahertz filter 12. The perforated off-axis parabolic mirror 13 concentrically focuses the focused probe beam and the filtered collimated terahertz wave, so that the focused probe beam and the filtered collimated terahertz wave are incident at the same focal point on the zinc telluride crystal 18. The probe beam from the zinc telluride crystal 18 is focused by the quarter-wave plate 19 and the second focusing lens 20 in sequence, and the resulting focused probe beam is incident on the Wollaston prism 21. The Wollaston prism 21 splits the incident focusing probe beam into a first beam and a second beam with mutually perpendicular polarization directions. The first beam and the second beam are focused onto the first probe and the second probe of the binocular photodiode balanced detector 22, respectively. The first probe and the second probe are used to obtain the differential signal of the first beam and the second beam, thereby obtaining the intensity signal of the terahertz wave.

[0026] In this invention, the focused probe beam and the filtered focused terahertz wave are incident on a zinc telluride crystal. The alternating electric field instantly changes the refractive index distribution of the crystal, and the greater the electric field strength, the more significant the refractive index change. The focused probe beam and the terahertz wave are simultaneously superimposed within the zinc telluride crystal. The probe beam is polarized and modulated during its transmission through the zinc telluride crystal, achieving different propagation speeds along the e-axis and o-axis of the crystal. The polarization change amplitude of the probe beam is proportional to the instantaneous intensity of the terahertz wave. The o-ray and e-ray of the probe beam are split by the Wollaston prism and incident on the two probes of the dual-lens photodiode balanced detector. Differential detection is used to obtain the polarization change of the probe beam, thereby detecting the radiation intensity of the terahertz wave.

[0027] Furthermore, the terahertz time-domain spectroscopy detection device employs a terahertz wave electro-optic sampling detection method to detect the terahertz time-domain spectral signal. The chopper 11 has a frequency of 180 Hz.

[0028] This invention provides a specific embodiment of a system for emitting ultra-intense broadband terahertz waves using a structured, diffraction-free, chirped femtosecond laser.

[0029] A structured, diffraction-free, chirp-free femtosecond laser system radiating ultra-intense broadband terahertz waves comprises, sequentially arranged in the optical path, a femtosecond laser 1, a beam splitter 2, an optical parametric amplifier 3, a first reflector 4, a laser pulse shaper 5, a reflective spatial light modulator 6, a second reflector 7, a first off-axis parabolic mirror 8, a BBO crystal 9, a second off-axis parabolic mirror 10, and air plasma, wherein: The femtosecond laser 1 emits a femtosecond laser with a wavelength of 800 nm. The 800 nm femtosecond laser is split by the beam splitter 2 to obtain a pump beam and a probe beam. The pump beam, after passing through the optical parametric amplifier 3, emits a femtosecond laser with a wavelength of 1550 nm. This 1550 nm femtosecond laser is reflected by the first reflector 4 to the laser pulse shaper 5, outputting a chirped femtosecond laser. The chirped femtosecond laser output from the laser pulse shaper 5 is incident on the reflective spatial light modulator 6. The reflective spatial light modulator 6 converts the Gaussian optical field distribution of the chirped femtosecond laser into a Bessel, ring Airy, or vortex beam, etc., special cross-sectional energy distribution beams. The angle between the incident beam and the reflected beam of the reflective spatial light modulator 6 is less than 10 degrees. The structured, diffraction-free optical field energy distribution beam reflected from the reflective spatial light modulator 6 passes through the second reflector 7, is focused by the first off-axis parabolic mirror 8, and then incident on the BBO crystal 9. The BBO crystal 9 converts a portion of the pump laser into a second harmonic with a wavelength of 775 nm. The 1550 nm pump light and the 775 nm... The second harmonics of nm are focused together in the air to generate air plasma, which radiates outwards as ultra-strong broadband terahertz waves.

[0030] In this embodiment, wherein, as Figure 1 As shown, it also includes a terahertz time-domain spectroscopy detection device, which in its optical path includes a second off-axis parabolic mirror 10, a chopper 11, a terahertz filter 12, a perforated off-axis parabolic mirror 13, an electric translation device 14, a third mirror 15, a fourth mirror 16, a first focusing lens 17, a zinc telluride crystal 18, a quarter-wave plate 19, a second focusing lens 20, a Wollaston prism 21, and a binocular photodiode balanced detector 22, wherein: The terahertz wave generated by the structured diffraction-free chirped femtosecond laser excitation of the air plasma is incident on the second off-axis parabolic mirror 10, and then incident on the perforated off-axis parabolic mirror 13 via the chopper 11 and the terahertz filter 12. The detection beam split by the beam splitter 2 passes sequentially through the electric translation device 14, the third reflecting mirror 15 and the fourth reflecting mirror 16, and is focused by the first focusing lens 17 before entering the perforated off-axis parabolic mirror 13 through the small hole on the back of the perforated off-axis parabolic mirror 13. The terahertz wave is focused by the perforated off-axis parabolic mirror 13 and then incident on the zinc telluride crystal 18 at the same focal point as the detection beam. After passing through the quarter-wave plate 19 and the second focusing lens 20, it is incident on the Wollaston prism 21 and split into two beams with mutually perpendicular polarization directions. The two beams with mutually perpendicular polarization directions are focused and incident on the two probes of the binocular photodiode balanced detector 22 to detect the differential signal of the two beams, thereby obtaining the intensity signal of the terahertz wave. The terahertz wave, incident on the zinc telluride crystal 18, causes its alternating electric field to instantaneously change the crystal's refractive index distribution; the greater the electric field strength, the more significant the refractive index change. The probe beam and the terahertz wave are simultaneously superimposed within the zinc telluride crystal 18. During transmission through the zinc telluride crystal 18, the probe beam is polarized and modulated, achieving different propagation speeds along the e-axis and o-axis of the crystal. The polarization change amplitude of the probe beam is proportional to the instantaneous intensity of the terahertz wave. The o-ray and e-ray of the probe beam are split by the Wollaston prism 21 and incident on the two probes of the binocular photodiode balanced detector 22. Differential detection is used to obtain the polarization change of the probe beam, thereby detecting the radiation intensity of the terahertz wave.

[0031] In this embodiment, the terahertz time-domain spectroscopy detection device employs a terahertz wave electro-optic sampling detection method to detect the terahertz time-domain spectral signal. Specifically, the terahertz wave intensity detector is a dual-eye photodiode balanced detector 22, and the chopper 11 has a frequency of 180 Hz to modulate the terahertz wave pulse, thereby improving detection accuracy and signal-to-noise ratio.

[0032] In this embodiment, the laser pulse shaper 5 uses a multiphoton pulse intra-pulse interference phase scanning method to modulate the amplitude and phase of the initial laser pulse, thereby achieving chirp compression, compensation, and modulation of the incident laser pulse.

[0033] In this embodiment, the reflective spatial light modulator 6 is a reflective spatial light modulator 6 suitable for 1550 nm wavelength laser. The angle between the incident light and the reflected light is less than 10 degrees.

[0034] In this embodiment, the reflective spatial light modulator 6 modulates its phase modulation spectrum through software control, thereby changing the beam distribution from Gaussian to structured, diffraction-free beams such as Bessel, ring Airy, and vortex beams. Specifically, the energy distribution of the beam is modulated by controlling the phase modulation spectrum parameters of each type of beam, including the central spot diameter, central peak intensity, and number of concentric rings of the Bessel beam; the main ring radius, main ring width, number of concentric rings, and main ring peak intensity of the ring Airy beam; and the phase singularity position, topological charge, and pulse intensity of the vortex beam.

[0035] This invention provides a method for radiating ultra-intense broadband terahertz waves using a structured, chirp-free femtosecond laser. Based on the aforementioned system for radiating ultra-intense broadband terahertz waves using a structured, chirp-free femtosecond laser, the method includes: Step S1: The femtosecond laser 1 of the structured, diffraction-free, chirped femtosecond laser radiation ultra-intense broadband terahertz wave system emits a first femtosecond laser of the first wavelength; after the first femtosecond laser is split by the beam splitter 2, a pump beam and a probe beam with mutually perpendicular propagation directions are obtained, and the probe beam is emitted to the terahertz time-domain spectroscopy detection device. Step S2: The optical parametric amplifier 3 modulates the pump beam into a second femtosecond laser of the second wavelength. The second femtosecond laser is reflected by the first reflector 4 to the laser pulse shaper 5. The laser pulse shaper 5 performs pulse compression, compensation and chirp modulation on the second femtosecond laser to obtain a third femtosecond laser. Step S3: The reflective spatial light modulator 6 converts the third femtosecond laser from a Gaussian beam into a structured, diffraction-free light field energy distribution beam, which is then used as the fourth femtosecond laser. Step S4: The first off-axis parabolic mirror focuses the incident fourth femtosecond laser onto the BBO crystal 9; the BBO crystal 9 converts part of the incident fourth femtosecond laser into a second harmonic of the third wavelength, and the remaining light in the incident fourth femtosecond laser and the second harmonic are focused together into the air to generate air plasma; the air plasma radiates terahertz waves with an electric field strength of not less than 100 MV / cm and a spectrum coverage of (0.1~50) THz.

[0036] Please note that the technical features of the above embodiments can be combined arbitrarily. For the sake of brevity, not all possible combinations of the technical features in the above embodiments have been described. However, as long as the combination of these technical features does not contradict each other, it should be considered within the scope of this specification. The above embodiments only illustrate several implementations of the present invention, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the invention patent. It should be pointed out that for those skilled in the art, several modifications and improvements can be made without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of the present invention should be determined by the appended claims.

Claims

1. A system for emitting ultra-intense broadband terahertz waves using structured, diffraction-free, chirped femtosecond lasers, characterized in that... The system includes: The system includes a femtosecond laser (1), a beam splitter (2), an optical parametric amplifier (3), a first reflector (4), a laser pulse shaper (5), a reflective spatial light modulator (6), a second reflector (7), a first off-axis parabolic mirror (8), a BBO crystal (9), and a second off-axis parabolic mirror (10), arranged sequentially on the optical path. The femtosecond laser (1) emits a first femtosecond laser of the first wavelength. After the first femtosecond laser is split by the beam splitter (2), a pump beam and a probe beam with mutually perpendicular propagation directions are obtained. The probe beam is emitted to the terahertz time-domain spectroscopy detection device. The optical parametric amplifier (3) modulates the pump beam into a second femtosecond laser of the second wavelength. The second femtosecond laser is reflected by the first mirror (4) to the laser pulse shaper (5) and outputs a chirped modulated third femtosecond laser. The reflective spatial light modulator (6) converts and reflects the incident third femtosecond laser, converting the Gaussian light field distribution of the third femtosecond laser into a structured non-diffraction light field distribution. The reflected structured non-diffraction light field distribution femtosecond laser is the fourth femtosecond laser. The fourth femtosecond laser is reflected by the second mirror (7) to the first off-axis parabolic mirror (8). The first off-axis parabolic mirror focuses the incident fourth femtosecond laser and then incident it onto the BBO crystal (9). The BBO crystal (9) converts part of the incident fourth femtosecond laser light into a second harmonic of the third wavelength. The remaining light in the incident fourth femtosecond laser light and the second harmonic are focused together into the air to generate air plasma. The air plasma radiates a terahertz wave with an electric field strength of not less than 100 MV / cm and a spectrum coverage of (0.1~50) THz. The terahertz wave is incident on the second off-axis parabolic mirror (10) and reflects a collimated terahertz wave. Among them, the structured non-diffraction light field distribution includes Bessel beam distribution, ring Airy beam distribution, and vortex beam distribution; the angle between the third femtosecond laser incident on the reflective spatial light modulator (6) and the reflected beam is less than 10 degrees.

2. The system according to claim 1, characterized in that, The femtosecond laser (1) is a titanium-doped sapphire femtosecond laser amplifier with a first wavelength of 800 nm, a second wavelength of 1550 nm, and a third wavelength of 775 nm.

3. The system according to claim 1, characterized in that, The laser pulse shaper (5) uses the multiphoton pulse interferometry phase scanning method to perform amplitude and phase modulation, thereby realizing the chirp compression, compensation and modulation of the second femtosecond laser.

4. The system according to claim 1, characterized in that, The reflective spatial light modulator (6) is a reflective spatial light modulator suitable for 1550 nm wavelength lasers.

5. The system according to claim 1, characterized in that, The terahertz time-domain spectroscopy detection device is independent of the structured, diffraction-free, chirped femtosecond laser radiation ultra-intense broadband terahertz wave system. The terahertz time-domain spectroscopy detection device includes, in its optical path, a chopper (11), a terahertz filter (12), a perforated off-axis parabolic mirror (13), an electric translation device (14), a third mirror (15), a fourth mirror (16), a first focusing lens (17), a zinc telluride crystal (18), a quarter-wave plate (19), a second focusing lens (20), a Wollaston prism (21), and a binocular photodiode balanced detector (22), wherein: The collimated terahertz wave reflected by the second off-axis parabolic mirror (10) of the structured, diffraction-free, chirped femtosecond laser radiating ultra-intense broadband terahertz waves is incident on the perforated off-axis parabolic mirror (13) via a chopper (11) and a terahertz filter (12); wherein the perforated off-axis parabolic mirror (13) has an opening on its back side; The probe beam obtained by the beam splitter (2) of the structured diffraction-free chirped femtosecond laser radiation ultra-intense broadband terahertz wave system passes through the electric translation device (14), the third mirror (15) and the fourth mirror (16) in sequence and is incident on the first focusing lens (17). The first focusing lens (17) focuses the incident probe beam. The focused probe beam is incident from the back of the perforated off-axis parabolic mirror (13) through the opening. The incident direction of the focused beam is perpendicular to the incident direction of the collimated terahertz wave filtered by the terahertz filter (12). The perforated off-axis parabolic mirror (13) performs concentric circle confocal focusing on the focused probe beam and the filtered collimated terahertz wave, so that the focused probe beam and the filtered collimated terahertz wave are incident on the zinc telluride crystal (18) at the common focal point. The probe beam from the zinc telluride crystal (18) is focused by passing through a quarter-wave plate (19) and a second focusing lens (20) in sequence, and the resulting focused probe beam is incident on the Wollaston prism (21). The Wollaston prism (21) splits the incident focused detection beam into a first beam and a second beam with mutually perpendicular polarization directions. The first beam and the second beam are focused onto the first probe and the second probe of the binocular photodiode balanced detector (22), respectively. The first probe and the second probe are used to obtain the differential signal of the first beam and the second beam, thereby obtaining the intensity signal of the terahertz wave.

6. A method for emitting ultra-intense broadband terahertz waves using a structured, diffraction-free, chirped femtosecond laser, employing the system described in any one of claims 1-5, characterized in that the method... include: Step S1: The femtosecond laser (1) of the structured, diffraction-free, chirped femtosecond laser radiating ultra-intense broadband terahertz waves emits a first femtosecond laser of the first wavelength; after the first femtosecond laser is split by a beam splitter (2), a pump beam and a probe beam with mutually perpendicular propagation directions are obtained, and the probe beam is emitted to the terahertz time-domain spectroscopy detection device. Step S2: The optical parametric amplifier (3) modulates the pump beam into a second femtosecond laser of the second wavelength. The second femtosecond laser is reflected by the first mirror (4) to the laser pulse shaper (5). The laser pulse shaper (5) performs pulse compression, compensation and chirp modulation on the second femtosecond laser to obtain a third femtosecond laser. Step S3: The reflective spatial light modulator (6) converts the third femtosecond laser into a structured, diffraction-free light field energy distribution beam by converting the Gaussian beam into the fourth femtosecond laser; Step S4: The first off-axis parabolic mirror focuses the incident fourth femtosecond laser onto the BBO crystal (9); the BBO crystal (9) converts part of the incident fourth femtosecond laser into a second harmonic of the third wavelength, and the remaining light in the incident fourth femtosecond laser and the second harmonic are focused together into the air to generate air plasma; the air plasma radiates terahertz waves with an electric field strength of not less than 100 MV / cm and a spectrum coverage of (0.1~50) THz.

Images