CN109445226B - Terahertz optical frequency comb generation device and method based on polar molecule coherent rotation - Google Patents
Terahertz optical frequency comb generation device and method based on polar molecule coherent rotation Download PDFInfo
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- CN109445226B CN109445226B CN201811400583.1A CN201811400583A CN109445226B CN 109445226 B CN109445226 B CN 109445226B CN 201811400583 A CN201811400583 A CN 201811400583A CN 109445226 B CN109445226 B CN 109445226B
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Abstract
The invention provides a terahertz optical frequency comb generating device and method based on polar molecule coherent rotation, wherein the device comprises: the device comprises a femtosecond laser, a first film beam splitting lens, a seed terahertz wave generating device, a first delay line and a polar molecule cavity; a nozzle is arranged in the polar molecule chamber and used for spraying polar molecules; the femtosecond laser signal enters a polar molecule cavity after being delayed by a first delay line, and is injected into polar molecules sprayed by a nozzle to excite the induced dipole moment of the polar molecules to interact with a laser field, so that the molecules generate coherent rotation transition and generate a rotation wave packet of the polar molecules; the femtosecond laser signal is processed by the seed terahertz wave generating device to generate seed terahertz waves, the generated seed terahertz waves enter the polar molecule cavity to irradiate the rotating wave packet of the polar molecules, and the terahertz frequency comb is obtained. The terahertz frequency comb is obtained by utilizing coherent rotation of polar molecules, and a brand new idea for obtaining the terahertz frequency comb is developed.
Description
Technical Field
The invention relates to the technical field of optical frequency combs, in particular to a terahertz optical frequency comb generation device and method based on polar molecule coherent rotation.
Background
The terahertz wave is electromagnetic radiation with the frequency of 0.1-10 THz and the wavelength of 0.03-3 mm. The terahertz frequency is between the infrared laser and the microwave, and the terahertz frequency has a wide application prospect in the aspects of nondestructive testing, spectral analysis, target imaging, aerospace communication and the like. The terahertz field is developed rapidly in recent years, but the current development in the field is mainly limited by the development and development of a light source and a detector, and particularly, the generation of the optical frequency comb of terahertz frequency has great research potential and application value in the aspects of imaging, characterization, rapid detection, high-precision measurement and the like.
The optical frequency comb is a novel stable light source following laser, has light radiation with equal intervals on a frequency spectrum, and is widely applied to aspects of ultra-high precision detection, chemical component rapid detection, optical communication and sensing, long-distance communication and the like. The optical frequency comb can be used as a ruler in frequency to perform accurate measurement on laser, atomic and gas physics. The manner in which the optical frequency comb is generated determines the minimum temporal accuracy with which detection is obtained. In general, it is difficult to prepare a coherent broadband light source by mode-locking the different color spectral lines of an extremely wide spectrum to obtain synchronously driven frequencies, the wider these synchronous frequency ranges, the higher the time resolution of the composed optical comb.
At present, the generation method of the optical frequency comb mainly comprises a micro-ring resonant cavity, a high-order nonlinear optical fiber, an F-P resonator, a photoelectric oscillator and the like, and the development of the optical frequency comb in the terahertz frequency band is still challenging. At present, a terahertz frequency comb generation method based on a modulator and a terahertz frequency comb method based on a quantum cascade laser have been proposed successively, but the frequency of the terahertz frequency comb generation method is far beyond the general definition range of 0.1-10 THz and is closer to an infrared light band (100 terahertz and above), and the terahertz frequency comb generation method depends on the quantum cascade laser close to absolute zero or a low-temperature environment, and meanwhile, the terahertz radiation efficiency of a high order is low.
Disclosure of Invention
Aiming at the problems in the prior art, the invention provides a terahertz optical frequency comb generation device and method based on polar molecule coherent rotation.
Specifically, the invention provides the following technical scheme:
in a first aspect, the present invention provides a terahertz optical frequency comb generating device based on polar molecule coherent rotation, including: the device comprises a femtosecond laser, a first film beam splitting lens, a seed terahertz wave generating device, a first delay line and a polar molecule cavity;
the femtosecond laser generated by the femtosecond laser is split by the first film beam splitting lens to obtain a first femtosecond laser signal and a second femtosecond laser signal;
a nozzle is arranged in the polar molecule chamber and used for spraying polar molecules;
the first femtosecond laser signal enters the polar molecule cavity after being delayed by the first delay line, and is injected into the polar molecules sprayed by the nozzle to excite the induced dipole moment of the polar molecules to interact with the laser field, so that the molecules generate coherent rotational transition and generate a rotational wave packet of the polar molecules;
and the second path of femtosecond laser signal is processed by the seed terahertz wave generating device to generate seed terahertz waves, the generated seed terahertz waves enter the polar molecule cavity and irradiate the rotating wave packet of the polar molecules to obtain the terahertz optical frequency comb.
Further, the seed terahertz wave generating apparatus includes: an optical path regulating device and a first optical path focusing device;
the light path regulating device is used for regulating and controlling the light path of the second femtosecond laser signal to generate a dual-color laser field with parallel polarization, time coincidence and adjustable relative phase;
the first light path focusing device is used for focusing the double-color laser field to generate plasma to obtain terahertz wide-spectrum radiation, and the terahertz wide-spectrum radiation is the seed terahertz wave.
Further, the optical path adjusting device includes: the optical frequency doubling crystal, the dispersion compensation crystal, the optical wedge pair and the polarization adjusting crystal are arranged in sequence;
the second femtosecond laser signal passes through the optical frequency doubling crystal to obtain frequency doubling emergent light, the polarization direction of the frequency doubling emergent light is vertical to the second femtosecond laser signal, and the central wavelength of the frequency doubling emergent light is half of that of the second femtosecond laser signal;
the dispersion compensation crystal is used for providing negative dispersion compensation for the second femtosecond laser signal and the frequency doubling emergent light;
the optical wedge pair consists of two optical wedges with the same structure and is used for providing positive dispersion for the second femtosecond laser signal and the frequency doubling emergent light, changing the transmission distance to change the positive dispersion of the bicolor laser field and controlling the relative phase of the bicolor laser field;
the polarization adjusting crystal is realized by a dual-wavelength wave plate, and the second femtosecond laser signal and the frequency doubling emergent light are oscillated along the same polarization direction through the polarization adjusting crystal.
Further, the polar molecule chamber comprises: the first window, the second window, the third window, the second light path focusing device and the third light path focusing device; the first window is positioned on the left side of the polar molecule cavity and used for injecting polar molecules sprayed by the nozzle after penetrating through the first femtosecond laser signal and being focused by the second light path focusing device, and exciting interaction between induced dipole moment of the polar molecules and a laser field to enable the molecules to generate coherent rotational transition so as to generate a rotational wave packet of the polar molecules; the second window is positioned at the upper left of the polar molecule cavity and used for irradiating the rotating wave packet of the polar molecule after penetrating the seed terahertz wave and being focused by the second light path focusing device to obtain the terahertz optical frequency comb; the third window is positioned at the lower right part of the polar molecular cavity and outputs the terahertz optical frequency comb after being processed by the third optical path focusing device; the first window is a quartz window, and the second window and the third window are high-resistance silicon windows, polyethylene windows or polypropylene windows.
Further, the polar molecule chamber is a vacuum chamber.
Further, the polar molecules are polar diatomic or polyatomic linear molecules, and the molecular rotation transition energy of the polar molecules is in the terahertz level.
Further, the nozzle adopts a continuous nozzle or a pulse type supersonic nozzle synchronized with the femtosecond laser.
Further, the terahertz optical frequency comb generating device based on polar molecule coherent rotation further comprises: and the terahertz optical frequency comb signal detection device is used for detecting the generated terahertz optical frequency comb by utilizing the phase locking relation between the femtosecond laser generated by the femtosecond laser and the generated terahertz optical frequency comb through an electro-optical sampling principle to obtain the time domain waveform of the terahertz optical frequency comb.
Further, the terahertz optical frequency comb signal detection device comprises: the second thin-film beam splitting lens, the second delay line, the fourth light path focusing device, the nonlinear crystal, the 1/4 lambda plate, the polarization beam splitting crystal, the first photoelectric detector, the second photoelectric detector and the phase-locked amplifier;
the femtosecond laser generated by the femtosecond laser is split by the second film beam splitting lens to obtain a third femtosecond laser signal; after the third femtosecond laser signal is delayed by the second delay line, the third femtosecond laser signal is focused by the fourth optical path focusing device and then coincides with the terahertz optical frequency comb in time, and the terahertz optical frequency comb excites the instantaneous Pockels effect in the nonlinear crystal, so that the femtosecond laser is converted into elliptical polarization from circular deflection after passing through an 1/4 lambda wave plate; and then two components of the ellipsometric terahertz wave are obtained after passing through the polarization beam splitting crystal, the two components are respectively detected by a first photoelectric detector and a second photoelectric detector, and a time domain waveform signal of the terahertz optical frequency comb is obtained through the phase-locked amplifier.
In a second aspect, the invention further provides a terahertz optical frequency comb generation method based on polar molecule coherent rotation, which includes:
s1, injecting polar molecules emitted from the nozzle by femtosecond laser, and exciting interaction between induced dipole moment of the molecules and a laser field by the femtosecond laser to enable the molecules to generate coherent rotation transition and generate a rotation wave packet of the polar molecules;
s2, the rotating wave packet of the polar molecule is irradiated by the seed terahertz wave obtained by femtosecond laser processing, because the excited rotating state of the polar molecule has coherence, the energy level difference corresponding to the rotating transition can be radiated at the reproduction moment of the rotating wave packet, the radiated terahertz waves have the characteristic of equal interval on the frequency spectrum, the radiation can be obtained at intervals of preset time in the time domain, and the terahertz frequency comb has the characteristics of phase locking and high repetition frequency, so the terahertz frequency comb is obtained.
According to the technical scheme, the terahertz frequency comb generation device based on polar molecule coherent rotation provided by the invention comprises: the device comprises a femtosecond laser, a first film beam splitting lens, a seed terahertz wave generating device, a first delay line and a polar molecule cavity; the femtosecond laser generated by the femtosecond laser is split by the first film beam splitting lens to obtain a first femtosecond laser signal and a second femtosecond laser signal; a nozzle is arranged in the polar molecule chamber and used for spraying polar molecules; the first femtosecond laser signal enters the polar molecule cavity after being delayed by the first delay line, and is injected into the polar molecules sprayed by the nozzle to excite the induced dipole moment of the polar molecules to interact with the laser field, so that the molecules generate coherent rotational transition and generate a rotational wave packet of the polar molecules; and the second path of femtosecond laser signal is processed by the seed terahertz wave generating device to generate seed terahertz waves, the generated seed terahertz waves enter the polar molecule cavity and irradiate the rotating wave packet of the polar molecules to obtain the terahertz optical frequency comb. The terahertz frequency comb is obtained by utilizing coherent rotation of polar molecules, and a brand new idea for obtaining the terahertz frequency comb is developed. According to the invention, ultrafast femtosecond laser is used for irradiating polar molecules emitted from a nozzle, after coherent rotation wave packets of the molecules are excited, seed terahertz light waves are injected into the coherent rotation wave packets, terahertz radiation related to molecular rotation transition energy can be generated at the rotation reproduction moment of the polar molecules, the radiation corresponds to the rotation energy level energy difference of the polar molecules, and the radiation has the characteristic of equal spacing and can be used as a coherent terahertz light frequency comb source with phase locking. The terahertz frequency comb is driven by femtosecond laser and radiated by coherent rotating wave packets, so that the obtained terahertz frequency comb has the coherent characteristic of phase locking. Meanwhile, the rotation reproduction time of the molecules is only in the picosecond order, so that the obtained terahertz frequency comb has the output characteristic of high repetition frequency.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below, and it is obvious that the drawings in the following description are some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to these drawings without creative efforts.
Fig. 1 is a schematic structural diagram of a terahertz optical-frequency comb generating device based on polar molecule coherent rotation according to an embodiment of the present invention;
fig. 2 is a schematic structural diagram of an optical path adjusting device according to an embodiment of the present invention;
FIG. 3 is a schematic structural diagram of a polar molecular chamber according to an embodiment of the present invention;
fig. 4 is a schematic structural diagram of a terahertz optical-frequency comb generating device based on polar molecule coherent rotation according to an embodiment of the present invention;
fig. 5 is a schematic diagram of a distribution of terahertz frequency-comb frequency domain signals measured by a carbon monoxide experiment according to an embodiment of the present invention;
fig. 6 is a flowchart of a terahertz optical frequency comb generation method based on polar molecule coherent rotation according to another embodiment of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
Fig. 1 shows a schematic structural diagram of a terahertz frequency comb generation device based on polar molecule coherent rotation according to an embodiment of the present invention. As shown in fig. 1, the terahertz optical frequency comb generating device based on polar molecule coherent rotation according to the embodiment of the present invention includes: a femtosecond laser (Ti in fig. 1: Sapphire laser), a first thin film beam splitting lens, a seed terahertz wave generating device, a first delay line (delay line 1 in fig. 1), and a polar molecule chamber;
the femtosecond laser generated by the femtosecond laser is split by the first film beam splitting lens to obtain a first femtosecond laser signal and a second femtosecond laser signal;
generating a seed terahertz wave after the second femtosecond laser signal enters the seed terahertz wave generating device;
the first femtosecond laser signal enters the polar molecule cavity after being delayed by the first delay line, a nozzle is arranged in the polar molecule cavity and used for spraying polar molecules, the first femtosecond laser signal entering the polar molecule cavity is injected into the polar molecules sprayed by the nozzle, and the induced dipole moment of the polar molecules is excited to interact with a laser field, so that the molecules generate coherent rotation transition, and a rotation wave packet of the polar molecules is generated;
the delay time of the first delay line is required to be greater than or equal to the time for coherent rotation of the first path of pumping polarity molecules, that is, the first femtosecond laser needs to perform time delay to irradiate the polarity molecules after the first path of pumping polarity molecules perform coherent rotation. At this time, as the rotation state of the molecule is driven by the femtosecond laser, the time-contained (including time) coherent evolution of the rotation wave packet can be carried out, and the period of the evolution is the rotation period of the molecule;
it should be noted that the first femtosecond laser passes through the delay line 1 shown in fig. 1 and is spatially overlapped with the seed terahertz wave generated by the second femtosecond laser.
And the second path of femtosecond laser signal is processed by the seed terahertz wave generating device to generate seed terahertz waves, the generated seed terahertz waves enter the polar molecule cavity and irradiate the rotating wave packet of the polar molecules to obtain the terahertz optical frequency comb.
In the present embodiment, the femtosecond laser is preferably a femtosecond laser (hereinafter abbreviated as 800 nm laser) with a titanium sapphire stimulated radiation center frequency approximately near 800 nm, and the focused laser peak power needs to reach 1013W/cm, so as to realize the excitation of the molecular rotation wave packet.
In this embodiment, it should be noted that through the evolution of the polar molecule rotation wave packet, especially for polar linear small molecules, energy required for rotation transition is in the terahertz wave band, and the polar small molecules are weak in vibration-rotation coupling, so that their rotation spectra can show a comb-tooth-shaped discrete spectral line structure with equidistant distribution, and are a natural terahertz optical frequency comb source. In addition, the energy difference of the molecule rotation transition corresponds to the rotation angle quantum number, the higher the angle quantum number is, the wider the cut-off energy of the terahertz optical frequency comb corresponding to the higher the angle quantum number is, and the rotation states of the molecules are driven by laser to transition, so that the radiation of the molecules is coherent and in-phase, and the requirements of the optical frequency comb on phase locking by radiation of different frequencies are met.
In this embodiment, the first femtosecond laser is used to inject polar molecules emitted from the nozzle, and the femtosecond laser excites interaction between induced dipole moment of the molecules and the laser field, so that the molecules generate coherent rotation transition and the rotation state is rearranged; then, irradiating the rotation wave packet of the polar molecules by using the seed terahertz wave generated by the second path of femtosecond laser; finally, because the excited rotation state of the polar molecules has good coherence, energy level differences corresponding to rotation transitions can be radiated at the reproduction moment of the rotation wave packet, the radiated terahertz waves have the characteristic of equal intervals in a frequency domain, and radiation is obtained at regular intervals in a time domain, so that the terahertz wave has the characteristics of phase locking and high repetition frequency.
It should be noted that the frequency interval of the THz band optical frequency comb obtained in this embodiment can be controlled by changing the molecular species, the frequency interval of the polar molecule is 2B0, the molecular frequency interval for carbon monoxide is about 0.116THz, the frequency interval for carbonyl sulfide is about 0.012THz, the intrinsic dipole moments of different polar molecules are different, and the heavier the molecular optical frequency comb is, the narrower the distance is. That is, the time interval for obtaining radiation at regular intervals in the time domain is related to the moment of inertia of the molecule, for example, the radiation reproduction time interval of carbon monoxide molecules is about 8.6 picoseconds, and the reproduction time interval of carbonyl sulfide molecules is about 82.1 picoseconds.
As can be seen from the above, in the embodiment, the ultrafast femtosecond laser is used to irradiate the polar molecules emitted from the nozzle, and after the coherent rotating wave packet of the molecules is excited, the seed terahertz wave is injected into the coherent rotating wave packet, and terahertz radiation related to the molecular rotation transition energy is generated at the time of the rotation reproduction of the polar molecules, and the radiation corresponds to the rotation energy level energy difference of the polar molecules, has the characteristic of equal spacing, and can be used as a phase-locked coherent terahertz optical frequency comb source. Therefore, the device is driven by femtosecond laser and radiated by coherent rotating wave packet, so that the obtained terahertz comb has coherent characteristic of phase locking. Meanwhile, the rotation reproduction time of the molecules is only in the picosecond order, so that the obtained terahertz frequency comb has the output characteristic of high repetition frequency.
Based on the content of the above-mentioned embodiment, in an alternative implementation, referring to fig. 1, the seed terahertz wave generating apparatus further includes: an optical path regulating device and a first optical path focusing device;
the light path regulating device is used for regulating and controlling the light path of the second femtosecond laser signal to generate a dual-color laser field with parallel polarization, time coincidence and adjustable relative phase;
the first light path focusing device is used for focusing the double-color laser field to generate plasma to obtain terahertz wide-spectrum radiation, and the terahertz wide-spectrum radiation is the seed terahertz wave.
In the embodiment, the light path regulating device is used for generating a double-color laser field, and then the light path focusing device is used for focusing the double-color laser field to generate plasma so as to obtain terahertz wide-spectrum radiation, so that seed terahertz waves are obtained. The following embodiment (as shown in fig. 2) shows a specific implementation of the optical path control device for generating a two-color laser field, which includes different crystals and the arrangement of the crystals. In other embodiments of the present invention, the seed terahertz wave generating apparatus may further include an optical path adjusting device capable of generating a three-color laser field or a multi-color laser field, and then the optical path focusing device focuses the three-color laser field or the multi-color laser field to generate plasma to obtain terahertz wide-spectrum radiation, so as to obtain seed terahertz waves. The specific implementation mode of the seed terahertz wave generating device is not limited, and the seed terahertz wave generating device can generate the wide-spectrum radiation seed terahertz wave with the frequency covering the polar molecule rotation transition energy. It should be noted that, compared to the optical path adjusting device shown in fig. 2, the optical path adjusting device capable of generating a three-color laser field or a multi-color laser field may be implemented by adding more crystals or changing some arrangement modes at the same time.
Fig. 2 is a schematic structural diagram of an optical path adjusting device according to an embodiment of the present invention. As shown in fig. 2, the optical path adjusting device further includes: the optical frequency doubling crystal, the dispersion compensation crystal, the optical wedge pair and the polarization adjusting crystal are arranged in sequence;
the second femtosecond laser signal passes through the optical frequency doubling crystal to obtain frequency doubling emergent light, the polarization direction of the frequency doubling emergent light is vertical to the second femtosecond laser signal, and the central wavelength of the frequency doubling emergent light is half of that of the second femtosecond laser signal;
the dispersion compensation crystal is used for providing negative dispersion compensation for the second femtosecond laser signal and the frequency doubling emergent light;
the optical wedge pair consists of two optical wedges with the same structure and is used for providing positive dispersion for the second femtosecond laser signal and the frequency doubling emergent light, changing the transmission distance to change the positive dispersion of the bicolor laser field and controlling the relative phase of the bicolor laser field;
the polarization adjusting crystal is realized by a dual-wavelength wave plate, and the second femtosecond laser signal and the frequency doubling emergent light are oscillated along the same polarization direction through the polarization adjusting crystal.
In this embodiment, the second femtosecond laser passes through the optical path control device shown in fig. 2 to obtain a dual-color laser field (such as 800 nm and 400 nm femtosecond light) with parallel polarization, time coincidence and adjustable relative phase, first, the 800 nm femtosecond laser passes through an optical frequency doubling crystal (usually adopting β -BBO for optical frequency doubling) to obtain 400 nm frequency doubling emergent light with polarization perpendicular to 800 nm incident pump, because the refractive indexes of the 800 nm and 400 nm light in the crystal are different, the two lasers introduce a time difference when passing through a normal dispersion medium, so that the two lasers cannot effectively generate seed terahertz wave radiation in time, therefore, the two lights need to be subjected to anomalous dispersion compensation by a dispersion compensation crystal (the adopted crystal provides negative dispersion for α -BBO or calcite), the polarization adjustment crystal is realized by a dual-wavelength wave plate, the 400 nm laser is a full-wave plate, the 800 nm laser is a half-wave plate, the 400 nm and 800 nm laser can be subjected to oscillation along the same polarization direction by a focusing air lens, the terahertz wave can be obtained by irradiating a half-wavelength metal mirror, and the terahertz light can be irradiated by a common focusing mirror to generate a terahertz reflection mirror, and the terahertz light source can be generated by a common focusing metal reflection mirror, and the terahertz light path can be generated by a common focusing metal reflection mirror.
Fig. 3 is a schematic structural diagram of a polar molecular chamber according to an embodiment of the present invention. As shown in fig. 3, the polar molecular chamber includes: the first window, the second window, the third window, the second light path focusing device and the third light path focusing device; the first window is positioned on the left side of the polar molecule cavity and used for injecting polar molecules sprayed by the nozzle after penetrating through the first femtosecond laser signal and being focused by the second light path focusing device, and exciting interaction between induced dipole moment of the polar molecules and a laser field to enable the molecules to generate coherent rotational transition so as to generate a rotational wave packet of the polar molecules; the second window is positioned at the upper left of the polar molecule cavity and used for irradiating the rotating wave packet of the polar molecule after penetrating the seed terahertz wave and being focused by the second light path focusing device to obtain the terahertz optical frequency comb; the third window is positioned at the lower right part of the polar molecular cavity and outputs the terahertz optical frequency comb after being processed by the third optical path focusing device; the third light path focusing device is used for converting the focused terahertz light emitted out again into parallel light, and then emitting out from the third window. It should be noted that the focal points of the second optical path focusing device and the third optical path focusing device need to be overlapped, so as to convert the focused light into parallel light and emit the parallel light. Wherein the first window may be a quartz window, and the second and third windows may be high-resistance silicon windows.
It should be noted that the terahertz wave generated by the second femtosecond laser is used as a seed pulse to irradiate the polar molecular rotation wave packet generated by the first femtosecond laser, and the terahertz wave optical frequency comb radiation is obtained by using the rotation transition between the molecular rotation states. As shown in fig. 3, the terahertz seed light enters from the second window at the upper left of the chamber, and the obtained terahertz optical frequency comb exits along the third window at the lower right of the chamber. In addition, the first window for taking charge of incidence and the third window for taking charge of emergence are not limited to the high-resistance silicon window, but may be polyethylene or polypropylene windows, and the transmission efficiency of the polyethylene or polypropylene windows may be higher.
Based on the foregoing description of the embodiments, in an alternative implementation, the polar molecule chamber is a vacuum chamber.
In this embodiment, the polar molecule chamber is a vacuum chamber, and the whole optical path of the terahertz wave is disposed in a vacuum environment, so that the reabsorption of the polar molecules on the generated terahertz frequency comb is reduced.
It should be noted that the nozzle for ejecting the polar molecules is in a vacuum environment, the diameter of the polar molecule ejection is about 100-300 microns, and a supersonic gas flow is radiated from the nozzle to the vacuum environment. Wherein the back pressure of the gas injected into the nozzle by the polar molecules is in the range of 0.5 to 6 standard atmospheres.
Based on the content of the above embodiments, in an alternative implementation, the polar molecule is a polar diatomic or polyatomic linear molecule, and the molecular rotational transition energy of the polar molecule is in the terahertz order.
In this embodiment, since the molecules are diatomic or polyatomic molecules, the vibration-rotation coupling effect is weak, and therefore the rotation energy levels of the molecules have a significant constant frequency interval characteristic, and can be used as a terahertz frequency comb source. For example, the molecule may be a polar diatomic or polyatomic linear molecule such as carbon monoxide, carbonyl sulfide, and the like.
Based on the above disclosure of the embodiments, in an alternative embodiment, the nozzle is a continuous nozzle or a pulsed supersonic nozzle synchronized with the femtosecond laser.
In the present embodiment, the nozzle may employ a continuous nozzle or a pulsed supersonic nozzle synchronized with a femtosecond laser. The pulsed supersonic nozzle is superior to the continuous nozzle because the higher the vacuum degree in the chamber when the pulsed nozzle sprays gas into the chamber, the faster the translation speed of the supersonic expansion of the sprayed gas molecules from the nozzle connected with the steel cylinder. At this time, the high density of collisions between the molecules in translation can transfer the rotational vibration energy of the molecules to the translation energy, thereby cooling the molecules. The cooled molecules are easier to be excited by femtosecond laser, so that a rotating wave packet with good coherence is obtained and evoluted.
Fig. 4 shows another schematic structural diagram of a terahertz optical-frequency comb generation device based on polar molecule coherent rotation according to an embodiment of the present invention. As shown in fig. 4, the terahertz optical frequency comb generating device based on polar molecule coherent rotation further includes: and the terahertz optical frequency comb signal detection device is used for detecting the generated terahertz optical frequency comb by utilizing the phase locking relation between the femtosecond laser generated by the femtosecond laser and the generated terahertz optical frequency comb through an electro-optical sampling principle to obtain the time domain waveform of the terahertz optical frequency comb.
Based on the content of the foregoing embodiment, in an alternative implementation manner, referring to fig. 4, the terahertz optical-frequency comb signal detection apparatus includes: a second thin film beam splitting mirror, a second delay line (delay line 2 in fig. 4), a fourth optical path focusing device, a nonlinear crystal (ZnTe crystal in fig. 4), an 1/4 λ plate, a polarization beam splitting crystal, a first photodetector (photodetector 1 in fig. 4), a second photodetector (photodetector 2 in fig. 4), and a lock-in amplifier (not shown in fig. 4);
referring to fig. 4, the femtosecond laser generated by the femtosecond laser is split by the second thin film beam splitter lens to obtain a third femtosecond laser signal; after the third femtosecond laser signal is delayed by the delay line 2, the third femtosecond laser signal is focused by the fourth optical path focusing device and then coincides with the terahertz optical frequency comb emitted from the polar molecular cavity in time, and the terahertz optical frequency comb excites the instantaneous bubble Kerr effect in the zinc telluride crystal, so that the femtosecond laser is converted into elliptical polarized light by circular deflection after passing through an 1/4 lambda wave plate; and then two components of the ellipsometric terahertz wave are obtained after passing through the polarization beam splitting crystal, the two components are respectively detected by the photoelectric detector 1 and the photoelectric detector 2, and a time domain waveform signal of the terahertz frequency comb is obtained through the lock-in amplifier.
In this embodiment, it should be noted that the third femtosecond laser signal passes through the delay line 2 shown in fig. 4, and is temporally and spatially overlapped with the terahertz optical comb exiting from the polar molecular cavity in the zinc telluride crystal. The third femtosecond laser signal mainly aims to measure the time domain waveform of the terahertz frequency comb, and meanwhile, the frequency domain information of the terahertz frequency comb can be obtained through Fourier transform. Fig. 5 provides a graph of the terahertz-frequency comb spectrum obtained using carbon monoxide polar molecules, the corresponding polar molecular rotational transitions having been marked in the graph. It should be noted that the third femtosecond laser signal is used for characterizing and detecting the terahertz optical frequency comb signal, and the first femtosecond laser signal and the second femtosecond laser signal are used for obtaining the terahertz optical frequency comb radiation. In the embodiment, a third femtosecond laser signal is adopted for detecting by using an electro-optical sampling principle by utilizing a phase locking relationship between the path of light and the generated terahertz frequency comb. The polar molecules used may cause different frequency resolution of the terahertz comb if replaced by other molecules, but the frequency resolution is related to the kind of molecules and is not limited to the above-mentioned carbon monoxide molecules. As shown in fig. 4, an electro-optical sampling method is adopted for the terahertz frequency comb characterization, the adopted crystal is a zinc telluride crystal, the characteristic response frequency is within 3 terahertz, and other nonlinear crystals such as gallium phosphide and the like can be adopted in an actual experiment, or an ABCD (air-binary coherent detection) method is adopted for detection, so that a wider frequency domain spectrum can be obtained.
By the technical scheme, the terahertz frequency comb generating device based on the polar molecule coherent rotation provided by the embodiment obtains the terahertz frequency comb by utilizing the coherent rotation of the polar molecule, and a brand new idea for obtaining the terahertz frequency comb is developed. According to the invention, ultrafast femtosecond laser is used for irradiating polar molecules emitted from a nozzle, after coherent rotation wave packets of the molecules are excited, seed terahertz light waves are injected into the coherent rotation wave packets, terahertz radiation related to molecular rotation transition energy can be generated at the rotation reproduction moment of the polar molecules, the radiation corresponds to the rotation energy level energy difference of the polar molecules, and the radiation has the characteristic of equal spacing and can be used as a coherent terahertz light frequency comb source with phase locking. The terahertz frequency comb is driven by femtosecond laser and radiated by coherent rotating wave packets, so that the obtained terahertz frequency comb has the coherent characteristic of phase locking. Meanwhile, the rotation reproduction time of the molecules is only in the picosecond order, so that the obtained terahertz frequency comb has the output characteristic of high repetition frequency.
Another embodiment of the present invention provides a terahertz optical frequency comb generation method based on polar molecule coherent rotation, and referring to fig. 6, the method includes the following steps:
step 101: the femtosecond laser is used for injecting the polar molecules emitted from the nozzle, and the femtosecond laser excites the induced dipole moment of the molecules to interact with the laser field, so that the molecules generate coherent rotation transition, and a rotation wave packet of the polar molecules is generated.
Step 102: the method comprises the steps that a seed terahertz wave obtained by femtosecond laser processing is used for irradiating a rotating wave packet of a polar molecule, because the excited rotating state of the polar molecule has coherence, energy level differences corresponding to rotating transitions can be radiated at the reproduction moment of the rotating wave packet, the radiated terahertz waves have the characteristic of equal intervals on a frequency spectrum, radiation can be obtained once at intervals of preset time in a time domain, and the characteristics of phase locking and high repetition frequency are achieved, so that the terahertz optical frequency comb is obtained.
It should be noted that, in the terahertz frequency comb generation method based on polar molecule coherent rotation provided in this embodiment, ultrafast femtosecond laser is used to irradiate polar molecules emitted from a nozzle, and after a coherent rotation wave packet of the molecules is excited, a seed terahertz light wave is injected into the coherent rotation wave packet, and terahertz radiation related to molecular rotation transition energy is generated at the time of rotation reproduction of the polar molecules, and the radiation corresponds to a rotational energy level energy difference of the polar molecules, has the characteristic of equal spacing, and can be used as a phase-locked coherent terahertz frequency comb source. The method is driven by femtosecond laser and radiated by coherent rotating wave packet, so that the obtained terahertz frequency comb has coherent characteristic of phase locking. Meanwhile, the rotation reproduction time of the molecules is only in the picosecond order, so that the obtained terahertz frequency comb has the output characteristic of high repetition frequency.
It should be noted that, since the method for generating the polar molecule coherent rotation-based terahertz frequency comb provided by this embodiment can be implemented by using the apparatus for generating the polar molecule coherent rotation-based terahertz frequency comb described in the foregoing embodiment, the specific operating principle and technical effect thereof are similar, and detailed description is omitted here, and specific contents can be referred to the description of the foregoing embodiment.
In the description of the present invention, it should be noted that the terms "upper", "lower", and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the drawings, which are merely for convenience in describing the present invention and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed in a specific orientation, and operate, and thus, should not be construed as limiting the present invention. Unless explicitly stated or limited otherwise. It is further noted that, herein, relational terms such as first and second, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Also, the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising an … …" does not exclude the presence of other identical elements in a process, method, article, or apparatus that comprises the element.
The above examples are only for illustrating the technical solutions of the present invention, and not for limiting the same; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present invention.
Claims (10)
1. A terahertz optical frequency comb generation device based on polar molecule coherent rotation is characterized by comprising: the device comprises a femtosecond laser, a first film beam splitting lens, a seed terahertz wave generating device, a first delay line and a polar molecule cavity;
the femtosecond laser generated by the femtosecond laser is split by the first film beam splitting lens to obtain a first femtosecond laser signal and a second femtosecond laser signal;
a nozzle is arranged in the polar molecule chamber and used for spraying polar molecules;
the first femtosecond laser signal enters the polar molecule cavity after being delayed by the first delay line, and is injected into the polar molecules sprayed by the nozzle to excite the induced dipole moment of the polar molecules to interact with the laser field, so that the molecules generate coherent rotation transition, and a coherent rotation wave packet of the polar molecules is generated;
the second path of femtosecond laser signal is processed by the seed terahertz wave generating device to generate seed terahertz waves, the generated seed terahertz waves enter the polar molecule cavity and irradiate the coherent rotating wave packet of the polar molecules to obtain a terahertz optical frequency comb;
wherein the polar molecular chamber comprises: the first window, the second window, the third window, the second light path focusing device and the third light path focusing device; the first window is positioned on the left side of the polar molecule cavity and used for injecting polar molecules sprayed by the nozzle after penetrating through the first femtosecond laser signal and being focused by the second light path focusing device, and exciting interaction between induced dipole moment of the polar molecules and a laser field to enable the molecules to generate coherent rotation transition so as to generate a coherent rotation wave packet of the polar molecules; the second window is positioned at the upper left of the polar molecule cavity and used for irradiating the coherent rotating wave packet of the polar molecules after penetrating the seed terahertz waves and being focused by the second light path focusing device to obtain the terahertz frequency comb; and the third window is positioned at the lower right part of the polar molecular cavity and outputs the terahertz frequency comb after being processed by the third optical path focusing device.
2. The polar molecule coherent rotation-based terahertz-frequency comb generation device as claimed in claim 1, wherein the seed terahertz wave generation device comprises: an optical path regulating device and a first optical path focusing device;
the light path regulating device is used for regulating and controlling the light path of the second femtosecond laser signal to generate a dual-color laser field with parallel polarization, time coincidence and adjustable relative phase;
the first light path focusing device is used for focusing the double-color laser field to generate plasma to obtain terahertz wide-spectrum radiation, and the terahertz wide-spectrum radiation is the seed terahertz wave.
3. The polar molecule coherent rotation-based terahertz optical frequency comb generation device as claimed in claim 2, wherein the optical path adjusting and controlling means comprises: the optical frequency doubling crystal, the dispersion compensation crystal, the optical wedge pair and the polarization adjusting crystal are arranged in sequence;
the second femtosecond laser signal passes through the optical frequency doubling crystal to obtain frequency doubling emergent light, the polarization direction of the frequency doubling emergent light is vertical to the second femtosecond laser signal, and the central wavelength of the frequency doubling emergent light is half of that of the second femtosecond laser signal;
the dispersion compensation crystal is used for providing negative dispersion compensation for the second femtosecond laser signal and the frequency doubling emergent light;
the optical wedge pair consists of two optical wedges with the same structure and is used for providing positive dispersion for the second femtosecond laser signal and the frequency doubling emergent light, changing the transmission distance to change the positive dispersion of the bicolor laser field and controlling the relative phase of the bicolor laser field;
the polarization adjusting crystal is realized by a dual-wavelength wave plate, and the second femtosecond laser signal and the frequency doubling emergent light are oscillated along the same polarization direction through the polarization adjusting crystal.
4. The polar molecule coherent rotation-based terahertz optical frequency comb generation device as claimed in claim 1, wherein the first window is a quartz window, and the second window and the third window are a high-resistance silicon window, a polyethylene window or a polypropylene window.
5. The polar molecule coherent rotation-based terahertz-frequency comb generation device as claimed in claim 1, wherein the polar molecule chamber is a vacuum chamber.
6. The polar molecule coherent rotation-based terahertz optical frequency comb generation device as claimed in claim 1, wherein the polar molecule is a polar diatomic or polyatomic linear molecule, and the molecular rotation transition energy is in the terahertz order.
7. The polar molecule coherent rotation-based terahertz-frequency comb generation device as claimed in claim 1, wherein the nozzle is a continuous nozzle or a pulsed supersonic nozzle synchronized with the femtosecond laser.
8. The terahertz frequency comb generation device based on the coherent rotation of polar molecules as claimed in any one of claims 1 to 7, further comprising: and the terahertz optical frequency comb signal detection device is used for detecting the generated terahertz optical frequency comb by utilizing the phase locking relation between the femtosecond laser generated by the femtosecond laser and the generated terahertz optical frequency comb through an electro-optical sampling principle to obtain the time domain waveform of the terahertz optical frequency comb.
9. The terahertz-frequency comb generation device based on polar molecule coherent rotation according to claim 8, wherein the terahertz-frequency comb signal detection device comprises: the second thin-film beam splitting lens, the second delay line, the fourth light path focusing device, the nonlinear crystal, the 1/4 lambda plate, the polarization beam splitting crystal, the first photoelectric detector, the second photoelectric detector and the phase-locked amplifier;
the femtosecond laser generated by the femtosecond laser is split by the second film beam splitting lens to obtain a third femtosecond laser signal; after the third femtosecond laser signal is delayed by the second delay line, the third femtosecond laser signal is focused by the fourth optical path focusing device and then coincides with the terahertz optical frequency comb in time, and the terahertz optical frequency comb excites the instantaneous Pockels effect in the nonlinear crystal, so that the femtosecond laser is converted into elliptical polarization from circular deflection after passing through an 1/4 lambda wave plate; and then two components of the ellipsometric terahertz wave are obtained after passing through the polarization beam splitting crystal, the two components are respectively detected by a first photoelectric detector and a second photoelectric detector, and a time domain waveform signal of the terahertz optical frequency comb is obtained through the phase-locked amplifier.
10. A terahertz optical frequency comb generation method based on polar molecule coherent rotation is characterized by comprising the following steps:
s1, injecting polar molecules emitted from the nozzle by femtosecond laser, and exciting interaction between induced dipole moment of the molecules and a laser field by the femtosecond laser to enable the molecules to generate coherent rotation transition and generate a coherent rotation wave packet of the polar molecules;
s2, the coherent rotating wave packet of the polar molecule is irradiated by the seed terahertz wave obtained by femtosecond laser processing, because the excited rotating state of the polar molecule has coherence, the energy level difference corresponding to the rotating transition can be radiated at the reproduction moment of the rotating wave packet, the radiated terahertz waves have the characteristic of equal interval on the frequency spectrum, the radiation can be obtained at intervals of preset time in the time domain, and the terahertz frequency comb has the characteristics of phase locking and high repetition frequency, so the terahertz frequency comb is obtained.
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