CN102879971B - Method for continuously regulating central frequency and spectrum width of THz (terahertz) wave - Google Patents
Method for continuously regulating central frequency and spectrum width of THz (terahertz) wave Download PDFInfo
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Abstract
The invention relates to a method for continuously regulating central frequency and spectrum width of THz (terahertz) wave. A bundle of ultrafast laser pulse passes through a movable birefringence crystal oblique split pair and a zero-order quarter wave plate to generate a novel specially-polarized light field by a polarizing gate method. The novel light field passes through a polarizer to obtain laser pulse with width of narrow pulse with linear polarization. Laser pulses different in width are obtained by controlling inserting quantity of the birefringence crystal oblique split pair, and accordingly the function of regulating central frequency and spectrum width of the THz wave is achieved. A device is simple and easy to operate, the width of the novel laser pulse can be changed only two crystals of the birefringence crystal oblique split pair simultaneously move oppositely at equal distance, and accordingly the central frequency and the spectrum width are adjusted. The method is suitable for light sources of various pulse widths and wavelengths.
Description
Technical field
The present invention relates to a kind of electromagnetic wave adjustment technology, particularly a kind of method of continuous adjustment THz wave centre frequency and spectrum width.
Background technology
In recent decades, THz wave, with its application prospect widely, has become an important subject of physical field in the world.Terahertz emission is the electromagnetic wave of frequency in 0.1 to 10 THz scopes, this wave band microwave and infrared between, there is the characteristics such as carry information amount is abundant, high temporal and spatial coherence, low photon energy, have huge using value in the scientific domain such as astronomy, biology, computing machine, communication.At present, main applied research has terahertz time-domain spectroscopic technology, THz imaging technology, safety inspection, Terahertz radar, uranology, the communication technology.
At present, the generation of broadband terahertz pulse mainly contain that photoconductive antenna excites, optical rectification method and these three kinds of methods of nonlinear transmission line.
It is the semiconductor utilizing photon energy to be greater than the ultrashort laser pulse pumping electric field bias of semiconductor material energy gap that photoconductive antenna excites, and makes its internal excitation charge carrier, produces electron-hole pair.Their accelerated motion under DC Electric Field, forms a transient photocurrents, then passes through an antenna at short notice to free-space propagation terahertz electromagnetic wave.Detect identical with producing equipment used, just photoconductor does not need to be biased voltage.The performance of terahertz electromagnetic wave emission coefficient depends mainly on three factors: photoconductor, antenna geometries and pumping laser pulse width.Centre frequency and the width of the pumping laser pulse generally provided are determined, the centre frequency of the THz wave therefore produced and spectrum width just uniquely determine.The method exists arbitrarily can not regulate produced THz wave centre frequency and the shortcoming of spectrum width.
Optical rectification method utilizes electro-optic crystal as nonlinear medium, utilizes the electro-optic crystals such as the laser pulse pumping ZnTe of picosecond magnitude, produce low frequency electric polarization field, give off terahertz electromagnetic wave thus by second order or high-order nonlinear optics process.Physical process is wherein instantaneous polarization process, requires that a branch of ultrashort laser pulse focuses on electrooptical material.The key issue of optical rectification is phase matching, and it can interaction in nonlinear material of amplifying laser and terahertz pulse and strengthen Terahertz and tell on.Conventional nonlinear medium has LiNbO
3, LiTaO
3, semiconductor GaAs, InTe, InP and organic crystal DAST etc.But the centre frequency of the terahertz electromagnetic wave obtained by this method is lower.
Nonlinear transmission line is distributed device, periodically loads nonlinear element by the transmission line of a high impedance.Nonlinear element is generally Schottky variable capacitance diode.Due to the balance between nonlinear characteristic and intrinsic scattering, in device, produce the wave of oscillation or solitary wave, the terahertz electromagnetic wave radiation of certain frequency can be produced.
On the other hand, the generation of narrow band terahertz band pulse obtains mainly through the method for optical parameter convert light mixing at present.Optically nonlinear crystal, such as LiNbO
3, during in near-infrared region by stronger laser pumped by pulsed laser, can produce and be excited Polarization scattering phenomenon.Frequency is W
ppump light when exciting nonlinear crystal, producing a frequency is W
ilight.Two bundles laser (wherein at least one is continuously adjustabe) carry out optical mixing in optical conductor, the photocurrent that a frequency is their differences can be produced, if frequency is within the scope of Terahertz, then photocurrent can pass through antenna free radiation, forms terahertz electromagnetic wave radiation.The terahertz electromagnetic wave of different frequency can be obtained by the incident angle changing pump light.But the requirement of whole device to device and laser is higher, involves great expense, and to expect that the THz wave of different center frequency and spectrum width not too easily realizes.
Generally speaking, current existing terahertz pulse produces the THz wave that technical method can only produce single centre frequency and certain spectrum width mostly.If need adjustment in experimentation, not only need accurate calculating, the adjustment of experimental provision is also very loaded down with trivial details.Based on these all not convenience experimentally, the experimental provision that can regulate THz wave centre frequency and spectrum width continuously is not also had to occur at present.
Summary of the invention
The present invention be directed to current terahertz pulse and produce the THz wave that technical method can only produce single centre frequency and certain spectrum width mostly, adjust frequency and the problem of spectrum width difficulty, propose a kind of method of continuous adjustment THz wave centre frequency and spectrum width, adopt polarization door method, with a branch of ultrashort pulse by moveable birefringece crystal tiltedly split to zeroth order quarter wave plate, produce the special new light field of polarization.New light field by the polarizer, just obtains the laser pulse that has the narrow pulse width of linear polarization again.Tiltedly split by controlling birefringece crystal the laser pulse that right insertion obtains different in width, thus realize the function regulating THz wave centre frequency and spectrum width.
Technical scheme of the present invention is: a kind of method of continuous adjustment THz wave centre frequency and spectrum width, the primary laser pulse that femto-second laser exports, it is right tiltedly to be split by moveable birefringece crystal, primary laser pulse is divided into the two bunch polarization lasers that polarization is mutual vertically, have time delay, obtains initial terahertz radiation successively after zeroth order quarter wave plate, the polarizer, condenser lens, frequency-doubling crystal; Birefringece crystal is tiltedly split two pieces of right crystal and moves identical distance round about simultaneously, decompose and obtain the two bunch polarization lasers different from front time delay, the new two bunch polarization lasers obtaining time delays pass through one piece of zeroth order quarter wave plate successively, form a left-handed and right-handed polarized light, time domain then synthesizes new light field; New light field is through the polarizer, obtain the laser pulse of the new pulse width of a linear polarization, new laser pulse is focused on by condenser lens, again by being positioned at the frequency-doubling crystal of condenser lens focused spot front end, produce plasma near focal point, the new terahertz radiation that formation spectrum width is different from initial terahertz radiation enters terahertz detection system.
Described birefringece crystal is tiltedly split can be obtained by various single axial birefringence crystal preparation, the optional calcite crystal of birefringece crystal, quartz crystal or ruby crystal, the polarization direction angle at 45 ° of right optical axis and primary laser pulse tiltedly split by birefringece crystal, primary laser pulse by birefringece crystal tiltedly split to after the two bunch polarization laser intensity that are divided into equal.
The optical axis of described zeroth order quarter wave plate is consistent with the polarization direction of primary laser pulse.
Described polarizer polarization direction is consistent with the polarization direction of primary laser pulse.
Described frequency-doubling crystal optional bbo crystal, lbo crystal or ktp crystal.
Beneficial effect of the present invention is: the present invention is a kind of method of continuous adjustment THz wave centre frequency and spectrum width, and device is simple, easily operates, can tiltedly split right insertion by free adjustment birefringece crystal.In actual mechanical process, only need that birefringece crystal is tiltedly split two pieces of right crystal and move identical distance round about simultaneously, just can realize the change of new laser pulse width, thus the centre frequency of adjustment THz wave and spectrum width; The method is all applicable for the light source of various pulse width and wavelength.
Accompanying drawing explanation
Fig. 1 is the apparatus structure schematic diagram that the present invention realizes regulating continuously THz wave centre frequency and spectrum width;
Fig. 2 is that the present invention regulates polarization door principle schematic in the method for THz wave centre frequency and spectrum width continuously;
Fig. 3 is that the present invention regulates new light field schematic diagram in the method for THz wave centre frequency and spectrum width continuously.
Embodiment
Realize the apparatus structure schematic diagram regulating THz wave centre frequency and spectrum width continuously as shown in Figure 1, by laser instrument 1, birefringece crystal is tiltedly split 2, zeroth order quarter wave plate 3, the polarizer 4, condenser lens 5, frequency-doubling crystal 6, and terahertz detection system 7 forms.Place one block of birefringece crystal tiltedly to split 2 in the exit of laser instrument 1, primary laser pulse is divided into the two bunch polarization lasers that polarization is mutual vertically, have certain hour delay.Because birefringece crystal is tiltedly split the optical axis of 2 and the polarization direction angle at 45 ° of primary laser pulse, so the two bunch polarization laser intensity be divided into are equal.Two bunch polarization lasers by one piece of zeroth order quarter wave plate 3, form a left-handed and right-handed polarized light, time domain then synthesize the special light field of polarization successively.The polarization of this light field changed along with the time, and pulse front edge is circular polarization, and during arrival two-beam time domain overlapping position, left-right rotary polarization is cancelled out each other and become linear polarization, becomes circular polarization gradually more subsequently.Light field, through the polarizer 4, obtains the linearly polarized laser pulse of a narrow pulse width.The width of gained linearly polarized laser pulse is relevant with the insertion that birefringece crystal is tiltedly split 2.Laser pulse through condenser lens 5, frequency-doubling crystal 6, forms wire drawing more in atmosphere, produces plasma, so form terahertz radiation, enters terahertz detection system 7.The THz wave spectrum width now obtained will much larger than the THz wave spectrum width directly obtained by primary laser pulse, and centre frequency also can correspondingly change.
Primary laser pulse can be the light pulse of various pulse width.It can be obtained by various single axial birefringence crystal preparation that birefringece crystal is tiltedly split 2, entering birefringece crystal tiltedly splits after 2, the two bunch polarization lasers be divided into are a branch of is o light, a branch of is e light, birefringece crystal is tiltedly split can not be too thick, in case the o light of decomposition gained and e light separate completely in time domain, birefringece crystal can be calcite crystal, quartz crystal or ruby crystal, the polarization direction angle at 45 ° of its optical axis and primary laser pulse.Zeroth order quarter wave plate 3, its optical axis is consistent with the polarization direction of primary laser pulse.The polarizer 4, its polarization direction is consistent with the polarization direction of primary laser pulse.Frequency-doubling crystal 6 can be bbo crystal, lbo crystal or ktp crystal.
In the following embodiments, to export the laser instrument that center wavelength of light is 800 nm, its all band is consistent with the implementation method of this wave band.
It is 800 nm that laser instrument exports center wavelength of light, pulse width is 40 fs, repetition frequency 1 KHz, the frequency doubled light of 400 nm is obtained for BBO frequency-doubling crystal 6, the adjustment process of specific implementation THz wave centre frequency and spectrum width is as follows: the primary laser pulse exported by femto-second laser 1, polarization door principle schematic as shown in Figure 2, tiltedly split 2 by moveable birefringece crystal, primary laser pulse is divided into the two bunch polarization lasers that polarization is mutual vertically, have certain hour delay; Two pieces of crystal that birefringece crystal is tiltedly split 2 are moved identical distance simultaneously round about, namely change birefringece crystal and tiltedly split insertion to 2, then the thickness of birefringece crystal changes thereupon, causes the time delay of decomposing between two bunch polarization lasers of gained to change; The new two bunch polarization lasers obtained, successively by one piece of zeroth order quarter wave plate 3, form a left-handed and right-handed polarized light, and new light field schematic diagram, time domain then synthesizes the new light field that polarization is special as shown in Figure 3; The polarization of this new light field changed along with the time, and pulse front edge is circular polarization, and arrive two-beam time domain overlapping position, the left-right rotary polarized light that intensity is equal is cancelled out each other and become linear polarization, becomes circular polarization gradually more subsequently; New light field, through the polarizer 4, obtains the laser pulse of the narrow pulse width of a linear polarization.The width of gained linearly polarized laser pulse is relevant with the insertion that birefringece crystal is tiltedly split 2.Split that right pitch angle is 10 ° for quartz material, tiltedly, the change of insertion from the birefringece crystal of 0.1 to 1 mm tiltedly split to the pulse width variation amount of, gained linearly polarized laser pulse will between 1 to 10 fs consecutive variations.Laser pulse centre wavelength is now still at 800 nm places, but pulse width is less than the width of primary laser pulse.New laser pulse is focused on by condenser lens 5.BBO frequency-doubling crystal 6 is positioned at the front end of laser spot, is produced the light source of 400 nm by the laser pulse of BBO frequency-doubling crystal 6.Then optical frequency mixing is utilized to produce terahertz emission in atmosphere.Because the pulse width of linearly polarized laser pulse can continuously change, the centre frequency of the THz wave therefore produced and spectrum width also can be continuously adjusted.In whole experimentation, the parameter of regulation and control is that right insertion tiltedly split by birefringece crystal.
Claims (6)
1. one kind regulates the method for THz wave centre frequency and spectrum width continuously, it is characterized in that, the primary laser pulse that femto-second laser exports, it is right tiltedly to be split by moveable birefringece crystal, primary laser pulse is divided into the two bunch polarization lasers that polarization is mutual vertically, have time delay, obtains initial terahertz radiation successively after zeroth order quarter wave plate, the polarizer, condenser lens, frequency-doubling crystal; Birefringece crystal is tiltedly split two pieces of right crystal and moves identical distance round about simultaneously, decompose and obtain the two bunch polarization lasers different from front time delay;
The new two bunch polarization lasers obtaining time delays pass through one piece of zeroth order quarter wave plate successively, form a left-handed and right-handed polarized light, time domain then synthesizes new light field; New light field is through the polarizer, obtain the laser pulse of the new pulse width of a linear polarization, new laser pulse is focused on by condenser lens, again by being positioned at the frequency-doubling crystal of condenser lens focused spot front end, produce plasma, form the new terahertz radiation that spectrum width is different from initial terahertz radiation, finally enter terahertz detection system.
2. regulate the method for THz wave centre frequency and spectrum width according to claim 1 continuously, it is characterized in that, described birefringece crystal is tiltedly split can be obtained by various single axial birefringence crystal preparation, the polarization direction angle at 45 ° to optical axis and primary laser pulse tiltedly split by birefringece crystal, and the two bunch polarization laser intensity that primary laser pulse is tiltedly split being divided into by birefringece crystal are equal.
3. regulate the method for THz wave centre frequency and spectrum width according to claim 2 continuously, it is characterized in that, the optional calcite crystal of described single axial birefringence crystal, quartz crystal or ruby crystal.
4. regulate the method for THz wave centre frequency and spectrum width according to claim 1 continuously, it is characterized in that, the optical axis of described zeroth order quarter wave plate is consistent with the polarization direction of primary laser pulse.
5. regulate the method for THz wave centre frequency and spectrum width according to claim 1 continuously, it is characterized in that, described polarizer polarization direction is consistent with the polarization direction of primary laser pulse.
6. regulate the method for THz wave centre frequency and spectrum width according to claim 1 continuously, it is characterized in that, described frequency-doubling crystal optional bbo crystal, lbo crystal or ktp crystal.
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| CN103411903B (en) * | 2013-07-29 | 2015-04-29 | 南开大学 | THz radiation reinforcing method through light line array |
| CN104375353B (en) * | 2014-10-29 | 2017-10-24 | 汕头大学 | The generation device and method of a kind of soliton of regulable center frequency |
| CN104577647A (en) * | 2015-01-09 | 2015-04-29 | 中国科学院上海光学精密机械研究所 | Elliptic polarization modulation method of terahertz waves |
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| CN106802185B (en) * | 2017-03-14 | 2018-09-28 | 深圳市鹏星光电科技有限公司 | A kind of narrow linewidth Terahertz light source and spectrometer, imager that frequency is continuously adjustable |
| CN107017542A (en) * | 2017-05-15 | 2017-08-04 | 华东师范大学 | A kind of device that Terahertz is produced based on femtosecond circular polarization two-color laser field in the same direction |
| CN109068042B (en) * | 2018-09-25 | 2020-09-25 | 中国科学院光电技术研究所 | Automatic focusing method suitable for light field camera |
| CN111076814A (en) * | 2019-12-13 | 2020-04-28 | 首都师范大学 | System and method for detecting terahertz waves by using liquid water based on fluorescence radiation |
| CN113437620A (en) * | 2021-06-25 | 2021-09-24 | 重庆邮电大学 | Terahertz wave frequency adjusting system based on BBO crystal and pump pulse energy |
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| JP5240858B2 (en) * | 2009-09-03 | 2013-07-17 | 独立行政法人理化学研究所 | Monochromatic tunable terahertz wave generation / detection system and method |
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| CN1034814A (en) * | 1987-12-11 | 1989-08-16 | 北京大学 | Super wideband and tunable light beam polarization plane rotator |
| EP1715377A1 (en) * | 2004-01-29 | 2006-10-25 | Zaidan Hojin Handotai Kenkyu Shinkokai | Electromagnetic wave generating device |
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