Диссертация (1102749), страница 6
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In contrast to the caseverse field, the angular radiation pattern and thecharacteristics of the terahertz wave remain thePHYSICSLETTERS 88, 261103 !2006"se without the external APPLIEDelectric field.We extendn-Cherenkov model by taking into account therent in filament driven by the external voltage.emission profile from laser-induced air plasmareproduces all the observed features of this amertz emission.hong,Nick Karpowicz, and X.-C. Zhanga!or Terahertz Research, Rensselaer Polytechnic Institute, Troy, New York 12180ved5 March 2006; accepted 7 May 2006; published online 27 June 2006"il: yi.liu@ensta.fr.hom correspondence should be addressed.
Electronic mail:ort the characterization of the emission profile ofwicz@ensta.fr.terahertzFIG.wavesgeneratedfour-wave1. !Coloronline" bySchematicsetup of the experiment.Рис. 1.6 СхемаТГц излученияпри филаментацииизлучения на основнойin the presence of laser-inducedplasma.генерацииHighly directionalterahertz waveswith anceanglesmallerthan10°at:aremeasured титан-сапфировогоin thepresence of longлазераplasmasв присутствиипродольного!!1093,051108-1©2008mm".AmericanInstitute of Physicsg08/93"5!/051108/3/$23.00contentis subjectto thetermshttps://publishing.aip.org/authors/rights-and-permissions.Downloadto IP: 93.180.54.156On: Tue,26 Julчастотевнешнегоncy-dependent interference structures in the angulardistribution of the radiation are observed2016 12:07:35ighter focusing conditions and are explained by intenseself-phase modulation ofполяthe opticalэлектростатическогоиз работы [68]n the plasma.
This study reveals that terahertz generation from four-wave mixing in airs is a promising source for spectroscopy and imaging in the terahertz range. © 2006an Institute of Physics. #DOI: 10.1063/1.2216025$§3. Характеристики ТГц излучения при коллинеарном распространении импульсовосновной и второй гармоник титан-сапфирового лазера в филаменте!CCD" images of the plasmas formed with the above focalwave emission from laser-induced plasmasconditions.
The existence of plasma has been proved by obor the first time in 1993.1,2 The emission,serving inducted voltage in the cable through lock-in amplithe propagation of the optical pulse, is exfierвторойwhen placingthe inner coreof a coaxial cable beneatheration of electrons and ions drivenby ponИспользованиегармоникититан-сапфировойлазерной системы дляthe plasma.9 Figure 1!c" illustrates the geometry that giveses. Radiation in the forward direction is notгенерацииТГцизлучения1.7) прифиламентациипредложена в работе [69].rise to (рис.the far-fieldradiationpattern and willбылаbe discussedn be steered from the sideby applyinga static3,4laterinthisletter.rpendicular to the plasma. Recently, effiТакаясхема,где ТГц Weизлучениегенерируетсяприраспространенииhave observedterahertz radiationbothколлинеарномin the forwardradiation in the forwarddirectionhas beenand sideways directions.
The forward direction is more thanfocusing a fundamentallaser beam основной!"" andимпульсови второй гармоник лазера в филаменте была названа двуцветной.5–7three orders stronger than that of the sideways radiation !theonic !2"" beam into the air. Studies showcontributionof ponderomotive force"within a comparableerahertz radiation is accompaniedby генерировалсяthe onТГц сигналпри четырехволновомсмешенииизлучения титан-сапфировогоsolidangle.Theforwardradiationprofileis acquired byr !plasma". The generation mechanism can bescanningacross гармоникойthe beam.
At в газах. Генерациялазерафс, 8001 кГц,a 2-mm-thick150 мкДж)aluminumс его slitвторой2" beamsin нм,ur-wave mixing amongthe ", (65each point of the angular distribution, the entire temporalгармоникикристаллеBBO Isquared,типа. andДетектированиеТГц излученияterahertz field вwaveform is recorded,integratedwaves radiated from второйlaser-inducedplasmas происходилаtogivetheenergyoftheterahertzpulse.Thecomaaberrationwave mixing process providean intenseсteraпроводилосьпомощьюZnTe.sinceПоказано,что beamиспользованиев экспериментеof theкристаллаlens is negligiblethe measuredsize issensing, spectroscopy, and imaging applicamuch smaller than the aperture size of the hyperbolic lensthat purpose, an understandingof the spatial нарядувторой гармоникис первой приводит к росту интенсивности ТГц сигнала на 3and the beam is well centered.10 It is noteworthy that thehe terahertz radiation is crucial.
In this letter,profileby slit scanningis not the true linearcrossчто acquiredТГц сигналтакже генерируетсяв кристаллеBBO вместе соprofile measurementsпорядка.of terahertzОбнаружено,waves rasectionimageoftheprofile,buttheintegratedintensityof a-wave mixing in the air with the presence ofplanar slice of the beam normal to the direction of travel ofвторойгармоникой,lasma, at various plasmalengths.As the однако сигнал BBO мал по сравнению с ТГц сигналом филамента.the slit. However, since the slit size is much smaller than thea increases, the terahertz radiation becomes.
By confining the effective radiation sourcelong the plasma, we confirm that this direcciated with plasma length.mental setup is schematically illustrated inlaser used is a Ti:sapphire laser !Spectrane" with 700 mW output power, 795 nm cen1 kHz repetition rate, and 130 fs pulse durad harmonic beam !2"" is generated in a 100Рис. crystal.1.7 Схемагенерации ТГц излучения при коллинеарном распространении импульсовI beta barium borate !BBO"Both thems are focused in the ambient air and theи второй гармоник титан-сапфирового лазера в филаменте из работы [70]z waves are collected by aосновной190 mm diameterethylene lens with a 66.3 mm focal length inПервоенаправленности ТГц сигнала двуцветного филамента былоA 1-mm-thick ZnTe crystal isused asисследованиеanector to sense the collected terahertz radia1.
!a"Schematicdiagram использовалосьof the experimental setup.L1: optical излучение на длине[70] FIG.(рис.1.8).В работелазерноеer of the input opticalпредставленоpump beam is 5.6вmmlens; B: BBO; S: slit; L2: plastic lens; P: pellicle; D: balanced detector.The lenses that focus the optical beam to!b" CCD imagesимпульсаof plasmas takenat differentfocusing повторенияconditions.волны 795 нм, с длительностью130 фси частотой1 кГц. Выходнаяmas have focal lengths of 50, 200, 300, and!c" Illustration of the simulated terahertz radiation generation concept.The terahertz field measured at point !L + x0 , y 0" on the observing plane isasmas are all opticallyмощностьvisible throughwhiteсоставляла700 мВт. Детектирование ТГц излучения проводилось в кристаллеcontributed from each emission source along the plasma: EL+x0,y0n; Fig.
1!b" shows charge-coupled device22Langxc@rpi.edu= %0 !A!x" / � + !L − x"$2 + y 0 "exp#−i!k� + !L − x"$2 + y 0 − "!x / c""$dx,where L is the length of the array and x0 is the distance between the end ofthe array and the plane.8"26!/261103/3/$23.0088, 261103-1© 2006 American Institute of Physics1 to 193.232.121.160. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions21ZnTe.
Для получения углового распределения ТГц излучения поперечное сечение пучкасканировалось с использованием толстой алюминиевой пластинки с щелью шириной 2 ммщели поперек пучка. В каждой точке углового распределения временная форма ТГц полярегистрировалась, возводилась в квадрат и интегрировалась для получения энергии ТГцимпульса. Обнаружено, что с ТГц сигнал имеет кольцевой пространственный профиль и сувеличением длины филамента (или фокусного расстояния линзы, фокусирующейлазерное излучение) становится более направленным.
Аналогичная динамика былаобнаружена в экспериментах [71], где лазерное излучение титан-сапфировой лазернойсистемы (на длине волны 800 нм, энергией 2.2 мДж, диаметром пучка 10 мм, частотойповторения 1 кГц) и его вторая гармоника фокусировались в воздух с помощью линз сразличными фокусными расстояниями (10, 20, 80, 100 см). Регистрация ТГц сигналапроводилась с помощью пироэлектрического детектора. В [70] показано также, что ТГцсигнал, распространяющийся вперед, на три порядка больше того, что распространяется внаправлении, перпендикулярном филаменту.261103-3Zhong, Karpowicz, and ZhangFIG.
4. Profile of !a" measured and !b" simulated frequency componentsAppl. Phys. Lett. 88, 261103 !2006assuming the intensity of &1 & 1015 W / cm2.15–18 Thmechanism of enhanced $!3" in four-wave mixing in plasmhas been observed and studied, but theoretical explanation athis particular four-wave mixing geometry is not availablyet.4–8,19–22 The simulated profile reproduces the frequencydependent structure on the top of the beam, but is widerAn exact description of the formation of the interferencstructure would include the spatial !and spectral" distributionof the self-phase modulation, which is not known at thimoment.In conclusion, we report the characterization of the terahertz radiation profile from four-wave mixing in ambient aiin the presence of laser-induced plasma. We showed thaterahertz wave radiates along the plasma, and the directionaradiation is associated with the plasma length.
The interference structures of terahertz beam profile at shorter plasmlengths are explained by the phase walk off between thterahertz and optical radiation propagating inside the focusThe highly directional terahertz wave radiated from longplasmas may provide an effective source for terahertz frequency imaging and spectroscopy applications at standofdistance.when usingдвуцветногоa lens of 50 mm focal length филаментаwith consideration of the selfРис. 1.8 Направленность ТГц сигналав экспериментах [70]phase modulation of the optical pulse.peated the same experiment with a 350 !m pinhole and ob-Двумерная картина пространственногораспределенияТГц сигнала была впервыеserved the same trendof terahertz pulse energy change.To understand the cause of the structures with the shorterplasmas, we examined the frequency dependence of the profiles. Figure 4!a" shows the terahertz beam profile generatedwith a lens of 50 mm focal length at 1, 1.5, and 2 THz,respectively.
Specifically, a dip appears at the center of thebeam at 1.5 THz and is wider at 2 THz. The structures areless prominent at lower input pulse energy. We speculate thatthis phenomenon is mainly due to phase walk off of theterahertz radiation inside the plasma, which is most likelycaused by the self-phase modulation of the optical beam inside the plasma.14 With longer plasmas, the self-phase modulation is weak and cannot be resolved.From a separate study which was conducted in a similarmanner as described in Ref. 13, we measured that the densities of plasmas created by using lenses with focal lengths of50 and 200 mm are in the range of 1015 cm−3. At these conditions, the threshold for self-guided pulse propagation mayhave been reached.15–18 However, it is reasonable to believethat the self-phase modulation dominates before the Kerr effect is balanced by the dispersion properties of the plasma,and thus causes a positive refractive index change"nKerr = !1 / n02c#0"$!3"I along the focus, where n0 is the refractive index of air.14 This imposes a phase delay betweenthe terahertz and optical pulses, since the terahertz pulse isnot as confined in time or space as the optical pulse.
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