from nanosecond to femtosecond science

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1、FromnanosecondtofemtosecondscienceN.BloembergenPierceHall,HarvardUniversity,Cambridge,Massachusetts02138Lasershavecausedrevolutionarychangesinmany®eldsofscienceandtechnology.Abriefreviewispresentedhereofhowlasershaveprovidedaccesstomeasurementsofshorttimeintervals.Since1960,sixordersofm

2、agnitude,from1029to10215seconds,havebeenaddedtotime-resolvedobservationoffastphenomena.Newsub®eldsofscience,includingfemtochemistryandfemtobiology,havebeencreated.SomerepresentativeexamplesoftransientRamanscatteringandofimpulsiveanddisplaciveexcitationsinmoleculesandcrystalsillustrateth

3、eusefulnessofpicosecondandfemtosecondpulsetechniques.Inaddition,thedomainofpower¯uxdensitieshasbeenextendedfrom1012to1019watts/cm2bytheuseofshortfocusedpulses.Thishasgivenexperimentalaccesstonewphenomena,includingultrafastphasetransitionsinelectronicstructure,above-thresholdionizationof

4、atoms,andhigh-orderharmonicgenerationandaccelerationofrelativisticelectronsbylightpulses.[S0034-6861(99)01002-8]I.HISTORICALINTRODUCTIONdensitiesexceeding1018watts/cm2.Someultrahigh-®eldphenomenathatarecurrentlyunderintenseinvestiga-Themeasurementofshorttimeintervalsduringthetioninclude

5、ultrafastphasetransitionsinelectronic®rsthalfofthiscenturywaslimitedtointervalslongerstructureandthecreationofhigh-density,high-thanonenanosecond.Before1900itwasalreadyknowntemperatureplasmas.Otherphenomenaincludeabove-thatelectricalsparksandKerr-cellshutterscouldhavethresholdionization

6、ofatoms,high-orderharmonicgen-responsetimesasshortas1028s.AbrahamandLem-eration,andtheaccelerationofrelativisticelectrons.oine(1899)usedadischargefromcondenserplatesThevastnessofthenewordersofmagnitudeintimewhichsimultaneouslyactivatedasparkgapandaKerr-andinradiativedensitythathavebeeno

7、peneduptocellshutter.Thecondenserplateswereplacedinacellexperimentalinvestigationbyshortlightpulsesmaybecontainingcarbondisul®debetweencrossedpolarizerillustratedbythefollowingtwoobservations:Theandanalyzer.Thelightpathbetweenthesparksourceminuteisthegeometricmeanbetweenthelife