<title>Pulsed Far Infrared Lasers</title>

Abstract

AbstractThe use of optical pumping as a means of producing intense far infrared and submillimeterradiation is discussed. Characteristics, configurations, and limitations of these systemsare reviewed.IntroductionThe generation of intense far infrared (FIR) and submillimeter radiation via laser actionpresents a somewhat difficult and challenging problem from the standpoint of conventionallaser techniques. The usual successful high powered laser is characterized by two distinctfeatures; a) relatively long upper lasing state lifetimes with correspondingly good energystorage, and b) the ability to be excited with a broadband pump which implies the presenceof an energy funneling mechanism leading to excitation of the narrow laser level. Twoexamples will illustrate these points further. Nd +3 in glass has a lifetime of about amsec and is excited by an incoherent broadband optical source. There are a number of broadabsorption bands a few hundred angstroms wide which would result in the excitation of stateswhich nonradiatively cascade into the upper laser level thus providing a funneling action.The second example is high pressure electrically excited CO2 which has a lifetime of a fewpsec at one atmosphere. Here the broadband pump is the electrons with a spread of a few eV.Excitation is accomplished by first exciting N2 into its lower vibrational levels and thentransferring the internal molecular energy to CO2, a process which happens to favor theupper laser level predominately.For the specific case of the FIR, one deals predominately with single rotational statesof molecules which typically have lifetimes in the range of 10 nsec at 1 Torr pressure.Hence because of the short lifetime there is generally little or no energy storage and thusthe usual technique of Q- switching is of limited use. Furthermore, the energy separationbetween upper and lower laser levels is comparable to or smaller than kT which generallyprecludes any useful funneling action by energy transfer or cascade excitation.