ENERGY TECHNOLOGY

We have developed a laser system that produces tunable, coherent A tunable beams of radiation covering the electromagnetic spectrnmfrom below 200 nm in the ultraviolet to above 1000 nm in the infrared. This laser system system uses a tunable dye laser in the visible region and converts the for the laser light to shorter and longer wavelengths by stimulated Raman scattering in molecular hydrogen or deuterium gas. For example, in ultraviolet, the ultraviolet region, we have generated wavelengths as short as 171.3 nm. Tunable ultraviolet beams can be applied to atomic and visible, molecular spectroscopy as well as to isotopic and chemical analyses. and infrared • regions Tunable dye lasers in the visible wavelengths as short as 185 nm region have opened up large areas have been produced in this man­ of scientific study and, in particular, ner, they have been generated ef­ have become important analytical ficiently only at wavelengths longer tools. 1 However, in the ultraviolet than 220 nm. As a result of our re­ (uv) region below 220 nm, con­ cent work, there is now a new, venient, continuously tunable, satisfactory method of generating coherent light sources have been continuously tunable light from the sought for many years with little infrared through the visible to the success. Although there have been ultraviolet to wavelengths below For further information contact many recent advances in tunable 220nm. Jeffrey Paisner (422-6211) or uv and vacuum ultraviolet (vuv) Our simple and versatile method Steven Hargrove (422-6178). generation below 220 nm, the is based on the so-called anti­ methods used generally require un­ Stokes stimulated Raman scatter­ usual materials or techniques, and ing (SRS) in molecular hydrogen the wavelength coverage is gas. We have successfully and ef­ generally very limited. ficiently generated continuously Continuously tunable uv light tunable wavelengths as short as can be generated by frequency­ 171.3 nm. To our knowledge, this doubling the output of a con­ is the shortest wavelength ever tinuously tunable dye laser produced by stimulated Raman operating in the visible region. This scattering. We believe that this frequency-doubling is accom­ technique can be further extended plished through the use of a non­ to wavelengths as short as 130 nm. linear, birefringent crystal. Although Stimulated Raman scattering Stimulated Raman scattering was extenSively studied in the earliest days of quantum elec­ tronics. In fact, the high-order Raman processes we use were first observed over a decade ago.

1 The physics of the Raman Raman process is said to be "first anti-Stokes radiation," process is illustrated in the energy "Stokes-shifted," and this particular denoted A51. The energetics of this level scheme depicted in Fig. 1a. output is called "first Stokes radia­ process are depicted in Fig. 1 b. The pump laser pulse at frequency tion," denoted 51. Raman generation of A51 is vpis incident upon a molecule in an As in conventional lasing, first sometimes called parametric four­ initial state, i, and can promote it to Stokes emission has a flux-density wave mixing because four waves a virtual state, indicated by the (or intensity) threshold. Below this (two at v p' one at v 51, and one at dashed line in Fig. 1a. If enough threshold, no 51 radiation appears, v A51) participate in the process. Un­ transitions are induced, a popula­ but once the pump laser flux den­ like first Stokes generation, this tion inversion can result in the gas sity exceeds the threshold, 51 grows process has no threshold once 51 between the virtual level and a final exponentially in intensity as the radiation appears. Higher-order level,! In a molecule such as pump intensity increases. As a anti-Stokes frequencies (v p + n v R, hydrogen, the initial state is the result, two intense waves (the for n = 2,3,4, ... ) are produced ground -state vibrational level, and pump and the 51) propagate in in an equivalent way as depicted in the final state is the first vibrational phase through the molecular gas. Fig. 1c. Parametric generation of level. Stimulated emission occurs at Since these two waves are in phase, higher-order Stokes frequencies a longer wavelength corresponding they harmonically beat against one also occurs. to the frequency v p - (v j- Vi) = another and produce an electronic V p - v R. For hydrogen, the Raman polarization in the gas at the Anti-Stokes scattering frequency, v R, is simply its - Raman frequency, v R. This in­ in molecular hydrogen fundamental vibrational frequency duced polarization induces a high­ Molecular hydrogen is an ex­ (v R =4155 cm-I, and 1 cm-1 = frequency sideband on the light. cellent gas for Raman scattering 30 GHz). The fraction of the pump This conversion of light to a higher because it has a very large Raman laser light that is converted in this frequency (v p + v R) is called the cross section. In addition, it has

(a) (b) (e)

v p

t----'-- f f f

i ~~------~--- i that v R v - Vi' (b) Generation offlrst anti-Stokes radia­ lFfi®o n Simplified energy-level diagram depicting = f (§) the energetics of the stimulated Raman tion,AS1,atjrequencyvASl = vp + vnbyfour-wave scattering phenomena. (a) Stimulated Stokes scattering in parametric mixing. (c) Parametric generation of the nth which a pump photon atfrequency vp is absorbed and a anti-Stokes Raman radiation (where n = 1,2,3, .. .). Here, Stokes photon atjrequency vS1 = vp - vR is emitted. Note ASo is just the pump laser.

2 ENERGY TECHNOLOGY widely spaced vibration levels the medium. Optical dispersion in Fig. 2. Starting with the fourth (IIR = 4155 em-I) so that large fre ­ would destroy this condition, harmonic of a neodymium­ quency shifts can be obtained using because the mixing waves would yttrium-aluminum-garnet the SRS process. Isotopic substitu­ propagate through the gas at dif­ (Nd-YAG) laser at 266nm we tion of deuterium for hydrogen ex­ ferent velocities. The larger the op­ have generated quite easily at least tends the wavelength coverage to tical dispersion, the shorter the dis­ five anti-Stokes and five Stokes different ranges because the tance over which the waves can in­ components under modest pump­ Raman shift in deuterium is II R = teract and produce the high-order ing conditions. In this case, the AS4 2991 em-I. Furthermore, hydrogen Raman processes. Ultimately, the wave occurs at 184.4 nm and the can withstand high optical fields number of short-wavelength AS ASs wave at 171.3 nm. Although before it breaks down. This allows sideband -component frequencies the AS6 occurs at 159.9 nm, it was the propagation of waves at those generated is limited by the optical not observed because it was totally intensities needed to produce the dispersion. Fortunately, the conver­ absorbed by the suprasil dispersing high-order, nonlinear Raman sion efficiency to the Raman­ prism used in these studies. All the processes we have discussed. generated waves increases beams emerge collinearly from the Perhaps the most important dramatically with frequency and Raman cell, and a simple prism was property of molecular hydrogen tends to offset this effect in used to disperse the separate com­ gas, however, is its very low optical molecular hydrogen. ponent waves. dispersion. We recall from the dis­ Focusable power is essential so cussion in the previous section that Generation of anti-Stokes that the threshold for Stokes high-order Raman processes (Sn ultraviolet radiation generation can be exceeded and and ASn , for n = 1,2,3, ... ) occur Fixed wavelengths. We have the efficient generation of shorter because the waves at two different exploited the SRS method using wavelengths can be achieved. In frequencies travel in phase through the experimental setup illustrated the experiment depicted, the

IFfj® ~ Schematic of the anti-Stokes Raman generator. The H 2 (§)O pressure in the 1-m Raman cell was typically 450 kPa. The 1-cm 2 input beam at 266 nm was focused by a 50-cm lens to a spot of about 15 J.tm in diameter.

Pump radiation, 266 nm Anti-Stokes Raman generator cell

Dispersing prism

3 intensity at the focus of the pump orders, for example, about 1 % to 52 (1048 nm); and with 280-nm radiation was about 1012 W/cm2. energy or about 0.7% photon con­ pump radiation we have observed We found that thresholds for 51 version in the A54 wave. With 15 f-LJ A54 (191.1 nm) to 56 (927.9 nm). generation and the associated of pump energy at 266 nm, we The generation of anti-Stokes parametric anti -Stokes processes have measured 150 f-LJ in the A54 waves at even higher orders below 200 nm were less than 2 mJ at 184.4 nm and 5 f-LJ in the A5s at probably occurs, although they (in a 5-ns pulse) under the condi­ 171.3 nm. could not be observed in this par­ tions shown in Fig. 2. This laser Tunable anti-Stokes mdiation. ticular experiment because of the power condition is easily met by For a tunable uv laser, we must use particular output window of the commercially available lasers, both a tunable pump laser in the visible Raman cell. In another test using tunable and fixed-frequency. region. With a commercially 65-mJ pulses (~5 ns long) from We have observed pump-wave available dye laser pumped by an the dye laser at 560 nm, we energy depletions of greater than Nd-YAG laser, we have obtained measured more than 25 f-LJ at A5s 75%. The energy partitioning results similar to those just (195.7 nm). favors the lower-order Stokes and described. Using a 560-nm pump The linewidth of the uv radiation anti-Stokes waves, buLan adequate at fluxes of 20 mJ in a 5-ns pulse, is another important parameter efficiency is achieved in the higher we have observed A5s (195.7 nm) that influences the usefulness of

n AS range Second harmonic range n n n Sn range _ Fundamental range SECOND HARMONIC Rh 6G Hydrogen 9876 5 4 3 2 0 2 3 4

Deuterium 987654 2 0 2 3 4 5

FUNDAMENTAL Rh 6G Hydrogen 9 8 7 6 5 4 3 2 o

Deuterium 987 6 5 4 3 2 o

130 200 280 360 440 520 600 Wavelength, A - nm Wavelength coverage possible between 130 and 600 nm. using a Raman-shifted rhodamine 6G dye laser_ We IFfi@oSS have observed ASs to S:2 using the fundamental and ASs to S 6 using the second harmonic of the rhodamine 6G dye laser_

4 ENERGY TECHNOLOGY this radiation. In general the In one example where we energy into the gas. The resulting linewidth of the Raman radiation desired large spectral coverage in pressure wave induces a signal in a depends both on the linewidth of the uv, we used the AS7 compo­ small microphone transducer in the the pump and on the details of the nent of the output of a rhodamine cell. Figure 4 is a trace of the mi­ stimulated Raman scattering. If the 6G dye laser to excite molecular crophone signal versus the scanned Raman processes are purely nitric-oxide vapor in a wavelength. This spectrum parametric as mentioned above, photoacoustic cell. When the AS7 covers a portion of the rotational the spectral linewidth, .:1 v , of the radiation is tuned to resonance spectrum of the ,),(1,0) band in anti-Stokes (Stokes) emissions is with a nitric-oxide transition, the ex­ nitric oxide. It is a spectral region simply that of the pump. Indeed, cited molecule deposits some of its previously inaccessible using other we have observed that the linewidth of ASs at 195.7 nm tracks the 0.5-cm-1 linewidth at the 560- (a) nm pump wavelength. As a result, ,,(2,0) optical resolutions, .:1V'A, of 10-5 to 10-6 even below 220 nm are routinely obtained. fc Other important characteristics ....,s:! e- of the uv beams are their polariza­ o .0'" tions and divergences. Because the « Raman scattering is purely vibrational [Q(l) model, the scat­ tered radiation is linearly polarized 225.0 220.0 215.0 210.0 205.0 200.0 ~Wavelength, A- nm with the pump radiation. Circular polarization of the pump is to be avoided, since it would induce ,..----_____J l ______rotational Raman components ( 1 [S(l) and S(O) modes] that would I I I I I reduce the conversion efficiency to (b) the vibrational Raman lines. The output beams are close to '"c being diffraction-limited. We at­ . ~ '"(.) tribute this to the diffraction-limited .~ ::l pump and the relatively low o pressures «790 kPa) at which we (.) ~ ~ ...,'"o ~ V operate the hydrogen cell. o ~ " ~ ~ j j" ~ 1 1 ~I ~~0 214.6 214.5 214.4 214.3 214.2 214.1 214.0 Full-spectrum coverage ~Wavelength, A-nm With the output powers available IFfi@o 41 Nitric-oxide absorption spectrum taken around 215 nm. (a) The from commercial dye lasers, spec­ C§) upper trace is an absorption spectrum taken at low resolution. tral coverage in the uv range can be The three major bands correspond to transitions between the v" = 0 vibrational achieved to below 175 nm using level of the ground electranic state, X 2n ;, ,and the Vi = 0,1,2 vibrational levels of SRS with only a few dyes. In fact, as the exdted electranic state, 2~+. (b) The lower trace is a portion ofthe resolved illustrated in Fig. 3, this tuning rotational spectrum of v' = +- v" = 0 taken photoacousticolly with the anti-Stokes stimulated Raman source described in the text and illustrated in fig. 2. range can be almost entirely spanned by using a rhodamine 6G dye laser alone if we include also the frequency-doubled output from such a laser.

5 laser methods. With our laser run­ Key words: anti·Stokes radiation; coherent radiation; infrared radiation; laser systems; ning at 10 pulses per second, this stimulated Raman scattering; Stokes radiation; spectrum was recorded in only 5 uv radiation; visible radiation. minutes.

Potential applications Notes and references 1. See, for example, "High-Resolution Dual­ The extremely versatile proper­ Laser Flow Cytometry," in the December ties of this laser system make it ap­ 1978 Energy and Technology Review plicable to the evaluation of ad­ (UCRL-S2000-78-12), p. 27, and "Laser Excitation and Probing: New Directions in vanced isotope separation schemes Photochemistry," in the January 1979 and technology and to Energy and Technology Review (UCRL- photochemical studies. In addition, 52000-79-1), p. 24. it is suitable for quantum elec­ tronics applications. The excellent spectral and spatial properties of the scattered radiation make anti­ Stokes generation an ideal signal source for injection-locking short­ wavelength excimer lasers. Further­ more, since SRS gain increases with the pump frequency, the parametric processes should have progressively lower thresholds at shorter wavelengths in spite of in­ creased dispersion and potential breakdown problems. The ultimate limit may be the absorption edge of H2 «130 nm). One possibility for achieving tunable output to 130 nm may be by using an ArF­ based system. ASs of the ArF out­ put at 193.3 nm would occur at 137.9 nm, while AS6 would occur at 130.4 nm. We plan to explore these intriguing possibilities in the near future.

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