COHERENT VUV GENERATION BY NONLINEAR PROCESSES

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COHERENT VUV GENERATION BY NONLINEAR

PROCESSES

J. Wynne

To cite this version:

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JOURNAL DE PHYSIQUE Colloque C4, supplkment au no 7, Tome 39, Juillet 1978, page C4-69

COHERENT VUV GENERATION

BY

NONLINEAR PROCESSES

J. J. WYNNE

IBM Thomas J. Watson Research Center,

P.O. Box 218, Yorktown Heights, New York 10598, U.S.A.

Rksumk. - Le melange des ondes optiques dans les vapeurs a et6 utilid pour produire des rayonne- ments coherents, intenses et de faible largeur de raie dans le VUV. Les vapeurs atomiques et mole- culaires ont permis de creer des longueurs d'onde aussi courtes que 38 nm et accordables d'une faqon continue dans la region entre 130-200 nm. Une revue des differentes mkthodes non lineaires de rayonnement VUV sera presentee. Le rBle de la resonance, qui permet d'accroitre la reponse du milieu non lineaire aux radiations incidentes, sera souligne.

Abstract. - Optical mixing in vapours has been used to produce intense, spectrally narrow, coherent radiation in the VUV. Atomic and molecular vapours have been used to generate light to wavelengths as short as 38 nm and continuously over the region from 130-200 nm. A review of the various methods of nonlinear VUV generation will be presented, with emphasis on the use of resonant enhancement to increase the response of the nonlinear medium to the input radiation.

1. Introduction. - The history of coherent sources

of electromagnetic radiation and their application is characterized by a steady trend toward shorter wavelengths. The frontier for coherent sources is now in the V W region of the spectrum. Coherent VUV sources fall into two groups - lasers and sources generated by nonlinear- optical interactions. The first group is discussed in the companion paper, Far and

Extreme Ultraviolet Lasers by A. V. Vinogradov [l]. This paper shall be concerned with methods of generating VUV by interactions of laser beams in nonlinear media.

Non-tunable laser sources may be used to produce VUV at fixed wavelengths. The non-tunable lasers, such as ruby and Nd : YAG, are powerful enough to produce strong VUV via high order nonlinear pro- cesses without resonant enhancement. The shortest wavelength generated coherently to date, 38 nm [2], corresponds to the 28th harmonic of Nd : YAG. On the other hand, readily available tunable lasers have output powers below that of non-tunable lasers. The limited output power of the tunable lasers neces- sitates resonant enhancement of nonlinear proc'esses to generate VUV of significant power. However, this VUV radiation has the great advantage of being tunable and therefore useful for spectroscopic pur- poses.

Early experiments in coherent UV generation involved Second Harmonic Generation (SHG) in acentric media and Third Harmonic Generation (THG) in crystalline media. However, despite a large effort by workers in the field, the short wavelength

limit for SHG is 217 nm in the crystal KB,O, . 4 H,O [3]. No other acentric crystals have been found with sufficient transparency and birefringence to allow efficient SHG into the VUV. Light has been generated in many crystals by THG. Amongst those crystals tried have been LiF 141, the crystal with the shortest wavelength transmission limit at 104 nm. This cubic crystal has a small nonlinear susceptibility and is not phase-matchable, so that THG is relatively weak. No one has reported using LiF or any other trans- parent crystalline material to generate VUV by THG. Ward and New measured THG in rare gas atomic vapours and in some common diatomic and triatomic molecular gases [5]. Again, the nonlinearities are weak.

2. Resonant enhancement.

-

Harris and co-wor-

kers [6-81 exploited resonant enhancement to increase the efficiency of THG. They used alkali metal atomic vapours with resonance lines near the frequency of the input laser to obtain nonlinear susceptibilities 1O6 greater than that of He [5]. Kung et al. [9] utilized Cd vapour to extend this technique into the VUV, reaching as far as 118.2 nm. However, this VUV radiation was not tunable since the input laser was derived from the 1 060 nm fundamental of a Nd :

YAG laser and its harmonics. Kung, Young and Harris [l01 subsequently used Xe vapour to produce strong VUV at 118.2 nm by THG.

Hodgson, Sorokin and Wynne (HSW) [ l l] utilized resonant enhancement from a two-photon transition in Sr vapour to produce VUV tunable between 200

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C4-70 J. J. WYNNE

and 150 nm, starting with visible dye lasers. They recognized that one could tune very close to a two- photon resonance and have strong resonant enhancement without the deleterious effects of strong linear absorption and dispersion. Wallace and co-workers have used Mg vapour [l21 to tune to 140 nm and have also used molecular vapours [I31 for tunable VUV generation to 130 nm. Using a two-photon transition in Cd vapour, Hodgson has tuned between 120.7 and 121.9 nm, a range which includes the wavelength of Lyman a (121.6 nm).

All of these recent methods use resonant enhancement of the nonlinear response of the nonlinear media, occurring when a laser photon energy, or sum of photon energies is equal to the transition energy from the ground state of the atomic or molecular vapour to an excited state. The excited state must be connected to the ground state by allowed transitions utilizing the correct number of photons. Ward and New [5] looked at atoms with energy levels far in excess of the photon energies, so the nonlinear response of these atoms were not resonantly enhanced. Harris and co-workers [6-101 utilized atomic vapours with much lower energy levels such that visible or near visible photons were much nearer to resonance. Here, the detuning from resonance was sufficient to avoid the associated problems of linear dispersion and linear absorption. Linear dispersion becomes a problem when it causes the various waves to get out of phase. Efficient VUV generation is engendered when the phase velocities of the various waves are nearly or completely matched (i.e., phase matched). The Stanford group [6-101 found that it was possible to correct for linear dispersion by adding a buffer gas with compensating linear dispersion characte- ristics. Linear absorption is a problem which cannot be corrected when tuning too close to singlephoton resonances. Either the input lasers or the generated VUV output will be absorbed and reduce the VUV output.

To avoid these problems and to greatly increase the nonlinearity, HSW [l11 chose to resonantly enhance two-photon transitions by tuning right on resonance. There is just as much resonant enhancement of the nonlinear response of a nonlinear medium from a two-photon resonance as from a one-photon resonance, but there are no associated problems of linear dispersion and linear absorption. Limitations do occur due to two-photon absorption and the resulting saturation of the nonlinearity [12, 14, 151, but these are higher order effects than the compa- rable one-photon effects. The scheme used by HSW is a generalization of THG called parametric sum mixing. Three input waves at frequencies v,, v, and v, are added together to produce light at

The sum v,

+

v, is tuned to be resonant with an appropriate two-photon transition. For convenience,

v, may be set equal to v, (both coming from the same laser) with 2 v, tuned to resonance. Then v,,, tunes in response to v,. The efficiency of parametric sum mixing depends on the nonlinearity of the medium, the strength of the input lasers, phase-matching and absorption of the input or the generated light. Miles and Harris [8] have discussed the interplay of all these factors. Here, the main concern is with the nonlinearity and its resonant enhancement.

3. Tunable VUV generation.

-

The experimental

approach to parametric sum mixing of HSW, using pulsed dye lasers tuned to a two-photon resonance, will now be reviewed in some detail. For the input lasers, one may use nitrogen-laser-pumped dye lasers. These lasers have the advantage of relatively high peak power (> 10 kW), short pulse width (-- 10 ns) to avoid saturation [12, 151, wide tunability (360- 950 nm), low shot-to-shot variation in output power, and good repetition rates. In the configuration shown in figure 1 the beams from two dye lasers with ortho-

DYE GLAN 3371 (OPTIONAL) LASER SOLAR BLIND PHOTOMULTIPLIER LASER

FIG. 1. - Experimental configuration for generating

in Sr. The monochromator may be removed when all other VUV signals are suppressed (see text).

gonal linear polarization are combined into a colinear beam which is focused into a pipe containing the nonlinear medium. The procedure for maximizing the tunable VUV output is first to look for resonant enhancement of the VUV output at v,,, = 3 v,

when a single dye laser (v,) is focused into the nonlinear medium. When the laser is tuned off resonance, weak THG may be observed. As the laser is tuned so that the condition 2 v, = v, (where v, is the frequency of a two-photon transition) is approached, THG gets stronger. On resonance, THG increases by many orders of magnitude. In Sr vapour, HSW observed a THG enhancement of six orders of magnitude when a Rhodamine 6G dye laser was tuned to 575.7 nm so that two photons were resonant with the two-photon transition from the 5s' 'S, ground state to the 5s 5d

'D, excited state at 34 727 cm-'. There are many other even-parity states, connected to the ground state by two-photon transitions, which may be used for resonant enhancement (Fig. 2).

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COHERENT VUV GENERATION BY NONLINEAR PROCESSES C4-7 1

l I I I I I

IONIZATION LIMIT

STRONTIUM

I

FIG. 2.

-

Energy level diagram of Sr showing several even-parity states suitable for two-photon resonant enhancement.

tuned in order to tune . ,,v, Then the wavenumber tuning rate and range of ,,,v is the same as that of v,.

One has, in effect, up-conversion from v, to . ,,v,

The VUV output emerging from the nonlinear medium will, in general, consist of both the THG light at 3 v, and the desired tunable component at 2 v,

+

v,.

It has been shown that frequency tripling in isotropic media is not allowed with circularly polarized light [l 61. In THG, three input photons are destroyed and one frequency tripled photon is created. With circular polarization, three units of angular momentum would be destroyed, and the created photon would at most restore one unit of angular momentum, making it impossible to conserve angular momentum. Thus, if selection rules allow it, the THG may be eliminated while preserving the sum mixing by circularly pola- rizing the light at v,. Starting with linearly polarized light- at v, tuned onto resonance (2 v, = v,) to maxi- mize THG, a ,414 plate is inserted to circularly polarize the beam and null out the THG signal. The only VUV output remaining is that at 2 v,

+

v,. The selection rule just mentioned requires that the two-photon transition from the ground state to the resonant intermediate state be a AM = 2 transition. When the ground state is 'S, as in the alkaline earths, then the resonant intermediate state must be J = 2 and of the

same parity (even) as the ground state. For the case of Sr, HSW used a series of 'D, iulermediate states which,

when coupled with various dyes for v,, covered the tuning range 150 to 200 nm in the VUV.

One may use a VUV monochromator to verify the

VUV wavelength and also to discriminate against the visible dye laser light. But the VUV light is much easier to use for spectroscopy if the monochromator can be eliminated. For the case of Sr, where dye laser wavelengths are longer than 465 nm, one may use a solar-blind photomultiplier to detect the VUV light with no additional filtering.

I " " '

-

-m--T-

Vuuu =2vI +v3 DYE LASERS

A, =5757nm [Rh 661

FIG. 3. - VUV signal generated in Sr using 5s 5d 'D, as resonant intermediate state and scanning the laser (Rhodamine 6G) at A,.

Figure 3 shows the VUV signal as a function of frequency when v, comes from the Rhodamine 6 G laser and v, is set at the half frequency of the 5s 5d 'D, level (34 727 cm-'). The signal is not normalized in any way. The fall-off at the ends of the scan corresponds to the fall-off of laser power (v,) as the laser is scanned over its fluorescent bandwidth. In this tuning range, with laser powers of

-

20 kW focused to

-

100 p diameter within

-

5 torr Sr vapour, conversion efficiencies were 10-6. With pulse durations of

-

10 ns, this corresponds to 10' photonslpulse of

VUV radiation. Since the laser linewidths are

-

0.2 cm-', this represents a spectrally bright source of tunable VUV radiation.

Wallace and Zdasiuk [l 21 have extended the tuning range using this method with Mg vapour. They reported tuning from 140-1 60 nm. Comparing power- conversion efficiencies of Sr and Mg, they found that Mg had a 103 higher conversion efficiency with inci- dent powers of 10 kW. With input laser powers of 30 kW, they saw a power conversion efficiency of 2 X 10-3 at 143.6 nm. Wallace has recently extended

the tuning in Mg to 123 nm. Working with Cd vapour, Hodgson tuned the second harmonic of a Rh B dye laser to the half energy of the 5s2 'S,-5s 6d 'D,

transition at 65 134.9 cm-'. A Rhodarnine 6 G laser

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C4-72 J. J. WYNNE

doubling them, VUV to wavelengths as short as 725 nm should be achievable.

In recently reported work, Innes, Stoicheff and Wallace [l31 observed sum mixing in molecular vapours NO, Br, and CH,. The VUV was tuned from 130-150 nm in NO. At low pressures, various rotational lines resonantly enhanced the VUV output. However, by going to high pressures (10 atm) the rotational substructure was eliminated by pressure broadening. Thus it was possible to use only one laser and see resonantly enhanced THG smoothly tunable over

--

100 cm-' range with conversion efficiencies of 10-7 (corresponding to outputs of

-

107 photons/ pulse). This represents a great simplification in the method of VUV generation. Only one laser is needed and the experimental problems associated with spatial and temporal overlap of two lasers are eliminated.

Using picosecond pulses, Kung [l 71 reported tunable VUV radiation from Xe vapour. The input waves were produced from an ADP parametric generator. Wavelengths as short as 117.3 nm were observed. Interestingly, when this source was tuned to emit at or near Lyman a, no output was observable. The problem is probably related to phase-matching dif- ficulties.

More recently, Hutchinson, Ling and Bradley [l81 have frequency tripled the tunable output of a Xe, laser. The Xe, laser is tunable in a modest range about

171 nm [19], and hence its third harmonic is tunable. Using the 3p6-3p5 5p two-photon transition in Ar for resonant enhancement, Hutchinson et al. [l81 tuned the output signal around 57 nm. This method is just a first step towards the eventual development of a widely tunable, coherent VUV source below 100 nm. In an experimental geometry where the monochromator of figure 1 can be eliminated, the solar blind photomultiplier can look directly at the collimated VUV light. Typically the nonlinear medium is contained in a cell with a VUV transmitting output window, such as LiF. A vacuum-tight sample chamber encloses the space between the output window and the photomultiplier. If the sample is gaseous, it can simply be admitted directly into the chamber. A liquid or solid sample may be placed in the chamber so that the VUV beam passes through it. Figure 4 shows the results of admitting

--

5 torr air into the

--

10 cm long path between the output window and the photomultiplier. The observed absorption bands are due to the Schumann-Runge band system of atmospheric 0,.

This demonstrates the utility of the VUV generation method as a VUV spectrometer.

4. Fixed frequency VUV generation. - In much

of the reported experimental work on VUV generation by THG or parametric sum mixing, the output light was not tuned. This was usually a consequence of the use of fixed frequency lasers. In their original report on THG in alkali metal vapours, Young

et al. [7] frequency tripled the output of a Nd : YAG

X,,,(nm) X, ,, (cm-') FIG. 4. - Absorption of VUV due to Schumann-Runge bands of atmospheric 0,. a) Scan taken with

-

5 torr of air in a

-

10 cm

path length ; 6 ) reference scan with < 0.1 torr air (i.e.,

-

no 0,).

laser in a phase-matched mixture of Rb and Xe. The subsequent extension of this technique into the VUV [9, 101 utilized THG in atomic vapours of light produced by crystals at the second or third harmonics of the Nd : YAG output. The resultant VUV had wavelengths of 177.3 and 118.2 nm respectively. Furthermore, by adding the Nd : YAG output to two photons at the third harmonic of Nd : YAG, they produced VUV at 152 nm. In these experiments, the primary emphasis was on efficient conversion of visible or near visible light to UV light.

Harris [20] discussed the use of higher order processes such as fifth and seventh harmonic generation as a method of extending the short wavelength limit farther into the VUV. These processes have been utilized by Reintjes and co-workers to generate VUV at 53.2 nm [21] and 38.0 nm [2], the fifth and seventh harmonics of 266.1 nm, respectively. The 266.1 nm light was produced by frequency quadrupling the output of a mode-locked Nd : YAG laser, using two stages of frequency doubling in acentric crystals [22]. The fifth harmonic process was observed both in Ne and He. In Ne, the nonlinear response was dominated by resonant enhancement from a near coincidence of the energy sum of four input photons with the tran- sition from the ground state to the 3p [3/2] J = 2 level. This sum was only 12 cm-' away from resonance. Measured conversion efficiencies were

-

10-6 for an input intensity (at 266.1 nm) of 3 X 1014 W/cm2.

While the fifth harmonic output frequency lies in the ionizing continuum of Ne, it lies below the continuum edge for He.

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COHERENT VUV GENERATION BY NONLINEAR PROCESSES C4-73

harmonic output at 38.0 nm lies well into the ionizing continuum of He. The 266.1 nm input was focused to an intensity of 10" W/cm2, and with 150 torr of He, conversion efficiencies of

-

10-8-10-

'

were observed. These experiments are the first reported results on fifth and seventh harmonic generation into the VUV and represent a big step forward in the development of coherent, VUV sources.

The use of ionic media opens up numerous new opportunities for finding systems in which to generate VUV by nonlinear mixing. One system that has already been used is Ca 11. Sorokin et al. 1231, using a single dye laser, produced VUV at 127.8 nm by THG. This process is depicted in figure 5. Two dye laser photons

0'- -4s2

IS 'P ID 3~ 2~ 'P 'D 2~

CO I con

FIG. 5. -Energy level diagrams of Ca I and Ca I1 showing observed T H G process.

photoionized Ca I, creating enough Ca I1 ions to provide sizable third harmonic signals when the laser was tuned to the half energy of the 4s-5s two-photon transition in Ca 11. The third harmonic signal was observed only in a very narrow tuning range

(< 1 cm-') around the 383.3 nm input wavelength. There is no obvious reason to prohibit tuning the VUV output by adding a second laser, although this has not yet been tried.

Recently, Hsu et al. [24] have used Hg vapour to generate light at 120.3 nm by summing light generated by a parametrically amplfied, frequency doubled dye laser and by a parametrically amplified parametric oscillator. The parametric amplifiers and oscillator were pumped by the fourth harmonic of a mode locked Nd : YAG laser. Although the VUV output should be

tunable, the selected wavelength, 120.3 nm, was chosen because it is at one-half the frequency of the 1s 'So-2s 'S, two-photon transition in He. Output peak power at 120.3 nm was measured to be 300 W.

These examples point out the multitude of systems which may be used for parametric sum mixing to generate coherent, VUV radiation.

5. Conclusion. - This paper has discussed ways of adding together the frequencies of available lasers in the visible and near UV to generate coherent light in the VUV. Obvious applications of these VUV sources included spectroscopy and atomic and molecular resonance fluorescence detection. For example, resonance fluorescence using pulsed Lyman a radiation should prove very useful for measuring the den- sity of H atoms as a function of time. With a spectrally tunable Lyman a source, it should be possible to measure the velocity of H atoms using the Doppler shift. Possible studies include photodissociating systems, molecular and atomic beam systems, and plasmas.

While extensive spectroscopic studies using these sources has yet to be done, there are several cases where the VUV generation process itself was used as a spectroscopic tool. Armstrong and Wynne [25] studied the resonant enhancement of VUV generation in Sr when the VUV output frequency corresponded to a transition from the ground state to an autoionizing level of Sr. Their results show that VUV generation can be used to study the properties of autoionizing levels. More recently, Royt and Lee [26] have used this method to study an autoionizing level in Sr with large enough oscillator strength to the ground state to show significant absorption of the generated VUV light. They have also done time domain spectroscopy by introducing a variable time delay between the two input lasers. They used modelocked dye lasers with

-

50 ps long pulses and observed that the two photon coherence of Sr decayed in

-

100 ps.

These applications, as well as straight absorption and ionization spectroscopic studied, should become more widespread as the techniques for coherent, VUV generation become more widely available.

Acknowledgments. - I wish to thank Dr. R. T. Hod-

gson for helpful comments and a careful reading of the manuscript. The U.S. Army Research Office supported this work.

References

[l] VINOGRADOV, A. V., J. Physique Colloq. 39 (1978) C4-61. 151 WARD, J. F. and NEW, G. H. C., Phys. Rev. 185 (1969) 57. [2] REINTJES. J., SHE, C. Y., ECKARDT, R. C.. KARANGELEN, N. E..

ANDREWS, R. A. and ELTON, R. C., Appl. Phys. Lett. 30 [6] HARRIS, S. E. and MILES, R. B., Appl. Phys. let^. 19 (1971)

(1 977) 480. 385.

[3] DEWEY, C. F. Jr., COOK, W. R. Jr., HODGSON, R. T. and L71 Y'?'JNG2 J. F., _{G .}

&&u?%fi.&~~.&!?:

WYNNE, J. J.. ADD[. Phvs. Lett. 26 (1975) 714. and HARRIS, S. E., Phys. Rev. Lezt. 27 (1971) 1551.

[4] MAKER, P. D. and TERH~NE, R. W., Phys.' Rev. 137 (1965) [8] MILES, R. B. and HARRIS, S. E., IEEE J. Quantum Electron.

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C4-74 J. J. WYNNE

[9] KUNG, A. H., YOUNG, J. F., BJORKLUND, G. C. and HARRIS, [l91 WALLACE, S. C. and DREYFUS, R. W., Appl. Phys Lett. 25 S. E., Phys. Rev. Lett. 29 (1972) 985. (1974) 498.

[l01 KUNG, A. H., YOUNG, J. F. and HARRIS, S. E., Appl. Phys. [20] HARR1s, S. E., Phys. Rev. Lett. 31 341.

Lett. 22 (1973) 301. [21] REINTJES, J., ECKARDT, R. C., SHE, C. Y., KARANGELEN, N. E., [ I l l HODGSON, R. T., SOROKIN, P. P. and WYNNE, J. J., Phys. Rev. ELTON, R. C. and ANDREWS, R. A., Phys. Rev. Lett. 37

Lett. 32 (1974) 343. (1976) 1540.

[22] REINTJES, J. and ECKARDT, R. C., Appl. Phys. Lett. 30 (1977)

[l21 WALLACE, S. C. and ZDASIUK, G., Appl. Phys. Lett. 28 (1976) _{0 1}

449. 2,.

[23] SOROKIN, P. P,, ARMSTRONG, J. A., DREYFUS, R. W., HODGSON, [l31 INNES, K. K., STOICHEFF, B. P. and WALLACE, S. C., Appl.

Phys. Lett. 29 (1976) 715. R. T., LANKARD, J. R., MANGANARO, L. H. and WYNNE, J. J., Lecture Notes in Physics, ed. by S. Haroche (Springer- [l41 HARRIS, S. E. and BLOOM, D. M,, Appl. Phys. Lett. 24 (1974) _{Verlag, Berlin) 1975, Vol. 43,}D. 46.

229. _{[24] Hsv, K. S ,}KUNG; A. A., LYZWA~SKI, L. J., YOUNG, J. F. and [l51 WANG, C. C. and DAVIS, L. I., Phys. Rev. Lert. 35 (1975) 650. HARRIS, S. E., ZEEE J. Quantum Electron. QE-12 (1976) 1161 BEY, P. P. and RABIN, H., Phys. Rev. 162 (1967) 794. 60.

'171 KUNG, A. H., Appl. Phys. Lett. 25 (1974) 653. [25] ARMSTRONG, J. A. and WYNNE, J. J., Phys. Rev. Lett. 33 (1974) :l81 HUTCHINSON, M. H. R., LING, C. C. and BRADLEY, D. H., 1183.