Generation of coherent radiation tunable from 201 nm to 212 nm R. E. Stickel, Jr., and F. B. Dunning Rice University, Department of Space Physics & Astrono- my, Houston, Texas 77001. Received 12 May 1977. Recent studies have shown that tunable coherent radia- tion at wavelengths below those currently attainable through frequency doubling may be conveniently generated by sum frequency mixing in potassium pentaborate 1 (KB5) and re- frigerated ammonium dihydrogen phosphate (ADP). 2,3 Radiation at wavelengths as short as 208 nm has been gener- ated in this manner. The output energies observed are typ- ically greater than those realized using more complex, alter- nate techniques. 4 In addition, since phase matching may be achieved by angle tuning a crystal, it is not necessary to em- ploy two tunable lasers to provide a tunable uv output. 1 - 3 An earlier study of sum frequency mixing in KB 5 in this labora- tory 1 suggested that this material would form a suitable me- dium in which to mix the fundamental output of a ruby laser at 694.3 nm with the output of a frequency doubled dye laser resulting in the generation of radiation at wavelengths ex- tending down to ~201 nm. The present Letter reports the 2356 APPLIED OPTICS / Vol. 16, No. 9 / September 1977
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Generation of coherent radiation tunable from 201 nm to 212 nm R. E. Stickel, Jr., and F. B. Dunning
Rice University, Department of Space Physics & Astronomy, Houston, Texas 77001. Received 12 May 1977. Recent studies have shown that tunable coherent radia
tion at wavelengths below those currently attainable through frequency doubling may be conveniently generated by sum frequency mixing in potassium pentaborate1 (KB5) and refrigerated ammonium dihydrogen phosphate (ADP).2,3
Radiation at wavelengths as short as 208 nm has been generated in this manner. The output energies observed are typically greater than those realized using more complex, alternate techniques.4 In addition, since phase matching may be achieved by angle tuning a crystal, it is not necessary to employ two tunable lasers to provide a tunable uv output.1-3 An earlier study of sum frequency mixing in KB 5 in this laboratory1 suggested that this material would form a suitable medium in which to mix the fundamental output of a ruby laser at 694.3 nm with the output of a frequency doubled dye laser resulting in the generation of radiation at wavelengths extending down to ~201 nm. The present Letter reports the
Fig. 1. Angle tuning curve for sum frequency mixing with 694.3-nm radiation in KB5. The interacting beams propagate in the ab plane,
at angles φruby and φUV to the b axis.
Fig. 2. Temperature tuning curve for sum frequency mixing with 694.3-nm radiation in KB5. The interacting beams propagate along
the b axis of the crystal.
generation of radiation in the 201-212-nm range in this manner.
The frequency doubled dye laser used in the present study is pumped by the second harmonic output of the ruby laser and provides a tunable uv output which has a linewidth of 0.01 nm, a pulse power of 5-10 kW, and a pulse length of 20 nsec. This output is combined with a fraction of the fundamental output of the ruby laser by use of a dichroic mirror. A 100-cm focal length fused quartz lens is used to weakly focus the superimposed beams info the KB5 crystal, which is located ~20 cm from the focus of the lens. The interacting beams have a half-angle of convergence of ~1.7 mrad in the crystal and a diameter of ~1 mm. Relatively large beam diameters are
employed to limit the input power density to a value below 1 GW cm-2. The KB5 crystal is mounted in a sealed cell equipped with fused quartz windows. A temperature control system permits the crystal temperature to be varied from -20°C to +40°C. The KB5 crystal is a cube of side 10 mm with entrance and exit faces perpendicular to the b axis and is oriented so the interacting beams propagate through the crystal in the ab plane, both polarized in the ab plane. Angle tuning is accomplished by rotation of the crystal about the c axis. The sum frequency output radiation is separated from the remaining input radiation by a fused quartz prism and is detected either by the fluorescence it generates on a sodium salicylate phosphor, or, after attenuation by appropriate neutral density filters, by a calibrated EMR type 541G-08-18 solar blind photomultiplier.
In Fig. 1 are shown the uv input wavelengths at which phase matched sum frequency mixing was observed in a room temperature (24°C) KB5 crystal as a function of the angle of incidence of the input beams. The design of the cell in which the KB5 crystal was mounted prevented the use of angles of incidence greater than 30°. It is evident from Fig. 1 that radiation may be generated at wavelengths extending well below those attainable by frequency doubling. The short wavelength limit of 201.6 nm is within 0.1 nm of the value calculated by use of the revised refractive index data for KB5 presented in Ref. 1. At other than normal incidence the angles of refraction φ of the two input beams are different. Since the input face of the present crystal is normal to the b axis, φ is also the angle between the direction of propagation of an input beam and the b axis of the crystal. The angles φ appropriate to each input wavelength are shown in Fig. 1. The difference in the angles of propagation of the interacting beams is small, ~9 mrad at an angle of incidence of 30°, and does not result in a significant demerging of the input beams over the length of the crystal.
Temperature tuning of the KB5 crystal was investigated in an attempt to extend the generation of sum frequency radiation toward shorter wavelengths. Figure 2 shows the uv input wavelengths at which phase matched sum frequency mixing was observed as a function of the temperature of the crystal, at normal incidence. It is apparent from Fig. 2 that, although refrigeration of the crystal does permit the generation of radiation at shorter wavelengths, the extension in the tuning range is not great.
The efficiency for upconversion of the uv input radiation to sum frequency radiation was measured and was found to increase linearly with ruby laser input power. At a ruby laser input power of 4 MW, the largest employed in this study, an upconversion efficiency of ~10% was achieved at normal incidence. This efficiency is about one-third that estimated on the basis of the small signal, plane wave theory5 using the value6 dsi = 1.09 × 10-10 esu. The experimental efficiency is, however, expected to be somewhat less than that calculated by the use of this simple theory as a result of the finite line-width and angle of convergence of the interacting beams.5
Angle tuning of the crystal was found to result in a significant decrease in the upconversion efficiency; for instance, an efficiency of only ~2% was realized at an uv input wavelength of 295 nm. The efficiencies for unfocussed beams were typically an order of magnitude smaller than those obtained with focusing. The present upconversion efficiencies are sufficient to permit the generation of sum frequency pulse energies of 2-10 μJ, corresponding to the generation of 2 × 1012-1013
photons per pulse. These pulse energies are the largest yet generated in this wavelength range and are more than sufficient to permit the study of a wide range of photon interaction processes.
The authors wish to express their appreciation to NASA Johnson Space Center for furnishing the ruby laser used in this study.
This research was supported by the Research Corporation and by the Division of Physical Research of the U.S. Energy Research & Development Administration. F. B. Dunning is an Alfred P. Sloan Fellow.
References 1. F. B. Dunning and R. E. Stickel, Jr., Appl. Opt. 15, 3131 (1976). 2. G. A. Massey, Appl. Phys. Lett. 24, 371 (1974). 3. G. A. Massey and J. C. Johnson, IEEE J. Quantum Electron.
QE-12,721 (1976). 4. See, for instance, R. T. Hodgson, P. P. Sorokin, and J. J. Wynne,
Phys. Rev. Lett. 32, 343 (1974); or K. K. Innes, B. P. Stoicheff, and S. C. Wallace, Appl. Phys. Lett. 29, 715 (1976).
5. F. Zernike and J. E. Midwinter, Applied Non Linear Optics, S. S. Ballard, Ed. (Wiley, New York, 1973).
6. H. J. Dewey, IEEE J. Quantum Electron. QE-12,303 (1976).
