A comparative study of magnetic-field-enhanced THz generation in semiconductor surfaces of InSb, InAs, InP, GaAs, and GaSb is reported. Applying an external magnetic field, the power of the generated THz radiation is increased for all examined semiconductor materials. The use of time-resolved measurements of the THz waveform allows to distinguish between the fraction of radiation originating from the surface depletion field and the fraction that is additionally generated by the magnetic field. It turns out that the power enhancement factor due to the magnetic field is inversely proportional to the effective electron mass.
We have observed the 1S-2S transition in atomic hydrogen and deuterium by Dopplerfree two-photon spectroscopy, using a frequency-doubled pulsed dye laser at 2430 A. Simultaneous recording of the absorption spectrum of the Balmer-/3 line at 4860 A, using the fundamental dye-laser output, allowed us to precisely compare the energy intervals IS-25 and 2S,P-4S 9 P,D and to determine the Lamb shift of the 15 ground state to be 8.3±0.3 GHz (D) and 8.6±0.8 GHz (H).We have observed transitions from the IS ground state of atomic hydrogen and deuterium to the metastable 2S state, using Doppler-free two-photon spectroscopy. 1 " 3 The atoms are excited by absorption of two photons of wavelength 2430 A, provided by a frequency-doubled pulsed dye laser, and the excitation is monitored by observing the subsequent collision-induced 2P-1S fluorescence at the L a wavelength 1215 A. Linewidths smaller than 2% of the Doppler width were achieved with two counter-propagating light beams, whose Doppler shifts cancel. The fundamental dye-laser wavelength at resonance 4860 A coincides with the visible Balmer-/3 line, and simultaneous recording of the absorption profile of this line permits a precise comparison of the energy intervals 1S-2S and 2S,P-4S,P,Z>. From our first preliminary measurements we have determined the Lamb shift of the IS ground state to be 8.3 ±0.3 GHz (D) and 8.6±0.8 GHz (H), in good agreement with theory. The only previous measurement of the Lamb shift of the IS state of deuterium, 7.9±1.1 GHz, has been reported by Herzberg, 4 who used a difficult absolute-wavelength measurement of the L a line. The hydrogen-lS Lamb shift has never been measured before.Numerous authors 2,3,5 have pointed out that it would be very desirable to observe the 1S-2S transition in hydrogen by Doppler-free two-photon spectroscopy. The ^--sec lifetime of the 2S state promises ultimately an extremely narrow resonance width. The resolution obtained in the present experiment is already better than that achieved in our recent study of the Balmer-o? line by saturation spectroscopy, 6 and the implications for a future even more precise measurement of the Rydberg constant are obvious.We utilized a dye-laser system, consisting of a pressure-tuned dye-laser oscillator with op-
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