We demonstrate efficient, pulsed, gas-phase, nonlinear frequency conversion in a quadruply resonant, double- Lambda system and, simultaneously, verify theoretical predictions of Rabi-frequency matching unique to absorbing nonlinear media. This system is used to up-convert ultraviolet light at 233 nm to the vacuum ultraviolet at 186 nm in atomic Pb vapor with small-signal conversion efficiencies exceeding 30% and with modest atomic density-length (NL) products (scale 10(14) cm(-2)) and optical power densities (10-100 kW/cm(2)).

We report the demonstration of a pulsed atomic lead (Pb) vapor-based vacuum ultraviolet frequency converter from 233 to 186 nm with unity photon-conversion eff iciency. This conversion is attained without phase matching. © 1999 Optical Society of America OCIS codes: 190.2620, 190.4410, 270.1670, 020.1670 The techniques of electromagnetically induced transparency (EIT) can be employed to create a nonlinear response at a given wavelength that is equal in magnitude to the linear response at that wavelength. Physically, this allows frequency converters wherein unity photon-to-photon conversion occurs within a single coherence length, i.e., in that distance that causes a p phase slip between the driving polarization and the generated electromagnetic wave. These ideas were first implemented by Jain and co-workers 1 to convert 425 nm to 293 nm with an energy conversion efficiency of 40%, in a medium consisting of a single isotope of Pb. The enhancement of nonlinear optical processes using EIT has driven much recent theoretical 2 and experimental 3,4 work. This Letter reports the demonstration of a vaporphase vacuum ultraviolet frequency converter from 233 to 186 nm with unity photon conversion efficiency. Up to 300 mJ of 186 nm is generated, with a peak power of 25 kW.Our technique is this: two strong pulsed laser fields (MW͞cm 2 -class) are applied that are two-photon resonant with a Raman transition of the Pb atoms, as shown in the inset of Fig. 1. Similar to previous EIT work, 5 a 283-nm probe laser and a 406-nm coupling laser adiabatically drive all the atoms of the ensemble into a maximally coherent superposition of the ground and metastable states. Following the level designations of Fig. 1, maximal coherence of the Raman ͑j1͘ ! j2͒͘ transition is def ined as the condition in which the density matrix elements satisfy r 11 r 22 jr 12 j 1͞2. Once the atoms are driven to this dark state, they are decoupled from the applied optical fields, and, hence, the high-intensity probe and coupling lasers maintain wave vectors equal to their vacuum values and propagate without loss or distortion. 6This permits substantial resonant enhancement of the fourwave-mixing nonlinear optical susceptibilities that govern the generation of the upconverted radiation. The atomic coherence, r 12 , acts as a local oscillator and a third applied frequency beats against it to produce sum and difference frequencies. This frequency converter has the special property that, when the atoms of the ensemble are driven at maximal coherence, 100% of the input photons may be upconverted within a single coherence length, thus obviating the need to phase match the propagating beams.This experiment has been carried out in naturalabundance Pb metal at a density-length NL product of 1.5 3 10 17 atoms͞cm 2 , under conditions where the transmission of a resonant weak-probe 283-nm beam, if alone, would be approximately exp͑22 3 10 5 ͒. Natural abundance Pb consists of three isotopes, 24%206 Pb, 23%207 Pb, and 53% 208 Pb. Of the three, the 207 Pb isotope has a ...

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An analysis of the single point reproducibility of TD-THz based paint thickness measurements demonstrated a precision of 130 nm, corresponding to 0.1% of the measured thickness. A detailed model of the anticipated TD-THz waveforms from samples of varying thickness indicates that an intrinsic uncertainty of 0.09% is anticipated in the absence of environmental fluctuations. Therefore, the influence of oscillations in the THz field associated with the initial reflection does not adversely impact the ability to extract accurate paint thickness information, and the noise associated with these oscillations could limit the measurement uncertainty of a calibrated instrument under optimum laboratory conditions. In the case of a deployed sensor, we anticipate that the accuracy will be degraded by environmental fluctuations.

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