High-resolution Interferometric Diagnostics for Ultrashort Pulses

8.9 Summary and outlookFor the phase-sensitive detection, one could use an interferometer similar to that of chapter 7,with the control field inserted into one arm using a dichroic mirror. The control field itself couldbe produced using an optical parametric amplifier (OPA). Current designs based on the nonlinearcrystal BBO are strong candidates [433–441, 441–443]. The amplitude and phase of the controlfield could be scanned in a variety of ways, with the most direct being a half-waveplate/polarizervariable attenuator and a rotating thin glass plate. Shutters could rapidly block and unblock thereference arm of the interferometer, enabling interferograms as well as intensity-only images tobe acquired. Shutters could also repeatedly block the control field, providing control-field-freeimages which would provide the interferometer reference phase and thus keep track of slow driftsduring the scan. It would be necessary to eliminate phase-matching effects and access, as closelyas possible, the single-atom response. A thin gas jet, combined with a flat-top beam profile [444,445] would achieve this end.Another possibility is noninterferometric experimental implementation, in which only the intensityof the harmonics from the drive+control field is obtained. In principle, the filtering stepshould still be possible, since beating between the quantum paths will appear as sidebands inthe Fourier domain. However, with n quantum paths there are n(n − 1) sidebands in the Fourierdomain (self-terms are at the origin), which may complicate the identification and isolation steps.It is also worthwhile to consider more complex control fields, which may offer better quantumpathselectivity. One possibility is a much shorter control field, which could select only a singlehalf-cycle of the drive field.215