text: Coherent microwave scattering from electrons in REMPI-induced plasma filaments adds the capability to measure selectively-ionized species with a high spatial and temporal resolution - allowing for nonintrusive determinations of concentration profiles without the use of physical probes or electrodes. It has been applied for the detection of species such as argon, xenon, nitric oxide, carbon monoxide, atomic oxygen, and methyl radicals both within enclosed cells, open air, and atmospheric flames.
  Microwave detection is based on homodyne or heterodyne technologies.  They can significantly increase the detection sensitivity by suppressing the noise and follow sub-nanosecond plasma generation and evolution.  The homodyne detection method mixes the detected microwave electric field with its own source to produce a signal proportional to the product of the two.  The signal frequency is converted down from tens of gigahertz to below one gigahertz so that the signal can be amplified and observed with standard electronic devices.  Because of the high sensitivity associated with the homodyne detection method, the lack of background noise in the microwave regime, and the capability of time gating of the detection electronics synchronous with the laser pulse, very high SNRs are possible even with milliwatt microwave sources.  These high SNRs allow the temporal behavior of the microwave signal to be followed on a sub-nanosecond time scale.  Thus the lifetime of electrons within the plasma can be recorded.  By utilizing a microwave circulator, a single microwave horn transceiver has been built, which significantly simplifies the experimental setup.
  Detection in the microwave region has numerous advantages over optical detection.  Using homodyne or heterodyne technologies, the electric field rather than the power can be detected, so much better noise rejection can be achieved.  In contrast to optical heterodyne techniques, no alignment or mode matching of the reference is necessary.  The long wavelength of the microwaves leads to effective point coherent scattering from the plasma in the laser focal volume, so phase matching is unimportant and scattering in the backward direction is strong.  Many microwave photons can be scattered from a single electron, so the amplitude of the scattering can be increased by increasing the power of the microwave transmitter.  The low energy of the microwave photons corresponds to thousands of more photons per unit energy than in the visible region, so shot noise is drastically reduced.  For weak ionization characteristic of trace species diagnostics, the measured electric field is a linear function of the number of electrons which is directly proportional to the trace species concentration.  Furthermore, there is very little solar or other natural background radiation in the microwave spectral region.