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PUBLISHED ONLINE: 7 NOVEMBER 2016 | http://dx.doi.org/10.1038/nphoton.2016.214
Web End =DOI: 10.1038/NPHOTON.2016.214
C. G. Wade, N. ibali, N. R. de Melo, J. M. Kondo, C. S. Adams and K. J. Weatherill*
Terahertz (THz) near-eld imaging is a ourishing discipline1,2,
with applications from fundamental studies of beam propagation3 to the characterization of metamaterials4,5 and waveguides6,7. Beating the diffraction limit typically involves rastering structures or detectors with length scale shorter than the radiation wavelength; in the THz domain this has been achieved using a number of techniques including scattering tips8,9 and aper
tures10. Alternatively, mapping THz elds onto an optical wavelength and imaging the visible light removes the requirement for scanning a local probe, speeding up image collection times11,12. Here, we report THz-to-optical conversion using a gas of highly excited Rydberg atoms. By collecting THz-induced optical uorescence we demonstrate a real-time image of a THz standing wave and use well-known atomic properties to calibrate the THz eld strength.
Atoms make excellent electromagnetic eld sensors because narrow-linewidth atomic transitions couple strongly to electromagnetic elds, giving atoms a sensitive, narrowband response. In addition, each atom of the same isotope is identical and has well-studied permanent properties that facilitate easy calibration to SI units. Atomic states that couple to multiple transitions offer an interface between different frequency regimes. In this way, atomic ground states have been used to map microwave elds onto an optical probe13. However, atomic ground states are only sensitive to a limited selection of microwave frequencies. In contrast, highly excited Rydberg atoms couple to strong, electric dipole transitions across a wide range of microwave and THz frequencies, making them ideal candidates for eld measurement and for frequency standards in the millimetre wave and THz range14. Previous methods for THz imaging with Rydberg atoms used the THz radiation to ionize the atoms15,16. More recently, optical readout of Rydberg states was demonstrated in a room-temperature alkali-metal vapour using electromagnetically induced transparency (EIT)17. The Rydberg EIT technique has since been exploited to readout radiofrequency elds18, to demonstrate precision microwave electrometry1921 and for subwavelength imaging of microwave elds22.
In distinction to the EIT technique we make direct use of THz-induced optical uorescence to demonstrate THz imaging. An overview of our THz imaging set-up is shown in Fig. 1a. Infrared laser beams...