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## THz Sensing and Imaging

In this project, we use highly excited (Rydberg) atoms to sense, and image, THz frequency fields.

Room temperature alkali-metal atom vapours are ideal candidates for the sensing of THz radiation, with sensitivities comparable to state-of-the-art bolometers [ref]. In addition, the atomic properties that parameterise the interactions with the THz fields are well-known, meaning that sensors based on atoms are naturally calibrated. Finally, the near room temperature operation will allow for the miniaturisation of sensors.

With a 3-step excitation process we create Rydberg atoms in a Caesium vapour confined within a glass cell. From here, transitions to nearby Rydberg states can be driven by THz frequency fields. The atoms subsequently emit light at visible wavelengths, which can easily be detected, allowing the presence of the THz field to be inferred.

By detuning the Rydberg laser (green) from resonance, the background fluoresence from the $|n\mathrm{P}\rangle$ state is suppressed. Meanwhile, the THz field drives a 2-photon Raman transition, meaning that the strong fluorescence is still observed in regions where the THz field is strong.

## THz Sensing

By scanning the frequency of the Rydberg laser and monitoring the transmission of the weak probe laser we observe three-photon electromagnetically induced transparency (EIT) spectra (right). The presence of a resonant THz field splits the EIT resonance by an amount proportional to the THz electric field strength, providing a sensitive method for measuring the intensities of THz fields.

## 1D THz Imaging

The reflection of the THz field by the walls of the glass cell forms a standing wave with a period equal to half the THz wavelength. A real colour image taken with a DSLR camera clearly shows the standing wave.

## 2D THz Imaging

Using the 3-step excitation lasers we create a two-dimentional light sheet with which to image the THz field in 2D.

## Latching THz Detector

The system can be configured as a latching THz detector, using the optical bistability properties of Caesium. In this case the probe transmission changes sharply when a THz field is applied, but then remains in the changed state after the THz field is turned off. The system can be reset by power cycling the Rydberg laser.