Terahertz (THz) near-field imaging is a flourishing discipline [1, 2], with applications from fundamental studies of beam propagation [3] to the characterisation of metamaterials [4,5] and waveguides [6,7]. 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 tips [8,9] and apertures [10]. Alternatively, mapping THz fields onto an optical wavelength and imaging the visible light removes the requirement for scanning a local probe, speeding up image collection times [11,12]. Here we report THz to optical conversion using a gas of highly excited 'Rydberg' atoms. By collecting THz-induced optical fluorescence we demonstrate a real-time image of a THz standing wave and we use well-known atomic properties to calibrate the THz field strength.Atoms make excellent electromagnetic field sensors because narrow line-width atomic transitions couple strongly to EM fields, giving atoms a sensitive, narrowband response. In addition, each atom of the same isotope is identical and has well studied, permanent properties which 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 fields onto an optical probe [13]. However atomic ground states are only sensitive to a limited selection of microwave frequencies. In contrast, highlyexcited Rydberg atoms couple to strong, electric dipole transitions across a wide range of microwave and THz frequencies, making them ideal candidates for field measurement and for frequency standards in the millimeter wave and THz range [14]. Previous methods for THz imaging with Rydberg atoms used the THz radiation to ionise the atoms [15,16]. More recently optical read-out 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 radio frequency fields [18], to demonstrate precision microwave electrometry [19][20][21] and for sub-wavelength imaging of microwave fields [22]. * c.g.wade@durham.ac.ukIn distinction to the EIT technique we make direct use of THz-induced optical fluorescence to demonstrate THz imaging. An overview of our THz imaging setup is shown in Figure 1a. Infrared laser beams forming a three-step ladder excitation scheme [23] are co-axially aligned with a continuous wave (CW) THz beam and pass through a caesium vapour in a 2 mm long quartz cell. In regions where both the THz field and the laser beams are present, atoms are excited to a Rydberg state and subsequently decay with fluorescence at visible wavelengths. The fluorescence is imaged by a consumer digital camera, and a typical 0.5 s exposure is shown in Figure 1b. In Figure 1c we show THz transition frequencies and calculated reduced dipole matrix elements,d, for...

We observe and characterize intrinsic optical bistability in a dilute Rydberg vapor. The bistability is characterized by sharp jumps between states of low and high Rydberg occupancy with jump up and down positions displaying hysteresis depending on the direction in which the control parameter is changed. We find that the shift in frequency of the jump point scales with the fourth power of the principal quantum number. Also, the width of the hysteresis window increases with increasing principal quantum number, before reaching a peak and then closing again. The experimental results are consistent with predictions from a simple theoretical model based on semiclassical Maxwell-Bloch equations including the effects of broadening and frequency shifts. These results provide insight to the dynamics of driven dissipative systems.

We report on the observation of electromagnetically induced transparency (EIT) and absorption (EIA) of highly excited Rydberg states in thermal Cs vapor using a four-step excitation scheme. The advantage of this four-step scheme is that the final transition to the Rydberg state has a large dipole moment and one can achieve similar Rabi frequencies to two- or three-step excitation schemes using two orders of magnitude less laser power. This scheme enables new applications such as dephasing free Rydberg excitation. The observed lineshapes are in good agreement with simulations based on multilevel optical Bloch equations.

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