Real-time near-field terahertz imaging with atomic optical fluorescence

Abstract

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...

A terahertz-driven non-equilibrium phase transition in a room temperature atomic vapour

“…1c). The bright orange fluorescence is due to optical decay from a large selection of Rydberg levels, indicating re-distribution of the atomic population 6,21 . As the Rydberg laser frequency is scanned we see hysteresis in the response, and the system undergoes abrupt changes as it switches from one state to another (Fig.…”

“…While the system would not be suitable for broad-band terahertz sensing, transitions between different Rydberg states span the terahertz-frequency spectrum 21 , and a detector could be ‘tuned’ to any of these frequencies by changing the Rydberg state to which the atoms are driven. In general higher energy Rydberg states would require more laser excitation power and correspond to lower terahertz frequencies, but result in stronger interactions (faster ionisation and stronger polarizability).…”

“…In general higher energy Rydberg states would require more laser excitation power and correspond to lower terahertz frequencies, but result in stronger interactions (faster ionisation and stronger polarizability). We anticipate further applications combining the phase transition with Rydberg electrometry 26 and Rydberg-fluorescence terahertz imaging 21 .…”

“…Terahertz devices have seen rapid development in recent decades 16 , with new technology based on media from super-conducting 17 and semi-conducting 18 solids to atomic vapour 19–21 . However, the traceable calibration of terahertz detectors, typically using cryogenic bolometers, still yields substantial uncertainties 22 .…”

“…Since then practical techniques have progressed significantly, most notably through the coherent optical detection of Rydberg states 24 , allowing coherent control of Rydberg states in compact, room temperature vapour cells 25 . These developments have facilitated precision microwave and mm-wave electrometry 21,26–28 , where Rydberg atom techniques promise to provide an SI traceable standard.…”