Atomic GHz/THz Sensor & Imager

Rydberg states are atoms with electrons excited to high principal quantum numbers. The electric dipole transitions between two Rydberg states of alkali atoms cover the microwave range and terahertz (THz) range.

Terahertz waves form a part of the electromagnetic spectrum in the frequency range from 100 GHz to 10 THz. It falls between microwave and infrared, called the ‘THz gap’. The interaction of Rydberg atoms to THz leads to the radiation of a bright visible fluorescence from the atoms decaying to lower states in the vapor cell. This fluorescence mechanism acts as a conversion of THz-to-optical photons, making the difficult-to-detect THz easy to detect with a normal optical camera. This vapor-cell-based technique is very promising for THz imaging with the advantage of high speed and sensitivity, low cost and easy operation and will have wide applications.

Objective: in macQsimal, partners will work closely together to design and develop a portable Rydberg atomic THz imaging system.

Related macQsimal publication

Full-Field Terahertz Imaging at Kilohertz Frame Rates Using Atomic Vapor
Downes, L.A., MacKellar, A.R., Whiting, D.J., Bourgenot, C., Adams, C.S., Weatherill, K.J. (2020) Physical Review X, 10(1), 011027. A preprint of this publication is available on arXiv.

Towards a frequency-tunable microwave magnetic field imager withultrathin atomic vapor cells
University of Basel (UNIBAS), 2020, micro-WOPM 2020, online.

Imaging principle

The principle of THz imaging here is to use Rydberg atoms to perform THz-to-optical frequency conversion, converting difficult-to-detect THz radiation into easy-to detect optical photons. The electric dipole transitions between neighboring Rydberg states of alkali atoms lie in the THz frequency range. These transitions have very large dipole moments, resulting in a high probability of interaction with a resonant THz field. When THz radiates the cell, there is bright visible fluorescence generated from the cell. We use the emitted visible photons to create an image of the incident THz field in a single exposure [1]. While some imaging systems raster their detector to capture the image pixel-by-pixel in sequence, our system captures all pixels in a 2D image simultaneously, increasing our image acquisition speed. The versatility of this approach means that any optical camera can be employed and can easily be substituted into the system.

Cell closeup and THz fluorescence

We used a cuboid quartz cell filled with alkali atoms to image THz. The atoms are firstly excited to a certain Rydberg state. When the THz is on, there is bright green fluorescence radiated from the cell. This bright fluorescence image can be taken by an optical camera.

Ceasium cell with THz induced fluorescence. When the THz is on, there is bright green fluorescence radiated from the cell.

THz image for chocolate

As THz can penetrate most wrappers materials, such as plastic and cardboard, THz can be very useful for food security or quality check without opening the food wrappers. Here we make use of the high penetration feature of THz to image a hazelnuts chocolate bar. From the image, we can clearly tell the hazelnuts distribution in the chocolate. This can potentially enable us to discriminate food contaminants and monitor food quality, and thus be widely used in the food production line.

THz image of hazelnutTHz image of hazelnuts chocolate bar with 0.55 THz in Cs atoms.

See how do we work – THz image labs

Now we have both the Rb imaging system (top) and Cs imaging system (bottom). Different atoms enable different possible THz transitions and different TH fluorescence colors. On each table, we have the 3 lasers with their locking systems to excite our alkali atoms into a Rydberg state sensitive to the THz frequency of interest, as well as the THz source and atomic cell.

Laboratory - Rb imaging system.
Laboratory Cs imaging system.