S. Chakraborty

Generation of Bessel beams using a terahertz quantum cascade laser

We report the generation of Bessel beams using polytetrafluoroethene conical lenses and a quantum cascade laser emitting at 2.8 THz. The formation of a central beam spot that retains its size over distances exceeding the characteristic Rayleigh range is demonstrated, and the power transport properties of these beams are compared with those obtained using parabolic reflectors. These lenses could provide an attractive alternative to parabolic reflectors for terahertz imaging applications where a large depth of focus and/or efficient and controllable coupling of radiation onto a small target are desirable.

Electronically tunable aperiodic distributed feedback terahertz lasers

Focussed ion beam milling can be used to introduce aperiodic distributed feedback (ADFB) gratings into fully packaged, operational terahertz (THZ) quantum cascade lasers to achieve electronically controlled, discretely tunable laser emission. These aperiodic gratings—designed using computer-generated hologram techniques—consist of multiple slits in the surface plasmon waveguide, distributed along the length of the laser cavity. Tuning behaviour and output power in ADFB lasers operating around 2.9u2009THz are investigated with a variety of slit dimensions and grating scales. Mode selectivity and grating losses are found to be strongly dependent on milling depth into the upper waveguide layers, dramatically increasing as the metallic layers are penetrated, then rising more slowly with deeper milling into the laser active region. Grating scale and placement along the laser cavity length are also shown to influence mode selection.

Intermodulation distortion in wide-band dual-mode bulk ferroelectric bandpass filters

An investigation into intermodulation (IM) distortion in bulk ferroelectric bandpass filters is presented. The main objective has been to study the effect of the response time of ferroelectric material on IM distortion. In order to investigate the response, time IM measurements with wide carrier frequency spacing must be performed. Thus a novel planar dual-mode bandpass filter, with 600 MHz bandwidth at 2 GHz, has been designed and fabricated on a bulk Barium Strontium Titanate (BST) substrate. The two tone third-order IM has been measured for a range of signal separations in order to estimate the material response time.

Diffuse reflection imaging at terahertz frequencies for security applications

We report diffuse reflection imaging of concealed powdered samples in atmospheric air using a quantum cascade laser operating at 2.83 THz. The imaging system uses a helium-cooled silicon bolometer for mapping radiation diffusely reflected and scattered from samples, and a room-temperature pyroelectric sensor for simultaneously acquiring a specular image. A range of powders concealed within plastic packaging and standard FedEx envelopes was imaged with a resolution of better than 0.5 mm, and it was possible to detect powdered samples concealed within packaging from which there was a strong component of surface reflection. The feasibility of performing dual-wavelength diffuse reflection imaging for identification of illicit drugs and explosives is discussed.

Modal perturbation of terahertz quantum cascade lasers

Focused ion beam etching has been used to introduce one-dimensional photonic crystals between the cleaved facets of terahertz quantum cascade lasers. These micro-structured lasers showed single-mode emission at specific wavelengths, governed by the respective photonic crystal lattice constants.