###### Abstract  
Knowledge of the beam profiles of terahertz emitters is required for the design of terahertz instruments and applications, and in particular for designing terahertz communications links. We report measurements of beam profiles of an optoelectronic silicon lens-integrated PIN-PD emitter at frequencies between 100 GHz and 1 THz and observe significant deviations from a Gaussian beam profile. The beam profiles were found to differ between the H-plane and the E-plane, and to vary strongly with the emitted frequency. Skewed profiles and irregular side-lobes were observed. Metrological aspects of beam profile measurements are discussed and addressed.  
terahertz; emitters; beam profile +
Footnote †: journal: Applied Sciences  
with attached bow-tie antenna is employed, which is mounted on a hyper-hemispheric silicon lens and packaged into a fibre-pigtailed housing. A detailed description of this emitter and its THz performance can be found elsewhere [33].  
While many aspects of PD devices and THz systems as a whole have been intensively studied, the emitter radiation pattern in the THz domain is generally assumed to be Gaussian. Since mirrors or lenses are commonly employed in spectroscopic and sensing applications, the beam profile of THz emitters remains widely unknown. However, in THz communication links, no beam forming elements can be placed in the free space between the emitter and the receiver. For all applications, but particularly for those involving medium and long-range free-space propagation such as communications, it is important to have a good understanding of the spatial profile of the THz beam. Therefore, an investigation of the beam profile is urgently required to develop transmitters, i.e., antenna structures, dedicated to THz communication links. In microwave and millimetre wave communication systems, the characterization of emitter (transmitter) beam profiles has long since been accepted as an essential tool. In this field, the techniques for antenna characterisation are extensively developed and well understood [34]. Furthermore, specialized facilities are available to perform the required measurements. None of these as yet exist for THz devices. THz radiation is often assumed to propagate as a perfect Gaussian beam, which the present study shows to be inaccurate.  
There is very little published literature on measurements of beam profiles of CW THz emitters, with the vast majority of beam profile data describing pulsed systems [35; 36; 37; 38]. This is primarily due to the widespread use of pulsed THz systems for spectroscopy, where beam profile properties may give rise to errors in spectroscopic data. Moreover, pulsed THz emitters have peak powers that are 3-4 orders of magnitude higher than CW devices, facilitating beam characterisation. Of the existing literature on CW beam profiles, most are for high power laser sources with a narrow tuning range such as quantum cascade lasers (QCLs) [39; 40] and gas lasers [41].  
In this study we performed detailed measurements of the beam profile of a commercial THz emitter based on a PIN photodiode at frequencies between 100 GHz and 1 THz, and for both E-plane and H-plane.