A new concept for the optical generation of THz radiation has been introduced by AMO GmbH, Germany. The approach – filed for patent application  by the company – uses grating structures made of two different metal materials which are configured on a semiconducting substrate. THz pulse generation is triggered by optical excitation of this structure through femtosecond near-infrared pulses and subsequent acceleration of the photo-induced charge-carriers. Scaling of the actively emitting areas into a range of square-mm sizes (helpful to avoid conversion efficiency degradation through pump saturation effects) is straightforward.
Earlier large-scale THz emitter concepts used voltage-biased metal-semiconductor-metal (MSM)-structures [2,3]. For these structures increasing emitter area means an increasing probability for device fade-out through a single short-cut defect. Now, the emitter is operated bias-free because inherent Schottky-fields present at the bimetallic/semiconductor interfaces are used for charge-carrier acceleration. As a result, the emitter stays functionally unimpaired even in case of short-cut defects.
Bias-free Terahertz emission – Schottky-field vs. photo-Dember
The introduced emitter is sharing this robustness with an earlier bias-free THz emitter concept based on lateral photo-Dember field induction , but it features the important further advantages of simple monolithic fabrication and higher efficiency. In order to generate lateral photo-Dember fields the fabrication of (mono-) metallic gratings with three-dimensional wedged profiles is required. These profiles are difficult to realize for arbitrary shaped gratings like radial or curved ones instead of shown linear gratings. More important, lateral charge carrier acceleration through Schottky-fields is not limited to semiconductor materials with pronounces differences in electron and hole mobilities and diffusion processes (as used for photo-Dember field induction). Consequently, the new approach enables bias-free Terahertz emitters with improved efficiency. A demonstration device in terms of a radial mode THz emitter has now been presented using the new bimetal grating concept.
No bimetal, no Terahertz emission
The key feature enabling Terahertz generation within the novel large-scale grating structure is based on the application of two different metal materials: While the lateral Schottky-fields in a monometallic grating always cancel out over a full grating area, there is a net lateral field at bimetallic gratings resulting from the Schottky-field difference between both metals applied. Fig. 1 (top) is showing the principle configuration of the investigated radial grating emitter. By choosing the medial sequential order of the applied metals (e.g. metal 1/metal 2/semiconductor instead of metal 2/metal 1/semiconductor) it is possible to flip the direction of the generated THz field by 180° as shown in the cross-section view at the bottom of Fig. 1. A representative surface-profile is shown in Fig. 2. Three height levels visible in this plot correspond to plateau areas representing the bare semiconductor surface (dark green), metal 1 or metal 2 (light green) and metal 1/metal 2 one above the other (yellow). No wedged structures have been formed in this case.
Advanced monitoring of THz near-field emission
The THz emission process has been measured using photoconductive microprobes (from the TeraSpike TD-800 series) developed in-house through AMO. The probes allow the selective time-domain sampling of every THz vector-field component in x-, y- and z-direction in terms of amplitude and phase. Fig. 3 is showing a single snap-shot of the field amplitude distribution in z-direction measured shortly after optical excitation at a pair of radial emitters. The measurement plane is on the emitter backside as sketched by the dashed red line in Fig. 1. As expected for the z-component of a radial mode, the largest field magnitudes are observed at the center of each emitter. Both emitters have been fabricated using the converse bimetal sequences also illustrated in Fig. 1. As a consequence, the field lines on both radial emitters are pointing in opposite direction which confirms that the Schottky-field induced THz generation is working as expected. A movie showing the time evolution of the excitation process can be watched by following the link in the caption of Fig. 3.
Bimetallic grating structures are highly attractive for the production of bias-free large-scale THz emitters of arbitrary shape and size. The given example of a radial-mode emitter demonstrates the flexibility of this approach very nicely. Further important attributes are robustness and efficiency. In addition to pulsed generation the concept should also be attractive for continuous wave (cw) Terahertz signal generation  using semiconducting materials with sufficiently short carrier lifetimes.
 M. Nagel, German patent application, DE 102012010926 A1
 A. Dreyhaupt et al., Appl. Phys. Lett. 86, 121114 (2005), http://dx.doi.org/10.1063/1.1891304
 M. Awad et al., Appl. Phys. Lett. 91, 181124 (2007); http://dx.doi.org/10.1063/1.2800885
 G. Klatt et al., Optics Express, Vol. 18, Issue 5, pp. 4939-4947 (2010), http://dx.doi.org/10.1364/OE.18.004939
 Dohler et al., Terahertz Science and Technology, IEEE Transactions on , Vol. 3 , Issue 5, pp. 532 – 544 (2013), http://dx.doi.org/10.1109/TTHZ.2013.2266541