Cannard, PJ

Davies, AG

Freeman, JR

Graham, CS

Hatem, O

Linfield, EH

Mitrofanov, O

Moodie, DG

Mueckstein, R

Natrella, M

Renaud, CC

Robertson, MJ

Seeds, AJ

White Rose Research Online

This is a repository copy of THz pulse generation by charge density gradients investigatedby near-field microscopy.White Rose Research Online URL for this paper:http://eprints.whiterose.ac.uk/89833/Version: Accepted VersionConference or Workshop Item:Mueckstein, R, Natrella, M, Hatem, O et al. (10 more authors) (2015) THz pulse generationby charge density gradients investigated by near-field microscopy. In: Optical Terahertz Science & Technology Conference, 08-13 Mar 2015, San Diego, CA. eprints@whiterose.ac.ukhttps://eprints.whiterose.ac.uk/Reuse Unless indicated otherwise, fulltext items are protected by copyright with all rights reserved. The copyright exception in section 29 of the Copyright, Designs and Patents Act 1988 allows the making of a single copy solely for the purpose of non-commercial research or private study within the limits of fair dealing. The publisher or other rights-holder may allow further reproduction and re-use of this version - refer to the White Rose Research Online record for this item. Where records identify the publisher as the copyright holder, users can verify any specific terms of use on the publisher’s website. Takedown If you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing eprints@whiterose.ac.uk including the URL of the record and the reason for the withdrawal request.   THz pulse generation by charge density gradients investigated by near-field microscopy   R. Mueckstein,1 M. Natrella,1 O. Hatem,2 J. R. Freeman,2 C. S. Graham,1 Cyril C. Renaud,1 A. J. Seeds,1  E. H. Linfield,2 A. G. Davies,2 P. J. Cannard,3 M. J. Robertson,3 D. G. Moodie,3 and Oleg Mitrofanov1  1 University College London, London, UK 2University of Leeds, UK 3CIP Technologies (now Huawei), Ipswich, UK   The processes of generation of THz pulses by optical excitation of semiconductor and metallic surfaces recently attracted renewed attention as novel THz emission mechanisms were discovered [1-2]. Among these mec anisms is the emission due to transient photo-currents in the plane of the optically excited surfaces, which has been considered negligible until recently. Using a THz near-field microscopy system, we correlate the emission of THz pulses with the spatial distribution of photo-excited carriers and demonstrate that the in-plane gradients of the photo- xcited charge carrier density can be the dominant source of THz emission in the excitation of semiconductor surfaces.  We investigate the process of THz generation from epitaxial layers of Fe-doped In0.53Ga0.47As excited at various angles of incidence. Figure 1 shows a diagram of the experimental system and the near-field space-time maps of the emitted THz field for two cases: the angle of incidence of 50° (the optimal angle for the process of THz generation) and 0°. The near-field investigations reveal that THz waves of similar amplitudes are generated in both cases. However the asymmetric field distribution for the normal incidence case produces no far-field emission. Remarkably, the generated THz pulses originate not from the whole photo-excited area, but primarily from the edge regions, indicating that the lateral photo-currents produce a dominant contribution to the process of THz pulse generation.  The ability to map the emitted THz wave in the near-field zone of a photo-excited semiconductor surface allows investigations of other THz generation mechanisms. The understanding of the dominant emission mechanism will be used to enhance the optical-to-THz conversion efficiency.    References  [1] G. Klatt et al., Opt. Express 18(5), 4939-4947 (2010). [2] G. Ramanandan et al., ACS Photonics 11, (2014).  Fig. 1. (a) Schematic diagram of the experimental system, where the semiconductor surface is excited at the angle of incidence of 50°. The emitted THz wave is mapped on the back side of the 30m thick sample along the yellow dashed line. (b,c) Near-field space-time map of the generated THz wave for the angle of incidence of 50° and 0°. 

THz pulse generation by charge density gradients investigated by near-field microscopy

https://eprints.whiterose.ac.uk/89833/1/Mueckstein%20et%20al%202015%20OTST%20THz%20pulse%20generation%20by%20charge%20density%20gradients%20investigated%20by%20near-field%20microscopy.pdf

THz pulse generation by charge density gradients investigated by near-field microscopy

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