Abstract
Generation of high intensity terahertz radiation in the low frequency region (f < 5 THz) is still a challenging task and only few experimental demonstrations exceeding 1 MV/cm have been reported so far. One viable option is the use of resonant metallic structures which act as amplifiers for the impinging radiation. Here with the aid of finite difference time domain simulations, we design and realize a set of isolated resonant elements which allow us to reach a 28-fold enhancement of freely propagating THz radiation at f ≈ 1 THz. These elements are deposited on a GaP sample allowing the direct measurement of the field enhancement using electro-optical sampling. Interestingly, we experimentally show strong modifications of the antennas resonance which is interpreted in terms of interference effects. These are particularly important in samples thinner than half the spatial pulse length.
Highlights
One viable option is the use of resonant metallic structures which act as amplifiers for the impinging radiation
The primary drawback of this approach is the extremely complex machining of the surface, requiring a coverage of large areas with features smaller than 1 μm in size [5, 12, 13]. They rely on the excitation of localized carrier which resonate and amplify the impinging radiation
A careful optimization of the focusing apparatus can help; very recently extremely large peak electric fields exceeding 50 MV/cm have been reported for source parameters that normally would lead to only a few MV/cm fields [17]
Summary
THz radiation has received increasing attention in the last decades, thanks in part to the availability of high efficiency optical rectification materials such as LiNbO3, and/or the most common organic crystals, i.e. (2-(3-(4-Hydroxystyryl)-5,5-dimethylcyclohex2-enylidene)malononitrile) [OH1], 4-N,N-dimethylamino-4’-N’-methyl-stilbazolium 2,4,6trimethylbenzenesulfonate [DSTMS], and 4-N,N-dimethylamino-4’-N’-methyl-stilbazolium tosylate [DAST] [1,2,3,4]. A careful optimization of the focusing apparatus can help; very recently extremely large peak electric fields exceeding 50 MV/cm have been reported for source parameters that normally would lead to only a few MV/cm fields [17] Both methods achieve high field amplitude without complex machining of the sample surface for frequency f < 5 THz, but pose strong limitations on the sample geometry and/or rely on an extremely precise positioning of sample and optical elements which is not always practical, together with the use of extremely powerful lasers. Still another approach is the use of the near-field (NF) enhancement of THz fields from isolated metallic elements deposited on the sample surface. By taking advantage of these effects, we are able to achieve enhancements larger than 28 times, resulting in peak fields of more than 5 MV/cm at a frequency around 1 THz and at a repetition rate of 1 KHz for externally coupled THz pulses
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