Diffraction-limited ultrabroadband terahertz spectroscopy.

M. Baillergeau,J. Tignon,E. H. Linfield,K. Maussang,L. H. Li,J. Mangeney,J. Palomo,S. Dhillon,T. Nirrengarten,A. G. Davies

doi:10.1038/srep24811

M. Baillergeau, J. Tignon + Show 8 more

Open Access

https://doi.org/10.1038/srep24811

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Abstract

Diffraction is the ultimate limit at which details of objects can be resolved in conventional optical spectroscopy and imaging systems. In the THz spectral range, spectroscopy systems increasingly rely on ultra-broadband radiation (extending over more 5 octaves) making a great challenge to reach resolution limited by diffraction. Here, we propose an original easy-to-implement wavefront manipulation concept to achieve ultrabroadband THz spectroscopy system with diffraction-limited resolution. Applying this concept to a large-area photoconductive emitter, we demonstrate diffraction-limited ultra-broadband spectroscopy system up to 14.5 THz with a dynamic range of 103. The strong focusing of ultrabroadband THz radiation provided by our approach is essential for investigating single micrometer-scale objects such as graphene flakes or living cells, and besides for achieving intense ultra-broadband THz electric fields.

Highlights

Distinct diffraction regimes according to the spectral range
Since the THz electric field emitted by the large-area photoconductive antennas (PA) is proportional to the optical pump intensity, the THz waist radius wTHz is expressed as wTHz =, where wopt is the optical spot radius on the surface of the large-area PA
(a) Schematic of the concept of a THz emitter excited by a spherical wavefront optical pulse in a reflected and transmitted configuration: the emitter is both illuminated by a divergent optical pump beam, the divergence angle of the optical pump intensity is chosen to match the numerical aperture of the collecting parabolic mirror

Summary

Conclusion

We experimentally and theoretically demonstrate diffraction-limited ultra-broadband TDS from up to 14.5 THz, relying on spherical-wavefront optical excitation of the THz emitter. This extremely easy-to-implement concept applied to a LTG GaAs large-area interdigitated PA emitter provides an ultrabroadband THz TDS system with up to 22 THz bandwidth associated with a dynamic range of 103. A strong support for the development of advanced ultra-broadband THz TDS systems that is currently growing owing to the proliferation of 15-fs pulse lasers. This work has the potential to greatly impact nonlinear THz applications as it can provide intense ultra-broadband THz electric field at the diffraction limit

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