Sources and detectors

Terahertz radiation sources and detectors
Terahertz time-domain spectroscopy has attracted great attention recently. The possibility of doing sub-picosecond time-resolved studies in the far-infrared region of the spectrum with both phase and amplitude resolution has provided spectroscopists unprecedented power to investigate a series of systems that range from macro-bio-molecules to superconductors. For those interested on a general introduction to the field, an excellent review article was written by Charles Schmuttenmaer (Yale) and the transcript of a magnificent overview presentation by Martyn Chamberlain (Durham) at a Royal Society meeting is also available.

Photoconductive emitters
The early development of terahertz emitters was possible after picosecond pulsed lasers appeared. Photoconductive switches date form 1975 when picosecond optically unduced switching was first reported [1]. In 1983 its application to generation of hertzian radiation [2] was also reported by D.H. Auston. Since then the technology of ultrafast lasers has inproved dramatically and it is now possible to find sub-10fs pulsed lasers comercially. This has allowed performing spectroscopy over the far-infrared (~100GHz to ~10THz) as well as the mid-infrared using the technique known as time-domain spectroscopy [3,4].
The enormous interest and demand for far infrared spectroscpy has attracted our attention on improving these kind of emitters. Recently we have published a series of articles where the ultrafast ion-implanted semiconductor carrier dynamics influence on terahertz emission has been studied theoretically using Monte-Carlo methods and experimental measurements [5,6,7].
Photoconductive emitters are formed by a pair of metallic contacts deposited on a high resistivity semiconductor surface. High voltage is applied between the contacts, of corse, very little current is generated owing to the high resistivity of the substrate. When an ultra-short optical pulse hits the gap between the contacts, photocarriers are generated and separated rapidly by the applied voltage. The acceleration of the carriers generates an electromagnetic transient ("Maxwell style") in the far field which is proportional to the time detivative of the current density.

Three-dimensional Monte-Carlo simulation of dipole formation in a photoconducive emitter.
Photoconductive detectors
After the discovery of photoconductive emitters the "inverse" technology appeared rapidly[8]. In recent years we have also work very hard on improving and extending photoconductinve detection technology. Given that time-domain spectroscopy is a relatively recent technique it is still under development. Until very recently the appropriate technology for detecting the polarisation of a time-domain terahertz transient was unavailable. The introduction of a three-contact photoconductive emitter design has allowed resolving the full transverse electric field of a terahertz pulse as function of time[9,10]. This allows characterising materials with anisotropic complex dielectric properties such as birefringent materials, and opens the possibility of performing terahertz-vibrational-circular-dichroism experiments that have enormous potential in the investigation of conformational dynamics of macto-bio-molecular systems such as proteins and nucleic acids.
We have also studied the influence of ion-implanted semiconductor carrier dynamics on the performance of photoconductive detectors [11].

Scanning electron micrograph of a polarisation-sensitive three-contact photoconductive receiver (Image by Gabriella Chapman, Oxford Materials).

Back to main page

Laboratorio de óptica ultrarrápida is part of
Centro de Investigaciones en Óptica A.C.
Loma del bosque 115, Lomas del Campestre, 37150, León, GTO. México.
Tel: +52 (477) 441-42-00, Fax: +52 (477) 441-42-09
For questions please contact the webmaster.
Last modified: 17-May-2012

La información contenida en este sitio es responsabilidad única de su autor o
quien la publica y se deslinda al CIO de toda responsabilidad al respecto.