Three-dimensional photoconductive transducer for terahertz signals or picosecond electrical pulses
A photoconductive transducer intended to generate or detect waves in the terahertz frequency domain or in the picosecond pulse domain is provided. The transducer comprises a three-dimensional structure that includes, in this order, a first planar electrode, an array of nano-columns embedded in a layer of resist and a second planar electrode parallel to the first planar electrode. The design of the transducer increases the optical-to-terahertz conversion efficiency by means of photonic and plasmonic resonances and by means of high and homogeneous electric fields. The height of the nano-columns as well as the thickness of the resist range between 100 nanometres and 400 nanometres. The width of the nano-columns is between 100 nanometres and 400 nanometres, the distance between two adjacent nano-columns is between 300 nanometres and 500 nanometres, the nano-columns are made of a III-V semiconductor. The second electrode is transparent, so as to allow the transmission of a laser source towards the photo-absorbing nano-columns.
1. A Photoconductive transducer intended to generate or detect waves in the terahertz frequency domain or in the picosecond pulse domain, wherein photoconductive transducer comprises—a three-dimensional structure that comprises a first planar electrode (E 1 ), second planar electrode (E 2 ) parallel to the first planar electrode, and an array of identical nano-columns (C) embedded in a layer of resist (R) situated between the first and the second planar electrodes, the resist and the second planar electrode being transparent at a given wavelength in the visible or in the near infrared region of the electromagnetic spectrum, the height of the nano-columns as well as the thickness of the resist ranging between 100 nanometres and 400 nanometres, the width of the nano-columns being between 100 nanometres and 400 nanometres, the distance between two consecutive nano-columns being between 300 nanometres and 500 nanometres, the nano-columns made of a III-V semiconductor and the top part of each nano-column comprising a metal contact (CE) that is electrically connected to the second electrode.
2. The photoconductive transducer according to claim 1 , wherein the width of the columns, the distance separating two adjacent columns and the refractive index of the resist are chosen in such a way that illuminating the photoconductive transducer at the given wavelength through the second planar electrode excites: guided optical photonic modes propagating through the heterogeneous layer consisting of the polymer resist and of the array of nano-columns; plasmonic resonances at the upper and lower surfaces of the structure; and
resonant cavity modes inside the nano-columns and in the vertical direction between the two electrodes.
3. The photoconductive transducer according to claim 1 , wherein the area of the three-dimensional structure is comprised between 1 μm 2 and 1000 μm 2 .
4. The photoconductive transducer according to claim 1 , wherein the second electrode is made of indium-tin oxide.
5. The photoconductive transducer according to claim 1 , wherein the cross section of the nano-columns is rectangular or circular or polygonal.
6. The photoconductive transducer according to claim 1 , wherein the material of the nano-columns is a III-V semiconductor chosen among gallium arsenide or indium-gallium arsenide or indium phosphide.
7. The photoconductive transducer according to claim 1 , wherein the resist is a negative epoxy photoresist.
8. A terahertz emitter comprising a photoconductive transducer according to claim 1 and a laser (L) that emits at said defined wavelength, the laser being arranged so as to irradiate the array of columns through the aforementioned second electrode, and means for establishing a potential difference between the first and second electrode.
9. A terahertz receiver comprising a photoconductive transducer according to claim 1 , a laser (L) that emits at said defined wavelength, the laser being arranged so as to irradiate the array of columns through the aforementioned second electrode, and a voltmeter (V) that measure the output signal resulting from a photogenerated current resulting from irradiation from the laser and an incoming Terahertz radiation (RT).
10. Process for producing a photoconductive transducer according to claim 1 , characterized by the production of the three-dimensional structure comprises the following steps:
etching the array of nano-columns (C) in a substrate made of III-V semiconductor;
depositing a metal layer (E 1 , CE) on the upper surface of the nano-columns and on the lower surface of the substrate that bears the nano-columns, the top of the nano-columns thus comprising of a metal contact (CE);
spin coating a layer of negative epoxy photoresist (R) to cover the aforementioned array of columns;
exposing the aforementioned layer of resist with an electron beam or UV lithography;
polishing or etching the aforementioned layer of resist until the metal contacts (CE) at the top-end of the nano-columns appear;
depositing a transparent metal layer (E 2 ) on the layer of resist so as to connect the various metal contacts (CE).