PHOTONICS TERAHERTZ EMITTER BASED ON A HIGLY DISTRIBUTED PHOTOMIXER PUMPED BY A DUAL FREQUENCY LASER – PHENIX

In PHENIX, we propose to study a new architecture of photomixers in order to develop a wideband and powerful photomixing source. It is based on a Highly Distributed Photoconductor (HDP). The HDP, through its highly distributed nature (i.e a length reaching a few THz wavelengths), will offer both high-frequency and high-power operation unreachable with vertically illuminated photodetectors or standard distributed photodetectors. A photodetector of length reaching 1 mm will absorb up to 1 W of optical power, a value at least ten times higher than the one in the current high frequency photodetectors. An output power of around 10 mW at 300 GHz and 1 mW at 1 THz is expected with such a wide band CW THz source.
In PHENIX we propose to develop a compact beatnote source working at lambda=800 nm, which will offer a high beat frequency stability and spectral purity. It will be based on a Solid state Dual Frequency Lasers (DFL) using a titanium doped sapphire as active medium and exhibiting an intrinsically narrow beatline (<30 kHz in free regime).

Traveling wave photomixer:

-50 % coupling efficiency demonstrated at 800 nm.
-Silicon nitride waveguide: 2.3 dB/mm loss at 800 nm
- 50/50 MMI coupler : insertion loss =5.4 dB.

Dual Frequency Laser (FOTON)

-Two Laser oscillations demonstrated silmutaneously in a same Ti :Sa crystal
-Monofrequency oscillation occuring with a unique etalon

First demonstration of the THz source forseen at T0+48

1. Highly distributed photoconductor for CW THz generation Proceedings of 41st International Conference on Infrared, Millimeter and Terahertz Waves, PEYTAVIT E.
IRMMW-THz 2016, Copenhagen, Denmark, september 25-30, 2016, paper T5P.19.04, 2 pages, ISBN 978-1-4673-8486-5 ; e-ISBN 978-1-4673-8485-8
2. « Photomixing THz sources: past, present and future »
PEYTAVIT E.
5th International Biennial Nanomaterials and Nanotechnology Meeting, NOM 2017, Ostrava, Czech Republic, may 23-25, 2017

A large number of chemical species have absorption lines lying in the Terahertz frequency range, which is thus a very interesting spectral range for molecular spectroscopy applied to earth, planetary, environmental and space science. In spite of this great scientific interest, the THz range still remains one of the less used spectral band. It is indeed called the THz gap because of the lack of low cost, compact and reliable sources and detectors as compared to its two adjacent frequency ranges, i.e. the microwave and the optical waves, which rely respectively on the efficiency of the photonics and electronics technology.

A dramatic improvement of the detectors performances have been done in the last years, motivated by the emergence of new applications such as security or medical imaging, DNA study or even painting analysis. Background limited THz detectors are now available. On the other hand, the generation of THz wave remains a challenge and a room-temperature powerful, narrow linewidth, and compact THz source is still needed. Spectral purity is indeed as important as power level. It is obvious when a heterodyne mixer is used as receiver, whose minimum detectable power is related to the frequency width of the intermediate frequency filter.

One of the most promising THz continuous-wave (CW) sources working at room temperature is based on the photodetection of the beating frequency generated by two spatially overlapped infrared lasers. It is the so called photomixing THz source. This down converting between very high frequency (~300 THz) infrared lasers is intrinsically wideband. Nevertheless, THz sources based on photomixing suffer, up to now, from a lack of power, around 10 µW at 1 THz. The output power is indeed limited by the trade-off between the small size of the photodetector used in order to minimize its electrical capacitance and the photocurrent needed in order to generate a high output power. A new type of photomixer is required in order to break this trade-off and reach the milliwatt level at 1 THz.

In PHENIX, we propose to study a new architecture of photomixers in order to develop a wideband and powerful photomixing source. It is based on a Highly Distributed Photoconductor (HDP). The HDP, through its highly distributed nature (i.e a length reaching a few THz wavelengths), will offer both high-frequency and high-power operation unreachable with vertically illuminated photodetectors or standard distributed photodetectors. A photodetector of length reaching 1 mm will absorb up to 1 W of optical power, a value at least ten times higher than the one in the current high frequency photodetectors. An output power of around 10 mW at 300 GHz and 1 mW at 1 THz is expected with such a wide band CW THz source.

Besides, the achievement of an efficient THz CW source through photomixing technique relies on the availability of a narrow linewidth optical beatnote at the frequency of interest and at the absorption wavelength of the photomixer (~800 nm). The straightforward solution consists of using two laser diodes since such diodes are commercially available and can be easily implemented in the system. This solution however suffers from a limited frequency stability of the beat note and a poor spectral purity.

In PHENIX we propose to develop a compact beatnote source working at lambda=800 nm, which will offer a high beat frequency stability and spectral purity. It will be based on a Solid state Dual Frequency Lasers (DFL) using a titanium doped sapphire as active medium and exhibiting an intrinsically narrow beatline (<30 kHz in free regime).

We have recently performed a preliminary demonstration. Nevertheless, several improvements are still required to end up with a compact continuously tunable and robust laser.