External Cavity Laser Based Upon Metasurfaces

The QC-VECSEL can generate high quality beams with large powers in continuous-wave mode in the THz frequency range, where existing laser sources have problems achieving.Similar concept using active metasurfaces can be applied for utilities in the mid-infrared frequency ranges as well. The metasurface reflector forms part of the external laser cavity such that multiple THz QC-laser sub-cavities are locked to a high quality (high Q) cavity mode, allowing for scalable power combining.Near-diffraction-limited circular TEM00 beams are sustained by external cavities and spatial filtering to prevent higher order mode oscillation. The antenna metasurface is strongly coupled to radiation for high optical coupling efficiency.Sparse areal coverage of the active material in the metasurface reduces the areal power dissipation density over larger active areas for high power and good cw performance.Enables external cavity laser tuning schemes.The antenna coupled metasurface can be engineered to provide spatially dependent gain, phase, or polarization response. This can enable more efficient model/gain coupling, reflectarray focusing for compact cavities, or beams with customized polarization.

THz laser sources are useful for THz imaging and spectroscopy applications: o   Spectroscopy and multi-spectral imaging of materials such as explosives and drugs o   Non-destructive evaluation imaging of films and coatings for industrial process control or corrosion detection Other possible applications include: o   Biomedical THz imaging o   THz communications (i.e. civil aviation and military applications based on THz radiation’s superior performance under poor weather conditions) o   THz imaging for security screening (i.e. enhancement of chemical, biological, nuclear, radiological and/or explosive detection) o   Detection of astrophysical objects

Researchers at UCLA have developed a THz QC-VECSEL formed by an active metasurface reflector and a flat output coupler reflector. The innovative active metasurface reflector is comprised of a sparse array of antenna-coupled THz QC-laser active sub-cavities allowing scalable power combining. A prototype of this THz QC-VECSEL is able to produce approximately 6 mW of peak power, a highly directive beam with approximately 5 degree full width half max divergence angle, and a close to Gaussian profile.

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State Of Development

Prototype of the described THz QC-VECSEL has been successfully tested. 

Background

It is challenging for many types of semiconductor lasers to achieve a symmetric, directive, diffraction-limited, and non-astigmatic high quality beam. This is particularly true as the waveguide size is increased to scale up the power, since multiple transverse mode oscillations can lead to sub-optimal beam quality as well as modal instabilities. Beam quality poses a more difficult challenge for terahertz (THz) quantum-cascade (QC) lasers because THz QC-lasers almost exclusively use sub-wavelength metallic and/or plasmonic waveguides, which leads to highly divergent beams with large side-lobes unless special surface-emitting or antenna coupled geometries (i.e. 2nd order distributed feedback (DFB) and photonic crystal cavities, arrays of 2nd order DFBs, and end-fire 3rd oder DFBs) are used. Strategies for further beam narrowing are limited as the beam divergence increases as the square root of the device length.

The vertical-external-cavity surface-emitting-laser (VECSEL) approach has demonstrated to deliver near diffraction-limited beam quality even at high output powers when used in the visible and near-infrared spectral range. However, it has been impossible to implement VECSEL for QC-lasers since the intersubband selection rule prevents interaction of the gain medium with surface incident beam. Meanwhile, THz QC-lasers that operate with both high output power and excellent beam quality are sorely needed for a range of spectroscopy and imaging applications. 

Related Materials

Xu, L., Curwen, C.A., Hon, P.W.C., Chen, Q., Itoh, T., Williams, B.S. (2015). Metasurface external cavity laser. Applied Physics Letters, 107, 221105-1 – 221105-5
Xu, L., Curwen, C.A., Hon, P.W.C., Itoh, T., Williams, B.S. (2016). Terahertz quantum cascade VECSEL. Vertical External Cavity Surface Emitting lasers (VECSELs) VI, SPIE Proceedings, 9734, 97340G-1 – 97340G-6

Additional Technologies by these Inventors

• Focusing And Amplifying Reflectarray Metasurfaces For Stable Laser Cavities

Tech ID/UC Case

27109/2015-797-0

Related Cases

2015-797-0

美国

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