Abstract
The terahertz (THz) spectral region, covering frequencies from 1 to 10 THz, is highly interesting for chemical sensing. The energy of rotational and vibrational transitions of molecules lies within this frequency range. Therefore, chemical fingerprints can be derived, allowing for a simple detection scheme. Here, we present an optical sensor based on active photonic crystals (PhCs), i.e., the pillars are fabricated directly from an active THz quantum-cascade laser medium. The individual pillars are pumped electrically leading to laser emission at cryogenic temperatures. There is no need to couple light into the resonant structure because the PhC itself is used as the light source. An injected gas changes the resonance condition of the PhC and thereby the laser emission frequency. We achieve an experimental frequency shift of 10−3 times the center lasing frequency. The minimum detectable refractive index change is 1.6 × 10−5 RIU.
Highlights
Optical microsensors based on high-Q cavities are very attractive for a wide range of chemical, biomedical or environmental applications
We demonstrate an integrated optical microsensor with an embedded light source
We have realized a gas sensor operating in the THz spectral region
Summary
Optical microsensors based on high-Q cavities are very attractive for a wide range of chemical, biomedical or environmental applications. They offer high sensitivity, fast system response, low detection limit and simple fabrication. Different types of microresonators are used: cylinders [1], rings [2], toroids [3], spheres [4] and photonic crystals (PhCs) [5]. The mentioned resonators except for PhCs confine the optical mode by total internal reflection; whispering gallery modes are created. In PhC cavities, the mode is confined by multiple Bragg reflections. Both mechanisms ensure a strong modal confinement and a high-Q factor of the resonator
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