Over the past decades, people have shown high interests in measuring material properties through various optical property variations. Recently, index sensors based on PhC have been demonstrated and could detect an area of only 1 μm2 which have been discussed in Chapter 1. In this thesis, we propose a PhC HSE microcavity optical index sensor with high sensitivity and small detectable refractive index.
In order to obtain high Rn value, the electrical-field of resonance mode should be extended into the air. Therefore, we applied PWE method and FDTD method to simulate the properties of slab-edge mode microcavity in Chapter 2. First we discuss the typical slab-edge-mode (SLEM) microcavity with different truncated facets. We obtain the highest Rn value of 744 as τ=0.8 and the minimum Δndet of 2.39×10-3 as τ=0.3. But obviously, this Δndet value is limited by the low Q factor of only 1,021.
Hence, we propose high Q PhC hetero-slab-edge (HSE) microcavities formed by two and three different truncated facets, and gradual barrier formed by shrinking and shifting air holes, where the high Q surface mode is confined by mode-gap effect. By optimizing the gradual barrier in 3D FDTD simulation, we obtain high Q factor of 6.6
×105 from PhC gradual barrier HSE microcavity. Besides, high index sensing response Rn and small detectable index variation Δndet of 625 nm/RIU and 3.6×10-6 are obtained in simulations. This value indicates that device could detect the variation of CO2 concentration in air about 2%.
The real devices are fabricated on an epitaxial structure consisting of four 10 nm
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InGaAsP quantum wells by EBL and a series of etching process. After the fabrication process, the devices are optically pumped by a micro-PL system. There are the best measurement results of PhC gradual barrier HSE microcavity. The surface mode lasing action with high Q factor and low threshold of 6400 and 0.55 mW is obtained from the real devices.
For index sensing application, we put the real device of the HSE microcavity formed by three different truncated facets into CO2 chamber. The different environmental refractive index could be varied by controlling different gas pressures.
According to this method, we obtain a 0.29 nm blue shift when the pressure from 1.24 atm (n=5.37×10-4) to 0.75 atm (n=3.25×10-4), which corresponds to a large Rn value of 1368 nm/RIU and the calculated Δndet is as small as 2.4×10-4. Both values indicate the great potential of PhC HSE microcavity serving as a high-sensitivity optical index sensor with very condensed device size, and which will be potential in applying in biological, chemical, and medical sensors in the future.
In the future, we expect our device can be applied in biological. For our active device, the Q factor will suddenly degrade when the samples are soaked in liquid.
Because the index contrast between dielectric region and liquid is too small and the TIR confinement will be weak. Thus, we can demonstrate a passive device to solve this difficulty in the future. The passive device could be operated in continuous wave condition, and the Rn value could be obtained from resonance peak shift.
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