Silicon-based Nanobeam Cavity With Polymer Cladding And Its Applications

Due to its high Q factor,small foot-print,small mode volume and no limitation of free spectrum range(FSR),silicon based photonic crystal(PhC)nanobeam(NB)cavity has attracted increasing attention.However,there are still two problems that limit the applications of the silicon based PhC NB cavity in the communication system:first,the resonant wavelength of the silicon based PhC NB cavity will shift as the change of the temperature;second,the resonant wavelength is difficult to be precisely controlled.In this thesis,we solved these two problems by covering the silicon based PhC NB cavity with polymer cladding.Moreover,we proposed the high sensitivity temperature sensor based on cascaded silicon PhC NB cavities.Firstly,to reduce the influence of the temperature on the resonant wavelength,we demonstrated the temperature insensitive lower-index-mode silicon based PhC NB cavity for the first time.The thermo-optical coefficient(TOC)of silicon can be compensated by covering the silicon based PhC NB cavity with polymer cladding.To distribute more resonant mode into the polymer material region with lower-index,we designed the lower-index-mode silicon PhC NB cavity.The measured temperature dependence of the silicon PhC NB cavity is 5.1 pm/°C over 50 ? temperature range,which is only 1/18 to that of the air cladding PhC NB cavity.Furthermore,the foot-print of the device is 13.5 x 0.78 ?m2,which is the smallest silicon based temperature insensitive cavity.Secondly,to precisely control the resonant wavelength,we post-tune the resonant wavelength by controlled electron beam exposure.By exposing the SU-8 cladding with different doses,the thickness of SU-8 can be precisely and individually controlled from 150 nm to 650 nm.Using this method to control the thickness of the SU-8 cladding,the resonant wavelengths of the silicon PhC NB cavities can be modified.We measured the transmission spectrums of the cavities before exposure,after exposure and after development,respectively,and recorded the resonant wavelengths.The resonant wavelength of the silicon PhC NB cavities can be post-tuned as large as 30 nm,which is the largest for once tuning.Thirdly,active thermal tuning can precisely control the resonant wavelength,however,the tuning efficiency is quite low.In order to improve the tuning efficiency,we designed the ultra-low power consumption tunable PhC NB cavity based on suspended ridge waveguide.By suspending the heater and the silicon PhC NB cavity,the heat can be well confined in this region.And the tuning efficiency can be improved to 45.8 pm/[iW,which is the highest efficiency ever reported.Since the heat can be fast conducted into the silicon PhC NB cavity by the silicon slab,the tuning speed will be enhanced.Finally,to improve the sensitivity of the temperature sensor,we designed and fabricated the high sensitivity temperature sensor based on cascaded PhC NB cavities.Due to the high negative TOC of SU-8 and high positive TOC of silicon,as the increase of the temperature,the resonant wavelength of the stack width modulated PhC NB cavity with SU-8 cladding will blue shift;while that for parabolic-beam PhC NB cavity with PMMA cladding will red shift.Thus,the sensitivity of the proposed temperature sensor can be improved greatly.The experimental results show that the sensitivity of the temperature sensor is 162.9 pm/?,which is almost twice as high as that of conventional silicon based resonator sensor.Moreover,the detection limit of the proposed temperature sensor is 0.08 ?.