Cavity optomechanics in vertically-coupled photonic crystal nanobeams and membranes

  • S. Westendorp

Student thesis: Master


In the last decade the field of optomechanics has emerged that explores the interactionbetween light and mechanical motions. Photonic crystal cavities are promising for observinglarge optical forces, because of their large quality factors and very small modevolumes. In addition the use of multilayer photonic crystal (PhC) cavities could introducean additional contribution to the optical forces due to a mechanical modulation ofthe coupling strength between the cavities and thus the splitting of cavity modes.Optomechanical effects in vertically-coupled PhC nanobeams and membranes are studiedin this work. First, a general description of the theory behind optomechanics is givenand an original derivation of the expression for the optical force in the case of dissipativecoupling is given. Then the experimental results in two different systems are shown. The first system consists of two gallium arsenide (GaAs) PhC nanobeams that are vertically-coupled.These are characterized and devices with optical and mechanical quality factorsup to Q = 1300 and QM = 1900 are found. The optical spring effect is also observed inthese nanobeams and from this effect optomechanical vacuum coupling rates in the orderof tenths of MHz are calculated, close to record values reported for other photonic crystaldevices. The second system consists of two vertically coupled GaAs PhC membranes thatare suspended in air. Along the center axis of the membrane a line defect (waveguide)with a L3 cavity in the center is created in the PhC. The cavity is coupled via the PhCwaveguides and suspended nanobeams to double ridge waveguides, which can guide lightfrom and to the edge of the sample. The losses in these gallium arsenide / aluminumgallium arsenide double ridge waveguides (RWG) are investigated and waveguides withreasonably small losses around 15 dB/cm are attained. Coupling into this double RWG is achieved with an efficiency of 1%. A preliminary characterization of the vertically-coupledmembranes show the existence of fundamental symmetric and anti-symmetricmodes with optical quality factors up to Q = 1900. Photoluminescence spectra collectedfrom the side of the double-membrane PhC cavities, for non-resonant pumping from thetop, show that the fundamental anti-symmetric cavity mode can couple to and propagatethrough the double ridge waveguides and fibers more efficiently than the symmetric one.Furthermore, resonances from the slow-light region of two waveguide modes have beenobserved both from the top and the side collection.
Date of Award31 Aug 2014
Original languageEnglish
SupervisorAndrea Fiore (Supervisor 1) & Rob W. van der Heijden (Supervisor 2)

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