Photonic crystal (PhC) cavities made in broadband luminescent material offer attractive possibilities for flexible active
devices. The luminescence enables the cavity to operate as an autonomous entity. New applications of this property are
demonstrated for cavities made in the InGaAsP underetched semiconductor membrane with embedded InAs Quantum
Dots that emit in the range of 1400-1600 nm.
Planar photonic crystal membrane nanocavities were released from the parent chip by mechanical nanomanipulation.
The released cavity particle could be bonded on an arbitrary surface, which was exploited to make a novel fiber-optic tip
sensor with a PhC cavity attached to the tip.
A single mode from a short cavity is shown to couple simultaneously to at least three cavity modes of a long cavity, as
concluded from level anticrossing data when the small cavity was photothermally tuned.
Reconfigurable and movable cavities were created by locally varying the infiltration status by liquid oil near a PhC
waveguide or defect cavity. Liquid was displaced locally on a micron scale using capillary force effects or laser-induced
evaporation and condensation phenomena.
Results are presented on the use of InGaAsP photonic crystal nanobeam slot waveguides for refractive index
sensing. These sensors are read remote-optically through photoluminescence, which is generated by built-in InGaAs
quantum dots. The nanobeams are designed to maximize the electromagnetic field intensity in the slot region, which
resulted in record-high sensitivities in the order of 700 nm/RIU (refractive index unit). A cavity, created by locally
deflecting the two beams towards each other through overetching, is shown to improve the sensitivity by about 20%.
Liquid crystal (LC, Merk 5 CB) is infiltrated into active, InAs quantum dots embedded, InGaAsP membrane type
nanocavities to investigate the possible effect of the LC orientation on active cavity tuning. The tuning is demonstrated
thermally and thermo-optically. The thermal tuning showed that the cavity modes can be tuned in opposite directions and
exhibits a sudden change at the clearing temperature. The mechanism relies on the existence of both ordinary and
extraordinary refractive indices of the liquid crystal due to its molecular alignment inside the voids. It shows that the
electric field distribution of cavity modes can have a substantial component parallel to the LC director. The average
electric field orientation with respect to the LC orientation can be mode dependent, so that different modes can be
dominated by either branch of the LCs refractive index. Thermo-optic tuning of the modes is obtained when the power of
the excitation laser is increased from 40 μW to 460 μW. A large and a reversible blueshift of more than 10 nm of the
cavity modes is observed which is attributed to temperature induced liquid transport. InGaAsP type of nanocavities,
without InAs quantum dots were infiltrated with PbSe colloidal quantum dots to obtain a comparison of internal light
sources either in the semiconductor or in the holes.
Hexagonal symmetry InGaAsP membrane type cavities with embedded InAs quantum dots as active emitters were investigated by room temperature photoluminescence experiments at wavelengths near 1.50 µm. Cavities consisting of simple defects of just removing one or seven air holes were studied as well as modified cavities with additional holes decreased in size and shifted in position. The latter include the H0 cavity, in which only two adjacent holes were modified, but none removed. Low-Q cavity modes were observed for the simple cavities while high-Q modes were observed after modification of the surrounding holes. The resonant frequencies were varied over a large range of lithographic parameters both by changing the lattice spacing or the size of the modified holes. More than 15 nm reversible dynamic optical tuning of the resonance modes was observed by changing the applied laser power up to 5 mW. For thermo-optic tuning, this corresponds to a heating of up to 200 °C.
Photonic crystal (PC) devices in the InP/InGaAsP/InP planar waveguide system exhibiting narrow bandwidth
features were investigated for use as ultrasmall and tunable building blocks for photonic integrated circuits at
the telecom wavelength of 1.55 μm. The H1 cavity, consisting of a single PC-hole left unetched, represents
the smallest possible cavity in a dielectric material. The tuning of this cavity by temperature was investigated
under the conditions as etched and after the holes were infiltrated with liquid crystal (LC), thus separating the
contributions of host semiconductor and LC-infill. The shift and tuning by temperature of the MiniStopBand
(MSB) in a W3 waveguide, consisting of three rows of holes left unetched, was observed after infiltrating the PC
with LC. The samples finally underwent a third processing step of local wet underetching the PC to leave an
InGaAsP membrane structure, which was optically assessed through the ridge waveguides that remained after
the under etch and by SNOM-probing.
Glycerol/water microdroplets take almost spherical shapes when standing on a superhydrophobic surface. Hence they are suitable to function as optical microcavities. Using Rhodamine B doped water microdroplets, large spectral tunability of the whispering gallery modes (WGMs) (>5 nm) was observed. Tunability was achieved by evaporation/condensation in a current controlled mini humidity chamber. Experiments revealed a mechanism stabilizing the volume of these microdroplets with femtoliter resolution. The mechanism relied on the interplay between the condensation rate that was kept constant and the size dependent laser induced heating. The radii of individual water microdroplets (>5 µm) stayed within a few nanometers during long time periods (several minutes). By blocking the laser excitation for 500 msec, the stable volume of individual microdroplets were changed stepwise. Laser emission was also observed from Rhodamine B doped glycerol/water microdroplets using a pulsed, frequency-doubled Nd:YAG laser (=532 nm) as the excitation source. The observed largely tunable WGMs and laser emission can pave way for novel applications in optical communication systems. Besides due to the sensitivity of the WGMs to the size and shape of the microdroplets, the results can find applications in characterizing superhydrophobic surfaces and investigating liquid-solid surfaces.
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