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A photonic crystal cavity-optical fiber tip nanoparticle sensor for biomedical applications
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Image of FIG. 1.
FIG. 1.

(a) Tilted SEM images of a completed GaAs fiberPC device. The two small blurry circles are where epoxy has been applied. The circular semiconductor template consists of an outer release region surrounding a central photonic crystal of size 20 × 25 μm. At the center of the PC is an L3 defect cavity as seen in the second inset. (b) Schematic of the optical setup for measuring nanoparticles. Pump light from a laser diode is sent to the fiberPC tip through a custom 830/1300 nm WDM where it is absorbed above band by the GaAs semiconductor. Internal QDs embedded in the GaAs membrane emit PL both outward from the device and back into the fiberPC core where it can be subsequently detected by a spectrometer.

Image of FIG. 2.
FIG. 2.

(a) PL spectrum of a fiberPC device in air and prior to testing when pumped at 250 μW. Optical image on the right shows the fiber tip face before testing. Arrows indicate epoxy droplets. (b) PL spectrum of the same sensor now in a 12.5 nM gold NP solution. The cavity fundamental mode has redshifted, increased in quality factor, and dropped in intensity. (c) PL spectrum in air again after the device has been retracted with the pump laser turned off during retraction. The spectrum is almost identical to that in (a) and the fiber tip image shows only a slight circular deposition of NPs on the outer rim. (d) PL spectrum in air of the same device but after having retracted the fiber tip with the laser pump turned on. The cavity modes are almost completely removed from the spectrum and a large circular aggregation of nanoparticles (indicated by the arrow) is seen in the microscope image (enhanced online). [URL: http://dx.doi.org/10.1063/1.4719520.1] [URL: http://dx.doi.org/10.1063/1.4719520.2] [URL: http://dx.doi.org/10.1063/1.4719520.3]10.1063/1.4719520.110.1063/1.4719520.210.1063/1.4719520.3

Image of FIG. 3.
FIG. 3.

(a) SEM images of a fiberPC sensor with a metal NP aggregation on the cavity. Inset shows a close-up image. (b) Schematic model of the nanoparticle aggregation effect. A solution droplet on the fiber tip begins to evaporate as indicated by the outward flowing arrows (the contact angle of the droplet here is exaggerated for clarity). Meanwhile, part of the optical pump transmits through the thin photonic crystal membrane. Water and nanoparticles in the weak focus absorb the pump laser light, raising the local temperature of the water and setting up hydrothermal gradients. Combined with evaporation, the hydrothermal gradients create Marangoni convective flow which circulates fluid in a toroidal pattern (shown by the circles). This circulation propels NPs into the center of the droplet where they begin to aggregate most likely due to photochemical processes. Eventually, all the water in solution evaporates and only a deposition of NPs on the cavity remains. During this whole process, the quantum dot PL back-coupled into the fiber is observed with a spectrometer.

Image of FIG. 4.
FIG. 4.

(a) Pump-power dependent wavelength shift of the fiberPC sensor from Fig. 2 now in a 0.8 nM solution of metal NPs. (b) Concentration dependent wavelength shift of a different sensor when the pump power is held at 2.45 mW. (c) PL spectrum of a different device before testing and associated optical microscope image. (d) PL spectrum of the same device as in (c) after retraction with a 12 μW pump laser turned on in a 25 nM metal NP solution and associated image.

Image of FIG. 5.
FIG. 5.

(a) PL spectrum of a fiberPC device and the associated optical image of the fiber face prior to testing with ferumoxytol. (b) PL spectrum of the sensor after retraction from a 400 μg/mL solution of ferumoxytol with a 1.75 mW pump. The spectrum changes are similar to those caused by metal NPs with a large redshift, reduction in Q-factor, and reduction in peak intensity. The optical image shows a circular aggregate at the fiber center as seen previously. (c) Close-up SEM image of a ferumoxytol aggregate. The scale bar is 2 μm.



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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: A photonic crystal cavity-optical fiber tip nanoparticle sensor for biomedical applications