8 Femtosecond Laser Direct Writing for 3D Microfluidic Biochip …
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and the microlenses were fabricated simultaneously using a single FLAE procedure,
after which the WG was integrated by fs laser direct writing. This design significantly enhanced the sensitivity of both fluorescence and absorption measurements.
Whispering-gallery-mode (WGM) microcavities are considered to show exceptional
promise with regard to high-sensitivity sensing applications due to their very high
Q factors and small mode volumes. Song et al. employed FLAE of fused silica to
fabricate a 3D optofluidic device in which a microfluidic structure and an optical
microresonator with a Q-factor of 3.21 × 10
5 in air were monolithically integrated
[79]. The fabrication process for this optofluidic device consisted of three main steps:
(1) FLAE of fused silica to create a 3D microfluidic channel embedded in glass and
a microdisk structure at the outlet of the microchannel; (2) selective reflow of the
silica disk structure by CO 2 laser irradiation to create a microresonator with a high
Q-factor; and (3) assembly of a fiber taper to the resonator by CO 2 laser welding. The
performance of the device (Fig. 8.3e) was demonstrated by measuring the refractive
index of purified water containing a very low concentration of NaCl, and a detection
limit of approximately 1.2 × 10
−4 RIU (refractive index units) was determined.
Another important application of optofluidic devices fabricated by fs laser 3D
processing is the investigation of the gliding mechanism of Phormidium moving
toward a seedling root. Phormidium gliding is a useful means of accelerating
vegetable growth through the formation of endosymbiotic associations. For this
investigation, Hanada et al. first fabricated a T-shaped microfluidic channel formed
in Foturan glass having three reservoirs at its ends [22]. When the Phormidium was
introduced into one reservoir and a seedling root into another, the Phormidium was
found to always glide toward the seedling rather than toward a third, empty reservoir. In contrast, filling the third reservoir with carbonic water varied the direction
of movement of the Phormidium depending on the carbonic concentration. Additionally, at a critical CO 2 concentration, the cyanobacterium glided neither toward
the seedling root nor toward the carbonic water, indicating that CO 2 secreted by
respiration of the root is a possible attractant for the microorganism. To confirm
this hypothesis and determine the quantity of CO 2 secreted by the seedling roots,
an optofluidic device based on integration of a straight microfluidic channel and
WGs were subsequently fabricated. Figure 8.4a shows the schematic of the system.
After fabricating a channel in the Foturan glass by FLAE, two WGs that intersected
the center of the microfluidic channel were written. The channel was filled with
water containing a pH indicator (bromothymol blue (BTB) solution) and white light
from a halogen lamp was coupled to the entrance facet of WG I by an objective
lens. The white light transmitted by WG I passed through the microchannel, which
was filled with a liquid sample, and was then coupled into WG II. The light transmitted by WG II was coupled into a spectrometer by another objective lens to allow
collection of the absorption spectrum (Fig. 8.4a). The absorbance was calculated by
subtracting the spectrum obtained with the sample in the microfluidic channel from
that observed without the sample. The green line in Fig. 8.4b indicates the absorption spectrum of the water containing the BTB solution, having an intense absorption
peak at approximately 620 nm. The intensity of this peak decreased with increasing
CO 2 concentration in the water due to the concurrent change in pH. The spectrum of
257
and the microlenses were fabricated simultaneously using a single FLAE procedure,
after which the WG was integrated by fs laser direct writing. This design significantly enhanced the sensitivity of both fluorescence and absorption measurements.
Whispering-gallery-mode (WGM) microcavities are considered to show exceptional
promise with regard to high-sensitivity sensing applications due to their very high
Q factors and small mode volumes. Song et al. employed FLAE of fused silica to
fabricate a 3D optofluidic device in which a microfluidic structure and an optical
microresonator with a Q-factor of 3.21 × 10
5 in air were monolithically integrated
[79]. The fabrication process for this optofluidic device consisted of three main steps:
(1) FLAE of fused silica to create a 3D microfluidic channel embedded in glass and
a microdisk structure at the outlet of the microchannel; (2) selective reflow of the
silica disk structure by CO 2 laser irradiation to create a microresonator with a high
Q-factor; and (3) assembly of a fiber taper to the resonator by CO 2 laser welding. The
performance of the device (Fig. 8.3e) was demonstrated by measuring the refractive
index of purified water containing a very low concentration of NaCl, and a detection
limit of approximately 1.2 × 10
−4 RIU (refractive index units) was determined.
Another important application of optofluidic devices fabricated by fs laser 3D
processing is the investigation of the gliding mechanism of Phormidium moving
toward a seedling root. Phormidium gliding is a useful means of accelerating
vegetable growth through the formation of endosymbiotic associations. For this
investigation, Hanada et al. first fabricated a T-shaped microfluidic channel formed
in Foturan glass having three reservoirs at its ends [22]. When the Phormidium was
introduced into one reservoir and a seedling root into another, the Phormidium was
found to always glide toward the seedling rather than toward a third, empty reservoir. In contrast, filling the third reservoir with carbonic water varied the direction
of movement of the Phormidium depending on the carbonic concentration. Additionally, at a critical CO 2 concentration, the cyanobacterium glided neither toward
the seedling root nor toward the carbonic water, indicating that CO 2 secreted by
respiration of the root is a possible attractant for the microorganism. To confirm
this hypothesis and determine the quantity of CO 2 secreted by the seedling roots,
an optofluidic device based on integration of a straight microfluidic channel and
WGs were subsequently fabricated. Figure 8.4a shows the schematic of the system.
After fabricating a channel in the Foturan glass by FLAE, two WGs that intersected
the center of the microfluidic channel were written. The channel was filled with
water containing a pH indicator (bromothymol blue (BTB) solution) and white light
from a halogen lamp was coupled to the entrance facet of WG I by an objective
lens. The white light transmitted by WG I passed through the microchannel, which
was filled with a liquid sample, and was then coupled into WG II. The light transmitted by WG II was coupled into a spectrometer by another objective lens to allow
collection of the absorption spectrum (Fig. 8.4a). The absorbance was calculated by
subtracting the spectrum obtained with the sample in the microfluidic channel from
that observed without the sample. The green line in Fig. 8.4b indicates the absorption spectrum of the water containing the BTB solution, having an intense absorption
peak at approximately 620 nm. The intensity of this peak decreased with increasing
CO 2 concentration in the water due to the concurrent change in pH. The spectrum of