br pipetting L BP nanosheets
pipetting 30 μL BP nanosheets solution into microchannel container to cover the fiber. When the solvent was evaporating, BP nanosheets were being gradually adsorbed on the fiber surface. Such evaporating and drying processes enhanced the physisorption between BP nanosheets and fiber surface. After five cycles of deposition, the BP-coated TFG was placed in a vacuum drying oven at 40 °C for 12 h to enhance the ad-hesion between BP nanosheets and fiber and to improve the uniformity of BP overlay.
2.5. Measurement system and data analysis
Renishaw Raman Microscope 1000 (with 632.8 nm light) was em-ployed to characterize Raman spectra of BP material. The thickness of BP nanosheets and BP deposited overlay were measured by AFM (Veeco Instruments Inc., di Dimension 3100). The surface coverage of BP-fiber was measured by SEM (Hitachi, S-520).
For optical interrogation system, broadband light source (BBS: Agilent HP83437A, Agilent Technologies Inc.) was used along with an optical spectrum analyzer (OSA, Agilent HP86142A, Agilent
Technologies Inc.). The OSA was connected to a computer and the optical spectra were recorded by a customized program. Data analysis was performed using the customized program which automatically defined resonance wavelength using the centroid calculation method.
All biochemical procedures were performed in a fume cupboard. To minimize the cross-sensitivities of temperature and bend, all experi-ments were conducted at a controlled room temperature of 22.0 ± 0.1 °C unless specified otherwise, and the fiber device was placed straightly in a custom-made microchannel container and all the chemicals and solvents were added and withdrawn by careful pipetting.
3. Results and discussion
3.1. BP-fiber surface morphological characterization
The surface morphological characterization was verified by AFM, SEM and Raman spectroscopy. As shown in AFM tapping mode topo-graphic images (Fig. 3b and c), an overlay has been successfully de-posited on the cylindrical fiber surface where a step boundary is clearly
where ncoeff, i
presented between bare and coated-sections. The height profile (inset in Fig. 3c) gives the BP overlay thickness of ∼202.8 nm after five coating cycles. The surface was further examined by SEM (Fig. 3d) to show a homogeneous coating over entire cylindrical fiber surface demon-strating a high-quality deposition. Raman spectra (Fig. 3e) confirm that the overlay is BP as its spectrum after the deposition maintaining the same characteristic peaks as BP nanosheets.
The developed i-LbL deposition technique secured high-quality nano-coating deposited on non-planar substrate with strong adhesion as well as a precise thickness control. By taking the flexibility of i-LbL approach, the overlay thickness can be precisely controlled by adjusting the deposition conditions, such as dispersion concentration, quantity, number of coating cycles, Angeli’s Salt time, or their combination.
3.2. BP-induced strong optical modulation and enhanced light-matter interaction
Due to the birefringence caused by the largely titled fringes in fiber core, the 82°-TFG exhibits intrinsic optical polarization properties. When the light is launched into fiber core and experiences the largely tilted structures, fauna is coupled to the forward-propagating cladding modes which appear two sets of comb-like split resonances in trans-mission spectra at discrete wavelengths given by the following phase-matching conditions (Erdogan, 1997):
is the eﬀective refractive index of fiber core, i represents
transverse-magnetic (TM) or transverse-electric (TE) polarization state,
is the mth eﬀective refractive index of cladding, Λg is the normal grating period, and θ is the tilted angle of grating.
An 8 mm-long 82°-TFG was UV-inscribed in a BGe single mode fiber. Fig. S1(a) shows an optical image of 82°-tilted fringes by the use of a high magnification microscope with oil immersion. Fig. S1(b) depicts the transmission spectra of 82°-TFG exhibiting two sets of comb-like polarization resonances. With the randomly polarized light launched, both sets exhibited ∼3 dB loss (∼50%) as the light was equally coupled to two orthogonal birefringence modes. When the input light was lin-early polarized in either the tilt plane or perpendicular to it, the comb-like cladding mode resonances became predominantly polarized ra-dially or azimuthally with fully coupled strength.
The light-matter interaction has been significantly enhanced by BP deposition with diﬀerent BP thickness. The optical properties were observed by monitoring the TFG transmission spectra in real-time during multi-cycle BP deposition. As plotted in Fig. 4a, both TM- and TE-modes demonstrate the trends with red-shift in wavelength and decrease in intensity while BP overlay thickness increased during the coating process. As shown in Fig. 4b, TE resonant intensity weakens 11.53 dB which is 216% larger than that of TM mode after five coating cycles (with BP overlay thickness of ∼202.8 nm), where the value of coeﬃcient of variation of intensity change at diﬀerent cycles for TM and TE mode is between 0.03 and 0.07. During the same process, the TE resonant wavelength shifts 2.12 nm, which is only 75% that of TM mode (Fig. S2). It should be emphasized that diﬀerent eﬀects between two orthogonal modes indicate BP-induced polarization-dependent modulation as well as thickness-tunable optical characteristics. By taking the advantage of in-fiber grating configuration, it might provide a versatile and practicable method for the overlay thickness control by simply monitoring the intensity change of grating resonance.