1. Introduction

Due to the attractive properties such as tight optical confinement, large evanescent field, miniaturized footprint, and tailorable waveguide dispersion, micro/nanowaveguides (MN-WGs) fabricated from a variety of materials have been constantly gaining interests for investigating light-mater interactions [1–3]. Of the well-studied MN-WGs, chemically synthesized semiconductor MN-WGs are promising due to their favorable properties including large refractive index, large nonlinear susceptibility, and material diversity for various spectral ranges. Because of the large refractive index of semiconductors, the reflectivity at the endfaces of MN-WGs is relatively large (e.g. over 0.1) and will offer strong backward reflected waves [4, 5]. The forward and backward waves will overlap and may cause some interesting effects, such as Fabry−Pérot (F−P) microcavity lasers that widely reported before [1, 6]. More recently, we have reported the transverse frequency conversion in semiconductor MN-WGs induced by this overlap effect, with attractive advantages including tunable spatial distributions and broad spectral conversion ranges [7, 8]. Usually in these MN-WGs, the two endfaces act as reflective mirrors, which means that effective cavity lengths are inherent and continuously adjusting their resonance modes are difficult. In past years, using nanoscale gaps, tunable-bandgap compositions, and self-absorption approaches have been reported to realize tunable lasers in semiconductor MN-WGs [9–13]. However, cutting approaches using tungsten probes or focused ion beams are usually needed and will bring damage to MN-WGs.

Evanescent-wave coupling technique using silica microfibers has been proved efficient and compact in various waveguides ranging from glass, semiconductor and polymer to metal structures, and has found wide applications in sensing, photodetection, amplication, and nonlinear optics [3, 14–16]. For example, more recently SnO₂ nanowire-based single-cell endoscopy has been demonstrated by using microfiber/nanowire coupling systems [17]. On the other hand, in these wavelength-scale structures, the large fraction of evanescent wave outside MN-WGs is very sensitive to surrounding environment [18]. When other structures are close to the MN-WGs, the forward guided modes will suffer some disturbance such as scattering, which may induce backward reflected waves [19]. Thus, the waves reflected by both the endfaces of MN-WGs and the contact points, will cause overlap and affect the spectral properties of the devices. In this work, we demonstrated optical mode tailoring in the subwavelength-dimensional semiconductor MN-WGs coupled with optical silica microfibers. By investigating the reflection spectra of the MN-WGs/microfiber systems, it was found that the coupling positions and the cross-sectional dimensions of the MN-WGs have great influences on the interference spectra of the system. In addition, tunable free spectral range (FSR) of photoluminescence (PL) emissions and humidity sensing by using this interference effect were also demonstrated.

2. Experimental

In this work, CdS is selected due to its wide applications in nanophotonic area such as lasing, photodetection, and nonlinear optics. Single-crystalline CdS MN-WGs including nanowires and nanoribbons were synthesized using a chemical vapor deposition process [20]. These MN-WGs have high crystalline quality with hexagonal wurtzite structure, and grow along the [001] direction, i.e., length direction. As-grown MN-WGs were dispersed onto a glass substrate. It is known that crystals are apt to break along their cleavage plane when under stress. Therefore, using a simple bend-to-fracture method, we can cut the MN-WGs easily along its grown axis at a predetermined point and achieve high endface reflectivity. As shown in Fig. 1(a), two silica nanotaper probes were used to bend one end of the MN-WG fractured via micromanipulation. Figure 1(b) shows a typical transmission electron microscopy (TEM) image of a CdS nanowire, in which a smooth endface is observed.

 

Fig. 1 (a) Micrographs of cutting a CdS MN-WG by a bend-to-fracture method using two nanotaper probes. (b) TEM image of a fractured CdS nanowire. (c) Schematic diagram of the semiconductor MN-WG/silica microfiber system, in which the reflected light is collected and investigated.

Download Full Size | PDF

The investigated semiconductor MN-WG/silica microfiber system is schematically shown in Fig. 1(c). An optical microfiber drawn from a standard optical silica fiber (SMF-28, Corning) is placed on an MgF₂ substrate, and the tip of the microfiber is protruded from the substrate with a distance of about 50 μm. Via micromanipulation under an optical microscope, a CdS MN-WG is picked up from the substrate and placed on the tip of the suspended microfiber with their axes parallelly. Due to the Vander Waals and electrostatic attraction, the CdS MN-WG is steadily attached on the microfiber. Based on the evanescent wave coupling technique [7, 8], the input light can be efficiently coupled into and guided along the waveguides to form the forward wave ( + ω). Due to the high refractive index of CdS MN-WGs (~2.3 at 1.59 μm) [21], some portion of the guided light is reflected by the free-standing endfaces and propagates backward. Then the forward ( + ω) and backward (−ω) waves overlap and cause interference, which will greatly modulate the spectra of the system.

The suspended lengths of the MN-WGs were tuned using a silica nanotaper probe [16, 22]. The input light from an amplified spontaneous emission (ASE) source was first launched into the input arm of a circulator (F-CIR-15-P-FP, Newport) and then coupled into the MN-WGs through the fiber tapers. The reflection signals were collected at the output arm of the circulator and then were directed to an optical spectrum analyzer (AQ6370C, Yokogawa).

3. Results and discussions

The black line in Fig. 2(a) provides a collected reflected spectrum from a bare optical microfiber with a diameter of ~1.8 μm, in which a broad and smooth spectral profile is observed, suggesting that no detectable intrinsic cavity effect were observed from the protruded silica microfiber and the MgF₂ substrate. Then a 0.6-μm-thick and 1.7-μm-wide CdS nanoribbon with a whole length of 131.4 μm was coupled to the microfiber. When the suspended length was 93.2 μm, an obvious interference fringe was observed, as shown in the red line in Fig. 2(a). The measured extinction ratio is about 8.3 dB around the resonant wavelength (λ_R) of 1595.04 nm, and is as high as 10.6 dB around λ_R = 1600.81 nm. These extinction values are comparable with results in MN-WG-based structures using glass microfibers or nanofibers, such as Mach-Zehnder interferometers, knot resonators and Sagnac loops [14, 23–26], clearly suggesting the great modulation of the reflection spectra in the CdS nanoribbon-microfiber system.

 

Fig. 2 Interference spectra using a CdS nanoribbon with different suspended lengths of (a, red line) 93.2 μm, (b) 70.6 μm and (c) 46.1 μm, and using a bare microfiber (a, black line); Schematic diagrams and micrographs of the CdS nanoribbon with different suspended lengths. The white arrows point out the microfiber tips in each case.

Download Full Size | PDF

The FSR of a F−P cavity illustrated in Fig. 1(c) is determined by the well-known expression

Figure 3(a) shows the FSR and the full width at half maximum (FWHM) of the CdS nanoribbon. The investigated wavelength is around 1584 nm for all different L_eff. It is observed that the dependence of FSR on L_eff shows a linear behavior, because n_g is constant and FSR is inversely proportional to the L_eff, which can be expected from the Eq. (1). And as L_eff increases, the FWHM decreases but shows saturation, which could be attributed to the increasing of waveguide loss as L_eff and thus the quality of F−P cavity decreases. Tens of thin CdS MN-WGs (diameters less than 1 μm) were investigated and similar results were obtained as stated above. It suggests that the microfiber tips combines with the free-standing MN-WG endfaces to serve as two reflecting mirrors and generate the observed interference effect. By using 3D-FDTD simulation, Fig. 3(b) shows the reflectivity of guided fundamental modes at the endface and microfiber tip in microfiber-coupled CdS MN-WGs with different diameters, in which the reflectivity have maximum around the diameter of 1 μm for both of the microfiber tips (red dots) and the MN-WG endfaces (blue dots). For example, for a 0.9-μm-diameter CdS nanowire, the calculated reflectivity is about 21% at the microfiber tip, and is about 28% at the endfaces. In very thin waveguides the fractions of evanescent wave are high and induce large propagation losses, as shown in green line; and in very thick waveguides only few fraction of evanescent wave is distributed outside and induces less scattered energy and reflection [18].

 

Fig. 3 (a) FSR and FWHM around 1584 nm of the CdS nanoribbon as a function of L_eff. (b) Reflectivity of guided fundamental modes at the endface and microfiber tip, and fractions of evanescent wave outside CdS MN-WGs with different diameters. (c) Interference spectra using a 1.8-μm-diameter CdS microwire, with different suspended lengths of 18.5 and 22.3 μm, respectively. (d) Interference spectrum using a CdS wire, with a gradually varied diameter from 1.5 μm to less than 0.3 μm.

Download Full Size | PDF

To further verify such interference behaviors, a 1.8-μm-diameter CdS microwire with a whole length of 26.9 μm was used. Figure 3(c) shows two interference spectra collected from the microwire with suspended lengths of 18.5 and 22.3 μm respectively, in which the measured corresponding FSR is 20.61 (λ_R = 1584.39 nm) and 19.63 nm (λ_R = 1593.28 nm) respectively. Here n_g of the microwire is numerically calculated as 2.19, thus the corresponding L_eff are about 29.1 and 28.1 μm, respectively. These two values are far away from the suspended lengths but near to the whole lengths, suggesting that the two endfaces of the microwire contributes the interference effect together.

For MN-WGs with one bad endfaces, an effective interference effect can be manually generated using this microfiber coupling approach. Here a CdS wire with a gradually varied diameter from 1.5 μm to less than 0.3 μm was used. On this condition, the thick endface can serve as an effective mirror; but the other endface is too thin to offer enough reflection. By coupling a microfiber at the thin part of the wire, obvious spectral interference was observed, as shown in Fig. 3(d). At λ_R = 1590.31 nm the measured FSR is 8.68 nm and the extinction ratio is 7.9 dB. The suspended wire length is 55.8 μm and the wire diameter at the coupling position with the microfiber tip is about 0.9 μm.

Active cavity effect of the optical microfiber-semiconductor MN-WGs coupling systems was also investigated. Here CdSe nanowires were selected as the active waveguides [20]. As illustrated in Fig. 4(a), a CdSe nanowire is coupled with a microfiber and excited with a 532-nm pulsed laser (10 ns, 1 kHz) from a microscope objective. The PL emissions are collected from the same objective and directed to a spectrometer (QE65 Pro, Ocean Optics) and a CCD camera. Figure 4(b) shows the PL emissions obtained using a 0.5-μm-diameter CdSe nanowire with a whole length of 93.2 μm. By changing the suspended lengths of the nanowire from 16.4 μm to 28.5 μm, the FSR were changed from 3.32 nm at λ_R = 705.8 nm to 2.43 nm at λ_R = 706.9 nm. Obvious scattering spots were also observed at the tip of the microfiber, as shown in the insets. Here n_g of the nanowire is measured as 4.3, which was experimentally obtained from the resonant PL spectrum of the individual nanowire on an MgF₂ substrate [27]. Thus the corresponding L_eff is about 17.7 μm and is about 24.3 μm, which agrees with the suspended values.

 

Fig. 4 (a) Schematic diagram of an experimental setup for investigating active cavity effect. (b) Interference spectra of PL emissions using a CdSe nanowire, with different suspended lengths of 16.4 and 28.5 μm, respectively. Insets show the micrographs of PL emissions. (c) Interference spectra shift of a CdS microwire as humidity varying from 33% to 69%.

Download Full Size | PDF

As an example for practical applications, we explored this interference effect for sensing of relative humidity (RH). The detailed sensing approach can be found in our former work [16, 22]. A microfiber-coupled 1.1-μm-diamter CdS microwire was used, with a suspended length of ~200 μm. As shown in Fig. 4(c), when the RH increased from 33% to 69%, a spectral shift of 0.57 nm at λ_R = 1587.53 nm is clearly seen. When the microwire-microfiber system was exposed to humid air, refractive index of the microwire will increase due to the adsorption of water molecules on its surface [26], and induced the shifts of the interference peaks.

Compared with microfiber humidity sensors reported before [26, 28, 29], such as gelatin layer-coated microfibers, polymer nanowires, and microfiber knot resonators, our device has two typical advantages. Firstly, most of reported humidity sensors are based on changes of optical transmission, thus one optical microfiber is needed to collect signal changes from sensing elements. Here our devices just detect the reflection signals. The simple strategy and the microscale dimension of MN-WGs make our devices very suitable in ultra-narrow environment, such as intracellular compartments of biological cells [17]. Secondly, using optical grating structures can realize sensing by just detect reflection signals [30]; however, complex and expensive fabrication methods such as lithography technique are usually needed. Here our semiconductor MN-WGs are fabricated very cheap and high yield, and the device fabrication is also simple via micromanipulation.

4. Conclusion

In conclusion, obvious mode tailoring in the coupling system between subwavelength-dimensional semiconductor MN-WGs and optical silica microfibers have been demonstrated. The measured extinction ratio in the interference patterns of the reflection spectra can be as high as ~10 dB. The microfiber tips can act effective reflection mirrors in relatively thin MN-WGs with subwavelength dimensions, and can also been used to tune the interference wavelengths by changing the contact points with the MN-WGs. This approach has also been applied to the active MN-WGs, as demonstrated in tuning FSR of PL emissions in CdSe nanowires. This tuning approach is convenient just by moving the fiber taper and will not bring damage to the MN-WGs compared with the cutting approach reported before. In addition, since the fiber taper is directly connected to a standard optical fiber, the microfiber/MN-WG coupling systems can act as optical micro/nano-probes, even as biological endoscopy reported recently [17], with attractive advantages of simple optical power readout, remote sensing, and continuous monitoring. Therefore, our results demonstrated here may find promising applications in miniature tunable lasers and sensing.