Silicon photonics has been developed successfully with a top-down fabrication technique to enable large-scale photonic integrated circuits with high reproducibility, but is limited intrinsically by the material capability for active or nonlinear applications. On the other hand, free-standing nanowires synthesized via a bottom-up growth present great material diversity and structural uniformity, but precisely assembling free-standing nanowires for on-demand photonic functionality remains a great challenge. Here we report hybrid integration of free-standing nanowires into silicon photonics with high flexibility by coupling free-standing nanowires onto target silicon waveguides that are simultaneously used for precise positioning. Coupling efficiency between a free-standing nanowire and a silicon waveguide is up to ~97% in the telecommunication band. A hybrid nonlinear-free-standing nanowires–silicon waveguides Mach–Zehnder interferometer and a racetrack resonator for significantly enhanced optical modulation are experimentally demonstrated, as well as hybrid active-free-standing nanowires–silicon waveguides circuits for light generation. These results suggest an alternative approach to flexible multifunctional on-chip nanophotonic devices.

On the basis of the transverse second harmonic generation (TSHG) in a highly nonlinear subwavelength-diameter CdTe nanowire, we demonstrate a single-nanowire optical correlator for femto-second pulse measurement with pulse energy down to femtojoule (fJ) level. Pulses to be measured were equally split and coupled into two ends of a suspending nanowire via tapered optical fibers. The couterpropagating pulses meet each other around the central area of the nanowire, and emit TSHG signal perpendicular to the axis of the nanowire. By transferring the spatial intensity profile of the transverse second harmonic (TSH) image into the time-domain temporal profile of the input pulses, we operate the nanowire as a miniaturized optical correlator. Benefitted from the high nonlinearity and the very small effective mode area of the waveguiding CdTe nanowire, the input energy of the single-nanowire correlator can go down to fJ-level (e.g., 2 fJ/pulse for 1064 nm 200 fs pulses). The miniature fJ-pulse correlator may find applications from low power on-chip optical communication, biophotonics to ultracompact laser spectroscopy.

A novel type of mid-IR microresonator, the chalcogenide glass (ChG) microfiber knot resonator (MKR), is demonstrated, showing easy fabrication, fiber-compatible features, resonance tunability, and high robustness. ChG microfibers with typical diameters around 3 μm are taper-drawn from As 2 S 3 glass fibers and assembled into MKRs in liquid without surface damage. The measured Q factor of a typical 824 μm diameter ChG MKR is about 2.84 × 10 4 at the wavelength of 4469.14 nm. The free spectral range (FSR) of the MKR can be tuned from 2.0 nm (28.4 GHz) to 9.6 nm (135.9 GHz) by tightening the knot structure in liquid. Benefitting from the high thermal expansion coefficient of As 2 S 3 glass, the MKR exhibits a thermal tuning rate of 110    pm · ° C − 1 at the resonance peak. When embedded in polymethyl methacrylate (PMMA) film, a 551 μm diameter MKR retains a Q factor of 1.1 × 10 4 . The ChG MKRs demonstrated here are highly promising for resonator-based optical technologies and applications in the mid-IR spectral range.

While these strategies work well for bulk lasing systems, it is difficult to apply them to low-dimensional lasing cavities such as nanowires. In the past decades, a number of approaches, including mechanical modulation of cavity length, [10,11] strain-induced bandgap reduction, [12] and bandgap-engineered self-absorption, [13] have been investigated to enable lasing wavelength tuning in single nanowires. Furthermore, the wavelength tunability can also be realized by utilizing several nanowires or nanoribbons with different bandgaps. [14,15] However, limited by the tuning reversibility, [10,11,13] speed, [12,16] and dynamic range, [10,12] fast lasing wavelength tuning in nanowires are yet to be demonstrated.The temperature-induced bandgap change in semiconductor materials, known as Varshni shift in absorption or luminescent spectra, has been well studied since its discovery in 1967. [17] When the temperature increases, the bandgap decreases due to enhanced lattice vibrations and electron-phonon interactions, [18] obeying an empirical Varshni relation [17] (see also Section S1, Supporting Information). Typical Varshni shifts can go up to tens of nanometers, [19][20][21] sufficiently broad for lasing wavelength tuning for a variety of applications. However, in conventional bulk semiconductors, the thermal response is relatively slow due to the large thermal inertia of the bulk materials, limiting the temperature tuning rate to less than 10 K s −1 (see Section S2, Supporting Information), corresponding to a wavelength tuning rate on the order of 1 nm s −1 for cadmium sulfide (CdS) materials. [20,22] Also, to avoid straininduced damage, the range of the spectral tuning is typically narrow. To date, as far as it is known, the effect of Varshni shift has not been explored for fast lasing wavelength tuning.The emergence of crystalline semiconductor nanowires offers an opportunity to realize nanoscale semiconductor lasers with new functionalities. [1,23,24] By miniaturizing a bulk semiconductor to create a 1D nanowire, the allowed strain and lattice mismatch can be significantly increased, [25,26] making it possible to obtain much larger Varshni shift in a nanowire than in its bulk counterpart, as have been experimentally demonstrated in photoluminescence (PL) or lasing emission in single nanowires at low temperatures. [23,27,28] However, in previous studies, the nanowires were supported by bulk substrates with large thermal inertia, which limits the rate of the temperature tuning (Section S2, Supporting Information). In contrast, in