Ultraefficient on-chip supercontinuum generation: bringing handheld sensing devices within reach

The mid-infrared (MIR) wavelength coincides with various molecular resonances. In particular, a 13-20 µm wavelength window has fingerprints of unique groups such as organometallic, halogenated, and aromatic bonds. In this work, for the first time, to the best of our knowledge, an on-chip supercontinuum generation (SCG) source based on cadmium telluride (CdTe)/ cadmium sulfide (CdS)/ silicon heterostructure is proposed to extend the on-chip SCG beyond 13 µm (spanning 3.5 to 20 µm). CdTe has an ultra-broad transparent spectral range up to 25 µm, and almost the largest third-order nonlinear coefficient (n2∼ 5×10-17 m2/W at 1.55 µm, 1.3×10-17 m2/W at 9 µm, several times larger than that of silicon) among the MIR materials, making CdTe an excellent candidate for long-wavelength MIR on-chip SCG. The waveguide structure is designed with CdS as the intermediate cladding layer to achieve a low waveguide loss and high mode confinement. A large-core CdTe waveguide is tailored to generate a low and flat dispersion (< 30 ps/nm/km) in a spectral range spanning from 5 to 20 µm, while balancing the large effective nonlinearity and the convenience of coupling. The simulation results solved by the nonlinear Schrödinger equation manifest that the engineered large cross-section waveguide with only 2.5-mm propagation distance broadens the MIR spectrum covering 3.5 to 20 µm pumped by a 9 µm femtosecond laser. Moreover, it is found that good coherence is achieved from the designed MIR waveguide, before severe soliton fission breaks the temporal profile. 5-fold self-compression of the pump pulse down to 1.6 optical cycles is observed while propagating inside the CdTe waveguide. The detailed simulation of the CdTe/CdS/Si waveguide design with the various waveguide parameters, polarizations, pump wavelengths, and pump power are provided. With the SC spectrum covering almost the entire fingerprint regime and the excellent coherence generated from the designed CdTe waveguide, it provides abundant new opportunities for MIR microphotonics.

Midinfraredspectroscopy is a universal way to identify chemicaland biological substances. Indeed, when interacting with a light beam,most molecules are responsible for absorption at specific wavelengthsin the mid-IR spectrum, allowing to detect and quantify small tracesof substances. On-chip broadband light sources in the mid-infraredare thus of significant interest for compact sensing devices. In thatregard, supercontinuum generation offers a mean to efficiently performcoherent light conversion over an ultrawide spectral range, in a singleand compact device. This work reports the experimental demonstrationof on-chip two-octave supercontinuum generation in the mid-infraredwavelength, ranging from 3 to 13 μm (that is larger than 2500cm–1) and covering almost the full transparencywindow of germanium. Such an ultrawide spectrum is achieved thanksto the unique features of Ge-rich graded SiGe waveguides, which allowsecond-order dispersion tailoring and low propagation losses overa wide wavelength range. The influence of the pump wavelength andpower on the supercontinuum spectra has been studied. A good agreementbetween the numerical simulations and the experimental results isreported. Furthermore, a very high coherence is predicted in the entirespectrum. These results pave the way for wideband, coherent, and compactmid-infrared light sources by using a single device and compatiblewith large-scale fabrication processes.

We report on what we believe is the first demonstration of on-chip supercontinuum generation through dual-wavelength pumping of a nonlinear photonic integrated circuit, fabricated from high-index doped silica glass. In addition to the bandwidth enhancement through cross-phase modulation, we also observe multiple temporal reflections that help to extend the bandwidth further through far-detuned radiations. The experimental results show good agreement with the phase-matching theory and numerical simulations. Additionally, our real-time, pulse-to-pulse, spectral measurements through the dispersive Fourier transform technique reveal a significant increase in relative intensity noise for dual-wavelength pumping compared to the case of single-wavelength pumping.

On-chip supercontinuum generation in visible is demonstrated experimentally in angle-etched diamond waveguides. The output spectrum spans from 670-920 nm in response to a 100 femtosecond pulse input at 15 mW average power and 80 MHz repetition rate.

We propose a novel silicon waveguide that exhibits four zero-dispersion wavelengths for the first time, to the best of our knowledge, with a flattened dispersion over a 670-nm bandwidth. This holds a great potential for exploration of new nonlinear effects and achievement of ultra-broadband signal processing on a silicon chip. As an example, we show that an octave-spanning supercontinuum assisted by dispersive wave generation can be obtained in silicon, over a wavelength range from 1217 to 2451 nm, almost from bandgap wavelength to half-bandgap wavelength. Input pulse is greatly compressed to 10 fs.

Mid-infrared spectroscopy attracts growing interests because of its coverage over the characteristic spectral absorption features for many molecules. This paper will review our study in mid-infrared photonics including material characterization, on-chip waveguide fabrication and supercontinuum generation for the spectroscopic applications.

We report mid-IR supercontinuum generation, from 2.9 to 7.7μm, in a CMOS compatible silicon-germanium waveguide. This 1.3 octave bright supercontinuum has been achieved in a low loss dispersion engineered air-clad Si0.6Ge0.4/Si waveguide.