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Zhe L, Hang Y, Wu H, et al. Self-phase modulation in integrated cadmium telluride polycrystalline waveguide[J]. Opto-Electron Eng, 2023, 50(2): 220313. doi: 10.12086/oee.2023.220313
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Self-phase modulation in integrated cadmium telluride polycrystalline waveguide
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Abstract
The mid-infrared (MIR) wavelength coincides with various molecular resonances and spectroscopy. It is a universal way to identify chemical and biological substances. Thus, the MIR supercontinuum generation (SCG) is widely used in biomedicine, spectroscopy, and environmental science. Cadmium telluride (CdTe) has an ultra-broad transparent spectral range, from 0.86 µm to 25 µm, and one of the largest third-order nonlinear coefficients. It makes CdTe become an excellent candidate for long-wavelength MIR on-chip SCG. As an important material of solar cells, there is a well-established thin film growth technology for CdTe. We designed a CdTe integrated waveguide on a low-refractive-index CdS film with a silicon substrate. The simulation results solved by the nonlinear Schrödinger equation manifest that the MIR SCG covering 4.1 µm to 9.7 µm can be generated from a 1 cm CdTe waveguide pumped by a 5.5 µm femtosecond laser. We experimentally fabricated the waveguide via the lithography and wet-etching techniques. The spectral broadening based on self-phase modulation from the large-core CdTe integrated waveguide is demonstrated by a femtosecond laser at the central wavelength of 1030 nm with a pulse width of 250 fs. The numerical simulations match well with the experimental results. These results pave the way for long-wavelength mid-infrared light sources and provide abundant new opportunities for MIR micro photonics.-
Keywords:
- integrated waveguide /
- supercontinuum /
- mid-infrared laser /
- ultrafast laser
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References
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Overview
The mid-infrared (MIR) wavelength coincides with various molecular resonances and spectroscopy, which is universally used to identify chemical and biological substances. In particular, the 13 µm~20 µm wavelength window has fingerprints of unique material groups such as organometallic, halogenated, and aromatic bonds. Thus, the MIR supercontinuum generation (SCG) is widely used in the fields of biomedicine, spectroscopy, and environmental science. Thanks to the mature semiconductor growth technology combined with the advanced CMOS integration technology, SCG in on-chip devices has been studied in recent years. Cadmium telluride (CdTe) has an ultra-broad transparent spectral range, from 0.86 µm to 25 µm, and one of the largest third-order nonlinear coefficients (n2~5×10−17 m2/W at 1.55 µm, 2×10−17 m2/W at 5.5 µm, which are several times larger than that of silicon) among the MIR materials, which makes CdTe become an excellent candidate for long-wavelength MIR on-chip SCG. As an important material of solar cells, there is a mature film growth and etching technology for CdTe. In this work, we designed a large-core CdTe integrated waveguide on a low-refractive-index cadmium sulfide (CdS) film with a silicon substrate. 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 in the MIR spectral range, while balancing the large effective nonlinearity and the convenience of coupling. The effective refractive index in the CdTe waveguide is obtained by finite element method. Then, the simulated results solved by the nonlinear Schrödinger equation manifest that a CdTe waveguide with a propagation distance as short as 1 cm can broaden the MIR spectrum covering 4.1 µm to 9.7 µm pumped by a 5.5 µm femtosecond laser. Experimentally, polycrystalline CdS and CdTe films were deposited by magnetron sputtering, and the CdTe waveguides were fabricated by photolithography followed by wet etching. In particular, the sidewall of the waveguide is almost perpendicular to the substrate due to the large difference in the longitudinal and transverse etching rates caused by the unique grain arrangement of the film. A near-infrared femtosecond laser centered at 1030 nm with a pulse width of 250 fs at a 500 kHz repetition rate is employed as the pump source, and an apparent spectral broadening based on self-phase modulation was observed. The numerical simulations match well with the experimental results. These results pave the way for long-wavelength mid-infrared light sources and provide abundant new opportunities for MIR microphotonics.
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Zhe L, Hang Y, Wu H, et al. Self-phase modulation in integrated cadmium telluride polycrystalline waveguide[J]. Opto-Electron Eng, 2023, 50(2): 220313. doi: 10.12086/oee.2023.220313
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Figure 1.
Design and simulation of the on-chip integrated CdTe waveguide. (a) The 3D structure diagram of the CdTe/CdS/Si waveguide. The width (WCdTe), height (HCdTe), and thickness (HCdS) of the CdTe waveguide are 20 µm, 8 µm, and 5 µm, respectively; (b) The propagation loss caused by the CdS layers with different thickness (3 µm, 5 µm, 10 µm, and 15 µm); (c) Total dispersion and mode confinement ratio of the TE mode in CdTe with 20 µm height and 8 µm width; (d) The SCG in the waveguide pumped with 5.5 µm, 15 kW pulse; (e) and (f) are the time-domain pulse and frequency-domain spectral evolution diagrams of the pulse propagated in a 1 cm-long CdTe waveguide, respectively
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Figure 2.
Waveguide fabrication flow chart and scanning electron microscope images. (a) The waveguide fabrication flow chart; (b) The waveguide surface topography. The height and width of the waveguide are 8.5 µm and 22 µm, respectively. The angle between the side wall of the waveguide and the CdS layer is 90°; (c) The side and top topography of the waveguide
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Figure 3.
(a) The experimental setup diagram. The inset shows the field distribution of the TE mode at 1030 nm wavelength in the waveguide; (b) The broadened spectrum from the waveguide at different pump power
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Figure 4.
Spectral broadening caused by self-phase modulation (SPM) in the CdTe waveguide based on GNLSE. (a) The output spectrum of the waveguide at different peak power (5 kW~20 kW) pumped at 1030 nm wavelength; (b), (d), (f), (h) The time-domain evolution diagrams of SCG process; (c), (e), (g), (i) The frequency-domain evolution diagrams of the SCG process
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