The full exploration of Si-based photonic integrated circuits is limited by the lack of an efficient light source that is compatible with the complementary metal− oxide−semiconductor process. Highly strained germanium (Ge) is a promising solution, as its band structure can be fundamentally altered by introducing tensile strain. However, the main challenge lies in the incorporation of an electrical structure while maintaining high strain with uniform distribution in the active region. Here we present highly strained Ge LEDs driven by lateral p−i−n junctions and report the strain-induced enhancement of electroluminescence (EL) from Ge. Raman characterization shows that 1.76% strain along the ⟨100⟩ direction with relatively uniform strain distribution is achieved. The observed strain-induced red-shifts of EL spectra agree well with the theoretical prediction, revealing that the direct band gap of Ge can be tuned in the range of 0.785 eV (1580 nm) to 0.658 eV (1885 nm). This work offers a pathway toward a strained Ge laser with low threshold current, as well as opens possibilities for new types of optoelectronics devices based on strain engineering.

Silicon-based stimulated Brillouin scattering (SBS) promotes the on-chip all-optical signal processing network by interfacing silicon photonic and phononic technologies. Controllable and strong Brillouin coupling in silicon is a key requirement for this purpose. Here, we demonstrate traveling-wave forward SBS and Brillouin gain through a class of hybrid photonic-phononic silicon waveguides on the silicon-on-insulator (SOI) platform. This design combines the advantages of a silicon ridge waveguide and phononic crystal slab, allowing the independent control on the confined optical and acoustic modes. The strong and tailorable Brillouin nonlinearity is demonstrated via the heterodyne four-wave mixing spectroscopy. Three-tone gain experiment reveals a small-signal Stokes gain of 0.9 dB in a 1.085 cm length straight waveguide device at moderate pump power. The limiting factors and further improvements of net Brillouin amplification in our system are also discussed. This design can also be applied to the intermodal SBS as well as other silicon-based material platforms, and thus it offers the pathway toward on-chip microwave photonic filters, Brillouin amplifiers, and nonreciprocal devices.

In recent years, piezoelectric materials, such as AlN, lithium niobate (LN) and GaAs, are frequently applied to acoustooptic (AO) coupling interactions with great potentials. [16][17][18][19][20][21][22][23][24] With a travelling SAW actuated by the interdigital transducer (IDT), the optical waves in waveguides are scattered by the travelling refractive index gratings and frequencyshifted through Doppler effect. [25] The AO coupling in piezoelectric materials offers a promising way to implement on-chip AO modulation, microwave photonic filtering, acousto-optic frequency shift, and nonreciprocal light propagation. [16,17,26,27] Different from the intrinsic optomechanical interactions in waveguides, [28][29][30][31][32][33][34] it is unnecessary to design suspended waveguides or optomechanical cavities for the AO interactions, which guarantees the stability and feasibility for on-chip applications. [16,35] Meanwhile, the electrically driven AO interactions exhibit higher AO scattering efficiency than the intrinsic optomechanical interactions, which can be adjusted in both optical domain and electrical domain. [36,37] Although LN (d 33 = 6 pC N -1 ) is a good candidate in AO interactions for its outstanding optical and piezoelectric properties, [38] it is not compatible with complementary metal-oxide-semiconductor (CMOS) technology to achieve large-scale fabrication. [20][21][22] As for AlN, it is generally deposited on silicon substrate through magnetic sputtering that is compatible with CMOS technology, but it poses a challenge to design waveguide structures in AlN layer because of the smaller refractive index than silicon substrate. [17,18] Therefore, suspended waveguide structures are indispensable for AlN on silicon substrate to carry out AO interactions. Surprisingly, a nonsuspended structure of AlN deposited on silicon-on-insulator (SOI) platform with a SiO 2 interlayer had realized AO modulation recently, where the optical wave is mostly confined in the nonpiezoelectric silicon layer instead of AlN layer. [39] In that structure, the SAW actuated by the IDT propagates from AlN layer across SiO 2 interlayer and finally modulates the optical waves in the silicon waveguides, where the SiO 2 interlayer greatly increases the SAW propagation loss and degrades the acoustic performances. [40] At last, with the same CMOS-compatible deposition as AlN and better piezoelectric properties than AlN, Aluminum scandium nitride (AlScN) has attracted extensive attention for its excellent piezoelectric properties in the micro-electromechanical system applications. In this work, AlScN is demonstrated to be a promising candidate for on-chip acousto-optic coupling interactions with outstanding piezoelectric properties as well. Based on piezoelectric Al 0.6 Sc 0.4 N film deposited on silicon-on-insulator platform, the proposed devices exhibit impressive acousto-optic coupling performances over a short interaction length of 210 µm with surface acoustic waves actuated by interdigital transducers. Meanwhile, the acousto-optic coupling...

Frame and frequency synchronization are essential for orthogonal frequency division multiplexing (OFDM) systems. The frame offset owing to incorrect start point position of the fast Fourier transform (FFT) window, and the carrier frequency offset (CFO) due to Doppler frequency shift or the frequency mismatch between the transmitter and receiver oscillators, can bring severe intersymbol interference (ISI) and inter-carrier interference (ICI) for the OFDM system. Relying on the relatively good correlation characteristic of the pseudo-noise (PN) sequence, a joint frame offset and normalized CFO estimation algorithm based on PN preamble in time domain is developed to realize the frame and frequency synchronization in the OFDM system. By comparison, the performances of the traditional algorithm and the improved algorithm are simulated under different conditions. The results indicate that the PN preamble based algorithm both in frame offset estimation and CFO estimation is more accurate, resource-saving and robust even under poor channel condition, such as low signal-to-noise ratio (SNR) and large normalized CFO.

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We demonstrate the forward stimulated Brillouin scattering (FSBS) in a partly suspended silicon nanowire racetrack resonator. To realize the tight confinement of the transverse acoustic modes in the nanoscale silicon core, the racetrack resonator is supported by the tiny pillar. The Brillouin amplification of 2.25 dB is achieved with the resonator radius of 100 μm under a low-power pump laser of 8 mW. The influences of the waveguide width and the top width of the tiny pillar on the Brillouin frequency shift and Brillouin gain are presented and analyzed. The Brillouin frequency shift is conveniently manipulated by the changes in waveguide widths. Our proposed approach furnishes an alternative towards harnessing FSBS in integrated photonic circuits.

We report holographic fabrication of nanoporous distributed Bragg reflector (DBR) films with periodic nanoscale porosity via a single-prism configuration. The nanoporous DBR films result from the phase separation in a material recipe, which consists of a polymerizable acrylate monomer and nonreactive volatile solvent. By changing the interfering angle of two laser beams, we achieve the nanoporous DBR films with highly reflective RGB colors. The reflection band of the nanoporous DBR films can be tuned by further filling different liquids into the pores inside the films, resulting in the color change accordingly. Experimental results show that such kind of nanoporous DBR films could be potentially useful for many applications, such as color filters and refractive index sensors.

The strain technology is accelerating the progress on the CMOS compatible Ge-on-Si laser source. Here, we report a monolithically integrated microbridge-based emitting-detecting configuration, equipped with lateral p–i–n junctions, waveguide and gratings. The operating wavelength range of the emitting bridge and the detecting bridge are matched through the designed same dimensions of the two microbridges, as well as the strain. Strain-enhanced spontaneous emission and the effect of spectra red-shifting on low-loss transmission of on-chip light are discussed. Temperature dependence experiments reveal that in devices with highly strain-enhanced structure, the strain variation can offset the effect of electron thermalization, so that the performance of the device remains stable when temperature changes around room temperature.