O-band TE- and TM-mode densely packed adiabatically bent waveguide arrays on the silicon-on-insulator platform.

An efficient, dual-polarization silicon waveguide array with low insertion losses and negligible crosstalks for both TE and TM polarizations has been reported using S-shaped adiabatically bent waveguides. Simulation results for a single S-shaped bend show an insertion loss (IL) of ≤ 0.03 dB and ≤ 0.1 dB for the TE and TM polarizations, respectively, and TE and TM crosstalk values in the first neighboring waveguides at either side of the input waveguide are lower than -39 dB and -24 dB, respectively, over the wavelength range of 1.24 µm to 1.38 µm. The bent waveguide arrays exhibit a measured average TE IL of ≈ 0.1 dB, measured TE crosstalks in the first neighboring waveguides are ≤ -35 dB, at the 1310 nm communication wavelength. The proposed bent array can be made by using multiple cascaded S-shaped bends to transmit signals to all optical components in integrated chips.

On-chip integration of 2D Van der Waals germanium phosphide (GeP) for active silicon photonics devices.

The outstanding performance and facile processability turn two-dimensional materials (2DMs) into the most sought-after class of semiconductors for optoelectronics applications. Yet, significant progress has been made toward the hybrid integration of these materials on silicon photonics (SiPh) platforms for a wide range of mid-infrared (MIR) applications. However, realizing 2D materials with a strong optical response in the NIR-MIR and excellent air stability is still a long-term goal. Here, we report a waveguide integrated photodetector based on a novel 2D GeP. This material uniquely combines narrow and wide tunable bandgap energies (0.51-1.68 eV), offering a broadband operation from visible to MIR spectral range. In a significant advantage over graphene devices, hybrid Si/GeP waveguide photodetectors work under bias with a low dark current of few nano-amps and demonstrate excellent stability and reproducibility. Additionally, 65 nm thick GeP devices integrated on silicon waveguides exhibit a remarkable photoresponsivity of 0.54 A/W and attain high external quantum efficiency of ∼ 51.3% under 1310 nm light and at room temperature. Furthermore, a measured absorption coefficient of 1.54 ± 0.3 dB/µm at 1310 nm suggests the potential of 2D GeP as an alternative infrared material with broad optical tunability and dynamic stability suitable for advanced optoelectronic integration.

Compact and broadband silicon TE-pass polarizer based on tapered directional coupler.

We demonstrate a novel TE-pass polarizer, to the best of our knowledge, on a silicon-on-insulator (SOI) platform. The device's working principle is based on the phase-matched coupling of the unwanted TM0 mode in an input waveguide to the TM1 mode in a tapered directional coupler (DC), which is then guided through a low-loss bend (180-degree) and scattered in a terminator section with low back reflections. However, the input TE0 mode is routed through the tapered section uncoupled with negligible loss. An S-bend is added before the output for filtering any residual TM0 mode present in the input waveguide. Tapering the DC helps maintain phase matching for broadband operation and increases the tolerance toward fabrication errors. The measurement shows low insertion loss (IL < 0.44 dB), high extinction ratio (ER > 15 dB), and wide bandwidth (BW = 80 nm). The overall device length is only 13 µm. A high performing TE-pass polarizer (IL < 0.89, ER > 30, and BW = 100 nm) is also demonstrated by cascading two proposed polarizers.

CMOS compatible ultra-compact MMI based wavelength diplexer with 60 Gbit/s system demonstration.

We design and experimentally demonstrate an ultra-compact 1310/1550 nm wavelength diplexer based on a multimode interference (MMI) coupler. The proposed device is designed at the first imaging length for 1550 nm wavelength resulting in an MMI length of only 41 µm. In order to improve the extinction ratio, the output ports are made asymmetric in width. A low insertion loss (< 1dB) and high extinction ratio (> 20 dB) is measured at the two operating wavelengths. It also displays a wide 3-dB bandwidth of 100 nm centered around 1310 nm and 1550 nm wavelengths. Furthermore, an on-chip wavelength demultiplexing experiment carried out on the fabricated device, with a non-return-to-zero (NRZ) on-off keying (OOK) signal at 60 Gbit/s, shows clear eye diagrams for both the wavelengths.

Short-wavelength infrared (SWIR) photodetector based on multi-layer 2D GaGeTe.

Recent theoretical studies proposed that two-dimensional (2D) GaGeTe crystals have promising high detection sensitivity at infrared wavelengths and can offer ultra-fast operation. This can be attributed to their small optical bandgap and high carrier mobility. However, experimental studies on GaGeTe in the infrared region are lacking and this exciting property has not been explored yet. In this work, we demonstrate a short-wavelength infrared (SWIR) photodetector based on a multilayer (ML) GaGeTe field-effect transistor (FET). Fabricated devices show a p-type behavior at room temperature with a hole field-effect mobility of 8.6 - 20 cm2 V-1s-1. Notably, under 1310 nm illumination, the photo responsivities and noise equivalent power of the detectors with 65 nm flake thickness can reach up to 57 A/W and 0.1 nW/Hz1/2, respectively, at a drain-source bias (Vds) = 2 V. The frequency responses of the photodetectors were also measured with a 1310 nm intensity-modulated light. Devices exhibit a response up to 100 MHz with a 3dB cut-off frequency of 0.9 MHz. Furthermore, we also tested the dependence of the device frequency response on the applied bias and gate voltages. These early experimental findings stimulate the potential use of multilayer GaGeTe for highly sensitive and ultrafast photodetection applications.

Experimental studies of plasmonics-enhanced optical physically unclonable functions.

We present an experimental analysis of optical Physically Unclonable Functions enhanced using plasmonic metal nanoparticles in a Silicon on Insulator based integrated structure. We experimentally demonstrate the behavior of possible configurations of simple PUF structures defined only by the nanoparticle distribution. The devices show a promising response when tested with transverse magnetic polarized light. This response offers an easy-to-implement methodology to enhance the behavior of previously proposed optical PUFs. We additionally make a comprehensive analysis of the power, thermal, and polarization stability of the devices for possible side-channels attacks to the systems.

Strong enhancement of direct transition photoluminescence at room temperature for highly tensile-strained Ge decorated using 5 nm gold nanoparticles.

Strain engineering of germanium has recently attracted tremendous research interest. The primary goal of this approach is to exploit mechanical strain to tune the electrical and optical properties of Ge to ultimately achieve an on-chip light source compatible with silicon technology. Additionally, this can result in enhanced electrical performance for high-speed optoelectronic applications. In this paper, we demonstrate the formation of highly tensile-strained Ge islands grown on a pre-patterned (110) GaAs substrate using a depth controlled nanoindentation process. Results show that a biaxial tensile strain, up to ∼2%, can be transferred from the mechanically stamped substrate to Ge islands by optimizing the parameters of the nanoindentation process. We verified our measurements by observing the islands' photoluminescence (PL) emission properties. A strong emission at room-temperature was observed around the wavelength of 1.9 µm (650 meV). This strain-induced redshift of the PL spectra is consistent with theoretical predictions, revealing a direct Ge bandgap formation. Furthermore, we demonstrate a significant 6.5x enhancement in the PL emission signal of the direct-transition when the Ge islands are decorated by 5 nm gold nanoparticles. This is attributed to a longer optical path length interaction and a plasmonic induced high-field enhancement which increases the light absorption in the Ge islands. Furthermore, results show that GNPs can significantly modulate the energy band structure and the carrier's transportation at the nanoscale metal-germanium Schottky interface. This maskless physical approach can offer a pathway towards a practical CMOS-compatible integrated laser. Additionally, it opens possibilities for designing innovative optoelectronic devices.

Compact silicon TE-pass polarizer using adiabatically-bent fully-etched waveguides.

A high-performance integrated silicon TE-pass polarizer is proposed and demonstrated. The polarizer uses a series of adiabatic waveguide bends that yield high extinction ratio for the TM polarization and low insertion loss for the TE polarization, and does not require special materials or complex fabrication steps. The polarizer, implemented on a silicon-on-insulator platform with a 220 nm silicon thickness, is measured to have insertion loss ≤ 0.37 dB (average 0.12 dB) and extinction ratio ≥ 27.6 dB (average 36.0 dB) over a 1.5 µm to 1.6 µm wavelength range, with a footprint of 63 µm × 9.5 µm. The trade-off between the footprint of the polarizer and its performance is established. While the analysis was done for a silicon-on-insulator platform, the concept is applicable to other waveguide geometries and integrated photonic platforms.

Gradient-index optofluidic waveguide in polydimethylsiloxane.

We demonstrate a gradient-index (GRIN) optofluidic waveguide using polydimethylsiloxane cured with a radial variation of temperature. The waveguide wraps the microfluidic channel and the GRIN profile localizes the light around it, making the device suitable for evanescent sensing applications. The fabricated waveguide shows good light confinement, with a propagation loss of 1.47 dB/cm at a wavelength of 632.8 nm.