Yonghao Liu

Approaching total absorption at near infrared in a large area monolayer graphene by critical coupling

We demonstrate experimentally close to total absorption in monolayer graphene based on critical coupling with guided resonances in transfer printed photonic crystal Fano resonance filters at near infrared. Measured peak absorptions of 35% and 85% were obtained from cavity coupled monolayer graphene for the structures without and with back reflectors, respectively. These measured values agree very well with the theoretical values predicted with the coupled mode theory based critical coupling design. Such strong light-matter interactions can lead to extremely compact and high performance photonic devices based on large area monolayer graphene and other two–dimensional materials.

High quality factor photonic crystal filter at k ≈0 and its application for refractive index sensing

We report here the refractive index (RI) sensing using the singly degenerate high quality factor (Q) modes in photonic crystal slabs (PCS) with the free-space coupled incident beam close to normal incidence. Q values of 3.2x104 and 1.8x104 were achieved for the fabricated PCS in air and aqueous solution, respectively. A spectral sensitivity (S) of 94.5 nm/RIU and a detection limit (DL) of 3x10-5 RIU were achieved with our device. Such a high-Q cavity for the singly degenerate mode close to normal incidence is very promising to achieve a lower DL for RI sensing.

Optofluidic Fano resonance photonic crystal refractometric sensors

We report an ultra-compact surface-normal optofluidic refractometric sensor based on a two-dimensional silicon photonic crystal on insulator. In contrast to the conventional symmetric Lorentzian resonance that is prevalently used in the label-free sensors, the asymmetric lineshape and steep peak-to-dip transition of a Fano resonance enable the enhanced detection sensitivity. The detection limit of 1.3 × 10−6 refractive index units is achieved, which is among the lowest reported experimentally in the defect-free photonic crystal sensors.

Polycrystalline GeSn thin films on Si formed by alloy evaporation

Polycrystalline GeSn thin films on Si substrates with a Sn composition up to 4.5% have been fabricated and characterized. The crystalline structure, surface morphology, and infrared (IR) absorption coefficient of the annealed GeSn thin films were carefully investigated. It was found that the GeSn thin films with a Sn composition of 4.5% annealed at 450 °C possessed a desirable polycrystalline structure according to X-ray diffraction (XRD) analyses and Raman spectroscopy analyses. In addition, the absorption coefficient of the polycrystalline GeSn thin films in the IR region was significantly better than that of the single crystalline bulk Ge.

Amorphous Si/SiO2 distributed Bragg reflectors with transfer printed single-crystalline Si nanomembranes

A distributed Bragg reflector (DBR) consisting of a single-crystal Si nanomembrane (NM) layer formed by the transfer printing technique on top of an evaporated amorphous Si (a-Si)/SiO2 DBR structure was demonstrated. The reflectivity of different DBR structures/pairs is measured and verified it by the simulation. An improved surface roughness of the top layer by employing a Si NM suggests that the smoother single crystalline surface not only minimizes light scattering loss but also can be an epitaxial template layer for subsequent Si growth without contributing any strain. The results indicate a simple pathway toward achieving high performance Si/SiO2 DBRs employing Si NM as a top layer. This method could also lead to the fabrication of large-area, high performance NM based DBRs at low cost with high throughput.

Optical Refractive Index Sensing Based on High-Q Bound States in the Continuum in Free-Space Coupled Photonic Crystal Slabs

High sensitivity (S) and high quality factor (Q) are desirable to achieve low detection limit in label-free optical sensors. In this paper, we theoretically demonstrate that single-layer and coupled bi-layer photonic crystal slabs (PCS) possess simultaneously high S and high Q near the bound states in the continuum (BIC). We theoretically achieved S > 800 nm/RIU and Q > 107 in refractive index sensing in the 1400–1600 nm telecom optical wavelength bands. We experimentally demonstrated an S of 94 nm/RIU and a Q of 1.2 × 104, with a detection limit of 6 × 10−5 refractive index unit. These sensor designs can find applications in biochemical sensing, environmental monitoring, and healthcare.

Athermal Photonic Crystal Membrane Reflectors on Diamond

We report here experimental demonstration of Fano resonance photonic crystal Si membrane reflectors on diamond for much improved thermal performance. At normal incidence, high reflection is obtained over a broad spectral band, with reflectivity greater than 95% from 1260 to 1450 nm. The measured reflection spectrum agrees well with the theoretical design. The improved thermal performance, with much reduced temperature rise, was also observed experimentally over a wide range of high incident optical power intensities. Such athermal Si membrane reflectors on diamond offer great opportunities for the applications in energy efficient lasers and high-power lasers.

Flexible Transient Optical Waveguides and Surface‐Wave Biosensors Constructed from Monocrystalline Silicon

Optical technologies offer important capabilities in both biological research and clinical care. Recent interest is in implantable devices that provide intimate optical coupling to biological tissues for a finite time period and then undergo full bioresorption into benign products, thereby serving as temporary implants for diagnosis and/or therapy. The results presented here establish a silicon-based, bioresorbable photonic platform that relies on thin filaments of monocrystalline silicon encapsulated by polymers as flexible, transient optical waveguides for accurate light delivery and sensing at targeted sites in biological systems. Comprehensive studies of the mechanical and optical properties associated with bending and unfurling the waveguides from wafer-scale sources of materials establish general guidelines in fabrication and design. Monitoring biochemical species such as glucose and tracking physiological parameters such as oxygen saturation using near-infrared spectroscopic methods demonstrate modes of utility in biomedicine. These concepts provide versatile capabilities in biomedical diagnosis, therapy, deep-tissue imaging, and surgery, and suggest a broad range of opportunities for silicon photonics in bioresorbable technologies.

Photonic crystal surface-emitting lasers on silicon substrates

We report here photonic crystal surface emitting lasers on bulk silicon substrates. Optically pumped lasers were demonstrated with single mode operation. Thermal resistance of such oxide-free cavity was investigated to evaluate the heat dissipation and lasing characteristics.

Free-space coupled silicon photonic crystal refractometric membrane sensors

We report free-space coupled single-layer and coupled bi-layer silicon photonic crystal refractive index sensing schemes to simultaneously achieve extremely high quality factor Q > 10⁷ and high sensitivity S of 800–900 nm/RIU, with corresponding theoretical detection limit around 10⁻⁸ RIU. Experimentally demonstrated detection limit of 10⁻⁶ RIU was achieved.