Subwavelength High-contrast Gratings Research Articles

Sub-Wavelength Grating Lenses With a Twist

Dielectric high-contrast sub-wavelength grating (SWG) structures have received much attention in recent years, offering a new paradigm for the integration of optical systems. Their nanoscale resonant properties can result in a complex and unintuitive far-field behavior that, if carefully crafted, allows the full control of the optical phase front from a thin sub-wavelength planar layer. To date, experimental demonstrations of these new devices have only been realized with polarized light in a reflective mode, greatly limiting their use for practical systems. In this letter, we demonstrate a highly efficient, sub-wavelength thick, transmissive grating lens configuration using symmetrical resonant posts to achieve polarization-independent operation. Our transmissive SWG lenses are easily fabricated using low-cost scalable semiconductor process technology. To illustrate their performance, we demonstrate the generation of high-order orbital angular momentum beams and their use in an optical mode-isolator application that achieves a suppression ratio of over 25 dB.

A new approach to designing circular grating focusing reflector

A new approach to designing planar, high numerical aperture, low loss, focusing reflectors using circular subwavelength high contrast gratings is presented. Through analyzing particular physical scene, a mathematical transformation from existing “focus line” convergent beam, which can be achieved by bar grating reflector, to the convergent beam with a “focus point”, is obtained. By changing the shape of the bar grating reflector with the mathematical transformation obtained, a circular grating reflector, which can achieve “focus point” convergent beam, is obtained. The focusing properties and reflection characteristic of the circular grating reflector are numerically studied with the finite element method. After the radially polarized light reflected from circular grating reflector with a diameter of 29.788 μm, the beam will focus at 10 μm away from the reflector, resulting in a numerical aperture of 0.8302 and a reflectivity of 0.9163. In the focal plane, the numerical simulation results present a field distribution with a full width half maximum value of 1.5548μm, which is extremely close to diffraction limit.

High-transmittivity non-periodic sub-wavelength high-contrast grating with large-angle beam-steering ability

We present a high-transmittivity non-periodic sub-wavelength high-contrast grating (HCG) with large-angle beam-steering ability for transmitted light. The phase front profile of transmitted light is a decisive factor to the beam-steering property of the HCG. By designing the structural parameters of the HCG, both beam steering and high transmittivity can be achieved. The properties of the beam steering and transmission are numerically studied with the finite element method. The results show that the transmittivity is up to 0.91 and the steering angle is 27.42° which is consistent with the theoretical 30°.

High efficiency two-dimensional grating reflectors with angularly tunable polarization efficiency

We report on two-dimensional high-contrast subwavelength gratings with a polarization efficiency that is tunable in the angular spectrum. Due to the high index contrast it is possible to realize highly efficient reflectors with nearly angular independent reflection for light with transverse-electric polarization. Simultaneously, for transverse-magnetic polarized light the minimum and maximum reflectance can be tuned to a defined angle of incidence. Configurations with minimized and maximized polarization efficiencies at incidence angles of 0° and 45° are exemplified for a design wavelength of 1550 nm. For the case of minimum polarization efficiency we present the experimental results for a non-monolithic and a monolithic realization.

Low loss hollow-core waveguide on a silicon substrate

Abstract Optical-fiber-based, hollow-core waveguides (HCWs) have opened up many new applications in laser surgery, gas sensors, and non-linear optics. Chip-scale HCWs are desirable because they are compact, light-weight and can be integrated with other devices into systems-on-a-chip. However, their progress has been hindered by the lack of a low loss waveguide architecture. Here, a completely new waveguiding concept is demonstrated using two planar, parallel, silicon-on-insulator wafers with high-contrast subwavelength gratings to reflect light in-between. We report a record low optical loss of 0.37 dB/cm for a 9-μm waveguide, mode-matched to a single mode fiber. Two-dimensional light confinement is experimentally realized without sidewalls in the HCWs, which is promising for ultrafast sensing response with nearly instantaneous flow of gases or fluids. This unique waveguide geometry establishes an entirely new scheme for low-cost chip-scale sensor arrays and lab-on-a-chip applications.

Wavelength Trimming of Vertical-Cavity Surface-Emitting Lasers with High-Contrast Subwavelength Grating

We proposed a trimming technology for the precise control of the lasing wavelength and lasing performance of high-contrast subwavelength grating (HCG) loaded vertical-cavity surface-emitting lasers (VCSELs). The trimming is based on the postprocess surface etching of HCG. Our result obtained using rigorous coupled wave analysis (RCWA) shows a possible precise wavelength tuning range of 3 nm for GaAs-based HCG-VCSELs. Our preliminary experiment reveals an expected smallest wavelength trimming of 0.07 nm.

Wavelength Trimming of Vertical-Cavity Surface-Emitting Lasers with High-Contrast Subwavelength Grating

Wavelength Trimming of Vertical-Cavity Surface-Emitting Lasers with High-Contrast Subwavelength Grating

Matrix Fabry–Perot resonance mechanism in high-contrast gratings

We present a simple analytic formalism to explain the unique resonance phenomenon in subwavelength high-contrast gratings (HCG). We show that the resonances are due to strong coupling between two surface-normal waveguide array modes resulting from abrupt and large index contrast. Simple expression for HCG quality factor is derived that agrees with spectral-fitting approaches reported in literature.

Long-Wavelength High-Contrast Grating Vertical-Cavity Surface-Emitting Laser

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> A novel long-wavelength vertical-cavity surface-emitting laser (VCSEL) structure based on a subwavelength high-contrast grating (HCG) as the output mirror has been realized. By design, these devices are highly polarization stable, are single mode at large apertures, and solve the VCSEL-mirror problem at long wavelengths in an elegant way. With cost-effective mass fabrication in mind, the top HCG reflector consists of amorphous silicon on isolator (amorphous silica). The single-mode laser emission is tailored to be around 1320-nm wavelength, targeting applications in high-speed optical data transmission, particularly those for passive optical networks. We report single-mode emission for devices with apertures as large as 11 <formula formulatype="inline"><tex Notation="TeX">$\mu\hbox{m}$</tex> </formula> operating in continuous wave with output powers in excess of 0.4 mW. Pulsed operation with output powers up to 4 mW at room temperature is demonstrated as well. This is the first electrically pumped VCSEL structure realized in this wavelength regime utilizing an HCG mirror. </para>

Planar high-numerical-aperture low-loss focusing reflectors and lenses using subwavelength high contrast gratings

We propose planar, high numerical aperture (NA), low loss, focusing reflectors and lenses using subwavelength high contrast gratings (HCGs). By designing the reflectance and the phase of non-periodic HCGs, both focusing reflectors and lenses can be constructed. Numerical aperture values as high as 0.81 and 0.96 are achieved for a reflector and lens with very low losses of 0.3 and 0.2 dB, respectively. The design algorithm is also shown to be readily extended to a 2D lens. Furthermore, HCG optics can simultaneously focus the reflected and transmitted waves, with important technological implications. HCG focusing optics are defined by one-step photolithography and thus can be readily integrated with many devices including VCSELs, saturable absorbers, telescopes, CCDs and solar cells.

A novel ultra-low loss hollow-core waveguide using subwavelength high-contrast gratings

We propose a novel ultra-low loss single-mode hollow-core waveguide using subwavelength high-contrast grating (HCG). We analyzed and simulated the propagation loss of the waveguide and show it can be as low as 0.006 dB/m, three orders of magnitude lower than the lowest loss of the state-of-art chip-scale hollow waveguides. This novel HCG hollow-core waveguide design will serve as a basic building block in many chip-scale integrated photonic circuits enabling system-level applications including optical interconnects, optical delay lines, and optical sensors.

Surface-normal emission of a high-Q resonator using a subwavelength high-contrast grating

We report a novel high-quality (Q) factor optical resonator using a subwavelength high-contrast grating (HCG) with in-plane resonance and surface-normal emission. We show that the in-plane resonance is manifested is by a sharp, asymmetric lineshape in the surface-normal reflectivity spectrum. The simulated Q factor of the resonator is shown to be as high as 500,000. A HCG-resonator was fabricated with an InGaAs quantum well active region sandwiched in-between AlGaAs layers and a Q factor of >14,000 was inferred from the photoluminescence linewidth of 0.07 nm, which is currently limited by instrumentation. The novel HCG resonator design will serve as a potential platform for many devices including surface emitting lasers, optical filters, and biological or chemical sensors.

Single mode high-contrast subwavelength grating vertical cavity surface emitting lasers

We present a single mode vertical-cavity surface-emitting laser (VCSEL) utilizing a single-layer, high-contrast subwavelength grating (HCG) based top mirror, which provides both the ultrahigh reflectivity and lateral transverse modes control. Experimentally, we demonstrate that merely 11 HCG stripes with a thickness of 230nm are required to achieve lasing operation. The finite area of HCG, in combination with an oxide aperture of 2–6μm, effectively discriminates the higher-order transverse modes in VCSELs with a side mode suppression ratio of ⩾40dB. Furthermore, a single (first high-order transverse) mode, high output power of 2.3mW laser was achieved from a VCSEL with a 10μm oxide aperture.

A nanoelectromechanical tunable laser

The ability to tune the frequency of an oscillator is of critical importance and is a fundamental building block for many systems, be they mechanical or electronic1,2. However, this very important function is still highly inadequate in optical oscillators, particularly in semiconductor laser diodes3,4. The limitations in tuning a laser frequency (or wavelength) include the tuning range and the speed of tuning, which is typically milliseconds or slower. In addition, the tuning is often not continuous and may require complex synchronization of several electrical control signals. In this Letter, we present a new tunable laser structure with a lightweight nanoelectromechanical mirror based on a single-layer, high-contrast subwavelength grating. The high-contrast subwavelength grating reflector enables a drastic reduction of the mirror mass, which increases the mechanical resonant frequency and hence tuning speed5. This allows for a wavelength-tunable light source with potential switching speeds of the order of tens of nanoseconds and suggests various new areas of practical application, such as bio- or chemical sensing6,7,8, chip-scale atomic clocks9 and projection displays10,11.