Monolithic High-contrast Grating Research Articles

Monolithic High Contrast Grating Integrated with Metal: Infrared Electrode with Exceptionally High Conductivity and Transmission

AbstractThe design of transparent conductive electrodes (TCEs) for optoelectronic devices requires a trade‐off between high conductivity and transmittivity, limiting their efficiency. This paper demonstrates the best ever achieved TCEs with the novel approach to fabricating TCEs that effectively alleviates this trade‐off: a monolithic high contrast grating integrated with metal (metalMHCG). The metalMHCG enables higher electrical conductivity than other TCEs, while providing transmissive and antireflective properties. It focuses on infrared spectrum TCEs, which are essential for sensing, thermal imaging, and automotive applications. However, due to elevated free carrier absorption, they are much more demanding than TCEs for the visible spectrum. It demonstrates a record 75% absolute transmittiance of unpolarized light, resulting in a record 108% transmittance relative to plain GaAs substrate. It achieves even larger absolute transmittance of polarized light, reaching 92% or 133% relative transmittance. Despite the record high transmittance, the sheet resistance of the metalMHCG is the best ever reported, several times lower than any other TCE, ranging from 0.5 to 1 Ω Sq−1.

Monolithic high contrast grating on GaSb/AlAsSb based epitaxial structures for mid-infrared wavelength applications.

We demonstrate monolithic high contrast gratings (MHCG) based on GaSb/AlAs0.08Sb0.92 epitaxial structures with sub-wavelength gratings enabling high reflection of unpolarized mid-infrared radiation at the wavelength range from 2.5 to 5 µm. We study the reflectivity wavelength dependence of MHCGs with ridge widths ranging from 220 to 984 nm and fixed 2.6 µm grating period and demonstrate that peak reflectivity of above 0.7 can be shifted from 3.0 to 4.3 µm for ridge widths from 220 to 984 nm, respectively. Maximum reflectivity of up to 0.9 at 4 µm can be achieved. The experiments are in good agreement with numerical simulations, confirming high process flexibility in terms of peak reflectivity and wavelength selection. MHCGs have hitherto been regarded as mirrors enabling high reflection of selected light polarization. With this work, we show that thoughtfully designed MHCG yields high reflectivity for both orthogonal polarizations simultaneously. Our experiment demonstrates that MHCGs are promising candidates to replace conventional mirrors like distributed Bragg reflectors to realize resonator based optical and optoelectronic devices such as resonant cavity enhanced light emitting diodes and resonant cavity enhanced photodetectors in the mid-infrared spectral region, for which epitaxial growth of distributed Bragg reflectors is challenging.

Threshold performance of pulse-operating quantum-cascade vertical-cavity surface-emitting lasers.

Quantum-cascade (QC) vertical-cavity surface-emitting lasers (VCSELs) could combine the single longitudinal mode operation, low threshold currents, circular output beam, and on-wafer testing associated with VCSEL configuration and the unprecedented flexibility of QCs in terms of wavelength emission tuning in the infrared spectral range. The key component of QC VCSEL is the monolithic high-contrast grating (MHCG) inducing light polarization, which is required for stimulated emission in unipolar quantum wells. In this paper, we demonstrate a numerical model of the threshold operation of a QC VCSEL under the pulse regime. We discuss the physical phenomena that determine the architecture of QC VCSELs. We also explore mechanisms that influence QC VCSEL operation, with particular emphasis on voltage-driven gain cumulation as the primary mechanism limiting QC VCSEL efficiency. By numerical simulations, we perform a thorough analysis of the threshold operation of QC VCSELs. We consider the influence of optical and electrical aperture dimensions and reveal the range of aperture values that enable single transversal mode operation as well as low threshold currents.

Experimental Demonstration of Light Focusing Enabled by Monolithic High-Contrast Grating Mirrors.

We present the first experimental demonstration of a planar focusing monolithic subwavelength grating mirror. The grating is formed on the surface of GaAs and focuses 980 nm light in one dimension on the high-refractive-index side of the mirror. According to our measurements, the focal length is 475 μm (300 μm of which is GaAs) and the numerical aperture is 0.52. The intensity of the light at the focal point is 23 times larger than that of the incident light. To the best of our knowledge, this is the highest value reported for a grating mirror. Moreover, the full width at half-maximum (FWHM) at the focal point is only 3.9 μm, which is the smallest reported value for a grating mirror. All of the measured parameters are close to or very close to the theoretically predicted values. Our realization of a sophisticated design of a focusing monolithic subwavelength grating opens a new avenue to technologically simple fabrication of the gratings for use in diverse optoelectronic materials and applications.

Nearly perfect transmission of unpolarized infrared radiation through a one-dimensional metal grating embedded in a monolithic high-contrast grating.

We demonstrate a conceptually simple polarization-independent mechanism for nearly perfect infrared light transmission through a subwavelength one-dimensional metal grating implemented in the grooves of a deep-subwavelength monolithic high-contrast grating (metalMHCG). We provide theoretical background explaining the transmission mechanism, which eliminates Fresnel reflection as well as significantly reduces metal absorption and the reflection of transverse electric and transverse magnetic light polarizations. Careful design of a metalMHCG implemented at the interface between the regions of high refractive index contrast enables the coincidence of high transmission conditions for both light polarizations, enabling up to 97% transmission of polarization-independent infrared radiation. Our analysis shows excellent electrical properties of the metalMHCG as evidenced by sheet resistance of 2 ΩSq-1 facilitating straightforward horizontal electron transport and vertical injection of the current into the semiconductor substrate on which the electrode is implemented.

Transparent electrode employing deep-subwavelength monolithic high-contrast grating integrated with metal.

This paper demonstrates designs of transparent electrodes for polarized light based on semiconductor deep-subwavelength monolithic high-contrast gratings integrated with metal (metalMHCG). We provide theoretical background explaining the phenomena of high transmittance in the gratings and investigate their optimal parameters, which enable above 95% transmittance for sheet resistance of 2 ΩSq-1 and over 90% transmittance for extremely small sheet resistance of 0.04 ΩSq-1 in a broad spectral range below the semiconductor band-gap. The analysis is based on our fully vectorial optical model, which has been verified previously via comparison with the experimental characteristics of similar structures. The transparent electrodes can be realized in any high refractive index material used in optoelectronics and designed for light in spectral ranges starting from ultra-violet with no upper limit for the wavelength of the electromagnetic waves. They not only enable lateral transport of electrons but can also be used as an electric contact for injecting current into a semiconductor.

Tuning of reflection spectrum of a monolithic high-contrast grating by variation of its spatial dimensions.

We report the first experimental parametric analysis of subwavelength monolithic high-contrast grating (MHCG) mirrors. To date, subwavelength grating mirrors have been fabricated by suspending a thin grating membrane in the air or placing it on a low refractive index material - a scheme that requires sophisticated processing and makes the gratings sensitive to mechanical stress, impeding current injection, and heat dissipation if used in active devices. Inherently MHCGs are well suited for optoelectronic devices because they can be fabricated in all possible material systems. Here we demonstrate above 90% optical power reflectance, strong polarization discrimination. Based on experimental analysis aided by numerical simulations, we demonstrate the possibility of tuning the spectral characteristics of MHCGs reflectance for more than 200 nm via modification of the duty cycle of the MHCG stripes. We show our MHCG tuning method is convenient to define the properties of MHCG devices during the device processing.

Polymer-based MHCG as selective mirror

In this paper, we present the simulation and preparation in IP-Dip material of a polymer grating, which is used as a highly reflective and spectrally selective mirror based on monolithic high-contrast grating (MHCG) geometry. We simulated the power reflectance as a function of the grating parameters and prepared the grating structure in IP-Dip material using three-dimensional laser lithography. The 3D morphology of the prepared devices was analyzed using a scanning electron microscope. Finally, their spectral power reflectance characteristics were measured and compared with the simulations. The prepared and simulated MHCG structures exceeded 70% power reflectance in the narrow spectral region of 740–810 nm.

Monolithic High Contrast Grating Nitride-Based VECSEL

This paper proposes a new design for a nitride-based vertical-external-cavity surface-emitting laser (VECSEL) emitting at 394 nm. The structure contains a monolithic high contrast grating (MHCG) instead of a conventional distributed Bragg reflector (DBR), makes the vertical design of the laser more compact. In a numerical analysis, we consider several designs for MHCG VECSELs and show that replacing the DBR with an MHCG improves laser performance by 50%, in terms of output energy and slope efficiency. Further optimization of the MHCG VECSEL design achieves an almost threefold improvement in both values.

Design and study of a long-wavelength monolithic high-contrast grating resonant-cavity-enhanced photodetector

Novel construction of a resonant-cavity-enhanced photodetector (RCE-PD) with monolithic high-contrast grating (HCG) is proposed to overcome the difficulty of fabricating a high reflective mirror of RCE-PD at 1 550 nm. In this structure, HCG serves as the top mirror of the RCE-PD, whereas InGaAs serves as a sacrificial layer to achieve monolithic integration. During the bandwidth optimization, the ratio of the thickness of the total intrinsic region and the absorption layer is introduced to realize the simultaneous optimization of the thickness of spacing layers and absorption layer. After structural optimization, the quantum efficiency of the device with diameter of 20 μm is 82% at 1 550 nm, and the 3-dB bandwidth is 34 GHz at a bias of 3 V.

Monolithic High-Contrast Grating Based Polariton Laser

High-reflectance gratings promise new generations of vertical-cavity lasers that allow flexible control of the lasing mode properties and better integration. To date, conventional and polariton vertical-cavity lasers have been created with high-contrast gratings surrounded by air. Suspended in air, these gratings are delicate to fabricate and use and are difficult for precise property control, electrical injection, or thermal management. Here, we demonstrate a high-quality vertical-cavity laser with a nonsuspended monolithic grating. High reflectance is achieved to allow the strong coupling regime, verified by angle-resolved photoluminescence measurements. We demonstrate polarized polariton lasing, manifested by a superlinear increase in the polariton emission and line width narrowing. We also demonstrate multiwavelength operation of the polariton laser, with the wavelength controlled by design.

Optimal parameters of monolithic high-contrast grating mirrors.

In this Letter a fully vectorial numerical model is used to search for the construction parameters of monolithic high-contrast grating (MHCG) mirrors providing maximal power reflectance. We determine the design parameters of highly reflecting MHCG mirrors where the etching depth of the stripes is less than two wavelengths in free space. We analyze MHCGs in a broad range of real refractive index values corresponding to most of the common optoelectronic materials in use today. Our results comprise a complete image of possible highly reflecting MHCG mirror constructions for potential use in optoelectronic devices and systems. We support the numerical analysis by experimental verification of the high reflectance via a GaAs MHCG designed for a wavelength of 980 nm.

Long-Wavelength Tunable Detector Using High-Contrast Grating

We report a tunable 1550-nm detector using a monolithic high-contrast grating (HCG) vertical-cavity surface-emitting laser. The detector is resonance enhanced, with a responsivity demonstrated as large as 1 A/W. It features a strong wavelength selectivity over a wide tuning range, enabled by the highly reflective microelectromechanically actutable HCG mirror. A tuning range of 33.5 nm and a spectral width of 1.2 nm are demonstrated. We show that it can select a specific data channel with 1 Gb/s data rate and reject adjacent channels 4 nm away. Higher detection speed at 10 Gb/s is also demonstrated. We show three special functionalities of this tunable detector. The first one is dual functionality-not only as a receiver but also a transmitter-with a simple change of its bias polarity. Though long sought after, a tunable bifunctional device had not been realized before. An error-free high-speed link over 25 km single-mode-fiber is demonstrated between two such devices. The second special property is its wavelength-self-tracking configuration, i.e., its resonance wavelength can automatically track the input light's wavelength. Finally, we demonstrate its use as a fast on-chip optical spectrometer, with an operation speed at 200 kHz. This tunable detector would be useful for a variety of wavelength-division-multiplexing network and lab-on-a-chip systems.