Mayank Bahl

Mixed-level optical simulations of light-emitting diodes based on a combination of rigorous electromagnetic solvers and Monte Carlo ray-tracing methods

Abstract. Over the last two decades, extensive research has been done to improve light-emitting diodes (LEDs) designs. Increasingly complex designs have necessitated the use of computational simulations which have provided numerous insights for improving LED performance. Depending upon the focus of the design and the scale of the problem, simulations are carried out using rigorous electromagnetic (EM) wave optics-based techniques, such as finite-difference time-domain and rigorous coupled wave analysis, or through ray optics-based techniques such as Monte Carlo ray-tracing (RT). The former are typically used for modeling nanostructures on the LED die, and the latter for modeling encapsulating structures, die placement, back-reflection, and phosphor downconversion. This paper presents the use of a mixed-level simulation approach that unifies the use of EM wave-level and ray-level tools. This approach uses rigorous EM wave-based tools to characterize the nanostructured die and generates both a bidirectional scattering distribution function and a far-field angular intensity distribution. These characteristics are then incorporated into the RT simulator to obtain the overall performance. Such a mixed-level approach allows for comprehensive modeling of the optical characteristic of LEDs, including polarization effects, and can potentially lead to a more accurate performance than that from individual modeling tools alone.

Optical simulations of organic light-emitting diodes through a combination of rigorous electromagnetic solvers and Monte Carlo ray-tracing methods

Over the last two decades there has been extensive research done to improve the design of Organic Light Emitting Diodes (OLEDs) so as to enhance light extraction efficiency, improve beam shaping, and allow color tuning through techniques such as the use of patterned substrates, photonic crystal (PCs) gratings, back reflectors, surface texture, and phosphor down-conversion. Computational simulation has been an important tool for examining these increasingly complex designs. It has provided insights for improving OLED performance as a result of its ability to explore limitations, predict solutions, and demonstrate theoretical results. Depending upon the focus of the design and scale of the problem, simulations are carried out using rigorous electromagnetic (EM) wave optics based techniques, such as finite-difference time-domain (FDTD) and rigorous coupled wave analysis (RCWA), or through ray optics based technique such as Monte Carlo ray-tracing. The former are typically used for modeling nanostructures on the OLED die, and the latter for modeling encapsulating structures, die placement, back-reflection, and phosphor down-conversion. This paper presents the use of a mixed-level simulation approach which unifies the use of EM wave-level and ray-level tools. This approach uses rigorous EM wave based tools to characterize the nanostructured die and generate both a Bidirectional Scattering Distribution function (BSDF) and a far-field angular intensity distribution. These characteristics are then incorporated into the ray-tracing simulator to obtain the overall performance. Such mixed-level approach allows for comprehensive modeling of the optical characteristic of OLEDs and can potentially lead to more accurate performance than that from individual modeling tools alone.

Accounting for coherent effects in the ray-tracing of light-emitting diodes with interface gratings via mixed-level simulation

Abstract. Ray-tracing (RT) has long been the workhorse technique for analyzing light-emitting diode (LED) dies and packages and has led to significant improvements in extraction efficiency and beam shaping. However, to achieve further enhancements, nano-/microscale features such as patterned substrates and surface textures have been explored. The coherent effects arising from these near/subwavelength features are difficult to include in the RT of the packaged device. We show that under certain conditions these effects can have a significant impact on LED performance, especially if back-reflectors are present. Furthermore, we demonstrate that coherence must be accounted for even in structures that would otherwise be considered as having relatively large feature sizes, such as gratings with periods many times the wavelength. We present comparisons between the optical responses of prototypical periodically patterned substrates modeled with RT alone and with a mixed-level approach that combines RT and rigorous electromagnetic simulation, such as rigorous coupled wave analysis and finite-difference time-domain. Several examples with varying lateral periods are computed with both methods. It is shown that these results may differ, and that these differences can be significant if back reflection is present. We conclude that a mixed-level approach is an efficient and accurate method to model light extraction in modern LEDs.

Broadband 3D optical modeling of HgCdTe infrared focal plane arrays

We present a modeling technique for the efficient broadband simulation of infrared HgCdTe-based focal plane arrays. The approach performs a single broadband FullWAVETM FDTD optical simulation and a series of discrete Fourier transforms for obtaining absorbed photon density profiles on a set of frequencies. These distributions are then aggregated, through a weighted average, and imported as a generation term into a single Sentaurus Device electrical simulation, which calculates the corresponding photocurrent and inter-pixel crosstalk. This technique can save an order of magnitude in memory and computation time compared to performing multiple monochromatic optical and electrical simulations.

Non-Monochromatic 3D Optical Simulation of HgCdTe Focal Plane Arrays

Combined optical and electrical simulations of infrared HgCdTe-based focal plane arrays under broadband, non-monochromatic illumination are critically relevant to the design of small-volume detectors with sub-wavelength pixel pitches. We present an efficient technique, based on a single finite-difference time-domain electromagnetic simulation, that provides the photogeneration rate profile due to realistic, broadband optical sources, avoiding multiple monochromatic simulations. This technique is applied to assess the effects of the temperature of blackbody optical sources on quantum efficiency and inter-pixel crosstalk of planar LWIR arrays.

Modeling diffractive effects due to micro-lens arrays on liquid crystal panels in projectors

The components in optical projectors are becoming increasingly smaller due to the need for increased output resolution and the desire for small form-factor devices. One such component is Liquid Crystal (LC) panels, that utilize periodic micro-lens arrays which become more sensitive to diffractive effects as the period becomes near/sub wavelength. This paper explores the diffraction effects within these systems through numerical modeling. Traditionally Ray tracing techniques have been used for analyzing projection systems and has led to significant improvements in illumination uniformity and efficiency. However, increasingly complex projector designs that incorporate smaller geometric features like micro/nano lens arrays, including coherent diffraction and interference effects arising from such structures, cannot be handled by ray-tracing approaches alone. Rigorous electromagnetic (EM) wave optics based techniques, such as finite-difference time-domain (FDTD) and rigorous coupled wave analysis (RCWA) which solve Maxwell’s equations must be used. These rigorous EM techniques, however, have difficulty in analyzing the larger projector structures due to computational resource limitations. We use a mixedlevel optical simulation methodology which unifies the use of rigorous EM wave-level and ray-level tools for analyzing projector performance. This approach uses rigorous EM wave based tools to characterize the LC panel through a Bidirectional Scattering Distribution function (BSDF) file. These characteristics are then incorporated into the ray-tracing simulator for the illumination and imaging system design and to obtain the overall performance. Such a mixed-level approach allows for comprehensive modeling of the optical characteristic of projectors, including coherent effects, and can potentially lead to more accurate performance than that from individual modeling tools alone.

Mixed-level simulation of opto-electronic devices

Presented here are two important devices that cannot be modeled accurately and/or tractably by a single simulation technique. Simulation flows to address each device are presented. The first is a patterned Light Emitting Diode (LED), the optical modeling of which requires a mixed-level simulation approach combining FDTD (or RCWA) and Ray Tracing. The second is a CMOS Image Sensors (CIS), which requires process, optical and electrical simulation techniques.