Optical Design and Simulation

Optical Design and Simulation

What is Optical Simulation Services?

In many cases, commercially available optical simulation software does not meet customer’s needs.

Optical Simulation Services

SCIVAX can offer customized solutions for sophisticated optical applications based on our staff’s extensive experience and expertise in electromagnetic field based simulation. 

Benefits to Customers: Optical Design/Simulation Services

Optical Design/Simulation Environment

SCIVAX has invested in computer system infrastructure to allow massive computer calculations needed for optical simulation

◾️Software​

1. FDTD Method Simulation Software

2. RCWA Method Simulation Software
3. BPM法 Method Simulation Software
4.Ray Tracing Simulation Software

■ Hardware
CPU Xeon 2.6GHz, Max 32 cores
Memory 512GByte
InfiniBand (High Speed Network) Connection

Optical Design/Simulation Services Flow

※ Based on structures involved and other factors, above flow may be altered.

Simulation Examples

The Nano-pattern formed on the sapphire wafer improves LED light extraction efficiency

LED external quantum efficiency simulations were performed using the FDTD method, and various designs were considered, but due the challenges of the large data calculations, no clear conclusion was reached regarding the optimal design. SCIVAX employed a proprietary simulation method in together with the RCWA method, and was succeeded in confirming an ideal design for optimizing the LED device external quantum efficiency in a short time.

Using the simulated nano-patterns, actual LEDs showed good improvements for external quantum efficiency. And also, measurement values and simulated results have good agreement. 

※SCIVAX has submitted a patent for this design

The structure above shows the LED device simulation results. The diffraction efficiency was investigated by varying the conditions little by little and performing multiple simulations. Those data were later analyzed to yield a calculation of the light output distribution and light extraction efficiencies. 

In Simulation Result Example 1, the patterned layer thickness is 1.8μm, but if the pattern is the same, analysis can be done for structured layer thicknesses up to 100μm.

Simulation Result 1

Simulation Result 2

LED Light Extraction Efficiency (RCWA Based)

Simulation Result 2 is the final output after post-processing of the data.

Diffraction direction for each order can be represented by a connection from the origin to the small polygon, and all of the diffraction efficiencies are shown in three dimensional plot. 

Simulation Result 3

Simulations are performed with nanostructure geometry entered as parameters.

A model is constructed based on data verified with AFM, TEM and other nano-precise instruments.

Example of work with Meijo University regarding LED External Quantum Efficiency

SCIVAX is cooperating with Meijo University’s Professors Akasaki and Kamiyama to develop nano-patterned sapphire substrates (nPSS) based on collaboration in optical simulation and actual nPSS fabrication with the objective of improving LED light extraction efficiency.

Research Overview

Most patterned sapphire substrates (PSS) enable some improvement in LED light extraction, but the degree of improvement depends on the patterning geometry.

Also, the layered topography of grown crystalline layers must be accounted for in making judgements regarding optimal designs.

This research relies on both optical simulation and nanoimprint lithography technologies

Unprocessed
Nano-patterned
Simulation Example

The LED devices themselves and the nano-patterns which are hundreds of times smaller must be considered together, so FDTD simulation requires long calculation times and huge amounts of memory. As a result, limited range of simulations are possible to execute. 

SCIVAX scientists realized that the periodic structure lent itself to combining the RCWA simulation method to generating a full 3D simulation and succeeded in producing the same. 

This simulation was verified by Meijo Universities Professor Kamiyama’s model.

Unprocessed (Flat)
Nano-patterned (structured)
Patterning of Sapphire Substrates

It is possible to use NIL technology to pattern sapphire substrates.

Example of Moth-Eye Antireflection Structure

Moth-Eye and other nanostructures can be formed on reflective surfaces to significantly reduce surface reflectivity.

The anti-reflection performance is greatly affected by the geometry and materials of the nanostructures.

SCIVAX performed a simulation on the effects based on the above factors as parameters.

※Measurements made with actual devices agreed with the SCIVAX simulation results to an extremely high degree. 

Simulation

Software Used: DiffractMOD (optical element analysis by the RCWA method)

Model Data (Refractive Index Distribution)

AFM Data Display

SEM Photo

Comparison to Actual Device Measurement

Measured Values

Simulation Result

Characteristics of SCIVAX's Originally Developed Ideal Moth-Eye Structure

Characteristics of SCIVAX's Originally Developed Ideal Moth-Eye Structure

With the anti-reflective structured surface entered as a parameter, continuum simulation was performed.

AFM, TEM and other data for the anti-reflective structures was used to make a model for pattern simulation and these simulations have been applied to master mold design and fabrication.

Example of Light Control Using DOE* Lens *DOE: Diffractive Optical Elements

Using a DOE lens, the direction of incident light, beam broadening angles, beam shape and focal length can be controlled.

SCIVAX can supply DOE lens technology from design to prototyping to volume production.

Model 1: Light Focusing

DOE Lens Specifications

■Refractive Index: 1.5   ■Light Source: Planewave     ■Wavelength: 0.5μm       ■Polarization: TE    ■Focal Length: 2mm

Compared to a Fresnel lens, DOE lenses can be made much thinner and can minimize losses due to stray light.

Model 2: Angular Control (-5°)

By optimizing the repeating triangular structure and geometry, it is possible to efficiently bend light to a specified angle. 

Model 3: Light Distribution Control

Light Distribution Control

■Refractive Index: 1.5   ■Light Source: Gaussian     ■Beam Spreading Angle: 20゜     ■Wavelength: 0.5μm     ■Polarization: TE     ■Light Distribution: Tophat              ■Beam Spreading Angle: 10゜ ■Light Source-DOE Distance: 5μm

It is possible to control light distribution by modifying the repeating triangular patterns and distribution based on consideration of the diffraction angle and diffraction efficiency. 

Regarding the simulation method

FDTD Method (Finite Difference Time Domain Method)

In this method, we breakdown space to a grid with a mesh pitch 1/10 of the launched wavelength, and the light propagates along with elapsed time. By applying the mesh grid, we can remove the quantum error and truly express Maxwell’s laws in the model.

When using this method, huge amounts of computer resource is required, so the simulation size will be limited to a few cubic microns with typical workstation.

Below, using the FDTD method, we modelled the behavior of light in the case of a planewave with a wavelength of 0.5μm passing through 10μm and 0.5μm slits opened in a beam blocker. In the case of the small slit, the light propagates outward as a ripple. It has not been possible model light phenomena at this level with conventional tools. This requires analysis at the electromagnetic field level.

Behavior in the case of the 10μm slit in the blocker
Behavior in the case of the 0.5μm slit in the blocker

Using SCIVAX’s proprietary simulation methods, RCWA method simulations can be applied to larger dimension optical simulations.

RCWA Method (Rigorous Coupled Wave Analysis)

The RCWA method requires launching planewave, and also periodic boundary conditions (i.e. based on infinitely repeating identical patterns) should be applied. This method takes slices of the structural object in the vertical direction (i.e. optical axis), and performs a Fourier transformation on the distribution of refractive indices in each slice and performs calculations on these base data. If the refractive index distribution is also repeating, it is possible to take each slice as one chunk of data. Thus by taking advantage of the features of the RCWA method, simulations of devices in actual size can be done, and previously this has been impossible with the FDTD method. 

Primary Output Data: Transmitted and reflected light’s diffractive efficiency (order of diffraction, diffraction angle, diffracted intensity)

 ※It makes sense that the total transmitted efficiency + the total reflected efficiency + the total absorption by structure = 1. 

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