Silicon carbide is best known as a wide-bandgap semiconductor for high-power and high-temperature electronics, but its optical properties are becoming increasingly important in photonics, infrared optics, nonlinear optics and quantum technologies.

Among these properties, the refractive index of SiC is particularly important because it determines how light propagates, reflects, refracts and is confined inside the material.

A common mistake is to describe silicon carbide using a single refractive-index value such as:

n ≈ 2.6

In reality, the refractive index of SiC depends on several factors:

• SiC polytype
• Wavelength
• Light polarization
• Crystal orientation
• Temperature
• Doping concentration
• Material quality

For the two most important hexagonal polytypes, 4H-SiC and 6H-SiC, the optical response is also anisotropic. This means that ordinary and extraordinary polarized light experience slightly different refractive indices.

This guide explains the refractive index of 4H-SiC and 6H-SiC, how it changes with wavelength, why birefringence occurs, and what these properties mean for optical and photonic applications.

What Is the Refractive Index of Silicon Carbide?

The refractive index, normally represented by n, describes how strongly light slows down when it enters a material.

In simplified form:

n = c / v

where:

• c = speed of light in vacuum
• v = phase velocity of light in the material

A higher refractive index generally means stronger optical confinement and greater refraction at the interface between two materials.

For visible and near-infrared wavelengths, silicon carbide typically has a refractive index in the approximate range of:

n ≈ 2.5–2.8

depending on polytype, wavelength and polarization.

This is relatively high compared with many conventional optical materials, making SiC attractive for compact waveguides, resonators and photonic devices.

4H-SiC vs. 6H-SiC

4H-SiC and 6H-SiC have the same basic chemical composition:

SiC

but differ in the stacking sequence of silicon-carbon bilayers along the crystal c-axis.

Their electrical properties differ significantly enough that 4H-SiC is now dominant in power electronics.

Optically, however, the two polytypes are much more similar.

Published optical studies show that the refractive indices of 4H-SiC and 6H-SiC generally differ by less than about 1% at the same wavelength, although the exact difference depends on polarization and wavelength.

A simplified comparison is:

The bandgap values of approximately 3.26 eV for 4H-SiC and 3.02 eV for 6H-SiC are widely reported for the two polytypes.

Ordinary and Extraordinary Refractive Index

Unlike isotropic optical materials, hexagonal SiC is a uniaxial crystal.

This means the optical response depends on the relationship between light polarization and the crystallographic c-axis.

Two refractive indices are therefore commonly used:

Ordinary refractive index

nₒ

The ordinary refractive index describes one polarization orientation relative to the optical axis.

Extraordinary refractive index

nₑ

The extraordinary refractive index applies to the orthogonal polarization state and is usually slightly higher in 4H-SiC and 6H-SiC.

This difference is called:

birefringence

or:

Δn = nₑ − nₒ

Because SiC is birefringent, an optical device using a SiC substrate may behave differently for TE- and TM-like polarization modes.

Representative 4H-SiC Refractive Index Values

Measurements of 4H-SiC clearly show that refractive index decreases as wavelength increases.

Representative room-temperature values reported for 4H-SiC include:

These values illustrate both dispersion and birefringence in 4H-SiC.

The key trend is simple:

As wavelength increases from the visible toward the near-infrared, the refractive index gradually decreases.

Representative 6H-SiC Refractive Index Values

6H-SiC shows very similar behavior.

Published visible-wavelength measurements include:

These results again show that:

nₑ > nₒ

and both indices decrease with increasing wavelength.

At telecom wavelengths near 1550 nm, a representative refractive index of approximately 2.59 is often used for 6H-SiC, compared with roughly 2.56 for 4H-SiC in simplified photonics calculations.

Why Does the SiC Refractive Index Change with Wavelength?

The wavelength dependence of refractive index is called:

optical dispersion

Electronic polarization inside the SiC crystal responds differently depending on the frequency of the electromagnetic field.

As the photon energy approaches electronic transitions near the band edge, refractive index generally rises.

At longer wavelengths, farther away from these electronic resonances, the index becomes lower and changes more slowly.

For example, 6H-SiC ordinary refractive-index data show values around:

• ~2.56 at photon energy 1 eV
• ~2.64 at 2 eV
• ~2.75 at 3 eV

demonstrating increasing refractive index toward shorter wavelengths and higher photon energies.

This behavior is important when designing optical devices over a wide wavelength range.

Silicon Carbide Dispersion

The relationship between refractive index and wavelength can be described using equations such as:

• Sellmeier equations
• Cauchy equations
• Oscillator models

For SiC photonics, Sellmeier equations are commonly used because they provide continuous refractive-index values across a wavelength range rather than only at individual measured wavelengths.

Researchers have measured both 4H-SiC and 6H-SiC at wavelengths from approximately 404.7 nm to 2325.4 nm and developed temperature-dependent Sellmeier equations for both polytypes.

This is much more useful for optical design software than simply entering:

n = 2.6

for every wavelength.

Refractive Index at 1550 nm

The wavelength around:

1550 nm

is particularly important for photonics because it lies in the standard telecommunications band.

Representative literature values are approximately:

These simplified values are often used for initial optical design.

However, for precise waveguide or resonator simulations, engineers should use polarization-dependent Sellmeier data because 4H-SiC and 6H-SiC are birefringent.

For 4H-SiC, for example, measured values around 1530 nm are approximately:

nₒ ≈ 2.564

and

nₑ ≈ 2.609.

A difference of about 0.045 is large enough to significantly influence photonic-device behavior.

Why Birefringence Matters

In a birefringent material, light polarized in different directions experiences different refractive indices.

This can produce:

• Different optical phase velocities
• Polarization-dependent resonance wavelengths
• TE/TM mode splitting
• Different waveguide effective indices
• Polarization rotation
• Phase-matching effects

For simple semiconductor power devices, birefringence may not be important.

For precision photonics, however, it can become a critical design parameter.

SiC Refractive Index and Wafer Orientation

Commercial 4H-SiC wafers are most commonly supplied with the surface close to the:

(0001) basal plane

with either on-axis or off-axis orientation.

The crystallographic c-axis is approximately normal to the wafer surface.

This orientation is important when determining whether the electric field of a guided optical mode samples the ordinary or extraordinary refractive index.

When designing:

• SiC waveguides
• Microresonators
• Optical cavities
• Polarization-sensitive components

the wafer orientation should therefore be included in the optical model.

Temperature Dependence of SiC Refractive Index

Refractive index is also temperature dependent.

This behavior is described by the thermo-optic coefficient:

dn/dT

Experiments on 4H-SiC and 6H-SiC from approximately 293 K to 493 K show that both ordinary and extraordinary refractive indices increase as temperature rises.

The thermo-optic effect becomes especially important in devices such as:

• Ring resonators
• Interferometers
• Optical filters
• Lasers
• Modulators
• Temperature sensors

A small change in refractive index can shift an optical resonance significantly.

Research at 1550 nm has therefore examined the temperature dependence of the thermo-optic coefficient of 4H-SiC for integrated photonics applications.

Does Doping Affect the Refractive Index?

Yes, particularly when carrier concentrations become high.

Free carriers can affect both:

• Real refractive index n
• Extinction coefficient k

The effect tends to become more important in infrared wavelengths.

Therefore, optical constants measured on:

• Semi-insulating SiC
• N-type SiC
• Heavily doped SiC

may not be identical.

For precision optical systems, the substrate specification should include doping type and resistivity rather than assuming all SiC wafers have identical optical constants.

Refractive Index vs. Extinction Coefficient

Optical behavior is more completely described by the complex refractive index:

N = n + ik

where:

• n = real refractive index
• k = extinction coefficient

The refractive index determines phase propagation.

The extinction coefficient describes absorption.

For transparent optical applications, engineers normally want:

highly controlled n + very low k

over the target wavelength range.

Spectroscopic ellipsometry studies have measured the optical constants of 3C-, 4H- and 6H-SiC from infrared through ultraviolet wavelengths, showing that both refractive index and absorption behavior change significantly across the spectrum.

4H-SiC Optical Band Edge

Bandgap also influences optical transmission.

4H-SiC has a bandgap of approximately:

3.26 eV

while 6H-SiC is approximately:

3.02 eV.

Using the approximate relationship between photon energy and wavelength, this places the fundamental absorption edge roughly around:

• 4H-SiC: ~380 nm
• 6H-SiC: ~410 nm

Therefore, high-quality semi-insulating SiC can transmit a significant portion of visible and infrared light above the absorption-edge wavelength, although actual transmission also depends on:

• Doping
• Defects
• Thickness
• Surface finish
• Impurities
• Free-carrier absorption

Why SiC Is Attractive for Integrated Photonics

A refractive index around 2.6 is significantly higher than that of many conventional optical cladding materials.

This creates strong refractive-index contrast.

For example, when SiC is combined with a lower-index oxide or air, light can be strongly confined in a relatively small waveguide.

Strong optical confinement allows:

• Compact bend radii
• Small resonator dimensions
• High optical intensity
• Reduced photonic-device footprint

These characteristics make SiC suitable for integrated photonics.

Optical Application 1: SiC Waveguides

Waveguides rely on a higher-index core surrounded by lower-index material.

SiC’s relatively high refractive index makes it an effective optical core material.

Possible structures include:

• SiC-on-insulator
• SiC-on-SiO₂
• Suspended SiC waveguides
• Ridge waveguides
• Rib waveguides

Accurate refractive-index data is necessary to calculate:

• Effective refractive index
• Mode size
• Mode confinement
• Cutoff conditions
• Dispersion
• Bend loss

Using an incorrect SiC index can shift simulated device performance significantly.

Optical Application 2: Microresonators

SiC can be fabricated into:

• Microrings
• Microdisks
• Whispering-gallery resonators

The resonant wavelength depends strongly on refractive index.

Therefore, changes caused by:

• Temperature
• Wafer orientation
• Fabrication geometry
• Material dispersion

can shift the resonance.

This is why accurate SiC dispersion and thermo-optic data are essential in resonator design.

Optical Application 3: Nonlinear Photonics

Hexagonal SiC has attracted interest for nonlinear optics because it supports both second-order and third-order nonlinear processes.

Potential effects include:

• Second-harmonic generation
• Third-harmonic generation
• Four-wave mixing
• Self-phase modulation
• Optical frequency conversion

The non-centrosymmetric structure of 4H-SiC and 6H-SiC allows second-order optical nonlinearities.

Photonic research has demonstrated the potential of SiC platforms for these nonlinear processes.

Accurate refractive-index dispersion is particularly important because nonlinear frequency conversion often requires:

phase matching

or engineered dispersion.

Optical Application 4: Quantum Photonics

SiC also contains optically active defects that can act as quantum emitters.

Examples include:

• Silicon vacancies
• Divacancies
• Other color centers

This makes SiC interesting for combining:

quantum emitters + optical cavities + integrated waveguides

on a single semiconductor platform.

The refractive index determines how emitted photons are confined and coupled into photonic structures.

Optical Application 5: Infrared and THz Optics

SiC optical properties extend beyond visible and telecom wavelengths.

Research has investigated 3C-, 4H- and 6H-SiC across the 2–16 µm infrared range, including temperature-dependent optical behavior and phonon resonances.

In the terahertz region, refractive index becomes significantly higher.

Measurements of 4H-SiC show an ordinary refractive index around:

3.14–3.15 between approximately 1 and 4 THz

with an extraordinary value around 3.24 at approximately 2 THz.

This demonstrates an important principle:

SiC refractive index should never be treated as one fixed number across the electromagnetic spectrum.

Optical Application 6: High-Temperature Optical Devices

One major advantage of SiC is its excellent thermal and chemical stability.

Compared with many conventional photonic materials, SiC can operate in harsher environments involving:

• Elevated temperatures
• High optical powers
• Radiation
• Chemical exposure
• Mechanical stress

Possible future applications include:

• High-temperature optical sensing
• Aerospace photonics
• Harsh-environment spectroscopy
• Industrial process monitoring

In these systems, temperature-dependent refractive-index data becomes particularly important.

4H-SiC or 6H-SiC for Optical Applications?

There is no universal answer.

From an optical-index perspective, the difference between 4H and 6H is relatively small.

Choose 4H-SiC when:

• Semiconductor-grade 4H wafers are readily available
• Integration with SiC electronics is required
• Modern SiC photonic device research is being followed
• Standard commercial substrates are preferred

Choose 6H-SiC when:

• Specific optical or nonlinear properties are required
• Existing device designs use 6H-SiC
• Research requires comparison between polytypes
• Legacy substrates or specialized crystal orientations are used

For most new semiconductor-integrated photonics projects, 4H-SiC is generally the more commercially relevant substrate.

Optical-Grade SiC Wafer Specifications

If a SiC wafer will be used for optical or photonic applications, electrical specifications alone may not be sufficient.

Useful RFQ parameters include:

For transmission or nonlinear optical experiments, double-side polished SiC wafers are often preferable because backside roughness can introduce scattering.

Why Surface Roughness Matters

Even if bulk SiC has excellent optical properties, poor surface quality can increase:

• Scattering
• Propagation loss
• Resonator loss
• Background noise
• Coupling loss

This becomes especially important for:

• Waveguides
• Optical windows
• Fabry-Pérot cavities
• High-Q resonators

Sub-nanometer surface roughness is often desirable for precision photonic structures.

Why Thickness Matters

Wafer thickness may influence optical behavior in transmission experiments.

Parallel polished surfaces can produce:

Fabry-Pérot interference

because light reflects repeatedly between the front and rear surfaces.

Thickness should therefore be controlled when:

• Measuring refractive index
• Measuring transmission
• Performing spectroscopy
• Designing etalons
• Building resonant optical structures

Measuring SiC Refractive Index

Common measurement methods include:

Spectroscopic Ellipsometry

Useful for determining:

• n
• k
• Film thickness
• Optical anisotropy

It is widely used for SiC films and wafers.

Prism Minimum-Deviation Method

High-quality SiC crystals can be machined into prism geometries to accurately measure bulk refractive index.

This method has been used for detailed wavelength- and temperature-dependent studies of 4H-SiC and 6H-SiC.

Transmission Interference

Parallel thin samples can produce interference fringes.

The fringe spacing can be used to calculate refractive index.

THz Time-Domain Spectroscopy

Used for terahertz-frequency optical constants.

This technique has been applied to 4H-SiC to measure both ordinary and extraordinary refractive indices.

Common Mistakes When Using SiC Refractive-Index Data

Mistake 1: Using n = 2.6 at Every Wavelength

This may be acceptable for a rough calculation but not precision photonic design.

Use wavelength-dependent data.

Mistake 2: Ignoring Birefringence

4H-SiC and 6H-SiC are anisotropic.

Use ordinary and extraordinary indices where necessary.

Mistake 3: Ignoring Temperature

Temperature changes refractive index and can shift resonances.

Mistake 4: Mixing Data from Different Polytypes

3C-SiC, 4H-SiC and 6H-SiC should not automatically be assigned identical optical constants.

Mistake 5: Ignoring Doping

High carrier concentration can alter infrared optical properties.

Mistake 6: Ignoring the Extinction Coefficient

Refractive index alone does not describe optical absorption.

For accurate device simulation, both n and k may be required.

FAQ

What is the refractive index of silicon carbide?

For visible and near-infrared wavelengths, SiC typically has a refractive index of approximately 2.5–2.8, depending on wavelength, polytype and polarization.

What is the refractive index of 4H-SiC at 1550 nm?

A commonly used approximate value is about 2.56.

More detailed measurements near 1530 nm give approximately nₒ = 2.564 and nₑ = 2.609 for 4H-SiC.

What is the refractive index of 6H-SiC at 1550 nm?

A representative simplified value is approximately 2.59, although the exact value depends on polarization and temperature.

Is 4H-SiC birefringent?

Yes.

4H-SiC is a positive uniaxial optical material, so its ordinary and extraordinary refractive indices are different.

Does SiC refractive index decrease with wavelength?

Generally yes through much of the visible and near-infrared transparency region.

For example, 4H-SiC ordinary refractive index decreases from approximately 2.76 near 405 nm to about 2.54 near 2.3 µm.

Does temperature affect SiC refractive index?

Yes.

Measurements show that ordinary and extraordinary refractive indices of both 4H-SiC and 6H-SiC increase with increasing temperature.

Which has a higher refractive index, 4H-SiC or 6H-SiC?

The values are very close.

Depending on wavelength and polarization, one may be slightly higher than the other. In most practical comparisons, the difference is relatively small and commonly below approximately 1%.

Conclusion

The refractive index of silicon carbide cannot be accurately represented by a single number.

For the most important hexagonal polytypes, 4H-SiC and 6H-SiC, refractive index depends strongly on:

wavelength + polarization + crystal orientation + temperature + material condition

In visible and near-infrared wavelengths, SiC generally has a relatively high refractive index of approximately 2.5–2.8, providing strong optical confinement for photonic devices.

At telecom wavelengths near 1550 nm, representative values are approximately:

4H-SiC: ~2.56

6H-SiC: ~2.59

while the extraordinary refractive index can be noticeably higher because both materials are birefringent.

These optical characteristics, combined with SiC’s wide bandgap, thermal stability and nonlinear properties, make it increasingly attractive for:

• Integrated photonics
• Optical waveguides
• Microresonators
• Nonlinear optics
• Quantum photonics
• Infrared optics
• THz devices
• Harsh-environment optical sensing

For precision optical design, engineers should therefore use wavelength- and polarization-dependent refractive-index data rather than a single nominal SiC refractive-index value.