Indium phosphide is the III-V compound semiconductor that actually emits and amplifies light on a chip, making it the incumbent gain material for photonic integrated circuits, but its low refractive index contrast caps integration density and it is now one of three platforms fighting over the PIC market rather than the obvious winner.
Summary
Indium phosphide (InP) is a compound semiconductor used both as a wafer substrate and as the active gain medium in photonic devices. Where silicon has to be coaxed into handling light and cannot generate it, InP lases natively: recent work demonstrated half-wave plasmonic lasing from InP nanoparticles roughly 115 nm long and 100 nm high on gold, emitting at 730 nm, described by the authors as one of the smallest active laser cavities reported. The same gain property underpins semiconductor optical amplifiers (SOAs), which can be used not just to boost signals but as an all-optical nonlinearity via cross-gain modulation 2022 Shi Soa Aonn Noise Depth Scaling.
The parameter that most constrains InP as a circuit platform is refractive index contrast. InP is explicitly categorised as a low index contrast platform, which makes waveguide bends lossy and forces large bend radii, limiting how densely a chip can be laid out. Work on elliptical width-modulated bends reports 0.22 dB per 90 degree turn at a 6 micrometre bend radius, about 40 per cent lower than a conventional circular bend and 27 per cent lower than a circular width-modulated bend, for a 15 per cent footprint penalty. That is a real gain, but it is an optimisation against a structural handicap that silicon-on-insulator does not have.
Commercially, InP is not a research material. Coherent signed a letter of intent for a CHIPS Act grant of up to $50m to expand an indium phosphide fab in Texas ref. Market analysis frames silicon photonics, InP and thin-film lithium niobate as three platforms competing to define the next generation of photonic integrated circuits, each holding a distinct competitive position rather than one displacing the others ref. The decisive questions for InP are therefore not whether it works, but where the boundary sits between native InP circuits and InP used only as a bonded or flip-chipped gain element on someone else’s substrate.
Separate physics work continues to characterise the material itself: InP nanocrystals on silicon nanotip arrays show radial breathing modes at 8 GHz and 10.3 GHz, with nonlinear frequency mixing appearing above 3 mJ/cm2 excitation fluence and a correlation between oxidation and the onset of nonlinear acoustic modes. First-principles calculations single out InP as a material where phonon scattering selection rules strongly amplify the longitudinal acoustic contribution to thermal conductivity, producing mode-polarised heat currents in nanofilms below 100 K. Neither is a product, but both indicate the material is still yielding new device-relevant behaviour.
Evidence base
- 16 June 2026: Coherent signed an LOI for up to $50m in CHIPS Act support to expand a Texas indium phosphide fab, indicating InP capacity expansion backed by public funding ref.
- 30 March 2026: market analysis frames silicon photonics, InP and thin-film lithium niobate as three platforms competing to define next-generation PICs, each in a distinct competitive position, read as evidence against any one platform taking >70 per cent of volume by 2030 ref.
- 16 June 2026: InP is classified as a low index contrast platform where bend loss is critical; an elliptical width-modulated bend achieves 0.22 dB per 90 degrees at 6 micrometre radius, 40 per cent and 27 per cent below circular and circular width-modulated bends, for a 15 per cent footprint increase.
- 7 July 2026: high-gain InP nanoparticles on ultrasmooth gold lased at 730 nm from a cavity estimated at ~115 nm long and ~100 nm high, described as one of the smallest active laser cavities reported.
- 2 September 2022: in an SOA all-optical neuron, cross-gain modulation compresses input noise such that output OSNR exceeds input OSNR when input OSNR is below about 19 dB, so ASE does not accumulate uncontrollably with layer depth; depth was emulated, not fabricated 2022 Shi Soa Aonn Noise Depth Scaling.
- 28 May 2026: InP nanocrystals on silicon nanotips show radial breathing modes at 8 GHz and 10.3 GHz, with nonlinear frequency mixing above 3 mJ/cm2 fluence and oxidation correlated with the onset of nonlinear acoustic modes, marking a mechanical damage threshold for InP nanostructures.
Open questions
- Does InP remain a full circuit platform, or does it collapse to a bonded gain die on silicon, TFLN or glass substrates? The sources describe the competition but give no share data.
- Do the simulated elliptical width-modulated bends (0.22 dB per 90 degrees at 6 micrometre radius) reproduce in fabricated InP devices, and does the 15 per cent footprint penalty hold at circuit scale?
- Does the SOA noise-compression result survive in a fabricated multilayer chip rather than an OSNR recursion emulated from one neuron 2022 Shi Soa Aonn Noise Depth Scaling?
- What are InP wafer diameter, yield and cost per die relative to silicon photonics? No supplied source contains these figures, and they determine everything about the platform boundary.
Assessment drafted 2026-08-31 from up to 11 KB sources using the technology-scorecard framework; scores are a draft read pending review.