Semiconductor Lasers: How Laser Diodes Produce Light

A semiconductor laser — usually called a laser diode — is a laser the size of a grain of sand, made from the same materials as computer chips. Instead of a gas tube or crystal rod, its active medium is a p-n junction: when electric current pushes electrons and holes together in the junction, they recombine and release photons, and with the right structure this light is amplified into a laser beam.

These tiny devices matter more than any other laser type: billions are made every year for fibre-optic internet, barcode scanners, laser printers, Blu-ray players and phone face-recognition. They turned the laser from a laboratory curiosity into a disposable component.

Key Concepts

• Laser diode: A forward-biased p-n junction (usually a double heterostructure) that produces laser light directly from electric current.
• Band gap (E_g): The energy difference between conduction and valence bands; the emitted photon energy equals E_g, fixing the laser's colour.
• Population inversion in diodes: Achieved by heavy doping plus forward bias, which floods the junction region with electrons and holes.
• Double heterostructure: A thin active layer sandwiched between wider-band-gap materials, trapping both charge carriers and light for efficient lasing.
• Cleaved facets: The crystal's naturally flat ends act as mirrors, forming the laser cavity — no external mirrors needed.
• Threshold current: The minimum current at which gain beats losses and lasing starts; below it the diode is just an LED.

Formulas

Emission wavelength: λ = hc/E_g, with hc = 1.986 × 10⁻²⁵ J·m. GaAs (E_g = 1.42 eV) gives infrared light near 870 nm; GaN (E_g = 3.4 eV) gives blue/violet near 365 nm. Threshold gain condition: g_th = α + (1/2L)·ln(1/R₁R₂), where α is internal loss, L the cavity length and R₁, R₂ the facet reflectivities — gain must exceed all losses for lasing.

Worked Example

What colour light does a gallium arsenide (GaAs) laser diode emit? E_g = 1.42 eV.

1. Convert the gap to joules: 1.42 × 1.602 × 10⁻¹⁹ = 2.275 × 10⁻¹⁹ J.
2. λ = hc/E_g = 1.986 × 10⁻²⁵ / 2.275 × 10⁻¹⁹ ≈ 8.73 × 10⁻⁷ m.
3. That is 873 nm — in the near-infrared, invisible to the eye.
4. This is why CD-player and fibre-optic laser diodes glow invisibly: the band gap of GaAs sits just beyond red. (DVD players use different materials to reach visible red at 650 nm.)

Common Misconceptions

• "A laser diode is just a bright LED." Below threshold current it is an LED — but above threshold, stimulated emission takes over and the output becomes coherent laser light, with a sharp jump in brightness and spectral purity.
• "All laser diodes are red." Colour is set by the band gap: infrared (GaAs), red (AlGaInP), green/blue/violet (GaN-based) diodes all exist.
• "They are too weak to be dangerous." Even milliwatt laser diodes can damage eyes because the beam is concentrated; the infrared ones are extra treacherous since you cannot see them.

Real-World Applications

• Fibre-optic networks run on 1310 nm and 1550 nm laser diodes carrying the world's internet traffic.
• Barcode scanners, laser printers and optical drives (CD/DVD/Blu-ray) each use a diode matched to their task.

Semiconductor lasers turn electric current directly into laser light at a p-n junction, with the band gap setting the colour via λ = hc/E_g — 1.42 eV GaAs gives 873 nm infrared. Cheap, tiny and efficient, the laser diode put lasers into billions of everyday devices.