Lasers, Semiconductors and STM: Working and Modern Uses

A laser produces an intense, narrow beam of light of one colour in which the waves march in step, a property called coherence. A semiconductor is a material whose electrical conductivity lies between a conductor and an insulator and can be controlled by doping, temperature or light. A scanning tunnelling microscope (STM) images surfaces atom by atom using quantum tunnelling current.

These three applications show quantum mechanics at work in everyday technology. Lasers read discs and carry internet data through fibres, semiconductors form every chip in phones and computers, and the STM lets scientists see and even move individual atoms. All three depend directly on quantised energy levels, band structure and tunnelling.

Key Concepts

• Stimulated emission: an incoming photon triggers an excited atom to emit an identical photon, amplifying light in a laser. Population inversion, more atoms excited than in the ground state, is required.
• Band gap (E_g): the forbidden energy range between valence and conduction bands in a semiconductor, in electron-volts. Silicon has E_g ≈ 1.12 eV at room temperature.
• Doping: adding tiny amounts of impurity to create n-type (extra electrons) or p-type (extra holes) material; a p–n junction is the basis of diodes and transistors.
• Tunnelling current: in an STM, electrons tunnel between a sharp tip and a surface across a gap of about a nanometre; the current changes exponentially with distance.

Formulas

Photon energy in a laser: E = h f = h c / λ. A red laser at λ = 650 nm gives E = (6.626 × 10⁻³⁴ × 3.0 × 10⁸) / (650 × 10⁻⁹) ≈ 3.06 × 10⁻¹⁹ J ≈ 1.91 eV per photon.

STM current dependence: I ∝ e^(−2κd), where d is the tip–surface gap and κ depends on the barrier height. Increasing d by just 0.1 nm can reduce the current roughly tenfold, which is why the STM resolves height differences far smaller than one atom.

Worked Example

An STM tip moves 0.05 nm closer to a surface and the tunnelling current rises from 1.0 nA to about 2.7 nA. Explain the sensitivity.

1. The current depends exponentially on distance, not linearly. Because κ for typical surfaces is near 1 per ångström (0.1 nm), halving that distance scale still multiplies the current strongly.
2. A change of 0.05 nm, half an ångström, gives a factor near e^(0.5 × 2 × 0.5) estimated from the measured ratio as roughly e¹ ≈ 2.7.
3. The feedback loop in the STM holds the current constant by moving the tip up and down, tracing atomic bumps with height resolution around 0.01 nm.

Common Misconceptions

• Lasers are just very bright torches. Wrong. Laser light is coherent and nearly monochromatic; brightness alone does not give the tight, in-step beam used in surgery and fibre optics.
• Semiconductors are simply poor conductors. Wrong. Their conductivity is deliberately engineered by doping and junctions, switching by factors of millions, which no ordinary poor conductor can do.
• The STM touches the surface to feel atoms. Wrong. The tip never touches; it senses tunnelling current across a vacuum gap, avoiding damage to delicate samples.

Real-World Applications

Lasers power fibre-optic communications, barcode scanners, eye surgery and precision cutting. Semiconductors make processors, solar cells, LEDs and camera sensors. STMs verify chip surfaces at atomic scale, study catalysts, and build experimental nanostructures atom by atom in research laboratories worldwide.

Together, these devices prove that quantised levels, controlled bands and tunnelling are not abstractions but the working principles of modern technology.