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A physicist who helped achieve continuous operation of semiconductor lasers at room temperature. A double heterostructure surrounded a thin active layer with different semiconductor layers, reducing the current needed to sustain lasing. This addressed a practical obstacle in early devices and helped make laser diodes useful for optical communication and other technologies.^[ref]

A laser diode is an electrically injected semiconductor laser. Forward biasing supplies electrons and electron holes to an active region, where radiative recombination can amplify a compatible optical mode by stimulated emission.

Reflecting facets or other cavity structures provide feedback. Below the laser threshold, emission is dominated by spontaneous processes; above it, stimulated emission into resonator modes becomes important. A light-emitting diode can also use carrier recombination but does not require this feedback-and-gain threshold. Output power, emitted spectrum and terminal current-voltage characteristic together characterize the device more fully than brightness alone.

An electrical engineer researching semiconductor lasers and optical fibers.^[ref]

Morton B. Panish helped develop semiconductor lasers using a double heterostructure. A narrow-gap active layer between wider-gap semiconductor layers confines injected charge carriers and guides the emitted light. This combined electronic and optical confinement lowered the threshold sufficiently for continuous operation at room temperature.^[ref]

A chemist who developed semiconductor lasers at Bell Laboratories.^[ref]

Adjacent longitudinal cavity modes obey $2nL=m{\lambda }_0$. Neglecting optical dispersion in estimating their spacing,Hence the gain interval accommodatespossible longitudinal cavity modes. The exact integer depends on the positions of the interval endpoints and resonances. This counts available resonances; mode competition and actual laser threshold can reduce the number that oscillates. More accurately, the spacing uses the group refractive index instead of $n$.

A broad multimode spectrum suffers differential delay through optical fiber group velocity dispersion, broadening transmitted pulses. Near $800\mathrm{nm}$, ordinary silica optical fiber attenuation is also larger than in the long-distance windows near $1.3$ and $1.55\mu \mathrm{m}$. These limitations concern this short-wavelength, simple multimode device: suitably engineered single-frequency semiconductor lasers at telecommunications wavelengths are useful communication sources.

Carrier confinement changes both the energies and the available optical transitions of a semiconductor. A quantum well confines one direction and leaves two continuous momentum coordinates; a wire confines two directions and leaves one; a quantum dot confines all three and has discrete ideal levels. Confinement becomes important when its level spacing is comparable with thermal, lifetime and disorder energy scales, and strong excitonic confinement involves comparison with the exciton Bohr radius.

Transition energies. In a simple infinite-well effective-mass model of width $L$, a same-index electron-hole transition has thresholdThus decreasing $L$ generally blue-shifts interband absorption and emission through the $L^{-2}$ confinement contribution. Finite barriers, band mixing, strain, nonparabolicity, dielectric environment and Coulomb interactions alter this elementary estimate. In dots, confinement shifts and exciton binding have different size dependences, so a single inverse-square law is not an exact nanocrystal spectrum.

Dimensional density of states. For a parabolic continuum the phase-space volume below kinetic energy $E$ scales as $E^{d/2}$, hence the density of states scales as $E^{d/2-1}$. Each subband threshold therefore contributes a square-root onset in three free dimensions, a constant step in two, and an inverse-square-root singularity in one. A zero-dimensional ideal dot has discrete lines rather than a continuum power law. Real temperature, disorder and lifetime broadening smooth singularities and lines. Optical absorption uses the joint electron-hole density of states multiplied by transition matrix elements and occupations; a bare electronic density of states is not the entire absorption spectrum.

Selection and Coulomb effects. Envelope-function overlap and Bloch-state dipole selection rules determine allowed transitions and their polarization. For equal ideal infinite-well envelopes, orthogonality favors matching transverse indices; finite barriers and asymmetric fields can relax that envelope rule. Reduced dimensionality can enhance electron-hole attraction and oscillator strength near an exciton resonance, but the binding energy depends on the actual confinement and dielectric screening. Intersubband transitions within a conduction band are different from interband recombination; in a quantum well they require an electric-field polarization component along the confinement direction.

Devices. Quantum wells, wires and dots allow wavelength engineering, carrier localization and altered optical gain for LEDs and semiconductor lasers. Concentrating states near selected transition energies can improve differential gain and reduce thresholds under appropriate injection and loss conditions. Dot ensembles can instead have substantial inhomogeneous broadening from size variation; nonradiative surface traps and carrier capture can limit an otherwise attractive design. Material confinement does not alone ensure high quantum efficiency.

An applied field across a quantum well shifts electron and hole envelopes apart, usually red-shifting the exciton transition and reducing overlap: the quantum-confined Stark effect. This supports electroabsorption modulation and tuneable optical transitions. Conduction-band intersubband energies can also be engineered for infrared detectors and cascade-laser designs. Optical waveguides and cavities confine photons and affect mode overlap or spontaneous-emission rates, but that photonic confinement is distinct from the carrier quantization derived above. Both must be specified when predicting a real device's threshold, spectrum and output.

A semiconductor laser uses a semiconductor as its laser gain medium. Pumping produces carrier populations that permit stimulated emission between suitable electronic states. An optical resonator supplies feedback, and oscillation begins when the gain compensates round-trip losses.

A laser diode supplies pumping by electrical injection; other semiconductor lasers can be optically pumped. The band gap, confinement and resonator determine the possible emission range. A measurement of optical output versus pump strength helps locate the laser threshold, while an emission spectrum tests the modes involved. Increasing output alone does not establish that the device has entered coherent laser oscillation.