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Joined 2026-09-21 Message
Incoming links: Chopped-light lock-in experiment
Light-emitting diode 2026-10-18
A light-emitting diode is a semiconductor diode producing electroluminescence under electric current. Forward bias injects Electrons and electron holes; radiative electron-hole recombination emits photons. A double heterostructure uses band offsets to retain both charge carriers in an active layer. A sufficiently thin layer is a quantum well.[ref]
https://journals.aps.org/rmp/pdf/10.1103/RevModPhys.87.1139. §I, p. 1139, Fig. 1; §IV.A, pp. 1143–1144: injection, active layer and quantum-well distinction
The optical and electrical questions are different: photon energy sets a color scale, while light-emitting diode efficiency determines how much supplied energy escapes as light. The blue light-emitting diode made compact white light-emitting diodes and full-color displays practical. A modulated light-emitting diode can also supply the optical signal in a chopped-light lock-in experiment.
Blue light-emitting diode: carrier injection, emitting structure and optical readout
. Original functional schematic of a representative indium gallium nitride quantum well device on sapphire, not a reconstruction of the 1989 or 1994 devices. Layer thicknesses are schematic. Separate electrical and optical readings determine different properties: terminal electric potential difference and electric current, spectrum, and calibrated escaping optical power. The arrows represent routes through the device rather than a calculated energy band profile. Source. Details.
Lock-in amplifier Created 2026-10-14 Updated 2026-10-18
A lock-in amplifier measures the component of an input signal synchronized with a reference at a chosen frequency. It multiplies the input by a reference sinusoid, then uses a low-pass filter to retain the slowly varying product. Two references in phase quadrature recover both amplitude and phase difference. A shared timing source keeps the relative wave phase stable: this is coherent detection.[ref]
Model SR830 DSP Lock-In Amplifier manual by Stanford Research Systems. Model SR830 DSP Lock-In Amplifier manual, revision 2.5, printed pp. 3-1–3-2: multiplication, filtering, reference locking and dual-phase detection
The useful trick is to move the wanted frequency to zero before averaging. An electronic amplifier alone enlarges both the wanted signal and unwanted noise; a lock-in amplifier instead selects a component by its timing. A photodiode's electric current can first be converted to an electric potential difference, so the same calculation can measure weak modulated light. The chopped-light lock-in experiment makes this selection tangible.