Laser Physics
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Direct answer
Ordinary light is a crowd — photons of many wavelengths, phases and directions, emitted spontaneously as electrons drop randomly. A laser beam is a disciplined column, and the discipline comes from stimulated emission — Einstein's 1917 insight that a photon passing an excited atom triggers release of a second photon identical in frequency, phase and direction. Building a working laser needs three ingredients: population inversion (more atoms in the upper state than the lower), a metastable upper level with lifetime near 10⁻³ s (a hundred thousand times the usual 10⁻⁸ s spontaneous decay) to hold the inversion, and pumping energy to feed it — a flashlamp for the ruby laser's 694.3 nm red line (Maiman, 1960), an electric discharge for the He-Ne laser's familiar 632.8 nm. Mirrors at both ends recycle the light through the gain medium, and above a threshold the stimulated cascade dominates.
What you must remember
- Three interaction processes: absorption (photon in, atom up), spontaneous emission (atom drops alone, random photon), stimulated emission (passing photon triggers an identical twin) — the three Einstein coefficients' processes.
- Coherence by construction: the stimulated photon matches the original in frequency, phase, polarization and direction — the physical root of laser coherence, monochromaticity and directionality.
- Population inversion: N_upper > N_lower, impossible at thermal equilibrium (Boltzmann populations forbid it), so a pump must drive atoms up and a metastable state must hold them.
- Metastable state: lifetime around 10⁻³ s versus about 10⁻⁸ s for an ordinary level; the shelf on which the inverted population waits.
- Optical pumping and the ruby laser: Cr³⁺ ions in Al₂O₃ absorb flashlamp green-blue, relax non-radiatively to a metastable level, and lase at 694.3 nm — a three-level system needing intense pumping since the lower level is the crowded ground state.
- He-Ne laser: helium excited by discharge transfers energy resonantly to neon's metastable level; the 632.8 nm red line is the familiar laboratory pointer — a four-level-style design that inverts more easily.
- Resonant cavity: two parallel mirrors, one partially transmitting, recycle light through the medium; lasing begins when gain per pass exceeds losses.
Population inversion in three and four levels
Walk the ruby laser's ladder to see why pumping is brutal. Atoms start almost entirely in the ground state — Boltzmann's doing. A flashlamp drives them to a broad absorption band; they leak down non-radiatively within 10⁻⁸ s to a metastable level and pile up for a millisecond. Once more than half the ions sit there, population inversion against the ground state exists, and one spontaneous 694.3 nm photon stimulates an avalanche, recycled by the mirrored ends. That "more than half" requirement is the three-level tax — the lower laser level is the crowded ground state, so pumping must deplete the majority population; hence ruby fires in pulses.
The four-level design lifts the tax. Its lower laser level sits well above the ground state and stays empty, so atoms pumped to the metastable upper level face a vacant lower level, and even a small inversion lases continuously. The He-Ne system works this way in spirit: the electric discharge excites helium (long-lived at 20.61 eV), helium hands its energy to a nearly-matching neon level by collision, and neon lases down to an emptier level, releasing 632.8 nm — why laboratory pointers run continuous and cool while ruby pulses.
Lasers in JEE
The definitional MCQ bank is stable: LASER expands to Light Amplification by Stimulated Emission of Radiation; stimulated versus spontaneous emission is contrasted (identical twin photons versus random ones); and population inversion means N_upper > N_lower — a non-equilibrium state. Second, why the metastable 10⁻³ s matters is a reason-based favourite: atoms must survive long enough to accumulate, while an ordinary 10⁻⁸ s level drains as fast as it fills. Third, applications anchor statement questions: LASIK eye surgery, optical fibre communication, barcode scanning, cutting and welding. Fourth, connections cross chapters: the laser's monochromaticity makes Young's double-slit fringes crisp, and photon energy E = hν at 632.8 nm (about 1.96 eV) links to the photoelectric chapter. Main keeps to definitions and level-diagram reading; Advanced sketches level schemes and asks which transition is the laser line and why the pumping is easier in one.
Frequently asked questions
What distinguishes stimulated from spontaneous emission?
Stimulated emission is triggered by a passing photon and produces a second photon identical in frequency, phase and direction; spontaneous emission occurs randomly and in an arbitrary direction and phase.
Why does a laser need population inversion?
So that stimulated emission outpaces absorption — otherwise a travelling photon is more likely to be absorbed than multiplied, and amplification never starts.
What role does the metastable state play?
It holds excited atoms for around a millisecond instead of 10 nanoseconds, letting the population accumulate and sustain the inversion long enough for the stimulated cascade to build.
How does a ruby laser differ from a He-Ne laser?
Ruby is optically pumped by a flashlamp, three-level, pulsed at 694.3 nm; He-Ne is discharge-pumped, effectively four-level, continuous at 632.8 nm with helium passing energy to neon by collision.
What do the mirrors at the ends of a laser do?
They form a resonant cavity that recycles light through the gain medium — gain per pass exceeding the mirror losses is the threshold condition for sustained lasing.