발광 (Luminescence) – emission of light by a material material absorbs light at one frequency & emits at another (lower) frequency. activator level Valence band Conduction band trapped states Eg Eemission How stable is the trapped state? If very stable (long-lived = >10-8 s) = phosphorescence (인광) If less stable (<10-8 s) = fluorescence (형광) Example: glow in the dark toys. Charge them up by exposing them to the light. Reemit over time. -- phosphorescence

Photoluminescence Hg uv electrode Arc between electrodes excites mercury in lamp to higher energy level. electron falls back emitting UV light (i.e., suntan lamp). Line inner surface with material that absorbs UV, emits visible Ca10F2P6O24 with 20% of F - replaced by Cl - Adjust color by doping with metal cations Sb3+ blue Mn2+ orange-red

Cathodoluminescence Used in T.V. set Bombard phosphor with electrons Excite phosphor to high state Relaxed by emitting photon (visible) ZnS (Ag+ & Cl-) blue (Zn, Cd) S + (Cu++Al3+) green Y2O2S + 3% Eu red Note: light emitted is random in phase & direction i.e., noncoherent

Electroluminescence LED (light emitting diode) ; pn접합의 순방향바이어스에서 주입된 자유전자와 정공이 재결합과정에서 에너지갭에 해당하는 광자를 방출. 직접밴드갭 반도체가 이용됨. GaAs, InP, GaN 등

광전도 (photoconduction) : 빛의 흡수에 의해 자유전자-정공쌍이 발생하므로 인해 전기전도도가 증가하는 현상. 빛을 차단한 상태의 전기전도도 ; σo = noeμn + poeμp 빛을 조사할 때의 전기전도도 ; σ = (no + Δn)eμn + (po + Δp)eμp Δn = Δp = τ∙GL 태양전지 (Solar cell) : p-n 접합의 접합경계영역 근처에서 빛을 흡수하여 발생한 자유전자와 정공은 내부전위차(built-in voltage)에 의해 각각 n영역과 p영역으로 drift하므로써 역방향 전류를 만들므로써 빛에너지를 전기에너지로 변환시킨다.

Is non-coherent light a problem? – diverges 레이저 (LASER) Is non-coherent light a problem? – diverges can’t keep tightly columnated How could we get all the light in phase? (coherent) LASERS Light Amplification by Stimulated Emission of Radiation Involves a process called population inversion of energy states

루비 레이저의 모식도 “pump” the lasing material to the excited state e.g., by flash lamp (non-coherent lamp). If we let this just decay we get no coherence.

Population inversion

LASER Cavity “Tuned” cavity: Stimulated Emission One photon induces the emission of another photon, in phase with the first. cascades producing very intense burst of coherent radiation. i.e., Pulsed laser

Continuous Wave LASER Can also use materials such as CO2 or yttrium- aluminum-garnet (YAG) for LASERS Set up standing wave in laser cavity – tune frequency by adjusting mirror spacing. Uses of CW lasers Welding Drilling Cutting – laser carved wood, eye surgery Surface treatment Scribing – ceramics, etc. Photolithography – Excimer laser

electron + hole  neutral + h Semiconductor LASER Apply strong forward bias to junction. Creates excited state by pumping electrons across the gap- creating electron-hole pairs. electron + hole  neutral + h excited state ground state photon of light

반도체 레이저의 작동원리

#1 use = compact disk player 반도체 레이저의 용도 #1 use = compact disk player Color? - red Banks of these semiconductor lasers are used as flash lamps to pump other lasers Communications Fibers often turned to a specific frequency (typically in the blue) only recently was this a attainable

광섬유 (Optical fiber) prepare preform as indicated in Chapter 13 preform drawn to 125 m or less capillary fibers plastic cladding applied 60 m

Step-index Optical Fiber Graded-index Optical Fiber