Light emission by solid-state lasers and light-emitting diodes

Here we explore the further consequences of light emission in solids, focusing our attention on ionic crystals and semiconductors used in the design of lasers and light emitting diodes. we discussed the conditions under which a material can become a laser and it would be helpful to review those concepts. The neodymium laser is an example of a four-level laser, in which the laser transition terminates in a state other than the ground state of the laser material (Fig. 20.62). In one form it consists of Nd³⁺ ions at low concentration in yttrium aluminium gar net (YAG, specifically Y₃Al₅O₁₂), and is then known as a Nd–YAG laser. The population inversion results from pumping a majority of the Nd3+ ions into an excited state by using an intense flash from another source, followed by a radiationless transition to another excited state. The pumping flash need not be monochromatic because the upper level actually consists of several states spanning a band of frequencies. A neodymium laser operates at a number of wavelengths in the infrared, the band at 1064 nm being most common. The transition at 1064 nm is very efficient and the laser is capable of substantial power output, either in continuous or pulsed (by Q-switching or mode-locking as discussed in Section 14.5) modes of operation. The titanium sapphire laser consists of Ti3+ ions at low concentration in a crystal of sapphire (Al₂O₃). The electronic absorption spectrum of Ti3+ ion in sapphire is very similar to that shown in Fig. 14.13, with a broad absorption band centred at around 500 nm that arises from vibronically allowed d–d transitions of the Ti³⁺ ion in an octahedral environment provided by oxygen atoms of the host lattice. As a result, the emission spectrum of Ti³⁺ in sapphire is also broad and laser action occurs over a wide range of wavelengths (Fig. 20.63). Therefore, the titanium sapphire laser is an example of a vibronic laser, in which the laser transitions originate from vibronic transitions in the laser medium. The titanium sapphire laser is usually pumped by another laser, such as a Nd–YAG laser or an argon-ion laser (Further information 14.1), and can be operated in either a continuous or pulsed fashion. Mode-locked titanium sapphire lasers produce energetic (20 mJ to 1 J) and very short (20–100 fs, 1 fs = 10⁻¹⁵ s) pulses. When considered together with broad wavelength tunability (700–1000 nm), these features of the titanium sapphire laser justify its wide use in modern spectroscopy and photochemistry. The unique electrical properties of p–n junctions between semiconductors can be put to good use in optical devices. In some materials, most notably gallium arsenide, GaAs, energy from electron–hole recombination is released not as heat but is carried away by photons as electrons move across the junction under forward bias. Practical light-emitting diodes of this kind are widely used in electronic displays. The wave length of emitted light depends on the band gap of the semiconductor. Gallium arsenide itself emits infrared light, but the band gap is widened by incorporating phosphorus, and a material of composition approximately GaAs_0.6 P_0.4 emits light in the red region of the spectrum.

A light-emitting diode is not a laser, because no resonance cavity and stimulated emission are involved. In diode lasers, light emission due to electron–hole recombination is employed as the basis of laser action. The population inversion can be sustained by sweeping away the electrons that fall into the holes of the p-type semiconductor, and a resonant cavity can be formed by using the high refractive index of the semiconducting material and cleaving single crystals so that the light is trapped by the abrupt variation of refractive index. One widely used material is Ga_1−xAl_xA_s, which produces infrared laser radiation and is widely used in compact-disc (CD) players. High-power diode lasers are also used to pump other lasers. One example is the pumping of Nd: YAG lasers by Ga_0.91 Al_0.09 As/Ga_0.7 Al_0.3 As diode lasers. The Nd: YAG laser is often used to pump yet another laser, such as a Ti: sapphire laser. As a result, it is now possible to construct a laser system for steady-state or time-resolved spectroscopy entirely out of solid-state components.

Fig. 20.62 The transitions involved in a neodymium laser. The laser action takes place between the 4F and 4I excited states.

Fig. 20.63 The transitions involved in a titanium sapphire laser. The laser medium consists of sapphire (Al₂O₃) doped with Ti³⁺ ions. Monochromatic light from a pump laser induces a ²E ← ²T₂ transition in a Ti³⁺ ion that resides in a site with octahedral symmetry. After radiationless vibrational excitation in the 2E state, laser emission occurs from a very large number of closely spaced vibronic states of the medium. As a result, the titanium sapphire laser emits radiation over a broad spectrum that spans from about 700 nm to about 1000 nm.

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مواضيع ذات صلة

Light absorption by molecular solids, metallic conductors, and semiconductors

Insulators and semiconductors

The occupation of orbitals

The formation of bands

Molecular solids and covalent networks

Energetics

Structure

Close packing

Structure refinement

The phase problem

Bragg’s law

X-ray diffraction