Laser in a test tube
(appeared in April 2015)

(link to main website)

Laser devices can now happen by mixing chemicals in the chemistry lab, says S.Ananthanarayanan.

semiconductor materials react to light by throwing out electric charges which can be made to run as currents. Or, if the free charges fall back, they can give off light energy. These properties are used in electronics, solar cells, lighting devices and lasers, with a very wide area of application.

But the devices typically call for precision fabrication at nanometer scales and cannot easily be mass produced. Now a group of scientists at the departments of chemistry, Columbia University and the University of Wisconsin, report in the journal, Nature Chemistry, that they have got a substance which is of interest in the manufacture of solar cells simply to deposit out of solution in the form of crystals that are just right to work as lasers.

Lasers and semiconductors

The laser is an arrangement where atoms in some materials get excited to higher energy states and pause a bit before they de-excite and give off a photon of light. But if an atom in the higher state is struck by a passing photon, then the atom de-excites at once, giving off a photon that is just ‘in step’ with the one that stimulated the emission, and the two photons are now the amplified, laser light. For lasers to work continuously, a good number of the atoms are kept excited, and part of the emitted light is fed back to the material, by reflectors that are tuned to synchronise. The key requirements are thus the atomic structure which allows the longer-lived excited state and the fabrication of the material, to have a pair of exactly placed reflecting surfaces at either end.

The material of lasers can be gases or solid crystals. One form of laser, called the semiconductor laser, gives off light not by the de-exciting of an atom, but by free electrons losing energy when they are absorbed by atoms. Now, metals are good conductors of electricity because their atoms have just one or two electrons in the outermost and weakly bound electron shell. These loosely bound electrons can thus move and help carry electricity. But atoms of semiconductors have outer shell electrons which are half-way between being loosely bound and tightly bound. They are hence not good as conductors. But if a trace impurity of an atom that has one more or one less outer electron is introduced into a lattice of semiconductor atoms, the impurity is accommodated, but with one electron either ‘free’ or ‘wanting’. This free electron, or the lack of one, which is called a ‘hole’, can then become a carrier of electricity.

Exciting some semiconductors with light also results in electrons building up and if there is a junction of semiconductors which have been oppositely ‘doped’, the electrons can flow only in one direction. On the one hand, a panel such junctions can create a stream of electrons that are given off when the panel is exposed to light, and this arrangement is the solar cell. On the other hand, if a pair of such oppositely doped semiconductors are separated by a region into which electrons and ‘holes’ are pumped when a voltage of the correct sense is applied, then the electrons, which have been freed by absorbing energy, ‘fall into the holes’ and in many cases, give off a flash of light. This is the working of the Light Emitting Diode (LED), which is called a diode because it conducts, and lights up, only when it is charge the correct way.

The semiconductor laser works like the LED, with the difference that the electrons and ‘holes’ do not meet to give off a photon at once, but float about, at least till a passing photon comes upon them. Then they combine at once, and give off a photon, ‘in step’ with the one that pushed them together, just like the excited atom in the normal laser. In the semiconductor laser as well, the light emitting region is enclosed by reflectors and there is back and forth movement of photons and ‘cascading’, to generate a laser beam.

The solar cell application of semiconductors, particularly, now has great relevance in the need for non polluting energy sources. There is hence feverish research going on to create more efficient, more economical and larger solar panels. The current technology is based mostly on processing silicon, which involves many steps, high temperatures and extreme purity. But organic materials with semiconductor properties have now been developed and an important new light harvesting material is a half organic-half inorganic compound, that contains lead or tin and atoms like chlorine, which form salts, and which has a crystal structure like a mineral called Perovskite.

The perovskite structure has many interesting electrical and magnetic properties and endows the organic-inorganic hybrids with merits for use in solar cells. A team at the Universities of Cambridge, Oxford and Munich had discovered that the hybrid converted light to electricity with great efficiency. The material can also be spread on substrates in the form of thin films using a chemical deposition process and this made it doubly attractive for use in solar cells.

Haiming Zhu, Yongping Fu, Fei Meng, XiaoxiWu, Zizhou Gong, Qi Ding, Martin V. Gustafsson, M. Tuan Trinh, Song Jin and X-Y. Zhu at Coumbia and Wisconsin note that the high efficiency of these materials in solar cells is because of the long time span for which excited states remain and the low rate of recombining without emission of light, the same qualities that are important for working as lasers. The group has then developed a simple method to grow organic-perovskites into elongated crystals, or nanowires, about 10 to 100 millionths of a meter long by about 400 billionths of a meter (nanometers) across. These elongated crystals, which grow at room temperature and almost defect-free, form with parallel, plane, reflective ends, just like the laser device needs. The nanowires take about 20 hours to grow once a glass plate coated with a solid reactant is submerged in a reactant. "There's no heat, no vacuum, no special equipment needed," says Song Jin, professor of chemistry at the University of Wisconsin-Madiso. "They grow in a beaker on the lab bench."

The paper in Nature Materials says the crystals act as lasers with a very low energy threshold, producing laser light of very high low frequency spread. And the process of lasing leads to nearly 100% efficiency of energy transfer to emitted light. And with this efficiency of performance, the paper says, there is the ease of production, and then, the capacity of easily varying the chemistry of the compounds to tune the frequency of the laser light emitted. “Halide Perovskites are ideal for the development of nanophotonics, in parallel with the rapid development in photovoltaics from the same materials,” the paper says.


Do respond to :