Cascade lasers pulse more powerfully

Ashley N. Paddock, ashley.paddock@photonics.com

New mid-infrared gallium arsenide (GaAs)-based cascade lasers with a pulse three times stronger than previous versions could prove useful for industrial and medical applications.

The Institute of Electron Technology (ITE) has developed technology to produce quantum cascade lasers (QCLs) of record-breaking power and has built prototypes. The new devices, made of aluminum-doped gallium arsenide (GaAs/AlGaAs), emit pulses of infrared radiation. At room temperature, the power of the pulse can reach up to several dozen milliwatts and, under cryogenic cooling, even up to 5 W.

The Institute of Electron Technology in Warsaw has developed technology to produce quantum cascade lasers of record-breaking power and has built prototypes. The microscopic image shows periodic structures of semiconductor layers in the new quantum cascade lasers. Courtesy of ITE.

QCLs are periodic structures made up of many semiconductor layers (up to several hundred), forming the superlattice. The layers’ thicknesses change according to a careful pattern and usually amount to a few nanometers. At ITE, layers are made of GaAs/AlGaAs deposited using molecular beam epitaxy.

QCLs rely on the mechanisms of conduction in semiconductor materials in a unique way. According to band theory of conduction, low-energy electrons in a semiconductor, whose energy levels lie within the basic (valence) band, are bound to semiconductor atoms. If the energy levels reach those from the conduction band, the electrons break away from the atoms and become, along with holes, charge carriers in the material.

Traditional semiconductor lasers rely on the recombination of electrons from the conduction band in a semiconductor, and on vacant states in the valence band to emit light. The wavelength of the radiation emitted by a semiconductor laser depends on the size of a bandgap between the valence and conduction bands.

In QCLs, the energy of the emitted radiation depends less on the material and more on the geometry of the superlattice – the composition and thickness of periodic semiconductor layers. An electron injected into the first period of the superlattice tunnels to a high region of the conduction band. A moment later, the electron jumps to a lower level, still within the conduction band (intersubband transition). Photon emission occurs during the jump. The electron then tunnels to a high region of the conduction band of the next period of the superlattice, and the process repeats. The gap between the excited and basic state within the conduction band can be altered, for example, by changing the thickness of superlattice layers.

The characteristics of QCLs make it relatively easy to design a semiconductor structure that emits electromagnetic waves of a specific wavelength. Cascade lasers can operate in the mid-infrared region of the electromagnetic spectrum (waves up to 9 to 10 µm in length), a highly attractive region of electromagnetic waves because many chemical compounds absorb radiation from that region.

QCLs are critical to building portable detectors that can detect trace amounts of chemical substances; for example, methane in mines or dangerous gases in the chemical industry. Medical applications are equally promising – QCLs in detectors could help sense even trace amounts of disease markers in the air exhaled by the patient. Given that infrared radiation passes through the human body, the lasers open up the possibility of safer medical examinations in a better resolution than that achieved in ultrasonography.