Nature News and Views

by David S. Citrin

    When we observe laser light, we typically measure its intensity, and so wave amplitude. The phase, which encodes further details of the laser¡¯s internal workings, was obscure ¡ª but fresh light is being shed on it.

    Watch waves on the ocean. They can be characterized by their height (amplitude) and by when the peaks arrive (phase); without amplitude and phase, there is no wave. Light is also a wave, but one that is composed of intimately linked electric and magnetic fields. Most of the time, we can infer from experiment only the amplitude of light waves: this quantity is given by the intensity averaged over time, which is easily measured.

    The phase of the wave is, by contrast, often ignored. But it, too, contains subtle, but no less important, information about the wave and the physical mechanisms that produced it. Knowing exactly what the phase is doing involves following dynamics that may be too fast to track directly, quite unlike ocean waves, which are easy to track with the eye. Typically, our only handle on the phase at the frequencies of optical light has been the interference patterns cast by the light waves. Not any more: on page 698 of this issue of Nature, Kröll et al. directly view both the amplitude and the phase of light in an operating semiconductor laser.

    Their laser is a quantum cascade laser, which is composed of numerous layers of two or more semiconductors. Under the influence of a d.c. electrical voltage (known as a bias), electrons in atoms of the semiconductor material fall down a staircase of quantum-mechanical levels, giving out a photon at each step. One of the most exciting aspects of quantum-cascade approaches is that the light thus emitted has very low frequencies that cannot be produced using conventional lasers. The lowest frequency demonstrated so far is about 1.6 terahertz (THz, 1012 Hz). This is close to the clock rates common in high-speed electronics, and so opens up the tantalizing prospect of a further convergence between photonic and electronic technologies. Conventional lasers, by comparison, operate at near-infrared or visible frequencies. This difference translates into oscillation periods of around a picosecond (10-12 s) for terahertz quantum cascade lasers, as against around a femtosecond (10-15 s) for visible or near-infrared lasers. On terahertz timescales, the task of resolving in time both amplitudes and phases is thus far easier.

    Continue reading more at Nature.com

    Nature 449, 669-670 (11 October 2007)