Making the invisible visible

Light is a packet of photons with increasingly important applications for both the scientific community and general public. For example, photonics researchers are harnessing photons as a means to visualize the motion of electrons in materials, as well as for use as noninvasive biological probes in medical imaging in humans. The use of photons has even revolutionized the way we send and store digital data, cost-effectively manufacture complex devices and generate electricity.

Photonics has also infiltrated our everyday lives. For instance, the circuit board of your cellular phone is likely to have been drilled by a laser and your car was probably laser welded. Using the Internet or sending an e-mail employs photons to carry the information. A hospital visit can involve x-ray cameras and optical endoscopes to look inside your body. If the nineteenth century was the century of steam and the twentieth century was the century of the electron, the twenty-first century will be remembered as the century of light (Fig. 1).

In the field of metrology, laser light is used to cool atoms and molecules down to extremely low temperatures of just a few nanokelvin. This capability makes it possible to realize ultra-precise optical clocks that are essential for tasks in communication and metrology, or to achieve new states of matter, such as Bose–Einstein condensates, and thus explore new features of quantum physics.

Laser research has also stimulated the development of nanophotonics. A long-held, universal law in optics is that the resolution of an optical image is limited by diffraction; thus, features smaller than half the wavelength of the light used cannot be visualized. This law means that the first lasers, which had a wavelength of just 1 micrometer, were unable to image nanostructures. Now, these tiny structures can be seen using lasers, thanks to the development of near-field optical technology and super-resolution imaging techniques, which can directly resolve subwavelength structures.

Advances in laser research have also provided the ability to generate radiation with a very long wavelength. Such lasers were developed by extending the concept of the maser, an amplifier that operates in the microwave region. Several of the first lasers, including the ruby, HeNe (helium–neon) and Nd:YAG (neodymium-doped yttrium aluminum garnet) lasers, operated only in the visible and near-infrared regimes of the electromagnetic spectrum. Consequently, the region that lies between the infrared and microwave bands—the so-called terahertz region—was left behind.

The terahertz region is very important, however, especially for sensing, imaging and spectroscopic applications. Many important chemicals, such as pharmaceuticals or explosives, have strong absorption signatures in the terahertz band. Terahertz waves are also capable of penetrating various materials, including clothes and paper, making the region important for security screening and structural monitoring. Nowadays, convenient and compact sources of terahertz radiation are available. The semiconductor quantum cascade laser, for example, greatly opens up this region of the electromagnetic spectrum.

In the terahertz area, RIKEN has made two big advances: the development of high-performance quantum cascade lasers operating in the terahertz region, and the generation of intense terahertz radiation. Increasing the operating temperature of quantum cascade lasers toward room temperature—around 300 kelvin—will make them more convenient and practical to use. The latest devices developed at RIKEN have now reached an operating temperature of around 160 kelvin. In the next 5 to 10 years, a temperature of 200 kelvin or more should be achievable. It will then be possible to use solid-state thermoelectric coolers known as Peltier devices instead of cryogenic gases to cool the lasers—an advance that will greatly expand the application of these lasers.

Another important development in the field has been the use of a new materials system for making quantum cascade lasers. In particular, RIKEN has successfully fabricated such lasers from gallium nitride (GaN). The wavelength range of operation of a laser is limited by the materials used. Employing GaN expands the wavelength possibilities into the 5–10 terahertz frequency band, which cannot be reached by traditional quantum cascade lasers based on the aluminum gallium arsenide (AlGaAs) materials system.

In addition, RIKEN has established ways to generate very intense terahertz waves that are useful for driving nonlinear processes and performing spectroscopy. This includes 100-kilowatt terahertz sources based on frequency conversion with a power output similar to that of a free-electron laser, a much larger and far more expensive instrument.