Professor Capasso's research cuts across several disciplines in basic physics, applied physics and engineering, which include optics, semiconductor physics, mesoscopic physics, solid-state electronics, optoelectronics and micromechanics.
A unifying theme of his research is the quantum design and study of new artificial materials and nanostructures with man-made electronic and optical properties, an approach that Professor Capasso pioneered and dubbed bandstructure engineering. These structures are grown by thin film deposition techniques such as Molecular Beam Epitaxy (MBE). These studies include both the investigation of quantum effects in lower dimensionality systems and the invention of photonic and electronic devices in which quantum effects on a mesoscopic scale (a few to ~ 100 nm) play a dominant role.
This multi-faceted research led Capasso and his collaborators to invent the quantum cascade (QC) laser, a fundamentally new light source whose emission wavelength can be designed to cover the entire spectrum from the mid to the far infrared by tailoring the active region layer thickness. You can read more about the development of the QC laser at Bell Labs and explore the technical details by viewing this slide show.
QC lasers are now commercially available and have wide ranging applications to molecular spectroscopy, chemical sensing and trace gas analysis (such as atmospheric chemistry, combustion diagnostics, breath analyzers in medicine, pollution monitoring and industrial process control, homeland security) and telecommunications.
At Harvard University Prof. Capasso's group has expanded QC laser research to new coherent light sources utilizing intracavity nonlinear optical effects. These include Raman injection lasers, lasers without inversion and difference frequency generators. Recently his group demonstrated the first Raman injection laser, a device based on resonant stimulated Raman scattering. This light source which currently operates in the mid-infrared, suitably scaled to the far-infrared,could lead to a widely tunable Terahertz source. Similarly a source of TeraHertz radiation based on intracavity frequency difference generation would have unprecedented tunability.
Capasso's group is also studying the ultrafast dynamics of QC lasers. Modelocking in these devices is poorly understood since the gain recovery time is expected to be much shorter than the cavity roundtrip time, so that present theories of modelocking cannot apply to QC lasers.
In bandstructure engineering one takes advantage of the control of the boundary conditions of the electron wavefunctions to design bottom-up the basic quantum mechanical features of materials (energy levels, optical matrix elements, scattering times, etc). One can play a similar game with phonons and photons and Professor Capasso's research has recently started to move into new directions by asking questions such as: can one make a phonon laser by analogy with the Quantum Cascade Laser?
Recently, the Harvard team collaborated with groups at Caltech and Bell Labs to develop a Quantum Cascade Photonic Crystal Surface Emitting Laser (QCPCSELS) that combines electronic and photonic band structure engineering to achieve vertical emission from the surface. The latter is perforated by a honeycomb of holes that form the photonic crystal. This work recently led Capasso to join a multiuniversity center involving Caltech, University of California at San Diego and Harvard. The technical focus of this effort will be on optofluidics, an exciting new research area based on the use of microfluidic devices to control optical processes, and which is expected to result in a new generation of small-scale, highly adaptable, and innovative optical devices. In particular Capasso's group effort is on optofluidic QC lasers in which holes are defined within the laser by focused ion beam or reactive ion etching to permit microfluidic delivery to the cavity. In this way not only one could control the emission properties of the laser but one hopes to also build a new class of on chip biochem/sensors in which the fluid delivers the analytes to the optical cavity.
In another research direction Capasso's group is also exploring new mid-infrared light sources based on surface plasmon and on the enhanced transmission of light through a periodic array of subwavelength holes.
Prof. Capasso has recently teamed up with Prof. John Joannopoulos and his group at MIT to investigate the radiation forces between microptical components such as microsphers and nano-optical fibers. They have found that under certain circumstances an attractive force can develop rather than the conventional radiation pressure force which is repulsive.
Another area of Capasso's research is the investigation of quantum electrodynamical phenomena such as the Casimir effect (the attractive force between uncharged parallel metallic plates. This effect is the manifestation of quantum mechanical vacuum fluctuations, i.e. the zero point energy of the electromagnetic field. Their spectrum can be altered by changing the boundary conditions of the electromagnetic field. This engineering of vacuum fluctuations can be used to design Casimir forces for specific applications.
Leveraging on earlier work work Capasso and his group will focus on designing geometries and investigating materials that will alter in nontrivial ways the Casimir force, including the investigation of repulsive Casimir force. A new experiment in Capasso's lab is searching for the predicted mechanical torque associated with vacuum fluctuations when two dielectric plates made of optically anisotropic materials are brought in close proximity.
Professor Capasso's goal is to eventually expand his research to the so-called dynamical Casimir effect. This elusive quantum electrodynamical phenomenon, predicted by Davies, Fulling and Schwinger in the 1970s, consists in the generation of non-thermal light out of vacuum by surfaces in a state of non-uniform acceleration (e.g. a vibrating microwave cavity). Its observation would be of truly fundamental significance. A related problem is the radiation from neutral molecules moving above a grating recently studied theoretically by Capasso and his collaborators.
A tutorial account of Professor Capasso's research on nanostructures has appeared in a book on materials research by Ivan Amato ("Stuff: the materials the world is made of", Basic Books, New York, NY, 1997). A tutorial description of Quantum Cascade Laser and quantum semiconductor structures is in the article "Diminishing Dimensions" of a special issue of Scientific American, entitled the Solid-State Century, 1997-1998.