Overview
Quantum Optics with Semiconductor Nanostructures
Nowadays it's quite easy to structure semiconductor crystals on a sub-nanometer length scale. In these tiny structures , the rules of Quantum Mechanics, allow us to tailor the energy levels of the electrons in the crystal; in effect we can make our own "artificial atoms, and they have optical properties that we can tailor to our needs.

Schematic of how the quantum mechanical scattering amplitudes of an electron can interfere, when their wavefunctions are made coherent with a control laser beam, to give "Electromagnetically Induced Transparency". The scattering wave amplitudes of green and yellow scattering pathways cancel, freezing the electron in the ground state, in a "matter wave" analogue of the Young's double slits experiment. The photo on gives some idea of what this would look like if we could do this with a human hand (which we can't, yet).

Using these we are copying a range of coherent optical effects first seen by the atom spectroscopists, but with the convenience of an all-solid state host. Electromagnetically-Induced-Transparency (EIT), uses a "control" beam, to generate a QM interferenec effects inside these artificial atom.. As long as the beam is on, the electrons are effectively "frozen" in their ground state. They stop interacting with light and the atom dissappears. We managed this vanishing trick with our "artificial atoms", semiconductor crystals, and it caused something of a stir in the papers.

The idea of "Gain-without inversion" goes a stage further, produciung optical gain without obeying Einsteins classically derived ideas baot population inversion.: The black/green/red/blue curves are the absorption spectra of an artificial atom sample in the presence of a control beam of progressively increasing intensity. By the highest intensity a gain feature appears, at a photon energy of ~187meV, even though the overall area under the peak tells us that >80% of the atoms are still in their ground state,|1>. At the same time the refractive index plot, the blue curve on the right, tells us that the light is slowed down to ~ c/40 in the amplifying region.Quantum Cascade Lasers with Strongly Coupled cavities: Proving Einstein Wrong.
Quantum Cascade Lasers (QCL's) are a new class of unipolar opto-electronic devices which generate radiation from electrons hopping between energy levels confined in nanostructured semiconductor multilayers. QCL's emit at previously unattainable wavelengths, spanning the mid-to far infrared ( wavelengths of 4 to 200 microns), or so called THz region. They're opening up all sorts of new technological and scientific possibilities.

Over 90 years ago, Einstein proved his "Population Inversion" theorem, i.e. that more than half the atoms in a laser needed to be excited before it could lase. He was assuming that the atoms were only weakly coupled to the laser light, and that they could also lose energy by radiating in random directions. Now we have engineered this coupling to be strong enough to make a new lasing medium, half matter and half light, and any energy put into it can leave only as laser radiation.

The result, for the first time, is a "thresholdless" laser that works even at the tiniest of drive levels. Its new metal-based photonic design can also instantaneously change an optical bitstreams' wavelength in a way that promises to up internet capacity dramatically.

This work uses "Plasmonic" effects in shiny gold layers to squeeze light into a microscopic space, one that's also full of tiny crystal nanostructures so small that we can use Quantum theory to design-in optical properties that are not found with natural atoms. The "Plasmonic" squeezing dramatically increases the coupling to the electronic transitions in the nanostructure.

The latest twist is that we can make the "artificial atom " asymmetric in a way that is impossible with natural ones. Now, when the electron probability cycles between two levels ( so-called "Rabi-flopping") underillumination, the centroid of the electron probability oscillates and emits tuneable radiation at close to the Rabi frequancy.
Experimental laser setup for the QCL novel device programme. The green glow comes from a 5W lab bench laser used to generate the near IR bandgap radiation. The mid-IR radiation is generated by the QCL device itself , there are about 12 of them (each one is a gold stripe) on the tiny semiconductor chip on the right.

Quantum Metamaterials for Plasmonics and "Strong Coupling"
We are studing an entirely new class of so-called metamaterials. They are made from semiconductor crystals which are structured at the nanoscale, in a way that allows us to design in electronic resonances using the theory of Quantum Mechanics.

At the moment, metamaterials use metal structures, with electromagnetic resonances engineered in on length scale shorter then the wavelenght of the radiation where they''''''''''''''''ll be used. They generate such fascinating designer optical effects as cloaking, negative refraction (see box) , and perfect lenses, which can beat the diffraction limit to imaging resolution. Our theoretical studies show that our new "Quantum Metamaterials" will allow us to make similar negatively-refracting devices and perfect lenses which perform 100 times better than the metal-based ones.
Left: Dispersion curves for a Quantum Metamaterial design, the negative gradient corresponde to light bending away from the normal, i.e. the "wrong way" when you make a prism out of it.

These new materials also promise a new class of "plasmonic" waveguide devices which give highly concentrated light fields ideal for compact optical circuits and for super sensitive chemical sensors. The extra versatility of our new Quantum Mechanical approach allows us to combine the advantages of these highly concentrated optical fields with very low propagation losses. Also the mode propagation can be controlled electrically and optically, both for the first time. This ushers in a new generation of active optical device concepts.

All of this is done with existing semiconductor fabrication technology. These devices will be easy and cheap to scale up in a manufacturing process, and they yield much higher and more reproducible device quality than is currently possible with metal based plasmonic designs.

To do all this we need be able to map out optical fields in the mid-infrared part of the spectrum, at sub-wavelength resolution. This will be achieved using the latest s-SNOM probe-based near field microscopy techniques which we will couple to a new tuneable IR laser, developed in-house.

As a spin-off benefit, this s-SNOM will open the way for a whole new range of high-resolution chemical mapping studies across the chemical, biological and medical sciences, and the laser itself will have applications in fields as diverse as industrial control, and environmental monitoring.

Most of the work uses home-built lasers and spectroscopy equipment developed to work in the spectral range where out "artificial atoms" absorb and refract.

Angle-resolved reflectivity plot showing how the "artifical atom" transition, at ~1100cm-1, anticrosses with the plasmon line. The anticrossing energy is many times the linewidth of either transition, showing that we are well into the "strong coupling" regime.

Tuneable mid-Infrared Imaging for Cancer Diagnosis.
Mid-IR (3μm