Carl R. Pidgeon

Coherent control of Rydberg states in silicon

Laser cooling and electromagnetic traps have led to a revolution in atomic physics, yielding dramatic discoveries ranging from Bose–Einstein condensation to the quantum control of single atoms. Of particular interest, because they can be used in the quantum control of one atom by another, are excited Rydberg states, where wavefunctions are expanded from their ground-state extents of less than 0.1 nm to several nanometres and even beyond; this allows atoms far enough apart to be non-interacting in their ground states to strongly interact in their excited states. For eventual application of such states, a solid-state implementation is very desirable. Here we demonstrate the coherent control of impurity wavefunctions in the most ubiquitous donor in a semiconductor, namely phosphorus-doped silicon. In our experiments, we use a free-electron laser to stimulate and observe photon echoes, the orbital analogue of the Hahn spin echo, and Rabi oscillations familiar from magnetic resonance spectroscopy. As well as extending atomic physicists’ explorations of quantum phenomena to the solid state, our work adds coherent terahertz radiation, as a particularly precise regulator of orbitals in solids, to the list of controls, such as pressure and chemical composition, already familiar to materials scientists.

InSb1-xNx growth and devices

Indium antimonide (InSb) has the smallest energy gap of any of the binary III–V materials, leading to a cut-off wavelength of 7 μm at 300 K. The addition of small proportions of nitrogen to InSb offers the prospect of extending the response wavelength into the 8–12 μm range, which is important for thermal imaging in that atmospheric transmission window and because it encompasses the absorption lines of several environmentally important gases and can therefore be used for monitoring the gases. We report on the growth, by a combination of molecular beam epitaxy and a nitrogen plasma source, of InSb1−xNx with up to 10% nitrogen. Structural characterisation techniques of TEM, AFM and SIMS have enabled some optimisation of material quality to be demonstrated by biasing the sample during growth. Measurements on light emitting diodes comprising a superlattice of InSb0.945N0.055/InSb show an emission wavelength of 10.5 μm, which is confirmed by free electron laser assessment. Comparison with first principles band-structure calculations indicate that approximately 10% of the nitrogen is active. Hall effect measurements of 1 μm thick bulk layers indicate an increasing n-type behaviour, the degeneracy effects of which mean, however, that this is only a lower limit.

Silicon as a model ion trap: Time domain measurements of donor Rydberg states

One of the great successes of quantum physics is the description of the long-lived Rydberg states of atoms and ions. The Bohr model is equally applicable to donor impurity atoms in semiconductor physics, where the conduction band corresponds to the vacuum, and the loosely bound electron orbiting a singly charged core has a hydrogen-like spectrum according to the usual Bohr–Sommerfeld formula, shifted to the far-infrared because of the small effective mass and high dielectric constant. Manipulation of Rydberg states in free atoms and ions by single and multiphoton processes has been tremendously productive since the development of pulsed visible laser spectroscopy. The analogous manipulations have not been conducted for donor impurities in silicon. Here, we use the FELIX pulsed free electron laser to perform time-domain measurements of the Rydberg state dynamics in phosphorus- and arsenic-doped silicon and we have obtained lifetimes consistent with frequency domain linewidths for isotopically purified silicon. This implies that the dominant decoherence mechanism for excited Rydberg states is lifetime broadening, just as for atoms in ion traps. The experiments are important because they represent a step toward coherent control and manipulation of atomic-like quantum levels in the most common semiconductor and complement magnetic resonance experiments in the literature, which show extraordinarily long spin lattice relaxation times—key to many well known schemes for quantum computing qubits—for the same impurities. Our results, taken together with the magnetic resonance data and progress in precise placement of single impurities, suggest that doped silicon, the basis for modern microelectronics, is also a model ion trap.

Toward Silicon-Based Lasers for Terahertz Sources

Producing an electrically pumped silicon-based laser at terahertz frequencies is gaining increased attention these days. This paper reviews the recent advances in the search for a silicon-based terahertz laser. Topics covered include resonant tunneling in p-type Si/SiGe, terahertz intersubband electroluminescence from quantum cascade structures, intersubband lifetime measurements in Si/SiGe quantum wells, enhanced optical guiding using buried silicide layers, and the potential for exploiting common impurity dopants in silicon such as boron and phosphorus to realize a terahertz laser

Si-based electroluminescence at THz frequencies

Experimental results of electroluminescence in the terahertz gap, at 6 THz (or 40 μm) from Si/SiGe multi quantum well structures, grown by a commercial chemical vapour deposition system are presented. Theoretical simulations were used to design the heterolayer structure and to explain the emission and absorption features. Electrical and materials characterisation is also presented to demonstrate the quality of the heterolayers.

Temperature dependence of the electron spin g factor in GaAs

The temperature dependence of the electron spin g factor in GaAs is investigated experimentally and theoretically. Experimentally, the g factor was measured using time-resolved Faraday rotation due to Larmor precession of electron spins in the temperature range between 4.5 K and 190 K. The experiment shows an almost linear increase of the g value with the temperature. This result is in good agreement with other measurements based on photoluminescence quantum beats and timeresolved Kerr rotation up to room temperature. The experimental data are described theoretically taking into account a diminishing fundamental energy gap in GaAs due to lattice thermal dilatation and nonparabolicity of the conduction band calculated using a five-level k·p model. At higher temperatures electrons populate higher Landau levels and the average g factor is obtained from a summation over many levels. A very good description of the experimental data is obtained indicating that the observed increase of the spin g factor with the temperature is predominantly due to band’s nonparabolicity.

The UK FEL project: Status and measurement of optical gain☆

Abstract The aim of the UK FEL project is to study the characteristics of a single-pass FEL over a wide operating range, using a four-section 5 m wiggler yielding a maximum K value of 2.7. The laser is driven by the 165 MeV Kelvin Laboratory linac. It has a design operating wavelength range of 2–20 μm, although higher harmonics are also under investigation. We report direct measurement of optical gain using a cw CO 2 laser between 9 and 11 μm. Simultaneous spectra are obtained of both gain and spontaneous emission as a function of linac energy or wiggler field as the tuning parameter. The linewidth obtained is close to that expected for homogeneous broadening, and the gain is consistent with that predicted theoretically for the current available. Strong enhancement of both spontaneous emission and measured gain is obtained in the presence of a high- Q FEL cavity, as expected by simple considerations of optical confinement. These results imply that despite the shortness of our macropulse (∼ 2.5 μm), it will be possible to diagnose buildup to oscillation from an injected signal.

Determination of the intersubband lifetime in Si/SiGe quantum wells

A direct pump and probe lifetime determination in the ps regime has been made in quantum wells of Si/Si1−xGex. We have used an rf linac‐pumped free‐electron laser to determine the relaxation time associated with intersubband absorption in Si/SiGe quantum wells with a subband separation smaller than the optical phonon energy. This measurement yields a lifetime of T1=30 ps.

Enhancement of FEL efficiency by using short electron pulses

In this paper we show that for an intermediate gain free electron laser oscillator, the efficiency is dramatically enhanced by operating with electron pulses shorter than the slippage distance. We discuss both the consequences of operating at longer wavelengths where lethargy effects are important and the implications of working with short electron pulses when inhomogeneous broadening is important. The discussion is supported by a one-dimensional multiparticle simulation that takes into account both saturation and short pulse effects.

Far-infrared optically detected cyclotron resonance observation of quantum effects in GaAs

For the first time optically detected cyclotron resonance (ODCR) has been demonstrated using a CO2 pumped far-infrared (FIR) laser instead of microwaves. Both the electron and the light-hole cyclotron resonances have been observed in GaAs, as well as the 1 s to 2p+ impurity transitions. Valence band quantum effects, well known in Ge, are resolved directly for the first time in GaAs and the electron cyclotron resonances show strong spin doublet in the highest quality MBE samples. The technique has remarkable resolution and sensitivity at low temperatures and, by contrast with other techniques that have been reported, the authors also observe the n=1 to 2 (polaron shifted) and higher spin doublet split resonances at helium temperatures and with low FIR laser power. The conduction band results are analysed on the five-band model and the implications of this model on the valence band results are discussed. The authors have determined the valence band inverse mass parameters to be: gamma 1=7.5, gamma 2=2.6, gamma 3=3.1, kappa =1.0.