Surface second harmonic generation

Optical second harmonic generation (SHG) requires an inversion asymetric material in order to be dipole allowed. For a material with bulk inversion symmetry, SHG is still dipole allowed at a surface or interface. This makes SHG a powerful surface-selective measurement technique.

The silicon/silicon dioxide interface is of particular technological relevance because this interface occurs between the channel and gate in integrated circuits. Since bulk crystalline silicon is inversion symmetric and silicon dioxide is transparent, SHG is extensively employed to study this interface.

Spin Dynamics in Semiconductors

Interest in the design of spin-based electronics (spintronics) or even quantum computing devices motivated research on spins in semiconductors. Surprisingly long spin coherence times – a prerequisite for efficient devices – were observed in bulk semiconductors. The origin of these long relaxation times and the role of doping carriers vs. optically excited carriers was initially unclear.

We investigated undoped and n-doped GaAs samples with varying doping densities by performing time-resolved Faraday rotation experiments at high optical excitation levels. In these experiments, a magnetic field was applied parallel to the sample surface. 100 fs laser pulses provided by a mode-locked Ti:sapphire laser tuned around the bandgap of the semiconductor create spin-polarized electrons. These electrons precess in the applied magnetic field (Larmor spin precession). By measuring the polarization rotation of a linearly polarized probe beam transmitted through the sample (Faraday rotation) using a polarization bridge one is able to monitor the spin precession of the electrons over time. By varying doping via optical pumping and across different samples, the role of many-body effects was investigated.

We also investigated spin diffusion by measuring transient spin gratings in GaAs quantum wells. In these experiments, two coincident laser pulses (pumps) with orthogonal linear polarizations interfered to form a spin grating, in which the electron spin orientation alternates across the sample. A delayed laser pulse (probe) was diffracted off of this grating, measuring the grating amplitude as a function of time. The spin grating decayed due to both spin relaxation and spin diffusion. In a doped quantum well, the spin grating lasts longer than the optically excited carriers, indicating a spin grating is formed in the electron gas. We were able to obtain the spin diffusion rates of the electron gas under various conditions, providing information relevant to spintronic applications.

Quantum Interference Control of Injected Currents in Semiconductors

The ontrol of ballistic currents due to quantum interference control (QUIC) between one- and two-photon paths in semiconductors was demonstrated by the groups of Henry van Driel and John Sipe at the University of Toronto. The carrier population can also be controlled via QUIC when imaginary part of the second-order nonlinear susceptibility χ⁽²⁾ is non-zero in a non-centrosymmetric medium, i.e. when optical fields are aligned along certain crystal axes in GaAs.

The quantum interference process was calculated in the presence of a static electric field by extending a theory of the one-photon Franz-Keldysh effect to multiphoton processes. The theory predicted that a DC field can combine with the pre-existing third-order nonlinear susceptibility χ⁽³⁾ to produce an effective χ⁽²⁾, which enables 1+2 population control in a semiconductor, without the requirement of a non-centrosymmetric medium.

We demonstrated electric field induced population control on (100) GaAs samples. To avoid trap-enhanced field and carrier screening, the electrodes are biased using a radio frequency bias technique to produce a uniform electric field. Only when the uniform electrical field is applied do we clearly observe the change in probe transmittance as a function of the phase parameter, with both positive and negative bias.

Further studies including bias dependence and optical polarization dependence were performed; the results are consistent with theoretical predictions. This field induced population control technique could be a promising strategy for studying carrier transportation across metal-semiconductor interfaces with applied electric field.

Franz-Keldysh Effect

Electroreflectance (ER) based on Franz-Keldysh effect (FKE) is a widely used electro-optic technique for studying band structure in semiconductors. The FKE results from wavefunctions leaking into band gap when an electric field is applied to a semiconductor. The FKE induces a change in optical absorption in a semiconductor. Typical experiments use longitudinal electrode geometry, with the electric field parallel to the optical wave vector. The less common transverse geometry, wehre the electric field is perpendicular to the light wave vector, has the advantage that the optical polarization can be varied with respect to the electric field. However, in the transverse geometry when metal electrodes are deposited directly on the surface of a semiconductor, i.e. GaAs, the electric field distribution has been found to be strongly concentrated near the anode, due to so called ‘Trap enhanced field effect’, caused by carrier-trapping impurities.

We developed a reliable technique to produce a more uniform electric field in transverse geometry. First, we deposited an insulating layer between GaAs and  the electrode to prevent direct carrier injection and thus avoided the highly localized field near anode due to trap enhanced field. Another problem associated with this sample geometry is the screening of the applied field by photoexcited carriers. To solve this issue the bias was modulated in the range of MHz synchronized with the laser pulses from the light source. Electroreflectance measurement showed that the effectively constant electric field distribution across two electrodes can be achieved by inserting insulating layer and applying rapidly oscillating bias.

This technique could be very useful in ultrafast phenomena experiment which requires a uniform transverse electric field such as THz generation. We demonstrated enhanced THz generation from photo-conductive antenna by using uniform transverse electric field.