In this proceedings we describe our recent results on semiconductor nonlinear optics, investigated using single-cycle
THz pulses. We demonstrate the nonlinear absorption and self-phase modulation of strong-field THz pulses in doped
semiconductors, using n-GaAs as a model system. The THz nonlinearity in doped semiconductors originates from the
near-instantaneous heating of free electrons in the ponderomotive potential created by electric field of the THz pulse,
leading to ultrafast increase of electron effective mass by intervalley scattering. Modification of effective mass in turn
leads to a decrease of plasma frequency in semiconductor and produces a substantial modification of THz-range material
dielectric function, described by the Drude model. As a result, the nonlinearity of both absorption coefficient and
refractive index of the semiconductor is observed. In particular we demonstrate the nonlinear THz pulse compression
and broadening in n-GaAs, as well as an intriguing effect of coexisting positive and negative refractive index
nonlinearity within the broad spectrum of a single-cycle THz pulse. Based on Drude analysis we demonstrate that the
spectral position of zero index nonlinearity is determined by (but not equal to) the electron momentum relaxation rate.
Single cycle pulses of light, irrespective of the frequency range to which they belong, inherently have an ultrabroadband
spectrum covering many octaves of frequencies. Unlike the single-cycle pulses in optical domain, the THz pulses can be
easily sampled with sub-cycle resolution using conventional femtosecond lasers. This makes the THz pulses accessible
model tools for direct observation of general nonlinear optical phenomena occurring in the single-cycle regime.
We report on nonlinear propagation of single-cycle THz pulses with peak electric fields reaching 300 kV/cm in n-type
semiconductors at room temperature. Dramatic THz saturable absorption effects are observed in GaAs, GaP, and Ge,
which are caused by the nonlinear electron transport in THz fields. The semiconductor conductivity, and hence the THz
absorption, is modulated due to the acceleration of carriers in strong THz fields, leading to an increase of the effective
mass of the electron population, as the electrons are redistributed from the low-momentum, low-effective-mass states to
the high-momentum, high-effective-mass states in the energy-momentum space of the conduction band. Further, we
observe the typical accompanying effects of saturable absorption on the THz pulses, such as an increase of the group
delay, as the peak electric field of the pulse increases. In this paper we present the results of nonlinear THz time-domain
spectroscopy, and of THz pump - THz probe spectroscopy.
Optical frequency shifters are essential components of many systems. In this paper, a compact integrated optical
frequency shifter is designed making use of the combination of surface acoustic waves and Mach-Zehnder
interferometers. It has a very simple operation setup and can be fabricated in standard semiconductor materials. The
performance of the device is analyzed in detail, and by using multi-branch interferometers, the sensitivity of the device to
fabrication tolerances can be drastically reduced.
We report on the fabrication of a metal-dielectric composite material with tunable optical properties. The developed
fabrication method relies on simultaneous DC sputtering of a metal and a suitable dielectric, creating an isotropic
material with optical properties that can be controllably varied over a wide range of wavelengths. Currently the research
is focusing on a combination of Ag and ZnO that is suitable for applications at the visible and telecommunication
frequencies. The material combination is well suited for the deposition method chosen, and physical characterizations
using AFM and SEM measurements show that the mixture forms homogeneous films with low surface roughness. In
order to test the validity of this approach films are deposited with a variety of deposition parameters, focusing mainly on
the relative deposition rates basically controlling the filling factor. Optical properties found from experiments using
spectroscopic ellipsometry as well as farfield reflection-transmission measurements are compared to those predicted by
the effective medium theory.
The design of a quantum well (QW) based high-saturation energy and low-loss gain region allows a high power density
which ensures efficient saturation of the absorber, increases the efficiency, and lowers the noise of monolithic modelocked
lasers. This is illustrated though 10 GHz all-active lasers with different number of quantum wells.
By comparing a 40 GHz quantum dot and a 40 GHz quantum well laser we discuss the physical difference in the
dynamics of the devices. The slow dynamics of quantum dots (QD), results in low self-phase modulation for picosecond
pulses and a strong damping of intensity fluctuations, which gives rise to clean optical spectra and very low noise for
We present work on design of monolithic mode-locked semiconductor
lasers with focus on the gain medium. The use of highly inverted
quantum wells in a low-loss waveguide enables both low quantum
noise, low-chirped pulses and a large stability region. Broadband
noise measurements are performed and used to confirm the design
We report on a new coherent phenomenon in semiconductor microcavities at polariton selective resonance excitation by two femtosecond pulses, propagating along k2 and k1, associated with exciton gratings, travelling in lateral direction ± (k2 - k1). Diffracted polaritons experience a frequency shift as observed in nondegenerate spectrally resolved transient four-wave mixing experiments.
We have produced GaAs-based quantum-dot edge-emitting lasers operating at 1.16 μm with record-low transparency current, high output power, and high internal quantum efficiencies. We have also realized GaAs-based quantum-dot lasers emitting at 1.3 μm, both high-power edge emitters and low-power surface emitting VCSELs. We investigated the ultrafast dynamics of quantum-dot semiconductor optical amplifiers. The dephasing time at room temperature of the ground-state transition in semiconductor quantum dots is around 250 fs in an unbiased amplifier, decreasing to below 50 fs when the amplifier is biased to positive net gain. We have further measured gain recovery times in quantum dot amplifiers that are significantly lower than in bulk and quantum-well semiconductor optical amplifiers. This is promising for future demonstration of quantum dot devices with high modulation bandwidth.
Coherent optical spectroscopy in the form of nonlinear transient four-wave mixing (TFWM) and linear resonant Rayleigh scattering (RRS) has been applied to investigate the exciton dynamics of low-dimensional semiconductor heterostructures. The dephasing times of excitons are determined from the decay of the spectrally resolved non- linear signal as a function of the delay between the incident pulses in a two-beam TFWM experiment, and from the real time analysis of single speckles in RRS experiments (pure dephasing). From the density- and temperature- dependence of the dephasing times the exciton-exciton and the exciton-phonon interactions are determined. The degree of coherence of the secondary emission is determined from the speckle analysis.
The dynamics of excitonic transitions in semiconductors have been investigated by degenerate four-wave mixing experiments. We have studied the coherence, interference and dephasing of free, bound and localized excitons in bulk semiconductors and of quasi-2D excitons in quantum well structures. The influence of inhomogeneous broadening is investigated and compared with quantum interference in a continuum of states. The nature of four-wave mixing beats in a system of bound excitons and biexcitons is discussed.
Dynamics of free and bound excitons in CdSe, localized excitons in CdSexS1-x, and confined two-dimensional excitons in GaAs/AlGaAs quantum wells have been studied by degenerate four-wave mixing and light-induced grating experiments. In the coherent range, the dephasing of excitons has been determined as a function of temperature, density, and energy. For incoherent excitons, we have determined recombination lifetimes and diffusion coefficients. In particular, we have studied the mobility of excitons in CdSexS1-x near the mobility edge. Quantum interferences are observed in the nonlinear signal from these exciton systems, and the nature of these four-wave mixing beats are being discussed.
Coherent nonlinear resonances due to extended and localized excitons in CdSe and
CdSeS1_ are investigated by picosecond time resolved degenerate four-wave mixing. Large
nonlinear coefficients (X(31O_9cm2/V2) are found, with coherence times in the picosecond
range. Preliminary results on picosecond optical switching in CdSe are presented, and the
possibility of a purely coherent optical switching with picosecond response time is discussed.