Ultrashort Optical Pulse Excitation Research Articles

Microcavity design for low threshold polariton condensation with ultrashort optical pulse excitation

We present a microcavity structure with a shifted photonic stop-band to enable efficient non-resonant injection of a polariton condensate with spectrally broad femtosecond pulses. The concept is demonstrated theoretically and confirmed experimentally for a planar GaAs/AlGaAs multilayer heterostructure pumped with ultrashort near-infrared pulses while photoluminescence is collected to monitor the optically injected polariton density. As the excitation wavelength is scanned, a regime of polariton condensation can be reached in our structure at a consistently lower fluence threshold than in a state-of-the-art conventional microcavity. Our microcavity design improves the polariton injection efficiency by a factor of 4, as compared to a conventional microcavity design, when broad excitation pulses are centered at a wavelength of λ = 740 nm. Most remarkably, this improvement factor reaches 270 when the excitation wavelength is centered at 750 nm.

Terahertz Wave Generation Device using Multi-Quantum Well with Transverse Electric Field

A multi-quantum-well (MQW) device with an electric field in the MQW plane was proposed as a high-power terahertz electromagnetic wave (THz wave) generator. High-power THz waves were generated by ultrashort optical pulse excitation of the MQW device. In the excitation wavelength dependence, the amplitude of the THz wave had a peak at around exciton absorption in the MQW, which implies that the THz wave is generated by the acceleration of photoexcited carriers in the MQW region. The amplitude was proportional to an applied voltage, and the power of the THz wave at an applied voltage of 500 V was 60 times higher than that of the bulk InAs reference.

Investigation of Ultrafast Carrier Dynamics in Quantum Wire by Terahertz Time-Domain Spectroscopy

We have investigated ultrafast carrier dynamics in a semiconductor crescent-shaped quantum wire by terahertz (THz) wave time-domain spectroscopy (THz-TDS). THz waves are generated by the motion of photoexcited carriers in semiconductor nanostructures, so that the time evolution of coherent polarization in the nanostructures is obtained by integration of observed THz waves. THz waves were generated by ultrashort optical pulse excitation of the quantum wire and detected using a free-space electrooptic (EO) sampling method. The dephasing time of coherent polarization in the quantum wire was varied by increasing the excitation energy. It was estimated that the dephasing time of coherent polarization is 300 fs at an excitation energy of 1.534 eV.

Excitation Wavelength Dependence of Terahertz Electromagnetic Wave Generation from Quantum Wire

We have investigated terahertz (THz) electromagnetic wave generation from a semiconductor crescent-shaped quantum wire structure. The THz electromagnetic wave was generated by ultrashort optical pulse excitation, and detected with the free-space electrooptic (EO) sampling method. THz electromagnetic wave generation from the quantum wire exhibited excitation wavelength dependence. The amplitude of the THz electromagnetic wave was at a maximum at a slightly lower energy than the photoluminescence (PL) peak, and decreased with reduction of the excitation energy. The amplitude of the THz electromagnetic wave decreased rapidly at the fundamental absorption band edge of the GaAs substrate. Therefore the THz electromagnetic wave might be generated due to instantaneous polarization in the quantum wire.

Terahertz Electromagnetic Wave Generation from Quantum Nanostructure

We have investigated terahertz (THz) electromagnetic wave generation from semiconductor quantum nanostructures.The samples used in our experiment are a GaAs/AlGaAs symmetric triple-coupled quantum well and a GaAs/AlGaAs crescent-shaped quantum wire. The THz electromagnetic waves were generated by ultrashort optical pulse excitation and detected by a spatial electrooptic (EO) sampling method. Generation of a THz electromagnetic wave, which originats from quantum beats with three frequency components of 0.8, 1.0, and 1.8 THz, was observed in the symmetric triple-coupled quantum well. In a quantum wire, THz electromagnetic wave generation due to the instantaneous polarization accompanied by the creation of excitons by ultrashort optical pulse excitation was observed.

Carrier transport affected byΓ−Xtransfer in type-I GaAs/AlAs superlattices

We studied how $\ensuremath{\Gamma}\ensuremath{-}X$ transfer affects electron transport in type-I GaAs/AlAs semiconductor superlattices under an electric field. $\ensuremath{\Gamma}$ to X transfer was found to degrade the sweep out of carriers, while X to $\ensuremath{\Gamma}$ resonance was observed to promote it. An anomalously delayed photocurrent was observed under ultrashort optical pulse excitation. This phenomenon is explained by a switch of the electron transport path from $\ensuremath{\Gamma}\ensuremath{-}\ensuremath{\Gamma}$ to $\ensuremath{\Gamma}\ensuremath{-}X\ensuremath{-}\ensuremath{\Gamma}\ensuremath{-}X,$ caused by a change in the subband alignment between the second $\ensuremath{\Gamma}$ state $({\ensuremath{\Gamma}}_{2})$ in the well and the ${X}_{z}$ ground state ${(X}_{1})$ in the adjacent barrier by the electric field. The delayed photocurrent could be reproduced by a simple simulation while considering the above carrier transport paths. These results provide evidence that $\ensuremath{\Gamma}\ensuremath{-}X\ensuremath{-}\ensuremath{\Gamma}$ real-space transfer considerably affects carrier transport even in type-I superlattices under an electric field.

Time-development of transient-carrier temperature, density, and gain spectrum in ultrashort optical pulse excited InGaAs multiquantum-well laser structure

We present a novel transient gain-spectra measurement method based on the traditional variable pump-stripe technique. Using the pump-stripe technique with ultra-short optical pulse excitation, time-resolved amplified spontaneous emission spectroscopy of an InGaAs-InGaAsP multiquantum-well (MQW) laser structure was measured, and time-development of the transient optical net gain spectra was obtained accordingly. By fitting the measured gain spectra with a model for band-to-band transitions including momentum conservation and an energy- and density-dependent lifetime broadening, dynamics of band filling, carrier density, carrier temperature and bandgap renormalization have been obtained. This opens the possibility to study simultaneously the influence of transient-carrier density and, in particular, transient-carrier temperature on the transient optical gain. Strong gain compression in the whole gain-spectra region due to transient high carrier temperature after ultra-short pulse injection is clearly demonstrated for the first time.

Evidence for Gamma -X transport in type-I GaAs/AlAs semiconductor superlattices.

We report the first evidence that the $\ensuremath{\Gamma}\ensuremath{-}X$ transfer mechanism plays a significant role in carrier transport in type-I semiconductor superlattices under an electric field. An anomalously delayed photocurrent was observed in a GaAs/AlAs type-I superlattice under ultrashort optical pulse excitation. This phenomenon can be explained by a switch of the electron transport path from $\ensuremath{\Gamma}\ensuremath{-}\ensuremath{\Gamma}$ to $\ensuremath{\Gamma}\ensuremath{-}X\ensuremath{-}\ensuremath{\Gamma}\ensuremath{-}X$, caused by an electric-field induced change of the subband alignment of the second $\ensuremath{\Gamma}$ state ($\ensuremath{\Gamma}2$) in the well and the $X1$ state in the adjacent barrier.

Delayed photocurrent affected by Γ-X resonance in GaAs/AlAs type-I short-period superlattices

Delayed photocurrents were observed in GaAs/AlAs type-I short-period superlattices by measuring time-resolved photoresponses under ultrashort optical pulse excitation. According to the envelope function calculations, the X1 state in AlAs barriers resonates with the Γ2 state in the adjacent GaAs wells at a bias voltage where the delayed photocurrents were conspicuous. These results strongly suggest that the dynamic carrier transport process is significantly influenced by X1-Γ2 resonance effects in the superlattices.

Ultrafast intensity switching and nonthermal carrier effects in semiconductor microcavity lasers

A microscopic nonequilibrium theory is applied to investigate the dynamical response of semiconductor microcavity lasers for ultrashort optical pulse excitation. It is predicted that femtosecond pulse induced carrier heating leads to picosecond switching and recovery of the laser emission.

Carrier transport effects and dynamics in multiple quantum well optical amplifiers

The gain recovery dynamics of multiple quantum well semiconductor optical amplifiers, following gain compression caused by ultrashort optical pulse excitation, have been studied for several devices of different structures. Fast, slow, and intermediate time constants are identified. The fast component (0.6 to 0.9 ps) corresponds to cooling of the dense, inverted electron-hole plasma. The slow component (150 to 300 ps) corresponds to replenishment of carriers from the external bias supply, with the dynamics dominated by spontaneous recombination (primarily Auger) of the electron-hole plasma. The intermediate time constant (2 to 14 ps) is caused by carrier capture by the quantum wells and is structure-dependent. For most of the devices, the capture process is dominated by diffusion-limited transport in the cladding/barrier region. The variation of carrier density and temperature also affects the refractive index profile of the devices and, hence, affects the waveguiding properties. Dynamical variation of the mode confinement factor is observed on the fast and slow timescales defined above.

Virtual excitons and the nonlinear optical response of polydiacetylene

We present data on the nonlinear optical response of the quasi-1D semiconductor polydiacetylene-PTS under ultrashort optical pulse excitation. For near resonant excitation, the effect is to “burn a hole” in the exciton line. The lineshape of the hole depends sensitively on the pump frequency, ω p, and its magnitude peaks when ω p = ω x − ω o where ω o and ω x are the phonon and exciton frequencies. We discuss these results in the context of a model stipulating strongly coupled excitons, photons and phonons.

Phonon-mediated optical nonlinearity in polydiacetylene.

We have studied the nonlinear optical response of the quasi-1D semiconductor polydiacetylene-toluene sulfonate under ultrashort optical pulse excitation. The effect of the excitation is to "burn a hole" in the exciton line. The line shape of the hole depends sensitively on the pump frequency, ${\ensuremath{\omega}}_{p}$, and its magnitude peaks when ${\ensuremath{\omega}}_{p}={\ensuremath{\omega}}_{x}\ensuremath{-}{\ensuremath{\omega}}_{o}$, where ${\ensuremath{\omega}}_{o}$ and ${\ensuremath{\omega}}_{x}$ are the phonon and exciton frequencies. We discuss these results with a model in which optical nonlinearity results from phonon-mediated interactions between virtual excitons.