Wannier Stark Research Articles

Wannier-Stark Localization in Superlattices

We present experimental and theoretical evidence for Wannier-Stark (WS) oscillations in the DC electric current through reverse-biased highly doped p-i-n GaAs diodes. The intrinsic region in the diode contained seven (GaAs)5/(AlAs)2 multi-quantum wells. Carrying out these transport experiments at low temperatures, we found periodic WS oscillations in the second derivative of the Zener current, which is in qualitative agreement with theoretical predictions based on a realistic multiband and multichannel scattering theory. These findings resolve long-standing controversies about the existence of Wannier-Stark levels in the Zener tunneling current.

A global stability analysis for symmetric self-electrooptic effect device systems using a potential function method

This paper proposes a novel analytical method for use in symmetric self-electrooptic effect device (S-SEED) systems, called the potential function method, based on a global stability analysis of differential equations for photocurrent in S-SEED circuits. The method provides intuitive views for analyzing the stability of the system, and is useful for tracking the temporal dynamics of S-SEED nonlinear electrical circuits. This paper also describes electro-absorption characteristics of SEED's, especially those based on Wannier Stark localization (WSL). In this type of SEED, the photocurrent versus reverse bias voltage characteristics has multiple peaks and multiple negative differential resistance regions, resulting from Stark ladder transitions due to thin barrier superlattices. Various types of stabilities, including the metastable state, as well as the temporal switching dynamics of WSL-S-SEEDs, can be explained clearly by using this potential function method.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

RECENT DEVELOPMENTS AND APPLICATIONS IN ELECTROABSORPTION SEMICONDUCTOR MODULATORS

State-of-the-art developments in electroabsorption modulators that utilize quantum well-semiconductors are reviewed. Optimized Multiple Quantum Well modulators in the GaAs/AlGaAs system have been driven by applications in photonic switching and optical interconnects. Surprisingly, these same structures exhibit a wealth of new behavior that ranges from Bloch oscillations to excitons in Coupled Well, Wannier–Stark and Shallow Well superlattices. Several types of excitonic phases have been identified optically and found to transform as the system dimensionality changes from 2D to 3D. In addition, new material systems have shown that quantum well excitonic absorption quality can be transferred to technologically important wavelengths at 1.06 μm or 1.55 μm.

Monolithic integration on InP of a Wannier Stark modulator with a strained MQW DFB 1.55-μm laser

We present the technical approach and the preliminary device results on the first integration of a Wannier Stark (WS) electroabsorption (EA) modulator with a DFB laser on InP. The WS modulator active layer consists of a lattice matched InGaAs-InAlAs superlattice (SL) grown by solid source MBE. It is butt-coupled to a laser grown by AP-MOVPE whose active layer includes a strained InGaAsP-InGaAsP MQW stack. Device results cover static performances of integrated lasers and modulators, and measurements of high frequency characteristics (small signal bandwidth and 10 Gb/s eye diagram).< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Dynamic Wannier–Stark effect in semiconductor superlattices

A novel effect, consisting of optically induced effective mass change due to the change in the degree of localization in the semiconductor superlattices is theoretically investigated. Possible application as a nonabsorbing differential light detector/switch is considered.

Optical differentiator based on rapid collapse of Wannier–Stark localization due to space charge field screening

An optical differentiator operation is demonstrated using a GaAs/AlAs short-period superlattice near an optical absorption band edge. This operation is clearly explained by a rapid collapse of Wannier–Stark localization due to electric field screening by photogenerated space charges, because the screening causes a positive feedback loop between restoration of blueshifted wavelength of absorption band edge to the red side and an increase in optical absorption which causes an additional field screening.

Optically multistable operation of a waveguide device based on the Wannier–Stark effect in an InGaAs/InP superlattice

An optically multistable device in waveguide configuration has been fabricated, which is based on the Wannier–Stark effect in an InGaAs/InP superlattice. It implements a waveguided version of the self-electro-optic effect device and it operates at room temperature at a wavelength of 1.55 μm.

Analog operation of a symmetric self-electro-optic effect device based on Wannier-Stark localization, and its gain characteristics: An all-optical analog transistor

We propose a novel all-optical analog three-terminal device using a symmetric self-electro-optic effect device based on Wannier–Stark localization (WSL-S-SEED). The optical gain factor is derived theoretically from the ratio between derivatives of the two crossed photocurrent versus voltage curves, and a large gain is predicted near an operating point where the ratio is nearly equal to unity. The principle was verified experimentally using a WSL-S-SEED having the above characteristics, and a sufficient analog modulation of the optical output was observed.

Coexistence of Wannier-Stark transitions and miniband Franz-Keldysh oscillations in strongly coupled GaAs-AlAs superlattices.

Using differential photocurrent spectroscopy we have studied the field dependent absorption in GaAs/AlAs superlattices, which have only one monolayer AlAs barriers, with unpreceded dynamical resolution. We are able to resolve a symmetric Wannier-Stark (WS) fan up to an index of \ifmmode\pm\else\textpm\fi{}9 and for the first time Franz-Keldysh (FK) oscillations across the whole energy width of the lowest combined miniband. A modulation of the WS transitions by FK oscillations (and vice versa) is clearly visible. We also present results of theoretical calculations which describe in detail this coexistence of the linear WS fan and the nonlinear fans of the FK oscillations.

Modulation of Wannier-Stark transitions by miniband Franz-Keldysh oscillations in strongly coupled GaAs-AlAs superlattices

We present calculations and optical experiments performed at 77 K on a GaAs/AlAs superlattice with only one monolayer AlAs barriers and a combined miniband width of 380 meV. In particular the field dependent transition from the 3D miniband system to the 2D Wannier Stark (WS) regime is investigated. Using Differential Photocurrent Spectroscopy we are able to resolve WS transitions with indices n between −9 and +9. At the same time Franz-Keldysh (FK) oscillations are observable at the lower and for the first time even at the upper miniband edge. A modulation of the WS transitions by the FK oscillations (and vice versa) is clearly visible. Our theory describes in detail the competition of the linear WS fan originating at the “center of mass energy” with the non-linear fans of the FK oscillations evolving from the lower and upper miniband edges.

Low power all-optical bistability in InGaAs-AlInAs superlattices: Demonstration of a wireless self-electro-optical effect device operating at 1.5 μm

We report the observation of optical transmission bistability at low temperature in unprocessed InGaAs-AlInAs superlattice p-i-n structures. Bistability results, in analogy with the self-electro-optical effect device, from a positive feedback mechanism due to the interplay between Wannier–Stark effect, built-in field, and screening by photocarriers.

Wannier–Stark localization in InGaAs/GaAs superlattices and its application to electro-optical devices

We have observed Wannier–Stark localization in strained In0.2Ga0.8As/GaAs superlattices by low- and room-temperature photocurrent spectra measurements. The experimental results are well in agreement with the theoretical predictions. A large field-induced modulation response of the absorption edge of the superlattices at room temperature suggests the possibilities of the application to the design of various kinds of electro-optical devices operating at a wavelength of 0.98 μm, based on Wannier–Stark localization effects.

CHIRP measurement and transmission experiment at 10 Gbit/s with Wannier-Stark modulator

The chirp parameter α of an electroabsorption Wannier–Stark modulator is measured for the first time, with a new, simple and accurate method. It varies widely from positive to negative values with applied bias. It is shown that a newly-introduced effective chirp parameter is zero as confirmed in a transmission experiment at 10 Gbit/s in a standard fibre.

Operation of an optoelectronic AlGaAs/GaAs waveguide neuron

A novel monolithic AlGaAs/GaAs waveguide neuron is proposed and implemented with a Wannier-Stark superlattice in the core. Dynamic weighting, summing, and thresholding of signals is done by chip-level integration of modulators and a saturable absorber on a rib waveguide power combiner. At 780 nm (below the band gap) the range for synaptic weights is between 1 and -25 dB/mm for reverse bias below 7 V, and the modulation depth is 25 dB for the thresholding element. For a two-to-one neuron, the output-input range ratio is 25 dB.

Wannier–Stark quantization by internal field in the HgTe/CdTe superlattice

The Stark ladder can be formed in a semiconductor superlattice by an applied electric field. The localization of electrons by an external electric field is known as Wannier–Stark quantization. We have performed the Shubnikov–de Haas measurements with a tilted magnetic field and the photoluminescence measurement on a HgTe/CdTe superlattice in the absence of an external electric field. From the observation of a two-dimensional electron gas and a blueshift of the photoluminescence spectrum, we conclude that the Stark ladder exists in the HgTe/CdTe superlattice and it is formed due to the Wannier–Stark quantization by the internal electrostatic field.

Electric-field effect on intersubband optical absorption in a Si/Si1−xGex superlattice

Electronic band structure and absorption coefficient for intersubband transitions in a Si/Si1−xGex superlattice under external electric fields are reported using a full Brillouin-zone energy-band description. Full spectra of the absorption coefficient are calculated from zero to high electric fields. The effect of Wannier–Stark localization on intersubband transitions is theoretically studied for the superlattice and the charge densities are given for various electric fields. A finite-length superlattice instead of an infinite superlattice is considered to show the effects of electric fields on the optical absorption. The Stark shift is observed toward the shorter photon wavelength for an intersubband transition in a quantum well.

Optical tri-state switching through a metastable state using short pulse excitation

Optical tri-state switching is obtained for the first time, using short pulse excitation in a symmetric self-electro-optic effect device based on the Wannier–Stark localization (WSL-SEED). A novel method, by controlling photoexcited charges, is proposed to set and reset the transmission states of the signal diode, which enables us to use a metastable state as an intermediate state in the tri-state device. By the use of a sequence of the optical pulses a clear switching among three stable states as well as optical adder/subtracter properties is demonstrated.

Electroabsorption modulator based on Wannier–Stark localization with 20 GHz/V efficiency

We report on a ridge-waveguide modulator based on Wannier–Stark localization in an InGaAs/InAlAs superlattice. Anisotropic absorption is measured and efficient modulation is obtained in the low-field domain and in the high-field domain with TE-polarized light. The device exhibits outstanding HF characteristics: in terms of bandwidth-to-drive-voltage ratio, we find that the Wannier–Stark localization is far more efficient than the quantum Stark effect.

Wannier localization in GaAs/GaAlAs superlattices under electric field

We have studied the Wannier–Stark effect in GaAs/GaAlAs short-period superlattices under applied electric field perpendicular to the layers by room- and low-temperature photocurrent measurements. The changes in the transition intensities with biasing are well fitted to a theoretical calculation based on the finite Kronig–Penney model on which the potential of an applied electric field is superposed. With increasing electric field, the 0h peak grows to a maximum while the −1h and +1h peaks monotonousely decrease. By a comparison of the spectra measured at different temperatures, the two peaks in the room temperature photocurrent spectra at relatively low electric field (1.0×104 V/cm) are identified to be caused by the Wannier localization effect instead of saddle-point excitons.

Dual wavelength optical bistability and multistability in a symmetric self-electro-optic effect device based on Wannier–Stark localization

A tunable optical switch has been studied of a symmetric self-electro-optic effect device based on Wannier–Stark localization (WSL) in GaAs/AlAs superlattices. This device is driven by two different wavelengths for a signal beam and a control beam. Clear bistability and/or triple stability are demonstrated by controlling the illumination wavelengths. The tunability of the switching properties is clearly explained by the WSL mechanism.