Excitonic Gain Research Articles

Coexistence of excitonic lasing with electron–hole plasma spontaneous emission in one-dimensional semiconductor structures

We report that excitonic lasing gain coexists with spontaneous optical emission characteristic of an electron–hole plasma in highly photoexcited one-dimensional semiconductors. The experiments probe quantum T-wire laser structures optimized for high photoexcitation. Evidence of dense electron–hole plasma is clearly seen in the spontaneous recombination measured when lasing emission displays distinct excitonic character. These findings differ strikingly from those in higher dimentional semiconductors, and offer insights on optical processes considered by recent theories of dense electron–hole plasmas.

Nonlinear absorption in semiconductor quantum dots

The density matrix approach has been employed to understand the mechanism of nonlinear absorption / gain in a single semiconductor quantum dot as well as in an inhomogeneous distribution of quantum dots. The analytical results show an enhancement of excitonic gain in the presence of biexcitons. The same has been confirmed by the numerical analysis carried out for a realistic system of semiconductor quantum dots of CdS embedded in a glass matrix. The numerical study exhibits a red shift of maximum gain at large distribution widths.

Excitonic gain and stimulated ultraviolet emission in nanocrystalline zinc-oxide powder

We have studied ultraviolet photoluminescence from high-purity zinc-oxide powder over a wide temperature range (2–293 K). At low temperatures, the spontaneous emission is due to radiative recombination of excitons bound to donors and acceptors. At high temperature (>90 K), it mainly consists of recombination of free excitons, with exciton–exciton collision-induced recombination dominating the spectrum at higher pumping intensities. Emission from the exciton–exciton collision process shows clear stimulated-emission behavior. At sufficiently high pumping intensity, the stimulated emission band shifts abruptly to a longer wavelength due to the formation of an electron–hole plasma. Sharp emission peaks are observed in the stimulated emission bands. The possible origins of these sharp peaks are discussed.

Excitonic absorption and gain in ZnSe

We present calculations of the linear optical absorption/gain in ZnSe including the relevant many-body effects which comply with a correct description of position and linewidth of the exciton and of the transition from gain to absorption at the chemical potential. The influence of an excitonic fraction is included considering the chemical equilibrium of bound e–h pairs and free carriers by a T-matrix approach. For temperatures from 40 upto 200 K, we found that increasing the excitation the chemical potential comes close to the excitonic absorption peak but does not cross it, as long as the latter survives. Moreover, there is only a minor fraction of excitons under gain conditions. We deduce, that the gain mechanism is governed by many-body correlations of a strongly Coulomb-correlated electron–hole plasma.

Excitonic coherent gain induced by giant Zeeman splitting in Cd 1− xMn xTe quantum wells

We have observed that anomalously large optical gain emerges at the spectral position of the lowest heavy-hole exciton transition in Cd 1− x Mn x Te/Cd 1− y Mg y Te quantum wells ( x=0.02, y=0.33) with the resonant excitation condition by 200 fs laser pulse at 7 T and 5 K. The gain is enhanced by increasing the applied field. The maximum gain value with the injection energy of 6.0 μJ/cm 2/pulse reaches 3.5×10 4 cm −1 which corresponds to 13 of the linear absorption maximum of the exciton transition at 7 T. Further, the gain grows rapidly along with the injection of pump pulse and relaxes with the decay time less than 1 ps. These experimental results suggest that the gain is induced by the optical polarization of excitons which is created by the optical field of pump pulse and also that gain relaxation is connected to the dephasing time of the exciton coherence. Further, we have observed that the exciton absorption band splits into three bands before the gain appears, indicating the splitting is induced by the optical Stark effect. The results are discussed in terms of the dressed-atom model.

Dynamics of carriers in resonantly excited quantum-well lasers studied by intersubband absorption

Using infrared picosecond pulses to probe the intersubband absorption of GaAs/AlGaAs quantum-well lasers following their optical excitation, we directly measure the dynamics of carriers in these devices. We find no evidence for excitonic gain even at cryogenic temperatures and resonant excitonic excitation.

Lasing via ground-subband transitions in V-groove quantum wire lasers

We present direct evidence of stimulated emission from the one-dimensional ground subband in optically pumped GaAs/AlGaAs V-groove quantum wire structures. We find that the lasing is sustained by excitonic gain. Spectra detected for low excitation power and above the lasing threshold demonstrate that lasing originates from localized excitons.

Quantum dots formed by ultrathin insertions in wide-gap matrices

We report on experimental and theoretical studies on a new type of quantum-dot (QD) structures obtained using ultrathin, i.e. below the critical thickness for 2D–3D transition, strained narrow gap insertions in wide bandgap matrices. We concentrate on submonolayer (SML) or slightly above 1 ML CdSe insertions in a wide-gap II–VI matrices and give the first results on ultrathin InGaN insertions in a GaN matrix. A discussion on detailed comparison of our original results with the results of other authors is presented. The formation of dense arrays (up to 1012 cm−2) of nanoscale two-dimensional (2D) islands is revealed in processed high-resolution transmission electron microscopy images. In the case of stacked sheets of SML insertions, the islands in the neighboring sheets are formed predominantly in correlated or anticorrelated way for thinner and thicker spacer layers, respectively. Different polarization of photoluminescence (PL) emission recorded in edge geometry for vertically-uncoupled and coupled QDs confirms the QD nature of excitons. By monitoring of sharp lines due to single QDs using cathodoluminescence the 3D confinement is manifested. We demonstrate significant squeezing of the QD exciton wavefunction in the lateral direction using magneto-optical experiments. We point to complete suppression of lateral motion of excitons bound to islands in case of wide-gap (ZnMgSSe) matrices, as follows from PL excitation studies. A resonant (0-phonon) lasing is observed in ultrathin CdSe insertions and proves the lifting of the k-selection rule for QD excitons. We show that lack of exciton screening in QDs up to high excitation densities enables strong resonant modulation of the refractive index in stacked ultrathin insertions and allows realization of resonant (excitonic) waveguiding and lasing. This enables the realization of a new type of heterostructure laser operating without external optical confinement by layers having lower average refractive indices. Ultrahigh QD excitonic gain in dense arrays of stacked QDs allows a new type of surface-emitting laser.

Ultrafast excitonic optical gain induced by giant-Zeeman splitting in Cd 1− xMn xTe quantum wells

We observed anomalously large optical gain at the spectral region of the lowest heavy-hole exciton transition in Cd 1− x Mn x Te/ Cd 1− y Mg y Te quantum wells ( x=0.022, y=0.33 ) with the resonant excitation condition by 200 fs laser pulse at 7 T and 5 K. The gain is enhanced with increasing magnetic field. The maximum gain value with the injection energy of 6.0 μJ/cm 2/pulse reaches 3.5×10 4 cm −1 which corresponds to 1 3 of the linear absorption maximum of the exciton transition at 7 T. Further, the gain grows rapidly along with the injection of pump pulse and relaxes with decay times of 0.18 and 0.98 ps. The gain induced by the magnetic field is always observed at the spectral position of the lowest exciton transition. These experimental results suggest that the gain is induced by the optical polarization of exciton which is created by the optical field of pump pulse and gain relaxation is connected to the dephasing time of the exciton coherence.

Dense excitonic luminescence and optical gain in ZnS-based quantum wells

Biexciton formation in ZnS epitaxial layers and Cd x Zn 1− x S–ZnS quantum wells has been studied by means of photoluminescence excitation spectroscopy. On the basis of the two-photon absorption of biexcitons, the binding energy of the biexciton in Cd x Zn 1− x S–ZnS quantum wells was estimated to be 34 meV, which was approximately 4 times larger than that in ZnS. In addition, the quantum-well biexciton was found to be localized by alloy disorder, and there existed the distribution of the biexciton binding energy. The contribution of biexcitons to optical gain formation and resultant stimulated emission was proposed in Cd x Zn 1− x S–ZnS quantum wells up to about 250 K.

Two-dimensional exciton dynamics and gain formation processes inInxGa1−xNmultiple quantum wells

We investigated exciton dynamics in ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ multiple quantum wells at 2 K and room temperature by means of time-resolved photoluminescence and pump-and-probe measurements. Under low excitation density, the temporal change of the spontaneous emission indicated slow dynamical features of the two-dimensional exciton localization, while, above a stimulation threshold density, the decay time of the emission was shortened to be less than $\ensuremath{\sim}30 \mathrm{ps}$ due to the stimulated emission process. Further, the time-resolved pump-and-probe measurement revealed a fast relaxation of photoexcited delocalized electron-hole $(e\ensuremath{-}h)$ pairs into localized states. Above stimulation threshold, localized states were saturated, and $e\ensuremath{-}h$ pairs at delocalized states were observed. Two-dimensional $e\ensuremath{-}h$ pairs at delocalized states relaxed into localized states and excitons were formed, from which the optical gain was formed in terms of the stimulated emission process.

Quantum dot lasers: The birth and future trends

Despite its early age, laser based on arrays of self-organized quantum dots modified all the basic commandments of the heterostructure laser. Excitonic gain mechanism and discrete energy spectrum in a quantum dot provide principally new ways to control optical properties of the media. Extension of the spectral range using the same substrate will probably soon lead to the appearance of quantum dot lasers on the market.

Dynamic theory of excitonic hyper-Raman gain

We present a dynamic theory of the hyper-Raman gain due to excitons in semiconductors coherently driven with femtosecond pulses. The gain spectrum is very different from the stationary result. The gain position is not determined by energy conservation between excitons, pump, and test photons, which signifies a strong contribution from continuum states. The numerical results are in good agreement with recent measurements [Phys. Rev. Lett. 79, 3716 (1997)].

Optical gain in (Zn, Cd)Se–Zn(S, Se) quantum wells

We have investigated the mechanism of stimulated emission in ZnCdSe–ZnSSe quantum wells through optically pumped measurements of the gain spectrum in a variety of structures from 270 to 77 K. We also calculated the optical gain, using a model that includes many-body effects, and found excellent agreement between the calculated gain line shapes and our measurements. Under the conditions studied, which are close to those found in an operating laser diode, we conclude that the stimulated emission arises from an electron–hole plasma in our samples, even down to 77 K. Although our measurements do not rule out exciton gain mechanisms at other temperatures or operating conditions, sensitive line-shape fitting does not require them in our case. However, our line-shape analysis does show that Coulomb enhancement is significant, even at room temperature.

Excitonic Gain in Coherently-Driven Semiconductors

Excitonic Gain in Coherently-Driven Semiconductors

Gain studies of (Cd, Zn)Se quantum islands in a ZnSe matrix

By inserting stacked sheets of nominally 0.7 monolayer CdSe into a ZnSe matrix we create a region with strong resonant excitonic absorption. This leads to an enhancement of the refractive index on the low-energy side of the absorption peak. Efficient waveguiding can thus be achieved without increasing the average refractive index of the active layer with respect to the cladding. Processed high-resolution transmission electron microscopy images show that the CdSe insertions form Cd-rich two-dimensional (Cd, Zn)Se islands with lateral sizes of about 5 nm. The islands act as quantum dots with a three-dimensional confinement for excitons. Zero-phonon gain is observed in the spectral range of excitonic and biexcitonic waveguiding. At high excitation densities excitonic gain is suppressed due to the population of the quantum dots with biexcitons.

Room-temperature gain spectra and lasing in microcrystalline ZnO thin films

We report room-temperature measurements of optical gain and gain spectra of ultraviolet emission from ZnO thin film. An optical gain of 300 cm −1 is measured at a fluence of 3.8 μJ/cm 2. The large gain is attributed to the modification of the spontaneous emission rate by the dielectric photonic structure of these films. The presence of excitonic gain is also observed in the room-temperature gain spectra.

Optical gain in (Zn,Cd)SeZn(S,Se) quantum wells as a function of temperature

We have investigated the mechanism of stimulated emission in ZnCdSeZnSSe quantum wells through measurements of the optical gain spectrum between 77 and 270 K. We also calculated the optical gain using a model which included many-body effects and found excellent agreement with our measurements. Our results are inconsistent with an excitonic gain mechanism and we conclude that the stimulated emission arises from an electron—hole plasma in our samples. However, we find that the electron-hole Coulomb interaction is still significant at room temperature in II–VI heterostructures.

Reduced threshold current density due to excitonic optical gain in the presence of dislocations and surface states in tensile strained ZnCdSe quantum wire lasers

Higher binding energies (30–60 meV) in II–VI wide-gap materials result in large exciton densities, making optical transitions due to excitons dominant over free electrons and holes. Optical gain and threshold current densities in ZnCdSe based multiple quantum wire lasers are computed including the effect of strain. It is found that the tensile-strained quantum wires yield lower threshold current densities than compressive strained or unstrained structures. The calculated threshold current density for a defect free tensile-strained ZnCdSe–ZnMgSSe quantum wire laser, realized on an InP substrate, has been computed to be 58 A/cm2. The exciton transitions assist in lowering the threshold current density which is adversely affected by the presence of dislocations and surface states. It is found that the threshold current density would increase to 435 A/cm2 assuming 1017 cm−3 trapping levels due to dislocations and surface states. However, taking into account the exciton transitions, the threshold current density is reduced to 79 A/cm2 when assuming the same trapping level density.

Lasing in ZnSe/ZnS0.18Se0.82 superlattices.

We investigated lasing in highly excited ZnSe/Zn${\mathrm{S}}_{0.18}$${\mathrm{Se}}_{0.82}$ superlattices. Despite the considerable exciton stability and the strong exciton-phonon coupling, we found that lasing originates from electron-hole plasma recombination. Exciton recombination is found to dominate the spontaneous emission up to carrier densities very close to the stimulated emission threshold. The modest enhancement of the exciton binding energy caused by the negligible conduction band offset prevents excitonic gain in $\mathrm{ZnSe}/\mathrm{Zn}{\mathrm{S}}_{x}{\mathrm{Se}}_{1\ensuremath{-}x}$ heterostructures. The impact of these findings for the interpretation of various spectroscopic experiments is discussed in the framework of a phase diagram model in which exciton and free-carrier phases coexist at high density in the quantum well.