Microscopic theory of the intracollisional field effect in semiconductor superlattices. Jörg Hade... more Microscopic theory of the intracollisional field effect in semiconductor superlattices. Jörg Hader, Torsten Meier, and Stephan W. Koch Fachbereich Physik und Zentrum für Materialwissenschaften, Philipps-Universität Marburg,. ...
Monthly Notices of the Royal Astronomical Society, 2011
Using an extended set of multi-epoch high resolution high signal-to-noise ratio optical spectra, ... more Using an extended set of multi-epoch high resolution high signal-to-noise ratio optical spectra, we readdress the multiplicity properties of the O-type stars in IC 2944 and in the Cen OB2 association. We present new evidence of binarity for five objects and we confirm the multiple nature of another two. We derive the first orbital solutions for HD 100099, HD 101436 and HD 101190 and we provide additional support for HD 101205 being a quadruple system. The minimal spectroscopic binary fraction in our sample is f min = 0.57. Using numerical simulations, we show that the detection rate of our observational campaign is close to 90%, leaving thus little room for undetected spectroscopic binary systems. The statistical properties of the O-star population in IC 2944 are similar, within the uncertainties, to the results obtained in the earlier papers in this series despite the fact that sample size effects limit the significance of the comparison. Using newly derived spectroscopic parallaxes, we reassess the distance to IC 2944 and obtained 2.3 ± 0.3 kpc, in agreement with previous studies. We also confirm that, as far as the O stars are concerned, the IC 2944 cluster is most likely a single entity.
Context. Plaskett's star (HD 47 129) is a very massive O + O binary that belongs to the Mon OB2 a... more Context. Plaskett's star (HD 47 129) is a very massive O + O binary that belongs to the Mon OB2 association. Previous work suggested that this system displays the Struve-Sahade effect although the measurements of the secondary radial velocities are very difficult and give controversial results. Both components have powerful stellar winds that collide, and produce a strong X-ray emission. Aims. Our aim is to study in detail the physical parameters of this system, and to investigate the relation between its spectral properties and its evolutionary status. Methods. We present here the analysis of an extensive set of high resolution optical spectra of HD 47 129. We use a disentangling method to separate the individual spectra of each star. We derive a new orbital solution and discuss the spectral classification of both components. A Doppler tomography technique applied to the emission lines Hα and He ii λ 4686 yields a Doppler map that illustrates the wind interactions in the system. Finally, an atmosphere code is used to determine the different chemical abundances of the system components and the wind parameters. Results. HD 47 129 appears to be an O8 III/I + O7.5 III binary system in a post RLOF evolutionary stage, where matter has been transferred from the primary to the secondary star. The He overabundance of the secondary supports this scenario. In addition, the N overabundance and C underabundance of the primary component confirm previous results based on X-ray spectroscopy and indicate that the primary is an evolved massive star. We also determine a new orbital solution, with M P sin 3 i = 45.4 ± 2.4 M ⊙ and M S sin 3 i = 47.3 ± 0.3 M ⊙ . Furthermore, the secondary star has a large rotational velocity (v sin i ∼ 300 km s −1 ) that deforms its surface, leading to a non-uniform distribution in effective temperature. This could explain the variations in the equivalent widths of the secondary lines with phase. We suggest that the wind of the secondary star is confined near the equatorial plane because of its high rotational velocity, affecting the ram pressure equilibrium in the wind interaction zone.
Context. The properties of the early-type binary Cyg OB2 #5 have been debated for many years and ... more Context. The properties of the early-type binary Cyg OB2 #5 have been debated for many years and spectroscopic and photometric investigations yielded conflicting results. Aims. We have attempted to constrain the physical properties of the binary by collecting new optical and X-ray observations. Methods. The optical light curves obtained with narrow-band continuum and line-bearing filters are analysed and compared. Optical spectra are used to map the location of the He ii λ 4686 and Hα line-emission regions in velocity space. New XMM-Newton as well as archive X-ray spectra are analysed to search for variability and constrain the properties of the hot plasma in this system. Results. We find that the orbital period of the system slowly changes though we are unable to discriminate between several possible explanations of this trend. The best fit solution of the continuum light curve reveals a contact configuration with the secondary star being significantly brighter and hotter on its leading side facing the primary. The mean temperature of the secondary star turns out to be only slightly lower than that of the primary, whilst the bolometric luminosity ratio is found to be 3.1. The solution of the light curve yields a distance of 925 ± 25 pc much lower than the usually assumed distance of the Cyg OB2 association. Whilst we confirm the existence of episodes of higher X-ray fluxes, the data reveal no phase-locked modulation with the 6.6 day period of the eclipsing binary nor any clear relation between the X-ray flux and the 6.7 yr radio cycle. Conclusions. The bright region of the secondary star is probably heated by energy transfer in a common envelope in this contact binary system as well as by the collision with the primary's wind. The existence of a common photosphere probably also explains the odd mass-luminosity relation of the stars in this system. Most of the X-ray, non-thermal radio, and possibly γ-ray emission of Cyg OB2 #5 is likely to arise from the interaction of the combined wind of the eclipsing binary with at least one additional star of this multiple system.
Aims. The line profile variability and photometric variability of the O9.5 Vp star HD 93521 are e... more Aims. The line profile variability and photometric variability of the O9.5 Vp star HD 93521 are examined in order to establish the properties of the non-radial pulsations in this star. Methods. Fourier techniques are used to characterize the modulations of the He i λλ 5876, 6678 and H α lines in several spectroscopic time series and to search for variations in a photometric time series. Results. Our spectroscopic data confirm the existence of two periods of 1.75 and 2.89 hr. The line profiles, especially those affected by emission wings, exhibit also modulations on longer time scales, but these are epoch-dependent and change from line to line. Unlike previous claims, we find no unambiguous signature of the rotational period in our data, nor of a third pulsation period (corresponding to a frequency of 2.66 d −1 ). Conclusions. HD 93521 very likely exhibits non-radial pulsations with periods of 1.75 and 2.89 hr with l ≃ 8 ± 1 and l ≃ 4 ± 1 respectively. No significant signal is found in the first harmonics of these two periods. The 2.89 hr mode is seen at all epochs and in all lines investigated, while the visibility of the 1.75 hr mode is clearly epoch dependent. Whilst light variations are detected, their connection to these periodicities is not straightforward.
We present a method to calculate the diffraction efficiency of a two dimensional photonic crystal... more We present a method to calculate the diffraction efficiency of a two dimensional photonic crystal (2D PC) surface structure for light emitting diodes. To give appropriate results not only the diffraction properties of the 2D PC is taken into account, but also the mode structure of the light emitting diode. The method is sufficiently general that any 2D PC with arbitrary hole depth and filling factor can be studied
IEEE/LEOS Summer Topi All-Optical Networking: Existing and Emerging Architecture and Applications/Dynamic Enablers of Next-Generation Optical Communications Systems/Fast Optical Processing in Optical Transmission/VCSEL and Microcavity Lasers., 2002
Summary form only given. We have for the first time implemented an optically pumped semiconductor... more Summary form only given. We have for the first time implemented an optically pumped semiconductor thin-disk laser (OPS-TDL) emitting at visible wavelengths around 660 nm. The gain element is based on the AlGaInP material system, grown lattice-matched on GaAs. The epitaxial structures consist of a resonant periodic gain structure with varying number of periods and stacks of 1 to 4
We demonstrate that suitable sample and circuit design of n-i-p-i photodetectors allows one to ac... more We demonstrate that suitable sample and circuit design of n-i-p-i photodetectors allows one to achieve linearity of the photoresponse over many orders of magnitude in optical power (more than 8 orders of magnitude in the present investigation) and, at the same time to maintain the extremely high photoconductive gain which is characteristic for n-i-p-i crystals (in the present case 0.3.106). Under sufficiently large reverse bias the n-i-p-i crystal exhibits opto-electronic switching. The switching power and the slope of the ramp for a given device can be adjusted within a wide range by appropriate circuit design. We show experimental results for optical switching of the conductance between lnS and 100~S (on/off ratio larger than 104) occuring at optical switching powers between 1 and 100 [tW/cm 2 with a width of the ramp of 0.5 gW/cm 2. We expect, that switching bistability can be achieved due to the light induced change of the absorption coefficient (Franz-Keldysh-oscillations).
ABSTRACT The modulation of the energy bands of a doping superlattice corresponds to periodic chan... more ABSTRACT The modulation of the energy bands of a doping superlattice corresponds to periodic changes of the optical refractive index in the direction of growth. Moreover the tunability of the electronic structure allows for variation of both strength and spatial dependence. These features imply the possibility of designing a highly wavelength-selective and tunable Bragg reflector. We calculate the expected properties for reasonable design parameters using a semiclassical model for the superlattice and a transfer matrix method. A peak reflectance up to 0.6 and an on-off-ratio of more than 1:30 is obtained.
We present visible high power AlInGaP lasers and laser bars from 635nm to 670nm. At room temperat... more We present visible high power AlInGaP lasers and laser bars from 635nm to 670nm. At room temperature we achieved 5.3W of cw power at 642nm from a 10mm bar with an overall efficiency of 17%.
ABSTRACT The optically pumped semiconductor thin-disk laser with external-cavity (OPS-TDL) is a n... more ABSTRACT The optically pumped semiconductor thin-disk laser with external-cavity (OPS-TDL) is a new type of semiconductor laser structure with the capability of achieving high output power while retaining good beam quality. We demonstrate the first AlGaInP-based red light emitting OPS-TDL structure. The device has been pumped optically with an argon-laser at 514~nm. The device has an epitaxial backside mirror and a multiple quantum well active region, consisting of strained InGaP quantum wells arranged in several groups as a periodic gain structure. A peak single-mode output power of more than 200mW at 660nm has been obtained in pulsed operation. Various designs for the active layer have been investigated.
ABSTRACT We report on a novel electro-optic modulator structure based on the two-dimensional Fran... more ABSTRACT We report on a novel electro-optic modulator structure based on the two-dimensional Franz- Keldysh effect (2D-FKE) in multiple quantum well (MQW) structures. Due to the increased electron-hole interaction in these quasi-two-dimensional systems, strong excitonic resonances are observed even at room temperature. If an electric field is applied parallel to the layers of a MQW structure, very low electric fields (10 - 30 kV/cm) are sufficient to cause field ionization of the excitons, because of their weak in-plane confinement. Large absorption changes as high as 7000 cm-1 with field changes of only 30 kV/cm have been observed in GaAs/AlGaAs-MQWs. In addition, an increase of the absorption below and oscillations of the absorption coefficient above each subband transition are obtained due to the two-dimensional Franz-Keldysh effect. These features have been applied in our novel electro- optic modulator structure. Using interdigitated metal-semiconductor-metal (MSM) contacts, high in-plane electric fields can be generated with moderate voltages. Furthermore the low capacitance of these MSM structures is particularly favorable for high speed applications. In a MSM-modulator structure, consisting of 75 GaAs/AlGaAs quantum wells with a distributed Bragg-reflector (DBR) below the MQW-layers, a maximum contrast ratio of 5:1 without using any cavity effects has been achieved with a voltage swing of 20 V.
ABSTRACT We have studied the field dependence of the absorption coefficient of three GaAs-AlAs su... more ABSTRACT We have studied the field dependence of the absorption coefficient of three GaAs-AlAs superlattices using a new modulation technique, the wavelength modulated photocurrent spectroscopy. At small applied electric fields we observe transitions corresponding to the edges of the joint miniband density of states between electrons and heavy holes as well as light holes. At intermediate fields Franz-Keldysh oscillations appear at the lower and upper band edges of the heavy and light hole joint miniband. With further increasing electric field these oscillations transform gradually into Wannier-Stark ladder transitions. The experimentally observed features are well reproduced by numerical calculations.
ABSTRACT We performed room temperature photo- and electro-modulation measurements on MBE-grown Ga... more ABSTRACT We performed room temperature photo- and electro-modulation measurements on MBE-grown GaAs pnp and pin structures with large layer thicknesses. In these crystals the optical properties are expected to be dominated by the local field-induced changes of the dielectric function rather than by subband-transitions. The dominating effect in the pnp-structure turns out to be the spatial dependence of the varying refractive index, resulting in a characteristic interference pattern. For the case of the pin structure we clearly observe Franz-Keldysh oscillations changing in amplitude and width with the internal field. Theoretical calculations using effective mass theory are in very good quantitative agreement with the experimental results. This agreement can be achieved only by the inclusion of excitonic effects in the calculation of the field-dependent absorption.
... The explicit results for the wave functions and absorp-tion spectra, however, are calculated ... more ... The explicit results for the wave functions and absorp-tion spectra, however, are calculated for a superlattice consisting of ... identical to zero.18 If all the remaining tunneling terms Xnt (ks) are neglected, a solution of Eq ... 17) is inserted, a calculation analogous to the derivation of Eq ...
ABSTRACT The optical properties of a GaAs/Al0.3Ga0.7As hetero-n-i-p-i crystal with GaAs quantum w... more ABSTRACT The optical properties of a GaAs/Al0.3Ga0.7As hetero-n-i-p-i crystal with GaAs quantum wells in the intrinsic region (‘‘type II’’) have been investigated by photoreflectance measurements. To understand the complex spectra we have varied several measurement parameters such as ac pump intensity, dc pump intensity, pump frequency, and temperature. The latter one represents a very useful tool to vary the degree of excitation of the sample over a wide range while keeping the pump and probe light intensities at a constant level. The role of temperature is described by a simple model which takes into account the strong temperature dependence of the thermally activated recombination kinetics. The photoreflectance spectra are compared with spectra calculated within the framework of a semiclassical Franz-Keldysh model and a model considering an infinite quantum well in an electric field (quantum-confined Franz-Keldysh model). From this comparison the internal electric field can be deduced. We demonstrate that it is essential to take into account the fact that the dielectric function has a spatial dependence with the period of the superlattice.
Physics and Simulation of Optoelectronic Devices XII, 2004
Operation-induced degradation of internal quantum efficiency of high-brightness (AlxGa1-x)0.5In0.... more Operation-induced degradation of internal quantum efficiency of high-brightness (AlxGa1-x)0.5In0.5P light-emitting devices (LEDs) is analysed experimentally and theoretically. A test series of LEDs was grown by MOCVD with identical layer sequence but different Aluminum content x in the active AlGaInP layer resulting in devices emitting light between 644 nm and 560 nm. The analysis yields the wavelength dependence of both the nonradiative recombination constant A as well as the carrier leakage parameter C of devices before and after aging. While test devices with lambda>615 nm are very stable, LEDs with shorter emission wavelengths exhibit both an increase of A and a slight decrease of C upon aging. Possible degradation mechanisms are discussed.
ABSTRACT We report results of photoreflectance (PR) investigations of GaAs/Alo.3Gao.7As hetero-n-... more ABSTRACT We report results of photoreflectance (PR) investigations of GaAs/Alo.3Gao.7As hetero-n-i-p-i crystals with either GaAs-QWs ("type II") in the intrinsic region or GaAs-QWs interspersed in the n-region ("type I"). To understand the complex PR-spectra of these samples we changed several measurement parameters such as ac-pump-intensity, dc-pump-intensity, pump frequency and temperature. Especially the spectra of type II showed a strong temperature dependence. We compare these spectra to spectra calculated with the model of an infinite quantum well in an electric field. Because of the dielectric function being periodic in space the z-dependence of the dielectric function is taken into account.
Microscopic theory of the intracollisional field effect in semiconductor superlattices. Jörg Hade... more Microscopic theory of the intracollisional field effect in semiconductor superlattices. Jörg Hader, Torsten Meier, and Stephan W. Koch Fachbereich Physik und Zentrum für Materialwissenschaften, Philipps-Universität Marburg,. ...
Monthly Notices of the Royal Astronomical Society, 2011
Using an extended set of multi-epoch high resolution high signal-to-noise ratio optical spectra, ... more Using an extended set of multi-epoch high resolution high signal-to-noise ratio optical spectra, we readdress the multiplicity properties of the O-type stars in IC 2944 and in the Cen OB2 association. We present new evidence of binarity for five objects and we confirm the multiple nature of another two. We derive the first orbital solutions for HD 100099, HD 101436 and HD 101190 and we provide additional support for HD 101205 being a quadruple system. The minimal spectroscopic binary fraction in our sample is f min = 0.57. Using numerical simulations, we show that the detection rate of our observational campaign is close to 90%, leaving thus little room for undetected spectroscopic binary systems. The statistical properties of the O-star population in IC 2944 are similar, within the uncertainties, to the results obtained in the earlier papers in this series despite the fact that sample size effects limit the significance of the comparison. Using newly derived spectroscopic parallaxes, we reassess the distance to IC 2944 and obtained 2.3 ± 0.3 kpc, in agreement with previous studies. We also confirm that, as far as the O stars are concerned, the IC 2944 cluster is most likely a single entity.
Context. Plaskett's star (HD 47 129) is a very massive O + O binary that belongs to the Mon OB2 a... more Context. Plaskett's star (HD 47 129) is a very massive O + O binary that belongs to the Mon OB2 association. Previous work suggested that this system displays the Struve-Sahade effect although the measurements of the secondary radial velocities are very difficult and give controversial results. Both components have powerful stellar winds that collide, and produce a strong X-ray emission. Aims. Our aim is to study in detail the physical parameters of this system, and to investigate the relation between its spectral properties and its evolutionary status. Methods. We present here the analysis of an extensive set of high resolution optical spectra of HD 47 129. We use a disentangling method to separate the individual spectra of each star. We derive a new orbital solution and discuss the spectral classification of both components. A Doppler tomography technique applied to the emission lines Hα and He ii λ 4686 yields a Doppler map that illustrates the wind interactions in the system. Finally, an atmosphere code is used to determine the different chemical abundances of the system components and the wind parameters. Results. HD 47 129 appears to be an O8 III/I + O7.5 III binary system in a post RLOF evolutionary stage, where matter has been transferred from the primary to the secondary star. The He overabundance of the secondary supports this scenario. In addition, the N overabundance and C underabundance of the primary component confirm previous results based on X-ray spectroscopy and indicate that the primary is an evolved massive star. We also determine a new orbital solution, with M P sin 3 i = 45.4 ± 2.4 M ⊙ and M S sin 3 i = 47.3 ± 0.3 M ⊙ . Furthermore, the secondary star has a large rotational velocity (v sin i ∼ 300 km s −1 ) that deforms its surface, leading to a non-uniform distribution in effective temperature. This could explain the variations in the equivalent widths of the secondary lines with phase. We suggest that the wind of the secondary star is confined near the equatorial plane because of its high rotational velocity, affecting the ram pressure equilibrium in the wind interaction zone.
Context. The properties of the early-type binary Cyg OB2 #5 have been debated for many years and ... more Context. The properties of the early-type binary Cyg OB2 #5 have been debated for many years and spectroscopic and photometric investigations yielded conflicting results. Aims. We have attempted to constrain the physical properties of the binary by collecting new optical and X-ray observations. Methods. The optical light curves obtained with narrow-band continuum and line-bearing filters are analysed and compared. Optical spectra are used to map the location of the He ii λ 4686 and Hα line-emission regions in velocity space. New XMM-Newton as well as archive X-ray spectra are analysed to search for variability and constrain the properties of the hot plasma in this system. Results. We find that the orbital period of the system slowly changes though we are unable to discriminate between several possible explanations of this trend. The best fit solution of the continuum light curve reveals a contact configuration with the secondary star being significantly brighter and hotter on its leading side facing the primary. The mean temperature of the secondary star turns out to be only slightly lower than that of the primary, whilst the bolometric luminosity ratio is found to be 3.1. The solution of the light curve yields a distance of 925 ± 25 pc much lower than the usually assumed distance of the Cyg OB2 association. Whilst we confirm the existence of episodes of higher X-ray fluxes, the data reveal no phase-locked modulation with the 6.6 day period of the eclipsing binary nor any clear relation between the X-ray flux and the 6.7 yr radio cycle. Conclusions. The bright region of the secondary star is probably heated by energy transfer in a common envelope in this contact binary system as well as by the collision with the primary's wind. The existence of a common photosphere probably also explains the odd mass-luminosity relation of the stars in this system. Most of the X-ray, non-thermal radio, and possibly γ-ray emission of Cyg OB2 #5 is likely to arise from the interaction of the combined wind of the eclipsing binary with at least one additional star of this multiple system.
Aims. The line profile variability and photometric variability of the O9.5 Vp star HD 93521 are e... more Aims. The line profile variability and photometric variability of the O9.5 Vp star HD 93521 are examined in order to establish the properties of the non-radial pulsations in this star. Methods. Fourier techniques are used to characterize the modulations of the He i λλ 5876, 6678 and H α lines in several spectroscopic time series and to search for variations in a photometric time series. Results. Our spectroscopic data confirm the existence of two periods of 1.75 and 2.89 hr. The line profiles, especially those affected by emission wings, exhibit also modulations on longer time scales, but these are epoch-dependent and change from line to line. Unlike previous claims, we find no unambiguous signature of the rotational period in our data, nor of a third pulsation period (corresponding to a frequency of 2.66 d −1 ). Conclusions. HD 93521 very likely exhibits non-radial pulsations with periods of 1.75 and 2.89 hr with l ≃ 8 ± 1 and l ≃ 4 ± 1 respectively. No significant signal is found in the first harmonics of these two periods. The 2.89 hr mode is seen at all epochs and in all lines investigated, while the visibility of the 1.75 hr mode is clearly epoch dependent. Whilst light variations are detected, their connection to these periodicities is not straightforward.
We present a method to calculate the diffraction efficiency of a two dimensional photonic crystal... more We present a method to calculate the diffraction efficiency of a two dimensional photonic crystal (2D PC) surface structure for light emitting diodes. To give appropriate results not only the diffraction properties of the 2D PC is taken into account, but also the mode structure of the light emitting diode. The method is sufficiently general that any 2D PC with arbitrary hole depth and filling factor can be studied
IEEE/LEOS Summer Topi All-Optical Networking: Existing and Emerging Architecture and Applications/Dynamic Enablers of Next-Generation Optical Communications Systems/Fast Optical Processing in Optical Transmission/VCSEL and Microcavity Lasers., 2002
Summary form only given. We have for the first time implemented an optically pumped semiconductor... more Summary form only given. We have for the first time implemented an optically pumped semiconductor thin-disk laser (OPS-TDL) emitting at visible wavelengths around 660 nm. The gain element is based on the AlGaInP material system, grown lattice-matched on GaAs. The epitaxial structures consist of a resonant periodic gain structure with varying number of periods and stacks of 1 to 4
We demonstrate that suitable sample and circuit design of n-i-p-i photodetectors allows one to ac... more We demonstrate that suitable sample and circuit design of n-i-p-i photodetectors allows one to achieve linearity of the photoresponse over many orders of magnitude in optical power (more than 8 orders of magnitude in the present investigation) and, at the same time to maintain the extremely high photoconductive gain which is characteristic for n-i-p-i crystals (in the present case 0.3.106). Under sufficiently large reverse bias the n-i-p-i crystal exhibits opto-electronic switching. The switching power and the slope of the ramp for a given device can be adjusted within a wide range by appropriate circuit design. We show experimental results for optical switching of the conductance between lnS and 100~S (on/off ratio larger than 104) occuring at optical switching powers between 1 and 100 [tW/cm 2 with a width of the ramp of 0.5 gW/cm 2. We expect, that switching bistability can be achieved due to the light induced change of the absorption coefficient (Franz-Keldysh-oscillations).
ABSTRACT The modulation of the energy bands of a doping superlattice corresponds to periodic chan... more ABSTRACT The modulation of the energy bands of a doping superlattice corresponds to periodic changes of the optical refractive index in the direction of growth. Moreover the tunability of the electronic structure allows for variation of both strength and spatial dependence. These features imply the possibility of designing a highly wavelength-selective and tunable Bragg reflector. We calculate the expected properties for reasonable design parameters using a semiclassical model for the superlattice and a transfer matrix method. A peak reflectance up to 0.6 and an on-off-ratio of more than 1:30 is obtained.
We present visible high power AlInGaP lasers and laser bars from 635nm to 670nm. At room temperat... more We present visible high power AlInGaP lasers and laser bars from 635nm to 670nm. At room temperature we achieved 5.3W of cw power at 642nm from a 10mm bar with an overall efficiency of 17%.
ABSTRACT The optically pumped semiconductor thin-disk laser with external-cavity (OPS-TDL) is a n... more ABSTRACT The optically pumped semiconductor thin-disk laser with external-cavity (OPS-TDL) is a new type of semiconductor laser structure with the capability of achieving high output power while retaining good beam quality. We demonstrate the first AlGaInP-based red light emitting OPS-TDL structure. The device has been pumped optically with an argon-laser at 514~nm. The device has an epitaxial backside mirror and a multiple quantum well active region, consisting of strained InGaP quantum wells arranged in several groups as a periodic gain structure. A peak single-mode output power of more than 200mW at 660nm has been obtained in pulsed operation. Various designs for the active layer have been investigated.
ABSTRACT We report on a novel electro-optic modulator structure based on the two-dimensional Fran... more ABSTRACT We report on a novel electro-optic modulator structure based on the two-dimensional Franz- Keldysh effect (2D-FKE) in multiple quantum well (MQW) structures. Due to the increased electron-hole interaction in these quasi-two-dimensional systems, strong excitonic resonances are observed even at room temperature. If an electric field is applied parallel to the layers of a MQW structure, very low electric fields (10 - 30 kV/cm) are sufficient to cause field ionization of the excitons, because of their weak in-plane confinement. Large absorption changes as high as 7000 cm-1 with field changes of only 30 kV/cm have been observed in GaAs/AlGaAs-MQWs. In addition, an increase of the absorption below and oscillations of the absorption coefficient above each subband transition are obtained due to the two-dimensional Franz-Keldysh effect. These features have been applied in our novel electro- optic modulator structure. Using interdigitated metal-semiconductor-metal (MSM) contacts, high in-plane electric fields can be generated with moderate voltages. Furthermore the low capacitance of these MSM structures is particularly favorable for high speed applications. In a MSM-modulator structure, consisting of 75 GaAs/AlGaAs quantum wells with a distributed Bragg-reflector (DBR) below the MQW-layers, a maximum contrast ratio of 5:1 without using any cavity effects has been achieved with a voltage swing of 20 V.
ABSTRACT We have studied the field dependence of the absorption coefficient of three GaAs-AlAs su... more ABSTRACT We have studied the field dependence of the absorption coefficient of three GaAs-AlAs superlattices using a new modulation technique, the wavelength modulated photocurrent spectroscopy. At small applied electric fields we observe transitions corresponding to the edges of the joint miniband density of states between electrons and heavy holes as well as light holes. At intermediate fields Franz-Keldysh oscillations appear at the lower and upper band edges of the heavy and light hole joint miniband. With further increasing electric field these oscillations transform gradually into Wannier-Stark ladder transitions. The experimentally observed features are well reproduced by numerical calculations.
ABSTRACT We performed room temperature photo- and electro-modulation measurements on MBE-grown Ga... more ABSTRACT We performed room temperature photo- and electro-modulation measurements on MBE-grown GaAs pnp and pin structures with large layer thicknesses. In these crystals the optical properties are expected to be dominated by the local field-induced changes of the dielectric function rather than by subband-transitions. The dominating effect in the pnp-structure turns out to be the spatial dependence of the varying refractive index, resulting in a characteristic interference pattern. For the case of the pin structure we clearly observe Franz-Keldysh oscillations changing in amplitude and width with the internal field. Theoretical calculations using effective mass theory are in very good quantitative agreement with the experimental results. This agreement can be achieved only by the inclusion of excitonic effects in the calculation of the field-dependent absorption.
... The explicit results for the wave functions and absorp-tion spectra, however, are calculated ... more ... The explicit results for the wave functions and absorp-tion spectra, however, are calculated for a superlattice consisting of ... identical to zero.18 If all the remaining tunneling terms Xnt (ks) are neglected, a solution of Eq ... 17) is inserted, a calculation analogous to the derivation of Eq ...
ABSTRACT The optical properties of a GaAs/Al0.3Ga0.7As hetero-n-i-p-i crystal with GaAs quantum w... more ABSTRACT The optical properties of a GaAs/Al0.3Ga0.7As hetero-n-i-p-i crystal with GaAs quantum wells in the intrinsic region (‘‘type II’’) have been investigated by photoreflectance measurements. To understand the complex spectra we have varied several measurement parameters such as ac pump intensity, dc pump intensity, pump frequency, and temperature. The latter one represents a very useful tool to vary the degree of excitation of the sample over a wide range while keeping the pump and probe light intensities at a constant level. The role of temperature is described by a simple model which takes into account the strong temperature dependence of the thermally activated recombination kinetics. The photoreflectance spectra are compared with spectra calculated within the framework of a semiclassical Franz-Keldysh model and a model considering an infinite quantum well in an electric field (quantum-confined Franz-Keldysh model). From this comparison the internal electric field can be deduced. We demonstrate that it is essential to take into account the fact that the dielectric function has a spatial dependence with the period of the superlattice.
Physics and Simulation of Optoelectronic Devices XII, 2004
Operation-induced degradation of internal quantum efficiency of high-brightness (AlxGa1-x)0.5In0.... more Operation-induced degradation of internal quantum efficiency of high-brightness (AlxGa1-x)0.5In0.5P light-emitting devices (LEDs) is analysed experimentally and theoretically. A test series of LEDs was grown by MOCVD with identical layer sequence but different Aluminum content x in the active AlGaInP layer resulting in devices emitting light between 644 nm and 560 nm. The analysis yields the wavelength dependence of both the nonradiative recombination constant A as well as the carrier leakage parameter C of devices before and after aging. While test devices with lambda>615 nm are very stable, LEDs with shorter emission wavelengths exhibit both an increase of A and a slight decrease of C upon aging. Possible degradation mechanisms are discussed.
ABSTRACT We report results of photoreflectance (PR) investigations of GaAs/Alo.3Gao.7As hetero-n-... more ABSTRACT We report results of photoreflectance (PR) investigations of GaAs/Alo.3Gao.7As hetero-n-i-p-i crystals with either GaAs-QWs ("type II") in the intrinsic region or GaAs-QWs interspersed in the n-region ("type I"). To understand the complex PR-spectra of these samples we changed several measurement parameters such as ac-pump-intensity, dc-pump-intensity, pump frequency and temperature. Especially the spectra of type II showed a strong temperature dependence. We compare these spectra to spectra calculated with the model of an infinite quantum well in an electric field. Because of the dielectric function being periodic in space the z-dependence of the dielectric function is taken into account.
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Papers by Norbert Linder