Periodic single metallic meander structures have been shown to exhibit extraordinary transmission in the visible
frequency domain within a well-defined pass band that can be shifted by geometry variation. Furthermore, meander
structures are not only linear polarizers but also induce phase retardation between s- and p-polarized light. In addition,
they are able to convert the polarization of light due to plasmonic excitations. Those features combined with the
advantages of plasmonic metamaterials in general, such as radiation stability, temperature independence and low weight
make them perfect candidates for optical devices in space instruments. We show analytically and numerically that an
optical depolarizer can be designed by spatially distributing meander structures in a pixel-like fashion and rotating each
element by a random angle. The depolarizing properties of meander structures, indicated by the Mueller matrix elements,
are investigated for various geometrical parameters and can be improved by stacking two meander structures onto each
other. The presented polarization scrambler can be flexibly designed to work anywhere in the visible wavelength range
with a bandwidth of up to 100 THz. Furthermore, the depolarization effect relies on optical activity rather than
scattering. With our preliminary design, we achieve depolarization rates larger than 60% for arbitrarily polarized,
monochromatic or narrow-band light, respectively. One advantage of our concept is the flexibility to tune the
polarization scrambler to a particular optical frequency or functionality. Circularly polarized light (S = [1, 0, 0, ±1]) for
instance could be depolarized by 95% at 600 THz.
Plasmonic microcavities are compact systems having the capability to confine light in an extremely small volume. Light
matter interactions can therefore be mediated very effectively by them. In this report we demonstrate experimentally that
dispersion of photonic cavity modes can be tuned to a large degree in a plasmonic microcavity with two identical
corrugated metallic films as resonant mirrors. The modification of the dispersion is induced by interactions between the
photonic and plasmonic modes. Additionally, the excited surface waves are strongly enhanced by the gratings, which is
important for coupling and enhancing evanescent fields. To realize such a cavity, we employed self-assembled
monolayer nanosphere crystals as a prepatterned substrate. Metal/dielectric/metal films were subsequently deposited on
it. The cavity length was used to tune the interaction strength. As a result, the original positively dispersive FP mode, i.e.,
the resonance frequency is increased with the incident angle, becomes independent or even negatively dependent on the
incident angle. Due to the hexagonal textured corrugation of the metal film and the existence of some line defects in a
large area, the optical response is isotropic and independent of the specific polarization. This behavior can have potential
applications for light emission devices, plasmonic color filters and subwavelength imaging.
The European Space Agency (ESA) in the frame of its General Study Program (GSP) has started to investigate the
opportunity of using metamaterials in space applications. In that context, ESA has initiated two GSP activities which
main objectives are 1) to identify the metamaterials and associated optical properties which could be used to improve in
the future the performances of optical payloads in space missions, 2) to design metamaterial based devices addressing
specific needs in space applications.
The range of functions for metamaterials to be investigated is wide (spectral dispersion, polarisation control, light
absorption, straylight control...) and so is the required spectral range, from 0.4μm to 15μm.
In the frame of these activities several applications have been selected and the designs of metamaterial based devices are
proposed and their performances assessed by simulations.
Two resonant surfaces, which allow the propagation of surface plasmon polaritons (SPPs) can mimic negative index
materials (NIM). Hence, it is possible to recreate the near-field imaging effects known from Pendry's perfect lens. The
metallic meander structure is a well-suited candidate for such a resonant surface due to the excitation and tunability of
the short (SRSPP) and long range surface plasmon (LRSPP) frequencies. Furthermore, the Fano-type pass band between
the SRSPP and LRSPP frequencies of a single meander sheet retains its dominant role when being stacked. We demonstrate how a stack consisting of two meander structures can perfectly image within this pass band region and propose a stack of meander structures with successively increasing periodicity. Such a stack might be capable to decrease the lateral wave vector until near-field to far-field transformation is achieved. The frequency shift of the pass band for
each sheet can be compensated by changing other geometrical parameters. We rigorously calculate the spectra of various
meander designs and show that meander stacks transfer energy resonantly over large distances with a high transmission.
Metallo-dielectric structured materials, or in other words metamaterials (MTM), are in principle a well established
composite to improve efficiency, functionality, and weight of micro-wave components. In recent times, it has been
demonstrated that the functionalities of metamaterials can be scaled down to optical frequencies by nano structuring
techniques. Examples include negative index materials in the near infrared and visible frequency range, cloaking
structures, filters, and structures for improved sensing of environmental gases.
The physical processes in plasmonic metamaterials depend strongly on the excitation of surface plasmons and the
interaction between them.
We have learned how to control the plasmon-photon and the plasmon-plasmon interaction for manipulating the
electromagnetic response in a metamaterial at wavelengths well below the vacuum wavelength. Many interesting
and novel optical applications and devices are expected. For instance sub-wavelength imaging, compact
communication devices as polarisation splitters, slow light media structures, compact colour filters, and resonators.
All-plasmonic circuits are also the basis for ultra-dense photonic integration not achievable through the conventional
optical integration.
With examples of several metamaterial structures we try to illustrate the application potential of MTMs and
comment on their fabrication feasibility to show whether metamaterials can hold their promise. Their investigation is
in any case a rewarding adventure.
It has been shown that surface plasmon polaritons (SPPs) have a dominant influence on the unique properties of negative
index materials (NIMs). Consequently, one could replace bulk NIMs by resonantly coupled surfaces that allow the
propagation of SPPs. We show that a metallic meander structure is perfectly suited as such a resonant surface due to the
tunability of the short range SPP (SRSPP) and long range SPP (LRSPP) frequencies by means of geometrical variation.
Furthermore, the pass band between the SRSPP and LRSPP frequencies of a single meander sheet, induced by two Fanotype
resonances, retains its dominant role when being stacked. In this report we demonstrate how a stack consisting of
two meander structures can mimic perfect imaging known from Pendry's lens within this pass band region. On the other
hand, to observe sub-wavelength features in the far-field more than (perfect) near-field imaging is necessary. We propose
a stack of meander structures with successively increasing periodicity capable to decrease the lateral wave vector until
near-field to far-field transformation is achieved. When stacking multiple meander structures with different periodicities,
the pass band shifts in frequency for each sheet in a different way. We rigorously calculate the spectra of various
meander designs and show that this shift can be compensated by changing other geometrical parameters of each single
sheet. Such meander stacks can transfer energy resonantly over large distances with a high transmission and might
enable sub-wavelength imaging.
The excitation and transfer of evanescent electromagnetic waves appears as key challenge for the realization of optical
imaging devices with super resolution. In this process surface plasmon polaritons (SPP) overtake the role as
indispensable mediators between source fields and propagating fields. Therefore, the interaction between SPPs and the
vacuum field in a double meander structure (DMS) is investigated. The occurrence of Fabry-Pérot (FP) modes within
such a cavity and the SPP modes of the meander structure is analyzed to understand the interaction of both mode systems
in the combined double meander structure. We show that the known Fano-type passband of single meander structures
keeps its dominant role in the DMS and demonstrate the frequency selective role of meander mirrors within this meander
cavity. The meander geometry determined passband frequency position also controls nearly solely the passband of the
DMS. For far field superlenses (FSL) the energy transfer at low loss over practically arbitrary distances inside the
structure is a key property. A resonant amplitude transfer can be obtained between resonantly coupled meander surfaces
for unlimited distances in practical cases. This property enables a controlled transformation of evanescent modes to
traveling wave modes of higher diffraction order useful for superlens operation.
We report systematic investigations on hot-electron degradation in GaN-based HEMTs with different gate recess depths, d r , fabricated by reactive ion etching. The experimental data stipulate two different mechanisms underlying the hot-electron degradations of the devices. During the initial phase of hot-electron injection significant changes were observed in the dc characteristics of the devices and the flicker noise power spectral density, SV(f).The degradations were partially recovered by annealing the devices at 100°C for 20 minutes. It is shown that for stress time ts≤25 hours the reverse bias gate current, IG, decreases systematically with ts, whereas SV(f) fluctuates randomly. Detailed analyses of SV(f) measured over a wide range temperatures show that the initial degradations originate from the percolation of carriers in the 2DEG. The significant increase in the flicker noise during the initial phase of high-voltage stress is due to the generation of H+ at the AlGaN/GaN interface. The fluctuations in the magnitudes of SV(f) for ts≤25 hours originate from the motion of the H+ in the direction of the electric field. This results in the modulation of the percolation path leading to significant variations in SV(f) as a function of ts. For ts>25 hours both IGand SV(f) are stabilized resulting from the drifting of the H+ away from the gate region. Further stressing beyond 25 hours indicate strong dependencies of the device lifetimes on dr suggesting significant material degradation due to the reactive ion etching process for the fabrication of the gate recesses. Detailed characterization of the noise show that the final irreversible degradation is due to the generation of traps at the AlGaN/GaN interface.
In this paper we discuss the problems of the AlGaInP material system and its consequences for the laser applications in vertical-cavity surface-emitting lasers (VCSEL). The epitaxial and technological solutions to overcome at least parts of the inherent problems were presented. Measured power-current curves of 660nm AlGaInP-based oxide-confined VCSEL are compared with calculated data by a cylindrical heat dissipation model to improve heat removal out of the device. Pulsed lasing operation of a 670nm VCSEL at +120°C heat sink temperature is demonstrated, where we exceeded 0.5mW and at +160°C still 25μW output power were achieved. We also studied the modulation bandwidth of our devices and achieved 4GHz and calculations lead to a maximum possible intrinsic -3dB frequency of 25GHz.
This contribution drafts the problems of the AlGaInP material system and its consequences for the laser applications in vertical-cavity surface-emitting lasers (VCSEL). The epitaxial and technological solutions to overcome at least parts of the inherent problems were discussed. Calculated data by a cylindrical heat dissipation model were compared with measured power-current curves of 660nm oxide-confined VCSEL to improve the heat removal out of the device. At high temperatures pulsed operation of a 670nm VCSEL is demonstrated, where we could exceeded 0.5mW at +120°C and at +160°C still 25µW optical output power were achieved.
This talk focuses on the high frequency characteristics of red VCSELs. After a short description of important fabrication issues the modulation behaviour of GaInP surface emitting lasers is discussed on the basis of the laser rate equations. The influence of the geometric dimensions of the laser structure and of the operating conditions is investigated. From the S-parameter analysis a modulation coefficient of 3 GHz/(mA)1/2 for VCSELs with a 7 µm aperture and a differential gain of 1.15•10-16 cm2 are deduced. A more detailed analysis reveals, that the modulation behaviour of red VCSELs nearly solely depends on their photon density inside the quantum wells as expected from the rate equations. These results imply that for a certain range of geometries diffusion and diffraction have a second order influence on the high frequency characteristics of red VCSELs. The K-factor analysis indicates very short carrier transfer and relaxation times around 5 ps and a maximum frequency of 25 GHz. Large signal modulation issues such as the properties of the eye diagram are also addressed. From the device characteristics it is concluded that the GaInP-VCSEL is suitable for data communication applications. Low cost fabrication makes the red VCSEL an attractive candidate for both automotive and high-speed data communication.
Vertical cavity surface emitting lasers (VCSEL) in the GaInP/AlGaInP material system have experienced a rapid development in their short history. In general lasers from that material system are suitable for a huge number of applications beginning with TV lasers and high power lasers for edge emitters, continuing with optical data storage, medical applications as well as data communication in cars, air planes, offices and between computers as application field for VCSELs. Especially automotive applications show the highest requirements on a laser with respect to operation temperature and power. In this talk we draw out the problems of the material system AlGaInP and its implications for laser applications. We discuss the epitaxial and technological solutions to overcome at least a part of these inherent problems. We will discuss the possible power that we can expect from VCSELs emitting in the range between 650 nm to 670 nm. We got from our lasers 5 mW, CW @ RT, 670nm and 2.5mW, CW@RT, 650 nm. We emphasize the role of doping, Bragg mirror grading, suitable detuning of cavity mode and gain, and optimisation of the contact layer and control of the oxide aperture in the VCSEL structure to get improved operation characteristics at higher temperatures. From the analysis of high frequency measurements, we could evaluate modulation bandwidths between 4 GHz and 10 GHz. The application of polyimide as a dielectric isolation material shows the potential to obtain modulation bandwidths beyond 10 GHz. For the intrinsic modulation bandwidth we get a value of 25 GHz, which is near the value edge emitters show. A more detailed discussion on photon lifetimes and carrier transport times will be given in the talk. Red light emitting VCSELS driven with short current pulses showed laser emission up to + 160°C case temperature. Thus, a CW operation up to +120°C can be expected after further improvement of power generation (decrease of series resistance) and heat spreading (optimized contacts and mounting). From these characteristics we can conclude that AlGaInP-surface emitting lasers have a real potential as low cost lasers for automotive applications as we all as data communication applications up to 10 GHz.
We present a new method for fabrication of tunable InGaAsP-InP single mode lasers without epitaxial overgrowth. These devices show the advantage of a considerably simplified fabrication process compared to conventional tunable laser types. The lasers comprise an active Bragg reflector integrated with an uncorrugated separately pumped gain region. To overcome the extensive and expensive overgrowth step we realized a surface grating on both sides of the ridge mesa, which provides DFB operation. By adjusting the current through the Bragg reflector, the wavelength can be tuned between 1590.8 nm and 1595.2 nm. A maximum of 11 wavelength channels with an average spacing of ~0.5 nm and a constant optical output power of ~0.5 mW are addressable.
In this article fabrication techniques and the analysis of AlGaInP semiconductor lasers for single mode emission and high power emission will be reported. Special emphasis will be spent on the appropriate vertical and longitudinal device structures. Furthermore we discuss low damage dry etching and epitaxial regrowth of DFB laser structures. The devices investigated are DBR- DFB- and MOPA-lasers.
Nanostructures based on III-V semiconductor materials have reached a status which enables basic physical studies on size effects in device and in nanostructures. The expected benefits of high modulation bandwidth, low laser threshold, and improved linewidth enhancement factor in DFB lasers, to say only a few, which are believed to be based mainly on the changed density of states (DOS) function in low dimensions might be counterbalanced by altered carrier energy relaxation and k-space filling in those structures. To investigate systematically size effects and device aspects, a continuous change of structure and active device size is needed from 2D to 0D dimensions. This requirement can be met by high resolution electron beam lithography in conjunction with low damage etch processes and epitaxial overgrowth. In this presentation we discuss the technology and design considerations of lasers with low dimensional active regions as well as DOS effects and device relevant carrier relaxation effects. The technology part will focus especially on low damage etch processes such as RIE- ECR. Nearly damage free structuring processes can be demonstrated. Based on this low damage dry etch process we obtained electrically pumped wire DFB lasers with relatively high output power (up to 6 mW) and operation temperature (60 degrees C). Time resolved optical ps-spectroscopy as well as high excitation spectroscopy on wire and dot nanostructures demonstrate strongly changed k-space filling and carrier relaxation mechanisms in low dimensions and represent a serious limitation of device speed. Results obtained from electrically pumped wire DFB lasers confirm the carrier relaxation and k-space filling effects in device structures which have been observed by optical pump experiments in nanostructures. Despite the band filling effects in low dimensional structures, the wire DFB lasers show clearly the expected feature of gain coupling and enhanced differential gain which might demonstrate the applicability of mesoscopic laser devices in common data communication approaches.
High frequency modulation properties of distributed feedback (DFB) lasers are analyzed by optical excitation with picosecond pulses. Typical time constants of the large signal response were investigated. Measurements at different temperatures demonstrate that capture of carriers from the adjacent barrier energy levels into the active region of the laser are dominant mechanisms for laser dynamics. This carrier transfer consists of the intrinsic processes of carrier-carrier scattering, carrier energy relaxation, carrier diffusion, and quantum mechanical capture of carriers into the quantum wells. In order to separate the limiting processes for laser speed, the influence of relaxation processes from different energy levels on the device response was tested by varying the wavelength of the excitation pulse. To investigate modulation limitations due to intrinsic band structure parameters, the laser emission wavelength was detuned against the gain spectrum by varying the grating period of a DFB laser. Longer time constants at higher wavelengths demonstrate that the differential gain constant is the most important contribution for modulation speed. A theoretical description based on a 3 level system model yields rate equations which explain our experimental results.
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