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We present TE- and TM-polarization-resolved photocurrent measurements on quantum well pin diodes under normal incidence. Usually, optical experiments performed in such a geometry yield information only about transitions involving in-plane (p x and p y ) components of the hole wave functions because of the in-plane (TE) polarization of the light. Information on transitions sensitive to the p z components can be obtained by focussing a radially polarized laser beam through a microscope objective with high numerical aperture (NA = 0.9). With our setup, the electrical field vector at the focal tail has a significant component along the optical axis (TM-polarization!) which enables excitation of transitions sensitive to p z components also. Additionally, the existence of a degenerate (azimuthally polarized) optical mode enables switching these p z components on and off easily. Experimental evidence of these features has been achieved by exploiting the selection rules for e-hh and e-lh transitions in a quantum well structure. We present a comparison of our recorded spectra with theoretical predictions obtained from simple geometric optics assumptions. For our quantum wells the polarization effects are small because our measurement averages the intensity distribution of the whole focal plane. We plan to extend our measurements to polarization resolved single quantum dot spectroscopy. By restricting the detection region to the spatial extent of a single dot, one can exploit the almost pure TM-polarization on the optical axis for obtaining high contrast between heavy- and light-hole exciton absorption.

We report the measurement of exceptionally bright, incoherent radiation in the THz region by frequency down-conversion of amplified spontaneous emission around 775-nm wavelength. The down-conversion technique is optical mixing in an interdigital photoconductive capacitor made from ultra fast ErAs:GaAs. The brightness temperature into a single spatial mode is approximately 1.1×105 K, making the new radiation at least 70 times more intense than common incandescent sources in the THz region.

A summary of ultrafast In0.53Ga0.47As photoconductors at λ = 1.55 µm is provided, with emphasis placed on recent ion-implanted and ErAs-nanoparticulate materials that have displayed response times

Conventional semiconductor-based photomixers are limited by the carrier transit-time-roll-off and the RC-roll-off of the device-capacitance. We have developed a THz-photomixer which can overcome both limitations and proves far superior to a conventional pin-photomixer.

An all-solid-state continuous-wave sweep oscillator has been developed that provides high-resolution, transmission measurements between ∼30 GHz and 3 THz. It is based on difference-frequency generation between two cw frequency-offset lasers driving an ultrafast photoconductive mixer (photomixer). The output power around 100 GHz is approximately 10 μW, falling to about 1 μW around 1 THz and 0.1 μW around 3 THz. The sweep oscillator is used with two types of detectors: a hot electron bolometer for high-sensitivity measurements of weak absorption features below 1 THz, and a room temperature Golay cell for coarse measurements anywhere between 30 GHz and 3 THz. The sweep oscillator facilitates the rapid characterization of a broad variety of materials including inorganic solids, biological materials, liquids, and gases with far greater resolution (∼10 MHz), frequency accuracy (∼0.1 GHz), and spectral density (∼1 μW/MHz) than competitive wideband instruments such as Fourier-transform or time-domain spectrometer...

Self-assembled ErAs islands were grown on GaAs between a two-dimensional electron gas (2DEG) and a backgate electrode by molecular-beam epitaxy. The islands have overlapping Schottky barriers that form an insulating potential barrier. A TiAu topgate was added by shadow mask evaporation. Thermal activation and charging experiments were employed to gain insight into the electronic properties of the ErAs island systems. In addition the 2DEG was characterized as a function of topgate and backgate voltage.

This letter reports the fabrication and demonstration of an ErAs:GaAs interdigitated photomixer as a tunable THz source ranging from ∼20GHzto∼2THz, with 12μW maximum power typically around ∼90GHz. Each photomixer is coupled to a composite dipole-spiral planar antenna that emits a Gaussian-type beam into free space. The beam switches from dipole to spiral antenna behavior as the frequency increases. A distributed Bragg reflector is embedded in the device beneath the photomixer to increase its external quantum efficiency. The photomixer has a 900A thick silicon nitride coating which serves as an antireflection and passivation layer, and also improves the reliability and heat tolerance of the device.

We demonstrate subpicosecond photocarrier lifetimes at 1.55μm in GaSb∕ErSb nanoparticle superlattices grown by molecular beam epitaxy. Pump–probe measurements were made with a 1.55μm mode-locked laser in transmission geometry to determine the photocarrier lifetime. The lifetime is found to be dependent on the size of the ErSb particles, amount of ErSb, and the distance between layers of particles. Through manipulation of these three parameters the photocarrier lifetime can be tuned down to less than 300fs, the temporal limit of our experiment.

We demonstrate the growth by molecular beam epitaxy of a metal/semiconductor composite consisting of epitaxial semimetallic ErSb particles in a GaSb matrix. The ErSb nucleates in an island growth mode leading to the spontaneous formation of nanometer-sized particles. These particles are found to preferentially grow along a [011] direction on a (100) GaSb surface. The particles can be overgrown with GaSb to form an epitaxial superlattice consisting of ErSb particles between GaSb spacer layers. The size of the ErSb particles increases monotonically with the deposition. The carrier concentrations in the superlattices are found to be dependent on both the size and density of the ErSb particles. Smaller particles and closer layer spacings reduce the hole concentration in the film.

This letter reports the measurement of photocarrier lifetime in ErAs:InGaAs epitaxial material grown on InP substrates. The measurement technique is ultrafast time-resolved phototransmission using a 1.55 μm mode-locked erbium-doped fiber laser. A lifetime of 3.56 ps is found in a sample containing an InAlAs smoothing layer, compared to 0.96 ps in a sample without any InAlAs. The difference is explained using a model of ambipolar diffusion of photocarriers.

We report electrical conductivity studies of highly-doped GaAs pn diodes containing a strongly n-doped low-temperature-grown (LT)–GaAs layer and pn junctions containing an approximately one monolayer thick ErAs layer. At room temperature, current densities of 1 kA/cm2 for the n-LT–GaAs samples and 6 kA/cm2 for the ErAs samples at 1 V forward bias have been measured. The I–V characteristics under forward bias for the n-LT–GaAs and ErAs samples exhibit significantly different behavior. At low temperatures, the n-LT–GaAs samples reveal a shoulder in the I–V characteristics, which can be explained by a model taking into account tunneling of carriers into LT midgap states. A similar model was able to explain the current transport in the ErAs diodes as tunneling of carriers into metallic regions inside the pn junction.

A sampling setup for a cryogenic environment has been developed using fiber-illuminated photoconductive switches fabricated from a material composed of equidistant layers of self-assembled ErAs nanoislands in a GaAs matrix. The setup includes dispersion compensation of the fibers and exhibits a time resolution better than 2.2 ps, which is insensitive to temperature and applied magnetic field and limited only by the properties of the coplanar waveguide circuitry. In cryogenic photocurrent autocorrelation measurements on single switches, a resolution exceeding 850 fs could be achieved.

We report here the fabrication and demonstration of the photomixers made from In0.53Ga0.47As epitaxial material lattice-matched to InP. The material consists of layers of ErAs nanoparticles separated by InGaAs and compensated with Be to reduce the photocarrier lifetime to picosecond levels and to increase the resistivity to ∼100 Ω cm. Interdigitated-electrode and planar-antenna structures were fabricated by e-beam lithography and tested for dc electrical characteristics, 1.55-μm optical responsivity, and difference-frequency photomixing. The measured responsivity of 8 mA/W and photomixer output of >0.1 μW beyond 100 GHz are already comparable to GaAs photomixers and suggest that coherent THz generation is now feasible using the abundant 1.55-μm-semiconductor-laser and optical-fiber technologies.

Self-assembled ErAs islands on GaAs embedded between a backgate electrode and a two-dimensional electron gas (2DEG) were grown by molecular-beam epitaxy. The nanometer-sized islands form Schottky barriers with overlapping depletion regions, which insulate the backgate from the 2DEG. From temperature-dependent measurements and charging experiments the effective barrier height between the islands and the Schottky barrier height onto the islands could be determined. In addition, the effects of illumination were studied.

We describe a radio frequency mechanical resonator that includes a quantum point contact, defined using electrostatic top gates. We can mechanically actuate the resonator using either electrostatic or magnetomotive forces. We demonstrate the use of the quantum point contact as a displacement sensor, operating as a radio frequency mixer at the mechanical resonance frequency of 1.5 MHz. We calculate a displacement sensitivity of about 3×10−12 m/Hz1/2. This device will potentially permit quantum-limited displacement sensing of nanometer-scale resonators, allowing the quantum entanglement of the electronic and mechanical degrees of freedom of a nanoscale system.

We have grown epitaxial superlattice structures of layers of semimetallic ErAs particles embedded in an InGaAs matrix on (001) Fe-doped InP substrates. Temperature-dependent Hall measurements, x-ray diffraction, and transmission electron microscopy were performed on the materials. The carrier mobility and the temperature dependence of the charge density imply conduction in the InGaAs matrix. We calculate an offset between the conduction-band minimum of the InGaAs matrix and the Fermi level of the ErAs particles that is strongly dependent on the amount of ErAs deposited. As the size of the ErAs particles increases, the Fermi level decreases from ∼0.01 eV above the InGaAs conduction-band edge to ∼0.2 eV below the InGaAs conduction-band edge and the electrical conduction properties change from metallic to semiconducting.

We experimentally study the thickness dependence of the terahertz (THz) response in {110}-oriented GaAs crystals for free space electro-optic sampling at 1.55 microm. The THz response bandwidths are analyzed and simulated under phase-matching condition with a model frequency response function. The results indicate that the detection bandwidth increases from 2 THz to 3 THz when the thickness of GaAs is reduced from 2 mm to 1 mm. Below 1 mm, the detected bandwidth is increasingly limited by the emitter characteristics and the finite probe pulse duration. The broadest bandwidth in experiment reaches 3.3 THz when using a 0.2 mm thick crystal, while it exceeds 5 THz in theory. The THz response sensitivity was studied experimentally and modeled taking into account the absorption of the THz radiation in the GaAs crystal. While absorption was found to be negligible for the crystal thickness range studied here, strong saturation is predicted theoretically for crystal thicknesses exceeding 5 mm.

ErAs:In 0.53 Ga 0.47 As superlattice THz emitters are fabricated and characterized at an excitation wavelength of 1.55 µm. Photocurrent-voltage characteristics, carrier lifetimes and bandwidth of the THz output are discussed as a function of the superlattice period.

We use a double quantum dot as a frequency-tunable on-chip microwave detector to investigate the radiation from electron shot-noise in a near-by quantum point contact. The device is realized by monitoring the inelastic tunneling of electrons between the quantum dots due to photon absorption. The frequency of the absorbed radiation is set by the energy separation between the dots, which is easily tuned with gate voltages. Using time-resolved charge detection techniques, we can directly relate the detection of a tunneling electron to the absorption of a single photon.

In scanning gate microscopy, where the tip of a scanning force microscope is used as a movable gate to study electronic transport in nanostructures, the shape and magnitude of the tip-induced potential are important for the resolution and interpretation of the measurements. Contaminations picked up during topography scans may significantly alter this potential. We present an in situ high-field treatment of the tip that improves the tip-induced potential. A quantum dot was used to measure the tip-induced potential., Comment: 3 pages, 1 figure, minor changes to fit published version

We present a detailed experimental study on the electrostatic interaction between a quantum dot and the metallic tip of a scanning force microscope. Our method allowed us to quantitatively map the tip-induced potential and to determine the spatial dependence of the tip's lever arm with high resolution. We find that two parts of the tip-induced potential can be distinguished, one that depends on the voltage applied to the tip and one that is independent of this voltage. The first part is due to the metallic tip while we interpret the second part as the effect of a charged dielectric particle on the tip. In the measurements of the lever arm we find fine structure that depends on which quantum state we study. The results are discussed in view of scanning gate experiments where the tip is used as a movable gate to study nanostructures., Comment: 7 pages, 5 figures, minor changes to fit published version

Nanolithography based on local oxidation with a scanning force microscope has been performed on an undoped GaAs wafer and a Ga[Al]As heterostructure with an undoped GaAs cap layer and a shallow two-dimensional electron gas. The oxide growth and the resulting electronic properties of the patterned structures are compared for constant and modulated voltage applied to the conductive tip of the scanning force microscope. All the lithography has been performed in non-contact mode. Modulating the applied voltage enhances the aspect ratio of the oxide lines, which significantly strengthens the insulating properties of the lines on GaAs. In addition, the oxidation process is found to be more reliable and reproducible. Using this technique, a quantum point contact and a quantum wire have been defined and the electronic stability, the confinement potential and the electrical tunability are demonstrated to be similar to the oxidation with constant voltage., Comment: 7 pages, 7 figures, accepted by J. Appl. Phys

We report experimental evidence for strong surface plasma resonances on semimetallic ErSb nanoparticles grown epitaxially in semiconducting GaSb by molecular beam epitaxy. The infrared transmission spectra of ErSb/GaSb superlattices grown by molecular beam epitaxy showed a pronounced attenuation peak below the band gap of GaSb, not observed in a GaSb epitaxial control layer containing no ErSb. We attribute the attenuation peak to a surface plasmon resonance on the ErSb particles. The position of this peak shifts to longer wavelengths with increased ErSb deposition, from 2.4 μm for the smallest deposition to 3.8 μm for the largest deposition. In addition, the attenuation strength increases with the total amount of ErSb contained in the sample. The strongest effective attenuation coefficient was measured to be 1.4×104 cm−1, which is nearly equal to the cross bandgap absorption of the bulk GaSb at 1 μm.

We report on a novel concept for THz‐photomixers based on quasi‐ballistic transport in an asymmetric nipnip‐doping‐superlattice. Due to tansport‐optimized i‐layers the emitted powers are not transit‐time‐limited up to 1 THz. Furthermore the capacitance and hence the RC‐roll‐off is minimized by increasing the number of pin‐periods. The frequency‐dependence of the nipnip‐emitter proofs to be superior to corresponding pin‐photomixers.

A novel concept for THz-photomixers based on semiconductor nanostructures with strongly increased conversion efficiency is presented. In THz-photomixers, typically based on photoconductivity in "low-temperature-grown-GaAs." (LT-GaAs), the performance is typically limited by the (very low) photoconductive gain and depends critically on an extremely short lifetime of photo-generated charge carriers in LT-GaAs. In contrast to those devices, in our photomixer the gain does not depend on the recombination lifetime but only on the transit time and on the path length of the photo-generated electrons within suitably designed p-i-n nanostructures, which both can be optimised for (primarily) ballistic transport. In addition, impedance matching to the attached antenna can be achieved.

We analyse electronic, photonic, and magnetic manipulation of electrons and nuclear spins in a variety of semiconductor nanostructures for quantum spin information processing in the solid state. Temporally-and spatially-resolved magneto-optical measurements on semiconductor quantum structures are also discussed.

Coupled semiconductor nanostructures with a high degree of tunability are fabricated using local oxidation with a scanning force microscope. Direct oxidation of the GaAs surface of a Ga[Al]As heterostructure containing a shallow two-dimensional electron gas is combined with the local oxidation of a thin titanium film evaporated on top. A four-terminal quantum dot and a double quantum dot system with integrated charge readout are realized. The structures are tunable via in-plane gates formed by isolated regions in the electron gas and by mutually isolated regions of the Ti film acting as top gates. Coulomb blockade experiments demonstrate the high quality of this fabrication process., Comment: 3 pages, 3 figures

The capture of an optical image in the plane of a semiconductor quantum well and the subsequent re-emission of this image in the form of a two-dimensional photon flux is demonstrated. Spatially resolved storage of photonic signals in a two-dimensional lateral potential landscape of the quantum well is employed to imprint optical images in the form of trapped photogenerated charges into the solid. The lateral two-dimensional potential modulation leads to very long storage times by a deliberate spatial separation of photogenerated electron–hole pairs. Once the potential modulation is lifted, the initial optical information is restored and the photographed image is released in a flash of light.

Reports on a new concept for THz photomixers, which are not (as usually) frequency limited by the carrier lifetime in low-temperature-grown GaAs. The concept is based on ballistic electron transport in a periodically doped heterostructure (p-i-n-p-i-n-superlattice) and allows for impedance matching to the attached antenna.

We report the growth by molecular beam epitaxy of composite epitaxial materials consisting of layers of semimetallic ErAs particles embedded in an In/sub 0.53/Ga/sub 0.47/As matrix. These structures are grown on [100]-oriented semi-insulating InP:Fe substrates at 490/spl deg/C. This material may serve as a fast photoconductor for applications at longer wavelengths than low temperature grown GaAs or ErAs:GaAs. In the ErAs:InGaAs material, the ErAs particles act as fast carrier recombination centers, while the In/sub 0.53/Ga/sub 0.47/As matrix allows for photoexcitation at wavelengths of 1.55 /spl mu/m.

In applications as diverse as fibre-optic communications and time-domain or terahertz spectroscopy, researchers are keen on ultrafast optoelectronic transducers that can be tailored to specific needs. The molecular beam epitaxy of photoconductors composed of equidistant layers of self-assembled ErAs-islands in a III-V semiconductor matrix, which act as efficient non-radiative carrier capture sites, enables this flexibility. Here, photocurrent autocorrelation techniques are applied to metal-semiconductor-metal photodetectors patterned on ErAs:GaAs superlattices. The experiments demonstrate that the electrical response speed can be conveniently tuned over at least two orders of magnitude starting from 190 fs by increasing the thickness of the GaAs spacer separating adjacent ErAs layers. The same concept is applied to the narrower bandgap InGaAs matrix. We demonstrate an electron lifetime of approximately 1 ps for this material. This brings closer the prospect of implementing terahertz technology at the important optical communication wavelengths of 1.3 and 1.55 microm.

Equidistant layers of self-assembled ErAs islands embedded in a GaAs matrix are a promising alternative to low-temperature-grown (LTG) GaAs for the fabrication of ultrafast photoconductive switches. Unlike in LTG-GaAs, the electronic properties of this new material can be engineered straightforwardly by varying the period of the ErAs island superlattice. The carrier dynamics in MBE grown ErAs:GaAs has been investigated using photo-current autocorrelation measurements on a single photoconductive switch as a function of the superlattice period L. The electron lifetime can be tuned from 10.6 ps down to 2.3 ps by varying L between 300 and 100 nm. The dependence of the electron lifetime on L is understood in terms of a diffusion model. Two such switches were incorporated in a coplanar waveguide and illuminated by short laser pulses transferred to the switches via single-mode fibers. This setup enables pulse generation, propagation and detection under cryogenic conditions and high magnetic fields. By compensating fiber dispersion, a time resolution of less than 3 ps was achieved, limited only by the waveguide dispersion. This setup is ideally suited for time domain characterization with ultrahigh resolution of mesoscopic devices integrated on the same wafer.

We present new high-resolution measurements of transient time-domain photoconductivity in ErAs:InGaAs superlattice nanocomposites intended for THz photoconductive switches and photomixers using a pure optical pump-probe method. We developed a model, using separate photocarrier trapping, recombination, and thermal reactivation processes, which very accurately fits the measurements. The measured material structures all exhibit a slow secondary decay process, which is attributed to thermal reactivation of the trapped carriers, either into the conduction band, or into high-energy defect states. We examined the influence of superlattice structure, dopants, DC bias, and temperature. Analysis shows that all of the THz energy produced by the photocarrier trapping and decay processes are at frequencies less than 1 THz, while the reactivation process only serves to create a large portion of the bias power dissipated. Energy higher than 1 THz must be created by a fast generation process or band-filling saturation. This allows pulsed THz generation even from a long-lifetime material. Pure optical pump-probe measurements are necessary to expose slow material processes, and eliminate the influence of electrical terminals and THz antennas. These measurements and modeling of THz photoconductive devices are necessary in order to optimize the output spectrum and power.

We investigate the carrier dynamics in a superlattice of self-assembled ErAs island-layers in a GaAs matrix on a picosecond timescale. We measure photocurrent autocorrelations on photoconductive switches as a function of the superlattice period. By altering the superlattice period from 100 nm to 300 nm, the response time of the switches can be tuned from 2.3 ps to 10.6 ps. This makes ErAs:GaAs a very promising candidate for future device applications such as THz sources or high-speed optoelectronic circuits.

We present a scanning gate experiment on the electrostatic interaction between a semiconductor quantum dot and the metallic tip of a scanning force microscope. With the help of a feedback mechanism we can map the lever arm of the tip, using the quantum dot in a given quantum state as a sensitive electrometer. Besides the geometrically expected shape at length scales of hundreds of nanometers, we observe fine structure on much shorter length scales.

We report on a novel concept for a THz-photomixer, which is not (as usually) limited by the carrier lifetime in GaAs and allows impedance matching to the attached antenna. The concept is based on quasi-ballistic electron transport in a doping superlattice.

We characterize ErAs:In0.53Ga0.47As superlattices as substrates for photoconductive terahertz emitters excited at 1.55 μm. The bandwidth of the emitted radiation is studied as a function of the superlattice period (or equivalently the electron lifetime) and the applied bias field. The results show that a variation in the electron lifetime from 0.2 to 6.3 ps does not considerably influence the bandwidth of the emitted radiation. However, the bandwidth increases linearly from 2.6 to 3.0 THz as the applied bias field is increased from 7 to 30 kV/cm. At higher bias fields, saturation is observed. The largest measured bandwidth is 3.1 THz.

The electron capture time in superlattice structures consisting of periodically spaced layers of self-assembled ErAs nanoislands and In0.53Ga0.47As is investigated on photoconductive switches as a function of the superlattice period using photocurrent autocorrelation and pulsed laser excitation at 1.55μm. The capture time can be tuned from picoseconds all the way down to 0.2ps by changing the periodicity. Two different Be doping schemes are explored to reduce the dark current. The resulting characteristics indicate that ErAs:InGaAs may serve as a high performance photoconductive material at this wavelength for pulsed terahertz emission and detection.

Femtosecond carrier dynamics in biased AlxGa1−xAs heterostructure diodes is investigated tracing transient modifications of the Franz-Keldysh absorption spectrum. The nonlinear optical response is sensitive to the number of electron-hole pairs in the high-field region of the sample. As a result, the dynamical buildup of a nonequilibrium carrier avalanche due to impact ionization for electric fields F⩾350kV∕cm is directly analyzed in the time domain. The time scale of the carrier multiplication is found to be in the order of 10ps depending on the number of photoinjected carriers. Monte Carlo simulations in a simplified band structure agree well with the experiment.

High-field transport in GaAs is investigated tracing ultrafast modifications of the Franz–Keldysh absorption spectrum of a AlxGa1−xAs heterostructure diode. A sophisticated sample design allows us to isolate the unipolar tranport properties of holes in combination with a nanometer scale definition of layers for both photoexcitation and detection of the propagating carrier distribution. Transient velocities and spatial broadening of the hole ensemble are directly measured for electric fields between 15 and 200 kV∕cm comparing room temperature operation to results for TL=4K. Even at low temperatures, the transient hole velocities are found not to exceed a value of 1.2×107cm∕s which is a result of ultrafast optical phonon emission with a scattering time below 25 fs.

We use high-angle annular dark-field imaging in scanning transmission electron microscopy to investigate the crystal structure and particle morphology of semimetallic ErAs nanoparticles embedded in epitaxial In0.53Ga0.47As layers. Deposition of increasing amounts of Er results in a higher density of particles and particles coalescence for high amounts. Despite overlap with the matrix through the thickness of the sample used for transmission electron microscopy, the crystal structure of ErAs particles is unambiguously identified as rock salt. The As sublattice is continuous across the interface between the particle and the zinc-blende semiconductor.

We have grown epitaxial metal/semiconductor superlattice materials by molecular beam epitaxy that exhibit subpicosecond photocarrier lifetimes at 1.55 μm. The superlattice samples consist of layers of semimetallic ErAs nanoparticles embedded in a semiconducting In0.53Ga0.47As matrix. Time-resolved optical measurements are performed using a fiber-based transmission pump-probe technique with an erbium-doped-fiber mode-locked laser. Photocarrier lifetimes decrease with increasing ErAs deposition and decreasing spacing between the ErAs layers. Further reduction in the lifetime is achieved by selective beryllium doping of the superlattice; measured lifetimes ⩽0.3ps were achieved in optimized structures.

We have grown composite epitaxial materials consisting of layers of semimetallic ErAs nanoparticles embedded in a semiconducting In0.53Ga0.47As matrix. Although the addition of ErAs particles into the InGaAs matrix increases the free-electron concentration, compensation of these free electrons is possible by depleting electrons from the metal particles through Be acceptor doping of the semiconductor. The room-temperature electron concentration of an ErAs:InGaAs superlattice sample with 0.05 monolayer ErAs per layer can be reduced by >104 by delta-doping the ErAs layers with 7×1012cm−2 of Be. The highest resistivity measured for a Be-doped ErAs:InGaAs superlattice was 350Ωcm.

We investigate a THz-photomixer based on quasi-ballistic transport in an asymmetric n-i-p-n superlattice. The measured frequency roll-off is due to the device RC time constant alone whilst the transit-time limitation is overcome in this device. Measured power levels compare well with those from standard LT-GaAs photomixers.