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In a Quantum Cascade Detector, photocurrent is generated by the absorption of infrared and terahertz radiation in the quantum-well-based modules arranged in series. Consequently, the current responsivity is by construction inversely proportional to the number of cascading modules. Upon absorption of a photon, the electron travels through only a single period of the detector, with a mean free path corresponding to the period length. Therefore, the shot noise power density is expected to decrease by the same factor under sufficiently high illumination, reflecting the same inverse relationship with the number of cascading modules. This phenomenon leads to sub-Poissonian noise characteristics. We experimentally observe this effect in a 90-period Quantum Cascade Detector operating at 4.5 μm, confirming a reduction in the shot noise contribution by the anticipated Fano factor of 1/90. This measurement underscores the suitability of these detectors for coherent detection scenarios, particularly where shot noise dominates. [ABSTRACT FROM AUTHOR]

Perturbation facilitated double-resonant four-wave mixing is applied to access high-lying vibrational levels of the X 1 Σ g + ( 0 g + ) ground state of Cu2. Rotationally resolved transitions up to v ″ = 102 are measured. The highest observed level is at 98% of the dissociation energy. The range and accuracy of previous measurements are significantly extended. By applying the near dissociation equation developed by Le Roy [R. J. Le Roy, J. Quant. Spectrosc. Radiat. Transfer 186, 197 (2017)], a dissociation energy of De = 16 270(7) hc cm−1 is determined, and an accurate potential energy function for the X 1 Σ g + ( 0 g + ) ground state is obtained. Molecular constants are determined from the measured transitions and by solving the radial Schrödinger equation using this function and are compared with results from earlier measurements. In addition, benchmark multi-reference configuration interaction computations are performed using the Douglas–Kroll–Hess Hamiltonian and the appropriate basis of augmented valence quadruple ζ type. Coupled-cluster single, double, and perturbative triple calculations were performed for comparison. [ABSTRACT FROM AUTHOR]

The low energy electronic structure of the copper dimer has been re-investigated using non-linear four-wave mixing spectroscopy and high level ab initio calculations. In addition to the measurement of the previously reported A, B, and C electronic states, a new state denoted A' is identified with T0 = 20 100.4090(16) cm-1 (63Cu2). Rotational analysis of the A'-X (0,0) and (1,0) transitions leads to the assignment of A' 1u. Ab initio calculations present the first theoretical description of the lowenergy states of the copper dimer in Hund's case (c) and confirm the experimental assignment. The discovery of this new low energy excited state emphasizes that spin-orbit coupling is significant in states with d-hole electronic configurations and resolves a decades-long mystery in the initial assignment of the A state. [ABSTRACT FROM AUTHOR]

Vibrational levels of the recently observed high-spin transition (1 5Πu - 1 5&3928;g) of dicarbon [P. Bornhauser et al., J. Chem. Phys. 142, 094313 (2015)] are explored by applying non-linear doubleresonant four-wave mixing and laser-induced fluorescence methods. The deperturbation of the d 3Πg, 3 = 8 and 1 5Πg, 3 = 3 states results in accurate molecular constants for the 3 = 3 "dark" quintet state. In addition, the spin-orbit interaction constant is determined and parameters for the upper Swan level d 3Πg, 3 = 8 are improved. The first excited vibrational state of 1 5Πu is observed by performing perturbation-assisted intersystem crossing via "gateway" states in the d 3Πg, 3 = 6 ~ 1 5Πg, 3 = 0 system. The rotationally resolved spectra yield 11 transitions to 1 5Πu, 3 = 1 that include four spinsubstates. Data reduction results in accurate molecular constants of this vibrational level in the shallow potential energy surface of this state. Finally, 3 = 1 and 2 of the lower quintet state (1 5Πg) are measured by performing perturbation-assisted double-resonant excitation to the 1 5Πu, 3 = 0 state and observing dispersed fluorescence. The obtained molecular constants are compared with high level ab initio computations at the multi-reference configuration interaction (MRCI) level of theory by using a large correlation consistent basis set or, alternatively, by applying the computationally less demanding method of explicitly correlated multi-reference configuration interaction (MRCI-F12). The spectroscopic accuracy of both methods is evaluated by comparison with the experimental findings. [ABSTRACT FROM AUTHOR]

The first high-spin transition in C2 (1 5Πu - 1 5Πg) is observed by perturbation-facilitated opticaloptical double resonance spectroscopy. The experiment is performed by applying unfolded two-color resonant four-wave mixing. C2 radicals in the initial a ³Πu, v = 5 state are produced by using a discharge source in a molecular beam environment. The final quintet state is excited via intermediate "gateway" states exhibiting both substantial triplet and quintet character due to a perturbation between the 1 5Πg, v = 0 and the d 5Πg, v = 6 states. Fifty seven rotational transitions in the P, Q, and R branches of all spin sub-states are measured and yield accurate molecular constants of the newly found upper level 1 5Πu. In addition, satellite transitions (ΔJ ≠ ΔN) are observed and allow an accurate determination of the spin-orbit constant. The results are compared with high-level ab initio computations at the multi-reference configuration interaction level of theory. The high-lying quintet state is found to be predissociative and displays a shallow potential that accommodates three vibrational levels only. [ABSTRACT FROM AUTHOR]

We present an antipodal Vivaldi antenna for broadband double metal waveguide terahertz quantum cascade lasers and frequency combs. Its exponentially curved flare profile results in an adiabatic in-plane mode expansion, producing an improved far-field with a single-lobed beam of (23 ° × 19 °) full width half maximum with an octave-spanning bandwidth. The antenna also acts as a wave retarder, rotating the polarization from vertical toward horizontal polarization by a frequency-dependent angle. The laser's emission spectrum and current–voltage characteristics are not affected, as well as frequency comb operation. Measurements agree well with numerical simulations, and the proposed antenna covers a broad spectral range (1.5–4.5 THz). [ABSTRACT FROM AUTHOR]

The extension of the operating frequency of Quantum Cascade Lasers (QCL) into the 20-50 μm regime is a desirable goal as it would bridge the gap between mid-infrared and THz devices. Coherent light emitters in this spectral range are also needed for spectroscopy and radio astronomy applications. Since little attention has been devoted to the subject in the past, we investigate the dominant loss mechanisms of QCLs in this spectral range. We report on an InGaAs/InAlAs QCL in an InP dielectric waveguide emitting at 23 μm wavelength whose electroluminescence spectrum shows an anomalous low-frequency cut which prevents laser action at low electric field. We also observe similar line shape in other GaAs/AlGaAs devices. The spectral features are analyzed and explained in terms of refractive index anomalies induced by phonon resonances. [ABSTRACT FROM AUTHOR]

Degradation mechanisms limiting the electrical reliability of GaN high-electron-mobility transistors (HEMTs) are generally attributed to defect generation by hot-electrons but specific mechanisms for such processes have not been identified. Here we give a model for the generation of active defects by the release of hydrogen atoms that passivate pre-exisiting defects. We report first-principles density-functional calculations of several candidate point defects and their interaction with hydrogen in GaN, under different growth conditions. Candidate precursor point defects in device quality GaN are identified by correlating previously observed trap levels with calculated optical levels. We propose dehydrogenation of point defects as a generic physical mechanism for defect generation in HEMTs under hot-electron stress when the degradation is not spontaneously reversible. Dehydrogenation of point defects explains (1) observed hot electron stress transconductance degradation, (2) increase in yellow luminescence, and opposite threshold voltage shifts in devices where the material was grown under nitrogen- and ammonia-rich conditions. [ABSTRACT FROM AUTHOR]

Terahertz time-domain spectroscopy is shown to provide a convenient and rapid means to measure the conductivity of individual layers in semiconductor heterostructures such as terahertz quantum cascade lasers. By modeling the complex transmission at terahertz frequencies, the electron density and the in-plane momentum scattering time of the active regions and doped contact layers were determined for both GaAs/AlGaAs and InGaAs/InAlAs epilayers. The measured temperature dependence of the electron scattering rate revealed the significance of impurity and LO phonon scattering. The implications for laser operation at room temperature are discussed by considering the changes in absorption and resonant tunneling current with temperature. [ABSTRACT FROM AUTHOR]

We present a new concept for waveguide acousto-optical devices based on coupled Mach–Zehnder interferometers driven by acoustic waves. These acousto-optical multiple interference devices use the periodic refractive index modulation induced by the acoustic wave to realize functionalities such as ON/OFF switching for an arbitrary time interval, as well as for efficient light modulation at high harmonics of the acoustic frequency and pulse shaping for, e.g., integrated Q-switches. We also discuss application of the concepts to light modulation by very high acoustic frequencies, where the acoustic wavelengths become much shorter than the optical ones. [ABSTRACT FROM AUTHOR]

The spontaneous formation of voids has been observed in Cu(In,Ga)Se2 films deposited by multistage evaporation processes but was not found commonly in single-stage evaporated material. These voids may be isolated in the grains or may occur at the grain boundaries. The voids exhibit strongly faceted geometries with interior surfaces being close-packed {112} planes of the chalcopyrite structure. A model based on the Kirkendal effect [Trans. AIME 171, 130 (1947)] and the interdiffusion and phase reaction in the bilayer process is proposed to explain the observation. The void shape is analyzed and an approximate Wulff construction [Z. Kristallogr. Mineral. 34, 449 (1901)] is used to estimate the relative surface energies of the predominant facets. Certain dislocation cores are also found to be unstable and convert to nanovoids in the presence of sufficient atomic vacancies moving in the lattice. [ABSTRACT FROM AUTHOR]

We present a terahertz quantum cascade laser operating on a thermoelectric cooler up to a record-high temperature of 210.5 K. The active region design is based on only two quantum wells and achieves high temperature operation thanks to a systematic optimization by means of a nonequilibrium Green's function model. Laser spectra were measured with a room temperature detector, making the whole setup cryogenic free. At low temperatures (∼40 K), a maximum output power of 200 mW was measured. [ABSTRACT FROM AUTHOR]

We design and fabricate large-area (1.1 mm × 1.1 mm) photonic crystal quantum cascade lasers, enabling single-mode (wavelength ∼ 8.5 µm) surface emission at room temperature, with a maximum peak power up to 176 mW. The beam divergence is <1∘ and with no side-lobes. Moreover, by introducing asymmetry into the photonic crystal pillar shape, a single-lobed far-field pattern is realized. The photonic band structure is measured with high spectral (0.72 cm−1) and angular (0.1°) resolution by using the photonic crystal quantum cascade laser itself as a detector. [ABSTRACT FROM AUTHOR]

Provides information on an experiment investigating transmission electron (TEM) and Raman spectroscopy InAlAs/InGaAs/InP HEMT heterostructures grown by molecular beam epitaxy (MBE). Methodology used to conduct the experiment; Information on surface corrugation in lattice matched samples; Findings of the experiment.

Bulk InGaAs layers were grown at 400 °C lattice-matched to InP semi-insulating substrates by molecular beam epitaxy. Si doping of the layers was performed by applying volume- and delta-doping techniques. The samples were characterized by capacitance-voltage, van der Pauw- Hall, secondary ion mass spectroscopy and photoluminescence measurements. Good agreement in terms of dependence of mobility and Burstein-Moss shift shift on doping concentration in samples doped by the two different techniques was obtained. Amphoteric behavior of Si was observed at doping concentrations higher than ~2.9×1019 cm-3 in both delta- and volume-doped samples. Degradation of InGaAs crystalline quality occurred in samples with Si concentrations higher than ~4×1019 cm-3. [ABSTRACT FROM AUTHOR]

The gain recovery time of a heterogeneous active region terahertz quantum cascade laser is studied by terahertz-pump-terahertz-probe spectroscopy. The investigated active region, which is based on a bound-to-continuum optical transition with an optical phonon assisted extraction, exhibits a gain recovery time in the range of 34–50 ps dependent on the operation condition of the laser. The recovery time gets shorter for stronger pumping of the laser while the recovery dynamics slows down with increasing operation temperature. These results indicate the important role of the intracavity light intensity for the fast gain recovery. [ABSTRACT FROM AUTHOR]

We present a two-quantum well THz intersubband laser operating up to 192 K. The structure has been optimized using non-equilibrium Green's function model. The result of this optimization was confirmed experimentally by growing, processing, and measuring a number of proposed designs. At high temperature (T>200 K), the simulations indicate that lasing fails due to a combination of electron-electron scattering, thermal backfilling, and, most importantly, re-absorption coming from broadened states. [ABSTRACT FROM AUTHOR]

In this work, we report the characterization of a quantum cascade laser frequency comb with an optical power of 1.05W at λ~8.2 μm. A 4.5mm long device has a high reflectivity coating on the back facet as well as a top cladding designed to lower the group velocity dispersion and is operated at 258 K. Very strong (more than 60 dB) narrow beatnotes are shown, and frequency comb operation is obtained on a bandwidth of 85 cm-1 in a very large range of light-versus current characteristics. A bandwidth of 82 cm-1 has a power per mode of more than 1 mW and an average power per mode of 4.1 mW. Finally, a multi-heterodyne spectrum with 215 lines covering an optical bandwidth of more than 70 cm-1 measured with lasers showing similar performances is presented with very good line separation. [ABSTRACT FROM AUTHOR]

We introduce a double metal terahertz quantum cascade laser meant for astrophysical heterodyne measurements. The laser ridge is embedded in benzocyclobutene, and the device exhibits single mode, continuous wave operation around 4.745 THz with a peak power of almost 1.8 mW at 10K and a power consumption of≈1.6 W. Moreover, thanks to the integration of a top metal contact with a patch array antenna for light out-coupling the beam of the emitted light has a lowdivergence single-lobe profile and an FWHM of≈30°. [ABSTRACT FROM AUTHOR]

We describe the design and characterization of superconducting coplanar waveguide cavities tailored to facilitate strong coupling between superconducting quantum circuits and single trapped Rydberg atoms. For initial superconductor-atom experiments at 4.2 K, we show that resonator quality factors above 104 can be readily achieved. Furthermore, we demonstrate that the incorporation of thick-film copper electrodes at a voltage antinode of the resonator provides a route to enhance the zero-point electric fields of the resonator in a trapping region that is 40 lm above the chip surface, thereby minimizing chip heating from scattered trap light. The combination of high resonator quality factor and strong electric dipole coupling between the resonator and the atom should make it possible to achieve the strong coupling limit of cavity quantum electrodynamics with this system. [ABSTRACT FROM AUTHOR]

The effective mass, sheet carrier concentration, and mobility of electrons within a two-dimensional electron gas in an AlGaN/GaN heterostructure were determined using a laboratory-based terahertz cyclotron resonance spectrometer. The ability to perform terahertz cyclotron resonance spectroscopy with magnetic fields of up to 31 T was enabled by combining a high-field pulsed magnet with a modified asynchronous optical sampling terahertz detection scheme. This scheme allowed around 100 transmitted terahertz waveforms to be recorded over the 14 ms magnetic field pulse duration. The sheet density and mobility were measured to be 8.0×1012cm-2 and 9000 cm2 V-1 s-1 at 77 K. The in-plane electron effective mass at the band edge was determined to be 0.228±0.002m0. [ABSTRACT FROM AUTHOR]

We analyze the peculiar case where the free carrier absorption arising from LO phonon absorptionassisted transitions becomes negative and therefore turns into a gain source for quantum cascade lasers. Such an additional source of gain exists when the ratio between the electronic and the lattice temperatures is larger than one, a condition that is usually fulfilled in quantum cascade lasers. We find a gain of few cm-1's at 200 K. We report the development of a terahertz quantum cascade laser operating in the negative free carrier absorption regime. [ABSTRACT FROM AUTHOR]

Dual-comb spectroscopy is emerging as an appealing application of mid-infrared frequency combs for high-resolution molecular spectroscopy, as it leverages on the unique coherence properties of frequency combs. Here, we present an on-chip dual-comb source based on mid-infrared quantum cascade laserfrequency combs. Control of the combs repetition and offset frequencies is obtained by integrating micro-heaters next to each laser. We show that a full control of the dual-comb system is possible, by measuring a multi-heterodyne beating corresponding to an optical bandwidth of 32cm-1 centered at 1330cm-1 (7.52μm), demonstrating that this device represents a critical step towards compact dual-comb systems. [ABSTRACT FROM AUTHOR]

We present a surface emitting laser array design based on distributed Bragg reflectors together with a second order extractor, providing a deterministic mode selection mechanism. The technology is implemented as a buried heterostructure compatible with continuous-wave operation and low dissipation. A proof of principle featured 10 regularly spaced single mode devices with a coverage of 175 cm-1 in the 8-10 μm wavelength range and milliwatt output powers with a far-field pattern full-width half-maximum of 8.3± in the longitudinal direction. [ABSTRACT FROM AUTHOR]

The spectral gain of a multi-stack terahertz quantum cascade laser, composed of three active regions with emission frequencies centered at 2.3, 2.7, and 3.0 THz, is studied as a function of driving current and temperature using terahertz time-domain spectroscopy. The optical gain associated with the particular quantum cascade stacks clamps at different driving currents and saturates to different values. We attribute these observations to varying pumping efficiencies of the respective upper laser states and to frequency dependent optical losses. The multi-stack active region exhibits a spectral gain full width at half-maximum of 1.1 THz. Bandwidth and spectral position of the measured gain match with the broadband laser emission. As the laser action ceases with increasing operating temperature, the gain at the dominant lasing frequency of 2.65 THz degrades sharply. [ABSTRACT FROM AUTHOR]

We demonstrate a sequentially operating dual-wavelength quantum cascade laser with electrically separated laser sections, emitting single-mode at 5.25 and 6.25 μ m. Based on a single waveguide ridge, this laser represents a considerable asset to optical sensing and trace gas spectroscopy, as it allows probing multiple gas species with spectrally distant absorption features using conventional optical setups without any beam combining optics. The laser capability was demonstrated in simultaneous NO and NO2 detection, reaching sub-ppb detection limits and selectivity comparable to conventional high-end spectroscopic systems. [ABSTRACT FROM AUTHOR]

We have performed a series of experiments using a spontaneous parametric down-conversion source to produce pairs of photons in either entangled or non-entangled polarization states. We determine the full quantum mechanical polarization state of one photon, conditioned on the results of measurements performed on the other photon. For non-entangled states, we find that the measured state of one photon is independent of measurements performed on the other. However, for entangled states, the measured state does depend on the results of measurements performed on the other photon. This is possible because of the nonlocal nature of entangled states. These experiments are suitable for an undergraduate teaching laboratory. [ABSTRACT FROM AUTHOR]

A study on far-infrared In0.53Ga0.47As/Al0.48In0.52As quantum cascade lasers operating with a double metal waveguide is presented. To increase the laser upper state lifetime, a diagonal bound-to-continuum transition scheme is used in the active region. The observed threshold current density at 50K is 5.7 kA/cm², and the maximum operation temperature is 240 K. The laser emission wavelength is 24.4 μm, which is the longest wavelength in the mid-infrared quantum cascade lasers so far reported. [ABSTRACT FROM AUTHOR]

Our work presents a slot and a patch array antenna at the front facet of a 4.7 THz quantum cascade laser as extractor, decreasing the facet reflectivity down to 2.6%. The resulting output power increases by a factor 2 and the slope efficiency by a factor 4. The simulated and the measured far-fields are in good agreement. [ABSTRACT FROM AUTHOR]

Electrical laser emission frequency tuning of a three terminal THz quantum cascade laser is demonstrated. A high electron mobility transistor structure is used in a surface plasmon waveguide to modulate the electron density in a channel, controlling the effective refractive index of the waveguide. The threshold current density was modulated by 28% via applying voltage from –3 to 2 V. The observed laser emission frequency shift by electric field was 2 GHz. By using the three terminal devices, pure frequency modulation of the output light is, in principle, achievable. [ABSTRACT FROM AUTHOR]

In this paper we describe a new variant of null ellipsometry to determine thicknesses and optical properties of thin films on a substrate at cryogenic temperatures. In the PCSA arrangement of ellipsometry the polarizer and the compensator are placed before the substrate and the analyzer after it. Usually, in the null ellipsometry the polarizer and the analyzer are rotated to find the searched minimum in intensity. In our variant we rotate the polarizer and the compensator instead, both being placed in the incoming beam before the substrate. Therefore the polarisation analysis of the reflected beam can be realized by an analyzer at fixed orientation. We developed this method for investigations of thin cryogenic films inside a vacuum chamber where the analyzer and detector had to be placed inside the cold shield at a temperature of T ≈ 90 K close to the substrate. All other optical components were installed at the incoming beam line outside the vacuum chamber, including all components which need to be rotated during the measurements. Our null ellipsometry variant has been tested with condensed krypton films on a highly oriented pyrolytic graphite substrate (HOPG) at a temperature of T ≈ 25 K. We show that it is possible to determine the indices of refraction of condensed krypton and of the HOPG substrate as well as thickness of krypton films with reasonable accuracy. [ABSTRACT FROM AUTHOR]

Terahertz intersubband electroluminescence from an InAs/AlSb quantum cascade structure based on a single quantum well active layer is studied. A sharp electroluminescence peak with a linewidth of 0.6 meV is observed at 4 THz. Low temperature magneto-transport measurements are also performed to investigate alignment of subbands in the active layer. The model taking account of a conduction band nonparabolicity reproduces well the subband energy in the InAs/AlSb THz quantum cascade structure. [ABSTRACT FROM AUTHOR]

We report the observation of transverse electric polarized electroluminescence from InAs/AlInAs quantum dash quantum cascade structures up to room temperature. The emission is attributed to the electric field confined along the shortest lateral dimension of the dashes, as confirmed by its dependence on crystallographic orientation both in absorption measurements on a dedicated sample and from electroluminescence itself. From the absorption, we estimate a dipole moment for the observed transition of = 1.7 nm. The electroluminescence is peaked at around 110 meV and increases with applied bias. Its temperature dependence shows a decrease at higher temperatures limited by optical phonon emission. [ABSTRACT FROM AUTHOR]

Coincidence counting is a necessary ingredient for quantum optics experiments at the undergraduate level, but cost has created an entry barrier for many schools. We present a design of a coincidence-counting module that replaces the traditional method based on time-to-amplitude conversion and pulse-height analysis. Our module accepts inputs from up to four detectors, has a coincidence-time window of less than 10 ns, and has a throughput of more than triple that of the traditional method. The cost of our coincidence-counting module is less than 5% of the cost of the traditional method. [ABSTRACT FROM AUTHOR]

While the classical, wavelike behavior of light (interference and diffraction) has been easily observed in undergraduate laboratories for many years, explicit observation of the quantum nature of light (i.e., photons) is much more difficult. For example, while well-known phenomena such as the photoelectric effect and Compton scattering strongly suggest the existence of photons, they are not definitive proof of their existence. Here we present an experiment, suitable for an undergraduate laboratory, that unequivocally demonstrates the quantum nature of light. Spontaneously downconverted light is incident on a beamsplitter and the outputs are monitored with single-photon counting detectors. We observe a near absence of coincidence counts between the two detectors—a result inconsistent with a classical wave model of light, but consistent with a quantum description in which individual photons are incident on the beamsplitter. More explicitly, we measured the degree of second-order coherence between the outputs to be g(2)(0) = 0.0177±0.0026, which violates the classical inequality g(2)(0)&ges; 1 by 377 standard deviations. [ABSTRACT FROM AUTHOR]

In this article we describe the test of a zone plate monochromator for a laboratory soft x-ray source which is a laser produced plasma on a liquid jet target. The monochromator consists of a zone plate and a pinhole. Due to the special zone plate used (condensor zone plate KZP7) the monochromator is particularly suitable for laboratory sources since it collects a relatively large solid angle in the present setup. Depending upon the diameter of the pinhole a monochromaticity of up to λ/Δλ=600 can be achieved. The usefulness of the linear monochromator was proven on the basis of a filter transmission measurement. The monochromator can be used for several applications. In particular it is suitable for time-resolved x-ray absorption spectroscopy and pump and probe experiments. The use for such investigations is discussed. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Explores the statistical properties of photocurrent produced when light is measured with a photodetector. Information on the generation of photocurrent; Classical and quantum mechanical theories of photoelectric detection; Importance of equipment used to conduct experiment; Statistical properties of light beams; Information on the quantum theory.

In this work, we have studied the frequency response to a direct current modulation of a mid-infrared quantum cascade laser that has been monolithically integrated within a microwave strip. By merging microwave and semiconductor laser technologies, we have demonstrated a flat frequency response up to ∼15 GHz for a device operating at 77 K and emitting at 9 μm. Laser performances are preserved and comparable to state of the art devices. Finally, in our devices, we have observed that the modulation bandwidth varies substantially upon the laser photon density. Improved frequency response is indeed achieved, while lasers operate well above threshold. [ABSTRACT FROM AUTHOR]