Giant Nonlinear Circular Dichroism from Intersubband Polaritonic Metasurfaces.

Nonlinear metasurfaces are advancing into a new paradigm of "flat nonlinear optics" owing to the ability to engineer local nonlinear responses in subwavelength-thin films. Recently, attempts have been made to expand the design space of nonlinear metasurfaces through nonlinear chiral responses. However, the development of metasurfaces that display both giant nonlinear circular dichroism and significantly large nonlinear optical response is still an unresolved challenge. Herein, we propose a method that induces giant nonlinear responses with near-unity circular dichroism using polaritonic metasurfaces with optical modes in chiral plasmonic nanocavities coupled with intersubband transitions in semiconductor heterostructures designed to have giant second and third order nonlinear responses. A stark contrast between effective nonlinear susceptibility elements for the two spin states of circularly polarized pump beams was seen in the hybrid structure. Experimentally, near-unity nonlinear circular dichroism and conversion efficiencies beyond 10-4% for second- and third-harmonic generation were achieved simultaneously in a single chip.

Giant nonlinear response from plasmonic metasurfaces coupled to intersubband transitions.

Intersubband transitions in n-doped multi-quantum-well semiconductor heterostructures make it possible to engineer one of the largest known nonlinear optical responses in condensed matter systems--but this nonlinear response is limited to light with electric field polarized normal to the semiconductor layers. In a different context, plasmonic metasurfaces (thin conductor-dielectric composite materials) have been proposed as a way of strongly enhancing light-matter interaction and realizing ultrathin planarized devices with exotic wave properties. Here we propose and experimentally realize metasurfaces with a record-high nonlinear response based on the coupling of electromagnetic modes in plasmonic metasurfaces with quantum-engineered electronic intersubband transitions in semiconductor heterostructures. We show that it is possible to engineer almost any element of the nonlinear susceptibility tensor of these structures, and we experimentally verify this concept by realizing a 400-nm-thick metasurface with nonlinear susceptibility of greater than 5 × 10(4) picometres per volt for second harmonic generation at a wavelength of about 8 micrometres under normal incidence. This susceptibility is many orders of magnitude larger than any second-order nonlinear response in optical metasurfaces measured so far. The proposed structures can act as ultrathin highly nonlinear optical elements that enable efficient frequency mixing with relaxed phase-matching conditions, ideal for realizing broadband frequency up- and down-conversions, phase conjugation and all-optical control and tunability over a surface.

III-V-on-Silicon Photonic Integrated Circuits for Spectroscopic Sensing in the 2-4 µm Wavelength Range.

The availability of silicon photonic integrated circuits (ICs) in the 2-4 µm wavelength range enables miniature optical sensors for trace gas and bio-molecule detection. In this paper, we review our recent work on III-V-on-silicon waveguide circuits for spectroscopic sensing in this wavelength range. We first present results on the heterogeneous integration of 2.3 µm wavelength III-V laser sources and photodetectors on silicon photonic ICs for fully integrated optical sensors. Then a compact 2 µm wavelength widely tunable external cavity laser using a silicon photonic IC for the wavelength selective feedback is shown. High-performance silicon arrayed waveguide grating spectrometers are also presented. Further we show an on-chip photothermal transducer using a suspended silicon-on-insulator microring resonator used for mid-infrared photothermal spectroscopy.

III-V-on-silicon 2-µm-wavelength-range wavelength demultiplexers with heterogeneously integrated InP-based type-II photodetectors.

2-µm-wavelength-range silicon-on-insulator (SOI) arrayed waveguide gratings (AWGs) with heterogeneously integrated InP-based type-II quantum well photodetectors are presented. Low insertion loss (2.5-3 dB) and low crosstalk (-30 to -25 dB) AWGs are realized. The InP-based type-II photodetectors are integrated with the AWGs using two different coupling approaches. Adiabatic-taper-based photodetectors show a responsivity of 1.6 A/W at 2.35 µm wavelength and dark current of 10 nA at -0.5 V, while photodetectors using grating-assisted coupling have a responsivity of 0.1 A/W and dark current of 5 nA at -0.5 V. The integration of the photodetector array does not degrade the insertion loss and crosstalk of the device. The photodetector epitaxial stack can also be used to realize the integration of a broadband light source, thereby enabling fully integrated spectroscopic systems.

2.3 µm range InP-based type-II quantum well Fabry-Perot lasers heterogeneously integrated on a silicon photonic integrated circuit.

Heterogeneously integrated InP-based type-II quantum well Fabry-Perot lasers on a silicon waveguide circuit emitting in the 2.3 µm wavelength range are demonstrated. The devices consist of a "W"-shaped InGaAs/GaAsSb multi-quantum-well gain section, III-V/silicon spot size converters and two silicon Bragg grating reflectors to form the laser cavity. In continuous-wave (CW) operation, we obtain a threshold current density of 2.7 kA/cm2 and output power of 1.3 mW at 5 °C for 2.35 µm lasers. The lasers emit over 3.7 mW of peak power with a threshold current density of 1.6 kA/cm2 in pulsed regime at room temperature. This demonstration of heterogeneously integrated lasers indicates that the material system and heterogeneous integration method are promising to realize fully integrated III-V/silicon photonics spectroscopic sensors in the 2 µm wavelength range.

GaSb-based vertical-cavity surface-emitting lasers with an emission wavelength at 3 µm.

GaSb-based electrically pumped vertical-cavity surface-emitting lasers (VCSELs) with a buried tunnel junction emitting at 3 µm are demonstrated. To achieve this, a low optical loss VCSEL concept with an undoped epitaxial distributed Bragg reflector and intracavity contact is presented. The devices operate up to 5°C continuous wave and up to 50°C in pulsed mode. Single-mode operation with a side-mode suppression ratio of 30 dB and electro-thermal tuning range of 19.7 nm is achieved.

Attosecond photoelectron streaking with enhanced energy resolution for small-bandgap materials.

Attosecond photoelectron streaking spectroscopy allows time-resolved electron dynamics with a temporal resolution approaching the atomic unit of time. Studies have been performed in numerous systems, including atoms, molecules, and surfaces, and the quest for ever higher temporal resolution called for ever wider spectral extent of the attosecond pulses. For typical experiments relying on attosecond pulses with a duration of 200 as, the time-bandwidth limitation for a Gaussian pulse implies a minimal spectral bandwidth larger than 9 eV translating to a corresponding spread of the detected photoelectron kinetic energies. Here, by utilizing a specially tailored narrowband reflective XUV multilayer mirror, we explore experimentally the minimal spectral width compatible with attosecond time-resolved photoelectron spectroscopy while obtaining the highest possible spectral resolution. The validity of the concept is proven by recording attosecond electron streaking traces from the direct semiconductor gallium arsenide (GaAs), with a nominal bandgap of 1.42 eV at room temperature, proving the potential of the approach for tracking charge dynamics also in these technologically highly relevant materials that previously have been inaccessible to attosecond science.

2 µm wavelength range InP-based type-II quantum well photodiodes heterogeneously integrated on silicon photonic integrated circuits.

The heterogeneous integration of InP-based type-II quantum well photodiodes on silicon photonic integrated circuits for the 2 µm wavelength range is presented. A responsivity of 1.2 A/W at a wavelength of 2.32 µm and 0.6 A/W at 2.4 µm wavelength is demonstrated. The photodiodes have a dark current of 12 nA at -0.5 V at room temperature. The absorbing active region of the integrated photodiodes consists of six periods of a "W"-shaped quantum well, also allowing for laser integration on the same platform.

Miniaturized sensor for particles in air using Fresnel ring lenses and an enhanced intensity ratio technique.

We present a miniaturized particle sensor collecting scattered light in two solid angle intervals by Fresnel ring lenses. The particle size is determined from the ratio of both scattering amplitudes (intensity ratio) in addition to a linear diversity combining technique, generating a 3D particle size matrix that reduces the ambiguity by the index of refraction on the particle size identification. A signal-to-noise ratio of 30.3 was achieved for 147 nm sized polystyrene latex particles. Measurements of polydisperse particle size distribution show good agreement with the results by a scanning mobility particle sizer.

Enhanced luminescence properties of InAs-InAsP core-shell nanowires.

Utilizing narrow band gap nanowire (NW) materials to extend nanophotonic applications to the mid-infrared spectral region (>2-3 µm) is highly attractive, however, progress has been seriously hampered due to their poor radiative efficiencies arising from nonradiative surface and Auger recombination. Here, we demonstrate up to ~ 10(2) times enhancements of the emission intensities from InAs NWs by growing an InAsP shell to produce core-shell NWs. By systematically varying the thickness and phosphorus (P)-content of the InAsP shell, we demonstrate the ability to further tune the emission energy via large strain-induced peak shifts that already exceed >100 meV at comparatively low fractional P-contents. Increasing the P-content is found to give rise to additional line width broadening due to asymmetric shell growth generated by a unique transition from {110}- to {112}-sidewall growth as confirmed by cross-sectional scanning transmission electron microscopy. The results also elucidate the detrimental effects of plastic strain relaxation on the emission characteristics, particularly in core-shell structures with very high P-content and shell thickness. Overall, our findings highlight that enhanced mid-infrared emission efficiencies with effective carrier confinement and suppression of nonradiative recombination are highly sensitive to the quality of the InAs-InAsP core-shell interface.

Monte Carlo study of terahertz difference frequency generation in quantum cascade lasers.

We present an extended ensemble Monte Carlo approach, allowing for the self-consistent modeling of terahertz difference frequency generation in quantum cascade lasers. Our simulations are validated against available experimental data for a current room temperature design. Tera-hertz output powers in the mW range are predicted for ideal light extraction.

Feasibility study of Zeeman modulation spectrometry with a hollow capillary fiber based gas cell.

For paramagnetic gases (e.g., O2, NO, NO2, OH) Zeeman modulation spectrometry is a method for spectrometric gas sensing with extraordinary selectivity. In this Letter it is combined with a hollow capillary based gas cell, where the gas is filled in long light-guiding capillary that is placed inside a toroidal coil. Over conventional Zeeman spectrometry this has the advantage of lower power consumption at long optical path length, since several loops of the hollow capillary fiber can be placed in the coil. Compared to wavelength modulation spectrometry the advantage is insensitivity to interference by multimode propagation in the fiber and absorption by other nonparamagnetic gases, which should enhance both sensor stability and sensitivity. Experimental and theoretical results are presented, showing the feasibility of the approach.

Highly sensitive laser-based sensor for nanoparticles in air using a dual-ring-mirror setup.

One of the most frequently applied techniques to detect nanoparticles in air is analyzing laser light scattering. This technique is very flexible while offering high accuracy and reliability. Yet its functionality highly depends on the sensitivity of the measurement system components. Especially for miniaturized sensor devices with limited space, additional techniques are needed to preserve high intensity of scattered light. In our work we demonstrate a technique using two spherical ring mirrors to identify nanoparticles with diameters below 100 nm in a forward-scattering setup. We succeeded measuring polystyrene particles with diameters of 92 nm with a signal-to-noise-ratio of more than 10.

Low-level and ultralow-volume hollow waveguide based carbon monoxide sensor.

We demonstrate an ultralow sample volume optical carbon monoxide sensor with detection sensitivity of 180 parts in 10(9) (1σ at 1 Hz). The utilization of a 2.3 µm surface-emitting laser directly coupled to a 3 m hollow capillary fiber as the gas cell is proven to be a compact, sensitive, and cost-efficient gas sensing concept. By mechanical vibration of the fiber, an absorbance resolution of 10(-5) is achieved, which is comparable to single-reflective (double-pass) cells. An improvement of sensitivity over the conventional single-reflective cell is thus approximately linearly scaled with the enhancement of the optical path length, which is usually more than 1 order of magnitude.

Resolution limits of laser spectroscopic absorption measurements with hollow glass waveguides.

In this paper, resolution limits of laser spectroscopy absorption measurements with hollow capillary fibers are investigated. Furthermore, a concept of sensitive near-infrared sensing utilizing hollow fiber directly coupled with vertical-cavity surface-emitting lasers is developed. By performing wavelength modulation spectroscopy, the smallest absorbance that can be detected by the fiber sensor was determined to be 10(-4), limited by a random modulation of the fiber transmission function (modal noise). By mechanically vibrating the fiber, a sensor resolution of 10(-5) in absorbance is achieved. Because the random modulation on the fiber transmission function limits the detection sensitivity, its physical reasons are analyzed. One contribution is found to be the partial integration of the far field, and the amplitude of the spectral features is inversely proportional to the square root of the integrated speckle points number. Therefore, careful design of the fiber-detector outcoupling is necessary. It turned out that incoupling alignment is not of much influence with respect to the spectral background. The residual spectral background is caused by mode-dependent effects and can be lowered by vibrating the fiber mechanically.

22-Gb/s Long Wavelength VCSELs.

1.55-microm vertical cavity surface-emitting low-parasitic lasers show open eyes up to 22-Gb/s modulation speed. Uncooled error-free operation over a wide temperature range up to 85 degrees C under constant bias conditions is demonstrated at 12.5-Gb/s data rate. At these fixed bias conditions the laser characteristics are practically invariant with temperature. These are the highest data-rates reported from a long-wavelength VCSEL structure to date.

Diffraction loss in long-wavelength buried tunnel junction VCSELs analyzed with a hybrid coupled-cavity transfer-matrix model.

Intra-cavity diffraction in VCSELs is a loss mechanism that potentially can cause a significant decrease in efficiency and a rise in the threshold current, particularly in cavities with small lateral features with a high index contrast. One such VCSEL type is the 2.3 microm GaSb-based buried tunnel junction (BTJ) VCSEL studied in this work, where the BTJ induced topology of the top layers gives rise to excess loss through diffraction. Diffraction loss is difficult to measure, and also the numerical estimation must be done with care because of the non-axial propagation of the diffracted fields. We present a simulation method with spatially varying dimensionality, such that the field is three-dimensional (3D) in the entire cavity, whereas the material structure of the cavity is modelled in 3D near the BTJ and the layers with a varying topology, but elsewhere is assumed to be 1D like in a regular DBR structure. We find that the diffraction loss displays a non-monotonic behaviour as a function of the BTJ diameter, but as expected it rapidly increases below a certain diameter of the BTJ and may even become the dominant cause of loss in some device designs. We also show that the diffraction loss can be much reduced if the layers above the BTJ can be deposited such that the surface profile becomes smoother with increasing distance from the BTJ.

Temperature dependence of electrical characteristics of Pt/GaN Schottky diode fabricated by UHV e-beam evaporation.

Temperature-dependent electrical characterization of Pt/n-GaN Schottky barrier diodes prepared by ultra high vacuum evaporation has been done. Analysis has been made to determine the origin of the anomalous temperature dependence of the Schottky barrier height, the ideality factor, and the Richardson constant calculated from the I-V-T characteristics. Variable-temperature Hall effect measurements have been carried out to understand charge transport at low temperature. The modified activation energy plot from the barrier inhomogeneity model has given the value of 32.2 A/(cm2 K2) for the Richardson constant A** in the temperature range 200 to 380 K which is close to the known value of 26.4A/(cm2 K2) for n-type GaN.

Chemometric approach for improving VCSEL-based glucose predictions.

Optical methods are one of the painless and promising techniques that can be used for blood glucose predictions for diabetes patients. The use of thermally tunable vertical cavity surface-emitting lasers (VCSELs) as the light source to obtain blood absorption spectra, along with the multivariate technique partial least squares for analysis and glucose estimation, has been demonstrated. With further improvements by using data preprocessing and two VCSELs, we have achieved a clinically acceptable level in the physiological range in buffered solutions. The results of previous experiments conducted using white light showed that increasing the number of wavelength intervals used in the analysis improves the accuracy of prediction. The average prediction error, using absorption spectra from one VCSEL in aqueous solution, is about 1.2 mM. This error is reduced to 0.8 mM using absorption spectra from two VCSELs. This result confirms that increasing the number of VCSELs improves the accuracy of prediction.

Wavelength modulation spectroscopy with a widely tunable InP-based 2.3 microm vertical-cavity surface-emitting laser.

We report on the successful application of recently developed 2.3 microm InP-based vertical-cavity surface-emitting lasers with a buried tunnel junction in a wavelength modulation spectroscopy measurement for carbon monoxide (CO) detection. The electrically pumped devices operate at room temperature under cw operation with stable single-mode emission and should allow for parts in 10(6) (ppm) level resolution measurement of CO with a standard optical setup.