Emil A. Kadlec

Microwave and terahertz wave sensing with metamaterials

We have designed, fabricated, and characterized metamaterial enhanced bimaterial cantilever pixels for far-infrared detection. Local heating due to absorption from split ring resonators (SRRs) incorporated directly onto the cantilever pixels leads to mechanical deflection which is readily detected with visible light. Highly responsive pixels have been fabricated for detection at 95 GHz and 693 GHz, demonstrating the frequency agility of our technique. We have obtained single pixel responsivities as high as 16,500 V/W and noise equivalent powers of 10(-8) W/Hz(1/2) with these first-generation devices.

Phased-array sources based on nonlinear metamaterial nanocavities

Coherent superposition of light from subwavelength sources is an attractive prospect for the manipulation of the direction, shape and polarization of optical beams. This phenomenon constitutes the basis of phased arrays, commonly used at microwave and radio frequencies. Here we propose a new concept for phased-array sources at infrared frequencies based on metamaterial nanocavities coupled to a highly nonlinear semiconductor heterostructure. Optical pumping of the nanocavity induces a localized, phase-locked, nonlinear resonant polarization that acts as a source feed for a higher-order resonance of the nanocavity. Varying the nanocavity design enables the production of beams with arbitrary shape and polarization. As an example, we demonstrate two second harmonic phased-array sources that perform two optical functions at the second harmonic wavelength (∼5 μm): a beam splitter and a polarizing beam splitter. Proper design of the nanocavity and nonlinear heterostructure will enable such phased arrays to span most of the infrared spectrum.

Direct minority carrier transport characterization of InAs/InAsSb superlattice nBn photodetectors

We present an extensive characterization of the minority carrier transport properties in an nBn mid-wave infrared detector incorporating a Ga-free InAs/InAsSb type-II superlattice as the absorbing region. Using a modified electron beam induced current technique in conjunction with time-resolved photoluminescence, we were able to determine several important transport parameters of the absorber region in the device, which uses a barrier layer to reduce dark current. For a device at liquid He temperatures, we report a minority carrier diffusion length of 750 nm and a minority carrier lifetime of 200 ns, with a vertical diffusivity of 3 × 10−2 cm2/s. We also report on the devices optical response characteristics at 78 K.

Infrared rectification in a nanoantenna-coupled metal-oxide-semiconductor tunnel diode

Direct rectification of electromagnetic radiation is a well-established method for wireless power conversion in the microwave region of the spectrum, for which conversion efficiencies in excess of 84% have been demonstrated. Scaling to the infrared or optical part of the spectrum requires ultrafast rectification that can only be obtained by direct tunnelling. Many research groups have looked to plasmonics to overcome antenna-scaling limits and to increase the confinement. Recently, surface plasmons on heavily doped Si surfaces were investigated as a way of extending surface-mode confinement to the thermal infrared region. Here we combine a nanostructured metallic surface with a heavily doped Si infrared-reflective ground plane designed to confine infrared radiation in an active electronic direct-conversion device. The interplay of strong infrared photon-phonon coupling and electromagnetic confinement in nanoscale devices is demonstrated to have a large impact on ultrafast electronic tunnelling in metal-oxide-semiconductor (MOS) structures. Infrared dispersion of SiO2 near a longitudinal optical (LO) phonon mode gives large transverse-field confinement in a nanometre-scale oxide-tunnel gap as the wavelength-dependent permittivity changes from 1 to 0, which leads to enhanced electromagnetic fields at material interfaces and a rectified displacement current that provides a direct conversion of infrared radiation into electric current. The spectral and electrical signatures of the nanoantenna-coupled tunnel diodes are examined under broadband blackbody and quantum-cascade laser (QCL) illumination. In the region near the LO phonon resonance, we obtained a measured photoresponsivity of 2.7 mA W(-1) cm(-2) at -0.1 V.

Auger recombination in long-wave infrared InAs/InAsSb type-II superlattices

The Auger lifetime is a critical intrinsic parameter for infrared photodetectors as it determines the longest potential minority carrier lifetime and consequently the fundamental limitations to their performance. Here, Auger recombination is characterized in a long-wave infrared InAs/InAsSb type-II superlattice. Auger coefficients as small as 7.1×10−26 cm6/s are experimentally measured using carrier lifetime data at temperatures in the range of 20 K–80 K. The data are compared to Auger-1 coefficients predicted using a 14-band K·p electronic structure model and to coefficients calculated for HgCdTe of the same bandgap. The experimental superlattice Auger coefficients are found to be an order-of-magnitude smaller than HgCdTe.

Minority carrier lifetime and dark current measurements in mid-wavelength infrared InAs0.91Sb0.09 alloy nBn photodetectors

Carrier lifetime and dark current measurements are reported for a mid-wavelength infrared InAs0.91Sb0.09 alloy nBn photodetector. Minority carrier lifetimes are measured using a non-contact time-resolved microwave technique on unprocessed portions of the nBn wafer and the Auger recombination Bloch function parameter is determined to be |F1F2|=0.292. The measured lifetimes are also used to calculate the expected diffusion dark current of the nBn devices and are compared with the experimental dark current measured in processed photodetector pixels from the same wafer. Excellent agreement is found between the two, highlighting the important relationship between lifetimes and diffusion currents in nBn photodetectors.

Optical and electrical properties of narrow-bandgap infrared W-structure superlattices incorporating AlAs/AlSb/AlAs barrier layers

Optical and electrical properties of nBn photodetectors using InAs/AlAs/AlSb/AlAs/InAs/InAs0.61Sb0.39W-structure superlattice (W-SL) absorbers are reported. Minority carrier lifetimes of 500 ± 50 ns and 400 ± 30 ns, and Auger coefficients of 2.1 × 10−26 cm6/s and 1.6 × 10−25 cm6/s, for samples with bandgap energies of 5.3 μm (W-SL A) and 7.5 μm (W-SL B) are reported at 100 K, respectively. Shockley–Read–Hall defect states are identified at 65 meV and 45 meV above the W-SL valence band edges for W-SLs A and B, respectively. Dark currents are also reported and compared with diffusion currents calculated using the carrier lifetime data, suggesting low vertical heavy hole diffusivity.

Demonstration of long minority carrier lifetimes in very narrow bandgap ternary InAs/GaInSb superlattices

Minority carrier lifetimes in very long wavelength infrared (VLWIR) InAs/GaInSb superlattices (SLs) are reported using time-resolved microwave reflectance measurements. A strain-balanced ternary SL absorber layer of 47.0 A InAs/21.5 A Ga0.75In0.25Sb, corresponding to a bandgap of ∼50 meV, is found to have a minority carrier lifetime of 140 ± 20 ns at ∼18 K. This lifetime is extraordinarily long, when compared to lifetime values previously reported for other VLWIR SL detector materials. This enhancement is attributed to the strain-engineered ternary design, which offers a variety of epitaxial advantages and ultimately leads to a reduction of defect-mediated recombination centers.

Enhanced infrared detectors using resonant structures combined with thin type-II superlattice absorbers

We examined the spectral responsivity of a 1.77 μm thick type-II superlattice based long-wave infrared detector in combination with metallic nanoantennas. Coupling between the Fabry-Perot cavity formed by the semiconductor layer and the resonant nanoantennas on its surface enables spectral selectivity, while also increasing peak quantum efficiency to over 50%. Electromagnetic simulations reveal that this high responsivity is a direct result of field-enhancement in the absorber layer, enabling significant absorption in spite of the absorbers subwavelength thickness. Notably, thinning of the absorbing material could ultimately yield lower photodetector noise through a reduction in dark current while improving photocarrier collection efficiency. The temperature- and incident-angle-independent spectral response observed in these devices allows for operation over a wide range of temperatures and optical systems. This detector paradigm demonstrates potential benefits to device performance with applications thr...

Significantly enhanced carrier lifetimes of very long-wave infrared absorbers based on strained-layer InAs/GaInSb superlattices

Abstract. Significantly improved carrier lifetimes in very long-wave infrared (VLWIR) InAs/GaInSb superlattice (SL) absorbers are demonstrated using time-resolved microwave reflectance (TMR) measurements. A nominal 47.0  Å InAs/21.5  Å Ga0.75In0.25Sb SL structure that produces an ∼25  μm response at 10 K has a minority carrier lifetime of 140±20  ns at 18 K, which is an order-of-magnitude improvement compared with previously reported lifetime values for other VLWIR detector absorbers. This improvement is attributed to the strain-engineered ternary SL design, which offers a variety of epitaxial advantages and ultimately leads to the improvements in the minority carrier lifetime by mitigating defect-mediated Shockley–Read–Hall (SRH) recombination centers. By analyzing the temperature dependence of TMR decay data, the recombination mechanisms and trap states that currently limit the performance of this SL absorber were identified. The results show a general decrease in the long-decay lifetime component, which is dominated by SRH recombination at temperatures below ∼30  K and by Auger recombination at temperatures above ∼45  K. Since the strain-balanced ternary SL design offers a reasonably good absorption coefficient and many epitaxial advantages during growth, this VLWIR SL material system should be considered as a competitive candidate for VLWIR photodetector technology.