Infrared Regime Research Articles

Germanium metalens for longwave infrared applications

Metalenses represent a paradigm shift in optics, offering unprecedented control over light manipulation. This study focuses on the design optimization of a polarization-insensitive germanium (Ge) metalens operating in the longwave infrared (LWIR) regime. Employing rigorous coupled-wave analysis (RCWA) and finite-difference time-domain (FDTD) simulations, a metalens with 1mm focal length was designed using nanopillars with 3.5µm height and radius ranging from 0.55µm to 1.2µm. Then, the impact of lattice size and numerical aperture (NA) on lens performance was investigated. The results indicate that smaller lattices allow finer phase control and enhanced transmittance stability across the phase profile if significant coupling effects are not verified. As the NA increases, the focal spot size decreases, albeit with diminishing returns towards the diffraction limit. To the best of our knowledge, it is the first work that shows high focal efficiency (~80%) across multiple NA’s for a LWIR metalens with a diameter under 1100µm. The proposed metalens is compatible with complementary metal-oxide-semiconductor (CMOS) technology and supports low-cost manufacturing.

From Bulk to one-dimensional MoS2 nanochains: evolution of electronic, mechanical, and optical properties

We theoretically investigate the stability of a MoS2 nanochain, reporting its electronic, mechanical, and optical properties. The nanochain presents a semiconductor structure with a minute band gap of 67m eV compared to the larger gap bulk and monolayer structures. It is more malleable, enduring a maximum compressive (tensile) strain of 6% (6.5%). It is dynamically stable, showing no negative frequencies along its Brillouin zone (BZ) path. The nanochain is thermally stable at 300K, making it possible to synthesize as a freestanding structure. The optical properties of the bulk, monolayer, and 1D MoS2 materials are evaluated using the time-dependent density functional perturbation theory (TDDFPT) and compared to those determined via the independent particle approximation (IPA). Along the nanochain’s periodic x direction, the reflectivity retains a maximum value of ∼68% in the infrared (IR) region. Furthermore, its optical conductivity also exhibits a peak within the IR regime. These two features make such nanochains suitable as coating materials in applications involving infrared radiation or can even be exploited as conductive substrates in near-IR devices.

Swastika-shaped rotationally symmetric quadruple-structured near-infrared metamaterial absorber for absorbing solar energy

In this work, silicon dioxide based four-fold swastika shaped metamaterial nano structure absorber for solar radiation absorption in the near infrared (NIR) regime is proposed. The dimension of the unit cell is 648.50 × 648.50 nm2 which is designed on dielectric silicon dioxide and metallic conductor aluminum. The proposed structure is investigated by the finite-deference time domain (FDTD) method based software CST microwave studio. The metamaterial absorber (MMA) unit cell is simulated within the frequency range (200–398) THz that yielded dual resonance peaks at 260 THz and 378.0 THz with 95.50 % and 97.32 % of absorbance of solar energy, respectively. Additionally, the proposed nano design structure of unit cell was boosted up to attain the dual absorption band by engineering the design parameters. The electromagnetic response of the MMA was theoretically inspected and the excellent properties of the proposed designed absorber have promising applications in plasma-enhanced photovoltaic, optical absorption switching and infrared modulator optical communication.

Study of Magnetic, Optical, Electronic and Thermodynamic Effects in Thallium Rare Earth Disulphides (TlRES2, RE= Tb- Er)

In the present research work, we have investigated density of electronic states, electronic band structure, magnetic structure, optical spectra and temperature dependent thermodynamic characteristics of thallium rare earth sulphides, TlRES2 (RE: Tb-Er) using FP-LAPW method and PBE-GGA exchange correlation within DFT. The electronic structure pointed out that TlRES2 compounds show half metallic character. The magnetic moment and spin polarization calculations prove that TlRES2 are fully spin polarized compounds with ferromagnetic nature. Optical spectra show that intraband transitions occur in infrared (IR) regime due to half metallic character of TlRES2 and behave as opaque in ultraviolet (UV) region. High refractive index in IR regime also show metallic character and high peak in UV regime show slow propagation of light in UV regime. The reflection coefficient is found maximum (~50–60%) in UV regime. Greater energy loss has been seen on the higher energy side, which corresponds to the stimulation of plasmons by electrons moving through the compounds. Plasma resonances lead to the frequent peaks at ~ 6.0 eV, 11.0 eV, and 12.0 eV. According to calculations made using the quasi-harmonic Debye model, TlRES2 compounds are dimensionally less stable (i.e., much hard), have poor thermal conductivity, and experience faster-than-average disorder with increasing the temperature.

Long-wave bilayer graphene/HgCdTe based GBp type-II superlattice unipolar barrier infrared detector

Type-II superlattice (T2SL) material is used to design highly efficient next-generation infrared (IR) detectors. The flexibility of the material allows the integration of unipolar barrier IR detector architecture to suppress generation-recombination currents by blocking the majority charge carriers and improve device performance. This paper presents a self-powered T2SL unipolar barrier detector that operates in the long-wave IR regime. It comprises a contact layer made of p+-bilayer graphene (G) and a barrier layer made of p–-Hg1–xCdx=0.41Te (B) placed over a p+-Hg1–xCdx=0.2217Te absorber layer (p) and is known as the GBp detector. The optoelectronic characterization of the unipolar GBp detector is carried out using the Silvaco Atlas TCAD software. Under self-powered mode operation, the proposed GBp detector exhibits an improvement in photocurrent density of about 108 times. The dark current density of 0.4nA/cm2, the photocurrent responsivity of 7.82 A/W, the quantum efficiency of 91.44%, and the detectivity of 4.48 × 1014 cmHZ1/2/W are found at the liquid nitrogen temperature with a bias of –0.5 V. A dichroic ratio of 249 and a response time of less than 1 ps are likewise exhibited by the GBp detector. The substantial electric field at the contact/barrier heterointerface and the high barrier height in the conduction band, which result in an increased photodetection response, are attributed to this improved performance.

Dielectric chiral metasurfaces for enhanced circular dichroism spectroscopy at near infrared regime.

Numerous applications of chiro-optical effects can be found in nanophotonics, including imaging and spin-selective absorption, particularly in sensing for separating and detecting chiral enantiomers. Flat single-layer metasurfaces composed of chiral or achiral sub-wavelength structures offer unique properties to manipulate the light due to their extraordinary light-matter interaction. However, at optical wavelengths, the generation of strong chirality is found to be challenging via conventional chiral metasurface approaches. This work intends to design and optimize a dielectric chiral meta-nano-surface based on a diatomic design strategy to comprehend giant chiro-optical effects in the near-infrared (NIR) regime for potential application in circular dichroism (CD) spectroscopy. Instead of using a single chiral structure that limits the CD value at optical wavelengths, the proposed metasurface used a diatomic (two meta-atoms with distinct geometric parameters) chiral structure as a building block to significantly enhance the chiro-optical effect. Combining both meta-atoms in a single periodicity of the building block introduces constructive and destructive interferences to attain the maximum circular dichroism value exceeding 75%. Moreover, using multipolar resonance theory, the physics behind the generation of giant chiro-optical effects have also been investigated. The proposed dielectric chiral meta-platform based on the extra degree of freedom can find application in compact integrated optical setups for CD spectroscopy, enantiomer separation and detection, spin-dependent color filters, and beam splitters.

Ultraviolet supercontinuum generation driven by ionic coherence in a strong laser field

Supercontinuum (SC) light sources hold versatile applications in many fields ranging from imaging microscopic structural dynamics to achieving frequency comb metrology. Although such broadband light sources are readily accessible in the visible and near infrared regime, the ultraviolet (UV) extension of SC spectrum is still challenging. Here, we demonstrate that the joint contribution of strong field ionization and quantum resonance leads to the unexpected UV continuum radiation spanning the 100 nm bandwidth in molecular nitrogen ions. Quantum coherences in a bunch of ionic levels are found to be created by dynamic Stark-assisted multiphoton resonances following tunneling ionization. We show that the dynamical evolution of the coherence-enhanced polarization wave gives rise to laser-assisted continuum emission inside the laser field and free-induction decay after the laser field, which jointly contribute to the SC generation together with fifth harmonics. As proof of principle, we also show the application of the SC radiation in the absorption spectroscopy. This work offers an alternative scheme for constructing exotic SC sources, and opens up the territory of ionic quantum optics in the strong-field regime.

Flexible Assembled Metamaterials for Infrared and Microwave Camouflage

AbstractLight, heat, and waves in electromagnetic energy are the foundation for the advancement of human being. Camouflage materials based on metamaterials are used to excel the performance limits by manipulating the electromagnetic energy. However, multispectral camouflage materials with flexibility are difficult to fabricate because required radiative properties in each spectral regime are different and have largely different scales of the unit cell in a single structure. The authors propose flexible assembled metamaterials (FAM) by assembling the flexible infrared (IR) emitter and flexible microwave absorber. The authors adopt the intermediate layer to assemble the IR emitter and the microwave absorber, leading to securing the selective emission in IR wave measured by Fourier‐transform infrared spectroscopy (FT‐IR) measurement and broadening the wavelength of microwave absorption. They calculate the energy dissipation of accumulated energy due to the lowering the radiative energy in the IR regime similar to that of conventional camouflage materials. The wavelength having required absorption (>0.9) for microwave camouflage is wider 2–12 GHz than the conventional microwave absorber. FAM demonstrate multispectral camouflage performances through the decrease in the contrast radiant intensity for IR wave by 75% and the radar cross section for microwave by 99% compared to reference surfaces.

Germanium nanowire microbolometer

Near-infrared detection is widely used for nondestructive and non-contact inspections in various areas, including thermography, environmental and chemical analysis as well as food and medical diagnoses. Common room temperature bolometer-type infrared sensors are based on architectures in the μm range, limiting miniaturization for future highly integrated ‘More than Moore’ concepts. In this work, we present a first principle study on a highly scalable and CMOS compatible bolometer-type detector utilizing Ge nanowires as the thermal sensitive element. For this approach, we implemented the Ge nanowires on top of a low thermal conducting and highly absorptive membrane as a near infrared (IR) sensor element. We adopted a freestanding membrane coated with an impedance matched platinum absorber demonstrating wavelength independent absorptivity of 50% in the near to mid IR regime. The electrical characteristics of the device were measured depending on temperature and biasing conditions. A strong dependence of the resistance on the temperature was shown with a maximum temperature coefficient of resistance of −0.07 K−1 at T = 100 K. Heat transport simulations using COMSOL were used to optimize the responsivity and temporal response, which are in good agreement with the experimental results. Further, lock-in measurements were used to benchmark the bolometer device at room temperature with respect to detectivity and noise equivalent power. Finally, we demonstrated that by operating the bolometer with a network of parallel nanowires, both detectivity and noise equivalent power can be effectively improved.

A low-profile consolidated metastructure for multispectral signature management

This work pertains to the design, numerical investigation, and experimental demonstration of an optically transparent, lightweight, and conformable metastructure that exhibits multispectral signature management capabilities despite its extremely low-profile configuration. In comparison to the existing hierarchical approaches of designing multispectral stealth solutions, attention has been paid to accommodate the conflicting requirements of radar and infrared (IR) stealth using a single metasurface layer configuration, which required a few constraints to be incorporated during the design stage to ensure compatibility. This methodology promulgates the desired multispectral response with minimal manufacturing footprint and facilitates an efficient integration with the other existing countermeasure platforms. The resulting design exhibits a polarization-insensitive and incident angle stable broadband microwave absorption with at least 90% absorption ranging from 8.2 to 18.4 GHz. Concomitantly it also exhibits an averaged IR emissivity of 0.46 in the 8–14 µm long-wave IR regime, along with high optical transparency (71% transmission at 632.8 nm). Notably, the total thickness of the metastructure stands at 0.10 ( corresponds to the wavelength at lowest frequency). The metastructure has been fabricated with indium tin oxide coated polyethylene terephthalate sheets, on which the frequency selective pattern is machined using Excimer laser micromachining, and the performances are verified experimentally. Furthermore, a hybrid theoretical model has been developed that not only provides crucial insights into the operation of metastructure but also presents a methodical semi-analytical approach to design.

Inverted c-functions in thermal states

We first compute the effect of a chiral anomaly, charge, and a magnetic field on the entanglement entropy in mathcal{N} = 4 Super-Yang-Mills theory at strong coupling via holography. Depending on the width of the entanglement strip the entanglement entropy probes energy scales from the ultraviolet to the infrared energy regime of this quantum field theory (QFT) prepared in a given state. From the entanglement entropy, we compute holographic c-functions and demonstrate an inverted c-theorem for them. That is, these c-functions in generic thermal states monotonically increase towards the infrared (IR) energy regime. This is in contrast to the c-functions in vacuum states which decrease along the renormalization group flow towards the IR regime of a renormalizable QFT. Furthermore, in thermal states and in the IR limit, the c-functions behave thermally, growing proportionally to the value of the thermal entropy. The chiral anomaly affects the c-functions more in the IR regime, and its effect is peaked at an intermediate value of the magnetic field at a fixed chemical potential and temperature.

Optimal Design and Noise Analysis of High-Performance DBR-Integrated Lateral Germanium (Ge) Photodetectors for SWIR Applications

This work presents the high-performance Si/SiO2 distributed Bragg reflector (DBR)-integrated lateral germanium (Ge) p-i-n photodetectors (PDs) for atmospheric gas sensing and fiber-optic telecommunication networks in the short-wave infrared (SWIR) regime. In addition, this study also proposes an optoelectronic compact small-signal noise equivalent circuit model (SSNECM) of the designed device to compute the noise performance at the detectors’ output. Various figure-of merits including current under dark and illumination, responsivity, detectivity, bandwidth, and the noise of the proposed device are estimated at the room temperature (RT) for an incident optical power of 0.5 μW. Furthermore, the impact of width and height scaling on dark current, responsivity, and bandwidth are investigated to optimize the proposed device. The validation of the proposed model is done by comparing various parameters including dark current, responsivity, and detectivity of the designed device with other Ge PDs. The estimated results show the reduced trade-off between responsivity and bandwidth of the designed device. At λ=1550 nm, the proposed device achieves a high detectivity and SNR of >2×1011Jones and 120 dB (at 3 THz), respectively, with the bias voltage of -2V. These encouraging results pave the path for the future development of low-noise and high-speed detectors.

Third-order nonlinear properties and reverse absorption behavior in layered Ti3C2 MXene in the near infrared region

Two-dimensional transition metal carbides/nitrides (MXenes) have attracted a lot of attention as optical materials owing to their unique electronic and optical features. In this paper, we demonstrated the nonlinear optical response of Ti3C2 MXene in the near infrared (NIR) regime at 1 and 1.3 µm. The nonlinear optical absorption behavior was systematically studied by the open/closed aperture (OA/CA) Z-scan techniques. Under a low excitation intensity, the saturable absorption dominated the optical nonlinear process, and the maximum effective nonlinear absorption coefficients βeff were −0.59 cm/MW and −0.68 cm/MW at 1 and 1.3 µm, respectively. The nonlinear refractive index n2 and the third-order susceptibility of Ti3C2 Mxene at 1 µm were −2.18 ×10−2 cm2/GW and 3.65×10−3 esu, respectively. While with the excitation laser at 1.3 µm, the nonlinear refractive index n2 was −1.64×10−2 cm2/GW and the third-order susceptibility was 2.75×10−3 esu. With the increase of incident optical intensity, the two-photon absorption (TPA) process was observed, leading to the reverse absorption phenomenon. Our results confirmed that Ti3C2 MXene possessed intensity-depended nonlinear optical properties, which could be applied in various nonlinear optical devices.

Silicon‐Based Intermediate‐Band Infrared Photodetector Realized by Te Hyperdoping

Si-based photodetectors satisfy the criteria of low-cost and environmental-friendly, and can enable the development of on-chip complementary metal-oxide-semiconductor (CMOS)-compatible photonic systems. However, extending their room-temperature photoresponse into the mid-wavelength infrared (MWIR) regime remains challenging due to the intrinsic bandgap of Si. Here, we report on a comprehensive study of a room-temperature MWIR photodetector based on Si hyperdoped with Te. The demonstrated MWIR p-n photodiode exhibits a spectral photoresponse up to 5 {\mu}m and a slightly lower detector performance than the commercial devices in the wavelength range of 1.0-1.9 {\mu}m. We also investigate the correlation between the background noise and the sensitivity of the Te-hyperdoped Si photodiode, where the maximum room-temperature specific detectivity is found to be 3.2 x 10^12 cmHz^{1/2}W^{-1} and 9.2 x 10^8 cmHz^{1/2}W^{-1} at 1 {\mu}m and 1.55 {\mu}m, respectively. This work contributes to pave the way towards establishing a Si-based broadband infrared photonic system operating at room temperature.

GALACTICNUCLEUS: A high-angular-resolution JHKs imaging survey of the Galactic centre

Context. The extreme extinction (AV ~ 30 mag) and its variation on arc-second scales towards the Galactic centre hamper the study of its stars. Analysis of them is restricted to the near infrared (NIR) regime, where the extinction curve can be approximated by a broken power law for the JHKs bands. Therefore, it is fundamental to correct for extinction at these wavelengths in order to analyse the structure and stellar population of the central regions of our Galaxy. Aims. We aim to, (1) discuss different strategies to de-redden the photometry and check the usefulness of extinction maps to deal with variable stars; (2) build extinction maps for the NIR bands JHKs and make them publicly available; (3) create a de-reddened catalogue of the GALACTICNUCLEUS (GNS) survey, identifying foreground stars; and (4) perform a preliminary analysis of the de-reddened Ks luminosity functions (KLFs). Methods. We used photometry from the GNS survey to create extinction maps for the whole catalogue. We took red clump (RC) and red giant stars of similar brightnesses as a reference to build the maps and we de-reddened the GNS photometry. We also discussed the limitations of the process and analysed non-linear effects of the de-reddening. Results. We obtained high resolution (~3″) extinction maps with low statistical and systematics uncertainties (≲5%) and computed average extinctions for each of the regions covered by the GNS. We checked that our maps effectively correct the differential extinction reducing the spread of the RC features by a factor of ~2. We assessed the validity of the broken power law approach computing two equivalent extinction maps AH using either JH and HKs photometry for the same reference stars and obtained compatible average extinctions within the uncertainties. Finally, we analysed de-reddened KLFs for different lines of sight and found that the regions belonging to the NSD contain a homogeneous stellar population that is significantly different from that in the innermost bulge regions.

Polarization insensitive plasmonic stacked multilayer metasurface with deep nanohole cavity as multi-band absorber

A metal-dielectric stacked multilayer plasmonic metasurface containing square array of deep cylindrical nanohole cavity in layer-pairs is demonstrated to achieve polarization insensitive multiresonant near perfect absorption in the visible-near infrared (Vis-NIR) regime (500–1600 nm). Finite difference time domain (FDTD) computation is carried out to study the optical absorption properties. Four resonant absorption peaks with absorption strength in the range of 80–100% are observed. Studies show low wavelength (<900 nm) resonant absorptions due to surface plasmon polariton (SPP) excitation and cavity modes, and high wavelength (>900 nm) absorptions due to magnetic resonance confining magnetic near-fields at the dielectric region between two metal layers. A systematic increase in number of layer-pairs shows increase in peak absorption strength and blue-shifts in resonant wavelengths. Resonant condition and hence, optimized structure can be obtained by manipulating suitably the layer-pair thickness, cavity filling materials and unit cell period. It is observed that the proposed metasurface is completely insensitive to linearly, and left and right circularly polarized lights (LCP, RCP), and has large launch angle tolerance of ~25 degrees. Simplistic design, multi-band absorptions, polarization insensitivity and launch angle tolerance make the metasurface highly suitable for integrated optoelectronic devices for Vis-NIR detection.

Simulation of High-Efficiency Resonant-Cavity-Enhanced GeSn Single-Photon Avalanche Photodiodes for Sensing and Optical Quantum Applications

Two novel resonant-cavity-enhanced (RCE) GeSn single-photon avalanche photodiode (SPAD) detectors are designed and simulated for high-efficiency single-photon detection at 1550 and 2000 nm wavelength at room temperature for sensing and optical quantum applications. The RCE GeSn SPAD consists of a PIPIN GeSn/Si heterostructures embedded in an optical cavity formed by a distributed Bragg reflector (DBR) and GeSn surface. The results show that high photon absorption efficiency and avalanche triggering probabilities can be achieved by careful design of DBR reflectors, GeSn absorber, doping concentrations of Si charge sheet layer and multiplication layer, which lead to a high single-photon detection efficiency (SPDE) of ~80%, which is promising for emerging quantum applications demanding high SPDE, such as linear optical quantum computing. The noise equivalent power (NEP) and dark count rate (DCR) as a function of threading dislocations density (TDD) are examined as well. It is found that the device could operate near room temperature with a similar DCR level to that of Ge SPAD operating at low temperature. A NEP of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 3\times 10^{-15}$ </tex-math></inline-formula> W/Hz <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{1/2}$ </tex-math></inline-formula> is observed from RCE GeSn SPAD for 1550 nm wavelength at room temperature. This work shows that the proposed RCE GeSn SPADs are promising candidates for high-efficiency single-photon detection in short-wave infrared (SWIR) regime for sensing and optical quantum applications.

Tunable absorber embedded with GST mediums and trilayer graphene strip microheaters

Investigation was made of the optical response of metal-dielectric stacks-based cavity structures embedded with graphene microheaters for the purpose of perfect absorption. The absorber configuration exploits the Ge2Sb2Te5 (GST) phase changing medium, and the effects of different parametric and operational conditions on the absorption spectra were explored. The refractive indices of GST layers can be manipulated by the external electrical pulses applied to microheaters. The amplitude and duration of electrical pulses define the crystallinity ratio of the used GST mediums. The results revealed achieving perfect absorption (> 99%) in the visible and infrared (IR) regimes of the electromagnetic spectrum upon incorporating two thin GST layers of different thicknesses (in the stack) in the amorphous state. The proposed configuration showed the capability of introducing independent transition state (amorphous and/or crystalline) for each GST layer—the visible regime could be extended to the IR regime, and the perfect absorption peak in the IR regime could be broadened and red-shifted. It is expected that the structure would find potential applications in active photonic devices, infrared imaging, detectors and tunable absorbers.

Silicon‐Based Intermediate‐Band Infrared Photodetector Realized by Te Hyperdoping

AbstractSi‐based photodetectors satisfy the criteria of being low‐cost and environmentally friendly, and can enable the development of on‐chip complementary metal‐oxide‐semiconductor (CMOS)‐compatible photonic systems. However, extending their room‐temperature photoresponse into the mid‐wavelength infrared (MWIR) regime remains challenging due to the intrinsic bandgap of Si. Here, we report on a comprehensive study of a room‐temperature MWIR photodetector based on Si hyperdoped with Te. The demonstrated MWIR p‐n photodiode exhibits a spectral photoresponse up to 5 µm and a slightly lower detector performance than the commercial devices in the wavelength range of 1.0–1.9 µm. The correlation between the background noise and the sensitivity of the Te‐hyperdoped Si photodiode, where the maximum room‐temperature specific detectivity is found to be 3.2 × 1012 cmHz1/2 W−1 and 9.2 × 108 cmHz1/2 W−1 at 1 µm and 1.55 µm, respectively, is also investigated. This work contributes to pave the way towards establishing a Si‐based broadband infrared photonic system operating at room temperature.

Species-dependent tunneling ionization of weakly bound atoms in the short-wave infrared regime

We investigate the intensity- and species-dependent strong-field ionization of alkali metal atoms; sodium, potassium, rubidium and caesium; by intense, few-cycle laser pulses in the short-wave infrared (sw-IR) regime at 1800 nm. The low ionization potential, Ip, of these atoms allows us to scale the interaction and study the tunneling regime at sw-IR wavelengths using low intensities and pulse energies. Measurements of above-threshold ionization spectra in the alkali species exhibit distinct differences to rare gas spectra at 800 and 1800 nm. However, pairing the low ionization potential of these atoms with longer wavelengths results in the reemergence of some well-know features of nobel gas spectra in the visible, e.g., the plateau. Our focus lies on the comparison of high-energy rescattered electron yield among the different alkali species. The highly unfavorable plateau scaling known from rare gases at longer wavelengths is successfully circumvented by switching to low-Ip targets. In the investigated parameter range, we identify potassium as the most efficient rescatterer. In addition, this paves the way to a carrier-envelope phasemeter operating in the sw-IR/mid-wave IR regime, employing alkali metal atoms as a target.