mid-IR Frequency Research Articles

Tackling the focal shift effect for metalenses

We present a theoretical analysis aimed at comprehending and mitigating the focal shift phenomenon in planar dielectric metalenses. To conduct this analysis, we introduce metalens designs consisting of silicon and germanium nanoblocks on a calcium fluoride substrate, operating in the mid-IR frequency range. The lensing performance of these metalenses is investigated using the finite-difference time-domain method, and they operate at wavelengths of 3 and 4 µm with a polarization conversion efficiency close to unity. Our findings indicate a strong correlation between the focal shift phenomena on dielectric metalenses and the numerical aperture (NA), revealing that increasing the Fresnel number is not always an effective approach to minimizing the focal shift. In contrast to previous studies, we define a critical NA, independent of the lens size, where the focal shift reaches a minimum, resulting in a symmetric focal intensity distribution and ultimately yielding a better-performing metalens. We demonstrate that for NAs greater than the determined critical value, a positive focal shift is observed on planar metalenses, diverging from the conventional negative shift predicted by existing models. Additionally, we show that by selecting a metalens within a specific NA range and with smaller diameters, high focusing efficiencies can be achieved. The focusing efficiency of the studied metalenses is measured as high as 70%, marking one of the best values reported for the IR range to date. These results serve as a guide for improving the agreement between experimental and designed metalens features, enhancing their practical applications.

Deeply subwavelength mid-infrared phase retardation with α-MoO3 flakes

Phase retardation is a cornerstone of modern optics, yet, at mid-infrared (mid-IR) frequencies, it remains a major challenge due to the scarcity of simultaneously transparent and birefringent crystals. Most materials resonantly absorb due to lattice vibrations occurring at mid-IR frequencies, and natural birefringence is weak, calling for hundreds of microns to millimeters-thick phase retarders for sufficient polarization rotation. Here, we demonstrate mid-IR phase retardation with flakes of α-MoO3 that are more than ten times thinner than the operational wavelength, achieving 90 degrees polarization rotation within one micrometer of material. We report conversion ratios above 50% in reflection or transmission mode, and wavelength tunability by several micrometers. Our results showcase that exfoliated flakes of low-dimensional crystals can serve as a platform for mid-IR miniaturized integrated low-loss polarization control.

Tunable magnetless optical isolation with twisted Weyl semimetals

Abstract Weyl semimetals hold great promise in revolutionizing nonreciprocal optical components due to their unique topological properties. By exhibiting nonreciprocal magneto-optical effects without necessitating an external magnetic field, these materials offer remarkable miniaturization opportunities and reduced energy consumption. However, their intrinsic topological robustness poses a challenge for applications demanding tunability. In this work, we introduce an innovative approach to enhance the tunability of their response, utilizing multilayered configurations of twisted anisotropic Weyl semimetals. Our design enables controlled and reversible isolation by adjusting the twist angle between the anisotropic layers. When implemented in the Faraday geometry within the mid-IR frequency range, our design delivers impressive isolation, exceeding 50 dB, while maintaining a minimal insertion loss of just 0.33 dB. Moreover, the in-plane anisotropy of Weyl semimetals eliminates one or both polarizers of conventional isolator geometry, significantly reducing the overall dimensions. These results set the stage for creating highly adaptable, ultra-compact optical isolators that can propel the fields of integrated photonics and quantum technology applications to new heights.

Electro-Optic Sampling of a Free-Running Terahertz Quantum-Cascade-Laser Frequency Comb

Quantum-cascade-laser (QCL) frequency combs are compact semiconductor light sources operating in the mid-IR and terahertz frequencies. Achieving subpicosecond laser pulses with high peak power is of vital importance for performing nonlinear time-resolved spectroscopy as well for exploring nonlinear phenomena. Therefore, investigation and characterization of time-resolved free-running laser emission is a key for further improvement and optimization of these intersubband devices. In this work, we demonstrate a direct electric field measurement of a free-running terahertz QCL frequency comb using electro-optic sampling in combination with computational phase correction, where we retrieve the electric field profile and access the comb parameters. The demonstrated method is of high interest for time-resolved low-power laser emission characterization, typical for ring terahertz QCL frequency combs and investigation of phase-compensated emission via waveguide dispersion engineering for broadband comb operation.

Highly coherent phase-lock of an 8.1 μm quantum cascade laser to a turn-key mid-IR frequency comb

A continuous-wave Fabry–Pérot quantum cascade laser (QCL) emitting at 8.1 μm operating in the single mode regime has been coherently phase locked to a turn-key low-noise commercial mid-Infrared (mid-IR) optical frequency comb. The stability of the comb used as a reference is transferred to the QCL resulting in an integrated residual phase error of 0.4 rad. The laser linewidth is narrowed by more than two orders of magnitude reaching sub-kHz level at 1 ms observation time, limited by the spectral purity of the mid-IR comb. Our experiment is an important step toward the development of both powerful and metrology-grade QCLs and fully stabilized QCL frequency comb and opens perspectives for precision measurements and frequency metrology in the mid-IR.

Dual-Band Coupling of Phonon and Surface Plasmon Polaritons with Vibrational and Electronic Excitations in Molecules

Strong coupling (SC) between light and matter excitations bears intriguing potential for manipulating material properties. Typically, SC has been achieved between mid-infrared (mid-IR) light and molecular vibrations or between visible light and excitons. However, simultaneously achieving SC in both frequency bands remains unexplored. Here, we introduce polaritonic nanoresonators (formed by h-BN layers on Al ribbons) hosting surface plasmon polaritons (SPPs) at visible frequencies and phonon polaritons (PhPs) at mid-IR frequencies, which simultaneously couple to excitons and molecular vibrations in an adjacent layer of CoPc molecules, respectively. Employing near-field optical nanoscopy, we demonstrate the colocalization of near fields at both visible and mid-IR frequencies. Far-field transmission spectroscopy of the nanoresonator structure covered with a layer of CoPc molecules shows clear mode splittings in both frequency ranges, revealing simultaneous SPP-exciton and PhP-vibron coupling. Dual-band SC may offer potential for manipulating coupling between exciton and molecular vibration in future optoelectronics, nanophotonics, and quantum information applications.

Twist-tunable polaritonic nanoresonators in a van der Waals crystal

Optical nanoresonators are key building blocks in various nanotechnological applications (e.g., spectroscopy) due to their ability to effectively confine light at the nanoscale. Recently, nanoresonators based on phonon polaritons (PhPs)—light coupled to lattice vibrations—in polar crystals (e.g., SiC, or h-BN) have attracted much attention due to their strong field confinement, high quality factors, and their potential to enhance the photonic density of states at mid-infrared (mid-IR) frequencies, where numerous molecular vibrations reside. Here, we introduce a new class of mid-IR nanoresonators that not only exhibit the extraordinary properties previously reported, but also incorporate a new degree of freedom: twist tuning, i.e., the possibility of controlling their spectral response by simply rotating the constituent material. To achieve this result, we place a pristine slab of the van der Waals (vdW) α-MoO3 crystal, which supports in-plane hyperbolic PhPs, on an array of metallic ribbons. This sample design based on electromagnetic engineering, not only allows the definition of α-MoO3 nanoresonators with low losses (quality factors, Q, up to 200), but also enables a broad spectral tuning of the polaritonic resonances (up to 32 cm−1, i.e., up to ~6 times their full width at half maximum, FWHM ~5 cm−1) by a simple in-plane rotation of the same slab (from 0 to 45°). These results open the door to the development of tunable and low-loss IR nanotechnologies, fundamental requirements for their implementation in molecular sensing, emission or photodetection applications.

Enhanced far-field coherent thermal emission using mid-infrared bilayer metasurfaces.

A classical thermal source, such as an incandescent filament, radiates according to Planck's law. The feasibility of super-Planckian radiation has been investigated with sub-wavelength-sized sources in the last decade. In such sources, a crystal-dependent coupling of photons and optical phonons is possible at thermal energies corresponding to that at room temperature. This interaction can be used to tailor the far-field thermal emission in a coherent manner; however, understanding heat transfer during this process is still nascent. Here, we used a novel measurement platform to quantify thermal signals in a Ge2Sb2Te5/SiO2 nanoribbon structure. We were able to separate and quantify the radiated and conducted heat transfer mechanisms. The thermal emission from the Ge2Sb2Te5/SiO2 nanoribbons was enhanced by 3.5× compared to that of a bare SiO2 nanoribbon. Our model revealed that this enhancement was directly due to polaritonic heat transfer, which was possible due to the large and lossless dielectric permittivity of Ge2Sb2Te5 at mid-IR frequencies. This study directly probes the far-field emission with a thermal gradient stimulated by Joule heating in temperature ranges from 100 to 400 K, which bridges the gap between mid-IR optics and thermal engineering.

Mid-IR frequency laser control with a comb for H2+ spectroscopy

We will report our recent progress toward H2+ spectroscopy by use of a SI-referenced Mid-IR source laser. H2+ molecular ions are very interesting candidates to improve the determination of fundamental constants, such as the proton to electron mass ratio mp/me and search for new physics beyond the standard model. At LKB, an erbium fibered frequency comb is phase locked to the LNE-SYRTE frequency standards thanks to the T-REFIMEVE network. By sum frequency generation in a AgGaSe2 crystal between a CO2 laser and an output of the comb at 1895 nm, a shifted frequency comb centered at 1560 nm is generated. The latter is then mixed with the original one to generate a beatnote used to stabilise the Mid-IR laser. As a first application, a narrow saturated absorption line in formic acid has been extensively studied. Pressure, power and modulation depth shifts and broadenings have been evaluated, leading to a determination of its central frequency at a sub ppt (10-12) resolution, high enough for H2+ spectroscopy and fundamental constant determination.

New potential candidates for astronomical searches discovered in the electrical discharge of the PAH naphthalene and acetonitrile

The formation and dissociation mechanisms of polycyclic aromatic hydrocarbons (PAHs) as well as their reactivity with other interstellar molecules are elusive. In this work, we have investigated the electrical discharge chemistry of the PAH naphthalene and acetonitrile, a molecule known to be present in interstellar environments, using a combination of mass-selective IR-UV ion dip spectroscopy with the free electron laser FELIX in the mid-IR frequency region (550 – 1800 cm−1) and quantum chemical calculations. In addition to the species known to be produced in the electrical discharge of pure naphthalene, –CH3 and –CN substituted unsaturated hydrocarbons have been identified. Most of them, in particular those containing a nitrogen atom in the molecular framework, such as 7H-benzo[7]annulenecarbonitrile, have a substantial dipole moment and, therefore, can be considered as potential candidates for radio astronomical searches. Among the species observed, the two isomers 1- and 2-cyanonaphthalene, which have been recently detected in the TMC-1, have been identified in our experiment, thus continuing to highlight the use of electrical discharge sources as a valuable tool to produce astronomically relevant species.

Active Graphene Plasmonic Tweezers: Size Based Nanoparticle Trapping and Sorting

In this article, a novel graphene based Optical tweezer (OT) and size based particle sorter is proposed and numerically investigated by finite difference time domain (FDTD) method and Maxwells stress tensor (MST) analysis. Each unit cell of the structure is composed of a graphene layer on top a metallic nano-ring which is embedded in SiO2. Strong hotspots with field enhancements reaching values as high as 140 are obtained inside the rings with no need to sharp edges conventional in plasmonic structures. Size based trapping can be achieved by controlling the fermi energy of the graphene through a gate. By cascading several unit cells, each operating at a different gate voltage, a particle size based sorter is designed which can effectively sort Polystyrene particles with their diameter in the range of smaller than 10 nm to 100 nm. The structure is illuminated by a laser beam in the mid-IR frequency range of 4-8 m and its wavelength and intensity are kept constant even for sorting functionality. Instead, tunable plasmonic characteristics of graphene is utilized for active particle sorting. The fermi level-particle size-gate voltage relationship of the proposed OT is fully investigated at the article. Moreover, it is shown that the proposed structure is capable of sorting particles according to their refractive indices as well.

Mid-infrared cylindrical vector beams enabled by dielectric metasurfaces

Over the last decade, photonics in the mid-infrared (mid-IR) frequency range had major advances in both generation and detection of light. However, efficient manipulation of the mid-IR light still faces many challenges. Spatially inhomogeneous control over the wavefront and polarization of mid-IR radiation is particularly difficult. Many standard techniques used for visible and near-infrared frequencies, such as liquid crystal-based spatial light modulation, are not applicable in the mid-IR due to unfavorable material properties in that spectral range. Here, we demonstrate spatially inhomogeneous polarization control of the mid-infrared light using custom-designed vortex retarders. Vortex retarders, while being widely used in the near-infrared and visible spectral ranges for generation of cylindrical vector beams, have been missing in the mid-infrared spectral range. Our implementation of mid-infrared vortex retarders is based on the concept of metasurfaces. We demonstrate metasurface-based vortex retarders at the wavelengths of 2.9 and 3.5 μm. We compare the performance of all-dielectric metasurface vortex retarders with the elements arranged in square and hexagonal lattices [Arbabi et al., Nat. Nanotechnol. 10, 937–943 (2015) and Arbabi et al., Nat. Commun. 6, 7069 (2015)]. Our work could accelerate the adoption of metasurfaces for the development of novel classes of mid-infrared optical components.

Chalcogenide Glass Microfibers for Mid-Infrared Optics

With diameters close to the wavelength of the guided light, optical microfibers (MFs) can guide light with tight optical confinement, strong evanescent fields and manageable waveguide dispersion and have been widely investigated in the past decades for a variety of applications. Compared to silica MFs, which are ideal for working in visible and near-infrared regions, chalcogenide glass (ChG) MFs are promising for mid-infrared (mid-IR) optics, owing to their easy fabrication, broad-band transparency and high nonlinearity, and have been attracting increasing attention in applications ranging from near-field coupling and molecular sensing to nonlinear optics. Here, we review this emerging field, mainly based on its progress in the last decade. Starting from the high-temperature taper drawing technique for MF fabrication, we introduce basic mid-IR waveguiding properties of typical ChG MFs made of As2S3 and As2Se3. Then, we focus on ChG-MF-based passive optical devices, including optical couplers, resonators and gratings and active and nonlinear applications of ChG MFs for mid-IR Raman lasers, frequency combs and supercontinuum (SC) generation. MF-based spectroscopy and chemical/biological sensors are also introduced. Finally, we conclude the review with a brief summary and an outlook on future challenges and opportunities of ChG MFs.

Mid-infrared sensor based on resonance excitation of graphene plasmon polariton-coupled Bloch surface modes at the interface of anisotropically truncated one-dimensional ternary photonic crystal

In this article, we have theoretically demonstrated the sensor performance in mid-IR frequency domain by resonance excitation of Bloch surface modes through energy coupling with graphene surface plasmon polaritons in truncated one-dimensional (1D) ternary photonic crystal (TPC) under Kretschmann configuration coupling technique. The involvement of optical anisotropy via graphene monolayer enhances light-matter interaction thus enabling excellent control over the resonance wavelength (RW) of the excited Bloch surface modes (EBSMs). For incident transverse magnetic-polarized electromagnetic waves, the band structure, reflection spectra and field profile are computed using 2 × 2 transfer matrix method. The control over the RW has been established by examining the effect of variation of different model parameters like thickness of truncation layer, refractive index of external medium, temperature, chemical potential and the periodicity of TPC. The variations in periodicity result in the expansion of the range of confinement of EBSMs in photonic band gaps from 19 THz to 65 THz . We proved that the variation in temperature between 300 ∘ K and 500 ∘ K confines the EBSMs with high coupling efficiency (CE). Also, at room temperature the best achievable quality factor (QF) is 1421 and a high Q.F. of 8170 is attained at 260 ∘ K .

Dual-band absorbers based on multiplexed gratings at mid-infrared frequencies

We design a dual-band absorber based on metal multiplexed gratings at mid-infrared (mid-IR) frequencies. The metal multiplexed gratings are realized with two constitutive gratings and separated from a metallic ground plate by a thick dielectric spacer in order to construct the metal–insulator–metal architecture. Simulations indicate that two different absorption frequencies from the excitation of surface plasmon–polariton modes can be achieved under normal incidence, due to the different field distributions of each resonance node numbers in supercell. The doublet resonance positions can be independently and arbitrarily adjusted by varying the constitutive grating periods. We also confirmed that the absorption performance of the resonance peak is related to the non-uniform distribution level of the corresponding constitutive grating ridge. The designed absorber can be easily fabricated with planar processing technology and used in many applications where multispectral control of mid-IR signals is required.

Ultrafast intraband Auger process in self-doped colloidal quantum dots

Summary Investigating the separate dynamics of electrons and holes has been challenging, although it is critical for the fundamental understanding of semiconducting nanomaterials. n-Type self-doped colloidal quantum dots (CQDs) with excess electrons occupying the low-lying state in the conduction band (CB) have attracted a great deal of attention because of not only their potential applications to infrared optoelectronics but also their intrinsic system that offers a platform for investigating electron dynamics without elusive contributions from holes in the valence band. Here, we show an unprecedented ultrafast intraband Auger process, electron relaxation between spin-orbit coupling states, and exciton-to-ligand vibrational energy transfer process that all occur exclusively in the CB of the self-doped β-HgS CQDs. The electron dynamics obtained by femtosecond mid-infrared spectroscopy will pave the way for further understanding of the blinking phenomenon, disproportionate charging in light-emitting diodes, and hot electron dynamics in higher quantum states coupled to surface states of CQDs.

Broadband Infrared Spectroscopy of Molecules in Solutions with Two Intrapulse Difference-Frequency-Generated Mid-Infrared Frequency Combs.

Mid-infrared (mid-IR) spectroscopy is an incisive tool for studying structures and dynamics of complicated molecules in condensed phases. Developing a compact and broadband mid-IR spectrometer has thus been a long-standing challenge. Here, we show that a highly coherent and broadband mid-IR frequency comb can be generated by using an intrapulse difference-frequency-generation with a train of pulses from a few-cycle pulse Ti:sapphire oscillator. By tightly focusing the oscillator output beam into a single-pass, fan-out-type periodically poled lithium niobate crystal and tilting the orientation of the crystal, we show that a mid-IR frequency comb with more than an octave spectral bandwidth from 1550 cm-1 (46 THz) to 3650 cm-1 (110 THz) and vanishing carrier-envelope-offset phase can be generated. Using two coherent mid-IR frequency combs with different repetition frequencies, we demonstrate that a broadband mid-IR dual-frequency comb spectroscopy of aromatic compounds or amino acids in solutions is feasible. We thus anticipate that researchers will find our mid-IR frequency combs useful for developing ultrafast and broadband linear and nonlinear IR spectroscopy of chemically reactive or biologically important molecules in condensed phases.

Seven-octave high-brightness and carrier-envelope-phase-stable light source

High-brightness sources of coherent and few-cycle-duration light waveforms with spectral coverage from the UV to the THz would offer unprecedented versatility and opportunities for a spectacular range of applications from bio-chemical sensing, to time-resolved and nonlinear spectroscopy, to attosecond light-wave electronics. Combinations of various sources with frequency conversion and supercontinuum generation can provide relatively large spectral coverage, but many applications require much broader spectral range and low-jitter synchronization for time-domain measurements. Here, we present a carrier-envelope-phase stable light source, seeded by a mid-IR frequency comb, with simultaneous spectral coverage across 7 optical octaves, from the UV (340 nm) into the THz (40,000 nm). Combining soliton self-compression and dispersive wave generation in an anti-resonant-reflection photonic crystal fibre with intra-pulse difference frequency generation in BaGa2GeSe6, the spectral brightness is 2-5 orders of magnitude above synchrotron sources. This enables high-dynamic-range spectroscopies and provides enticing prospects for attosecond physics and material sciences.

Mid-infrared frequency doubling using strip-loaded silicon nitride on epitaxial barium titanate thin film waveguides.

Broadband mid-infrared (mid-IR) frequency doubling was demonstrated using nonlinear barium titanate (BTO) thin films. The device has a strip-loaded waveguide structure consisting of top silicon nitride (SiN) strips and an underneath BTO guiding layer. The epitaxial BTO was deposited on a strontium titanate (STO) substrate by pulsed-laser deposition. Through a SiN grating coupler, the pumping mid-IR light at wavelength λ=3.30-3.45µm was coupled into the nonlinear BTO layer, where the spectrum of the near-infrared (NIR) second-harmonic generation was characterized. The developed BTO waveguides provide a platform for mid-IR nonlinear integrated photonics and on-chip quantum optics.

Electrically tunable optical devices based on graphene-split-ring-resonator periodic multilayers at mid-infrared frequencies

In this work, we demonstrate that photonic crystals made with alternating layers of graphene and nanometer-scale split-ring-resonator metamaterials can be treated as single negative-index materials with broad zero-ϕeff gaps in the mid-IR frequency range. These provide a versatile platform for the fabrication of anti-interference multichannel filters whose tunability can be realized flexibly by adjusting the conductivity of the graphene layer. Specifically, by inserting dielectric defects into the periodic system, one can obtain tunable tunneling modes inside the zero-ϕeff gap, which are highly robust against scaling and structural disorder. Moreover, without altering the structure of the system, the number of defect modes multiplies as the graphene chemical potential is decreased appropriately. In addition, the tunneling modes are nearly invariant with the incident angle in the range of 0°–5°. Also, the bandwidths of the tunneling modes are compressed by decreasing the chemical potential, which could be utilized to improve the Q values of the filters. Furthermore, THz amplification could also be accomplished when taking into account the damping constant of the permittivity of graphene. As a result, these characteristics may facilitate the design of optical devices in the mid-IR range, especially leading to more practical applications of these filters.