Mid-infrared Frequency Research Articles

2 μm soliton lasers in a bidirectional nonlinear polarization evolution Tm3+-doped fiber oscillator

We experimentally demonstrate a novel 2 μm soliton laser using a bidirectional nonlinear polarization evolution mode-locked Tm3+-doped fiber oscillator. The mode-locking operations between the laser signals propagating in clockwise and counterclockwise directions can be achieved through the intracavity polarization state adjustments independently, and the maximum achieved output powers are 332.5 and 367.8 mW with transfer efficiencies of 17.2 % and 18.3 %, respectively. Hysteresis in the output powers and spectra transformation between the two laser signals propagating in counter directions are observed and investigations are conducted in detail. Benefiting from the robust birefringence filtering effect, the central wavelength of this soliton laser can be continuously tuned from 1951.51 to 2015.89 nm. Stable high-order harmonic mode-locking (HML) operations are observed, and up to 633.02 MHz repetition rate with a central wavelength of 1992.55 nm is achieved. The monitored intensity spectra stability and average output power fluctuations exhibit excellent performance of this proposed 2 μm soliton fiber laser. The phase noise levels in two counter directions propagating light signals are characterized at different average output powers. Our experiment provides a potential technique for the generation of 2 μm solitons fiber laser that can be widely applied in ultrafast time-resolved measurements, mid-infrared frequency metrology, molecular spectroscopy, etc.

Phase-controllable multi-laser system with coaxially combined near-infrared and subcycle mid-infrared and THz pulsed beams

With the progress of infrared high-intensity pulse technology, the ultrafast control of electronic states has been attracting considerable attention. Most elementary excitations of solids have resonance energy in the mid-infrared (MIR) frequency domain. Therefore, to understand these dynamics in detail and open up new possibilities, new technological innovations are desired. In this study, we have developed a system that combines 8 fs near-infrared pulsed light with carrier envelope phase-controllable subcycle MIR and THz pulsed lights. This technology has produced a new method that enables the analysis of phase-sensitive phenomena in the near-infrared to far-infrared region.

High-performance dielectric nano-cavities for near- and mid-infrared frequency applications

We present a judicious design approach for optimizing semiconductor nanocavities, starting from single photonic atoms to build photonic molecules functioning as high-performance nanocavities. This design approach is based on exact analytical solutions to the Maxwell equations for collective Mie resonances. Conceptually, we distinguish different concepts of cavity modes including Mie mode, collective Mie mode, photonic-crystal (PC) band-edge mode, and Feshbach-type bound states in the continuum (BIC) mode. Using the design approach, we present a unique structure of nanocavity supporting the Feshbach-type BIC mode, capable of enhancing the emission rate of a dipolar emitter by orders of magnitude. This high-performance nanocavity suppresses radiative loss channels strongly via destructive interference and consequently channels the emission light efficiently into an in-plane bi-directional beam with a divergence angle of 10°. Engineering the geometrical parameters of the nanocavity for near-infrared frequency applications requires a fabrication tolerance of ±5 nm. This high accuracy is challenging for the mass production of devices. The fabrication accuracy can be relaxed greatly for mid-infrared frequency devices. As a showcase, we analyze and optimize the well-known PC L3 defect nanocavity for mid-infrared frequency applications in the framework of Feshbach resonance. We show that the optimal structure of this defect nanocavity requires a fabrication tolerance of ±50 nm. Our nanocavity design approach may be useful for near- and mid-infrared frequency applications.

Midinfrared Compressed Fourier-Transform Spectroscopy with an Optical Frequency Comb

Broadband precision spectroscopy is an invaluable tool for high-fidelity species diagnostics as well as multispecies sensing in complex real-world environments. However, concurrent demands for broad spectral coverage and high spectral resolution present great challenges to both optical measurement systems and data acquisition and processing. We report methodological and experimental demonstration of a midinfrared direct optical frequency comb (OFC) spectrometer, using Fourier-transform spectroscopy (FTS) with a compressed sensing (CS) algorithm. Efficient data sampling and accurate spectral recovery are demonstrated with comb-teeth-resolving resolution of $0.0083\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ (250 MHz) over a spectral range of $300\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. The ${v}_{3}$ band of ${\mathrm{CH}}_{4}$ is measured, and validated against conventional FTS results and theoretical calculations. Studies on the effect of compression ratio show that a compressed interferogram using as few as 2% of the regular sample points can effectively recover the high-resolution broadband spectrum with an accuracy of 1.9%. The developed diagnostic system bears promising potential for accurate, real-time, multispecies sensing that is pressingly needed for studying the dynamics of complex multicomponent systems.

High‐Power Mid‐IR Few‐Cycle Frequency Comb from Quadratic Solitons in an Optical Parametric Oscillator

AbstractPowerful and efficient optical frequency combs in the mid‐infrared (MIR) spectral region are highly desirable for a broad range of applications. Despite extensive efforts utilizing various techniques, MIR frequency comb sources are still lacking power, efficiency, or bandwidth for many applications. Here, the generation of an intrinsically locked frequency comb source centered at 4.18 µm from an optical parametric oscillator (OPO) operating in the simulton regime is reported, in which formation of purely quadratic solitons leads to enhanced performance. Advantages of operation in the simulton regime in direct experimental comparisons to the conventional regime are shown, which are also supported by simulation and theory. 565 mW of average power, 900 nm of instantaneous 3 dB bandwidth, 350% slope efficiency, and 44% conversion efficiency are achieved, a performance that is superior to previous OPO demonstrations and other sources in this wavelength range. Here, a new avenue toward MIR frequency comb generation with high power and efficiency is opened, and the great potential of soliton generation based on quadratic nonlinearity in the MIR spectral region is suggested.

Parametrically Tunable Mid-Infrared Resonant Radiation by Four-Wave Mixing in Silicon Nitride Nanophotonic Waveguides

The direct generation of mid-infrared light is of significant interest for various applications including spectroscopy, detection system, optical sensing and biophotonics. Ultrashort-pulse nonlinear interaction can be viewed as four-wave mixing process, which can generate new spectral components under appropriate conditions. Here, we numerically demonstrate a fully, yet effective, spectral conversion scheme where the effective portion and components of femtosecond pulses can access the mid-infrared region directly by four-wave mixing process in silicon nitride integrated photonics waveguide. Wideband conversions occur mainly towards the mid-infrared region around the traditional red-shift phase-matching point without the adjustment of waveguide geometry. The corresponding conversion mechanisms can be attributed to nonlinear interaction of optical waves, resulting in the dispersive wave generation, the soliton-probe collision and the phase-sensitive scattering process. Our results provide an alternative route to realize wideband tunable mid-infrared frequency conversion in silicon nitride system on a chip.

Mid-infrared broadband optical frequency comb generated in MgF2 resonators

Microresonator-based optical frequency combs are broadband light sources consisting of equally spaced and coherent narrow lines, which are extremely promising for applications in molecular spectroscopy and sensing in the mid-infrared (MIR) spectral region. There are still great challenges in exploring how to improve materials for microresonator fabrication, extend spectral bandwidth of parametric combs, and realize fully stabilized soliton MIR frequency combs. Here, we present an effective scheme for broadband MIR optical frequency comb generation in a MgF 2 crystalline microresonator pumped by the quantum cascade laser. The spectral evolution dynamics of the MIR Kerr frequency comb is numerically investigated, revealing the formation mechanism of the microresonator soliton comb via scanning the pump-resonance detuning. We also experimentally implement the modulation instability state MIR frequency comb generation in MgF 2 resonators covering from 3380 nm to 7760 nm. This work proceeds microresonator-based comb technology toward a miniaturization MIR spectroscopic device that provides potential opportunities in many fields such as fundamental physics and metrology.

Performance evaluation of silicon-chip-based mid-infrared Kerr optical frequency combs with ridge cross section

We present a complementary metal-oxide-semiconductor (CMOS) compatible platform for on-chip frequency comb generation in the mid-infrared region based on a silicon-on-insulator (SOI) microring resonator with ridge cross section. Flat dispersion tailoring is performed with dispersion variation of 4.04 × 10−6 ps/nm/km by adjusting the geometry parameter and the low-loss SOI microring resonator with total quality factor (Q) up to 106 can be realized at wavelengths from 3.3 to 3.6 µm. Furthermore, the thresholdless frequency combs consisting of 50 comb lines spanning from 3.1 to 4.0 µm (over 900 nm) can be realized using SOI microring resonator with 50 mW pump power. Besides, the study shows that the frequency interval of the comb is related to the selection of the dual-pumped wavelength. The influences of the coupling coefficient and the radius of microring on the bandwidth of mid-infrared OFC are also investigated numerically which shows that remarkable enhancement of mid-infrared OFC bandwidth can reach 449 nm when the coupling coefficient varies from 0.012 to 0.05. This research work is instructive for realizing highly integrated photonics and achieving the experimental generation of mid-infrared optical frequency combs, which could enable substantial progress in spectroscopy applications.

Study on linewidth compression at 3.8 μm with multiple F-P etalon pumped by compound intra-cavity optical parametric oscillator

A compact, multiple F-P etalon and narrow linewidth intra-cavity single-resonant mid-infrared optical parametric oscillator based on MgO:PPLN is reported, which is composed of a 1064 nm gain cavity and a mid-infrared frequency conversion cavity. The narrow linewidth at 3.8 μm can be obtained indirectly through the linewidth compression at the fundamental wavelength and the signal wavelength. The linewidth at 3.8 μm is 0.73 nm without adjacent mode when the fundamental wavelength is acted by the single F-P etalon with the thickness of 2.5 mm and the signal wavelength is acted by the dual F-P etalon with the thickness of 2.5 mm and 0.5 mm. Meanwhile, the output power of 1.06 W is obtained, corresponding to the conversion efficiency of 2.41%, the pulse width of 11.17 ns is obtained synchronously, and the beam quality factor M2 at 3.8 μm are Mx2 = 1.92 and My2 = 1.87, respectively. It provides a method for intra-cavity pumped mid-infrared linewidth compression.

Controllable photonic spin hall effect of bilayer graphene

Bilayer graphene, composed of two layers of monolayer graphene in AB stacking order, has emerged as an alternative platform for atomically thin plasmonic and optoelectronic devices. However, its behavior of photonic spin hall effect remains largely unexplored. In this work, we have theoretically observed that bilayer graphene has two obvious discontinuities but monolayer graphene only has a single step in the spectra of the spin shifts as a function of wavelength at the Brewster angle over the midinfrared frequency range, which enables a possible route of distinguishing monolayer graphene and bilayer graphene. Additionally, the magnitudes and positions of the peak and valley values in the spectrum of spin shifts of bilayer graphene can be tuned by its Fermi energy. We also achieved the enhanced out-of-pane spin shift of the glass-AB stacking bilayer graphene-air structure at both the Brewster angle () and the critical angle () with the aid of the high order of Laguerre–Gaussian beam. The realization of large and controlled spin shift in bilayer graphene indicates its promising applications in precision measurements and refractive index sensors at the midinfrared frequency region.

A Self-Adapting, Pixelized Planar Antenna Design for Infrared Frequencies

Infrared antennas with reconfigurable characteristics offer several advantages in the medical, military, telecommunication and energy harvesting areas, while their design and implementation is a particularly challenging task for the researchers. This paper proposes a pixel antenna, designed for mid-infrared frequencies with a bandwidth more than 25 THz, consisting of 3 × 3 square metallic planar patches. Bolometer-based switches are placed between the adjacent pixels in order to obtain the adaptable characteristics, optimized for the incoming infrared radiation. The incident wave from a certain direction will heat up the bolometers. Consequently, the conductivity of these bolometers (PTC) will be decreased, and as a result they can be considered to turn to OFF state. The simulation results suggest that the proposed structure can steer the antenna pattern toward the direction of the incident radiation in an adaptable manner, thereby considerably increasing the antenna gain. The gain of the antenna can be increased up to 2 dB with respect to the reference one, which makes it a promising structure for various applications.

Versatile optical frequency combs based on multi-seeded femtosecond optical parametric generation.

This study proposes and demonstrates a versatile method for near- and mid-infrared optical frequency comb generation using multi-seeded femtosecond optical parametric generation. The method allows one to divide the repetition rate by an arbitrarily large integer factor, freely tune the offset frequency, and adjust the common phase offset of the comb modes. Since all possible degrees of freedom are adjustable, the proposed method manifests itself as versatile optical frequency synthesis.

Polarization properties in grating-gated AlN/GaN HEMTs at mid-infrared frequencies.

The plasmon resonances of grating-gated AlN/GaN HEMTs can occur in wide frequency regions at mid-infrared frequencies. However, the lack of polarization properties research in grating-gated AlN/GaN HEMTs prevents the application potential. In order to solve the problem, the polarization properties in grating-gated AlN/GaN HEMTs at mid-infrared frequencies were studied in the paper. After using the optical transfer matrix method to calculate the dispersion curves in grating-gated AlN/GaN HEMTs, the plasmon polaritons in conductive channel and phonon polaritons in GaN layer occur under TM incident waves rather than TE incident waves. The phenomenon illustrates the potential of polarization-selectivity has existed in grating-gated AlN/GaN HEMTs. To study the polarization properties of grating-gated AlN/GaN HEMTs in detail, the electric field distribution and transmission properties of the structure were simulated in COMSOL. The results show the excellent polarization-selectivity at mid-infrared frequencies in grating-gated AlN/GaN HEMTs. The studies of these characteristics indicate the vast potential for using grating-gated AlN/GaN HEMTs to design mid-infrared polarizers, mid-infrared polarization state modulators and other devices in the future.

Is Ge an Excellent Material for Mid-IR Kerr Frequency Combs Around 3-μm Wavelengths?

Mid-infrared (Mid-IR) Kerr frequency combs, which emit board, discrete, and evenly spaced laser frequencies in the spectral region of 2–20 μm, are expected to bring us tremendous applications from molecular spectroscopy to astronomy science. To develop mid-IR Kerr frequency combs, Ge has attracted significant attention due to its unique merits of a wide transparency window (2–14 μm), high refractive index (∼4), giant third-order nonlinear refractive index (∼10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−16</sup> m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /W), and fully CMOS-compatible device fabrication. However, it is still debatable whether Ge is an excellent material for developing Kerr frequency combs in the short-wavelength mid-IR region due to the harmful multiphoton and free-carrier absorption effects. Here, we seek to provide physical insights into the feasibility of developing mid-infrared Ge-based Kerr frequency combs around 3-μm wavelengths. Our method is based on a comprehensive model developing from a modified Lugiato-Lefever equation via taking into account high-order dispersion, multiphoton absorption, free-carrier absorption, free-carrier dispersion, and thermal-optical phase shift effect. The results show that, only with the pump wavelength of above 3.5 μm, a frequency comb could be generated by adjusting the carrier lifetime and pump power appropriately. The study is expected to provide useful guidance for developing mid-IR Kerr frequency combs by using the CMOS technology.

Ultra-broadband mid-infrared frequency combs produced by optical subharmonic generation

Optical frequency combs have revolutionised accurate frequency and time measurements and have enabled broadband and simultaneously high resolution spectroscopic measurements that were not previously possible. This paper is an overview of the main results of the previously performed work, describing a new approach to extending frequency combs to the mid-infrared ‘molecular signature’ range using a subharmonic generator based on an optical parametric oscillator operating in degenerate mode. Such an instrument acts as an efficient frequency divider that rigorously down-converts and augments the spectrum of a pump laser frequency comb while maintaining its coherence. Our recent result is the demonstration of a subharmonic system with a two-octave spectrum, 3 – 12 μm, which covers vibrational resonances for most molecular species. Potentially, through frequency division in the coherent subharmonic optical parametric amplifier regime, this method can be used to obtain intense long-wavelength pulses for high-field physics applications.

Simultaneous generation of a broadband MIR and NIR frequency comb in a GaP microring.

Midinfrared (MIR) optical frequency combs are of great significance as broadband coherent light sources used in extensive areas such as coherent communications and molecule detections. Conventional MIR combs are usually restricted in size and power, while most microcombs are focused in the near-infrared (NIR) region because of the limited accessible Q-factor of microrings and the poor performances of available pumps. In this paper, we numerically demonstrate the simultaneous generation of a broadband MIR and NIR comb in a GaP microring with an additive waveguide. The achieved octave-spanning (1890-4050 nm) MIR microcomb at a low pump power of 34 mW can be effectively converted to the second-harmonic NIR comb covering 1120-1520 nm with separate dispersion optimization of the ring cavity and straight waveguide. The proposed system has the advantage of simple structure and low power threshold, which could find potential in highly integrated MIR optical sources and related applications.

Analog FM free-space optical communication based on a mid-infrared quantum cascade laser frequency comb.

Quantum cascade laser frequency combs are nowadays well-appreciated sources for infrared spectroscopy. Here their applicability for free-space optical communication is demonstrated. The spontaneously-generated intermodal beat note of the frequency comb is used as carrier for transferring the analog signal via frequency modulation. Exploiting the atmospheric transparency window at 4 µm, an optical communication with a signal-to-noise ratio up to 65 dB is realized, with a modulation bandwidth of 300 kHz. The system tolerates a maximum optical attenuation exceeding 35 dB. The possibility of parallel transmission of an independent digital signal via amplitude modulation at 5 Mbit/s is also demonstrated.

Stopping and image forces on a charged particle moving parallel to an anisotropic two-dimensional material

We use a classical dielectric response framework to evaluate the force components acting on a point charge that moves parallel to an anisotropic two-dimensional material representing doped phosphorene. The anisotropic response of phosphorene is captured by a simple analytical conductivity model that combines contributions from its inter- and intraband electronic transitions, thereby giving rise to a topological transition between elliptic and hyperbolic isofrequency curves for its plasmon modes in the terahertz to the midinfrared frequency range. Pairing this model with a simple dielectric description of a ${\mathrm{SiO}}_{2}$ substrate further reveals the effects of plasmon hybridization with optical phonons in the substrate. We have found that the force on the incident charge has three components, which all exhibit strong velocity-dependent anisotropy when its direction of motion is varied with respect to the principal directions of the phosphorene's conductivity tensor. Besides the longitudinal stopping force and the dynamic image force, there arises a transverse stopping force, with a magnitude comparable to that of the longitudinal force, which acts parallel to the target surface and perpendicular to the trajectory of the particle, and can be therefore experimentally observable, even after its magnitude is reduced by the presence of the substrate. In addition, we have found that the dynamic image force can become repulsive when the particle moves along the armchair direction of a freestanding phosphorene, but the presence of the substrate eliminates that surprising but implausible consequence of the adopted model for anisotropic conductivity of that material.

Ultrafast modulation of vibrational polaritons for controlling the quantum field statistics at mid-infrared frequencies

Controlling the quantum field statistics of confined light is a long-standing goal in integrated photonics. We show that by coupling molecular vibrations with a confined mid-infrared cavity vacuum, the photocount and quadrature field statistics of the cavity field can be reversibly manipulated over sub-picosecond timescales. The mechanism involves changing the cavity resonance frequency through a modulation of the dielectric response of the cavity materials using femtosecond UV pulses. For a single anharmonic molecular vibration in an infrared cavity under ultrastrong coupling conditions, the pulsed modulation of the cavity frequency can adiabatically produce mid-infrared light that is simultaneously sub-Poissonian and quadrature squeezed, depending on the dipolar behavior of the vibrational mode. For a vibration-cavity system in strong coupling, non-adiabatic polariton excitations can be produced after the frequency modulation pulse is over, when the system is initially prepared in the lower polariton state. We propose design principles for the generation of mid-infrared quantum light by analyzing the dependence of the cavity field statistics on the shape of the electric dipole function of the molecule, the cavity detuning at the modulation peak and the anharmonicity of the Morse potential. Feasible experimental implementations of the modulation scheme are suggested. This work paves the way for the development of molecule-based mid-infrared quantum optical devices at room temperature.

Ultrafast multidimensional spectroscopy with field resolution and noncollinear geometry at mid-infrared frequencies

Energetic correlations and their dynamics govern the fundamental properties of condensed matter materials. Ultrafast multidimensional spectroscopy in the mid infrared is an advanced technique to study such coherent low-energy dynamics. The intrinsic many-body phenomena in functional solid-state materials, in particular few-layer samples, remain widely unexplored to this date, because complex and weak sample responses demand versatile and sensitive detection. Here, we present a novel setup for ultrafast multidimensional spectroscopy with noncollinear geometry and complete field resolution in the 15–40 THz range. Electric fields up to few-100 kV cm−1 drive coherent dynamics in a perturbative regime, and an advanced modulation scheme allows to detect nonlinear signals down to a few tens of V cm−1 entirely background-free with high sensitivity and full control over the geometric phase-matching conditions. Our system aims at the investigation of correlations and many-body interactions in condensed matter systems at low energy. Benchmark measurements on bulk indium antimonide reveal a strong six-wave mixing signal and map ultra-fast changes of the band structure with access to amplitude and phase information. Our results pave the way towards the investigation of functional thin film materials and few-layer samples.