Mid-infrared Fourier-transform spectroscopy with a high-brilliance tunable laser source: investigating sample areas down to 5 μm diameter

Broadband and photovoltaic THz/IR response in the GaAs-based ratchet photodetector

High-performance broadband infrared (IR)/terahertz (THz) detection is crucial in many optoelectronic applications. However, the spectral response range of semiconductor-based photodetectors is limited by the bandgaps. This paper proposes a ratchet structure based on the GaAs/Al_xGa_1-xAs heterojunction, where the quasi-stationary hot hole distribution and intravalence band absorption from light or heavy hole states to the split-off band overcome the bandgap limit, ensuring an ultrabroadband photoresponse from near-IR to THz region (4 to 300 THz). The peak responsivity of the proposed structure can reach 7.3 A/W, which is five orders of magnitude higher than that of the existing broadband photon-type detector. Because of the ratchet effect, the proposed photodetector has a bias-tunable photoresponse characteristic and can operate in the photovoltaic mode with a broad photocurrent spectrum (18 to 300 THz). This work not only demonstrates a broadband photon-type THz/IR photodetector but also provides a method to study the light-responsive ratchet.

Measuring Molecular Diffusion Through Thin Polymer Films with Dual-Band Plasmonic Antennas

A general and quantitative method to characterize molecular transport in polymers with good temporal and high spatial resolution, in complex environments, is an important need of the pharmaceutical, textile, and food and beverage packaging industries, and of general interest to the polymer science community. Here we show how the amplified infrared (IR) absorbance sensitivity provided by plasmonic nanoantenna-based surface enhanced infrared absorption (SEIRA) provides such a method. SEIRA enhances infrared (IR) absorbances primarily within 50 nm of the nanoantennas, enabling localized quantitative detection of even trace quantities of analytes and diffusion measurements in even thin polymer films. Relative to a commercial attenuated total internal reflection (ATR) system, the limit of detection is enhanced at least 13-fold, and as is important for measuring diffusion, the detection volume is about 15 times thinner. Via this approach, the diffusion coefficient and solubility of specific molecules, including l-ascorbic acid (vitamin C), ethanol, various sugars, and water, in both simple and complex mixtures (e.g., beer and a cola soda), were determined in poly(methyl methacrylate), high density polyethylene (HDPE)-based, and polypropylene-based polyolefin films as thin as 250 nm.

Broadband, few-cycle mid-infrared continuum based on the intra-pulse difference frequency generation with BGSe crystals

We demonstrate for the first time the generation of octave-spanning mid-infrared using a BGSe nonlinear crystal. A Cr:ZnS laser system delivering 28-fs pulses at a central wavelength of 2.4 µm is used as the pump source, which drives the intra-pulse difference frequency generation inside the BGSe crystal. As a result, a coherent broadband mid-infrared continuum spanning from 6 to 18 µm has been obtained. It shows that the BGSe crystal is a promising material for broadband, few-cycle mid-infrared generation via frequency down conversion with femtosecond pump sources.

Intra-pulse difference-frequency generation of mid-infrared (2.7-20 μm) by random quasi-phase-matching

We present a mid-infrared (MIR) source based on intra-pulse difference-frequency generation under the random quasi-phase-matching condition. The scheme enables the use of non-birefringent materials whose crystal orientations are not perfectly and periodically poled, widening the choice of media for nonlinear frequency conversion. With a 2 μm driving source based on a Ho:YAG thin-disk laser, together with a polycrystalline ZnSe element, an octave-spanning MIR continuum (2.7-20 μm) was generated. At over 20 mW, the average power is comparable to regular phase-matching in birefringent crystals. A 1 μm laser system based on a Yb:YAG thin-disk laser was also tested as a driving source in this scheme. The new approach provides a simplified way for generating coherent MIR radiation with an ultrabroad bandwidth at reasonable efficiency.

Broadband mid-infrared coverage (2-17 μm) with few-cycle pulses via cascaded parametric processes

A myriad of existing and emerging applications could benefit from coherent and broadband mid-infrared (MIR) light. Yet, existing tabletop sources are often complex or sensitive to interferometric optical misalignment. Here we demonstrate a significantly simplified scheme of broadband MIR generation by cascading the intra-pulse difference-frequency generation process in a specific nonlinear crystal. This allows pulses generated directly from mode-locked lasers to be used without further nonlinear temporal compression. The system, together with the driving beam, can provide an ultra-broadband coherent radiation coverage ranging from 2 to 17 μm with femtosecond pulse durations. To the best of our knowledge, this is the first demonstration of cascaded DFG in the MIR range, which brings emerging time-domain spectroscopic techniques closer to real-world applications.

Waveguide-Integrated Compact Plasmonic Resonators for On-Chip Mid-Infrared Laser Spectroscopy

The integration of nanoplasmonic devices with a silicon photonic platform affords a new approach for efficient light delivery by combining the high field enhancement of plasmonics and the ultralow propagation loss of dielectric waveguides. Such a hybrid integration obviates the need for a bulky free-space optics setup and can lead to fully integrated, on-chip optical sensing systems. Here, we demonstrate ultracompact plasmonic resonators directly patterned atop a silicon waveguide for mid-infrared spectroscopic chemical sensing. The footprint of the plasmonic nanorod resonators is as small as 2 μm², yet they can couple with the mid-infrared waveguide mode efficiently. The plasmonic resonance is directly measured through the transmission spectrum of the waveguide with a coupling efficiency greater than 70% and a field intensity enhancement factor of over 3600 relative to the evanescent waveguide field intensity. Using this hybrid device and a tunable mid-infrared laser source, surface-enhanced infrared absorption spectroscopy of both a thin poly(methyl methacrylate) film and an octadecanethiol monolayer is successfully demonstrated.

Optical parametric generation in orientation-patterned gallium phosphide

We report an optical parametric generator (OPG) based on the new nonlinear material, orientation-patterned gallium phosphide (OP-GaP). Pumped by a Q-switched nanosecond Nd:YAG laser at 1064 nm with 25 kHz pulse repetition rate, the OPG can be tuned across 1721-1850 nm in the signal and 2504-2787 nm in the idler. Using a 40-mm-long crystal in single-pass configuration, we have generated a total average output power of up to ∼18  mW, with ∼5  mW of idler power at 2670 nm, for 2 W of input pump power. The OPG exhibits a passive stability in total output power better than 0.87% rms over 1 h, at a crystal temperature of 120°C, compared to 0.14% rms for the input pump. The output signal pulses, recorded at 1769 nm, have duration of 5.9 ns for input pump pulses of 9 ns. Temperature-dependent loss measurements for the pump polarization along the [100] axis in the OP-GaP crystal have also been performed, for the first time, indicating a drop in transmission from 28.8% at 50°C to 19.4% at 160°C.

Nanoantenna-assisted plasmonic enhancement of IR absorption of vibrational modes of organic molecules

Nanoantenna-assisted plasmonic enhancement of IR absorption and Raman scattering was employed for studying the vibrational modes in organic molecules. Ultrathin cobalt phthalocyanine films (3 nm) were deposited on Au nanoantenna arrays with specified structural parameters. The deposited organic films reveal the enhancement of both Raman scattering and IR absorption vibrational modes. To extend the possibility of implementing surface-enhanced infrared absorption (SEIRA) for biological applications, the detection and analysis of the steroid hormone cortisol was demonstrated.

Surface-Enhanced Infrared Spectroscopy Using Resonant Nanoantennas

Infrared spectroscopy is a powerful tool widely used in research and industry for a label-free and unambiguous identification of molecular species. Inconveniently, its application to spectroscopic analysis of minute amounts of materials, for example, in sensing applications, is hampered by the low infrared absorption cross-sections. Surface-enhanced infrared spectroscopy using resonant metal nanoantennas, or short "resonant SEIRA", overcomes this limitation. Resonantly excited, such metal nanostructures feature collective oscillations of electrons (plasmons), providing huge electromagnetic fields on the nanometer scale. Infrared vibrations of molecules located in these fields are enhanced by orders of magnitude enabling a spectroscopic characterization with unprecedented sensitivity. In this Review, we introduce the concept of resonant SEIRA and discuss the underlying physics, particularly, the resonant coupling between molecular and antenna excitations as well as the spatial extent of the enhancement and its scaling with frequency. On the basis of these fundamentals, different routes to maximize the SEIRA enhancement are reviewed including the choice of nanostructures geometries, arrangements, and materials. Furthermore, first applications such as the detection of proteins, the monitoring of dynamic processes, and hyperspectral infrared chemical imaging are discussed, demonstrating the sensitivity and broad applicability of resonant SEIRA.

Ultra-stable high average power femtosecond laser system tunable from 1.33 to 20 μm

A highly stable 350 fs laser system with a gap-free tunability from 1.33 to 2.0 μm and 2.13 to 20 μm is demonstrated. Nanojoule-level pulse energy is achieved in the mid-infrared at a 43 MHz repetition rate. The system utilizes a post-amplified fiber-feedback optical parametric oscillator followed by difference frequency generation between the signal and idler. No locking or synchronization electronics are required to achieve outstanding free-running output power and spectral stability of the whole system. Ultra-low intensity noise, close to the pump laser's noise figure, enables shot-noise limited measurements.

Miniature Fourier transform spectrometer with a dual closed-loop controlled electrothermal micromirror

A large piston-displacement electrothermal micromirror with closed-loop control of both piston scan and tilting of the mirror plate is demonstrated for use in a miniature Fourier transform spectrometer. Constant scan velocity in an ultra large piston scan range has been demonstrated by the proposed closed-loop piston control scheme which can be easily implemented without considerably increasing system complexity. The experimental results show that the usable linear scan range generated by the micromirror has been extended up to 505 μm. The measured spectral resolution in a compact spectrometer reaches 20 cm^-1, or 0.57 nm at 532 nm wavelength. Compared to other presented systems, this microspectrometer will benefit from the closed-loop thermal actuator approach utilizing both the piston servo and tilt control to provide more consistent spectral response, improved spectral resolution and enhanced robustness to disturbances.

Narrowing of the linewidth of an optical parametric oscillator by an acousto-optic modulator for the realization of mid-IR noise-immune cavity-enhanced optical heterodyne molecular spectrometry down to 10⁻¹⁰ cm⁻¹ Hz⁻¹/²

The linewidth of a singly resonant optical parametric oscillator (OPO) has been narrowed with respect to an external cavity by the use of an acousto-optic modulator (AOM). This made possible an improvement of the sensitivity of a previously realized OPO-based noise-immune cavity-enhanced optical heterodyne molecular spectrometry instrument for the 3.2 - 3.9 µm mid-infrared region by one order of magnitude. The resulting system shows a detection sensitivity for methane of 2.4 × 10(-10) cm(-1) Hz(-1∕2) and 1.3 × 10(-10) cm(-1) at 20 s, which allows for detection of both the environmentally important (13)CH(4) and CH(3)D isotopologues in atmospheric samples.

Multi-Watt femtosecond optical parametric master oscillator power amplifier at 43 MHz

We present a high repetition rate mid-infrared optical parametric master oscillator power amplifier (MOPA) scheme, which is tunable from 1370 to 4120nm. Up to 4.3W average output power are generated at 1370nm, corresponding to a photon conversion efficiency of 78%. Bandwidths of 6 to 12nm with pulse durations between 250 and 400fs have been measured. Strong conversion saturation over the whole signal range is observed, resulting in excellent power stability. The system consists of a fiber-feedback optical parametric oscillator that seeds an optical parametric power amplifier. Both systems are pumped by the same Yb:KGW femtosecond oscillator.

High-power mid-infrared high repetition-rate supercontinuum source based on a chalcogenide step-index fiber

We demonstrate a tunable and robust femtosecond supercontinuum source with a maximum output power of 550 mW and a maximum spectral width of up to 2.0 μm, which can cover the mid-infrared region from 2.3 μm up to 4.9 μm by tuning the pump wavelength. As2S3 chalcogenide step-index fibers with core diameters of 7 and 9 μm are pumped at different wavelengths from 2.5 μm up to 4.1 μm with femtosecond pulses by means of a post-amplified optical parametric oscillator pumped by an Yb:KGW laser. The spectral behavior of the supercontinuum is investigated by changing the pump wavelength, core diameter, fiber length, and pump power. Self-phase modulation is identified as the main broadening mechanism in the normal dispersion regime. This source promises to be an excellent laboratory tool for infrared spectroscopy owing to its high brilliance as demonstrated for the CS2-absorption bands around 3.5 μm.