Field-resolved infrared spectroscopy of biological systems.

The proper functioning of living systems and physiological phenotypes depends on molecular composition. Yet simultaneous quantitative detection of a wide variety of molecules remains a challenge1-8. Here we show how broadband optical coherence opens up opportunities for fingerprinting complex molecular ensembles in their natural environment. Vibrationally excited molecules emit a coherent electric field following few-cycle infrared laser excitation9-12, and this field is specific to the sample's molecular composition. Employing electro-optic sampling10,12-15, we directly measure this global molecular fingerprint down to field strengths 107 times weaker than that of the excitation. This enables transillumination of intact living systems with thicknesses of the order of 0.1 millimetres, permitting broadband infrared spectroscopic probing of human cells and plant leaves. In a proof-of-concept analysis of human blood serum, temporal isolation of the infrared electric-field fingerprint from its excitation along with its sampling with attosecond timing precision results in detection sensitivity of submicrograms per millilitre of blood serum and a detectable dynamic range of molecular concentration exceeding 105. This technique promises improved molecular sensitivity and molecular coverage for probing complex, real-world biological and medical settings.

Designing broadband dispersive mirrors in the mid-infrared spectral range: a theoretical study.

The study reports practically important and interesting results on designing dispersive mirrors (DMs) operating in the mid-infrared spectral range from 3 to 18 µm. The admissible domains of the most important design specifications, the mirror bandwidth and group delay variation, were constructed. Estimations of the required total coating thickness, thickness of the thickest layer, and expected number of layers are obtained. The results are confirmed by an analysis of several hundreds of DM design solutions.

Optical Interference Coating Design Contest 2022: a black box coating and a filter for an outdoor 3D cinema challenge [invited].

The design problems for the Optical Interference Coating (OIC) 2022 Topical Meeting include black box coatings to reverse engineer and a pair of white-balanced, multi-bandpass filters for three-dimensional cinema projection in cold and hot outdoor environments. There were 14 designers from China, France, Germany, Japan, Russia, and the United States, submitting 32 total designs for problems A and B. The design problems and the submitted solutions are described and evaluated.

Reverse engineering of e-beam deposited optical filters based on multi-sample photometric and ellipsometric data.

A post-production characterization approach based on spectral photometric and ellipsometric data related to a specially prepared set of samples is proposed. Single-layer (SL) and multilayer (ML) sets of samples presenting building blocks of the final sample were measured ex-situ, and reliable thicknesses and refractive indices of the final ML were determined. Different characterization strategies based on ex-situ measurements of the final ML sample were tried, reliability of their results was compared, and the best characterization approach for practical use, when preparation of the mentioned set of samples would be a luxury, is proposed.

Ultra-rapid electro-optic sampling of octave-spanning mid-infrared waveforms.

We demonstrate ultra-rapid electro-optic sampling (EOS) of octave-spanning mid-infrared pulses centered at 9 µm, implemented by mechanically scanning a mirror with a sonotrode resonating at 19 kHz (forward and backward acquisition at 38 kHz). The instrument records the infrared waveform with a spectral intensity dynamic range of 1.6 × 105 for a single scan over a 1.6-ps delay range, acquired within 26 µs. The purely reflective nature of the delay scanning technique is compatible with broad optical bandwidths, short pulse durations (16 fs, centered at 1030 nm) and high average powers (Watt-level). Interferometric tracking of the sonotrode motion in combination with a predictor-corrector algorithm allows for delay-axis determination with down to single-digit attosecond precision. Ultra-rapid mid-infrared EOS will advance applications such as molecular fingerprinting of static samples as well as tracking of biological processes and chemical reactions and is likely to find new fields of application such as infrared-spectroscopic flow cytometry.

Molecular Origin of Blood-Based Infrared Spectroscopic Fingerprints*.

Infrared spectroscopy of liquid biopsies is a time- and cost-effective approach that may advance biomedical diagnostics. However, the molecular nature of disease-related changes of infrared molecular fingerprints (IMFs) remains poorly understood, impeding the method's applicability. Here we probe 148 human blood sera and reveal the origin of the variations in their IMFs. To that end, we supplemented infrared spectroscopy with biochemical fractionation and proteomic profiling, providing molecular information about serum composition. Using lung cancer as an example of a medical condition, we demonstrate that the disease-related differences in IMFs are dominated by contributions from twelve highly abundant proteins-that, if used as a pattern, may be instrumental for detecting malignancy. Tying proteomic to spectral information and machine learning advances our understanding of the infrared spectra of liquid biopsies, a framework that could be applied to probing of any disease.

Optimum Sample Thickness for Trace Analyte Detection with Field-Resolved Infrared Spectroscopy.

The strong absorption of liquid water in the infrared (IR) molecular fingerprint region constitutes a challenge for applications of vibrational spectroscopy in chemistry, biology, and medicine. While high-power IR laser sources enable the penetration of ever thicker aqueous samples, thereby mitigating the detrimental effects of strong attenuation on detection sensitivity, a basic advantage of heterodyne-measurement-based methods has-to the best of our knowledge-not been harnessed in broadband IR measurements to date. Here, employing field-resolved spectroscopy (FRS), we demonstrate in theory and experiment fundamental advantages of techniques whose signal-to-noise ratio (SNR) scales linearly with the electric field over those whose SNR scales linearly with radiation intensity, including conventional Fourier-transform infrared (FTIR) and direct absorption spectroscopy. Field-scaling brings about two major improvements. First, it squares the measurement dynamic range. Second, we show that the optimum interaction length with samples for SNR-maximized measurements is twice the value usually considered to be optimum for FTIR devices. In order to take full advantage of these properties, the measurement must not be significantly affected by technical noise, such as intensity fluctuations, which are common for high-power sources. Recently, it has been shown that subcycle, nonlinear gating of the molecular fingerprint signal renders FRS robust against intensity noise. Here, we quantitatively demonstrate this advantage of FRS for thick aqueous samples. We report sub-µg/mL detection sensitivities for transmission path lengths up to 80 µm and a limit of detection in the lower µg/mL range for transmission paths as long as 200 µm.

Design, fabrication and measurement of highly-dispersive mirrors with total internal reflection.

High group delay dispersion (GDD) is often required for ultrafast laser applications. To achieve GDD level higher than -10000 fs2 in a single mirror setting is difficult due to the high sensitivity to unavoidable production inaccuracies. To overcome the problem, total internal reflection (TIR) based dispersive mirrors have been proposed in theory. In this paper, we report our continuous effort to further design, fabricate and measure TIR based dispersive mirrors.

Characterization of e-beam evaporated Ge, YbF3, ZnS, and LaF3 thin films for laser-oriented coatings.

Thin films of Ge, ZnS, YbF3, and LaF3 produced using e-beam evaporation on ZnSe and Ge substrates were characterized in the range of 0.4-12 µm. It was found that the Sellmeier model provides the best fit for refractive indices of ZnSe substrate, ZnS, and LaF3 films; the Cauchy model provides the best fit for YbF3 film. Optical constants of Ge substrate and Ge film as well as extinction coefficients of ZnS, YbF3, LaF3, and ZnSe substrate are presented in the frame of a non-parametric model. For the extinction coefficient of ZnS, the exponential model is applicable. Stresses in Ge, ZnS, YbF3, and LaF3 were estimated equal to (-50)MPa, (-400)MPa, 140 MPa, and 380 MPa, respectively. The surface roughness does not exceed 5 nm for all films and substrates.

Optical interference coating design contest 2019: a non-polarizing beam splitter and a color-mixing challenge [Invited].

A non-polarizing beam splitter and a light color-mixing challenge were the topics of the design contest held in conjunction with the 2019 Optical Interference Coatings topical meeting of the Optical Society of America. A total of 10 designers from China, France, Germany, Japan, and the United States submitted over 70 designs for problems A and B. The design problems and the submitted solutions are described and evaluated.

Comparative study of NIR-MIR beamsplitters based on ZnS/YbF3 and Ge/YbF3.

Two beamsplitters operating across the near-infrared (770-1050 nm) and mid-infrared (4-8 µm) spectral ranges are developed. For the first time, the beamsplitters based on thin-film materials combinations of ZnS/YbF3 and Ge/YbF3 are investigated. The multilayers operate at the Brewster angle of ZnSe substrate. There are no special temperature conditions. The dichroic coatings are designed, produced, and carefully characterized. Potentials of the ZnS/YbF3 and Ge/YbF3 thin-film material combinations are discussed based on analytical estimations, as well as on optical and non-optical characterization results. The ZnS/YbF3 pair provides high reflectance in the near-infrared spectral range. The Ge/YbF3 solutions exhibit very broadband reflection zones. The Ge/YbF3 coatings are thinner and comprise fewer layers than ZnS/YbF3 multilayers. Ge/YbF3 pair has high potential for design and production of NIR-MIR coatings.

Complementary Si/SiO2 dispersive mirrors for 2-4†µm spectral range.

Complementary pair of dispersive multilayers operating in the 2-4 µm spectral range were designed and produced for the first time. The mirrors comprise layers of Si and SiO2 thin-film materials. The pair exhibits unparalleled reflectance exceeding 99.7% and provides a group delay dispersion of (-200) fs2. The mirrors can be used in Cr:ZnS/Cr:ZnSe femtosecond lasers and amplifiers.

2/3 octave Si/SiO2 infrared dispersive mirrors open new horizons in ultrafast multilayer optics.

Dispersive mirrors operating in a broadband infrared spectral range are reported for the first time. The mirrors are based on Si/SiO2 thin-film materials. The coatings exhibit reflectance exceeding 99.6% in the spectral range from 2 to 3.2 µm and provide a group delay dispersion of -100 fs2 and -200 fs2 in this range. The fabricated mirrors are expected to be key elements of Cr:ZnS/Cr:ZnSe femtosecond lasers and amplifiers. The mirrors open a new avenue in the development of ultrafast dispersive optics operating in the infrared spectral range.

Cavity-enhanced noncollinear high-harmonic generation.

Femtosecond enhancement cavities have enabled multi-10-MHz-repetition-rate coherent extreme ultraviolet (XUV) sources with photon energies exceeding 100 eV - albeit with rather severe limitations of the net conversion efficiency and of the duration of the XUV emission. Here, we explore the possibility of circumventing both these limitations by harnessing spatiotemporal couplings in the driving field, similar to the "attosecond lighthouse," in theory and experiment. Our results predict dramatically improved output coupling efficiencies and efficient generation of isolated XUV attosecond pulses.

Broadband dispersive Ge/YbF3 mirrors for mid-infrared spectral range.

Broadband dispersive mirrors operating in the mid-infrared spectral range of 6.5-11.5 µm are developed for the first time, to the best of our knowledge. The mirrors comprise Ge and YbF3 layers, which have not been used before for manufacturing of multilayer dispersive optics. The design and production processes are described; mechanical stresses of the coatings are estimated based on experimental data; and spectral and phase properties of the produced mirrors are measured. The mirrors compensate group delay dispersion of ultrashort laser pulses accumulated by propagation through 4 mm ZnSe windows and additional residual phase modulation of an ultrashort laser pulse.

Nonlinear pulse compression in a gas-filled multipass cell.

We present an efficient method for compressing sub-picosecond pulses at 200 W average power with 2 mJ pulse energy in a multipass cell filled with different gases. We demonstrate spectral broadening by more than a factor of five using neon, argon, and nitrogen as nonlinear media. The 210 fs input pulses are compressed down to 37 fs and 35 GW peak power with a beam quality factor of 1.3×1.5 at a power throughput of >93%. This concept represents an excellent alternative to hollow-core fiber-based compression schemes and optical parametric amplifiers (OPAs).

Experimental and numerical study of the nonlinear response of optical multilayers.

Dielectric multilayer coatings exhibiting steep reflectance in an extremely narrow transition zone, highly sensitive to any variations of layer refractive indices and therefore suitable for studying the nonlinear properties are produced and characterized. Increase of reflectance at growing intensity reveals the presence of the optical Kerr effect. A new method calculating intensity dependent spectral characteristics of multilayer optical coatings in the case of nonlinear interaction with high intensity laser pulses is developed. The method is based on the numerical solution of a boundary-value problem derived from the system of Maxwell equations describing the propagation of light through a multilayer system. The method opens a way to synthesis of optical coatings with predictable nonlinear properties. Comparison of our numerical modelling with experimental data enabled us to accurately determine the Kerr coefficients n2 of the widely-used thin-film materials Ta2O5 and Nb2O5.

Synthesis, fabrication and characterization of a highly-dispersive mirrors for the 2 µm spectral range.

We report a challenging design, fabrication and post-production characterization problem of a dispersive mirror supporting the spectral range from 2000 nm to 2200 nm and providing a group delay dispersion of -1000 fs2. The absolute reflectance in the working range is over 99.95%. The reported mirror is a critical element for Tm and Ho based lasers and paves the way for the development of ultrafast 2 µm lasers with sub-100 fs pulse duration.

Optical interference coating design contest 2016: a dispersive mirror and coating uniformity challenge.

A dispersive mirror and a coating uniformity challenge were the topics of the design contest held in conjunction with the 2016 Optical Interference Coatings topical meeting of The Optical Society (OSA). A total of 18 designers from China, France, Germany, Japan, and the United States submitted 38 total designs for problems A and B. Michael Trubetskov submitted the winning designs for all four design challenges. The design problems and the submitted solutions are described and evaluated.

Group delay dispersion measurements in the mid-infrared spectral range of 2-20 µm.

We present two measurement devices which both allow the direct measurement of the group delay (GD) and group delay dispersion (GDD) of laser optics, covering the near- and mid-infrared (MIR) spectral range from 2 to 20 µm (500-5,000 cm-1). Two different kinds of devices were developed to measure the GDD of multilayer interference coatings. One is a resonant scanning interferometer (RSI) and the other is a white light interferometer (WLI). The WLI is also capable of measuring the GDD in transmission, for instance of bulk material. GDD measurements of a high dispersive mirror for wavelengths from 2.0 to 2.15 µm and one of a multilayer mirror from 8.5 to 12.0 µm are presented. A measurement of a zinc selenide (ZnSe) substrate in transmission was made with the WLI demonstrating the full bandwidth of the device from 1.9 to 20 µm. The comparison of all measurements with their related theoretical values shows a remarkable correspondence.