In-line synthesis of multi-octave phase-stable infrared light.

Parametric downconversion driven by modern, high-power sources of 10-fs-scale near-infrared pulses, in particular intrapulse difference-frequency generation (IPDFG), affords combinations of properties desirable for molecular vibrational spectroscopy in the mid-infrared range: broad spectral coverage, high brilliance, and spatial and temporal coherence. Yet, unifying these in a robust and compact radiation source has remained a key challenge. Here, we address this need by employing IPDFG in a multi-crystal in-line geometry, driven by the 100-W-level, 10.6-fs pulses of a 10.6-MHz-repetition-rate, nonlinearly post-compressed Yb:YAG thin-disk oscillator. Polarization tailoring of the driving pulses using a bichromatic waveplate is followed by a sequence of two crystals, LiIO3 and LiGaS2, resulting in the simultaneous coverage of the 800-cm-1-to-3000-cm-1 spectral range (at -30-dB intensity) with 130 mW of average power. We demonstrate that optical-phase coherence is maintained in this in-line geometry, in theory and experiment, the latter employing ultra-broadband electro-optic sampling. These results pave the way toward coherent spectroscopy schemes like field-resolved and frequency-comb spectroscopy, as well as nonlinear, ultrafast spectroscopy and optical-waveform synthesis across the entire infrared molecular fingerprint region.

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.

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.

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.

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.

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.

Mid-infrared long-pass filter for high-power applications based on grating diffraction.

A gold-coated silicon grating has been developed, enabling efficient spatial separation of broadband mid-infrared (MIR) beams with wavelengths >5 µm from collinearly propagating, broadband, high-power light in the near-infrared (NIR) spectral range (centered at 2 µm). The optic provides spectral filtering at high powers in a thermally robust and chromatic-dispersion-free manner such as that necessary for coherent MIR radiation sources based on parametric frequency downconversion of femtosecond NIR lasers. The suppression of a >20 W average-power, 2 µm driving pulse train by three orders of magnitude, while retaining high reflectivity of the broadband MIR beam, is presented.

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.

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.

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.

Kerr effect in multilayer dielectric coatings.

We report the utilization of the optical Kerr effect in multilayer dielectric coatings, previously discussed only theoretically. We present the design and realization of multilayer dielectric optical structures with layer-specific Kerr nonlinearities, which permit tailoring of the intensity-dependent effects. The modulation depth in reflectance reaches up to 6% for the demonstrated examples of dielectric nonlinear multilayer coatings. We show that the nonlinearity is based on the optical Kerr effect, with the recovery time faster than the laser pulse envelope of 1 ps. Due to high flexibility in design, the reported dielectric nonlinear multilayer coatings have the potential to open hitherto unprecedented possibilities in nonlinear optics and ultrafast laser applications.

Broadband beamsplitter for high intensity laser applications in the infra-red spectral range.

We report on design, production and characterization of an extremely broadband multilayer beamsplitter, covering wavelength range from 0.67 - 2.6 µm. The group delay dispersion has small oscillations in the above mentioned working range. We used a new algorithm with floating constants allowing us to obtain a smooth and near constant GDD. The optical element based on the beamsplitter is used for dividing a low-energy super-octave spectrum into several sub-spectral regions which are later amplified and coherently combined.

Highly-dispersive mirrors reach new levels of dispersion.

A highly-dispersive mirror with the unprecedented group delay dispersion of -10000 fs2 in the wavelength range of 1025-1035 nm is reported. Reproducible production of a coating with such a high dispersion was possible due to the recently developed robust synthesis technique. Successful employment of the new highly-dispersive mirror in an oscillator is demonstrated.

Calculation of optical and electronic properties of modeled titanium dioxide films of different densities.

The electronic and optical properties of TiO2 atomic structures representing simulated thin films have been investigated using density functional theory. Suitable model parameters and system sizes have been identified in advance by validation of the results with experimental data. Dependencies of the electronic band gap and the refractive index have been calculated as a function of film density. The results of the performed calculations have been compared to characterized optical properties of titania single layers deposited using different coating techniques. The modeled dependencies are consistent with experimental observations, and absolute magnitudes of simulated values are in agreement with measured optical data.

Stress compensation with antireflection coatings for ultrafast laser applications: from theory to practice.

Each complicated coating, in particular, a dispersive mirror consists of dozens of layers. Thin films layers have mechanical stresses. After summing up stresses from all layers, the resulting stress is high enough to bend even a relatively thick substrate. To avoid this effect we suggest depositing an antireflection coating (AR) at the back-side of the substrate which together with suppression of unwanted reflections from the back side will also compensate this stress. We demonstrate unique, extremely thick and sophisticated AR coating consisting of 71 layers with the total physical thickness of 7.5 µm. This AR coating completely compensates stress from the dispersive mirror coated on the front side and minimizes unwanted reflections.

Quality control of oblique incidence optical coatings based on normal incidence measurement data.

We demonstrate selection of reliable approaches for post-production characterization of oblique incidence multilayer optical coatings. The approaches include choice of input information, selection of adequate coating model, corresponding numerical characterization algorithm, and verification of the results. Applications of the approaches are illustrated with post-production characterization of oblique incidence edge filter, oblique incidence beam splitter and oblique incidence 43-layer quarter-wave mirror.

Analytical estimations for the reference wavelength reflectance and width of high reflection zone of two-material periodic multilayers.

Periodic multilayer structures of quarterwave and multiple quarterwave stacks with shifted ratios of high and low index layers in the half-wave pairs are considered. Analytical dependencies of the reference wavelength reflectance and the width of high reflectance zone on the number of layers, fraction quarterwave and layer refractive indices are obtained. The structures are used as starting designs for notch filters. Obtained dependencies allow one to estimate in advance parameters required to achieve target spectral characteristics.

Computational manufacturing as a tool for the selection of the most manufacturable design.

Applications of computational manufacturing experiments (CMEs) for selection of the most manufacturable designs among a variety of different design solutions are demonstrated. We compare design solutions with respect to estimations of their production yields. Computational experiments are performed using two simulation software tools. In the course of CMEs, we take into account all major factors causing errors in our deposition process. Real deposition experiments are in agreement with CMEs; the most manufacturable design exhibits better target performances compared to other designs.