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Dual-comb spectroscopy (DCS) normally operates with two independent, relatively low power and actively synchronized laser sources. This hinders the wide adoption for practical implementations and frequency conversion into deep UV and VUV spectral ranges. Here, we report a fully passive, high power dual-comb laser based on thin-disk technology and its application to direct frequency comb spectroscopy. The peak power (1.2 MW) and the average power (15 W) of our Yb:YAG thin-disk dual-comb system are more than one-order-of-magnitude higher than in any previous systems. The scheme allows easy adjustment of the repetition frequency difference during operation. Both combs share all cavity components which leads to an excellent mutual stability. A time-domain signal recorded over 10 ms without any active stabilization was sufficient to resolve individual comb lines after Fourier transformation. Achieving high output powers in dual-comb sources is important for possible applications like deep UV high resolution spectroscopy. Here the authors demonstrate a fully passive scheme of generating a set of high-power dual-combs from a thin-disc gain medium.

The demand for and usage of broadband coherent mid-infrared sources, such as those provided by synchrotron facilities, are growing. Since most organic molecules exhibit characteristic vibrational modes in the wavelength range between 500 and 4000 cm-1 , such broadband coherent sources enable micro- or even nano-spectroscopic applications at or below the diffraction limit with a high signal-to-noise ratio1, 2, 3 . These techniques have been applied in diverse fields ranging from life sciences, material analysis, and time-resolved spectroscopy. Here we demonstrate a broadband, coherent and intrinsically carrier-envelope-phase-stable source with a spectrum spanning from 500 to 2250 cm-1 (-30 dB) at an average power of 24 mW and a repetition rate of 77 MHz. This performance is enabled by the first mode-locked thin-disk oscillator operating at 2 μm wavelength, providing a tenfold increase in average power over femtosecond oscillators previously demonstrated in this wavelength range4 . Multi-octave spectral coverage from this compact and power-scalable system opens up a range of time- and frequency-domain spectroscopic applications.

Tailoring the electric-field waveform of ultrashort light pulses forms the basis for controlling nonlinear optical phenomena on their genuine, attosecond timescale. Here we extend waveform control from the visible and near-infrared—where it was previously demonstrated—to the mid-infrared spectral range. Our approach yields single-cycle infrared pulses over several octaves for the first time. Sub-10-fs pulses from a carrier-envelope-phase-stabilized, Kerr-lens-mode-locked, diode-pumped Cr:ZnS laser drive cascaded intrapulse difference-frequency generation and control the electric-field evolution of the resulting coherent emission over 0.9–12.0 μm. Sub-cycle field control in this wavelength range will be instrumental for launching and steering few-femtosecond electron/hole wavepackets in low-gap materials, extending the bandwidth of electronic signal processing to multi-terahertz frequencies, as well as for electric-field-resolved molecular fingerprinting of biological systems.Continuously adjustable single-cycle waveform spanning from 0.9 to 12.0 μm is obtained by cascaded intrapulse difference-frequency generation in a ZnGeP2 crystal. The cascade-associated phase response—distinct for different spectral bands—provides a new tuning parameter for waveform adjustment.

Given the advancement of digital technologies, individuals can curate multiple news sources (multi-source) and readily engage in political discussion and expression (multi-type). They can also use multiple social media platforms of different consumptive and expressive affordances (multi-platform) to engage in political communication and maintain distinct social networks across different platforms (multi-network). To address how the high-choice and multifaceted political communication ecology is associated with democratic functioning in the increasingly polarized context of the United States, this dissertation proposes two approaches to address the implications of multi-source, multi-type, multi-platform, and multi-network aspects of political communication: (1) their conjoint effects and (2) their interrelations.Study 1 of this dissertation examines how multi-source, multi-type, and multi-platform aspects collectively shape citizens’ political communication repertoires and how these repertoires relate to different participatory and trust outcomes. The analysis identified seven repertoires: center-left legacy, minimalists, social media devotees, conservative broadcast, conservative online, liberal multi-platform, and omnivorous expressers. All the repertoires, except for minimalists, exhibited similar likelihoods of offline political participation. Moreover, the liberal and conservative repertoires tend to drive distrust in partisan outgroups to a larger extent than promoting trust in partisan ingroups. For institutional trust, the conservative repertoires are associated with lower trust in institutions deemed to be aligned with liberals, such as colleges and labor unions, whereas the liberal repertoires are associated with higher trust in these institutions and, surprisingly, show no less trust in the police.Study 2 investigates how the multi-platform and multi-network aspects relate to incidental exposure and political discussion. Higher immersion in either multiple politically heterogeneous networks or homogeneous networks was found to predict higher counter-attitudinal incidental exposure, despite the lack of distinct relationships between network heterogeneity on most platforms and such exposure. Furthermore, the analysis revealed that more heterogeneous networks on individual platforms predicted less political discussion on social media, but higher immersion in multiple heterogeneous networks across platforms was associated with more discussion.Together, these two studies highlight the value of understanding the multiplexity of individuals’ political communication and shed light on the impact of the multifaceted and high-choice communication ecology on democratic functioning and deliberation.

Hollow-core photonic-crystal fibers (PCFs) enable sustained interaction between light and gas. This thesis studies and utilizes some of the intriguing effects that arise from this interaction, with a focus on nonlinear processes in the femtosecond regime. The work is based on a particular type of hollow-core PCF—kagomé-PCF. It offers a high damage-threshold, broadband and low-loss guidance, and a weak and anomalous group-velocity dispersion (GVD). Variations in gas species and pressures filling the fiber core further facilitate the convenient tailoring of optical parameters. Using kagomé-PCFs filled with different noble gases, pulse compression is demonstrated via two techniques. Firstly, self-phase-modulation (SPM) is utilized under the normal dispersion regime. 103 fs (FWHM), 10.3 μJ pulses from a Ti:sapphire amplifier operating at 1 kHz repetition rate are compressed to 10.6 fs together with chirped-mirrors. The second technique involves SPM under anomalous GVD, invoking higher-order soliton dynamics and compressing 24 fs pulses to 6.8 fs at 6.6 μJ. The two techniques are further applied to a high repetition rate (38 MHz), high average power (40 W) thin-disk oscillator, compressing in two stages 250 fs pulses to 9.1 fs at 0.4 μJ, corresponding to an average output power of 14.5 W. The soliton dynamics involved can also efficiently transfer energy into narrow spectral bands by a process known as dispersive wave emission, with ∼0.5 W generated at 420 nm using the thin-disk oscillator. This coherent emission is widely tunable by varying the pressures and species of the filling-gas, with emission from the visible down to 176 nm demonstrated using the Ti:sapphire amplifier output. The power measured at 247 nm amount to 6.6% of the total output, and emission down to 135 nm is possible. Ionization processes represent another perturbation on the soliton dynamics, and are experimentally observed to blue-shift solitons by over 200 nm, with the emission of dispersive waves predicted as they approach the zero-dispersion wavelength. In addition, the early onset of ionization at very high input power is observed to heavily modify the soliton self-compression dynamics. These results demonstrate the versatility of gas-filled kagomé-PCFs, enabling the development of new laser sources and expanding the parameter spaces available to nonlinear optics and beyond.

Lasers based on Cr\\(^{2+}\\)-doped II-VI material, often known as the Ti:Sapphire of the mid-infrared, can directly provide few-cycle pulses with super-octave-spanning spectra, and serve as efficient drivers for generating broadband mid-infrared radiation. It is expected that the wider adoption of this technology benefits from more compact and cost-effective embodiments. Here, we report the first directly diode-pumped, Kerr-lens mode-locked Cr\\(^{2+}\\)-doped II-VI oscillator pumped by a single InP diode, providing average powers of over 500 mW and pulse durations of 45 fs - shorter than six optical cycles at 2.4 \\(\\mu\\)m. These correspond to a sixty-fold increase in peak power compared to the previous diode-pumped record, and are at similar levels with respect to more mature fiber-pumped oscillators. The diode-pumped femtosecond oscillator presented here constitutes a key step towards a more accessible alternative to synchrotron-like infrared radiation, and is expected to accelerate research in laser spectroscopy and ultrafast infrared optics.

For the first time to our knowledge, a dual-comb laser based on thin-disk technology and its application to direct frequency comb spectroscopy are presented. The peak power (0.6 MW) and the average power (12 W) of our Yb:YAG thin-disk dual-comb system are more than one-order-of-magnitude higher than in any previous systems. The scheme allows easy adjustment of the repetition frequency difference during operation. A time-domain signal recorded over 10 {\\mu}s without any active stabilization was sufficient to resolve individual comb lines after Fourier transformation. The demonstration should enable a wider adoption of dual-comb systems towards practical applications in research laboratories. Its simplicity and compactness especially for the realization of tri- and multi-comb systems and conversion into the still poorly covered UV and VUV ranges makes it a promising next-generation technology.

We report on the generation of a three-octave-wide supercontinuum extending from the vacuum ultraviolet (VUV) to the near-infrared, spanning at least 113 to 1000 nm (i.e., 11 to 1.2 eV), in He-filled hollow-core kagome-style photonic crystal fiber. Numerical simulations confirm that the main mechanism is a novel and previously undiscovered interaction between dispersive-wave emission and plasma-induced blueshifted soliton recompression around the fiber zero dispersion frequency. The VUV part of the supercontinuum, which modeling shows to be coherent and possess a simple phase structure, has sufficient bandwidth to support single-cycle pulses of 500 attosecond duration. We also demonstrate, in the same system, the generation of narrower-band VUV pulses, through dispersive-wave emission, tunable from 120 to 200 nm with efficiencies exceeding 1% and VUV pulse energies in excess of 50 nJ.

BackgroundTreatment cessation in chronic HBV infection may be durable in certain patient subgroups before hepatitis B surface antigen (HBsAg) seroclearance. The role of serum HBV RNA in determining treatment cessation suitability has not been well-investigated.MethodsNucleos(t)ide analogue (NUC) treatment was discontinued in non-cirrhotic patients with chronic HBV with serum HBsAg <200 IU/mL and fulfilling internationally recommended criteria for treatment cessation. Patients were monitored till 48 weeks with baseline and serial measurements of serum HBsAg, HBV RNA and hepatitis B core-related antigen. NUCs were resumed when HBV DNA reaches >2000 IU/mL regardless of alanine aminotransferase (ALT) levels.Results114 entecavir-treated patients (median age 58.4 years, median serum HBsAg 54.4 IU/mL) with median treatment duration of 6.7 years were recruited. The 48-week cumulative rate of HBV DNA >2000 IU/mL was 58.1%. End-of-treatment serum HBV RNA and off-treatment serial HBV RNA were both independently associated with HBV DNA >2000 IU/mL (HR 2.959, 95% CI 1.776 to 4.926, p<0.001; HR 2.278, 95% CI 1.151 to 4.525, p=0.018, respectively). Patients with HBV RNA ≥44.6 U/mL had a cumulative 48-week rate of 93.2%, while combining HBV RNA undetectability and HBsAg <10 IU/mL had a cumulative 48-week rate of 9.1%. 24 patients (38.7%) developed off-treatment ALT elevation, highest peak ALT was 1515 U/L. 8 patients (median serum HBsAg 2.6 IU/mL) developed HBsAg seroclearance.ConclusionSerum HBV RNA measurement is essential for deciding on entecavir cessation in patients with chronic HBV, especially with low HBsAg levels. Patients can be stratified on their risk of off-treatment relapse based on both viral determinants.Trial registration number NCT02738554