DSTMS-Based Ultrabroadband Terahertz Time-Domain Spectroscopy

Repetition rate tuning enables the fast acquisition of THz pulse profiles. By using this method we demonstrate a compact and broadband terahertz time domain spectroscopy system (THz TDS) driven by ytterbium doped fiber laser source. The importance of this method is realized in that Yb:doped fiber lasers can be amplified to sub-millijoule pulse strengths more easily than other types of fiber lasers. Hence, it has the potential to be used in excite-THz probe experiments. Furthermore, the repetition rate-tuning adds flexibility in the excite-probe techniques. These attributes as well as THz generation and detection are investigated with the laser that was developed.

The development of femtosecond lasers with high repetition rates combined with high average power is making this technology more relevant to the needs of the manufacturing community particularly in the field of micromachining. Although the scientific community has extensively studied the interaction of femtosecond laser pulses with materials, there remain aspects of the machining process that merit further study, particularly in relation to hybrid and assisted laser processes. This paper reviews the implementation of a multi-functional femtosecond laser and diagnostic system designed to allow a comprehensive analysis of ultrafast machining processes. The laser system is based on a diode-pumped Ytterbium laser with a sub 500 fs pulse width at 1030 nm, 515 nm, and 343 nm with repetition rates up to 300 kHz. Shorter pulse durations (sub 100 fs) at these wavelengths are achieved using spectral broadening in a Photonic Crystal Fiber and subsequent pulse compression. Oscillator laser output at 29 MHz is also available. The process diagnostic system includes online Raman spectroscopy, Schlieren imaging, and high speed video analysis.The development of femtosecond lasers with high repetition rates combined with high average power is making this technology more relevant to the needs of the manufacturing community particularly in the field of micromachining. Although the scientific community has extensively studied the interaction of femtosecond laser pulses with materials, there remain aspects of the machining process that merit further study, particularly in relation to hybrid and assisted laser processes. This paper reviews the implementation of a multi-functional femtosecond laser and diagnostic system designed to allow a comprehensive analysis of ultrafast machining processes. The laser system is based on a diode-pumped Ytterbium laser with a sub 500 fs pulse width at 1030 nm, 515 nm, and 343 nm with repetition rates up to 300 kHz. Shorter pulse durations (sub 100 fs) at these wavelengths are achieved using spectral broadening in a Photonic Crystal Fiber and subsequent pulse compression. Oscillator laser output at 29 MHz is also av...

It has been shown that the spectrum of intense few-cycle terahertz radiation generated in a DAST organic crystal can be controlled by chirping 1.24-μm pump femtosecond laser radiation of a chromium forsterite laser system. It has been found that an increase in the linear chirp of generating radiation results in the narrowing of the spectrum of terahertz radiation and its redshift. The simulation of the generation of terahertz radiation within the model of three-wave mixing has shown that this effect is due to a change in the phase matching width of the degenerate generation of the difference frequency of terahertz range. In addition, the comparative analysis of terahertz radiation spectra generated in DAST, DSTMS, OH1, and BNA organic crystals indicates that the spectral–temporal properties of terahertz radiation can be more roughly controlled by choosing an appropriate crystal. The proposed approach to control the terahertz radiation spectrum by chirping the pump pulse provides the foundation for spectroscopic studies using intense terahertz radiation with controlled spectral–temporal properties.

This work reports on the creation of luminescence inside organic crystal using a femtosecond laser on micron-scale in 3D. We thus show the possibility to modify physical properties by a particular photochemistry and space-selective functionalization.

We report on the efficient generation of intense terahertz radiation from the organic crystal N-benzyl-2-methyl-4-nitroaniline pumped by chirped Ti:sapphire femtosecond laser pulses. The THz energy and spectrum as a function of the pump fluence and duration of the chirped laser pulses are studied systematically. For the appropriate positively chirped pump pulses, a significant boost in the THz generation efficiency by a factor of around 2.5 is achieved, and the enhancement of high-frequency components (>1 THz) shortens the THz pulse duration. Via complete characterization of THz properties and transmitted laser spectra, this nonlinear behavior is attributed to the extended effective interaction length for phase matching as a result of the self-phase modulation of the intense pump laser pulses. Numerical calculations well reproduce the experimental observation. Our results demonstrate a robust, efficient, strong-field (up to several MV/cm) THz source using the common sub-10 mJ and sub-100 fs Ti:sapphire laser systems without optical parametric amplifiers.

Improving unbiased terahertz photoconductive antenna based on dissimilar Schottky barriers using plasmonic mode excitation

Antenna-coupled Ti-microbolometers for High-sensitivity Terahertz Imaging

High-power femtosecond mid-infrared (MIR) lasers are of vast importance to both fundamental research and applications. We report a high-power femtosecond master oscillator power amplifier laser system consisting of a single-mode Er:ZBLAN fiber mode-locked oscillator and pre-amplifier followed by a large-mode-area Er:ZBLAN fiber main amplifier. The main amplifier is actively cooled and bidirectionally pumped at 976 nm, generating a slope efficiency of 26.9%. Pulses of 8.12 W, 148 fs at 2.8 μm with a repetition rate of 69.65 MHz are achieved. To the best of our knowledge, this is the highest average power ever achieved from a femtosecond MIR laser source. Such a compact ultrafast laser system is promising for a wide range of applications, such as medical surgery and material processing.

During the last decades femtosecond lasers have proven their vast benefit in both scientific and technological tasks. Nevertheless, one laser feature bearing the tremendous potential for high-field applications, delivering extremely high peak and average powers simultaneously, is still not accessible. This is the performance regime several upcoming applications such as laser particle acceleration require, and therefore, challenge laser technology to the fullest. On the one hand, some state-of-the-art canonical bulk amplifier systems provide pulse peak powers in the range of multi-terawatt to petawatt. On the other hand, concepts for advanced solid-state-lasers, specifically thin disk, slab or fiber systems have shown their capability of emitting high average powers in the kilowatt range with a high wall-plug-efficiency while maintaining an excellent spatial and temporal quality of the output beam. In this article, a brief introduction to a concept for a compact laser system capable of simultaneously providing high peak and average powers all along with a high wall-plug efficiency will be given. The concept relies on the stacking of a pulse train emitted from a high-repetitive femtosecond laser system in a passive enhancement cavity, also referred to as temporal coherent combining. In this manner, the repetition rate is decreased in favor of a pulse energy enhancement by the same factor while the average power is almost preserved. The key challenge of this concept is a fast, purely reflective switching element that allows for the dumping of the enhanced pulse out of the cavity. Addressing this challenge could, for the first time, allow for the highly efficient extraction of joule-class pulses at megawatt average power levels and thus lead to a whole new area of applications for ultra-fast laser systems.

Quinolinium-based organic electro-optic crystals: Crystal characteristics in solvent mixtures and optical properties in the terahertz range

In last couple of decades, the THz science and technology has made huge leap of progress and THz sources and detectors in limited field strength and frequency bandwidth are being made available commercially for applications in spectroscopy, sensing, imaging and medical diagnostics. This is primarily because of the mature NIR femtosecond laser technology and the coherent nature of the THz radiation allowing researchers to create simple table top experimental tools for their THz studies. However, there is a growing demand for intense THz sources which can allow perform nonlinear optics and spectroscopy and develop new methodologies. For nonlinear THz radiation and matter interaction, one needs typically THz-fields comparable to the atomic fields $(\sim 10\wedge 6\mathrm{V}/$cm) such as those provided by lasers in other regions of the electromagnetic spectrum. Photoconductive antennas based on narrow band-gap materials such as InGaAs have seen a great improvement in their performances in terms of optical-to-THz efficiencies and maximum output pulsed THz radiation powers at frequencies above 1 THz due to the availability of high power laser sources operating at the optical telecommunication wavelengths. Even more powerful THz emitters have generally been developed by using nonlinear optical crystals where optical rectification of intense near infrared femtosecond laser pulses produces broadband THz radiation of field strengths reaching few hundreds of kV/cm to even few thousands kV/cm. Zinc telluride and lithium niobate crystals are very popular in this case [1]. Another popular mechanism for generating strong THz fields is by creating plasma in air and phase mismatch between the fundamental and the second harmonic fields of NIR femtosecond pulses [2]. Hetero-structures of layers of ferromagnetic materials and nonferromagnetic films have recently been used to produce THz radiation by excitation with femtosecond laser pulses [3].

The development of photonic and optoelectronic technologies, such as laser systems, high‐speed optical communication, quantum information science, and medical imaging, relies significantly on nonlinear optical (NLO) materials. Understanding and predicting their nonlinear optical responses is paramount for material innovation. Density Functional Theory (DFT) is important computational method for predicting electronic structure and calculating optical material properties. The present review gives an overview of application of DFT to study NLO materials that can be classified into organic molecules, inorganic crystals, metal‐organic frameworks (MOFs), hybrid perovskites, and 2D materials. We have also discussed the challenges, recent advancements, and emerging research areas in this field.

Effective upconversion emission from an organic crystal of cyano-substituted oligo (p-phenylenevinylene) (CNDPASDB) based on two-photon absorption is presented. Frequency upconverted cavityless lasing, or amplified spontaneous emission, from the crystal pumped by a femtosecond laser of 800 nm was observed when the excitation energy exceeded the threshold of 1.3 mJpulse(-1)cm(-2). Its polarization contrast was estimated to be approximately 0.93. This large ratio is due to the unified unidirectional configuration of the molecular long axis in crystal, beneficial to the stimulated emission with a low threshold. These results indicate that the present CNDPASDB crystal has a potential for upconversion laser device application.

Highly-integrated high-reliability widely-tunable femtosecond laser sources have important application values in various research and application fields, such as ultrafast spectroscopy, quantum optics, remote sensing and bio-imaging. In multi-photon excited fluorescence microscopy, femtosecond laser sources with moderate pulse energy and wide wavelength tunable range can not only meet the application requirements of the different tissue structures for the peak power and excitation wavelength, but also improve the nonlinear fluorescence efficiency and imaging resolution of the sample, and thus enhancing the penetration depth. Considering the extensive application prospect and important scientific research significance of the widely tunable femtosecond laser, in this paper we conduct an experimental research of the high repetition rate multi-wavelength femtosecond laser generation in compact sized and low-cost configuration based on the nonlinear propagation scheme of the high peak power femtosecond laser pulses in single-mode fiber. In experiment, we first construct a highly-integrated reliable all-polarization-maintaining fiber femtosecond laser amplifier, which mainly consists of an environmentally stable all-polarization-maintaining fiber mode-locked laser oscillator, single-mode fiber stretcher, a single-mode power pre-amplifier, a dual-cladding Yb-fiber amplifier, and transmission grating-pair compressor. Self-starting mode-locked operation is assured with a semiconductor saturable absorber mirror, and intra-cavity dispersion compensation is realized by a chirped fiber Bragg grating in the mode-locked oscillator. The mode-locked oscillator, which delivers laser pulses with center wavelength peaked at 1035 nm, is robust operation as temperature changes from 10℃ to 40℃ and the measured power fluctuation is less than 1% RMS over 168 hours at 23℃. The amplified high repetition rate laser pulses are compressed in a double-pass 1000 lines/mm transmission grating-pair compressor. After compression, laser pulses with 5.83 W average power and 264 fs pulse duration at 34 MHz repetition rate can be obtained. Simultaneously, we also study the dependence of the compressed pulse duration on the amplified output power. Employing a home-made high reliable compact sized all-polarization-maintaining fiber femtosecond laser as a pump source and low-cost single-mode fiber as a nonlinear medium, the generation technology of the widely tunable femtosecond laser in only fiber format is also studied based on the self-phase modulation nonlinear spectral broadening mechanism. Simultaneously, in order to reduce the effect of the dispersion on the spectral broadening as much as possible, an 80-mm-long fiber is used in experiment. The used single-mode spectral broadening fiber has a 6-m-diameter core and 20 fs2/mm dispersion coefficient. By coupling the femtosecond pump laser pulses into the 6-m-diameter fiber core, the output spectrum presents a significant nonlinear broadening. The coupled pump power can be continuously adjusted by a combination of a half-wave plate and a Glan laser polarizer. After bandpass filtering the leftmost and rightmost spectral lobes in self-phase modulation and self-steeping induced broadened spectrum with bandpass filters centered at 980, 1000, 1050, 1070 and 1100 nm, the laser pulses with 203, 195, 196, 187, and 194 fs pulse duration can be obtained at the corresponding center wavelengths. The experimental scheme presented in this paper, which is based on the nonlinear spectral broadening of the highreliability femtosecond laser pulse in single-mode fiber and the spectral selectivity technology, provides a new research approach to the realization of the highly-compacted reliable widely-tunable femtosecond laser sources and has important research significance.

Heat-sensitive materials, such as polymers, are used increasingly in various industrial sectors such as medical device manufacturing and organic electronics. Medical applications include implantable devices like stents, catheters and wires, which need to be structured and cut with minimum heat damage. Also the flat panel display market moves from LCD displays to organic LED (OLED) solutions, which utilize heat-sensitive polymer substrates. In both areas, the substrates often consist of multilayer stacks with different types of materials, such as metals, dielectric layers and polymers with different physical characteristic. The different thermal behavior and laser absorption properties of the materials used makes these stacks difficult to machine using conventional laser sources. Femtosecond lasers are an enabling technology for micromachining of these materials since it is possible to machine ultrafine structures with minimum thermal impact and very precise control over material removed. An industrial femtosecond Spirit HE laser system from Spectra-Physics with pulse duration 120 μJ and average output powers of >16 W is an ideal tool for industrial micromachining of a wide range of materials with highest quality and efficiency. The laser offers process flexibility with programmable pulse energy, repetition rate, and pulse width. In this paper, we provide an overview of machining heat-sensitive materials using Spirit HE laser. In particular, we show how the laser parameters (e.g. laser wavelength, pulse duration, applied energy and repetition rate) and the processing strategy (gas assisted single pass cut vs. multi-scan process) influence the efficiency and quality of laser processing.