Terahertz Generation in Lithium Niobate Driven by Ti:Sapphire Laser Pulses and its Limitations

We experimentally investigate the limits to 800 nm-to-terahertz (THz) energy conversion in lithium niobate at room temperature driven by amplified Ti:Sapphire laser pulses with tilted-pulse-front. The influence of the pump central wavelength, pulse duration, and fluence on THz generation is studied. We achieved a high peak efficiency of 0.12% using transform limited 150 fs pulses and observed saturation of the optical to THz conversion efficiency at a fluence of 15 mJ/cm2. We experimentally identify two main limitations for the scaling of optical-to-THz conversion efficiencies: (i) the large spectral broadening of the optical pump spectrum in combination with large angular dispersion of the tilted-pulse-front and (ii) free-carrier absorption of THz radiation due to multi-photon absorption of the 800 nm radiation.Comment: 4 pages, 6 figure

Spectral Phase Control of Interfering Chirped Pulses for High-Energy Narrowband Terahertz Generation

Highly-efficient optical generation of narrowband terahertz (THz) radiation enables unexplored technologies and sciences from compact electron acceleration to charge manipulation in solids. State-of-the-art conversion efficiencies are currently achieved using difference-frequency generation (DFG) driven by temporal beating of chirped pulses but remain, however, far lower than desired or predicted. Here we show that high-order spectral phase fundamentally limits the efficiency of narrowband DFG using chirped-pulse beating and resolve this limitation by introducing a novel technique based on tuning the relative spectral phase of the pulses. For optical terahertz generation, we demonstrate a 13-fold enhancement in conversion efficiency for 1%-bandwidth, 0.361 THz pulses, yielding a record energy of 0.6 mJ and exceeding previous optically-generated energies by over an order of magnitude. Our results prove the feasibility of millijoule-scale applications like terahertz-based electron accelerators and light sources and solve the long-standing problem of temporal irregularities in the pulse trains generated by interfering chirped pulses.Comment: 25 pages, 5 figures, updated to the state before review at Nature Communications (updated the affiliations, title, some content, methods, etc.

High energy multi-cycle terahertz generation

Development of compact electron accelerators and free-electron lasers requires novel acceleration schemes at shorter driving wavelengths. The Axsis project seeks to developterahertz based electron acceleration as well as the high energy terahertz sources required. This thesis explores the methods and optical material required for the generation of highenergymulti-cycle terahertz pulses. Two experimental concepts to generate high energy terahertz radiation are presented. In addition the theoretical background and the optical properties of pertinent optical materials in the terahertz range are discussed. Investigations of the materials are performed with a terahertz time domain spectrometer and a Fourier transform infrared spectrometer. The nonlinear optical crystal lithium niobate as well as other crystals suitable for the terahertz generation and in addition polymers and other radiation attenuators are characterized in the range from 0.2 to 1 THz. The theory describing the generation of narrowband terahertz radiation is evaluated. The experimental setups to generate terahertz radiation and to characterize its properties are described. The specific crystals (periodically poled lithium niobate (PPLN)) used in the experiments to generate the multi-cycle terahertz radiation are examined to determine e.g. the poling period. The first experimental concept splits the ultra fast, broadband pump pulses into a pulse train in order to pump the PPLN at a higher uence while increasing the damage limit. The measurements confirm that a pulse train of ultra short, broadband pump pulses increases not only the terahertz energy but also the energy conversion efficiency. The second experimental concept utilizes chirped and delayed infrared laser pulses. This pulse format makes it possible to pump the crystal with high energy pulses resulting in high energy terahertz radiation. The concept is optimized to reach energies up to 127 μJ exceeding the existing results of narrowband terahertz sources by two orders of magnitude. These results endorse the proposed methods of generating multi-cycle THz pulses at the mJ-level and show a pathway for further scaling to the multi-mJ-level required for theconstruction of the electron accelerator in the Axsis project

Temperature dependent refractive index and absorption coefficient of congruent lithium niobate crystals in the terahertz range

Optical rectification with tilted pulse fronts in lithium niobate crystals is one of the most promising methods to generate terahertz (THz) radiation. In order to achieve higher optical-to-THz energy efficiency, it is necessary to cryogenically cool the crystal not only to decrease the linear phonon absorption for the generated THz wave but also to lengthen the effective interaction length between infrared pump pulses and THz waves. However, the refractive index of lithium niobate crystal at lower temperature is not the same as that at room temperature, resulting in the necessity to re-optimize or even re-build the tilted pulse front setup. Here, we performed a temperature dependent measurement of refractive index and absorption coefficient on a 6.0 mol% MgO-doped congruent lithium niobate wafer by using a THz time-domain spectrometer (THz-TDS). When the crystal temperature was decreased from 300 K to 50 K, the refractive index of the crystal in the extraordinary polarization decreased from 5.05 to 4.88 at 0.4 THz, resulting in ~1° change for the tilt angle inside the lithium niobate crystal. The angle of incidence on the grating for the tilted pulse front setup at 1030 nm with demagnification factor of −0.5 needs to be changed by 3°. The absorption coefficient decreased by 60% at 0.4 THz. These results are crucial for designing an optimum tilted pulse front setup based on lithium niobate crystals

Terahertz time domain spectrometer to characterize nonlinear materials for efficient terahertz generation

Strong-field THz radiation is essential to several applications such as terahertz (THz) time-resolved spectroscopy and electron acceleration.Optical rectification in nonlinear materials such as lithium niobate (LN), lithium tantalate (LT) and serveral organic crystals, holdpromise in energy scalabilty of single-cycle THz generation. Fundamental properties of these materials are yet unexplored or poorly reportedin this wavelength regime. We investigate the spectroscopic properties of nonlinear materials employed for broadband THz generation withan in-house THz time-domain spectrometer (THz-TDS) -based on a 85 MHz Ti:Sapphire oscillator with 400 mWaverage power and 50 fs pulseduration. We employ our THz-TDS to study the absorptive, reflective, and amplitude/phase properties of LN and LT at cryogenic- and roomtemperature.We also report experimental results of strong-field THz generation optimized from our nonlinear materials study

On the effect of third-order dispersion on phase-matched terahertz generation via interfering chirped pulses

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On the effect of third-order dispersion on phase-matched terahertz generation via interfering chirped pulses

High-energy narrowband terahertz (THz) pulses, relevant for a plethora of applications, can be created from the interference of two chirped-pulse drive lasers. The presence of third order dispersion, an intrinsic feature of many high-energy drive lasers, however, can significantly reduce the optical-to-THz conversion efficiency and have other undesired effects. Here, we present a detailed description of the effect of third-order dispersion (TOD) in the pump pulse on the generation of THz radiation via phase-matching of broadband highly chirped pulse trains. Although the analysis is general, we focus specifically on parameters typical to a Ti:Sapphire chirped-pulse amplification laser system for quasi-phase-matching in periodically-poled lithium niobate (PPLN) in the range of THz frequencies around 0.5 THz. Our analysis provides the tools to optimize the THz generation process for applications requiring high energy and to control it to produce desired THz waveforms in a variety of scenarios

On the effect of third-order dispersion on phase-matched terahertz generation via interfering chirped pulses

International audienceHigh-energy narrowband terahertz (THz) pulses, relevant for a plethora of applications, can be created from the interference of two chirped-pulse drive lasers. The presence of third order dispersion, an intrinsic feature of many high-energy drive lasers, however, can significantly reduce the optical-to-THz conversion efficiency and have other undesired effects. Here, we present a detailed description of the effect of third-order dispersion (TOD) in the pump pulse on the generation of THz radiation via phase-matching of broadband highly chirped pulse trains. Although the analysis is general, we focus specifically on parameters typical to a Ti:Sapphire chirped-pulse amplification laser system for quasi-phase-matching in periodically-poled lithium niobate (PPLN) in the range of THz frequencies around 0.5 THz. Our analysis provides the tools to optimize the THz generation process for applications requiring high energy and to control it to produce desired THz waveforms in a variety of scenarios

Efficient generation of terahertz radiation at 800 nm wavelength

Highly efficient generation of strong-field terahertz (THz) pulses by using very short laser pulses ~30 fs has some challenges due to chromatic aberrations. Here, we demonstrate an optical-to-THz conversion efficiency of 0.2% using a 3 mJ pump pulse energy. This result paves the way for strong-field applications of THz radiation. Commercial Ti:sapphire laser systems delivering more than 20 mJ output pulse energy at the central wavelength of 800 nm with 150 fs pulse width is appropriate for table-top, compact terahertz (THz) sources [1]. The expected maximum THz output pulse energy can be scaled up to ~100 μJ, when employing optical rectification using tilted-pulse-fronts (TPF) and cryogenic cooling to mitigate THz absorption in the lithium niobate crystal. We have already demonstrated 0.2% optical-to-THz energy efficiency by using 150 fs Ti:sapphire laser pulses.In order to further scale up the THz output energy from the μJ to mJ-level, customized Ti:sapphire systems delivering J-level output pulse energy are promising. However, this kind of laser system has an extremely broadband infrared spectrum. When these ultrashort laser pulses (30 fs) are used for THz generation with the conventional TPF technique, there are several limitations [2]: (i) Effective interaction length for efficient THz generation will be shorter than longer pulses (150 fs). (ii) The diffracted optical beam from the grating will be expanded to an unmanageably large size due to the large bandwidth. (iii) Different spectral components will be imaged into different spatial volumes in the crystal. We systematically investigatedifferent imaging schemes including one concave mirror (f=-100-mm), two concave mirrors (f1=-200 mm, f2=-100 mm) and one bi-convex lens (f=60 mm) for THz generation using TPF in lithium niobate driven by 30 fs Ti:sapphire laser pulses. The best results of 6 μJ THz output energy, 0.2% optical-to-THz conversion efficiency with 20 MV/m electric field in lithium niobate at room temperature pumped at 3 mJ is achieved from the simplest scheme with one bi-convex lens as the imaging element, shown in Fig. 1 (a). As exhibited in Fig. 1 (b) and (c), the single-cycle THz pulse holds a peak frequency at 0.32 THz. The maintenance of 0.2% optical-to-THz efficiency from 150 fs to 30 fs is helpful for scaling up THz output energy from μJ to mJ-level when employing J-level ultrashort Ti:sapphiure laser pulses. Future work will be focused on cryogenic cooling of the generationcrystal, improving out-coupling of the THz pulse at the interface of lithium niobate and air, newly designing the generation lithium niobate crystals, trying contact-grating method to make a linear generation geometry, and finally scaling up the output THz energy by impinging the generation crystal with J-level laser pulses for THz generation