The invention relates to methods of generating THz (terahertz) radiation, in particular by irradiating a conversion crystal device with optical input radiation providing and generating the THz radiation by an optical-to-THz-conversion process. Furthermore, the invention relates to a THz source apparatus, being adapted for generating THz radiation and comprising an input radiation source device and a conversion crystal device for generating the THz radiation by the optical-to-THz-conversion process. Applications of the invention are available in driving strong field processes and especially acceleration of particles. According to a preferred application of the invention, the THz source apparatus is employed in a THz accelerator of charged particles, e.g., electrons.
In the present specification, reference is made to the following publications cited for illustrating prior art techniques.    [1] U.S. Pat. No. 7,764,422 B2;    [2] U.S. Pat. No. 7,400,660 B2;    [3] U.S. Pat. No. 8,554,083 B2;    [4] S. Carbajo et al. in “Optics Letters”, vol. 40, p. 5762-5765, 2015;    [5] K. Vodopyanov et al. in “Applied Physics Letters”, vol. 99, p. 041104, 2011;    [6] EP 2 309 325 A1;    [7] U.S. Pat. No. 7,953,128 B2.    [8] C. Klieber et al. in “Applied Physics Letters”, vol. 98, p. 211908, 2011;    [9] Z. Chen et al. in “Applied Physics Letters”, vol. 99, p. 071102, 2011;    [10] S. R. Tripathi et al. in “Optics Letters”, vol. 39, no. 6, p. 1649, 2014;    [11] K. Kawase et al. in “Journal of Physics D: Applied Physics”, vol. 35, pp. R1-R13, 2002;    [12] U.S. Pat. No. 8,699,124 B2;    [13] U.S. Pat. No. 8,305,679 B2;    [14] T. D. Wang et al. in “Optics Express”, vol. 21, no. 2, p. 2452, 2013;    [15] J. R. Danielson, et al. in “Journal of Applied Physics”, vol. 104, p. 033111, 2008;    [16] M. Cronin-Golomb in “Optics Letters”, vol. 29, no. 17, page 2046, 2004;    [17] K. Vodopyanov et al. in “Optics Express”, vol. 14, no. 6, page 2263, 2006; and    [18] L. Pengxiang et al. in “Journal of Lightwave Technology”, vol. 31, no. 15, pages 2508-2514, 2013.
It is generally known that THz radiation can be generated with free-electron lasers, vacuum electronic devices, such as gyrotrons, molecular lasers, or photoconductive switches. These techniques have disadvantages with regard to at least one of complexity and costs, limited frequency ranges, low conversion efficiencies, limited THz pulse energy ranges and limited peak power and/or average power ranges. Furthermore, THz radiation can be generated with laser-based approaches which use laser pulses from an input radiation source to pump a nonlinear crystal with non-zero second order nonlinearity and generate THz radiation via an optical-to-THz-conversion process, including difference-frequency-generation (DFG) or optical rectification. DFG employs two distinct narrowband input lasers for THz generation, while optical rectification is based on a single broadband input pulse creating THz radiation by intra-pulse DFG in periodically poled crystals.
Proposals for low power terahertz generation systems based on the optical-to-THz-conversion process have been described e.g., in [1], [2] and [3]. However, these systems have not so far generated THz radiation with 10 MW to GW peak powers and mJ to several mJ pulse energies as required e.g. for particle accelerator applications. The best optical to THz energy conversion efficiencies (or energy conversion efficiency or conversion efficiency) reported with the conventional laser-based THz generation methods is 0.1% [4]. Limitations for scaling conversion efficiencies to higher levels mainly result from details of the employed input radiation source, the nonlinear crystals and the optical-to-THz-conversion processes as described in the following.
Firstly, high energy laser sources for scaling conversion efficiencies with sufficient 0.1 kHz to few kHz repetition rates of interest and Joule level pulse energies are most accessible with wavelengths below 1.1 μm, e.g., 1.1 μm to 800 nm. However, a common feature of nonlinear crystals conventionally used for laser-based THz generation, like e.g., Gallium Arsenide (GaAs), Gallium Phosphide (GaP), other semiconductor materials or organic materials, is the requirement of input radiation sources of wavelengths greater than 1.3 μm. In particular, GaAs with a bandgap energy of 1.42 eV and GaP or Zinc Telluride (ZnTe) with bandgap energies of 2.26 eV are unrealistic candidates for scaling THz energies and conversion efficiencies since they are prone to multi-photon absorption of 1 μm/800 nm lasers. It is extremely challenging to engineer laser sources at the 1.3 μm or longer wavelengths producing a fraction of 1 J, let alone more of pump energy.
Phase-matching or velocity matching in the nonlinear crystal is required for efficient THz generation. Conventionally employed phase-matching techniques comprise phase-matching of the optical input and THz radiation by e.g., tilted-pulse-front technique, quasi phase-matching, and waveguide based phase matching. Limitations for scaling conversion efficiencies exist in particular for the tilted-pulse-front technique having a high complexity for high energy pump pulses and the waveguide based phase matching using engineering the phase velocity of the THz radiation. In particular, as waveguide dimensions have to be on the order of the THz wavelength (about mm), waveguide structures (in particular optical fibers) cannot be pumped by Joule level optical pulse energies due to damage limitations.
Conventional input radiation sources for pumping the nonlinear crystal and generating THz radiation by DFG use e.g., two or more quasi-monochromatic lasers having frequencies that are separated by the THz frequency to be generated and having synchronized laser pulse repetition rates and phases. However, applications of these input radiation sources using at least two lasers have been limited to low power THz generation systems.
An example of optical rectification with intra-pulse DFG is described in [4]. As schematically illustrated in FIG. 26 (prior art), a single pass configuration is provided, wherein optical input radiation, like e.g., a single broadband laser pulse, from an input radiation source 10′ is imaged with an imaging system 20′ into the nonlinear crystal 30′, where the THz radiation is generated by a single passage of the optical input radiation. The input radiation source 10′ includes an ultra-short pulsed mode-locked oscillator, which is amplified to high energies, typically of several mJ at kHz repetition rates by using state-of-the art solid-state amplifiers. However, even by cryogenic cooling and optimization of the pulse bandwidth, conversion efficiency is limited to the 0.1% for terahertz generation in periodically poled lithium niobate crystals (PPLN) [4].
Numerical simulations of the conversion efficiency as a function of transform limited pulse duration of the pump laser for various terahertz frequencies in cryogenically cooled Lithium Niobate (with quasi phase matching period of crystal optimized for this THz frequency) show that the peak conversion efficiency occurs at shorter transform limited pulse durations for larger THz frequencies. This is natural since THz radiation is basically generated by beating of spectral components of the optical spectrum. At long pulse durations, there is not enough spectral content for efficient generation of the higher terahertz frequencies. However, in practice obtaining compressed pulses smaller than 500 fs at joule level pump energies and high repetition rates is very challenging and virtually unprecedented. Practically, ps long pulses are more feasible at high pulse energies. However, in this parameter range, the conversion efficiency is well below the 1% level.
As an alternative, THz generation can be based on conventional DFG using optical parametric oscillators (devices based on cavities) (OPO) specifically close to degeneration [6]. However, this also places limitations on the optical-to-THz energy conversion efficiency. According to [5] and as schematically shown in FIG. 27 (prior art), a series of frequency lines was generated with an input radiation source 10′ including an optical parametric amplifier to generate THz radiation in the nonlinear crystal 30′ made of GaAs using light at wavelengths between 1.3 and 2 μm. The nonlinear crystal 30′ is arranged in a resonant cavity 20′ creating multiple passages of the optical input radiation in the nonlinear crystal 30′. The disadvantages with this approach are firstly that lower repetition rates on the order of kHz are hard to realize using an OPO configuration. Furthermore, this demonstration produced only 200 μW of average THz power for 20 to 30 W of average optical pump power. This corresponds to low conversion efficiencies on the order of 0.001%. Finally, the resonant cavity 20′ restricts the range of optical pump frequencies which can be used for THz generation. This is because the cavity shall be stable only for a small set of frequencies. This limits the optical-to-THz energy conversion thus achieved. Therefore, even if the challenge of producing high energy Joule-level 2 μm optical input radiation were to be solved, the approach is fundamentally limited in its conversion efficiency.
Another parametric amplifier system using terahertz as a seed in a non-collinear phase matched generation process in bulk lithium niobate crystals pumped by a 1.064 μm optical pump source at room temperature (i.e. 300 K) was demonstrated, producing 10 nJ THz pulses ([10], [11], [12], and [13]). The optical-to-THz energy conversion efficiencies of such configurations are very low due to the pJ to nJ level THz seed. Secondly, the non-collinear configuration is experimentally tedious. Thirdly, the use of a THz seed offers limited possibilities in achieving high conversion efficiencies since THz seed energies of high value are hard to obtain. In [14], OPA behavior in periodically poled lithium niobate was studied (but not demonstrated) under conditions of large absorption (corresponding to room temperature operation) for generation at the 0.1% level or within the Manley-Rowe limit.
A THz generation system using difference frequency generation with one of the pump signals generated by an optical parametric oscillator scheme was introduced [7]. Multiple terahertz generation stages were included providing multiple DFG stages driven by OPO sources. However the construction of many OPO's is cumbersome and impractical.
Finally, with a chirp and delay approach, an ultrafast broadband optical pulse is chirped, split into two and then re-combined with a relative delay to generate an interferometric pattern tuned to the desired terahertz frequency. This technique has been demonstrated in ZnTe [15] and also for the tilted-pulse-front technique in lithium niobate [9]. For ZnTe, the conversion efficiency was about 0.0003%. However as discussed, ZnTe does not lend itself to being amenable for scaling to very high conversion efficiencies due to its incompatibility with the most probable 800 nm/1 μm laser technology. Furthermore, as discussed the tilted-pulse-front technique has fundamental limitations which prevent its use with high energy beams.
Further studies of THz generation based on DFG are described in [16] and [17], wherein the DFG process is based on two lines of equal strength. Furthermore, if ωP is the frequency of the pump photon and ωTHz is the frequency of the generated terahertz frequency, the cascaded DFG regime obtained by [16] has a conversion efficiency of
      η    =          N      ⁢                        ω          THz                          ω          P                      ,with maximum N, that can be theoretically achieved is around 2 only.
According to the THz generation described in [18], Cerenkov phase-matching is employed, which is intrinsically non-collinear and involves multiple reflections of the generated THz wave. Thus, [18] does not use a single-pass device. This is particularly true since [18] uses a nonlinear process, where the THz wave is first generated, it then propagates non-collinearly, reflects off the walls of the slab and then re-joins the propagating optical field in the crystal. The model that is used to describe the system is in fact an approximation which might not hold in a dramatically cascaded regime. Furthermore, the waveguide like geometry used in [18] induces significant dispersion, limiting the cascading process. Secondly, absorption of terahertz will also limit the extent of cascading. The spectrum described here once again involves 2 lines of equal strength.