Investigation of ultra-broadband terahertz generation from sub-wavelength lithium niobate waveguides excited by few-cycle femtosecond laser pulses

Abstract

The generation of coherent, ultra-broadband terahertz (THz) radiation pulses spanning more than a few octaves is vital to understanding the ultrafast response of elementary excitations, molecules, nanostructures, materials, and explore device functionality across a wide spectrum. In this work, we use 2D finite-difference time-domain simulations to show that ultra-broadband (0.18-106 THz) Cherenkov radiation can be produced from SiO:MgO-LiNbO:SiO waveguides having core dimensions that are sub-wavelength with respect to the optical pump pulse being guided. These sub-wavelength core dimensions allow the ultra-broad Cherenkov radiation to be emitted at an angle between 47.2° and 47.5° (dictated by the Si cladding layer dispersion), making these waveguide structures superior to the THz generation arrangements in bulk MgO-LiNbO crystals. When excited by a 7 fs, 780 nm laser pulse having an energy of 2 nJ, a 300 µm-long waveguide with transverse core dimensions of 500 nm × 2 mm can generate a sub-ps, kV/cm electric field pulse. Unlike THz pulse generation in bulk MgO-LiNbO crystals, having sub-wavelength core dimensions reduce the absorption from the MgO-LiNbO reststrahlen bands. These sub-wavelength SiO:MgO-LiNbO:SiO waveguides are ideal for on-chip applications that require ultra-broadband, compact THz sources.

Nonlinear Photonic Waveguides: A Versatile Platform for Terahertz Radiation Generation (a Review)

“…[58] A SiO 2 -LiNbO 3 -polymer planar waveguide (LiNbO 3 layer thickness of 530 nm and a waveguide length of 10 mm) was Adapted with permission. [57] Copyright 2017, The Optical Society. b) Spectral power of the OR THz radiation produced using a SiO 2 -LiNbO 3 -SiO 2 planar waveguide, where the inset shows the generated THz electric field pulse.…”

“…[ 57 ] THz radiation generation was numerically investigated in a 100 µm long planar waveguiding arrangement that had a LiNbO 3 thickness of 500 nm, which was excited using a 780 nm, 10 fs pulse. [ 57 ] The generated THz electric field pulse had a duration of hundreds of femtoseconds (see inset of Figure 5b), while the associated spectrum had a bandwidth extending up to nearly 100 THz (see Figure 5b). [ 57 ] Notably, unlike OR THz radiation generation from a bulk LiNbO 3 crystal, the sub‐wavelength core dimensions of the waveguide reduced the absorption from the LiNbO 3 reststrahlen bands and allowed the spectrum to approach 100 THz.…”

“…[ 57 ] Notably, unlike OR THz radiation generation from a bulk LiNbO 3 crystal, the sub‐wavelength core dimensions of the waveguide reduced the absorption from the LiNbO 3 reststrahlen bands and allowed the spectrum to approach 100 THz. [ 57 ] OR THz radiation generation from a similar arrangement was experimentally observed using a longer‐duration optical excitation pulse. [ 58 ] A SiO 2 –LiNbO 3 –polymer planar waveguide (LiNbO 3 layer thickness of 530 nm and a waveguide length of 10 mm) was excited using an 800 nm, 50 fs excitation pulse polarized along the LiNbO 3 c crystallographic axis.…”

“…This arrangement was shown to be capable of generating ultrabroadband (i.e., 0.18–81 THz) OR THz radiation. [ 57 ] By contacting Si prisms to the planar waveguide, the THz radiation avoided total internal reflection and was emitted into free space as Cherenkov waves. [ 57 ] THz radiation generation was numerically investigated in a 100 µm long planar waveguiding arrangement that had a LiNbO 3 thickness of 500 nm, which was excited using a 780 nm, 10 fs pulse.…”

Stable and compact RF-to-optical link using lithium niobate on insulator waveguides

“…Lithium niobate on insulator (LNOI) has gained a lot of momentum in recent years as a platform for photonic integrated circuits (PICs) 1,2 . Impressive demonstrations include for example efficient electro-optic modulators [3][4][5] , an on-chip spectrometer 6 , lasers and amplifiers [7][8][9] , optical parametric oscillators 10,11 , efficient second harmonic generation 12,13 , acousto-optic modulation 14 , THz generation 15,16 , Kerr frequency combs [17][18][19] , supercontinuum generation [20][21][22] and self-referencing of frequency combs 23,24 . These results show the versatility of the LNOI platform and its great potential for future applications in fields such as sensing, distance measurement, communication, metrology and many more 2 .…”