Thresholdless deep and vacuum ultraviolet Raman frequency conversion in hydrogen-filled photonic crystal fiber

Abstract

Coherent ultraviolet (UV) light has many uses, for example in the study of molecular species relevant in biology and chemistry. Very few if any laser materials offer UV transparency along with damage-free operation at high photon energies and laser power. Here we report efficient generation of deep and vacuum UV light using hydrogen-filled hollow-core photonic crystal fiber (HC-PCF). Pumping above the stimulated Raman threshold at 532 nm, coherent molecular vibrations are excited in the gas, permitting highly efficient thresholdless wavelength conversion in the UV. The system is uniquely pressure-tunable, allows spatial structuring of the out-coupled radiation, and shows excellent performance in the vacuum UV. It can also in principle operate at the single-photon level, when all other approaches are extremely inefficient. Main Text:Molecular species in biology, photochemistry and medicine (1) have outer-shell electronic transitions in the vacuum (VUV) and deep ultraviolet (DUV)-from ~100 nm to ~300 nm. Spectroscopy at these wavelengths requires tunable, compact and spectrally narrow UV light sources. Excimer lasers provide direct UV lasing transitions but are fixed-wavelength, inefficient and deliver poor beam quality. Although sum-frequency generation in nonlinear crystals provides a common alternative (2), to be efficient and tunable it requires stringent phase-matching over a broad range of wavelengths (almost impossible to realize in collinear geometries) along with high pump intensities and good spatial overlap between the interacting fields. Moreover, the wavelength-tunability of such systems is in general restricted because very few if any nonlinear crystals provide low dispersion, high transparency and resistance to photo-induced damage in the DUV/VUV. Although these issues have to some degree been addressed by Raman mixing in widebore capillaries and cells filled with gas (3, 4), that technique requires intense pump pulses and phase-matching.Gas-filled hollow-core photonic crystal fiber (HC-PCF), guiding by anti-resonant-reflection, has emerged as a promising alternative that is free from these restrictions. In addition to providing guidance from the VUV to the mid-infrared (5, 6), these fibers offer ultralong light-matter interaction lengths in a hollow channel only few tens of microns wide, together with pressuretunable dispersion (6). These fibers have reduced the threshold for stimulated Raman scattering (SRS) by orders of magnitude (7), paving the way for multi-octave Raman combs (8) reaching into the VUV (9), and broadband frequency conversion in the near infrared (10).In this paper we report efficient thresholdless frequency conversion of arbitrary DUV signals in hydrogen (9-11), which has the highest Raman gain and frequency shift (~125 THz for the Q(1) vibrational transition) of any gas and is transparent down to the VUV. We used a 40-cm-long kagomé-type HC-PCF ("kagomé-PCF") with a core 22 microns in diameter. When gas-filled, the

Intense Brillouin amplification in gas using hollow-core waveguides

“…(6) in the main manuscript, we obtain a Brillouin linewidth of 4.3 MHz, which is also close to the measured linewidth 3.65 MHz. So far, hydrogen gas shows the highest Raman gain (at a detuning frequency of 125 THz for the Q(1) vibrational transition) of any gas [9]. The peak plane-wave steady-state Raman-gain coefficient, g R (in units of cm/W), for the Q(1) transition for pump-laser wavelengths from 190 nm to 2 µm, densities of 1-100 amagats, at room temperature (298 K) is given as [10,11]:…”

Efficient Generation of Coherent Stokes Field in Hydrogen Gas-Filled Hollow Core Photonic Crystal Fibres

“…The energy transfer mechanism between pump -Stokes -coherence fields were explained in Ref. [11]. The formation and evolution, in the time-space phase plane (t, z), of the coherence and population inversion are presented in Fig.…”

“…However, the achievement of that regime in gas is difficult, it requires the applied pump pulse must be intense, ultrashort [2,4]. Gas filled HC-PCFs (hollow-core photonic crystal fibres) with the excellent properties opens the opportunity to study the transient SRS regime in gas by very long pump pulse duration [5,6], and discover a lot of interesting processes in SRS [6][7][8][9][10][11]. In SRS, the pump pulse width and the gas pressure filled inside HC-PCFs play an important role to optimize of interaction Raman efficiency and to generate Stokes frequency, especially to choose input parameters for desired Stokes field.…”

A Novel Gold Film-Coated V-Shape Dual-Core Photonic Crystal Fiber Polarization Beam Splitter Covering the E + S + C + L + U Band

“…Up to now, the fabrication technology of the PCFs with the circular air holes arranged in a hexagonal lattice has been developed more mature [ 26 , 27 , 28 , 29 , 30 , 31 , 32 ]. Many fabrication methods have been reported, including stack-and-draw, 3D printing, femtosecond laser drilling, etc.…”