Physicists have generated high harmonics in magnesium-doped lithium niobate (Mg:LiNbO3) and amorphous silicon (a-Si) using a macroscopic quantum state of light — a squeezed bright vacuum. This generation turned out to be about five times more efficient than standard harmonic generation with coherent light. The paper was published in Nature Physics.
High harmonic generation is the basis of many fundamental studies and practical applications. For example, it is used to obtain attosecond pulses (for which the Nobel Prize was awarded last year). High harmonics are usually obtained by pumping a sample using lasers — coherent light. However, the efficiency of such pumping is limited by various factors, such as photon energy, saturation, and loss of phase synchronization. The efficiency of generation could be increased by using compressed states of light. Scientists are already using such states, for example, in a photonic processor. For more information about compressed states of light, read our article “Quantum Pencil Sharpener”. However, in the application to high harmonic generation, the use of compressed states of light has so far been considered only theoretically.
A team of scientists from Germany, Israel and Canada led by Maria Chekhova and Francesco Tani from the Max Planck Institute for the Science of Light generated high harmonics in solids using a quantum state of light – a compressed bright vacuum. To do this, they used samples of magnesium-doped lithium niobate and amorphous silicon, in which they generated high harmonics using a classical and quantum method.
The same optical titanium-sapphire laser system with a central wavelength of 800 nanometers, a pulse width of 45 femtoseconds, and a repetition rate of one kilohertz served as a pump laser in both methods. By combining this system with a femtosecond optical parametric amplifier, the scientists obtained classical coherent light for generating high harmonics. To obtain a quantum state of light, the scientists, in parallel with the classical scheme, passed a laser beam through a three-millimeter-thick β-barium borate crystal, and then reflected the resulting state back into the same crystal using a flat silver mirror to reveal the fundamental spatial harmonic of the quantum state of light. As a result, the scientists obtained a quantum superposition of states with an even number of photons — a compressed bright vacuum.
Physicists irradiated samples using both methods and observed the generation of high harmonics in them. As a result, scientists found that at the same pumping intensity, harmonic generation by the quantum state of light is 5-15 times more efficient, depending on the order of the harmonic. Moreover, scientists showed that with an increase in pumping intensity, generation by the classical method can lead to optical damage to samples, while the quantum state of light does not harm the samples.
Read our article “Shortening the Impulse” to find out why scientists, including last year’s Nobel laureates, strive to obtain higher harmonics and shorter pulses.