Physicists generated high-order harmonics in magnesium-doped lithium niobate (Mg:LiNbO3) and amorphous silicon (a-Si) using a macroscopic quantum state of light—a compressed bright vacuum. This generation proved to be approximately five times more efficient than standard harmonic generation using coherent light. The paper was published in Nature Physics.
High harmonic generation underlies many fundamental research and practical applications. For example, it is used to generate attosecond pulses (for which a Nobel Prize was awarded last year). High harmonics are typically obtained by pumping a sample with lasers—coherent light. However, the efficiency of such pumping is limited by various factors, such as photon energy, saturation, and loss of phase synchronization. Generation efficiency could be increased by using squeezed states of light. Scientists are already using such states, for example, in a photonic processor. For more information on squeezed states of light, see our article "Quantum Pencil Sharpener." However, in applications to high harmonic generation, the use of squeezed states of light has so far only been considered 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 the quantum state of light—a compressed, bright vacuum. They used magnesium-doped lithium niobate and amorphous silicon samples, generating high harmonics using classical and quantum methods.
For the pump laser in both methods, the physicists used 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. 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 the laser beam through a three-millimeter-thick barium β-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. They found that, at the same pump intensity, harmonic generation using the quantum state of light is 5 to 15 times more efficient, depending on the order of the harmonic. Furthermore, they demonstrated that, with increased pump intensity, classical generation can cause optical damage to the samples, while the quantum state of light does not harm them.
Read our article "Shortening the Pulse" to learn why scientists, including last year's Nobel laureates, are striving to obtain higher harmonics and shorter pulses.
