The pyrochlore magnet ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ shows a lack of magnetic order to low temperatures and is considered to be a quantum spin liquid candidate. We perform time-domain THz spectroscopy on high-quality ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ crystals and study the low-energy excitations as a function of [111] magnetic field with high energy resolution. The low-energy crystal-field excitations change their energies anomalously under magnetic field. Despite several sharp field-dependent changes, we show that the material's spectrum can be described not by phase transitions but by field-dependent hybridization between the low-energy crystal-field levels. We highlight the strong coupling between spin and lattice degrees of freedom in ${\mathrm{Tb}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ as evidenced by the magnetic-field tunable crystal-field environment. Calculations based on single ion physics with field-induced symmetry reduction of the crystal-field environment can reproduce our data.

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• Frustrated magnetism
• Magneto-optical spectra
• Quantum spin liquid
• Pyrochlores
• Terahertz spectroscopy
• Terahertz time-domain spectroscopy