I. Ultrafast spin dynamics in ferromagnets excited by fs-laser pulses
- New method for generation and control of magnons (spin waves) in magnetic dielectric films by femtosecond laser pulses was developed and demonstrated. It was shown that 100-fs-laser pulses focused in a spot of about several microns in diameter on transparent magnetic film excite in this film spin waves. The physical mechanism is related to the stimulated Raman scattering and Inverse Faraday effect.
- It was shown that periodic influence of femtosecond laser pulses on a magnetic film makes it possible to create a magnon cloud with the following properties: possibility of frequency tuning; spin waves amplitude could be significantly increased; narrow spectrum; spin waves amplitude at a distance of about 100 μm from the source is practically constant; directivity.
- Development of a novel approach for tuning wavelength of spin waves generated by a train of fs-laser pulses. We launched spin waves by a sequence of fs-laser pulses with pulse interval much shorter than the relaxation time of the magnetization oscillations. This lead to the cumulative phenomenon and allowed us to generate magnons in a specific narrow range of wavenumbers. The wavelength of spin waves can be tuned from 15 μm to hundreds of microns by sweeping the external magnetic field by only 10 Oe or by slight variation of the pulse repetition rate.
- We performed switching between different types of spin waves by variation of fs-laser beam diameter. In particular, we modified the parameters of the circularly polarized optical pump beams emitted by femtosecond laser to reveal surface spin waves in bismuth iron garnet thin film. Beams that are larger than 10 μm in diameter generate both surface and volume spin waves with only one spectral peak near the ferromagnetic resonance. On the contrary, narrower beams excite predominantly surface spin waves of higher frequency, providing an additional peak in the spin wave spectrum. Thus, different interference patterns of the magnetization dynamics are achievable.
II. Plasmonic-mediated confinement and enhancement of the magneto-optical effects
- We proposed, fabricated and demonstrated novel nanostructured material – longitudinally magnetized plasmonic crystal, which makes it possible to efficiently control the intensity of transmitted or reflected radiation and plasmon-polaritons by external magnetic field. Magnetic plasmonic crystal consists of homogeneous layer of ferromagnetic dielectric (rare-earth iron-garnet with bismuth substitution) on nonmagnetic substrate with a layer of noble metal (gold) on its surface perforated with periodic system of slits or holes.
- Giant longitudinal intensity effect (~25%) arising in magnetic plasmonic crystals with waveguide layer magnetized in the longitudinal configuration, i.e. perpendicular to lattice gaps of the crystal and in the film plane, was experimentally demonstrated. Usually, magneto-optics provides intensity modulation by lase a percent, but with plasmonic cover we managed to achieve much higher values.
- Ultrasensitive magnetoplasmonic biosensor was proposed and demonstrated. Change in refractive index of the medium is registered via observation over changes in spectrum of transverse magneto-optical Kerr effect. The structure consists of a one-dimensional photonic crystal covered with nm-thick ferromagnetic layer. Ultra-long-range magnetoplasmons (propagation distance exceeds 100 μm) can be excited in this structure. In the far-field it provides ultra-narrow (0.06°) resonance in reflectance and even narrower resonance (0.02°) of the magneto-optical effect. The magneto-optical Kerr effect reaches 11%, which allows the detection limit of 10-6 RIU.
- Magnetoplasmonic quasicrystals were proposed and demonstrated. We have introduced a new concept for getting significant magneto-optical effects in the broadband wavelength range by using magnetoplasmonic quasicrystals. They are based on the gold grating perforated with the subwavelength slits forming Fibonacci binary sequence and magnetic dielectric film. While transmission spectra of the periodic and quasiperiodic patterns are quite similar, the TMOKE spectra demonstrate significant difference. Namely, for the quasicrystal the magneto-optical response is much more abundant. It demonstrates that TMOKE spectroscopy is an efficient tool for investigation of the peculiarities of plasmonic quasicrystals.
- Photoinduced segregation of Te from CdTe nanolayer in plasmonic crystals was achieved. We demonstrate efficient 3.6 THz modulation of light reflected from hybrid metal/semiconductor plasmonic crystals caused by lattice vibrations in a few-nm-thick layer of elemental tellurium. Surface plasmons at the gold/semiconductor interface provide energy localization that leads to the efficient formation of Te segregation at the interface.
- Transverse magneto-optical Kerr Effect in active magneto-plasmonic structures is theoretically described. It is shown that it can be enhanced by optical amplification of the doped ferromagnetic. This enhancement is a resonant effect. The stimulated emission of dopants compensates the losses of the plasmonic waves and, consequently, increases Q-factor of the magneto-plasmonic resonance of active nanostructure in comparison with the passive one.
- We have studied Schrödinger plasmon–solitons in Kerr nonlinear heterostructures with magnetic manipulation. In details, surface plasmon–soliton (SPS) propagation in transverse magnetic field in heterostructures with Kerr nonlinearity was investigated. The effect of the magneto-optical nonreciprocity in the Schrödinger equation is found. The estimations show that the effect is the strongest for the double-interface structure with a magnetic substrate in the vicinity of the resonant plasmonic frequency.
- Transverse magnetic field impact on waveguide modes of photonic crystals. We showed that the propagation constants of the TM waveguide modes are sensitive to the transverse magnetization and the spectrum of the transverse magneto-optical Kerr effect has resonant features at mode excitation frequencies. Two types of structures are considered: a non-magnetic photonic crystal with an additional magnetic layer on top and a magnetophotonic crystal with a magnetic layer within each period. We found that the magneto-optical non-reciprocity effect is greater in the first case: it has a magnitude of δ ∼ 10-4.
- Electric-field-driven magnetic domain wall in iron-garnet films was demonstrated as a microscale magneto-optical shutter. This represents a new concept for the light control via electric field applied locally to a magnetic domain wall playing the role of nanodevice. In detail, we charged a 15-μm-thick metallic tip to generate strong non-uniform electric field in the vicinity of the domain wall in the iron garnet film. The electric field influences the domain wall due to flexomagnetoelectric effect and causes the domain wall shift. The resulting displacement of the domain wall is up to 1/3 of domain width and allows to demonstrate a novel type of the electrically controlled magneto-optical shutter. Polarized laser beam focused on the electricfield-driven domain wall was used to demonstrate the concept of a microscale Faraday modulator. Such variability to control of domain wall’s displacement with spatial scale of about 10 μm makes the proposed concept very promising for nanophotonics and spintronics.
III. Ultrasensitive room temperature magnetometry and biosensing
- Highly sensitive magnetic field sensor based on epitaxial iron-garnet films for magnetocardiographic measurements was developed and demonstrated. Sensitivity of the sensor is 80 fT/Hz1/2 with potentially achievable sensitivity of 1 fT/Hz1/2 at room temperature. Key factors providing such high level of sensitivity are: using a ferromagnetic material (rear-earth iron-garnet) with large concentration of exchange-coupled spins, monocrystalline quality of the magnetic film, and special shape of film profile (fabricated by microstructured etching in orthophosphoric acid). We expect that by further improving technology of the film fabrication, their composition and proper profile shaping we will reach the sensitivity of 10 fT/Hz1/2 in the nearest future.
- High sensitivity of the developed magnetometer was demonstrated by measuring magnetocardiographic signal from small animals (rats) and humans. QRS-peak from rats were about 3 pT, while from a human heart – 100 pT. All main elements of the MCG peaks were recorded with good quality. A comparison of the MCG and ECG patterns of a healthy heart showed almost perfect coincidence of the temporal variation of magnetic field and electric field components parallel to the chest plane.
- New magneto-optical magnetic field sensor based on magnetoplasmonic crystal was developed and demonstrated. The novel type of the magneto-optical magnetic field sensor is based on the detection of previously discovered by our group the longitudinal magnetophotonic intensity effect in the magnetoplasmonic structure of the iron garnet film and a thin gold layer with periodic slit array. The proof-of-concept sample of the magnetoplasmonic sensor allows to achieve the sensitivity of 1 nT/Hz1/2. Potentially the sensitivity of the proposed magnetoplasmonic sensor can be significantly improved to 100 fT/Hz1/2 with lateral spatial resolution of about several microns.
- New vector magneto-optical magnetic field sensor based on epitaxial iron-garnet films with large cubic anisotropy was developed and demonstrated. The key element of the vector magnetometer is a transparent high Faraday activity magnetic film with a cubic crystal lattice. Magnetocrystalline anisotropy of the film leads to the three dimensional trajectory of the film magnetization when the magnetization is rotated by the control magnetic field. It makes the magnetization sensitive to all three components of the external magnetic field. The demonstrated vector magnetometer is promising for mapping and visualization of ultra-small magnetic fields.