Collinear interaction of photons with orbital angular momentum Apurv Chaitanya N Photonics science Laboratory, PRL

Introduction Motivation Experimental set up Results and discussion Conclusion Outline of the talk

Introduction Images taken from internet

Helical Wave-front Optical Vortex beam carries an orbital angular momentum of lħ per photon 22 44 66 22 0 Propagation Direction Vortex order (topological charge), l l=1l=2l=3 Vortex beam

Applications of optical vortex Laser ablation J. Hamazaki et al., Optical-vortex laser ablation. Opt. Express 18, 2144 (2010) Compared with Gaussian Clearer outline of the ablated zone Less debris Less ablation threshold fluence Smoother processed surface Improvement in characteristics with order

Applications of optical vortex Laser ablation Recently, we discovered that the helicity of a circularly polarized optical vortex can be directly transferred to an irradiated metal sample, resulting in the formation of chiral nanoneedles [16–18]. This is the first demonstration, to the best of our knowledge, of nanostructures created by structured light with angular momenta, and it clearly represents a new scientific phenomenon.[16–18] Chiral needles K. Toyoda, et al., Transfer of light helicity to nanostructures. Phys. Rev. Lett. 110(14), (2013)

Applications of optical vortex Laser ablation Chiral needles Super resolution microscopyPump Probe Image taken from internet

Applications of optical vortex Laser ablation Chiral needles Super resolution microscopy Optical tweezers Video taken from YouTube

Applications of optical vortex Laser ablation Chiral needles Super resolution microscopy Optical tweezers It would be nice to have Optical vortex with the following characteristics High power, High order (and of) Desired wavelength

Generation of optical vortex Holographic technique (Using Spatial light modulator (SLM)) Image taken from internet

Generation of optical vortex Holographic technique (Using Spatial light modulator (SLM)) Spiral Phase plate Image taken from internet

Generation of optical vortex limitations Holographic technique (Using Spatial light modulator (SLM)) Spiral Phase plate Higher orders possible High power not possible Wavelength limit High power possible Higher order technologically challenging Difficulty increases with decrease in wavelength Way out is make a higher order phase mask for higher wavelength and then somehow change the wavelength to lower wavelengths. Images taken from internet

Refraction Reflection Transmission Birefringence The wavelength remains same for all phenomenon you see around on a normal day

This happens due to the higher order response of the medium One of the second order processes is second harmonic generation (SHG) Hence using this nonlinear technique is a nice preposition to access high power optical vortices in lower wavelength range. Then we have to extensively study this interaction of photons with OAM at high power Once the intensity is high  Nonlinear regime  New color

Theoretical models predicting optimum focusing condition G. D. Boyd and D. A. Kleinman, J. Appl. Phys. 39, 3897–3641 (1968). Apurv Chaitanya N., A. Aadhi, R. P. Singh, G. K. Samanta, Opt. Lett., 39, (2014). High power, ultrafast No theoretical model which predicts the optimum focusing for achieving high efficiency

Proposed theoretical model for SHG of pump with OAM SHG

Experimental set up Fiber Laser, 5 W, 260 fs, 78 MHz Motorized Attenuator Polarizer Δλ=15 nm, central λ= 1060 nm 530 nm 1060 nm BIBO L λ/2 M2 λ/4 SPP1 Vortex Doubler PBS SPP2 M1 λ/2

Generated high power ultrafast vortex upto m=12 Pump SHG Order, m p Order, m s =2 m p Fiber Laser, 5 W, 260 fs, 78 MHz Motorized Attenuator Polarizer Δλ=15 nm, central λ= 1060 nm 530 nm 1060 nm BIBO L λ/2 M2 λ/4 SPP1 Vortex Doubler PBS SPP2 M1 λ/2

Results and discussions P=3.5 W

Results and discussions

First experimental realisation of high power ultrafast SHG of pump with OAM upto order 6. We have experimentally found that the conversion efficiency of SHG is decreasing with the increase in the OAM of the input photons. This finding is in good agreement with the theoretical model which we developed. Other important parameters like spectral and temporal width of the generated radiation, and the angular acceptance band width of this interaction does not vary with OAM 0.89 W of optical vortex in green with a pulse width of 308 fs with single- pass conversion efficiency as high as 18% is achieved. Conclusion Thanks for your attention