Group Seminar at MPQ:  Masters work in Andrea Bertoldi’s group

Tajamul Islam, Institut d'Optique Graduate School
Group Seminar at MPQ lecture hall and Zoom
Thursday, June 1st, 09:00am (MEZ)

Abstract for first part of talk

Semiconducting single-walled carbon nanotubes are one dimensional (1D) quantum nanostructures, and their unique optical properties arise from stable 1D excitons with huge binding energies. Here we review the studies on the optical response of the semiconducting nanotubes. We present the nonlinearities of the semiconducting nanotubes under the two-level approximation. We highlight the role of one- and two-photon transitions assisted by excitonic transitions in their strong third-order nonlinear susceptibility. We predict a negative sign for the nonlinear refractive index nearby 1550 nm for two types of semiconducting nanotubes, in accordance with preliminary nonlinear characterizations achieved in the group. We have designed and aligned a set-up to couple a laser beam between a single-mode optical fibre to a SiN waveguide. The coupling is achieved through grating couplers and we have reached a coupling efficiency of -10dB per coupler. It constitutes the first part of a set-up that will be used to characterise the nonlinear properties of carbon nanotubes deposited on nanostructured SiN waveguides.

Abstract for second part of talk

We study the interaction between cold atoms and laser light within a bow-tie cavity. The main aim is to create an ultracold 87Rb atom cloud in the central region of a bow tie cavity, which operates at a wavelength of 1560 nm. To achieve this, we employ a far-off-resonant telecom dipolar trap. We utilize the Sub-Doppler cooling technique, making use of dark states, commonly referred to as gray molasses. This type of cooling should enable us to continuously cool the atoms from outside to deep inside the trap. However, there is a significant loss at the centre of the trap for the D2 line due to the hindrance in the formation of dark states caused by hyperfine states above the cooling line. To address this, we use the D1 line of the rubidium for cooling where there is no effect from the higher hyperfine states. We stabilised an external-cavity diode laser (ECDL) and built a double-pass spectroscopic set-up based on the D1 line which will be used to cool the atoms in the far-off resonant dipole trap. We have designed a demodulation circuit for obtaining the error function and locking the laser, by employing PID (Proportional-Integral-Derivative) feedback control and using an EOM operating at 5MHz.