Shaping Light Sources for Atomic Sensors

This thesis investigates the miniaturization of optical interrogation systems required for the next generation of chip-scale atomic sensors, such as atomic clocks, magnetometers, and gyroscopes. Since these sensors heavily rely on the precise interaction between atomic vapor and circularly polarized (CP) light, current solutions using bulky discrete optical components, such as quarter wave plates, severely limit device portability. To overcome this, the research explores two complementary platforms: passive integrated photonic circuits for complex beam manipulation and active vertical-cavity surface-emitting lasers (VCSELs) engineered for inherent CP emission.
In the domain of passive integrated photonics, a comprehensive silicon nitride (Si3N4) chip architecture was developed to precisely route and shape optical beams. The design incorporates tunable Mach-Zehnder splitters, thermo-optic phase shifters, and a novel “photonic lens” created via graded-index-like SiO2 trenching to collimate expanding waveguide modes. These components coherently drive an optimized two-dimensional quasi-periodic out-coupler. Rigorous simulations confirm this structure can synthesize arbitrary polarization states, generating millimeter-scale, collimated beams with a circular polarization contrast exceeding 30 dB, effectively replacing bulk beam-expanders and external waveplates.
To directly address the light source, the thesis presents a modified VCSEL architecture that produces CP emission while relying entirely on standard, scalable semiconductor processes. Overcoming the residual linear polarization issues of prior designs, this approach introduces a two-stage, C2-symmetric cascaded structure that embraces the VCSEL’s inherent linear polarization stability. A highly optimized, partially etched GaAs linear grating first pins the polarization. The light is then converted by a silicon-rich SiN high-contrast grating acting as a quarter-wave plate, achieving a CP contrast 0.98 and strong cavity resonance.
Further advancing VCSEL miniaturization, the research proposes a novel, fully DBR-free VCSEL architecture. By replacing the bulky top distributed Bragg reflector (DBR) with a compact, highly selective chiral mirror structure, fabrication complexity is significantly reduced. This structure utilizes a cascaded quarter-wave layer and a chiral photonic crystal with C2 rotational symmetry, proven mandatory for maintaining high-reflectivity, handedness-preserving zero-order reflection. Optimized for the AlGaAs platform, full cavity simulations of this DBR-free design demonstrate excellent performance, yielding a Q-factor of 6,440 and high CP contrast of over 0.97, while remaining resilient to standard fabrication tolerances.
In summary, this research establishes a complete, lithographically defined optical toolkit for atomic sensor integration. By shifting complex beam shaping, collimation, and circular polarization control from discrete optics to highly integrated on-chip structures and novel VCSEL cavities, this work provides a scalable pathway toward the mass production of next-generation quantum sensors.

Ph.D. student Under the supervision of Prof. Meir Orenstein.