arXiv subjects

Ali Khalatpour

Publications and source records attributed to Ali Khalatpour.

5 recordsLinked to original sources

arXiv2026

Quantum cascade laser roadmap

Quantum cascade lasers (QCLs) are unipolar semiconductor lasers first demonstrated in 1994. Since then, they have played a central role in advancing mid-infrared and terahertz photonics, becoming among the most reliable light sources in these regions of the electromagnetic spectrum. Their importance is further reinforced by their ability to generate self-starting optical frequency combs, whose investigation is motivated both by fundamental physics and by a wide range of applications, including molecular spectroscopy and free-space optical communications. This Roadmap provides a unified overview of current advances and emerging directions in QCL research. The chapters are organized into three main sections: device design and technology; frequency combs and pulse formation; and applications of QCLs. Each chapter reviews the relevant background, summarizes the current state of the art, and identifies key challenges and future directions within its specific research area.

physics.optics↗

arXiv2025

Low-loss, fabrication-tolerant, and highly-tunable Sagnac loop reflectors and Fabry-P\'erot cavities on thin-film lithium niobate

We present low-loss ($<1.5\%$) and power-efficient Mach-Zehnder interferometers (MZIs) on thin-film lithium niobate. To accurately measure low MZI losses, we develop a self-calibrated method using tunable Sagnac loop reflectors (SLRs) to build cavities. Fabry-P\'erot cavities constructed from these fabrication-tolerant SLRs achieve an intrinsic quality factor of $2 \times 10^6$. By implementing thermal isolation trenches, we also demonstrate a $>10\times$ reduction in power consumption for thermo-optic phase shifters, achieving a $\pi$-phase shift ($P_\pi$) with just 2.5 mW. These tunable and efficient components are key for scaling up to complex photonic integrated circuits.

physics.optics↗

arXiv2025

Roughness-Limited Performance in Ultra-Low-Loss Lithium Niobate Cavities

Achieving low optical loss is critical for scaling complex photonic systems. Thin-film lithium niobate (TFLN) offers strong electro-optic and nonlinear properties in a compact platform, making it ideal for quantum and nonlinear optics. While $Q$ factors above $10^7$ have been achieved, they remain below the intrinsic material limit. We present a systematic study of scattering losses due to roughness in TFLN racetrack cavities, isolating contributions from sidewall and interface roughness. Quality factors up to $27 \times 10^6$ are demonstrated in waveguides with widths of $2.2\lambda$ ($\sim3.5\,\mu$m), where interface roughness dominates, and up to $1.2 \times 10^7$ in narrower waveguides $0.8\lambda$ wide ($\sim1.2\,\mu$m), where sidewall roughness is the primary limitation. Our modeling framework, based on 3D wave simulations informed by AFM-measured roughness, is material-independent and broadly applicable across integrated photonic platforms.

physics.optics↗

arXiv2025

Surface and Bulk Two-Level System Losses in Lithium Niobate Acoustic Resonators

Lithium niobate (LN) is a promising material for building acoustic resonators used in quantum applications, but its performance is limited by poorly understood material defects called two-level systems (TLS). In this work, we fabricate high-performance acoustic resonators from LN with quality factors up to $6\times10^7$ and use them to separate bulk and surface contributions to TLS loss. By comparing these bulk acoustic wave (BAW) resonators with previous surface acoustic wave and phononic crystal studies, we show that devices with high surface participation ratios are limited by surface TLS, while our BAW devices reveal an intrinsic bulk TLS limit. Through systematic surface treatments and microscopy, we demonstrate that BAW resonator performance remains unchanged despite surface modifications, confirming operation in a bulk-limited regime. Our work establishes quantitative bounds on both surface and bulk TLS losses in LN, within the context of material growth and fabrication approaches we have pursued, and provides guidance for future device engineering and materials development.

cond-mat.mes-hall↗

arXiv2022

Terahertz semiconductor laser source at -12 C

Room temperature operation of Terahertz Quantum Cascade Lasers (THz QCLs) has been a long-pursued goal to realize compact semiconductor THz sources. The progress toward high-temperature operation in THz QCLs has been relatively slow compared to infrared QCLs owing to more significant challenges at THz frequencies. Recently, the maximum operating temperature of THz QCLs was improved to 250 K, and the achievement revitalized hope in the THz community in pursuit of higher temperature operations. In this paper, we report on further improvement in operating temperature to ~261 K (-12 0C) by judiciously optimizing key parameters and discuss the challenges ahead in achieving room temperature operation.

physics.app-ph↗