SearcharxivSearch

arXiv subjects

Prashanta Kharel

Publications and source records attributed to Prashanta Kharel.

18 recordsLinked to original sources

Autonomous agentic design for photonics

We introduce an automated, agent-driven approach to the design of photonic devices. We instruct large language models (LLMs) to solve photonic design problems, given access to software tools for performance evaluation (through numerical simulations) and quantitative acceptance criteria (e.g., fabrication rules, geometric constraints, physical-consistency checks). Within this context, agents run autonomous design loops (propose, simulate, evaluate, iterate) and generate devices with state-of-the-art performance. We demonstrate this approach in two stages: First, we run it individually on four canonical problem classes in photonic chip design: a) passive components (waveguide bends, splitters, crossings, etc.); b) active devices (silicon microring modulators (MRMs)); c) radio-frequency (RF) devices (traveling-wave electrodes for a Mach-Zehnder modulator (MZM)); d) chip layout (electrical routing). Then, we combine the previous studies in one demonstration to produce a silicon photonic modulator, incorporating layout, charge transport, optical mode, and RF electrode design. The approach generalizes to any problem that combines a numerical simulator with performance criteria that an LLM can evaluate.

physics.optics

Simultaneous Brillouin and piezoelectric coupling to a high-frequency bulk acoustic resonator

Bulk acoustic resonators support robust, long-lived mechanical modes, capable of coupling to various quantum systems. In separate works, such devices have achieved strong coupling to both superconducting qubits, via piezoelectricity, and optical cavities, via Brillouin interactions. In this work, we present a novel hybrid microwave/optical platform capable of coupling to bulk acoustic waves through cavity-enhanced piezoelectric and photoelastic interactions. The modular, tunable system achieves fully resonant and well-mode-matched interactions between a 3D microwave cavity, a high-frequency bulk acoustic resonator, and a Fabry Perot cavity. We realize this piezo-Brillouin interaction in x-cut quartz, demonstrating the potential for strong optomechanical interactions and high cooperativity using optical cavity enhancement. We further show how this device functions as a bidirectional electro-opto-mechanical transducer, with quantum efficiency exceeding $10^{-8}$, and a feasible path towards unity conversion efficiency. The high optical sensitivity and ability to apply large resonant microwave field in this system also offers a new tool for probing anomalous electromechanical couplings, which we demonstrate by investigating (nominally-centrosymmetric) CaF$_2$ and revealing a parasitic piezoelectricity of 83 am/V. Such studies are an important topic for emerging quantum technologies, and highlight the versatility of this new hybrid platform.

quant-ph

Micro-fabricated mirrors with finesse exceeding one million

The Fabry-Pérot resonator is one of the most widely used optical devices, enabling scientific and technological breakthroughs in diverse fields including cavity QED, optical clocks, precision length metrology and spectroscopy. Though resonator designs vary widely, all high-end applications benefit from mirrors with the lowest loss and highest finesse possible. Fabrication of the highest finesse mirrors relies on centuries-old mechanical polishing techniques, which offer losses at the part-per-million (ppm) level. However, no existing fabrication techniques are able to produce high finesse resonators with the large range of mirror geometries needed for scalable quantum devices and next-generation compact atomic clocks. In this paper, we introduce a new and scalable approach to fabricate mirrors with ultrahigh finesse ($\geq 10^{6}$) and user-defined radius of curvature spanning four orders of magnitude ($10^{-4}-10^{0}$ m). We employ photoresist reflow and reactive ion etching to shape and transfer mirror templates onto a substrate while maintaining sub-Angstrom roughness. This substrate is coated with a dielectric stack and used to create arrays of compact Fabry-Pérot resonators with finesse values as high as 1.3 million and measured excess loss $<$ 1 ppm. Optical ringdown measurements of 43 devices across 5 substrates reveal that the fabricated cavity mirrors -- with both small and large radii of curvature -- produce an average coating-limited finesse of 1.05 million. This versatile new approach opens the door to scalable fabrication of high-finesse miniaturized Fabry-Pérot cavities needed for emerging quantum optics and frequency metrology technologies.

physics.optics

Reduced Material Loss in Thin-film Lithium Niobate Waveguides

Thin-film lithium niobate has shown promise for scalable applications ranging from single-photon sources to high-bandwidth data communication systems. Realization of the next generation high-performance classical and quantum devices, however, requires much lower optical losses than the current state of the art ($\sim$10 million). Unfortunately, material limitations of ion-sliced thin-film lithium niobate have not been explored, and therefore it is unclear how high-quality factor can be achieved in this platform. Here we evaluate the material limited quality factor of thin-film lithium niobate photonic platform can be as high as $Q\approx 1.8\times10^{8}$ at telecommunication wavelengths, corresponding to a propagation loss of 0.2 dB/m.

physics.optics

Femtosecond Pulse Generation via an Integrated Electro-Optic Time Lens

Integrated femtosecond pulse and frequency comb sources are critical components for a wide range of applications. The leading approaches for on-chip pulse generation rely on mode locking inside microresonator with either third-order nonlinearity or with semiconductor gain. These approaches, however, are limited in noise performance, wavelength tunability and repetition rates. Alternatively, sub-picosecond pulses can be synthesized without mode-locking, by modulating a continuous-wave (CW) single-frequency laser using a cascade of electro-optic (EO) modulators. This method is particularly attractive due to its simplicity, robustness, and frequency-agility but has been realized only on a tabletop using multiple discrete EO modulators and requiring optical amplifiers (to overcome large insertion losses), microwave amplifiers, and phase shifters. Here we demonstrate a chip-scale femtosecond pulse source implemented on an integrated lithium niobate (LN) photonic platform18, using cascaded low-loss electro-optic amplitude and phase modulators and chirped Bragg grating, forming a time-lens system. The device is driven by a CW distributed feedback (DFB) chip laser and controlled by a single CW microwave source without the need for any stabilization or locking. We measure femtosecond pulse trains (520 fs duration) with a 30-GHz repetition rate, flat-top optical spectra with a 10-dB optical bandwidth of 12.6 nm, individual comb-line powers above 0.1 milliwatt, and pulse energies of 0.54 picojoule. Our results represent a tunable, robust and low-cost integrated pulsed light source with CW-to-pulse conversion efficiencies an order of magnitude higher than achieved with previous integrated sources. Our pulse generator can find applications from ultrafast optical measurement to networks of distributed quantum computers.

physics.optics

Breaking voltage-bandwidth limits in integrated lithium niobate modulators using micro-structured electrodes

Electro-optic modulators with low voltage and large bandwidth are crucial for both analog and digital communications. Recently, thin-film lithium niobate modulators have enable dramatic performance improvements by reducing the required modulation voltage while maintaining high bandwidths. However, the reduced electrode gaps in such modulators leads to significantly higher microwave losses, which limit electro-optic performance at high frequencies. Here we overcome this limitation and achieve a record combination of low RF half-wave voltage of 1.3 V while maintaining electro-optic response with 1.8-dB roll-off at 50 GHz. This demonstration represents a significant improvement in voltage-bandwidth limit, one that is comparable to that achieved when switching from legacy bulk to thin-film lithium niobate modulators. Leveraging the low-loss electrode geometry, we show that sub-volt modulators with $>$ 100 GHz bandwidth can be enabled.

physics.app-ph

Wafer-scale low-loss lithium niobate photonic integrated circuits

Thin-film lithium niobate (LN) photonic integrated circuits (PICs) could enable ultrahigh performance in electro-optic and nonlinear optical devices. To date, realizations have been limited to chip-scale proof-of-concepts. Here we demonstrate monolithic LN PICs fabricated on 4- and 6-inch wafers with deep ultraviolet lithography and show smooth and uniform etching, achieving 0.27 dB/cm optical propagation loss on wafer-scale. Our results show that LN PICs are fundamentally scalable and can be highly cost-effective.

physics.app-ph

Microwave filtering using forward Brillouin scattering in photonic-phononic emit-receive devices

Microwave photonic systems are compelling for their ability to process signals at high frequencies and over extremely wide bandwidths as a basis for next generation communication and radar technologies. However, many applications also require narrow-band $(\sim\text{MHz})$ filtering operations that are challenging to implement using optical filtering techniques, as this requires reliable integration of ultra-high quality factor $(\sim 10^8)$ optical resonators. One way to address this challenge is to utilize long-lived acoustic resonances, taking advantage of their narrow-band frequency response to filter microwave signals. In this paper, we examine new strategies to harness a narrow-band acoustic response within a microwave-photonic signal processing platform through use of light-sound coupling. Our signal processing scheme is based on a recently demonstrated photon-phonon emitter-receiver device, which transfers information between the optical and acoustic domains using Brillouin interactions, and produces narrow-band filtering of a microwave signal. To understand the best way to use this device technology, we study the properties of a microwave-photonic link using this filtering scheme. We theoretically analyze the noise characteristics of this microwave-photonic link, and explore the parameter space for the design and optimization of such systems.

physics.app-ph

Shaping nonlinear optical response using nonlocal forward Brillouin interactions

We grow accustomed to the notion that optical susceptibilities can be treated as a local property of a medium. In the context of nonlinear optics, both Kerr and Raman processes are considered local, meaning that optical fields at one location do not produce a nonlinear response at distinct locations in space. This is because the electronic and phononic disturbances produced within the material are confined to a region that is smaller than an optical wavelength. By comparison, Brillouin interactions can result in a highly nonlocal nonlinear response, as the elastic waves generated through the Brillouin process can occupy a region in space much larger than an optical wavelength. The nonlocality of these interactions can be exploited to engineer new types of processes, where highly delocalized phonon modes serve as an engineerable channel that mediates scattering processes between light waves propagating in distinct optical waveguides. These types of nonlocal optomechanical responses have been recently demonstrated as the basis for information transduction, however the nontrivial dynamics of such systems has yet to be explored. In this work, we show that the third-order nonlinear process resulting from spatially extended Brillouin-active phonon modes involves mixing products from spatially separated, optically decoupled waveguides, yielding a nonlocal 'joint-susceptibility'. We further explore the coupling of multiple acoustic modes and show that multi-mode acoustic interference enables a tailorable nonlocal-nonlinear susceptibility, exhibiting a multi-pole frequency response.

physics.app-ph

Multimode strong coupling in cavity optomechanics

Optomechanical systems show great potential as quantum transducers and information storage devices for use in future hybrid quantum networks and offer novel strategies for quantum state preparation to explore macroscopic quantum phenomena. Towards these goals, deterministic control of optomechanical interactions in the strong coupling regime represents an important strategy for efficient utilization of quantum degrees of freedom in mechanical systems. While strong coupling has been demonstrated in both electromechanical and optomechanical systems, it has proven difficult to identify a robust optomechanical system that features the low loss and high coupling rates required for more sophisticated control of mechanical motion. In this paper, we demonstrate robust strong coupling between multiple long-lived phonon modes of a bulk acoustic wave (BAW) resonator and a single optical cavity mode. We show that this so-called "multimode strong coupling" regime can be a powerful tool to shape and control decoherence pathways through nontrivial forms of mode hybridization. Using frequency- and time-domain measurements, we identify hybridized modes with lifetimes that are significantly longer than that of any mode of the uncoupled system. This surprising effect, which results from the interference of decay channels, showcases the use of multimode strong coupling as a general strategy to mitigate extrinsic loss mechanisms. Moreover, the phonons supported by BAW resonators have a collection of properties, including high frequencies, long coherence times, and robustness against thermal decoherence, making this optomechanical system particularly enticing for applications such as quantum transduction and memories. These results show that our system can be used to study novel phenomena in a previously unexplored regime of optomechanics and could be an important building block for future quantum devices.

physics.optics

Creation and control of multi-phonon Fock states in a bulk acoustic wave resonator

Quantum states of mechanical motion can be important resources for quantum information, metrology, and studies of fundamental physics. Recent demonstrations of superconducting qubits coupled to acoustic resonators have opened up the possibility of performing quantum operations on macroscopic motional modes, which can act as long-lived quantum memories or transducers. In addition, they can potentially be used to test for novel decoherence mechanisms in macroscopic objects and other modifications to standard quantum theory. Many of these applications call for the ability to create and characterize complex quantum states, putting demanding requirements on the speed of quantum operations and the coherence of the mechanical mode. In this work, we demonstrate the controlled generation of multi-phonon Fock states in a macroscopic bulk-acoustic wave resonator. We also perform Wigner tomography and state reconstruction to highlight the quantum nature of the prepared states. These demonstrations are made possible by the long coherence times of our acoustic resonator and our ability to selectively couple to individual phonon modes. Our work shows that circuit quantum acousto-dynamics (circuit QAD) enables sophisticated quantum control of macroscopic mechanical objects and opens the door to using acoustic modes as novel quantum resources.

quant-ph

High-frequency cavity optomechanics using bulk acoustic phonons

To date, micro- and nano-scale optomechanical systems have enabled many proof-of-principle quantum operations through access to high-frequency (GHz) phonon modes that are readily cooled to their thermal ground state. However, minuscule amounts of absorbed light produce excessive heating that can jeopardize robust ground state operation within such microstructures. In contrast, we demonstrate an alternative strategy for accessing high-frequency ($13$ GHz) phonons within macroscopic systems (cm-scale). Counterintuitively, we show that these macroscopic systems, with motional masses that are $>20$ million times larger than those of micro-scale counterparts, offer a complementary path towards robust quantum operations. Utilizing bulk acoustic phonons to mediate resonant coupling between two distinct modes of an optical cavity, we demonstrate the ability to perform beam-splitter and entanglement operations at MHz rates on an array of phonon modes, opening doors to applications ranging from quantum memories and microwave-to-optical conversion to high-power laser oscillators.

physics.optics

Nonreciprocal Inter-band Brillouin Modulation

Achieving nonreciprocal light propagation in photonic circuits is essential to control signal crosstalk and optical back-scatter. However, realizing high-fidelity nonreciprocity in low-loss integrated photonic systems remains challenging. In this paper, we experimentally demonstrate a device concept based on nonlocal acousto-optic light scattering to produce nonreciprocal single-sideband modulation and mode conversion in an integrated silicon photonic platform. In this process, a traveling-wave acoustic phonon driven via optical forces in a silicon waveguide is used to modulate light in a spatially separate waveguide through a linear inter-band Brillouin scattering process. We demonstrate up to 38 dB of nonreciprocity with 37 dB of single-sideband suppression. In contrast to prior Brillouin- and optomechanics-based schemes for nonreciprocity, the bandwidth of this scattering process is set through optical phase-matching, not acoustic or optical resonances. As a result, record-large bandwidths in excess of 125 GHz are realized, with potential for significant further improvement through optical dispersion engineering. Tunability of the nonreciprocal modulator operation wavelength over a 35 nm bandwidth is demonstrated by varying the optical pump wavelength. Such traveling-wave acousto-optic modulators provide a promising path toward the realization of broadband, low-loss isolators and circulators in integrated photonic circuits.

physics.app-ph

Ultra-high-Q phononic resonators on-chip at cryogenic temperatures

Long-lived, high-frequency phonons are valuable for applications ranging from optomechanics to emerging quantum systems. For scientific as well as technological impact, we seek high-performance oscillators that offer a path towards chip-scale integration. Confocal bulk acoustic wave resonators have demonstrated an immense potential to support long-lived phonon modes in crystalline media at cryogenic temperatures. So far, these devices have been macroscopic with cm-scale dimensions. However, as we push these oscillators to high frequencies, we have an opportunity to radically reduce the footprint as a basis for classical and emerging quantum technologies. In this paper, we present novel design principles and simple fabrication techniques to create high performance chip-scale confocal bulk acoustic wave resonators in a wide array of crystalline materials. We tailor the acoustic modes of such resonators to efficiently couple to light, permitting us to perform a non-invasive laser-based phonon spectroscopy. Using this technique, we demonstrate an acoustic $Q$-factor of 28 million (6.5 million) for chip-scale resonators operating at 12.7 GHz (37.8 GHz) in crystalline $z$-cut quartz ($x$-cut silicon) at cryogenic temperatures.

physics.optics

Integrated RF-photonic Filters via Photonic-Phononic Emit-Receive Operations

The creation of high-performance narrowband filters is of great interest for many RF-signal processing applications. To this end, numerous schemes for electronic, MEMS-based, and microwave photonic filters have been demonstrated. Filtering schemes based on microwave photonic systems offer superior flexibility and tunability to traditional RF filters. However, these optical-based filters are typically limited to GHz-widths and often have large RF insertion losses, posing challenges for integration into high-fidelity radiofrequency circuits. In this article, we demonstrate a novel type of microwave filter that combines the attractive features of microwave photonic filters with high-Q phononic signal processing using a photonic-phononic emit-receive process. Through this process, a RF signal encoded on a guided optical wave is transduced onto a GHz-frequency acoustic wave, where it may be filtered through shaping of acoustic transfer functions before being re-encoded onto a spatially separate optical probe. This emit-receive functionality, realized in an integrated silicon waveguide, produces MHz-bandwidth band-pass filtering while supporting low RF insertion losses necessary for high dynamic range in a microwave photonic link. We also demonstrate record-high internal efficiency for emit-receive operations of this type, and show that the emit-receive operation is uniquely suitable for the creation of serial filter banks with minimal loss of fidelity. This photonic-phononic emitter-receiver represents a new method for low-distortion signal-processing in an integrated all-silicon device.

physics.app-ph

Quantum acoustics with superconducting qubits

The ability to engineer and manipulate different types of quantum mechanical objects allows us to take advantage of their unique properties and create useful hybrid technologies. Thus far, complex quantum states and exquisite quantum control have been demonstrated in systems ranging from trapped ions to superconducting resonators. Recently, there have been many efforts to extend these demonstrations to the motion of complex, macroscopic objects. These mechanical objects have important applications as quantum memories or transducers for measuring and connecting different types of quantum systems. In particular, there have been a few experiments that couple motion to nonlinear quantum objects such as superconducting qubits. This opens up the possibility of creating, storing, and manipulating non-Gaussian quantum states in mechanical degrees of freedom. However, before sophisticated quantum control of mechanical motion can be achieved, we must realize systems with long coherence times while maintaining a sufficient interaction strength. These systems should be implemented in a simple and robust manner that allows for increasing complexity and scalability in the future. Here we experimentally demonstrate a high frequency bulk acoustic wave resonator that is strongly coupled to a superconducting qubit using piezoelectric transduction. In contrast to previous experiments with qubit-mechanical systems, our device requires only simple fabrication methods, extends coherence times to many microseconds, and provides controllable access to a multitude of phonon modes. We use this system to demonstrate basic quantum operations on the coupled qubit-phonon system. Straightforward improvements to the current device will allow for advanced protocols analogous to what has been shown in optical and microwave resonators, resulting in a novel resource for implementing hybrid quantum technologies.

quant-ph

Noise and dynamics in forward Brillouin interactions

In this paper, we explore the spatio-temporal dynamics of spontaneous and stimulated forward Brillouin scattering. This general treatment incorporates the optomechanical coupling produced by boundary-induced radiation pressures (boundary motion) and material-induced electrostrictive forces (photo-elastic effects), permitting straightforward application to a range of emerging micro- and nano-scale optomechanical systems. Through a self-consistent fully coupled nonlinear treatment, developed within a general Hamiltonian framework, we establish the connection between the power spectral density of spontaneously scattered light in forward Brillouin interactions and the nonlinear coupling strength. We show that, in sharp contrast to backward Brillouin scattering, noise-initiated stimulated forward Brillouin scattering is forbidden in the majority of experimental systems. In fact, the single-pass gain, which characterizes the threshold for energy transfer in back-scattering processes, is negative for a large class of forward Brillouin devices. Beyond this frequent experimental case, we explore mechanisms for dispersive symmetry breaking that lead to amplification and dynamics reminiscent of backward Brillouin scattering.

physics.optics

On-Chip Detection of Entangled Photons by Scalable Integration of Single-Photon Detectors

Photonic integrated circuits (PICs) have emerged as a scalable platform for complex quantum technologies using photonic and atomic systems. A central goal has been to integrate photon-resolving detectors to reduce optical losses, latency, and wiring complexity associated with off-chip detectors. Superconducting nanowire single-photon detectors (SNSPDs) are particularly attractive because of high detection efficiency, sub-50-ps timing jitter, nanosecond-scale reset time, and sensitivity from the visible to the mid-infrared spectrum. However, while single SNSPDs have been incorporated into individual waveguides, the system efficiency of multiple SNSPDs in one photonic circuit has been limited below 0.2% due to low device yield. Here we introduce a micrometer-scale flip-chip process that enables scalable integration of SNSPDs on a range of PICs. Ten low-jitter detectors were integrated on one PIC with 100% device yield. With an average system efficiency beyond 10% for multiple SNSPDs on one PIC, we demonstrate high-fidelity on-chip photon correlation measurements of non-classical light.

physics.optics