Real-time ultrasound sensing with a mode-optimized photonic crystal slab.

Integrated photonic sensors can provide large scale, flexible detection schemes. Photonic crystal slabs (PCSs) offer a miniaturized platform for wideband, sensitive ultrasound detection by exploiting the photoelastic effect in water. However, poor modal overlap with the sensing medium and non-negligible absorption loss of the aqueous medium have previously limited PCS sensor performance. In this study, we detail the development and optimization of a PCS-based acoustic sensor by adding to it a low-loss high-index polymer cladding layer. Exploiting a mode-optimized TM-like optical resonance present in a PCS, with high bulk index sensitivity (>600nm/RIU) and quality factor Q (>8000), we demonstrate real-time ultrasound sensing at a noise equivalent pressure of 170 Pa (1.9Pa/Hz). The PCS sensor is backside-coupled to an optical fiber, which, along with its intensity-based ultrasound-sensing architecture, will allow us to scale up easily to a 2D array. This work paves the way to a sensitive compact ultrasound detector for photoacoustic-based diagnostics and monitoring of tissue.

Broadband fiber-based entangled photon-pair source at telecom O-band.

In this Letter, we report a polarization-entangled photon-pair source based on type-II spontaneous parametric downconversion at telecom O-band in periodically poled silica fiber (PPSF). The photon-pair source exhibits more than 130 nm (∼24THz) emission bandwidth centered at 1306.6 nm. The broad emission spectrum results in a short biphoton correlation time, and we experimentally demonstrate a Hong-Ou-Mandel interference dip with a full width of 26.6 fs at half-maximum. Owing to the low birefringence of the PPSF, the biphotons generated from type-II SPDC are polarization-entangled over the entire emission bandwidth, with a measured fidelity to a maximally entangled state greater than 95.4%. The biphoton source provides the broadest bandwidth entangled biphotons at O-band to our knowledge.

Dispersion measurement assisted by a stimulated parametric process.

Dispersion plays a major role in the behavior of light inside photonic devices. Current state-of-the-art dispersion measurement techniques utilize linear interferometers that can be applied to devices with small dispersion-length products. However, linear interferometry often requires beam alignment and phase stabilization. Recently, common-path nonlinear interferometers in the spontaneous regime have been used to demonstrate alignment-free and phase-stable dispersion measurements. However, they require single-photon detectors, resulting in high system cost and long integration times. We overcome these issues by utilizing a nonlinear interferometer in the stimulated regime and demonstrate the ability to measure the dispersion of a device with a dispersion-length product as small as 0.009 ps/nm at a precision of 0.0002 ps/nm. Moreover, this regime allows us to measure dispersion with shorter integration times (in comparison to the spontaneous regime) and conventional optical components and detectors.

Refractive-index-based ultrasound sensing with photonic crystal slabs.

We demonstrate ultrasound detection with 500-µm-diameter photonic-crystal slab (PCS) sensors fabricated from CMOS-compatible technology. An ultrasound signal impinging a PCS sensor causes a local modulation of the refractive index (RI) of the medium (water) in which the PCS is immersed, resulting in a periodic spectral shift of the optical resonance of the PCS. The acoustic sensitivity is found to scale with the index sensitivity S and quality factor Q. A noise equivalent pressure (NEP) of 650 Pa with averaging (7.4 Pa/Hz) and relative wavelength shifts of up to 4.3×10-5 MPa-1 are measured. The frequency response of the sensors is observed to be flat from 1 to 20 MHz, with the range limited only by our measurement apparatus.

Alignment-free dispersion measurement with interfering biphotons.

Measuring the dispersion of photonic devices with small dispersion-length products is challenging due to the phase-sensitive and alignment-intensive nature of conventional methods. In this Letter, we demonstrate a quantum technique to extract the second- and third-order chromatic dispersion of a short single-mode fiber using a fiber-based quantum nonlinear interferometer. The interferometer consists of two cascaded fiber-based biphoton sources, with each source acting as a nonlinear beam splitter. A fiber under test is placed between these two sources and introduces a frequency-dependent phase that is imprinted on the biphoton spectrum (interferogram) at the output of the interferometer. This interferogram contains the dispersion properties of the test fiber. Our technique has three novel features: (1) the broadband nature of the biphoton sources used in our setup allows accurate dispersion measurements on test devices with small dispersion-length products; (2) our all-fiber common-path interferometer requires no beam alignment or phase stabilization; and (3) multiple phase-matching processes supported in our biphoton sources enable dispersion measurements at different wavelengths, which yields the third-order dispersion achieved for the first time, to the best of our knowledge, using a quantum optical technique.

Breaking the Energy-Symmetry-Based Propagation Growth Blockade in Magneto-Optical Rotation.

The magneto-optical polarization rotation effect has myriad applications in many research areas spanning the scientific spectrum, including space and interstellar research, nanotechnology, material science, biomedical imaging, and subatomic particle research. In the nonlinear magneto-optical rotation (NMOR) effect, the angle of rotation of a linearly polarized optical field in a magnetized medium is dependent upon its intensity. However, typical NMOR signals of conventional single-beam Λ-scheme atomic magnetometers are peculiarly small, requiring sophisticated magnetic shielding and high-frequency phase-sensitive detection. Here, we show the presence of an energy-symmetry-based propagation growth blockade that undermines the NMOR effect in conventional single-beam Λ-scheme atomic magnetometers. We further demonstrate, both experimentally and theoretically, an inelastic wave-mixing technique that breaks this NMOR blockade, resulting in more-than-2-orders-of-magnitude enhancement of the NMOR signal power amplitude that cannot be achieved with conventional single-beam Λ-scheme atomic magnetometers. This technique, demonstrated here with substantially reduced light intensities at near-room temperatures, may lead to many applications, especially in the field of biomagnetism and high-resolution low-field magnetic imaging.

Compensation-free broadband entangled photon pair sources.

Quantum sources that provide broadband biphotons entangled in both polarization and time-energy degrees of freedom are a rich quantum resource that finds many applications in quantum communication, sensing, and metrology. Creating such a source while maintaining high entanglement quality over a broad spectral range is a challenge, which conventionally requires various compensation steps to erase temporal, spectral, or spatial distinguishabilities. Here, we point out that in fact compensation is not always necessary. The key to generate broadband polarization-entangled biphotons via type-II spontaneous parametric downcoversion (SPDC) without compensation is to use nonlinear materials with sufficiently low group birefringence that the biphoton bandwidth becomes dispersion-limited. Most nonlinear crystals or waveguides cannot meet this condition, but it is easily met in fiber-based systems. We reveal the interplay of group birefringence and dispersion on SPDC bandwidth and polarization entanglement quality. We show that periodically poled silica fiber (PPSF) is an ideal medium to generate high-concurrence (>0.977) polarization-entangled photons over a broad spectral range (>77nm), directly and without compensation. This is the highest polarization-entanglement concurrence reported that is maintained over a broad spectral range from a compensation-free source.

Polarization-entangled photon pair sources based on spontaneous four wave mixing assisted by polarization mode dispersion.

Photonic-based qubits and integrated photonic circuits have enabled demonstrations of quantum information processing (QIP) that promises to transform the way in which we compute and communicate. To that end, sources of polarization-entangled photon pair states are an important enabling technology. However, such states are difficult to prepare in an integrated photonic circuit. Scalable semiconductor sources typically rely on nonlinear optical effects where polarization mode dispersion (PMD) degrades entanglement. Here, we directly generate polarization-entangled states in an AlGaAs waveguide, aided by the PMD and without any compensation steps. We perform quantum state tomography and report a raw concurrence as high as 0.91 ± 0.01 observed in a 1,100-nm-wide waveguide. The scheme allows direct Bell state generation with an observed maximum fidelity of 0.90 ± 0.01 from another (800-nm-wide) waveguide. Our demonstration paves the way for sources that allow for the implementation of polarization-encoded protocols in large-scale quantum photonic circuits.

Correlated photon pair generation in AlGaAs nanowaveguides via spontaneous four-wave mixing.

We demonstrate a source of correlated photon pairs which will have applications in future integrated quantum photonic circuits. The source utilizes spontaneous four-wave mixing (SFWM) in a dispersion-engineered nanowaveguide made of AlGaAs, which has merits of negligible two-photon absorption and low spontaneous Raman scattering (SpRS). We observe a coincidence-to-accidental (CAR) ratio up to 177, mainly limited by propagation losses. Experimental results agree well with theoretical predictions of the SFWM photon pair generation and the SpRS noise photon generation. We also study the effects from the SpRS, propagation losses, and waveguide lengths on the quality of our source.

High-visibility two-photon interference of frequency-time entangled photons generated in a quasi-phase-matched AlGaAs waveguide.

We demonstrate experimentally the frequency-time entanglement of photon pairs produced in a CW-pumped quasi-phased-matched AlGaAs superlattice waveguide. A visibility of 96.0±0.7% without background subtraction has been achieved, which corresponds to the violation of the Bell inequality by 52 standard deviations.

Characterizing short dispersion-length fiber via dispersive virtual reference interferometry.

The ability to characterize fibers with near-zero dispersion-length products is of considerable practical interest. We introduce dispersive virtual reference interferometry (DVRI) as a technique for the characterization of short length (<1m) fibers with near-zero disperison-length. DVRI has an accuracy equivalent to standard balanced spectral interferometry (on the order of 10(−3) ps and 10(−5) ps/nm for the group delay and dispersion-length measurements respectively) but does not require wide spectral bandwidths or multiple spectral scans. Following experimental validation, the DVRI technique is used to characterize a 23.3-cm erbium-doped gain fiber (dispersion-length product <0.002 ps/nm), using a tunable laser with a bandwidth of 145 nm. Furthermore, the dispersion in a 28.6-cm commercial dispersion shifted fiber is characterized across the zero-dispersion wavelength and the zero-disperison-wavelength and slope were determined to be 1566.7 nm and 8.57 × 10(−5) ps/(nm2∙m) with a precision of ± 0.2 nm and ± 0.06 × 10(−5) ps/(nm2∙m), respectively.

Direct generation of polarization-entangled photon pairs in a poled fiber.

We experimentally demonstrate the direct generation of polarization-entangled photon pairs in an optical fiber at room temperature by exploiting type-II phase-matched spontaneous parametric down-conversion. A second-order nonlinearity is artificially induced in the 17-cm-long weakly birefringent step-index fiber through the process of thermal poling, and quasi-phase-matching allows for the generation of entangled photons in the 1.5-micron telecom band when the fiber is pumped at 775 nm. A greater-than 80:1 coincidence-to-accidental ratio is achieved, limited mainly by multiphoton pair generation. Without the need to subtract accidentals or to compensate for walk-off, the raw two-photon interference visibility is found to be better than 95%, and violation of Bell's inequality is observed by more than 18 standard deviations. This makes for a truly alignment-free, plug-and-play source of polarization-entangled photon pairs.

Measurement of chi((2)) symmetry in a poled fiber.

We measure the values of individual chi((2)) tensor components in a birefringent periodically poled silica fiber through spectrally separated type I and type II second-harmonic generation. We demonstrate that the chi((2)) tensor symmetry is consistent with that of chi((3)) in silica and thereby provide experimental evidence that this chi((2)) originates from a chi((3)) process.

Proposal for in-fiber generation of telecom-band polarization-entangled photon pairs using a periodically poled fiber.

We treat spontaneous parametric downconversion in a periodically poled fiber, quasi-phase matched to allow for the generation of photon pairs at wavelengths within the low-loss telecommunications window. For an appropriate pump polarization, the unusual properties of such a fiber's effective chi(2) result in a biphoton wave function that is symmetric upon simultaneous exchange of downconverted photon frequencies and polarizations and that is nonzero over a wide range of downconverted frequencies. This could lead to a significant technical simplification of sources of in-fiber telecom-band polarization-entangled photons.