Abstract: We present here systems and methods for generating a heterodyne signal using the naturally occurring chirp of a pulsed single-mode laser. The electrical square-wave pulse used to drive the laser heats the laser cavity, causing the laser frequency to change or chirp during the emission of the optical pulse. This chirped optical pulse can be split into a chirped signal pulse that interacts with a sample and a chirped reference pulse that interferes with the chirped signal pulse on a detector to produce a heterodyne modulation whose instantaneous phase and amplitude depend on the sample's dispersion and absorption, respectively. The chirp is reproducible, so the heterodyne modulation, instantaneous phase, and/or instantaneous amplitude can be average over many measurements, either with multiple pulses from the same laser or multiple pulses from different lasers, each emitting at a different wavelength.

Abstract: In quartz-enhanced photoacoustic spectroscopy (QEPAS), an analyte (typically in gas phase) generates a pressure wave in response to incident laser light. A quartz tuning fork (QTF) resonant at the frequency of the pressure wave transduces the pressure wave into an electrical signal. Pulsing the laser briefly reduces the amount of thermal chirp and increases the fraction of time that the laser emits at the wavelength(s) of interest. This increases the measurement efficiency. Pulsing the incident laser light with bursts of short pulses at the QTF resonant frequency increases signal strength. Exciting the sample with a two pulses at different laser wavelengths, separated by a half QTF period yields signal and background acoustic waves that partially cancel when integrated by the QTF, producing a differential measurement. Pulsing the incident laser light at a frequency faster than the gas response cut off frequency can improve the noise performance of a QEPAS measurement.

Abstract: A spectral encoder includes a thin layer of lossy dielectric material whose thickness varies transversely from 0 to a thickness of about ?/4n (e.g., <100 nm), where ? is the wavelength of incident radiation and n is the dielectric material's refractive index. The dielectric layer reflects (and/or transmits) light at a wavelength that depends on the layer's thickness. Because the dielectric layer's thickness varies, different parts of the dielectric layer may reflect (transmit) light at different wavelengths. For instance, shining white light on a dielectric layer with a linearly varying thickness may produce a rainbow-like reflected (and/or transmitted) beam. Thus, the spectral encoder maps different wavelengths to different points in space. This mapping can be characterized by a transfer matrix which can be used to determine the spectrum of radiation incident on the spectral encoder from the spatial intensity distribution of the radiation reflected (and/or transmitted) by the spectral encoder.

Abstract: A spectral encoder includes a thin layer of lossy dielectric material whose thickness varies transversely from 0 to a thickness of about ?/4n (e.g., <100 nm), where ? is the wavelength of incident radiation and n is the dielectric material's refractive index. The dielectric layer reflects (and/or transmits) light at a wavelength that depends on the layer's thickness. Because the dielectric layer's thickness varies, different parts of the dielectric layer may reflect (transmit) light at different wavelengths. For instance, shining white light on a dielectric layer with a linearly varying thickness may produce a rainbow-like reflected (and/or transmitted) beam. Thus, the spectral encoder maps different wavelengths to different points in space. This mapping can be characterized by a transfer matrix which can be used to determine the spectrum of radiation incident on the spectral encoder from the spatial intensity distribution of the radiation reflected (and/or transmitted) by the spectral encoder.

Abstract: A radiation-emitting device (e.g., a laser) includes an active region configured to generate a radiation emission linearly polarized along a first polarization direction and a device facet covered by an insulating layer and a metal layer on the insulating layer. The metal layer defines an aperture through which the radiation emission from the active region can be transmitted and coupled into surface plasmons on the outer side of the metal layer. The long axis of the aperture is non-orthogonal to the first polarization direction, and a sequential series of features are defined in or on the device facet or in the metal layer and spaced apart from the aperture, wherein the series of features are configured to manipulate the surface plasmons and to scatter surface plasmons into the far field with a second polarization direction distinct from the first polarization direction.

Abstract: A radiation-emitting device (e.g., a laser) includes an active region configured to generate a radiation emission linearly polarized along a first polarization direction and a device facet covered by an insulating layer and a metal layer on the insulating layer. The metal layer defines an aperture through which the radiation emission from the active region can be transmitted and coupled into surface plasmons on the outer side of the metal layer. The long axis of the aperture is non-orthogonal to the first polarization direction, and a sequential series of features are defined in or on the device facet or in the metal layer and spaced apart from the aperture, wherein the series of features are configured to manipulate the surface plasmons and to scatter surface plasmons into the far field with a second polarization direction distinct from the first polarization direction.

Abstract: A radiation-emitting device (e.g., a laser) includes an active region configured to generate a radiation emission linearly polarized along a first polarization direction and a device facet covered by an insulating layer and a metal layer on the insulating layer. The metal layer defines an aperture through which the radiation emission from the active region can be transmitted and coupled into surface plasmons on the outer side of the metal layer. The long axis of the aperture is non-orthogonal to the first polarization direction, and a sequential series of features are defined in or on the device facet or in the metal layer and spaced apart from the aperture, wherein the series of features are configured to manipulate the surface plasmons and to scatter surface plasmons into the far field with a second polarization direction distinct from the first polarization direction.

Abstract: A broadband quantum cascade laser (QCL) source includes one or more QCLs having an active region designed based on a diagonal laser transition. The QCL source may include multiple QCLs formed in an array or the QCL source may comprise a single QCL device. Although each QCL provides an emission spectrum comprising a small range of wavelengths at a given applied voltage, changes in the applied operating voltage result in changes in the emission spectrum of the QCL due to the Stark shift. When the QCL source comprises a plurality of QCLs formed in an array, at least some of the elements in the array may receive different applied operating voltages such that the combined output spectrum of the array is broader than that achievable by a single QCL.

Abstract: A broadband quantum cascade laser includes multiple gain regions and a spacer layer disposed between at least two of the gain regions. The arrangement and characteristics of the gain regions and the spacer layer may be configured to reduce cross absorption between the gain regions. For example, one gain region may be configured to produce gain in an energy range in which another gain region produces absorptive effects. The thickness of the spacer layer may be selected to separate optical modes produced by adjacent gain regions while still producing a single broadband output from the quantum cascade laser. Gain competition between gain stages within a gain region may be mitigated by dividing gain stages with overlapping gain curves among multiple gain regions.

Abstract: A radiation-emitting device (e.g., a laser) includes an active region configured to generate a radiation emission linearly polarized along a first polarization direction and a device facet covered by an insulating layer and a metal layer on the insulating layer. The metal layer defines an aperture through which the radiation emission from the active region can be transmitted and coupled into surface plasmons on the outer side of the metal layer. The long axis of the aperture is non-orthogonal to the first polarization direction, and a sequential series of features are defined in or on the device facet or in the metal layer and spaced apart from the aperture, wherein the series of features are configured to manipulate the surface plasmons and to scatter surface plasmons into the far field with a second polarization direction distinct from the first polarization direction.