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Introduction
My group uses nanoscale engineering to push beyond the limits of conventional electronics and photonics, exploiting quantum phenomena to create devices that interact with the world in extraordinary new ways. We engineer new coherent processes to make devices that utilize the entire electromagnetic spectrum—particularly the extremely challenging terahertz and mid-infrared wavelength regimes.
We not only seek to perform basic science and to use newly-discovered phenomena to create devices, but also using those devices to create systems that seek to address the world’s key challenges.
Publications
Publications (84)
Terahertz light can be used to identify numerous complex molecules, but has traditionally remained unexploited due to the lack of powerful broadband sources. Pulsed lasers can be used to generate broadband radiation, but such sources are bulky and produce only microwatts of average power. Conversely, although terahertz quantum cascade lasers are co...
In this work, we demonstrate all-electronically tunable terahertz quantum cascade lasers (THz QCLs) with MEMS tuner structures. A two-stage MEMS tuner device is fabricated by a commercial open-foundry process performed by the company MEMSCAP. This provides an inexpensive, rapid, and reliable approach for MEMS tuner fabrication for THz QCLs with a h...
Using terahertz time-domain spectroscopy, the gain of scattering-assisted terahertz quantum cascade lasers is measured. By examining the intersubband gain and absorption over a wide range of bias voltages, we experimentally detect energy anticrossings—revealing information about the mechanism of laser action—and compare the resonant-tunneling injec...
A terahertz pulse emitter monolithically integrated with a quantum cascade laser (QCL) is demonstrated. The emitter facet is excited by near-infrared pulses from a mode-locked Ti:sapphire laser, and the resulting current transients generate terahertz pulses that are coupled into an electrically isolated QCL in proximity. These pulses are used to me...
The interfaces of a dielectric sample are resolved in reflection geometry using light from a frequency agile array of terahertz quantum-cascade lasers. The terahertz source is a 10-element linear array of third-order distributed-feedback QCLs emitting at discrete frequencies from 2.08 to 2.4 THz. Emission from the array is collimated and sent throu...
This article presents an overview of the current status and future prospects of integrated nonlinear photonics in the long-wave infrared (LWIR) spectrum, spanning 6 to 14 μm. This range is well-suited for applications such as chemical identification, environmental monitoring, surveillance, search and rescue, and night vision. Nevertheless, the adva...
Quantum cascade lasers (QCLs) have emerged as promising candidates for generating chip-scale frequency combs in mid-infrared and terahertz wavelengths. In this work, we demonstrate frequency comb formation in ring terahertz QCLs using the injection of light from a distributed feedback (DFB) laser. The DFB design frequency is chosen to match the mod...
Fast, sensitive, and compact devices that implement nonlinear activation functions are needed to form fully connected photonic neural networks (PNNs). However, even in highly nonlinear media, optical nonlinearities are relatively weak. We propose here a scheme for implementing nonlinear activation functions that relies on band-structure-engineered...
The longwave infrared (LWIR) region of the spectrum spans 8 to 14 μm and enables high-performance sensing and imaging for detection, ranging, and monitoring. Chip-scale LWIR photonics has enormous potential for real-time environmental monitoring, explosive detection, and biomedicine. However, realizing technologies such as precision sensors and bro...
Frequency-modulated (FM) combs with a linearly-chirped frequency and nearly constant intensity occur naturally in certain laser systems; they can be most succinctly described by a nonlinear Schrödinger equation with a phase potential. In this work, we perform a comprehensive analytical study of FM combs in order to calculate their salient propertie...
Topological crystalline insulators -- topological insulators whose properties are guaranteed by crystalline symmetry -- can potentially provide a promising platform for terahertz optoelectronic devices, as their properties can be tuned on demand when layered in heterostructures. We perform the first optical-pump terahertz-probe spectroscopy of topo...
The three-dimensional images generated by digital holography are usually limited to a single color. A new technique exploiting frequency combs generates holograms with hundreds of colors at once.
The longwave infrared (LWIR) region of the spectrum spans 8 to 14 {\mu}m and enables high-performance sensing and imaging for detection, ranging, and monitoring. Chip-scale integrated LWIR photonics has enormous potential for real-time environmental monitoring, explosive detection, and biomedicine. However, realizing advanced technologies such as p...
Frequency-modulated (FM) combs with a linearly-chirped frequency and nearly constant intensity occur naturally in certain laser systems; they can be most succinctly described by a nonlinear Schr\"odinger equation with a phase potential. In this work, we perform a comprehensive analytical study of FM combs in order to calculate their salient propert...
Multiheterodyne techniques using frequency combs—radiation sources whose lines are perfectly evenly-spaced—have revolutionized science. By beating sources with the many lines of a comb, their spectra are recovered. Even so, these approaches are fundamentally limited to probing coherent sources, such as lasers. They are unable to measure most spectr...
In many laser systems, frequency combs whose output is frequency-modulated (FM) can form, producing light whose frequency sweeps linearly. While this intriguing result has been replicated experimentally and numerically, a compact description of the core physics has remained elusive. By creating a mean-field theory for active cavities analogous to t...
Chip-scale electrically pumped optical frequency combs (OFCs) are expected to play a fundamental role in applications ranging from telecommunications to optical sensing. To date, however, the availability of such sources around 2 μm has been scarce. Here, we present a frequency-modulated OFC operating around 2060 nm of wavelength exploiting the inh...
Frequency combs are light sources with coherent evenly-spaced lines. It has been observed that in certain laser systems, combs can form whose output is frequency-modulated (FM) in time. In this state, they produce an output whose frequency sweeps linearly and periodically. These results have been replicated numerically, but a thorough understanding...
We introduce a new, to the best of our knowledge, passive technique of mitigating the phase noise in optical frequency combs (FC) by reducing the drift of offset frequency. This can be achieved by customizing the dispersion to attain a power law dependence of the wave vector on frequency, k(ω)∼ωα, ensuring a constant ratio between group and phase v...
Multiheterodyne techniques using frequency combs -- light sources whose lines are perfectly evenly-spaced -- have revolutionized optical science. By beating an unknown signal with the many lines of a comb, its spectrum is recovered. However, these techniques have been restricted to measuring coherent sources, such as lasers. In this work, we demons...
Terahertz laser frequency combs based on quantum cascade lasers provide coherent, broadband, electrically pumped, THz radiation sources for use in future spectroscopic applications. Here, we explore the feasibility of such lasers in a dual-comb spectroscopy configuration for detection of multiple molecular samples in the gas phase. The lasers span...
SWIFT spectroscopy (Shifted Wave Interference Fourier Transform Spectroscopy) is a coherent beatnote technique that can be used to measure the temporal profiles of periodic optical signals. While it has been essential in understanding the physics of various mid-infrared and terahertz frequency combs, its ultimate limits have not been discussed. We...
We present the first semiconductor laser frequency comb in the 2 μm region. The source relies of an inherent gain nonlinearity in a quantum well laser diode, which promotes frequency-modulated operation with sub-THz spectral coverage.
We present quantum well diode laser frequency combs suitable for free-running dual-comb spectroscopy in the 2 pm region. A 600-GHz-wide dual-comb spectrum of a low- finesse etalon is measured in 1 millisecond with 4.1% precision.
Frequency combs are light sources whose lines are perfectly evenly-spaced. We will discuss how computational techniques can be used to elucidate their hidden properties and can be used to create novel sensors based on combs.
Precise knowledge of a laser's wavelength is crucial for applications from spectroscopy to telecommunications. Here, we present a wavemeter that operates on the Talbot effect. Tone parameter extraction algorithms are used to retrieve the frequency of the periodic signal obtained in the sampled Talbot interferogram. Theoretical performance analysis...
Precise knowledge of a laser's wavelength is crucial for applications from spectroscopy to telecommunications. Here we present a wavemeter which operates on the Talbot effect. Tone parameter extraction algorithms are used to retrieve the frequency of the periodic signal obtained in the sampled Talbot interferogram. Theoretical performance analysis...
Chip-scale frequency combs such as those based on quantum cascade lasers (QCLs) or microresonators are attracting tremendous attention because of their potential to solve key challenges in sensing and metrology. Though nonlinearity and proper dispersion engineering can create a comb—light whose lines are perfectly evenly spaced—these devices can en...
Hyperspectral imaging is a spectroscopic imaging technique that allows for the creation of images with pixels containing information from multiple spectral bands. At terahertz wavelengths, it has emerged as a prominent tool for a number of applications, ranging from nonionizing cancer diagnosis and pharmaceutical characterization to nondestructive...
We demonstrate a computational phase correction algorithm that is able to correct for phase and timing fluctuations of arbitrary dual comb spectra. By augmenting a Kalman filter with a global search and decoupling the interferogram estimation, we show that dual comb signals having a wide range of structures can be predicted and corrected. Furthermo...
Hyperspectral imaging is a spectroscopic imaging technique that allows for the creation of images with pixels containing information from multiple spectral bands. At terahertz wavelengths, it has emerged as a prominent tool for a number of applications, ranging from nonionizing cancer diagnosis and pharmaceutical characterization to nondestructive...
2019 SPIE. Quantum cascade laser-based frequency combs have attracted much attention as of late for applications in sensing and metrology, especially as sources for chip-scale spectroscopy at mid-infrared fingerprint wavelengths. A frequency comb is a light source whose lines are evenly-spaced, and only two frequencies are needed to describe the sy...
Quantum cascade laser frequency combs have substantial potential in sensing. We show that by blending them with microelectromechanical comb drives, one can directly manipulate the dynamics of the laser and fully control the comb state.
Hyperspectral imaging is a technique that allows for the creation of multi-color images. At terahertz wavelengths, it has emerged as a prominent tool for a number of applications, ranging from non-ionizing cancer diagnosis and pharmaceutical characterization to non-destructive artifact testing. Contemporary terahertz imaging systems typically rely...
For many applications, optical frequency combs (OFCs) require a high degree of temporal coherence (narrow linewidth). Commonly, OFCs are generated in nonlinear media from a monochromatic narrow linewidth laser source or from a mode-locked laser pulse, but in all the important mid-infrared (MIR) and terahertz (THz) regions of the spectrum, OFCs can...
For many applications Optical Frequency Combs (OFCs) require a high degree of temporal coherence (narrow linewidth). Commonly OFCs are generated in nonlinear media from a monochromatic narrow linewidth laser sources or from a mode-locked laser pulses but in the all-important mid-infrared (MIR) and terahertz (THz) regions of spectrum OFCs can be gen...
Broadband terahertz radiation potentially has extensive applications, ranging from personal health care to industrial quality control and security screening. While traditional methods for broadband terahertz generation rely on bulky and expensive mode-locked lasers, frequency combs based on quantum cascade lasers (QCLs) can provide an alternative c...
We consider here a time domain model representing the dynamics of quantum cascade lasers (QCLs) generating frequency combs (FCs) in both THz and long wave infrared (LWIR λ = 8-12µm) spectral ranges. Using common specifications for these QCLs we confirm that the free running laser enters a regime of operation yielding a pseudo-randomly frequency mod...
By incorporating a two-section geometry featuring an electrically tunable dispersion compensator, we demonstrate terahertz quantum cascade lasers exhibiting comb operation from Ith to Imax, covering the entire bias range of lasing.
We demonstrate THz dual comb spectroscopy of molecular samples using dispersion compensated quantum cascade lasers. The system achieves an optical bandwidth of ~150 GHz at 2.9 THz and is used to measure ammonia in gas phase.
Frequency combs based on quantum cascade lasers (QCLs) are finding promising applications in high-speed broadband spectroscopy in the terahertz regime, where many molecules have their “fingerprints.” To form stable combs in QCLs, an effective control of group velocity dispersion plays a critical role. The dispersion of the QCL cavity has two main p...
Recent research has shown that free-running quantum cascade lasers are capable of producing frequency combs in midinfrared and THz regions of the spectrum. Unlike familiar frequency combs originating from mode-locked lasers, these do not require any additional optical elements inside the cavity and have temporal characteristics that are dramaticall...
We study the feasibility of dispersion compensated terahertz quantum cascade lasers combs operating around 2.65-2.95 THz to perform multiheterodyne spectroscopy. The devices show short-term intermode beat note frequency drift after 30 μs, which indicates that reliable multiheterodyne spectroscopy over extended time-scale requires phase and timing c...
In recent years, quantum cascade lasers (QCLs) have shown tremendous potential for the generation of frequency combs in the mid-infrared and terahertz portions of the electromagnetic spectrum. The research community has experienced success both in the theoretical understanding and experimental realization of QCL devices, capable of generating stabl...
Optical frequency combs are light sources that consist of many evenly-spaced lines. I will discuss recent developments in frequency combs based on terahertz quantum cascade lasers, which emit broadband comb light in a compact package.
Optical frequency combs are light sources that consist of many evenly-spaced lines. I will discuss recent developments in terahertz quantum cascade laser based frequency combs, and will demonstrate their utility in compact dual comb spectroscopy.
A key parameter underlying the efficacy of any nonlinear optical process is group velocity dispersion. In quantum cascade lasers (QCLs), there have been several recent demonstrations of devices exploiting nonlinearities in both the mid-infrared and the terahertz. Though the gain of QCLs has been well studied, the dispersion has been much less inves...
The generation of frequency combs in the mid-infrared and terahertz regimes from compact and potentially cheap sources could have a strong impact on spectroscopy, as many molecules have their rotovibrational bands in this spectral range. Thus, quantum cascade lasers (QCLs) are the perfect candidates for comb generation in these portions of the elec...
Due to their broad spectral bandwidth and superior temperature performance, resonant phonon quantum cascade laser (QCL) designs have become the active-region of choice for many of the leading groups in terahertz (THz) QCL research. These gain media can vary substantially in the number of wells and barriers as well as their corresponding thicknesses...
Dual comb spectroscopy allows for high-resolution spectra to be measured over broad bandwidths, but an essential requirement for coherent integration is the availability of a phase reference. Usually, this means that the combs' phase and timing errors must be measured and either minimized by stabilization or removed by correction, limiting the tech...
Frequency combs based on terahertz quantum cascade lasers feature broadband coverage and high output powers in a compact package, making them an attractive option for broadband spectroscopy. Here, we demonstrate the first multi-heterodyne spectroscopy using two terahertz quantum cascade laser combs. With just 100 $\mu$s of integration time, we achi...
Utilizing the Kalman filter-based averaging scheme, we demonstrated THz dual-comb spectroscopy covering 282 GHz at ~2.8 THz with unstabilized quantum cascade laser frequency combs. The peak signal-to-noise ratio(SNR) is 60 dB within 100 us averaging.
The temporal profile of frequency combs based on quantum cascade lasers has been unclear for some time. We show how the SWIFTS technique directly measures such properties, obtaining combs' intensities and frequencies versus time.
Recently, much attention has been focused on the generation of optical frequency combs from quantum cascade lasers. We discuss how fast detectors can be used to demonstrate the mutual coherence of such combs, and present an inequality that can be used to quantitatively evaluate their performance. We discuss several technical issues related to shift...
In recent years, quantum cascade lasers have emerged as mature semiconductor sources of light in the terahertz range, the frequency range spanning 1 to 10 THz. Though technological development has pushed their operating temperatures up to 200 Kelvin and their power levels up to Watt-level, they have remained unsuitable for many applications as a re...
We demonstrate broadband terahertz laser frequency combs, compact semiconductor devices that combine the high power of lasers with the broad spectra of pulsed sources.
A terahertz pulse emitter is fabricated alongside a quantum cascade laser with a metal� metal waveguide. Terahertz pulses are used to measure the gain of the laser ridge, which is clamped above threshold to 18 cm �1 . OCIS codes: 140.5965, 300.6495, 300.6500 In the past few years, terahertz time domain spectroscopy (THzTDS) has been demonstrated as...
Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2009. This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. Cataloged from student-submitted PDF version of thesis. Includes bibliographical reference...
