Iman Kundu

Apertureless near-field terahertz imaging using the self-mixing effect in a quantum cascade laser

We report two-dimensional apertureless near-field terahertz (THz) imaging using a quantum cascade laser(QCL)source and a scattering probe. A near-field enhancement of the scattered field amplitude is observed for small tip-sample separations, allowing image resolutions of ∼1 μm (∼λ/100) and ∼7 μm to be achieved along orthogonal directions on the sample surface. This represents the highest resolution demonstrated to date with a THz QCL. By employing a detection scheme based on self-mixing interferometry, our approach offers experimental simplicity by removing the need for an external detector and also provides sensitivity to the phase of the reinjected field.

Discrete Vernier tuning in terahertz quantum cascade lasers using coupled cavities

Discrete Vernier frequency tuning of terahertz quantum cascade lasers is demonstrated using a device comprising a two-section coupled-cavity. The two sections are separated by a narrow air gap, which is milled after device packaging using a focused ion beam. One section of the device (the lasing section) is electrically biased above threshold using a short current pulse, while the other section (the tuning section) is biased below threshold with a wider current pulse to achieve controlled localized electrical heating. The resulting thermally-induced shift in the longitudinal cavity modes of the tuning section is engineered to produce either a controllable blue shift or red shift of the emission frequency. This discrete Vernier frequency tuning far exceeds the tuning achievable from standard ridge lasers, and does not lead to any corresponding change in emitted power. Discrete tuning was observed over bandwidths of 50 and 85 GHz in a pair of devices, each using different design schemes. Interchanging the lasing and tuning sections of the same devices yielded red shifts of 20 and 30 GHz, respectively.

Quasi-continuous frequency tunable terahertz quantum cascade lasers with coupled cavity and integrated photonic lattice

We demonstrate quasi-continuous tuning of the emission frequency from coupled cavity terahertz frequency quantum cascade lasers. Such coupled cavity lasers comprise a lasing cavity and a tuning cavity which are optically coupled through a narrow air slit and are operated above and below the lasing threshold current, respectively. The emission frequency of these devices is determined by the Vernier resonance of longitudinal modes in the lasing and the tuning cavities, and can be tuned by applying an index perturbation in the tuning cavity. The spectral coverage of the coupled cavity devices have been increased by reducing the repetition frequency of the Vernier resonance and increasing the ratio of the free spectral ranges of the two cavities. A continuous tuning of the coupled cavity modes has been realized through an index perturbation of the lasing cavity itself by using wide electrical heating pulses at the tuning cavity and exploiting thermal conduction through the monolithic substrate. Single mode emission and discrete frequency tuning over a bandwidth of 100 GHz and a quasi-continuous frequency coverage of 7 GHz at 2.25 THz is demonstrated. An improvement in the side mode suppression and a continuous spectral coverage of 3 GHz is achieved without any degradation of output power by integrating a π-phase shifted photonic lattice in the laser cavity.

Mode Selection and Tuning Mechanisms in Coupled-Cavity Terahertz Quantum Cascade Lasers

We present a model for longitudinal mode competition in coupled-cavity (CC) terahertz (THz) quantum cascade lasers (QCLs) by using a scattering matrix method and multi-mode reduced rate equations (RREs). The dependence of the mode selection and tuning characteristics on various device parameters is systematically investigated, including the net waveguide loss, the optical length of the passive cavity, and the heat-sink temperature for different relationship between the active and passive cavity lengths. The changes in eigenmode frequencies due to variations of device parameter are calculated before solving the RREs. The mode selection and tuning results obtained from solving the nonlinear RREs could be well explained by the linear scattering matrix analysis. The mode tuning process simulated by the proposed model is compared with experimentally measured data, yielding good agreement. Comprehensive study of the influence of the key device parameters on the performance of CC THz QCLs provides potential design rules for single-mode operation with either wide frequency tunability or high stability.

Terahertz frequency quantum cascade lasers: Optical feedback effects and applications

Remarkable progress in terahertz (THz) technology over the past decade has been driven by the potential applications of THz waves in areas such as biomedical imaging, long-range screening, and organic materials identification [1]. This growth is in no small measure related to the success of the quantum cascade laser (QCL) which has established itself as one of the most promising radiation sources at terahertz frequencies [2]. The appeal of these novel semiconductor lasers stems from their compact size, broad spectral coverage (~ 1-5 THz), and high output powers [3]. The ability of THz QCLs to generate coherent emission with quantum noise-limited linewidths, make them particularly suited to the development of interferometric THz sensing and imaging systems.

Origin of terminal voltage variations due to self-mixing in terahertz frequency quantum cascade lasers.

We explain the origin of voltage variations due to self-mixing in a terahertz (THz) frequency quantum cascade laser (QCL) using an extended density matrix (DM) approach. Our DM model allows calculation of both the current-voltage (I-V) and optical power characteristics of the QCL under optical feedback by changing the cavity loss, to which the gain of the active region is clamped. The variation of intra-cavity field strength necessary to achieve gain clamping, and the corresponding change in bias required to maintain a constant current density through the heterostructure is then calculated. Strong enhancement of the self-mixing voltage signal due to non-linearity of the (I-V) characteristics is predicted and confirmed experimentally in an exemplar 2.6 THz bound-to-continuum QCL.

Multi-spectral terahertz sensing: proposal for a coupled-cavity quantum cascade laser based optical feedback interferometer

We propose a laser feedback interferometer operating at multiple terahertz (THz) frequency bands by using a pulsed coupled-cavity THz quantum cascade laser (QCL) under optical feedback. A theoretical model that contains multi-mode reduced rate equations and thermal equations is presented, which captures the interplay between electro-optical, thermal, and feedback effects. By using the self-heating effect in both active and passive cavities, self-mixing signal responses at three different THz frequency bands are predicted. A multi-spectral laser feedback interferometry system based on such a coupled-cavity THz QCL will permit ultra-high-speed sensing and spectroscopic applications including material identification.

Ultrafast switch-on dynamics of frequency-tuneable semiconductor lasers

Single-mode frequency-tuneable semiconductor lasers based on monolithic integration of multiple cavity sections are important components, widely used in optical communications, photonic integrated circuits and other optical technologies. To date, investigations of the ultrafast switching processes in such lasers, essential to reduce frequency cross-talk, have been restricted to the observation of intensity switching over nanosecond-timescales. Here, we report coherent measurements of the ultrafast switch-on dynamics, mode competition and frequency selection in a monolithic frequency-tuneable laser using coherent time-domain sampling of the laser emission. This approach allows us to observe hopping between lasing modes on picosecond-timescales and the temporal evolution of transient multi-mode emission into steady-state single mode emission. The underlying physics is explained through a full multi-mode, temperature-dependent carrier and photon transport model. Our results show that the fundamental limit on the timescales of frequency-switching between competing modes varies with the underlying Vernier alignment of the laser cavity.Single-mode, tuneable monolithic semiconductor lasers are important light sources for integrated photonics. Here, Kundu et al. observe the switch-on dynamics and mode competition of a terahertz quantum cascade laser and explain the behaviour with a carrier and photon transport model.

Frequency Tunability and Spectral Control in Terahertz Quantum Cascade Lasers With Phase-Adjusted Finite-Defect-Site Photonic Lattices

We report on the effect of finite-defect-site photonic lattices (PLs) on the spectral emission of terahertz frequency quantum cascade lasers, both theoretically and experimentally. A central π-phase adjusted defect incorporated in the PL is shown to favor emission selectively within the photonic bandgap. The effect of the duty cycle and the longitudinal position of such PLs is investigated, and used to demonstrate three distinct spectral behaviors: single-mode emission from devices in the range 2.2−5 THz, with a side-mode suppression ratio of 40 dB and exhibiting continuous frequency tuning over >8 GHz; discrete tuning between two engineered emission modes separated by ∼40 GHz; and multiple-mode emission with an engineered frequency spacing between emission lines.

Optical feedback effects on terahertz quantum cascade lasers: modelling and applications

Terahertz (THz) quantum cascade lasers (QCLs) are compact sources of radiation in the 1–5 THz range with significant potential for applications in sensing and imaging. Laser feedback interferometry (LFI) with THz QCLs is a technique utilizing the sensitivity of the QCL to the radiation reflected back into the laser cavity from an external target. We will discuss modelling techniques and explore the applications of LFI in biological tissue imaging and will show that the confocal nature of the QCL in LFI systems, with their innate capacity for depth sectioning, makes them suitable for skin diagnostics with the well-known advantages of more conventional confocal microscopes. A demonstration of discrimination of neoplasia from healthy tissue using a THz, LFI-based system in the context of melanoma is presented using a transgenic mouse model.