Nick K. Hon

The third-order nonlinear optical coefficients of Si, Ge, and Si1−xGex in the midwave and longwave infrared

Using a combination of semiconductor theory and experimental results from the scientific literature, we have compiled and plotted the key third-order nonlinear optical coefficients of bulk crystalline Si and Ge as a function of wavelength (1.5−6.7 μm for Si and 2–14.7 μm for Ge). The real part of third-order nonlinear dielectric susceptibility (χ(3)′), the two-photon absorption coefficient (βTPA), and the Raman gain coefficient (gR), have been investigated. Theoretical predictions were used to curve-fit the experimental data. For a spectral range in which no experimental data exists, we estimate and fill in the missing knowledge. Generally, these coefficient-values appear quite useful for a host of device applications, both Si and Ge offer large χ(3)′ and gR with Ge offering the stronger nonlinearity. In addition, we use the same theory to predict the third-order nonlinear optical coefficients of Si1−xGex alloy. By alloying Si and Ge, device designers can gain flexibility in tuning desired optical coeffic...

Glass-clad single-crystal germanium optical fiber.

Long lengths (250 meters) of a flexible 150 microm diameter glass-clad optical fiber containing a 15 microm diameter crystalline and phase-pure germanium core was fabricated using conventional optical fiber draw techniques. X-ray diffraction and spontaneous Raman scattering measurements showed the core to be very highly crystalline germanium with no observed secondary phases. Elemental analysis confirmed a very well-defined core-clad interface with a step-profile in composition and nominally 4 weight-percent oxygen having diffused into the germanium core from the glass cladding. For this proof-of-concept fiber, polycrystalline n-type germanium of unknown dopant concentration was used. The measured infrared transparency of the starting material was poor and, as a likely outcome, the attenuation of the resultant fiber was too high to be measured. However, the larger Raman cross-section, infrared and terahertz transparency of germanium over silicon should make these fibers of significant value for fiber-based mid- to long-wave infrared and terahertz waveguides and Raman-shifted infrared light sources once high-purity, high-resistivity germanium is employed.

Silica-clad crystalline germanium core optical fibers

Silica-clad optical fibers comprising a core of crystalline germanium were drawn using a molten core technique. With respect to previous fibers drawn using a borosilicate cladding, the present fibers exhibit negligible oxygen despite being fabricated at more than twice the melting point of the germanium. The counterintuitive result of less oxygen when the fiber is drawn at a higher temperatures is discussed. The measured propagation loss for the fiber was 0.7 dB/cm at 3.39 μm, which is the lowest loss reported to date.

Giant tunable optical dispersion using chromo-modal excitation of a multimode waveguide

The ability to control chromatic dispersion is paramount in applications where the optical pulsewidth is critical, such as chirped pulse amplification and fiber optic communications. Typically, devices used to generate large amounts (>100 ps/nm) of chromatic dispersion are based on diffraction gratings, chirped fiber Bragg gratings, or dispersion compensating fiber. Unfortunately, these dispersive elements suffer from one or more of the following restrictions: (i) limited operational bandwidth, (ii) limited total dispersion, (iii) low peak power handling, or (iv) large spatial footprint. Here, we introduce a new type of tunable dispersive device, which overcomes these limitations by leveraging the large modal dispersion of a multimode waveguide in combination with the angular dispersion of diffraction gratings to create chromatic dispersion. We characterize the devices dispersion, and demonstrate its ability to stretch a sub-picosecond optical pulse to nearly 2 nanoseconds in 20 meters of multimode optical fiber. Using this device, we also demonstrate single-shot, time-wavelength atomic absorption spectroscopy at a repetition rate of 90.8 MHz.

Tailoring the biodegradability of porous silicon nanoparticles

Porous silicon nanoparticles (PSiNPs) are attractive carriers for targeted drug delivery in nanomedicine. For in vivo applications, the biodegradation property of PSiNPs provides a pathway for their safe clearance from the body. Particles sizes of 80-120 nm are of particular interest as they are important for cellular applications, such as drug delivery for cancer therapy, because these nanoparticles can take advantage of the enhanced permeability and retention effect to deliver drug preferentially to tumors with leaky vasculature, yet large enough to avoid renal clearance. However, the biodegradability rate of such particles is often too fast, which limits particle half-life and potentially reduces their in vivo delivery efficiency. In this work, we focus on the degradation of nanoscale particles and study the effect of both thermal oxidation and silica coating on the stability of PSiNPs in phosphate buffered saline solution (a close mimic of a basic biological fluid). Using thermal oxidation, the half-life of PSiNPs can be varied from 10 min up to 3 h. Using silica coating, the half-life can be extended further to 8 h. The particles produced using both these techniques can be functionalized using standard silica surface chemistries developed for applications in drug delivery.

Stress-induced χ (2) in silicon — Comparison between theoretical and experimental values

We provide a new theoretical estimation of stress-induced χ(2) in silicon and highlight the fact that there exists a large difference between theoretical and experimentally measured values. Possible reasons for this discrepancy are discussed.

Real-time wavelength and bandwidth-independent optical integrator based on modal dispersion.

High-throughput real-time optical integrators are of great importance for applications that require ultrafast optical information processing, such as real-time phase reconstruction of ultrashort optical pulses. In many of these applications, integration of wide optical bandwidth signals is required. Unfortunately, conventional all-optical integrators based on passive devices are usually sensitive to the wavelength and bandwidth of the optical carrier. Here, we propose and demonstrate a passive all-optical intensity integrator whose operation is independent of the optical signal wavelength and bandwidth. The integrator is implemented based on modal dispersion in a multimode waveguide. By controlling the launch conditions of the input beam, the device produces a rectangular temporal impulse response. Consequently, a temporal intensity integration of an arbitrary optical waveform input is performed within the rectangular time window. The key advantage of this device is that the integration operation can be performed independent of the input signal wavelength and optical carrier bandwidth. This is preferred in many applications where optical signals of different wavelengths are involved. Moreover, thanks to the use of a relatively short length of multimode waveguide, lower system latency is achieved compared to the systems using long dispersive fibers. To illustrate the versatility of the optical integrator, we demonstrate temporal intensity integration of optical waveforms with different wavelengths and optical carrier bandwidths. Finally, we use this device to perform high-throughput, single-shot, real-time optical phase reconstruction of phase-modulated signals at telecommunications bit rates.

Periodically-Poled Silicon

As a centrosymmetric crystal, bulk silicon lacks the crucial second-order nonlinearity χ(2) - the cornerstone of parametric light conversion. Although it has been demonstrated that mechanical stress can create χ(2) effects [1,2], efficient parametric χ(2) processes require a means to achieve phase matching. A powerful approach for efficient parametric conversion is quasi-phase matching (QPM) by periodic poling. However, conventional poling methods are not applicable to silicon because it lacks an intrinsic dipole moment. Here we show through numerical simulations, that (1) periodic stress gradients along a silicon waveguide can be realized by integrated thin films, (2) the stress gradient create χ(2) and modulate its sign in a periodic fashion, and (3) the structure can lead to efficient generation of mid-wave infrared (MWIR) from commonly available near-infrared (NIR) laser sources. This so-called Periodically Poled Silicon (PePSi) brings the powerful periodic poling capability to silicon, where the excellent materials properties and mature processing technology can be exploited for χ(2) processes.

Silicon PIN quasi-continuous wave Terahertz emitter

Enhanced external efficiency, improved coherence and tunability are demonstrated for silicon PIN THz emitters by utilizing the internal reflection and multi optical pulse excitation.

Periodically poled silicon

Bulk centrosymmetric silicon lacks second-order optical nonlinearity χ(2) - a foundational component of nonlinear optics. Here, we propose a new class of photonic device which enables χ(2) as well as quasi-phase matching based on periodic stress fields in silicon - periodically-poled silicon (PePSi). This concept adds the periodic poling capability to silicon photonics, and allows the excellent crystal quality and advanced manufacturing capabilities of silicon to be harnessed for devices based on χ(2)) effects. The concept can also be simply achieved by having periodic arrangement of stressed thin films along a silicon waveguide. As an example of the utility, we present simulations showing that mid-wave infrared radiation can be efficiently generated through difference frequency generation from near-infrared with a conversion efficiency of 50% based on χ(2) values measurements for strained silicon reported in the literature [Jacobson et al. Nature 441, 199 (2006)]. The use of PePSi for frequency conversion can also be extended to terahertz generation. With integrated piezoelectric material, dynamically control of χ(2)nonlinearity in PePSi waveguide may also be achieved. The successful realization of PePSi based devices depends on the strength of the stress induced χ(2) in silicon. Presently, there exists a significant discrepancy in the literature between the theoretical and experimentally measured values. We present a simple theoretical model that produces result consistent with prior theoretical works and use this model to identify possible reasons for this discrepancy.